Synthesis and Reactivity of 1,1‐Diborylalkanes towards C–C Bond Formation and Related Mechanisms

Abstract

Abstractgem‐Diborylalkanes have emerged as efficient reagents for synthesizing organoboron compounds through selective C−C bond‐forming reactions. Activation of the 1,1‐diborylalkanes generates carbanions with enhanced stability that are able to react with a series of electrophiles, carbonyl compounds, imines and epoxides to promote formation of a new C−C bond. These new sets of reactions have become general for a wide range of substrates and they can be understood by alternative mechanisms that justify the potential use of these reagents. The formation of C–C(B) bonds can be achieved with chemo‐, diastereo‐ and enantioselectivity, because the nucleophilc α‐boryl or α‐diboryl carbanions attack in a stereoselective manner, by means of the catalyst involved. The synthesis of gem‐diborylalkanes has also been promoted by innovative methods and facilitates access to multiborylated reagents with different substituents and properties.magnified image

Stereoselective Desymmetrization of gem‐Diborylalkanes by “Trifluorination”

“…gem ‐Diborylalkanes have recently emerged as a versatile class of intermediates for preparing alkylboron compounds through chemoselective transformations . These bisnucleophilic compounds (termed sp 3 ‐geminated organodimetallics) have increasingly attracted much attention from synthetic chemists, particularly for the construction of C−C bonds, for example, the pioneering work reported by Shibata in which he developed the first Pd‐catalyzed Suzuki–Miyaura cross‐coupling of 1,1‐diborylalkanes . Although many traditional approaches to the formation of C−C bonds have been made available by using symmetrical gem ‐diborylalkanes (e.g., 10 Scheme D), methods for cross‐coupling with unsymmetrical gem ‐diborylalkanes by C−B bond activation have not been satisfactorily addressed and could unveil new opportunities for sequential bond formation.…”

Selective Boryl‐Anion Migration in a Vinyl sp2−sp3 Diborane Induced by Soft Borane Lewis Acids

“…[11][12][13][14][15][16][17] We hypothesised that an alternative route to induce intramolecular 1,2-boryl-anion migration would be the activation of an unsaturated R À group (e.g. [19][20][21][22] Thes elective (for intramolecular 1,2-boryl migration) activation of [(vinyl)B 2 Pin 2 ] À (complex B,Scheme 1, bottom), requires judicious choice of the borane,B R 3 ,a sarange of outcomes are feasible,i ncluding:i )vinyl-anion transfer from B to BR 3 ;i i) binding of BR 3 to an oxygen atom in B and subsequent CÀOorB ÀOcleavage;iii){BPin} À anion transfer from B to BR 3 ;i v) BR 3 activation of the vinyl p system and intramolecular {BPin} À transfer.W hile (i)a nd (ii)a re undesirable,p athway (iii)w ould be an attractive route to unsymmetrical diboranes using commercial Grignard reagents as activators.E qually notable and our primary focus,i ntramolecular 1,2-boryl migration (pathway (iv)) would be anew and simple route to 1,1,2-triborylated alkanes. This approach is attractive as it avoids prefunctionalization of the carbanion activator.I ti sc onceptually related to the Zweifel reaction, [18] but the use of borane Lewis acids and {BPin} À as the migrating group will lead to differentially functionalised 1,1,2-triborylated alkanes in one step.Related 1,1-diborylated alkanes have emerged as highly versatile reagents used in selective CÀCb ond formation by the Suzuki-Miyaura coupling reaction or by deprotonation/ deborylation of the diborylated carbon atom.…”

“…The successful formation of [2] À was indicated by 11 BNMR spectroscopy,w hich showed two new resonances:o ne at 37.3 ppm (three-coordinate boron) and the other at 4.8 ppm (four-coordinate boron), analogous to the spectrum reported for [(Ph)B 2 Pin 2 ] À (39.2 and 4.0 ppm, respectively). The 19 FNMR spectrum confirmed [RO À B(C 6 F 5 ) 3 ] À formation, with ESIMS analysis supporting the formation of an [RO À B(C 6 F 5 ) 3 ] À species derived from ring opening of one BPin moiety in [2] À .With two additional 11 Bresonances observed at 48.0 and 29.2 ppm, we tentatively assign the product as derived from B(C 6 F 5 ) 3 activation of pinacol bound to the four-coordinate boron atom (Scheme 2, top). [23] The addition of B(C 6 F 5 ) 3 (1 equiv) to [2] À (at À78 8 8C) led after 2hto asingle new 11 Bresonance at À3.2 ppm, consistent with an [ROÀB(C 6 F 5 ) 3 ] À species (in contrast, [alkylÀB(C 6 F 5 ) 3 ] À anions have a 11 Br esonance at ca.…”

Copper‐Catalyzed Triboration: Straightforward, Atom‐Economical Synthesis of 1,1,1‐Triborylalkanes from Terminal Alkynes and HBpin

“…While multiple borylated compounds such as gem ‐diborylalkanes are important synthetic intermediates for preparing organoboron compounds via C−C bond formation,– by comparison, the use of 1,1,1‐tris(boronates) is much less developed . Herein, we describe an alkoxide‐promoted deborylative alkylation of 1,1,1‐tris(boronates) via the generation and electrophilic trapping of α‐boryl carbanions.…”