Zinc carbenoids mechanism

In contrast to the extensive body of work on the preparation of these zinc carbenoids, few investigations are on record concerning the mechanism of the Furu-kawa method for carbenoid formation. Two limiting mechanisms can be envisioned - a concerted metathesis via a four-centered transition structure or a stepwise radical cleavage-recombination (Scheme 3.11). 

The possibility of a radical mechanism is supported by the observation of the accelerating effect of molecular oxygen on the cyclopropanation. Miyano et al. discovered that the addition of dioxygen accelerated the formation of the zinc carbenoid in the Furukawa procedure [24a, b]. The rate of this process was monitored by changes in the concentration of ethyl iodide, the by-product of reagent formation. Comparison of the reaction rate in the presence of oxygen with that in the... 

III. MECHANISM OF THE CYCLOPROPANATION OF ALKENES USING ZINC CARBENOIDS... 

The reduction takes place at the surface of the zinc catalyst. In this reaction, alcohols are not postulated as intermediates, because subjection of the corresponding alcohols to these same reaction conditions does not lead to alkanes. The following proposal employs the intermediacy of zinc carbenoids to rationalize the mechanism of the Clemmensen Reduction ... 

Regarding the addition of a carbenoid 7 to an olefin 8, resulting in the formation of cyclopropane 9, theoretical calculations point towards a concerted mechanism involving a transition state 10 (Scheme 2) [21]. For a better understanding of the transition state of cyclopropanation with zinc carbenoids a reflection on tbe well-studied lithium carbenoids is profitable. Hoffmann et al. 

Zn(Cu)] and the Wittig-Schwarzenbach reagent [CH2N2 4- Znl2] give rise to the formation of one and the same carbenoid. Zinc chloride catalyzes the reaction (5) and an explanation in terms of ZnClg-assisted elimination of chloride ion via a transition state (X) was suggested to be most probable (546). The possibility of a two-step (addition and elimination) mechanism [Eq. (6)], 200) which was proposed for the reaction of... 

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