Asymmetric Propargylation of Ketones Using Allenylboronates Catalyzed by Chiral Biphenols

Abstract

Chiral biphenols catalyze the enantioselective asymmetric propargylation of ketones using allenylboronates. The reaction uses 10 mol % of 3,3′-Br2-BINOL as the catalyst and allenyldioxoborolane as the nucleophile, in the absence of solvent, and under microwave irradiation to afford the homopropargylic alcohol. The reaction products are obtained in good yields (60 – 98%) and high enantiomeric ratios (3:1 – 99:1). Diastereoselective propargylations using chiral racemic allenylboronates result in good diastereoselectivities (dr > 86:14) and enantioselectivities (er > 92:8) under the catalytic conditions.

Asymmetric Organocatalytic C‐C Bond Forming Reactions with Organoboron Compounds: A Mechanistic Survey

“…The scope of the enantioselective organocatalyzed boronate 1,2-additions was expanded by the development of the asymmetric propargylation of ketones with allenylboronate 118 catalyzed by (S)-3,3′-dibromo-BINOL (S)-49 (Scheme 22). [49] The reactions were run at 60°C for 1 h under microwave irradiation in neat conditions and applied to 22 ketones with a great diversity of functional groups. When α, -unsaturated ketones were used, complete chemoselectivity towards the 1,2-addition products 119 was observed.…”

Enantioselective Synthesis of anti- and syn-Homopropargyl Alcohols via Chiral Brønsted Acid Catalyzed Asymmetric Allenylboration Reactions

“…The stereochemistry of the methyl group introduced into 2 and 3 is controlled by the chirality of the allenylboronate ( M )- 1 , but use of the appropriate enantiomer of the chiral phosphoric acid catalyst is required in order to achieve synthetically useful diastereochemical control of these reactions. This study represents a striking case of a growing number of examples 4d, 13l, 14 in which the diastereoselectivity of an inherently non-diastereoselective but highly enantioselective transformation (e.g., entry 1, Table 1) is rectified by using a chiral catalyst to alter the relative energetics of the competing transition states in order to control the stereochemistry of the center (e.g., the hydroxyl group in 2 and 3 ) that is not controlled by using the primary chiral reagent alone (the allenylboronate ( M )- 1 , for the work described here). The reactions we describe here differ fundamentally from traditional examples of double asymmetric synthesis, which involve the use of a chiral substrate in combination with the appropriate enantiomer of a chiral reagent.…”

Microwave Reactors: A Brief Review on Its Fundamental Aspects and Applications

“…A number of 2,4-disubstituted quinolines have been synthesized in excellent yield (69% -93%) under mild reaction condition [72] and the catalyst is reused up to four times. 12) 3,3′-Br 2 -BINOL has been shown to catalyze the enantioselective asymmetric propargylation of ketones using allenyldioxoborolane as nucleophile, in absence of solvent and under microwave irradiation to afford homopropargylic alcohols in good yields (60% -98%) and high enantiomeric ratios (3:1 -99:1) [73]. Diastereoselective propargylations using chiral racemic allenylboronates result in good diastereoselectivities (dr > 86: 14) and enantioselectivities (er > 92:8) under the catalytic conditions.…”