Catalytic Alkyne Allylboration: A Quest for Selectivity

Catalytic methodologies that enable the synthesis of complex organic molecules from simple and readily available starting materials represent a goal in modern synthetic chemistry. In particular, multicomponent carboboration reactions that provide stereoselective access to densely functionalized building blocks are particularly valuable to achieve molecular diversity. This Perspective covers the developments in the area of catalytic allylboration of alkynes and highlights the key features that have allowed for the control of the regio-, diastereo-, and enantioselectivity in these transformations.

Uncatalyzed Carbometallation Involving Group 13 Elements: Carboboration and Carboalumination of Alkenes and Alkynes

Carbometallation of alkenes and alkynes are powerful carbon-carbon bond-forming reactions. The use of compounds containing bonds between carbon and group 13 elements, particularly boron and aluminum, are particularly attractive because of the versatility of subsequent transformations. Uncatalyzed carboboration and carboalumination represent less common classes of reactions. This Short Review discusses uncatalyzed carboboration and carboalumination reactions of alkenes and alkynes, including the reaction design and mechanism.

C-Boron Enolates Enable Palladium Catalyzed Carboboration of Internal 1,3-Enynes

A new family of carbon-bound boron enolates, generated by a kinetically controlled halogen exchange between chlorocatecholborane and silylketene acetals, is described. These C-boron enolates are demonstrated to activate 1,3-enyne substrates in the presence of a Pd⁰ /Senphos ligand complex, resulting in the first examples of a carboboration reaction of an alkyne with enolate-equivalent nucleophiles. Highly substituted dienyl boron building blocks are produced in excellent site-, regio-, and diastereoselectivity by the described catalytic cis-carboboration reaction.

Modular Synthesis of a Versatile Double-Allylation Reagent for Complex Diol Synthesis

Double-allylation reagents allow for the construction of highly complex molecules in an expedient fashion. We have developed an efficient, modular, and enantioselective approach towards accessing novel variants of these reagents through Cu/Pd-catalyzed alkenylboration of alkenylboron derivatives. Importantly, we demonstrate novel use of an allylBdan reagent directly in a stereocontrolled allylation without initial deprotection to the boronic ester. These allylation products are employed in a second intermolecular allylation to access complex diol motifs, which has yet to be shown with these types of double-allylation reagents. Overall, the modularity of this approach and the ease in which complex structural motifs can be accessed in a rapid manner signify the importance and utility of this method.

Highly Diastereo- and Enantioselective Synthesis of 3,6'-Bisboryl-anti-1,2-oxaborinan-3-enes: An Entry to Enantioenriched Homoallylic Alcohols with A Stereodefined Trisubstituted Alkene

A Cu-catalyzed regio-, diastereo-, and enantioselective carboboration of 1,1-bisboryl-1,3-butadiene is developed to generate enantioenriched 3,6'-bisboryl-anti-1,2-oxaborinan-3-enes. DFT calculations indicate that the initial diene 1,2-borocupration forms a ³ η-allylic copper as the most stable intermediate. Subsequent aldehyde addition, however, operates under Curtin-Hammett control via a more reactive α,α-bisboryl tertiary allylcopper species to furnish products with high enantioselectivities. The three boryl groups in the products are properly differentiated and can undergo a variety of chemoselective transformations to produce enantioenriched homoallylic alcohols with a stereodefined trisubstituted alkene.

Linkage Isomerism Leading to Contrasting Carboboration Chemistry: Access to Three Constitutional Isomers of a Borylated Phosphaalkene

We describe the reactivity of two linkage isomers of a boryl-phosphaethynolate, [B]OCP and [B]PCO (where [B]=N,N'-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-1,3,2-diazaboryl), towards tris- (pentafluorophenyl)borane (BCF). These reactions afforded three constitutional isomers all of which contain a phosphaalkene core. [B]OCP reacts with BCF through a 1,2 carboboration reaction to afford a novel phosphaalkene, E-[B]O{(C6 F5 )2 B}C=P(C6 F5 ), which subsequently undergoes a rearrangement process involving migration of both the boryloxy and pentafluorophenyl substituents to afford Z-{(C6 F5 )2 B}(C6 F5 )C=PO[B]. By contrast, [B]PCO undergoes a 1,3-carboboration process accompanied by migration of the N,N'-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-1,3,2-diazaboryl to the carbon centre.

Modular Synthesis of Furans with up to Four Different Substituents by a trans-Carboboration Strategy

Propargyl alcohols, on treatment with MHMDS (M=Na, K), B₂ (pin)₂ , an acid chloride and a palladium/copper co-catalyst system, undergo a reaction cascade comprised of trans-diboration, regioselective acylation, cyclization and dehydration to give trisubstituted furylboronic acid pinacol ester derivatives in good yields; subsequent Suzuki coupling allows a fourth substituent of choice to be introduced and hence tetrasubstituted (arylated) furans to be formed. In terms of modularity, the method seems unrivaled, not least because each product can be attained by two orthogonal but convergent ways ("diagonal split"). This asset is illustrated by the "serial" formation of a "library" of all twelve possible furan isomers that result from systematic permutation of four different substituents about the heterocyclic core.

Palladium(0)-Catalyzed Directed syn-1,2-Carboboration and -Silylation: Alkene Scope, Applications in Dearomatization, and Stereocontrol by a Chiral Auxiliary

We report the development of palladium(0)-catalyzed syn-selective 1,2-carboboration and -silylation reactions of alkenes containing cleavable directing groups. With B2 pin2 or PhMe2 Si-Bpin as nucleophiles and aryl/alkenyl triflates as electrophiles, a broad range of mono-, di-, tri- and tetrasubstituted alkenes are compatible in these transformations. We further describe a directed dearomative 1,2-carboboration of electron-rich heteroarenes by employing this approach. Through use of a removable chiral directing group, we demonstrate the viability of achieving stereoinduction in Heck-type alkene 1,2-difunctionalization. This work introduces new avenues to access highly functionalized boronates and silanes with precise regio- and stereocontrol.

Synthesis and Reactivity of Cyclic Borane-Amidine Conjugated Molecules Formed by Direct 1,2-Carboboration of Carbodiimides with 9-Borafluorenes

Efficient 1,2-carboboration reactions to the C=N bond of carbodiimides with 9-borafluorenes, which give rise to cyclic borane-amidine conjugates with a seven-membered BNC₅ ring, are reported. The resulting cyclic borane-amidine conjugates can be hydrolyzed into an acyclic bifunctional biaryl compound carrying both boronic acid and amidine groups, rendering the utility of the two-step protocol for the synthesis of multi-functionalized molecular systems with a potential as a supramolecular building block. Furthermore, the conjugated structure of the cyclic boron-amidine compounds can be changed upon alkylation of the boron atom that increases the coordination number of boron. The combination of Lewis acid (borane) and conjugated base (amidine) provides rich structural diversity of heteroatom-containing π-conjugated systems.

Directed, Palladium(II)-Catalyzed Enantioselective anti-Carboboration of Alkenyl Carbonyl Compounds

A substrate-directed enantioselective anti-carboboration reaction of alkenes has been developed, wherein a carbon-based nucleophile and a boron moiety are installed across the C=C bond through a 5-membered palladacycle intermediate. A preliminary result also shows it is possible to extend this reaction to alkenes that are more distal from the directing group and react via a 6-membered palladacycle. The reaction is promoted by a palladium(II) catalyst and a monodentate oxazoline ligand. A range of enantioenriched secondary alkylboronate products were obtained with moderate to high enantioselectivity that could be further upgraded by recrystallization. This work represents an efficient method to synthesize versatile and valuable alkylboronate building blocks. Building on an earlier mechanistic proposal by Peng, He, and Chen, a revised model is proposed to account for the stereoconvergent nature of this transformation.

Catalyst-Controlled 1,2- and 1,1-Arylboration of α-Alkyl Alkenyl Arenes

Two methods are reported for the 1,2- and 1,1-arylboration of α-methyl vinyl arenes. In the case of 1,2-arylboration, the formation of a quaternary center occurred through a rare cross-coupling reaction of a tertiary organometallic complex. 1,1-Arylboration was enabled by catalyst optimization and occurred through a β-hydride elimination/reinsertion cascade. Enantioselective variants of both processes are presented as well as mechanistic investigations.

Metal-Free Acetylene Coupling by the (C6 F5 )2 B-X 1,2-Halogenoboration Reaction

(C₆ F₅ )₂ B-halides were conveniently prepared by treatment of (C₆ F₅ )₂ BH with tritylchloride or -bromide, respectively. With cyclopropylacetylene, (C₆ F₅ )₂ BBr underwent sequential cis-1,2-halogenoboration followed by 1,2-carboboration to give the 4-bromo-2,4-dicyclopropylbutadienyl-B(C₆ F₅ )₂ product. It reacted further with additional cyclopropylacetylene to give the linear triene and tetraene products in a metal-free alkyne oligomerization reaction. The pyridine adduct of the initial diene product was characterized by X-ray diffraction. (C₆ F₅ )₂ BCl reacted analogously. Similar (C₆ F₅ )₂ BX induced oligomerization reactions were carried out with two conjugated enynes.

The Propargyl Rearrangement to Functionalised Allyl-Boron and Borocation Compounds

A diverse range of Lewis acidic alkyl, vinyl and aryl boranes and borenium compounds that are capable of new carbon-carbon bond formation through selective migratory group transfer have been synthesised. Utilising a series of heteroleptic boranes [PhB(C6 F5 )2 (1), PhCH2 CH2 B(C6 F5 )2 (2), and E-B(C6 F5 )2 (C6 F5 )C=C(I)R (R=Ph 3 a, nBu 3 b)] and borenium cations [phenylquinolatoborenium cation ([QOBPh][AlCl4 ], 4)], it has been shown that these boron-based compounds are capable of producing novel allyl- boron and boronium compounds through complex rearrangement reactions with various propargyl esters and carbamates. These reactions yield highly functionalised, synthetically useful boron substituted organic compounds with substantial molecular complexity in a one-pot reaction.

Functionalization of Intramolecular Frustrated Lewis Pairs by 1,1-Carboboration with Conjugated Enynes

The vicinal P/B frustrated Lewis pair (FLP) Mes2PCH2CH2B(C6F5)2 undergoes 1,1-carboboration reactions with the Me3Si-substituted enynes to give ring-enlarged functionalized C3-bridged P/B FLPs. These serve as active FLPs in the activation of dihydrogen to give the respective zwitterionic [P]H(+)/[B]H(-) products. One such product shows activity as a metal-free catalyst for the hydrogenation of enamines or a bulky imine. The ring-enlarged FLPs contain dienylborane functionalities that undergo "bora-Nazarov"-type ring-closing rearrangements upon photolysis. A DFT study had shown that the dienylborane cyclization of such systems itself is endothermic, but a subsequent C6F5 migration is very favorable. Furthermore, substituted 2,5-dihydroborole products are derived from cyclization and C6F5 migration from the photolysis reaction. In the case of the six-membered annulation product, a subsequent stereoisomerization reaction takes place and the resultant compound undergoes a P/B FLP 1,2-addition reaction with a terminal alkyne with rearrangement.

A 1,1-Carboboration Route to Bora-Nazarov Systems

Hydroboration of the conjugated enynes 1 a and 1 b with Piers' borane [HB(C6F5)2] gave the respective dienylboranes trans-2 c and trans-2 d. Their photolysis resulted in the formation of the dihydroborole products 3 c and 3 d. Both were converted to their pyridine adducts 5 c and 5 d, respectively. Compounds 3 c and 5 c,d were characterized by X-ray diffraction. The reaction of the bis(enynyl)boranes 6 a and 6 b with B(C6F5)3 resulted in the formation of the dihydroboroles 7 a and 7 b, respectively. This reaction is thought to proceed by 1,1-carboboration of one of the enynyl substituents at boron to generate the dienylborane intermediates 8 a/8 b, followed by thermally induced bora-Nazarov ring-closure and subsequent stabilizing 1,2-pentafluorophenyl group migration from boron to carbon. Compound 7 a was characterized by X-ray diffraction and solid-state (11)B NMR spectroscopy.