Elimination-Addition Route The Allenic Mechanism

A proton on a y-carbon may be eliminated in competition with a proton on the a-carbon, and subsequent addition of the nucleophile to the central carbon atom of the intermediate substituted allene (189) would result in an overall substitution (equation 18). In this j8,y-elimination- 

Beltrame et al. (1964) suggested this route for the reaction of 1,1-di-phenyl-2-halopropene (190) with EtO- ion which gives the substitution product (191) and a cyclobutane (192) which is a dimer of the alleged intermediate allene (193). The reaction is characterized by an element effect (fcBr/ ci = 2-54-2-79 at 80-125°), and by rate coefficients which are 

200-fold higher than those of the l,l-diphenyl-2-haloethylenes which react by addition-elimination. The vinylic ether (191) is the main product although more (192) is formed when X = Br than when X = C1. The ratio (191)/(192) is dependent on the ratio (EtO-)/(190), being e.g. 3-6 and 10 for ratios of 2-5 and 10 of EtO- to (190, X=C1) at 125-130°. 

In principle, the nucleophile can attack the allene at two different positions, but the products show exclusive attack at the central carbon atom, similarly to other nucleophilic additions to allenes (Eglinton et al., 1954 Stirling, 1964b Taylor, 1967). This may result from the stabilization of the carbanion (194), formed by attack at this position, by the two phenyl groups. The ion (194) may be protonated at either one of the terminal positions of the allenic system, and low amounts of 

Competition between the j8,y- and the a,/ -elimination-addition modes is possible in the systems studied by Bottini and coworkers. N-(2-Bromoallyl)alkylamine (199) with NaNH2 in liquid NH3 gives mainly l-alkyl-2-methyleneaziridine (200) together with a lower amount of N-alkylpropargylamine (201) (Pollard and Parcell, 1951 Bottini and Roberts, 1957). The driving force for this intramolecular substitution of the unactivated vinyl bromide (199) is probably the presence of the 

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