Stereodefined Substituted Cyclopropyl Zinc Reagents from Gem-Bismetallics

Carbozincation Reactions of Carbon–Carbon Multiple Bonds

“…To explain the origin of the relative configuration of the metalated carbon observed in the product (trans to the alkyl and allyl groups), a mechanism involving an inversion-inversion at both centers through a W-shaped conformer is postulated. 151 The diastereoselective synthesis of a tetrasubstituted cyclopropane can also be achieved using the same procedure (Scheme 124). 151 In this case, a gauche interaction exists in the W-shaped transition state structure 19 between trimethylsilyl and the allyl moiety that is released in structure 20; such an interaction is absent in the nonsubstituted structure 21.…”

“…151 The diastereoselective synthesis of a tetrasubstituted cyclopropane can also be achieved using the same procedure (Scheme 124). 151 In this case, a gauche interaction exists in the W-shaped transition state structure 19 between trimethylsilyl and the allyl moiety that is released in structure 20; such an interaction is absent in the nonsubstituted structure 21. An alternative strategy to prepare tertiary cyclopropylzinc derivatives is shown in Scheme 125).…”

“…An alternative strategy to prepare tertiary cyclopropylzinc derivatives is shown in Scheme 125). 151 Replacing the butyl group with a phenyl group changes the stereochemical outcome of the cyclization reaction. Although the carbometalation is performed at lower temperatures, the organo-gem-dimetallic reagent cyclizes immediately to afford the trans-cyclopropane with a diastereomer ratio of 95:5.…”

“…Also, the use of a more coordinating solvent (THF instead of ether) disrupts this association and lower diastereoselectivity is observed. 151 Yet another approach to 1-alkyl-2-vinylcyclopropanes 152,153 involves addition of (3-ethoxyallyl)zinc bromide to an alkenylmagnesium reagent to afford an organogem-dimetallic species that undergoes a 1,3-elimination leading to the corresponding cyclopropane (Scheme 127). …”

Carbometalation of Vinyllithiums by Zincated Allyltrimethylsilane

“…When the alcohol is protected as a MOM ether, the presence of three equivalents of ZnBr 2 is also beneficial, but in this case, the addition of MgBr 2 was precluded, since it would promote a γ-elimination of the OMOM group. [3,6] However, with this protection, the reaction is faster and takes place within 24 h at 0°C and, except for reagent 2f, a unique diastereomer is obtained after the initial mixture of E and Z isomers has been warmed up to room temperature (entries 8Ϫ11). In the case of 2f (entries 12Ϫ13), epimerization of the CϭC bond to the thermodynamic E isomer is much slower and only an E/Z ratio of 73:27 is attained within 24 h at room temperature (entry 13).…”