TetrahedronLetters No.55, pp. 57%-579tJ, 1968. Perwm Press. Printed in Great Britain. DEUTERIM ISOTOPE EFFRCTS IN THE NORRISH TYPE II ELIMINATIONOF A KETONE WITH MULTIPIE AVAILABLE SITES. Albert Padwa* and William Bergmark Department of Chemistry State University of New York at Buffalo Buffalo, New York 14214 (Receivedin USA 13 August 1968; received in UK for publioation 15 October 1968) Recent studies on the Norrish Type II photoelimination have provided considerableinsight into the nature of the carbonyl excited state. (1) The present evidence suggests that abstractionof the y-hydrogen is reversible and can occur from either the first excited singlet state or from the triplet state. (2-4) Substitutionof the y-hydrogen with deuterium is known to exert a profound effect on the photoelimination.(5-7) Coulson and Yang (7) have pointed out that the enhanced quantum yield and triplet lifetimeof 2-hexanone-5,5-d2 over 2-hexanone can be interpretedby assuming that deuterium either (a) enhances the intersystemcrossing efficiency (b) causes a reduction in the rate of radiationlessdecay (c) retards the significantreturn of the biradical formed by y-abstractionto the ground state of starting ketone. We recently reported that the differences in product distributionarising from specific deuterium substitutionin the photolysis of aroylazetidinesto be most satisfactorily explained by the prominent reversibility of the hydrogen abstractionstep. (8) In connection with these studies we found it desirable to examine the influenceof y-deuteriumsubstitution on the quantum efficiency and distributionof photoproductsfrom 5-decanone (I) and 5-decanone- 2,2-d2 (II). In this communication we describe results pertaining to deuterium isotope effects in the Norrish Type II photoelimination. The labeled 5-decanone-2,2-d2 was synthesizedby the cuprous catalyzedaddition of ethyl magnesium iodide-1,1-d2to 1-octen-3-one and purified for use by preparativevpc. It was assayed for >97% d2 by mass spectral analysis. Degassed pentane solutionsof I and II (0.0266M) and various concentrations of cis-1,2-dicloroethylene were photolyzed to 6% - conversion using 300 mu radiation. The progress of reaction and quantum yields of formation * Alfred P. Sloan Foundation Fellow 1968-1970. 5795