Rh-Catalyzed Coupling of Aldehydes with Allylboronates Enables Facile Access to Ketones

We present herein a novel strategy for the preparation of ketones from aldehydes and allylic boronicesters. This reaction involves the allylation of aldehydes with allylic boronicesters and the Rh-catalyzed chain-walking of homoallylic alcohols. The key to this successful development is the protodeboronation of alkenyl borylether intermediate via a tetravalent borate anion species in the presence of KHF 2 and MeOH. This approach features mild reaction conditions, broad substrate scope, and excellent functional group tolerance. Mechanistic studies also supported that the tandem allylation and chain-walking process was involved.

Ligand-Controlled Regiodivergent Enantioselective Rhodium-Catalyzed Alkene Hydroboration

Regiocontrol in the rhodium-catalyzed boration of vinyl arenes is typically dominated by the presence of the conjugated aryl substituent. However, small differences in TADDOL-derived chiral monophosphite ligands can override this effect and direct rhodium-catalyzed hydroboration of β-aryl and β-heteroaryl methylidenes by pinacolborane to selectively produce either chiral primary or tertiary borated products. The regiodivergent behavior is coupled with enantiodivergent addition of the borane. The nature of the TADDOL backbone substituents and that of the phosphite moiety function synergistically to direct the sense and extent of regioselectivity and enantioinduction. Twenty substrates are shown to undergo each reaction mode with regioselectivity values reaching greater than 20:1 and enantiomer ratios reaching up to 98:2. A variety of subsequent transformations illustrate the potential utility of each product.

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.

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.

Beyond benzyl grignards: facile generation of benzyl carbanions from styrenes

Benzylic functionalization is a convenient approach towards the conversion of readily available aromatic hydrocarbon feedstocks into more useful molecules. However, the formation of carbanionic benzyl species from benzyl halides or similar precursors is far from trivial. An alternative approach is the direct reaction of a styrene with a suitable coupling partner, but these reactions often involve the use of precious-metal transition-metal catalysts. Herein, we report the facile and convenient generation of reactive benzyl anionic species from styrenes. A Cu(I)-catalyzed Markovnikov hydroboration of the styrenic double bond by using a bulky pinacol borane source is followed by treatment with KOtBu to facilitate a sterically induced cleavage of the C-B bond to produce a benzylic carbanion. Quenching this intermediate with a variety of electrophiles, including CO(2), CS(2), isocyanates, and isothiocyanates, promotes C-C bond formation at the benzylic carbon atom. The utility of this methodology was demonstrated in a three-step, two-pot synthesis of the nonsteroidal anti-inflammatory drug (±)-flurbiprofen.