Conventional direct methods for the synthesis of alkenes do not wholly address issues of stereo- and regio-control. A unified solution to controlled connective alkene synthesis will be described based on the eliminative cross-coupling of pairs of enantioenriched sp3 hybridized carbenoids. Herein, stereochemical information encoded within the carbenoid building blocks is translated via a series of three stereospecific processes (electrophilic substitution, 1,2-metalate rearrangement, and beta-elimination) into any desired configuration of the targeted unsaturated bond. Significantly, no special biasing features (e.g., electronic, steric, coordinating) are required within substituents to achieve complete stereo- and regio-control. Implementation of the concept by pairing alpha-(carbamoyloxy)alkyl boronates with alpha-(carbamoyloxy)alkyllithiums will be presented. For example, reaction of (S)-1-[(diisopropylamino)carbonyloxy]-1-phenylethyllithium (97% ee) with (S)-2-[1-[(diisopropylamino)carbonyloxy]-3-phenylpropyl]-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (97% ee) in diethyl ether at –78 deg C lead to (E)-2,5-diphenyl-2-pentene in 62% yield and E:Z = 95:5 after warming to rt and treatment with NaOH. By contrast, the analogous heterochiral eliminative cross-coupling reaction (i.e., (R)-lithiated carbamate with (S)-boryl carbamate) gave (Z)-2,5-diphenyl-2-pentene in 70% yield and E:Z = 6:94.

Stereospecific connective synthesis of alkenes by eliminative cross-coupling of enantioenriched carbenoids

251st National Meeting (2016)

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