Woodward-Hoffmann forbidden and allowed

In reality, the reaction could not be persuaded to go exactly as shown in Scheme 21.1, because the Cl—C3 bond would certainly break at very nearly the same rate as Cl—C2. In the experiments actually conducted by Baldwin et al., this problem was resolved by deuterium labeling both C2 and C3—creahng diastereomericaUy pure, but achiral molecules. Even then, there remained a large number of technical difficulties, which in the end the researchers were able to overcome. Their results indicated that the four stereochemical courses for the reaction run at 300 °C were sr 23%, si 40%, ar 13%, and ai 24%. These numbers do not ht the expectations from either mechanism. Clearly, the Woodward-Hoffmann forbidden and allowed products are formed in nearly equal amounts ([sr] + [ai] =47% [si] + [ar] = 53%)— hardly what one would expect for a pericyclic reaction. On the other hand, the stereochemical paths do not show the pairwise equalities expected from the stepwise mechanism. 

Scalar relativistic corrections, 209 Spin-spin interaction, 211 Turnover rule, 124 Woodward-Hoffmann forbidden and allowed... 

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