Transition states allenylzinc reagent additions

Zweifel and Hahn found that deprotonation of terminal allenes with -BuLi and subsequent addition of ZnCb leads to terminal allenylzinc reagents, which afford anti adducts upon addition to various aldehydes (Table 5)7. Branching in the aldehyde and allene substituents enhanced the anti syn ratio of adducts, in keeping with the previously proposed cyclic transition state for such additions. 

The factors that control the stereochemical outcome of such rections can be illustrated by additions of enantiomeric allenylzinc reagents to (S)-lactic aldehyde derivatives [114]. The matched S/S pairing proceeds via the cyclic transition state A in which addition to the aldehyde carbonyl assumes the Felkin-Anh orientation with an anti arrangement of the allenyl methyl and aldehyde substituents (Scheme 9.29). The alternative arrangement B is disfavored both by the anti-Felkin-Anh arrangement and eclipsing of the allenylmethyl and aldehyde substituents. 

Normant and Poisson prepared allenylzinc bromide reagents from TMS acetylenes along the lines of Epsztein and coworkers5, by sequential lithiation with s-BuLi to yield a lithiated species, and subsequent transmetallation with ZnBr2 (equation 35)27,28. Additions to racemic /J-silyloxy aldehydes proceed with low diastereoselectivity to afford mixtures of the anti,anti and anti,syn adducts (Table 17). The latter adducts are formed via an anti Felkin-Anh transition state. Additions to the racemic IV-benzylimine analogs, on the other hand, proceed with nearly complete Felkin-Anh diastereoselectivity to yield the anti,anti amino alcohol adducts (Table 18). 

The addition of a chiral allenyl metal to an aldehyde generating a 2-substituted butynyl structure is named Marshall-Tamaru MT) reaction (Scheme 5-13). An allenyl palladium species is generated via a formal Sn2 substitution of the mesylate, which in turn undergoes a transmetalation with diethylzinc yielding a nucleophilic species (Scheme 5-13). The reaction of the electrophile proceeds via a similar -ester enolate transition state as depicted in Scheme 5-13. Corresponding allenylindium reagents can also be used instead of allenylzinc intermediates. ... 

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