Cyclic organomagnesium reagent

An important consequence of the C2 symmetry of the cyclic boronic esters is that both faces of the trigonal boron atom are equivalent. Consequently, there is only one possible intermediate borate, which can also be generated by reaction of a (dichloromelhyl)boronic ester with an organolithium or organomagnesium reagent at — 78 C2126. 

In the cyclic series several examples, as summarized above, have been provided. The diastereoselectivity, l or u, essentially can be controlled by the choice of the halide in the Grignard reagent and the additive, i.e., employing either organomagnesium bromide with copper(I) bromide and a phosphine, or organomagnesium chloride with copper(I) bromide and chlorotrimethylsilane (see also syn/anti selectivity in Section 1.1.1.2.). 

Chiral sulfinimines 236 are very useful intermediates for the preparation of enantiomer-ically pure primary amines 237 (equation 158) . This reaction has been applied to the synthesis of a-amino acids . For sulfinimines obtained from simple ketones, lithium reagents are preferable for the addition , while for cyclic ketones organomagnesium compounds gave the best results. Addition of alkyl and aryl Grignard compounds to sulfinimines, derived from 3- and 4-substituted cyclohexanones, proceeds with excellent diastereoselectivity, depending on the stereochemistry of the ring substituents rather than the sulfinyl group . 

The cyclic mechanism, in which the concertedness of the reaction is so clearly depicted, has found applications in several other instances in organomagnesium chemistry. This is particularly true for the reduction reaction with the aid of Grignard compounds. It was reported in 1950, in a paper on the asymmetric reduction of ketones with the aid of chiral Grignard reagents [58], that Whitmore had already presented the idea of a cyclic mechanism (Scheme 23) at an American Chemical Society meeting in 1943 [59] ... 

It has been made clear, in Chapter 11, that the nature of the Grignard reagent has a great influence on the mechanism of its reactions. The extremes in the reactivity spectrum of organomagnesium compounds in their interaction with ketones, for example, are (I) reactions in which a single electron transfer occurs as the first step, or (2) reactions in which a concerted bond-breaking-bond-making takes place within a cyclic transition state. 

Biickvall [48] also showed that the regioselectivity of the reaction of addition of a Grignard reagent on a cyclic allylic chloride in the presence of CuCl 2LiCl, can be influenced by the nature of the organomagnesium species [Eq. (30],... 

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