A NEW [2-60 WITH ALKYL-SUBSTITUTED 1,3-BUTADIENES - A MECHANISTIC APPROACH - STEREOCHEMISTRY AND ISOTOPE EFFECTS(2] FUNCTIONALIZATION OF C)

The stereochemistry and secondary isotope effects of the [2 + 2] photo cycloaddition of trans, trans- (7), cis,cis- (8), and cis, trans-2,4-h exadiene (9), 2,5-dimethyl-2,4-hexadiene (1), and its deuterated analo gues 1-d(1), 1-d(6), and trans-1-d(3) to C-60 have been investigated. A loss of stereochemistry in the cyclobutane ring for photocycloadditi on of all three 2,4-hexadiene isomers 7, 8, and 9 to C-60 was observed (the trans stereochemistry in the cyclobutane ring predominates in al l cases), while the unreactive double bond retained its stereochemical integrity in the adducts. The cis double bond of 9 is 1.5 times more reactive than the trans. The [2 + 2] photocycloaddition of (E)-2,4-dim ethyl-2,4-hexadiene (10) to C-60 is regiospecific, affording two diast ereomeric adducts, 10a and 10b, by addition on the methyl monosubstitu ted terminal double bond. These results, when taken in conjunction wit h the small inverse intramolecular secondary isotope effect (k(H)/k(D) = 0.90 +/- 0.05) in the [2 + 2] photocycloaddition of 1-d(6) to C-60, favor the formation of an open biradical intermediate in the rate-det ermining step.