Applications of Transition Metal-Catalyzed ortho-Fluorine-Directed C-H Functionalization of (Poly)fluoroarenes in Organic Synthesis

The synthesis of organic compounds efficiently via fewer steps but in higher yields is desirable as this reduces energy and reagent use, waste production, and thus environmental impact as well as cost. The reactivity of C-H bonds ortho to fluorine substituents in (poly)fluoroarenes with metal centers is enhanced relative to meta and para positions. Thus, direct C-H functionalization of (poly)fluoroarenes without prefunctionalization is becoming a significant area of research in organic chemistry. Novel and selective methodologies to functionalize (poly)fluorinated arenes by taking advantage of the reactivity of C-H bonds ortho to C-F bonds are continuously being developed. This review summarizes the reasons for the enhanced reactivity and the consequent developments in the synthesis of valuable (poly)fluoroarene-containing organic compounds.

Fluoride Abstraction Induced by Tris(pentafluoroethyl)difluorophosphorane: A Convenient Way to Synthesize Cationic N-Heterocyclic Carbene- and Cyclic (Alkyl)(amino)carbene-Ligated Copper Alkyne and Arene Complexes

We herein report the convenient synthesis of different N-heterocyclic carbene (NHC)- and cyclic (alkyl)(amino)carbene (cAAC)-ligated copper cations using the weakly coordinating tris(pentafluoroethyl)trifluorophosphate counterion (FAP anion, [(C2F5)3PF3]-). The reaction of the fluorido complexes [(carbene)CuF] (carbene = NHC, cAACMe) 2a-2f and the tris(pentafluoroethyl)difluorophosphorane (C2F5)3PF2 in the presence of alkynes or arenes led to fluoride transfer from Cu to the phosphorane with formation of the cationic transition metal complexes [(carbene)Cu(L)]+ and the weakly coordinating counteranion [(C2F5)3PF3]- (FAP). Using this method, the complexes [(IDipp)Cu(L)]+FAP- (IDipp = 1,3-bis(2,6-di-iso-propylphenyl)-imidazolin-2-ylidene; L = PhC≡CPh, 4d; PhC≡CMe, 5d), [(cAACMe)Cu(L)]+FAP- (cAACMe = 1-(2,6-di-iso-propylphenyl)-3,3,5,5-tetramethyl-pyrrolidin-2-ylidene; L = PhC≡CPh, 4f; PhC≡CMe, 5f), [(SIDipp)Cu(C6Me6)]+FAP- (6e), (SIDipp = 1,3-bis(2,6-di-iso-propylphenyl)-imidazolidine-2-ylidene), and [(cAACMe)Cu(C6Me6)]+FAP- (6f) have been synthesized and characterized. The complexes [(IDipp)Cu(C6Me6)]+FAP- (6d) and [(cAACMe)Cu(C6Me6)]+FAP- (6f) have been used as catalysts for the copper(I)-catalyzed cycloaddition of benzyl azide to terminal alkynes.

Pd-Catalyzed Oxidative C-H Arylation of (Poly)fluoroarenes with Aryl Pinacol Boronates and Experimental and Theoretical Studies of its Reaction Mechanism

We report the synergistic combination of Pd(OAc)2 and Ag2O for the oxidative C-H arylation of (poly)fluoroarenes with aryl pinacol boronates (Ar-Bpin) in DMF as the solvent. This procedure can be conducted easily in air, and without using additional ligands, to afford the fluorinated unsymmetrical biaryl products in up to 98 % yield. Experimental studies suggest that the formation of [PdL2(C6F5)2] in DMF as coordinating solvent does not take place under the reaction conditions as it is stable to reductive elimination and thus would deactivate the catalyst. Thus, the intermediate [Pd(DMF)2(ArF)(Ar)] must be formed selectively to give desired arylation products. DFT calculations predict a low barrier (5.87 kcal/mol) for the concerted metalation deprotonation (CMD) process between C6F5H and the Pd(II) species formed after transmetalation between the Pd(II)X2 complex and aryl-Bpin which forms a Pd-Arrich species. Thus a Pd(Arrich)(Arpoor) complex is generated selectively which undergoes reductive elimination to generate the unsymmetrical biaryl product.

Phosphine-catalyzed 1,2-cis-diboration of 1,3-butadiynes

Trialkyl phosphines PMe3 and PEt3 catalyze the 1,2-cis-diboration of 1,3-butadiynes to give 1,2-diboryl enynes. The products were utilized to synthesize 1,1,2,4-tetraaryl enynes using a Suzuki-Miyaura protocol and can readily undergo proto-deborylation.

NHC-ligated indenyl- and fluorenyl-substituted Alanes and Gallanes: synthons towards indenyl- and fluorenyl-bridged (AlC)n-heterocycles (n = 2,3)

Indenyl-(Ind) and fluorenyl-(Fl) substituted NHC-stabilized alanes and gallanes (NHC)·EH2R 1-12 (NHC = IiPrMe, IiPr, IMeMe; E = Al, Ga; R = Ind, Fl) were prepared via reaction of the corresponding NHC-iodoalanes and -gallanes with LiInd and LiFl, respectively. Analogously, the alane adducts with two Ind/Fl substituents (NHC)·AlHR213-18 (NHC = IiPrMe, IiPr, IMeMe; R = Ind, Fl) were obtained by using two equivalents of LiInd/LiFl. Elimination of indene and fluorene was induced thermally affording unusual dimeric and trimeric NHC-alane adducts {(NHC)·AlH2}2-μ-Fl 19-20 and {(NHC)·AlH-μ-R}n21-23 (R = Ind, Fl; n = 2, 3) with bridging indenyl and fluorenyl ligands.

NHC-ligated nickel(II) cyanoborate complexes and salts

A comprehensive study on the synthesis and characterization of NHC-ligated nickel(II) cyanoborates (CBs) is presented (NHC = N-heterocyclic carbene). Nickel(II) cyanoborates Ni[BH2(CN)2]2·H2O (Ib·H2O), Ni[BH(CN)3]2·0.5H2O (Ic·0.5H2O), Ni[B(CN)4]2·H2O (Id·H2O) were reacted with selected NHCs of different steric size. The reaction of the nickel cyanoborates with small to medium-sized NHCs Me2ImMe and iPr2Im (R2Im = 1,3-di-organyl-imidazolin-2-ylidene; R2ImMe = 1,3-di-organyl-4,5-dimethyl-imidazolin-2-ylidene) afforded cyanoborate salts containing the rare homoleptic fourfold NHC-ligated nickel(II) cations [Ni(NHC)4]2+ (NHC = Me2ImMe (1c-d), iPr2Im (2c-d)) and cyanoborate counter-anions. Bulkier NHCs such as Mes2Im and Dipp2Im afforded complexes trans-[Ni(NHC)2(CB)2] (trans-4b, trans-5c). For the combination of the cyanoborate anion [BH2(CN)2]- and iPr2ImMe the salt of the tris-NHC complex [Ni(iPr2ImMe)3(NC-BH2CN)][BH2(CN)2] (3b) was isolated. Salt metathesis of NHC-ligated nickel(II) halides (Ni(NHC)2X2) (X = Cl, Br) with silver(I) and alkali metal cyanoborates were used to synthesize mono- and disubstituted coordination compounds of the type cis- or trans-[Ni(NHC)2(CB)X] (cis-10c, cis-11c, trans-12b) and cis- or trans-[Ni(NHC)2(CB)2] (cis-13b, cis-14a-c, trans-14a-b, trans-15b, trans-5b). Further investigations reveal that NHC-ligated cyanoborate complexes can act as building blocks for coordination polymers, as observed for structurally characterized 1∞{trans-[Ni(Mes2Im)2(μ2-[NC-BH2-CN])2]·2Ag(μ2-[BH2(CN)2])} (trans-5b·Ag). This study demonstrates the diverse character of cyanoborates in coordination chemistry as both, non-coordinating counter-anions, and weakly to medium coordinating anions forming novel transition metal complexes and salts. It provides evidence that a proper choice of cyanoborate and a proper choice of co-ligand can lead to a rich coordination chemistry of cyanoborate anions.

How to Decarbonize N-Heterocyclic Carbenes (NHCs): The simple Alane Adducts (NHC) ⋅ AlR3 (R=H, Me, Et)

The reaction of the amine-stabilized alane (NMe3) ⋅ AlH3 1 with the backbone-saturated N-heterocyclic carbene (NHC) SIDipp (SIDipp=1,3-bis-{2,6-di-iso-propyl-phenyl}-imidazolidin-2-ylidene) at 0 °C yielded the NHC alane adduct (SIDipp) ⋅ AlH3 2. Reaction at elevated temperatures or prolonged reaction at room temperature gave the product of a ring expansion reaction (RER) of the NHC, (NMe3) ⋅ AlH(RER-SIDippH2) 3 ⋅ (NMe3). Subsequent reaction of the latter with sterically less hindered NHCs (IMeMe {=1,3,4,5-tetramethyl-imidazolin-2-ylidene}, IiPrMe {=1,3-di-iso-propyl-4,5-dimethyl-imidazolin-2-ylidene}, and IiPr {=1,3-di-iso-propyl-imidazolin-2-ylidene}) afforded the NHC-stabilized RER-products (NHC) ⋅ AlH(RER-SIDippH2) 3 ⋅ (NHC) (NHC=IMeMe, IiPrMe, IiPr), while no reaction was observed with the sterically more demanding NHCs IDipp (=1,3-bis-{2,6-di-iso-propyl-phenyl}-imidazolin-2-ylidene), SIDipp and ItBu (=1,3-di-tert-butyl-imidazolin-2-ylidene). The compounds 3 ⋅ (NHC) were also obtained starting from (SIDipp) ⋅ AlH3 2 and NHC at room temperature. Heating solutions of (SIDipp) ⋅ AlH3 2 without additional base to 95 °C resulted in decarbonization of the NHC and substitution of the carbene carbon atom with aluminum hydride under loss of ethene. Subsequent dimerization afforded cis-[AlH{μ-N(Dipp)CH2CH2N(Dipp)}]2 4_dimer. Heating solutions of the NHC-ligated aluminum alkyls (SIDipp) ⋅ AlR3 2R (R=Me, Et) to 145 °C instead led to complete scission of the NHC backbone with evolution of ethene and isolation of the dialkylaluminium(III) amidinates {DippNC(R)NDipp}AlR2 5R (R=Me, Et).

Nickel(II) Cyanoborates and Cyanoborate-Ligated Nickel(II) Complexes

Nickel(II) cyanoborates Ni[BH2(CN)2]2·H2O (1b·H2O), Ni[BH(CN)3]2·0.5H2O (1c·0.5H2O), and Ni[B(CN)4]2·0.5H2O (1d·0.5H2O) were synthesized, and their reactivity with respect to dppeO2 (=1,2-bis-(diphenylphosphinoethane dioxide)), pyNO (=pyridine-N-oxide), dppe (=1,2-bis-(diphenylphosphinoethane), and DMSO (=dimethyl sulfoxide) was examined. Using these ligands, either cyanoborate (CB) complex salts of [Ni(dppe)2]2+ (2b-d) and [Ni(pyNO)6]2+ (3c-d) were isolated or complexes [Ni(DMSO)4{NC-B(CN)3}2] (1dDMSO) and [Ni(dppeO2)2{NC-B(CN)3}2] (1ddppeO2) were formed. Salt metathesis of [Ni(dppe)Cl2] with alkali metal cyanoborates resulted in mono- and disubstituted coordination compounds [Ni(dppe){NC-BH(CN)2}Cl] (5c) and [Ni(dppe){NC-BH2CN)2}] (4b), which decomposed to salts 2b-d. The synthetical pathways explored offer convenient routes to nickel(II) cyanoborates, nickel(II) complexes ligated with cyanoborates, and nickel(II) complex salts of cyanoborates. Further, our studies demonstrate the diverse character of cyanoborates in coordination chemistry as noncoordinating counteranions and also as medium coordinating anions forming novel transition-metal complexes and salts.

NHC-Stabilized Dialanes(4) of Al2 Mes4

The synthesis and characterization of novel N-heterocyclic carbene (NHC) stabilized dialanes Al2 Mes4 as well as first investigations concerning the reactivity of these compounds are reported. The synthesis of these compounds proceeds via the mesityl-substituted alanes (NHC)⋅AlHMes2 (NHC=IMeMe {=1,3,4,5-tetramethyl-imidazolin-2-ylidene}, IiPrMe {=1,3-di-iso-propyl-4,5-dimethylimidazolin-2-ylidene}) and iodo-alanes (NHC)⋅AlIMes2 (NHC=IMeMe , IiPrMe ). Metallic reduction of (NHC)⋅AlIMes2 afforded the new NHC-stabilized dialanes (NHC)2 ⋅Al2 Mes4 (NHC=IMeMe , IiPrMe ). The NHC-ligated dialanes are thermally robust and storable synthons for the dialane Al2 Mes4 . First reactivity studies on (IMeMe )2 ⋅Al2 Mes4 towards small molecules confirm this, as this compound shows controlled and selective reactions with several substrates. Reaction with CuCl leads to oxidation of the dialane and formation of (IMeMe )⋅AlClMes2 , reactions with pyridine N-oxide and t Bu-N=C=S, respectively, gave the chalcogenide-bridged dimers {(IMeMe )⋅AlMes2 }2 -μ-E (E=O, S), and reaction with acetylene afforded the dimetallaacetylide {(IMeMe )⋅AlMes2 }2 -μ-(C≡C).

On the Reactivity of a NHC Nickel Bis-Boryl Complex: Reductive Elimination and Formation of Mono-Boryl Complexes

The synthesis of the first terminal mono-boryl complexes of nickel, which are not stabilized by a pincer ligand, is reported. The reaction of the nickel bis-boryl complex cis-[Ni(i Pr2 ImMe )2 (Bcat)2 ] 1 (cat=1,2-O2 C6 H4 ) with the small donor ligand PMe3 led to a complete ligand exchange at nickel with reductive elimination of B2 cat2 and formation of the bis-NHC adduct [B2 cat2  ⋅ (i Pr2 ImMe )2 ] 3 and [Ni(PMe3 )4 ] 2 as the metal-containing species. Electrophilic attack of MeI on complex 1 or ligand dismutation of 1 with trans-[Ni(i Pr2 ImMe )2 Br2 ] led to loss of only one boryl ligand of 1 and afforded the nickel mono-boryl complexes trans-[Ni(i Pr2 ImMe )2 (Bcat)Br] 4 a and trans-[Ni(i Pr2 ImMe )2 (Bcat)I] 4 b.

Aluminum(III) Cations [(NHC) ⋅ AlMes2 ]+ : Synthesis, Characterization, and Application in FLP-Chemistry

The three-coordinate aluminum cations ligated by N-heterocyclic carbenes (NHCs) [(NHC) ⋅ AlMes2 ]+ [B(C6 F5 )4 ]- (NHC=IMeMe 4, IiPrMe 5, IiPr 6, Mes=2,4,6-trimethylphenyl) were prepared via hydride abstraction of the alanes (NHC) ⋅ AlHMes2 (NHC=IMeMe 1, IiPrMe 2, IiPr 3) using [Ph3 C]+ [B(C6 F5 )4 ]- in toluene as hydride acceptor. If this reaction was performed in diethyl ether, the corresponding four-coordinate aluminum etherate cations [(NHC) ⋅ AlMes2 (OEt2 )]+ [B(C6 F5 )4 ]- 7-9 (NHC=IMeMe 7, IiPrMe 8, IiPr 9) were isolated. According to a theoretical and experimental assessment of the Lewis-acidity of the [(IMeMe ) ⋅ AlMes2 ]+ cation is the acidity larger than that of B(C6 F5 )3 and of similar magnitude as reported for Al(C6 F5 )3 . The reaction of [(IMeMe ) ⋅ AlMes2 ]+ [B(C6 F5 )4 ]- 4 with the sterically less demanding, basic phosphine PMe3 afforded a mixed NHC/phosphine stabilized cation [(IMeMe ) ⋅ AlMes2 (PMe3 )]+ [B(C6 F5 )4 ]- 10. Equimolar mixtures of 4 and the sterically more demanding PCy3 gave a frustrated Lewis-pair (FLP), i.e., [(IMeMe ) ⋅ AlMes2 ]+ [B(C6 F5 )4 ]- /PCy3 FLP-11, which reacts with small molecules such as CO2 , ethene, and 2-butyne.

N-Heterocyclic carbene and cyclic (alkyl)(amino)carbene ligated half-sandwich complexes of chromium(II) and chromium(I)

The synthesis and characterization of a series of Cr(II) N-Heterocyclic Carbene (NHC) complexes of the type [{Cr(NHC)Cl(μ-Cl)}2] and [(Cyp)Cr(NHC)X] (Cyp = η5-C5H5, cyclopentadienyl; η5-C5Me5, pentamethylcyclopentadienyl; X = Cl, η3-C3H5; NHC = IMeMe, IiPrMe, IMes, IDipp) as well as the cyclic (alkyl)(amino)carbene cAACMe ligated complexes [(η5-C5H5)Cr(cAACMe)X] (X = Cl, NPh2), [(η5-C9H7)Cr(cAACMe)Cl] (C9H7 = Ind, indenyl) and [(η5-C13H9)Cr(cAACMe)Cl] (C13H9 = Fl, fluorenyl) are reported. The reduction of [(η5-C5Me5)Cr(IMeMe)Cl] with KC8 in the presence of CO afforded the NHC ligated Cr(I) metallo-radical [(η5-C5Me5)Cr(IMeMe)(CO)2]. Quantum chemical calculations performed on [(η5-C5Me5)Cr(IMeMe)(CO)2] confirm for this complex a predominantly chromium centered radical.

Solution and solid-state studies of hydrogen and halogen bonding with N-heterocyclic carbene supported nickel(II) fluoride complexes

Nickel fluoride complexes of the type [Ni(F)(L)2(ArF)] (L = phosphine, ArF = fluorinated arene) are well-known to form strong halogen and hydrogen bonds in solution and in the solid state. A comprehensive study of such non-covalent interactions using bis(carbene) complexes as acceptors and suitable halogen and hydrogen bond donors is presented. In solution, the complex [Ni(F)(iPr2Im)2(C6F5)] forms halogen and hydrogen bonds with iodopentafluorobenzene and indole, respectively, which have formation constants (K300) an order of magnitude greater than those of structurally related phosphine supported nickel fluorides. Co-crystallisation of this complex and its backbone-methylated analogue [Ni(F)(iPr2Me2Im)2(C6F5)] with 1,4-diiodotetrafluorobenzene produces halogen bonding adducts which were characterised by X-ray analysis and 19F MAS solid state NMR analysis. Differences in the chemical shifts between the nickel fluoride and its halogen bonding adduct are well in line with data that were obtained from titration studies in solution.

Anionic N-heterocyclic carbenes featuring weakly coordinating perfluoroalkylphosphorane moieties

The anionic 1-methyl-3-(tris(pentafluoroethyl)difluorophosphorane)imidazoline-2-ylidenate 3 and the 1,3-bis(tris(pentafluoroethyl)difluorophosphorane)imidazoline-2-ylidenate dianion 4 were obtained in high yield by deprotonation of {(C₂F₅)₃PF₂}-methylimidazole 1 and the {(C₂F₅)₃PF₂}₂-imidazolate anion 2. Carbenes 3 and 4 are first examples for a novel class of NHCs carrying weakly coordinating anions (WCA-NHCs). First reactions of these new ligands with elemental selenium and chloro(phosphine)gold(I) complexes to result in an anionic selenium adduct (5) and WCA-NHC gold complexes (6 and 7) have been undertaken. The structural and spectroscopic properties of these NHC derivatives in conjunction with data from quantum chemical calculations provide an insight into the electronic and steric properties of the WCA-NHCs 3 and 4. Especially the combination of properties such as weakly coordinating periphery combined with the coordinative carbene center, negative charge, large buried volume (%V_bur), and strong σ-donor as well as efficient π-acceptor ability render NHCs 3 and 4 unique and promising new ligands.

Subvalent group 13 molecules by carbene-induced hydrogen abstraction

The N-Heterocyclic Carbene (NHC) alane and gallane adducts (NHC)·Cp*AlH₂ (NHC = Me₂Im^Me5, iPr₂Im^Me6, Dipp₂Im 7) and (NHC)·Cp*GaH₂ (NHC = Me₂Im^Me8, iPr₂Im^Me9, Dipp₂Im 10; R₂Im = 1,3-di-organyl-imidazolin-2-ylidene; Dipp = 2,6 diisopropylphenyl; Me₂Im^Me = 1,3,4,5-tetra-methyl-imidazolin-2-ylidene; Cp* = C₅Me₅) were prepared either via the reaction of (AlH₂Cp*)₃1 with the NHC or by the treatment of (NHC)·GaH₂I (NHC = Me₂Im^Me2, iPr₂Im^Me3, Dipp₂Im 4) with KCp*. The reaction of (AlH₂Cp*)₃1 with the backbone saturated NHC Dipp₂Im^H led to NHC ring expansion instead with the formation of (RER-Dipp₂Im^HH₂)AlCp* 12. Heating solutions of the gallium compounds 8-10 triggered reductive elimination of Cp*H and afforded Cp*Ga^I16. The reaction of the alane adduct (Me₂Im^Me)·Cp*AlH₂5 with cAAC^Me led to the insertion of cAAC^Me into the Al-H bond with the formation of the compound rac-(Me₂Im^Me)·AlHCp*(cAAC^MeH) rac-14. Heating a solution of rac-14 led to irreversible isomerisation with the formation of (Me₂Im^Me)·AlHCp*(cAAC^MeH) meso-14. The alane adducts (iPr₂Im^Me)·Cp*AlH₂6 and (Dipp₂Im)·Cp*AlH₂7 react with cAAC^Me with the release of the NHC and formation of the exceptionally stable oxidative addition product (cAAC^MeH)AlHCp* 15. Reactions of the gallane adducts 8-10 with cAAC^Me led to reductive elimination of cAAC^Me-H₂ and the formation of Cp*Ga^I16.

An easy-to-perform evaluation of steric properties of Lewis acids

Steric and electronic effects play a very important role in chemistry, as these effects influence the shape and reactivity of molecules. Herein, an easy-to-perform approach to assess and quantify steric properties of Lewis acids with differently substituted Lewis acidic centers is reported. This model applies the concept of the percent buried volume (%V _Bur) to fluoride adducts of Lewis acids, as many fluoride adducts are crystallographically characterized and are frequently calculated to judge fluoride ion affinities (FIAs). Thus, data such as cartesian coordinates are often easily available. A list of 240 Lewis acids together with topographic steric maps and cartesian coordinates of an oriented molecule suitable for the SambVca 2.1 web application is provided, together with different FIA values taken from the literature. Diagrams of %V _Bur as a scale for steric demand vs. FIA as a scale for Lewis acidity provide valuable information about stereo-electronic properties of Lewis acids and an excellent evaluation of steric and electronic features of the Lewis acid under consideration. Furthermore, a novel LAB-Rep model (Lewis acid/base repulsion model) is introduced, which judges steric repulsion in Lewis acid/base pairs and helps to predict if an arbitrary pair of Lewis acid and Lewis base can form an adduct with respect to their steric properties. The reliability of this model was evaluated in four selected case studies, which demonstrate the versatility of this model. For this purpose, a user-friendly Excel spreadsheet was developed and is provided in the ESI, which works with listed buried volumes of Lewis acids %V _{Bur_LA} and of Lewis bases %V _{Bur_LB}, and no results from experimental crystal structures or quantum chemical calculations are necessary to evaluate steric repulsion in these Lewis acid/base pairs.

Nickel boryl complexes and nickel-catalyzed alkyne borylation

The first nickel bis-boryl complexes cis-[Ni( ⁱ Pr₂Im^Me)₂(Bcat)₂], cis-[Ni( ⁱ Pr₂Im^Me)₂(Bpin)₂] and cis-[Ni( ⁱ Pr₂Im^Me)₂(Beg)₂] are reported, which were prepared via the reaction of a source of [Ni( ⁱ Pr₂Im^Me)₂] with the diboron(4) compounds B₂cat₂, B₂pin₂ and B₂eg₂ ( ⁱ Pr₂Im^Me = 1,3-di-iso-propyl-4,5-dimethylimidazolin-2-ylidene; B₂cat₂ = bis(catecholato)diboron; B₂pin₂ = bis(pinacolato)diboron; B₂eg₂ = bis(ethylene glycolato)diboron). X-ray diffraction and DFT calculations strongly suggest that a delocalized, multicenter bonding scheme dictates the bonding situation of the NiB₂ moiety in these square planar complexes, reminiscent of the bonding situation of "non-classical" H₂ complexes. [Ni( ⁱ Pr₂Im^Me)₂] also efficiently catalyzes the diboration of alkynes using B₂cat₂ as the boron source under mild conditions. In contrast to the known platinum-catalyzed diboration, the nickel system follows a different mechanistic pathway, which not only provides the 1,2-borylation product in excellent yields, but also provides an efficient approach to other products such as C-C coupled borylation products or rare tetra-borylated compounds. The mechanism of the nickel-catalyzed alkyne borylation was examined by means of stoichiometric reactions and DFT calculations. Oxidative addition of the diboron reagent to nickel is not dominant; the first steps of the catalytic cycle are coordination of the alkyne to [Ni( ⁱ Pr₂Im^Me)₂] and subsequent borylation at the coordinated and, thus, activated alkyne to yield complexes of the type [Ni(NHC)₂(η²-cis-(Bcat)(R)C[double bond, length as m-dash]C(R)(Bcat))], exemplified by the isolation and structural characterization of [Ni( ⁱ Pr₂Im^Me)₂(η²-cis-(Bcat)(Me)C[double bond, length as m-dash]C(Me)(Bcat))] and [Ni( ⁱ Pr₂Im^Me)₂(η²-cis-(Bcat)(H₇C₃)C[double bond, length as m-dash]C(C₃H₇)(Bcat))].

Boranes Paving the Way to Anionic Cyclic (Alkyl)(amino)carbenes (Ani-cAACs)

First examples of anionic cyclic (alkyl)(amino)carbenes (Ani-cAACs) that contain borane substituents have been synthesized. The nature of the borane substituents allows a modulation of the σ-donor or π-acceptor abilities compared to their neutral analogues. A B(CN)₃ -substituted Ani-cAAC has been generated and used in situ. The corresponding C₂ F₅ BF₂ -Ani-cAAC 6 was obtained in high yield on a multigram scale. First reactions of these novel ligands with elemental selenium and chloro(triphenylphosphine)gold(I) led to the anionic selenium adducts 7 and 8 and the Ani-cAAC gold complex 9. The properties of these compounds and data derived from theoretical calculations provide an insight into the electronic and steric properties of these novel anionic cAACs. Especially the ease of synthesis and the combination of properties such as negative charge, large buried volume, and good σ-donor and π-acceptor ability renders Ani-cAACs unique and promising new building blocks.

Activation of Ge-H and Sn-H Bonds with N-Heterocyclic Carbenes and a Cyclic (Alkyl)(amino)carbene

A study of the reactivity of several N-heterocyclic carbenes (NHCs) and the cyclic (alkyl)(amino)carbene 1-(2,6-di-iso-propylphenyl)-3,3,5,5-tetramethyl-pyrrolidin-2-ylidene (cAAC^Me ) with the group 14 hydrides GeH₂ Mes₂ and SnH₂ Me₂ (Me=CH₃ , Mes=1,3,5-(CH₃ )₃ C₆ H₂ ) is presented. The reaction of GeH₂ Mes₂ with cAAC^Me led to the insertion of cAAC^Me into one Ge-H bond to give cAAC^Me H-GeHMes₂ (1). If 1,3,4,5-tetramethyl-imidazolin-2-ylidene (Me₂ Im^Me ) was used as the carbene, NHC-mediated dehydrogenative coupling occurred, which led to the NHC-stabilized germylene Me₂ Im^Me ⋅GeMes₂ (2). The reaction of SnH₂ Me₂ with cAAC^Me also afforded the insertion product cAAC^Me H-SnHMe₂ (3), and reaction of two equivalents Me₂ Im^Me with SnH₂ Me₂ gave the NHC-stabilized stannylene Me₂ Im^Me ⋅SnMe₂ (4). If the sterically more demanding NHCs Me₂ Im^Me , 1,3-di-isopropyl-4,5-dimethyl-imidazolin-2-ylidene (iPr₂ Im^Me ) and 1,3-bis-(2,6-di-isopropylphenyl)-imidazolin-2-ylidene (Dipp₂ Im) were employed, selective formation of cyclic oligomers (SnMe₂ )_n (5; n=5-8) in high yield was observed. These cyclic oligomers were also obtained from the controlled decomposition of cAAC^Me H-SnHMe₂ (3).

Transition Metal-Free Regio- and Stereo-Selective trans Hydroboration of 1,3-Diynes: A Phosphine-Catalyzed Access to (E)-1-Boryl-1,3-Enynes

We report a transition metal-free, regio- and stereo-selective, phosphine-catalyzed method for the trans hydroboration of 1,3-diynes with pinacolborane that affords (E)-1-boryl-1,3-enynes. The reaction proceeds with excellent selectivity for boron addition to the external carbon of the 1,3-diyne framework as unambiguously established by NMR and X-ray crystallographic studies. The reaction displays a broad substrate scope including unsymmetrical diynes to generate products in high yield (up to 95 %). Experimental and theoretical studies suggest that phosphine attack on the alkyne is a key process in the catalytic cycle.

N-heterocyclic carbene and cyclic (alkyl)(amino)carbene adducts of plumbanes and plumbylenes

Lewis-acid/base adducts of N-heterocyclic carbenes (NHCs) and the cyclic (alkyl)(amino)carbene cAAC^Me (1-(2,6-di-iso-propylphenyl)-3,3,5,5-tetramethyl-pyrrolidin-2-ylidene) with selected lead(II) and lead(IV) compounds are presented. The reaction of the NHCs Me₂Im^Me (1,3,4,5-tetramethyl-imidazolin-2-ylidene), iPr₂Im^Me (1,3-di-isopropyl-4,5-dimethyl-imidazolin-2-ylidene), Dipp₂Im (1,3-bis-(2,6-di-isopropylphenyl)-imidazolin-2-ylidene) and cAAC^Me (1-(2,6-di-iso-propylphenyl)-3,3,5,5-tetramethyl-pyrrolidin-2-ylidene) with PbI₂ yielded the NHC-containing plumbylenes NHC·PbI₂ (NHC = Me₂Im^Me (1), iPr₂Im^Me (2), Dipp₂Im (3) and cAAC^Me·PbI₂ (4)). Using the Pb(IV) compound PbCl₂Ph₂, the plumbane adducts NHC·PbCl₂Ph₂ (NHC = Me₂Im^Me (5), iPr₂Im^Me (6), Dipp₂Im (7)) and cAAC^Me·PbCl₂Ph₂ (8)) were isolated in high yields. Reduction of the lead(IV) adducts 5 and 6 with excess KC₈ afforded the diaryl substituted plumbylenes Me₂Im^Me·PbPh₂ (9) and iPr₂Im^Me·PbPh₂ (10), which are stable in the solid state but decompose in solution.

N-Heterocyclic carbene and cyclic (alkyl)(amino)carbene complexes of vanadium(III) and vanadium(V)

[VCl₃(THF)₃] offers a convenient entrance point into the chemistry of carbene stabilized V(III) complexes. Herein we report the paramagnetic mono- and biscarbene complexes [VCl₃(cAAC^Me)] 1, [VCl₃(cAAC^Me)(THF)] 1(thf), [VCl₃(IMes)] 2, [{VCl₂(IiPr^Me)(μ-Cl)}₂] 3, [VCl₃(IDipp)] 4, [VCl₃(SIDipp)] 5, [VCl₃(SIDipp)(THF)] 5(thf), [VCl₃(ItBu)] 6, [VCl₃(cAAC^Me)₂] 7 and [VCl₃(IiPr^Me)₂] 8. Reaction of 1 with MesMgCl, MesLi and LiNPh₂ afforded the complexes [VCl₂(Mes)(cAAC^Me)] 9, [cAAC^MeH]⁺[VCl₂Mes₂]^-10 and [VCl₂(NPh₂)(cAAC^Me)] 11. The V(V) complexes [V(O)Cl₃(IDipp)] 12 and [V(O)Cl₃(SIDipp)] 13 were selectively prepared from oxygen oxidation of 4 and 5. [V(O)Cl₃(IDipp)] 12 and [V(O)Cl₃(IMes)] react with isocyanates to yield the NHC-ligated imido complexes [V(N-p-CH₃C₆H₄)Cl₃(IDipp)] 14, [V(N-p-FC₆H₄)Cl₃(IDipp)] 15, [V(N-p-CH₃C₆H₄)Cl₃(SIDipp)] 16, [V(N-p-FC₆H₄)Cl₃(SIDipp)] 17, [V(N-p-CH₃C₆H₄)Cl₃(IMes)] 18 and [V(N-p-FC₆H₄)Cl₃(IMes)] 19.

Tricyanoborane‐Functionalized Anionic N‐Heterocyclic Carbenes: Adjustment of Charge and Stereo‐Electronic Properties

The 1‐methyl‐3‐(tricyanoborane)imidazolin‐2‐ylidenate anion (2) was obtained in high yield by deprotonation of the B(CN)₃‐methylimidazole adduct 1. Regarding charge and stereo‐electronic properties, anion 2 closes the gap between well‐known neutral NHCs and the ditopic dianionic NHC, the 1,3‐bis(tricyanoborane)imidazolin‐2‐ylidenate dianion (IIb). The influence of the number of N‐bonded tricyanoborane moieties on the σ‐donating and π‐accepting properties of NHCs was assessed by quantum chemical calculations and verified by experimental data on 2, IIb, and 1,3‐dimethylimidazolin‐2‐ylidene (IMe, IIa). Therefore NHC 2, which acts as a ditopic ligand via the carbene center and the cyano groups, was reacted with alkyl iodides, selenium, and [Ni(CO)₄] yielding alkylated imidazoles 3 and 4, the anionic selenium adduct 5, and the anionic nickel tricarbonyl complex 8, respectively. The results of this study prove that charge, number of coordination sites, buried volume (%V_bur) and σ‐donor and π‐acceptor abilities of NHCs can be effectively fine‐tuned via the number of tricyanoborane substituents.

Selective, Transition Metal-free 1,2-Diboration of Alkyl Halides, Tosylates, and Alcohols

Defunctionalization of readily available feedstocks to provide alkenes for the synthesis of multifunctional molecules represents an extremely useful process in organic synthesis. Herein, we describe a transition metal‐free, simple and efficient strategy to access alkyl 1,2‐bis(boronate esters) via regio‐ and diastereoselective diboration of secondary and tertiary alkyl halides (Br, Cl, I), tosylates, and alcohols. Control experiments demonstrated that the key to this high reactivity and selectivity is the addition of a combination of potassium iodide and N,N‐dimethylacetamide (DMA). The practicality and industrial potential of this transformation are demonstrated by its operational simplicity, wide functional group tolerance, and the late‐stage modification of complex molecules. From a drug discovery perspective, this synthetic method offers control of the position of diversification and diastereoselectivity in complex ring scaffolds, which would be especially useful in a lead optimization program.

N Heterocyclic Carbene and Cyclic (Alkyl)(amino)carbene Complexes of Molybdenum(IV) and Tungsten(IV)

The synthesis and characterization of N heterocyclic carbene (NHC) and cyclic (alkyl)(amino)carbene (cAAC) complexes of molybdenum(IV) and tungsten(IV) chloride is reported. Reaction of two equivalents of the NHCs IMes (1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene),...

NHC induced radical formation via homolytic cleavage of B–B bonds and its role in organic reactions

New borylation methodologies have been reported recently, wherein diboron(4) compounds apparently participate in free radical couplings via the homolytic cleavage of the B–B bond. We report herein that bis-NHC adducts of the type (NHC)₂·B₂(OR)₄, which are thermally unstable and undergo intramolecular ring expansion reactions (RER), are sources of boryl radicals of the type NHC–BR₂˙, exemplified by Me₂Im^Me·Bneop˙ 1a (Me₂Im^Me = 1,3,4,5-tetramethyl-imidazolin-2-ylidene, neop = neopentylglycolato), which are formed by homolytic B–B bond cleavage. Attempts to apply the boryl moiety 1a in a metal-free borylation reaction by suppressing the RER failed. However, based on these findings, a protocol was developed using Me₂Im^Me·B₂pin₂3 for the transition metal- and additive-free boryl transfer to substituted aryl iodides and bromides giving aryl boronate esters in good yields. Analysis of the side products and further studies concerning the reaction mechanism revealed that radicals are likely involved. An aryl radical was trapped by TEMPO, an EPR resonance, which was suggestive of a boron-based radical, was detected in situ, and running the reaction in styrene led to the formation of polystyrene. The isolation of a boronium cation side product, [(Me₂Im^Me)₂·Bpin]⁺I⁻7, demonstrated the fate of the second boryl moiety of B₂pin₂. Interestingly, Me₂Im^Me NHC reacts with aryl iodides and bromides generating radicals. A mechanism for the boryl radical transfer from Me₂Im^Me·B₂pin₂3 to aryl iodides and bromides is proposed based on these experimental observations.

Bis-NHC adducts of the type (NHC)₂·B₂(OR)₄ are sources of boryl radicals of the type NHC–BR₂˙, which are formed by homolytic B–B bond cleavage.

Cu-mediated vs. Cu-free selective borylation of aryl alkyl sulfones

A Cu-catalysed borylation of aryl alkyl sulfones was developed for the high yield synthesis of versatile arylboronic esters using a readily prepared NHC-Cu catalyst. In addition, the selective cleavage of either alkyl(C)-sulfonyl or aryl(C)-sulfonyl bonds of a cyclic sulfone via Cu-free or Cu-mediated processes generates the corresponding sulfinate salts, which can be further derivatised to provide sulfonyl-containing boronate esters, such as sulfones and sulfonyl fluorides.

First-Row d-Block Element-Catalyzed Carbon-Boron Bond Formation and Related Processes

Organoboron reagents represent a unique class of compounds because of their utility in modern synthetic organic chemistry, often affording unprecedented reactivity. The transformation of the carbon-boron bond into a carbon-X (X = C, N, and O) bond in a stereocontrolled fashion has become invaluable in medicinal chemistry, agrochemistry, and natural products chemistry as well as materials science. Over the past decade, first-row d-block transition metals have become increasingly widely used as catalysts for the formation of a carbon-boron bond, a transformation traditionally catalyzed by expensive precious metals. This recent focus on alternative transition metals has enabled growth in fundamental methods in organoboron chemistry. This review surveys the current state-of-the-art in the use of first-row d-block element-based catalysts for the formation of carbon-boron bonds.

Base-Mediated Radical Borylation of Alkyl Sulfones

A practical and direct method was developed for the production of versatile alkylboronic esters via transition metal-free borylation of primary and secondary alkyl sulfones. The key to the success of the strategy is the use of bis(neopentyl glycolato) diboron (B 2 neop 2 ), with a stoichiometric amount of base as a promoter. The practicality and industrial potential of this protocol are highlighted by its wide functional group tolerance, the late-stage modification of complex compounds, no need for further transesterification, and operational simplicity. Radical clock, radical trap experiments, and EPR studies were conducted which show that the borylation process involves radical intermediates.

Case Study of N-i Pr versus N-Mes Substituted NHC Ligands in Nickel Chemistry: The Coordination and Cyclotrimerization of Alkynes at [Ni(NHC)2 ]

A case study on the effect of the employment of two different NHC ligands in complexes [Ni(NHC)₂] (NHC=ⁱPr₂Im^Me1^Me, Mes₂Im 2) and their behavior towards alkynes is reported. The reaction of a mixture of [Ni₂(ⁱPr₂Im^Me)₄(μ‐(η² : η²)‐COD)] B/ [Ni(ⁱPr₂Im^Me)₂(η⁴‐COD)] B’ or [Ni(Mes₂Im)₂] 2, respectively, with alkynes afforded complexes [Ni(NHC)₂(η²‐alkyne)] (NHC=ⁱPr₂Im^Me: alkyne=MeC≡CMe 3, H₇C₃C≡CC₃H₇4, PhC≡CPh 5, MeOOCC≡CCOOMe 6, Me₃SiC≡CSiMe₃7, PhC≡CMe 8, HC≡CC₃H₇9, HC≡CPh 10, HC≡C(p‐Tol) 11, HC≡C(4‐^tBu‐C₆H₄) 12, HC≡CCOOMe 13; NHC=Mes₂Im: alkyne=MeC≡CMe 14, MeOOCC≡CCOOMe 15, PhC≡CMe 16, HC≡C(4‐^tBu‐C₆H₄) 17, HC≡CCOOMe 18). Unusual rearrangement products 11 a and 12 a were identified for the complexes of the terminal alkynes HC≡C(p‐Tol) and HC≡C(4‐^tBu‐C₆H₄), 11 and 12, which were formed by addition of a C−H bond of one of the NHC N‐ⁱPr methyl groups to the C≡C triple bond of the coordinated alkyne. Complex 2 catalyzes the cyclotrimerization of 2‐butyne, 4‐octyne, diphenylacetylene, dimethyl acetylendicarboxylate, 1‐pentyne, phenylacetylene and methyl propiolate at ambient conditions, whereas 1^Me is not a good catalyst. The reaction of 2 with 2‐butyne was monitored in some detail, which led to a mechanistic proposal for the cyclotrimerization at [Ni(NHC)₂]. DFT calculations reveal that the differences between 1^Me and 2 for alkyne cyclotrimerization lie in the energy profile of the initiation steps, which is very shallow for 2, and each step is associated with only a moderate energy change. The higher stability of 3 compared to 14 is attributed to a better electron transfer from the NHC to the metal to the alkyne ligand for the N‐alkyl substituted NHC, to enhanced Ni‐alkyne backbonding due to a smaller C_NHC−Ni−C_NHC bite angle, and to less steric repulsion of the smaller NHC ⁱPr₂Im^Me.

[Ni(NHC)2 ] as a Scaffold for Structurally Characterized trans [H-Ni-PR2 ] and trans [R2 P-Ni-PR2 ] Complexes

The addition of PPh2 H, PPhMeH, PPhH2 , P(para-Tol)H2 , PMesH2 and PH3 to the two-coordinate Ni0 N-heterocyclic carbene species [Ni(NHC)2 ] (NHC=IiPr2 , IMe4 , IEt2 Me2 ) affords a series of mononuclear, terminal phosphido nickel complexes. Structural characterisation of nine of these compounds shows that they have unusual trans [H-Ni-PR2 ] or novel trans [R2 P-Ni-PR2 ] geometries. The bis-phosphido complexes are more accessible when smaller NHCs (IMe4 >IEt2 Me2 >IiPr2 ) and phosphines are employed. P-P activation of the diphosphines R2 P-PR2 (R2 =Ph2 , PhMe) provides an alternative route to some of the [Ni(NHC)2 (PR2 )2 ] complexes. DFT calculations capture these trends with P-H bond activation proceeding from unconventional phosphine adducts in which the H substituent bridges the Ni-P bond. P-P bond activation from [Ni(NHC)2 (Ph2 P-PPh2 )] adducts proceeds with computed barriers below 10 kcal mol-1 . The ability of the [Ni(NHC)2 ] moiety to afford isolable terminal phosphido products reflects the stability of the Ni-NHC bond that prevents ligand dissociation and onward reaction.

1,3-Bis(tricyanoborane)imidazoline-2-ylidenate Anion-A Ditopic Dianionic N-Heterocyclic Carbene Ligand

The 1,3-bis(tricyanoborane)imidazolate anion 1 was obtained in high yield from lithium imidazolate and B(CN)3 -pyridine adduct. Anion 1 is chemically very robust and thus allowed the isolation of the corresponding H5 O2 + salt. Furthermore, monoanion 1 served as starting species for the novel dianionic N-heterocyclic carbene (NHC), 1,3-bis(tricyanoborane)imidazoline-2-ylidenate anion 3 that acts as ditopic ligand via the carbene center and the cyano groups at boron. First reactions of this new NHC 3 with methyl iodide, elemental selenium, and [Ni(CO)4 ] led to the methylated imidazolate ion 4, the dianionic selenium adduct 5, and the dianionic nickel tricarbonyl complex 6. These NHC derivatives provide a first insight into the electronic and steric properties of the dianionic NHC 3. Especially the combination of properties, such as double negative charge, different coordination sites, large buried volume and good σ-donor and π-acceptor ability, make NHC 3 a unique and promising ligand and building block.

Transition Metal Catalyst-Free, Base-Promoted 1,2-Additions of Polyfluorophenylboronates to Aldehydes and Ketones

A novel protocol for the transition metal-free 1,2-addition of polyfluoroaryl boronate esters to aldehydes and ketones is reported, which provides secondary alcohols, tertiary alcohols, and ketones. Control experiments and DFT calculations indicate that both the ortho-F substituents on the polyfluorophenyl boronates and the counterion K+ in the carbonate base are critical. The distinguishing features of this procedure include the employment of commercially available starting materials and the broad scope of the reaction with a wide variety of carbonyl compounds giving moderate to excellent yields. Intriguing structural features involving O-H⋅⋅⋅O and O-H⋅⋅⋅N hydrogen bonding, as well as arene-perfluoroarene interactions, in this series of racemic polyfluoroaryl carbinols have also been addressed.

Ni-Catalyzed Borylation of Aryl Sulfoxides

A nickel/N-heterocyclic carbene (NHC) catalytic system has been developed for the borylation of aryl sulfoxides with B2 (neop)2 (neop=neopentyl glycolato). A wide range of aryl sulfoxides with different electronic and steric properties were converted into the corresponding arylboronic esters in good yields. The regioselective borylation of unsymmetric diaryl sulfoxides was also feasible leading to borylation of the sterically less encumbered aryl substituent. Competition experiments demonstrated that an electron-deficient aryl moiety reacts preferentially. The origin of the selectivity in the Ni-catalyzed borylation of electronically biased unsymmetrical diaryl sulfoxide lies in the oxidative addition step of the catalytic cycle, as oxidative addition of methoxyphenyl 4-(trifluoromethyl)phenyl sulfoxide to the Ni(0) complex occurs selectively to give the structurally characterized complex trans-[Ni(ICy)2 (4-CF3 -C6 H4 ){(SO)-4-MeO-C6 H4 }] 4. For complex 5, the isomer trans-[Ni(ICy)2 (C6 H5 )(OSC6 H5 )] 5-I was structurally characterized in which the phenyl sulfinyl ligand is bound via the oxygen atom to nickel. In solution, the complex trans-[Ni(ICy)2 (C6 H5 )(OSC6 H5 )] 5-I is in equilibrium with the S-bonded isomer trans-[Ni(ICy)2 (C6 H5 )(SOC6 H5 )] 5, as shown by NMR spectroscopy. DFT calculations reveal that these isomers are separated by a mere 0.3 kJ/mol (M06/def2-TZVP-level of theory) and connected via a transition state trans-[Ni(ICy)2 (C6 H5 )(η2 -{SO}-C6 H5 )], which lies only 10.8 kcal/mol above 5.

Base-Free Pd-Catalyzed C-Cl Borylation of Fluorinated Aryl Chlorides

Catalytic C-X borylation of aryl halides containing two ortho-fluorines has been found to be challenging, as most previous methods require stoichiometric amounts of base and the polyfluorinated aryl boronates suffer from protodeboronation, which is accelerated by ortho-fluorine substituents. Herein, we report that a combination of Pd(dba)2 (dba=dibenzylideneacetone) with SPhos (2-dicyclohexylphosphino-2',6'-dimethoxybiphenyl) as a ligand is efficient to catalyze the C-Cl borylation of aryl chlorides containing two ortho-fluorine substituents. This method, conducted under base-free conditions, is compatible with the resulting di-ortho-fluorinated aryl boronate products which are sensitive to base.