Rh-catalyzed addition of β-carbonyl pinacol alkylboronates to aldehydes: asymmetric synthesis of γ-butyrolactones

Three-component vicinal-diarylation of alkenes via direct transmetalation of arylboronic acids

Transmetalation-initiated three-component vicinal-diarylation of alkenes.

Herein, we report three-component vicinal-diarylation of non-conjugated alkenes initiated by transmetalation of arylboronic acids, which provides complementary access to β,γ-diaryl carbonyl compounds. We have also screened a large number of chiral ligands for developing an enantioselective version of this reaction and obtained the preliminary results (up to 79 : 21 e.r.). Notably, the methodology developed herein represents the first three component syn-vicinal-dicarbofunctionalization of non-conjugated alkenes involving palladium catalysis.

Benzylation of Imines with Activated Boronate Nucleophiles

Benzylation reactions of N-tosyl imines and N-tert-butanesulfinyl imines using benzylboronic acid pinacol ester are reported. s-Butyllithium was used to activate the boronic ester, rendering it nucleophilic. The reaction was compatible with electronically diverse substituents on the imine in both substrate classes. Good diastereoselectivity was observed in additions to N-tert-butylsulfinylaldimines. The diastereoselectivity observed in these reactions is consistent with an open transition state for the addition. Examples of a secondary alkylboronic ester nucleophile and an N-tert-butanesulfinyl trifluoromethylketimine electrophile are also included.

Chemoselective Benzylation of Aldehydes Using Lewis Base Activated Boronate Nucleophiles

A benzylation of aldehydes using primary and secondary benzylboronic acid pinacol esters is reported. Activation of the boronic ester with s-butyllithium rendered it nucleophilic toward aldehydes. The activated nucleophile chemoselectively transfers the benzyl group over the s-butyl group, providing excellent yields of the benzylated products. ¹¹B NMR experiments were performed to study the mechnism of this transformation.

Stereospecific functionalizations and transformations of secondary and tertiary boronic esters

This feature article discusses the range of stereospecific transformations available to enantioenriched boronic esters, and their applications in synthesis.

Palladium-Catalyzed Enantioselective Redox-Relay Heck Arylation of 1,1-Disubstituted Homoallylic Alcohols

An enantioselective redox-relay oxidative Heck arylation of 1,1-disubstituted alkenes to construct β-stereocenters was developed using a new pyridyl-oxazoline ligand. Various 1,2-diaryl carbonyl compounds were readily obtained in moderate yield and good to excellent enantioselectivity. Additionally, analysis of the reaction outcomes using multidimensional correlations revealed that enantioselectivity is tied to specific electronic features of the 1,1-disubstituted alkenol and the extent of polarizability of the ligand.

Synthesis of Chiral Tertiary Boronic Esters by Oxime-Directed Catalytic Asymmetric Hydroboration

Chiral boronic esters are useful intermediates in asymmetric synthesis. We have previously shown that carbonyl-directed catalytic asymmetric hydroboration (CAHB) is an efficient approach to the synthesis of functionalized primary and secondary chiral boronic esters. We now report that the oxime-directed CAHB of alkyl-substituted methylidene and trisubstituted alkene substrates by pinacolborane (pinBH) affords oxime-containing chiral tertiary boronic esters with yields up to 87% and enantiomeric ratios up to 96:4 e.r. The utility of the method is demonstrated by the formation of chiral diols and O-substituted hydroxylamines, the generation of quaternary carbon stereocenters through carbon-carbon coupling reactions, and the preparation of chiral 3,4,4-trisubstituted isoxazolines.

Enantioselective desymmetrization via carbonyl-directed catalytic asymmetric hydroboration and Suzuki-Miyaura cross-coupling

The rhodium-catalyzed enantioselective desymmetrization of symmetric γ,δ-unsaturated amides via carbonyl-directed catalytic asymmetric hydroboration (directed CAHB) affords chiral secondary organoboronates with up to 98% ee. The chiral γ-borylated products undergo palladium-catalyzed Suzuki-Miyaura cross-coupling via the trifluoroborate salt with stereoretention.

Enantioselective 1,4-addition of cyclopropylboronic acid catalyzed by rhodium/chiral diene complexes

Rhodium-catalyzed asymmetric addition of cyclopropylboronic acids to electron-deficient alkenes proceeded to give high yields of the corresponding 1,4-addition products with high enantioselectivity.

Mechanistic Insights into Carbonyl-Directed Rhodium-Catalyzed Hydroboration: ab Initio Study of a Cyclic γ,δ-Unsaturated Amide

A two-point binding mechanism for the cationic rhodium(I)-catalyzed carbonyl-directed catalytic asymmetric hydroboration of a cyclic γ,δ-unsaturated amide is investigated using density functional theory. Geometry optimizations and harmonic frequency calculations for the model reaction are carried out using the basis set 6-31+G** for C, O, P, B, N, and H and LANL2DZ for Rh atoms. The Gibbs free energy of each species in THF solvent is obtained based on the single-point energy computed using the PCM model at the ECP28MWB/6-311+G(d,p) level plus the thermal correction to Gibbs free energy by deducting translational entropy contribution. The Rh-catalyzed reaction cycle involves the following sequence of events: (1) chelation of the cyclic γ,δ-unsaturated amide via alkene and carbonyl complexation in a model active catalytic species, [Rh(L2)₂S₂]⁺, (2) oxidative addition of pinacol borane (pinBH), (3) migratory insertion of the alkene double bond into Rh–H (preferred pathway) or Rh–B bond, (4) isomerization of the resulting intermediate, and finally, (5) reductive elimination to form the B–C or H–C bond with regeneration of the catalyst. Free energy profiles for potential pathways leading to the major γ-borylated product are computed and discussed in detail. The potential pathways considered include (1) pathways proceeding via migratory insertion into the Rh–H bond (pathways I, I-1, and I-2), (2) a potential pathway proceeding via migratory insertion into the Rh–B bond (pathway II), and two potential competing routes to a β-borylated byproduct (pathway III). The results find that the Rh–H migratory insertion pathway I-2, followed in sequence by an unanticipated isomerization via amide rotation and reductive elimination, is the most favorable reaction pathway. A secondary consequence of amide rotation is access to a competing β-hydride elimination pathway. The pathways computed in this study are supported by and help explain related experimental results.

Convergent, asymmetric synthesis of vicinal amino alcohols via Rh-catalyzed addition of α-amido trifluoroborates to carbonyls

The convergent, asymmetric synthesis of vicinal amino alcohols is achieved by the Rh-catalyzed addition of α-amido trifluoroborates to carbonyl compounds.

Synthesis and applications of α-trifluoromethylated alkylboron compounds

RBF3 K is a chemist's BFF: A metal-free synthetic route to unprecedented organoboron compounds bearing an α-trifluoromethyl substituent, employing a variety of trifluoroborate (RBF3 K) starting materials, is reported. These substrates represent the first isolated α-trifluoromethylated alkylboron building blocks, and these reagents lead to a variety of useful bench-stable, synthetic intermediates. Pin=pinacol.