Optically active, bulky tris(oxazolinyl)borato magnesium and calcium compounds for asymmetric hydroamination/cyclization

Jumping in the Chiral Pool: Asymmetric Hydroaminations with Early Metals

The application of early-metal-based catalysts featuring natural chiral pool motifs, such as amino acids, terpenes and alkaloids, in hydroamination reactions is discussed and compared to those beyond the chiral pool. In particular, alkaline (Li), alkaline earth (Mg, Ca), rare earth (Y, La, Nd, Sm, Lu), group IV (Ti, Zr, Hf) metal-, and tantalum-based catalytic systems are described, which in recent years improved considerably and have become more practical in their usability. Additional emphasis is directed towards their catalytic performance including yields and regio- as well as stereoselectivity in comparison with the group IV and V transition metals and more widely used rare earth metal-based catalysts.

Calcium catalyzed enantioselective intramolecular alkene hydroamination with chiral C2-symmetric bis-amide ligands

The chiral building block (R)-(+)-2,2'-diamino-1,1'-binaphthyl, (R)-BINAM, which is often used as backbone in privileged enantioselective catalysts, was converted to a series of N-substituted proligands R1-H2 (R = CH2tBu, C(H)Ph2, PPh2, dibenzosuberane, 8-quinoline). After double deprotonation with strong Mg or Ca bases, a series of alkaline earth (Ae) metal catalysts R1-Ae·(THF)n was obtained. Crystal structures of these C2-symmetric catalysts have been analyzed by quadrant models which show that the ligands with C(H)Ph2, dibenzosuberane and 8-quinoline substituents should give the best steric discrimination for the enantioselective intramolecular alkene hydroamination (IAH) of the aminoalkenes H2C[double bond, length as m-dash]CHCH2CR'2CH2NH2 (CR'2 = CPh2, CCy or CMe2). The dianionic R12- ligand in R1-Ae·(THF)n functions as reagent that deprotonates the aminoalkene substrate, while the monoanionic (R1-H)- ligand formed in this reaction functions as a chiral spectator ligand that controls the enantioselectivity of the ring closure reaction. Depending on the substituent R in the BINAM ligand, full cyclization of aminoalkenes to chiral pyrrolidine products as fast as 5 minutes was observed. Product analysis furnished enantioselectivities up to 57% ee, which marks the highest enantioselectivity reported for Ca catalyzed IAH. Higher selectivities are impeded by double protonation of the R12- ligand leading to complete loss of chiral information in the catalytically active species.

Snapshots of magnesium-centred diborane heterolysis by an outer sphere SN2 process

Reactions of a magnesium diboranate as a source of [Bpin]^– anions are initiated by ‘outer sphere’ attack of C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N bonded substrates.

Reactions of the β-diketiminato magnesium diboranate derivative, [(BDI)Mg{pinB(n-Bu)Bpin}] (BDI = HC{(Me)CNDipp}₂; Dipp = 2,6-i-Pr₂C₆H₃), with N,N′-dialkyl and N,N′-diaryl carbodiimides provided the corresponding C-borylated magnesium borylamidinates. This reactivity occurs with the displacement of n-BuBpin and with the apparent addition of a nucleophilic {Bpin} anion to the electrophilic unsaturated carbodiimide carbon centres. In contrast, while analogous reactions of [(BDI)Mg{pinB(n-Bu)Bpin}] with N-alkyl or N-aryl aldimines and ketimines also resulted in facile displacement of n-BuBpin, they provided the organomagnesium products of {Bpin} addition to the imine nitrogen atom rather than the more electrophilic trigonal imine carbon. Computational assessment by density functional theory (DFT) indicated that, although the energetic differences are marginal, the organomagnesium products may be considered as the kinetic outcome of these reactions with respect to the generation of alternative amidomagnesium regioisomers. This latter deduction was borne out by the thermally-induced conversion of two such organomagnesium species to their C-borylated amidomagnesium isomers, both of which occur with negligible entropies of activation indicative of purely intramolecular processes. Detailed analysis by DFT of the reaction of [(BDI)Mg{pinB(n-Bu)Bpin}] with PhN Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CHPh indicated that B–N bond formation is initiated by attack of the imine nitrogen at the three-coordinate boron atom of the diboranate anion rather than the more crowded magnesium centre. Consistent with an effectively spontaneous reaction, the resultant cleavage of the B–B bond of the diboranate unit is accomplished via the traversal of two very modest barriers of only 8.3 and 6.7 kcal mol^–1. This analysis was also supportive of a subsequent intramolecular B–N to B–C isomerisation process. Of greater general significance, however, the addition of the {Bpin}^– anion to the reducible aldimine is best rationalised as a consequence of the electrophilic character of this three-coordinate boron centre rather than any intrinsic nucleophilicity associated with the B–B bond of the [pinBB(n-Bu)pin]^– anion.

Brønsted acid-catalysed intramolecular hydroamination of unactivated alkenes: metal triflates as an in situ source of triflic acid

Hydroamination of alkenes or alkynes is one of the most straightforward methods to form C-N bonds and nitrogen-containing heterocycles. A simple Lewis acid Al(OTf)₃ was found to be an effective precatalyst for the hydroamination of unactivated primary and secondary alkenylamines between 110 and 150 °C. Subsequent studies show that other metal triflates are also effective precatalysts for the hydroamination reactions. For metal triflate salts, mechanistic studies, including kinetics, are consistent with the in situ generation of triflic acid, which likely serves as the active catalyst.

One-Component Aluminum(heteroscorpionate) Catalysts for the Formation of Cyclic Carbonates from Epoxides and Carbon Dioxide

New neutral and zwitterionic chiral NNO-donor scorpionate ligands 1 and 2 were designed to obtain new mononuclear and dinuclear NNO-heteroscorpionate aluminum complexes. Reaction of 1 with [AlR₃ ] (R=Me, Et) in a 1:1 or 1:2 molar ratio afforded the neutral mononuclear alkyl complexes [AlR₂ (κ² -bpzappe)] {R=Me (3), Et (4); bpzappeH=2,2-bis(3,5-dimethylpyrazol-1-yl)-1-[4-(dimethylamino)phenyl]-1-phenylethanol} and bimetallic complexes [{AlR₂ (κ² -bpzappe)}(μ-O){AlR₃ }] [R=Me (5), Et (6)]. By reaction of complexes 3-6 with PhCH₂ Br, mononuclear and dinuclear cationic aluminum complexes [AlR₂ (κ² -bbpzappe)]Br {R=Me (7), Et (8); bbpzappeH=N-benzyl-4-[2,2-bis(3,5-dimethyl-1H-pyrazol-1-yl)-1-hydroxy-1-phenylethyl]-N,N-dimethylbenzenaminium bromide} and [{AlR₂ (κ² -bbpzappe)}(μ-O){AlR₃ }]Br [R=Me (9), Et (10)] were synthesized. Both neutral aluminum complexes in the presence of Bu₄ NBr and cationic aluminum complexes were investigated as catalysts for cyclic carbonate formation from epoxides and carbon dioxide. Amongst them, complex 10 was found to be an efficient one-component catalyst for the synthesis of cyclic carbonates from both monosubstituted and internal epoxides and was shown to have broad substrate scope.

Enantioselective magnesium-catalyzed transformations

This review updates the major progress in the field of enantioselective transformations promoted by chiral magnesium catalysts, covering the literature since 2007, illustrating the power of these green catalysts to promote many types of reactions.

Recent developments in alkene hydro-functionalisation promoted by homogeneous catalysts based on earth abundant elements: formation of C-N, C-O and C-P bond

This Perspective article provides an overview of the recent advancements in the field of intra- and inter-molecular C-N, C-O and C-P bond formation by hydroamination, hydroalkoxylation, hydrophosphination, hydrophosphonylation or hydrophosphinylation of unactivated alkenes, including allenes, 1,3-dienes and strained alkenes, promoted by (chiral) homogeneous catalysts based on earth abundant elements of the s and p blocks, the first row transition metals and the rare-earth metals. The relevant literature from 2009 until late 2014 has been covered.

Cyclopentadienyl-based Mg complexes in the intramolecular hydroamination of aminoalkenes: mechanistic evidence for cationic versus neutral magnesium derivatives

Mechanistic evidence in the catalytic hydroamination of aminoalkenes for a cationic magnesium derivative.

The first scorpionate ligand based on diazaphosphole

The reaction of PhBCl2 with 1H-1,2,4-λ(3)-diazaphosphole in the presence of NEt3 gives a new scorpionate ligand, phenyl-tris(1,2,4-diazaphospholyl)borate (PhTdap). The coordination behaviour of this ligand toward transition and non-transition metals has been comprehensively studied. In the thallium(I) complex, Tl(PhTdap), κ(2)-N,N bonding supported with intramolecular η(3)-phenyl coordination has been observed in the solid state. Tl(PhTdap) also shows unusual intermolecular π-interactions between five-membered diazaphosphole rings and the thallium atom giving infinite molecular chains in the crystal. In the square planar complex [Pd(C,N-C6H4CH2NMe2)(PhTdap)], κ(2)-bonded scorpionate has been detected in both solution and in the solid state. For other studied compounds with the central metal ion Ti(IV), Mo(II), Mn(I), Fe(II), Ru(II), Co(II), Co(III), Ni(II) and Cd(II), the κ(3)-N,N,N coordination pattern was observed. Electronic properties of PhTdap and its ligand-field strength were elucidated from UV-Vis spectra of transition-metal species. The CH/P replacement on going from tris(pyrazolyl)borate to the ligand PhTdap causes a slight increase in electronic density rendered to the central metal atom. The following order of ligand-field strength has been established: HB(3,5-Me2pz)3 < PhB(pz)3 < HB(1,2,4-triazolyl) < HB(pz)3 < PhB(1,2,4-triazolyl) < PhTdap. The crystal structures of ten metal complexes bearing the new ligand are reported. The possibility of PhTdap coordination through the phosphorus atom is also briefly discussed.

Hydroamination of diphenylbutadiyne with secondary N-methyl-anilines using the dipotassium tetrakis(2,6-diisopropylanilino)calciate precatalyst

The approved precatalyst [K2Ca{N(H)Dipp}4] was employed to study the hydroamination of diphenylbutadiyne with N-methyl-anilines in tetrahydrofuran at room temperature. The hydroamination occurs regioselectively within a few hours yielding (N-methyl)-(N-aryl)-1,4-diphenylbut-1-ene-3-yne-1-ylamine with phenyl (1a), 4-tolyl (1b) and 4-fluorophenyl groups (1c). In all cases a mixture of E- and Z-isomers is obtained. The second hydroamination step requires drastically extended reaction times and is successful only for the reaction of diphenylbutadiyne with N-methyl-aniline and N-methyl-4-fluoroaniline giving 1,4-diphenyl-1,4-bis(N-methyl-anilino)buta-1,3-diene [R = H (2a) and F (2c)]; a mixture of E,E-, E,Z- and Z,Z-isomers is obtained. The X-ray structures of E-1a, E-1b and E-1c show a slightly shortened N-C bond to the alkene moieties. Due to enhanced steric strain the anilino units of Z,Z-2c and Z,Z-3 turn away from the butadiene unit and consequently, the lone pair at the planar nitrogen atoms slightly interacts with the adjacent aryl groups.

Catalytic bond forming reactions promoted by amidinate, guanidinate and phosphaguanidinate compounds of magnesium

The synthesis and catalytic properties of a series of magnesium compounds consisting of monoanionic, N,N'-chelating ligands (N∩N = amidinates, guanidinates, phosphaguanidinates) is reported. The compounds were synthesized by (i) insertion of a carbodiimide into an existing Mg-C or Mg-N bond, or (ii) protonolysis of an organomagnesium compound by a neutral pre-ligand. Structural analyses of mono- or bis-(chelate) compounds with general formula Mg(N∩N)X(L)n and Mg(N∩N)2(L)n (X = halide, aryloxide, amide; L = Et2O, THF; n = 0, 1 or 2) have been performed and the influence that the ligand substituent patterns have on the solid-state structures has been probed. Selected examples of the compounds were tested as (pre)catalysts for the polymerization of lactide, the dimerization of aldehydes and the hydroacetylenation of carbodiimides.

Zincation of 4,4-dimethyloxazoline using TMPZnCl·LiCl. A new preparation of 2-aryloxazolines

4,4-Dimethyloxazoline is directly zincated using TMPZnCl·LiCl and further functionalized by Pd-catalyzed cross-couplings, as well as Cu-mediated acylation and allylation reactions.

Highly enantioselective zirconium-catalyzed cyclization of aminoalkenes

Aminoalkenes are catalytically cyclized in the presence of cyclopentadienylbis(oxazolinyl)borato group 4 complexes {PhB(C5H4)(Ox(R))2}M(NMe2)2 (M = Ti, Zr, Hf; Ox(R) = 4,4-dimethyl-2-oxazoline, 4S-isopropyl-5,5-dimethyl-2-oxazoline, 4S-tert-butyl-2-oxazoline) at room temperature and below, affording five-, six-, and seven-membered N-heterocyclic amines with enantiomeric excesses of >90% in many cases and up to 99%. Mechanistic investigations of this highly selective system employed synthetic tests, kinetics, and stereochemistry. Secondary aminopentene cyclizations require a primary amine (1-2 equiv vs catalyst). Aminoalkenes are unchanged in the presence of a zirconium monoamido complex {PhB(C5H4)(Ox(4S-iPr,Me2))2}Zr(NMe2)Cl or a cyclopentadienylmono(oxazolinyl)borato zirconium diamide {Ph2B(C5H4)(Ox(4S-iPr,Me2))}Zr(NMe2)2. Plots of initial rate versus [substrate] show a rate dependence that evolves from first-order at low concentration to zero-order at high concentration, and this is consistent with a reversible substrate-catalyst interaction preceding an irreversible step. Primary kinetic isotope effects from substrate conversion measurements (k'obs((H))/k'obs((D)) = 3.3 ± 0.3) and from initial rate analysis (k2((H))/k2((D)) = 2.3 ± 0.4) indicate that a N-H bond is broken in the turnover-limiting and irreversible step of the catalytic cycle. Asymmetric hydroamination/cyclization of N-deutero-aminoalkenes provides products with higher optical purities than obtained with N-proteo-aminoalkenes. Transition state theory, applied to the rate constant k2 that characterizes the irreversible step, provides activation parameters consistent with a highly organized transition state (ΔS(++) = -43(7) cal·mol(-1) K(-1)) and a remarkably low enthalpic barrier (ΔH(++) = 6.7(2) kcal·mol(-1)). A six-centered, concerted transition state for C-N and C-H bond formation and N-H bond cleavage involving two amidoalkene ligands is proposed as most consistent with the current data.