Vinyl and fofmyl groups undergo rearrangements of the type X = CHCMe2-CHI + CMe2-CH2-CH=X (X = CHI or 0) with comparable rates (106-107 I/mol. s, 20°C). This result is in line wi.th high-level ab initio calculations performed on the basic systems, i. e. X = CH -CHI -CH2, which give barriers of activation of 14.9 (CH? = CH -CH2 -cH2) and 19.2 kcal/mol (0 = CH -CH2 -CH2). respectively. The calculations suggest that the rearrangements proceed via cyclopropane-like intermediates.Experimentelle und theoretische Untersuchungen zum Mechanismus der radikalischen 1,2-Umlagerung von Vinyl-und Formylgruppen Die Umlagerungen von Vinyl-und Formylgruppen vom Typ X = CH -CMe2-CH2 + CMe2-CH2-CH = X (X = CH2 oder 0) erfolgen mit vergleichbaren Geschwindigkeiten (lo6 -lo7 l/mol. s, 20°C). In Ubereinstimmung mit diesen kinetischen Messungen stehen ab-initio-Rechnungen, nach denen fur die Grundkorper Aktivierungsenergien von 14.9 (CH2 = CH -CH2 -CH3 und 19.2 kcal/mol (0 = CH -CH2 -CHI) berechnet werden.Die Rechnungen sagen Cyclopropanderivate als Zwischenstufen der Umlagerung voraus. Whereas 1,2-migrations in cations') and in anions') are thoroughly studied, less is known about the mechanism of 1,2-migrations in radicals, although radical rearrangements are discussed in various reactions, e. g. vitamin-BI2-catalyzed isomerizations ' ) or gas-phase reactions of radical cations4). Experimental" and theoretical6) studies show, that the 1,2-migration in radicals depends essentially upon the nature of the migration group. Three mechanisms have to be distinguished: (1) The rearrangement follows a dissociation/recombination mechanism, (2) the 1,2-migration is concerted, or (3) an intermediate is formed. All three cases have been discussed6) and it is known that H, alkyl, OH, and NH2 groups migrate very slowly, whereas OH:, N H f , and C(OH)$ substituents show a higher tendency for rearrangements. In the migration of vinyl and acyl groups (1-3) it was of interest for us, (1) whether a vinyl group (X = CH2) migrates faster than a 0 VCH Verlagsgesellschaft mbH, D-6940 Weinheim, 1986