REINTERPRETATION OF THE MECHANISMS OF CONCERTED CYCLOADDITION AND CYCLODIMERIZATION OF ALLENES

The cycloaddition and cyclodimerization reactions of alienes have been evaluated in terms of a concerted [π2S + (π2S + π2S)] mechanism in which the dienophile interacts with one p AO of each double bond of the aliene, the exocyclic double bond of the product being formed from the two remaining orthogonal p AO's by rotation of the terminal aliene methylene group. The application of second-order theory to this process results in predictions of chemo-, regio-, and stereoselectivities consistent with experimental observations. Similar calculations on the (π2S + π2a) process predict no definite situ- or regioselectivities should be observed and result in incorrect predictions in a couple of cases. © 1979, American Chemical Society. All rights reserved.