Allenes stereochemical model

The approximations inherent in the EH method make the small energy preference calculated for 4 (0, 90, 90) over 4 (0, 90, 0) seem inadequate grounds for firm conclusions. We simply note that the preference for 4 (0, 90, 90) as a stereochemical model for the intermediate in allene-olefin cycloadditions deduced from experimental data is consonant with the EH result. The very small dependence in energy upon the angle y for 4 (0, 90, y) may be cause for suspecting that at least in some substituted cases, rotation through y = 180 ° in the intermediate 4 (0, 90, y) may be kinetically competitive with ring closure. 

A stereochemical model was proposed to account for the chiral discrimination, which was consistent with the absolute configurations observed in the enantioselec-tive cycloaddition reactions. As the absolute sense of stereochemistry is predictable, this reaction could be used to assign the absolute configurations of new, optically active allenes. 

With the hypothesis that allene-olefin cycloadditions and methylenecyclobutane rearrangements have a common intermediate comes the prediction that common stereochemical modes for the various rotations involved will prevail. The cycloaddition model outlined above involves disrotatory ring closure with the same sense of rotation about C(4)C(5) and C(5)C(2). For the methylenecyclobutane rearrangement, the same stereochemistry is anticipated for the reverse reaction. 

Le Bel published his stereochemical ideas two months later, in November 1874, under the title, The relations that exist between the atomic formulas of organic compounds and the rotatory power of their solutions" (14). An English translation is presented in Le Bel (15). Le Bel approached the problem from a different direction from van t Hoff. His hypothesis was based on neither the tetrahedral model of the carbon atom nor the concept of fixed valences between the atoms. He proceeded purely from symmetry arguments he spoke of the asymmetry, not of individual atoms, but of the entire molecule, so that his views would nowadays be classed under the heading of molecular asymmetry. Only once does he mention the tetrahedral carbon atom, which he regarded as not a general principle but a special case. Today, substituted allenes, spiranes, and biphenyls are but a few examples of asymmetric molecules that do not contain any asymmetric carbons, thus confirming Le Bel s views on molecular asymmetry. The reason for the different approaches by van t Hoff and le Bel is easy to understand, van t Hoff came from the camp of structural chemists, and he... 

Reactions of chiral allenes proceed with a preference for the formation of the syn diastereomer. The stereochemical outcome of these reactions can be rationalized by invoking an open transition state model for the addition reactions (Figure 12), which depicts an antiperiplanar orientation of the chiral allenylsi-lane to the aldehyde carbonyl. In this model, steric repulsion between the allenyl methyl and the aldehyde substituent is most likely responsible for the destabilization of transition state (B), which leads to the anti (minor) stereoisomer. This destabilizing interaction is minimized in transition state (A). Table 5 illustrates representative examples and summarizes the scope of the regiocontrolled synthesis of homopropargylic alcohols using allenylsilanes. 

---