Photochemistry of Metal Olefin Complexes. Isomerization and Stereospecific Cyclodimerization of Silver Cycloalkene Complexes

Abstract

PhotoIysis of AgCI04 -1-methylene cyclohexane complex in pure polycrystalline state or in methanol solution leads to isomerization of the complex to AgCIO4 -1-methylcyclohexene. The isomerized complex undergoes further photochemical cyclodimerization giving the stereospecific exo-trans-exo dimer of the 1-methylcyclohexene and colloidal silver. Similar photoisomerization and a less stereospecific photodimerization were observed in the photolysis of the polycyclic AgC104 -p-pinene complex.La photolyse du complexe mtthylene-1 cyclohexane -AgCIO, soit sous forme polycristalline soit en solution dans le mtthanol conduit B une isomtrisation du complexe en mtthyl-1 cyclohexene -AgClO,.Le complexe isomtrist subit une cyclodimtrisation photochimique ulttrieure pour donner d'une f a~o n sttrtosptcifique le dimere exo-trans-exo du mtthyl-1 cyclohexene et de I'argent collofdal. De la mdme f a~o n , une photoisomtrisation et une photodimtrisation moins sttrtospecifique sont observtes dans la photolyse du complexe polycyclique P-pinhe -AgCIO4.Canadian Journal of Chemistry. 49, 2760 (1971) Although recent interests in the Agf-catalyzed rearrangements of strain polycyclic alkanes are developing rapidly (1-6), surprisingly little effort has been devoted to explore the photochemical reactivity of the well-known silver-olefin complexes. We report here the photoisomerization and the stereospecific photocyclodimerization of AgC10, complexes of 1-methylene cyclohexane and 1-methylcyclohexene. Similar photochemical reactions of the silver -P-pinene complex are also observed. These results indicate that the photochemical reactivity of these silver-olefin complexes are distinctly different from those of the free ligands and may be of general interest to synthetic and mechanistic organic photochemistry.The complexes were prepared by addition of the corresponding olefins to an acetone solution of AgC10, at 0 "C. The white crystalline products were filtered, washed, and dried over P205. Silver analyses of these solid complexes indicated a 2:l olefin to Ag ratio. Since the i.r. and n.m.r. data characterizing these complexes are not available in the literature, they are presented here in Table 1.The AgClO, -I-methylene cyclohexane complex was found to be stable in the dark at 0 "C for several weeks. Irradiation of a methanol solution of this complex (0.1 M) with U.V.(2537 A) effects firstly an isomerization to silver -1-methylcyclohexetle. The photoisomerization can be readily followed by n.m.r. and/or i.r. analyses showing the conversion of a complexed exocyclic olefin to a complexed endocycloalkene. The rate of the photoisomerization process is apparently independent of temperature within the range 10-30 "C. However, photolysis of the complex in the pure solid state leads to a tenfold increase in the rate of conversion. Prolonged irradiation of the complex either in solution or in the solid state for several hours results in secondary photochemical reaction of the isomer product giving the sole exo-trans-exo cyclic dimer of the 1-rnethy...