Chromium(II) mediated threo selective synthesis of homoallyl alcohols

Chromium‐Catalyzed Pinacol‐Type Cross‐Coupling: Studies on Stereoselectivity

“…[17] The shape of the transition state, whether it is chair-like (10 or 11) or skew-boat/twist-boat-like (10 b or 11 b), does not change the results qualitatively. In every case the energetically preferred position for the (large) alkyl residue, R 2 , of the aldehyde should be in the (pseudo)equatorial position (R 2e = alkyl; R 2a = H).…”

“…[17,18] We therefore tried to use solvents other than DMF in order to increase diastereoselectivities, which are generally highly substrate dependent.…”

“…Skew-boat-like transitions states with pivalaldehyde, analogous to Nozakis and Hiyamas, described in the literature. [17] While the pinacol coupling reaction proceeded in DMF in high yields but with low diastereoselectivity, in other solvents the desired product could be isolated only in low yields of < 15 %. In THF the reaction was dramatically changed, with the cyclopropanol 15 being formed diastereoselectively as the main product.…”

“…Similar transition states have been described by Takai for chromium-mediated coupling reactions of vinyl ketones with aldehydes [12] and by Nozaki and Hiyama for smaller R 2 residues. [17] Compound (Z)-9 should form transition state 10, while (E)-9 should lead to transition state 11. Both diastereomeric pinacols (syn and anti) can be obtained from either transition state, depending on the orientation of the aldehyde.…”

“…Compound 10 b represents an analogue of the transition state proposed previously by Nozaki and Hiyama for pivalaldehyde. [17] When the steric demand of R 2 is decreased, 10 will possibly give rise to a slight decrease in diastereoselectivity (Table 1, …”