A STUDY OF THE EFFECTS INTRAMOLECULAR π-COMPLEXATION ON THE REACTIVITY OF TRANSIENT ARYL(3-BUTENYL)GERMYLENES

Abstract

<p>Three novel 1-aryl-1-(3-butenyl)germacyclopent-3-enes (26a, 26b and 26c) were synthesized and their photochemistry in hexane solution was studied by steady state and laser flash photolysis (LFP) methods. Steady state photolysis of 1-(3-butenyl)-3,4-dimethyl-1-phenylgermacyclopent-3- ene (26a) was found to proceed cleanly to afford the corresponding germylene derived product along with 2,3-dimethyl-1,3-butadiene (DMB) in the presence of acetic acid, methanol and isoprene, suggesting that free (3-butenyl)phenylgermylene (GeBuPh) is the primary photochemically generated species. However, we were unable to detect the germylene by laser flash photolysis with this compound, due to rapid “self –quenching” of the germylene by the precursor. The direct detection of the germylene in solution by laser flash photolysis requires the use of a more strongly absorbing derivative. Indeed, 3-butenylphenylgermylene (GeBuPh) was successfully detected directly by laser flash photolysis of 1-(3-butenyl)-3-methyl-1,4-diphenylgermacyclopent-3-ene (26b) in hexane solution, where it exhibits a UV-Vis absorption band centered at λmax= 490 nm and decays with second order kinetics on the microsecond timescale. In the absence of reactive substrates the decay of GeBuPh is accompanied by the growth of a second transient absorption, assigned to Ge2Bu2Ph2 (34) λmax = 420 nm; the assignment is based on a comparison to the laser flash photolysis of 1,3-dimethyl-1,4- diphenylgermacyclopent-3-ene (26d). Rate constants have been determined for reaction iv of the germylene with selected germylene substrates in order to evaluate the effects of intramolecular π-complexation on its reactivity. The results indicate that GeBuPh exhibits similar reactivity to GeMePh under otherwise identical experimental conditions, and thus show no significant indication of intramolecular π-complexation. With this in mind we have synthesized and studied 1-(3-butenyl)-3-methyl-4- phenyl-1-[3,5-bis(trifluoromethyl)phenyl]germacyclopent-3-ene (26c), which was designed to produce a more strongly electrophilic Ge(II) center in the corresponding germylene. The germylene 46 is detectable as a weakly absorbing transient species with λmax = 490 nm by laser flash photolysis of 26c in hexane solution. Generation of germylene 46 in the presence of THF leads to the formation of the corresponding Lewis acid base complex 48 at λmax = 330 nm. The reactivity of germylene 46 with selected substrates such as AcOH, THF and isoprene has been examined and the results compared to analogous data for the parent germylene GeBuPh. The forward rate constant for germylene 46 with acetic acid is slightly higher than that for GeBuPh, and no evidence for intramolecular complexation with the remote C=C bond could be obtained.</p>