Direct detection of methylphenylgermylene and 1,2-dimethyl-1,2-diphenyldigermene — Kinetic studies of their reactivities in solution

Abstract

Photolysis of 1,3,4-trimethyl-1-phenylgermacyclopent-3-ene (5) in hydrocarbon solvents containing isoprene, methanol, or acetic acid affords 2,3-dimethyl-1,3-butadiene (DMB) and the expected trapping products of methyl phenylgermylene (GeMePh) in chemical yields exceeding 90%. The germylene has been detected in hexane solution by laser flash photolysis as a short-lived species (τ ~ 2 µs) exhibiting a UV-vis absorption spectrum centered at λmax = 490 nm. It decays with second-order kinetics and a rate constant close to the diffusion-controlled limit, with the concomitant growth of a second longer-lived transient (λmax = 420 nm) that is assigned to a mixture of (E)- and (Z)-1,2-dimethyl-1,2-diphenyldigermene (4). Absolute rate constants have been determined for the reactions of the germylene with primary and tertiary amines (n-BuNH2 and Et3N, respectively), acetic acid (AcOH), a terminal alkyne and alkene, isoprene, DMB, CCl4, and the group 14 hydrides Et3SiH and Bu3SnH. GeMePh is slightly more reactive than GePh2 towards all the reagents studied in this work; both are significantly less reactive than GeMe2 toward the same substrates. Absolute rate constants for the reactions of 4 have also been measured or assigned upper limits in every case and are compared to previously reported values for tetraphenyl- and tetramethyl-digermene with the same reagents.Key words: germylene, digermene, kinetics, laser flash photolysis, germirane, germirene, vinylgermirane, complex, UV–vis spectrum, insertion, addition.

The photochemistry of a potential diphenylsilylene precursor. Time-resolved spectroscopic studies of the reactivity of a transient vinylsilirane

“…The photochemical behavior of 2 contrasts sharply with that of the germanium homologue (1), whose photolysis yields GePh 2 and DMB cleanly and with a quantum yield = 0.55 under similar conditions (Equation (1)) [23]. It is difficult to determine with certainty whether or not the corresponding vinylgermirane (11a) is also formed in the photolysis of 1 in fluid solution; while no evidence for its formation could be obtained from steady state trapping studies, our subse- quent studies of the reactions of GePh 2 and GeMePh with isoprene and DMB [24,34] show that if it were formed as a primary photochemical product, it would dissociate to GePh 2 and DMB too rapidly for it to be trapped efficiently under typical experimental conditions [35]. Nevertheless, a closely related species could be detected by flash photolysis of 1 in the presence of 3-50 mM isoprene [24].…”

“…The results of this work indicate that 1-silacyclopent-3-ene derivatives will likely not show the same general suitability as their germanium [23,24,34,40] and tin [41] homologues as photochemical precursors to reactive metallylenes for study by time-resolved UV/vis absorption methods in solution. Nevertheless, they clearly show considerable potential as precursors for more detailed kinetic studies of the chemistry of reactive vinylsiliranes, the key primary intermediates in the reaction of silylenes with conjugated dienes [18,27,28,[42][43][44].…”

“…The germylene signals do, in fact, generally exhibit a distinct (50-80 ns) growth that lags slightly behind the laser pulse; the transient absorptions in the 270-300 nm range are very difficult to sort out because they consist of contributions from at least three species of varying lifetimes [23]. Subsequent studies of the reactions of GeMePh with isoprene and DMB indicate that the forward rate constants are similar for the two dienes, but the equilibrium constant for addition to DMB is significantly lower than that for addition to isoprene [34]. This suggests that the rate constants for dissociation of DMB-derived vinylgermiranes are somewhat greater than those of isoprene-derived derivatives, all else being equal.…”

Kinetic and mechanistic studies of the reactions of diarylgermylenes and tetraaryldigermenes with carbon tetrachloride

“…Kinetic studies of the reactions of CCl 4 with Ge 2 Me 4 , 11,43 Ge 2 Et 4 , 43 (GeMePh) 2 , 47 and Ge 2 Ph 4 20 in hydrocarbon solvents reveal a ca. 20-fold span in the rate constants, which decrease in the order k Ge 2 Et 4 % 2k Ge 2 Me 4 % 6k GeMePh 2 % 20k Ge 2 Ph 4 .…”

Carbenes and Nitrenes

“…107 The germylene was detected directly and by analysis of reaction products. Rate constants for the reactions with amines, acids, silanes, stannanes, oxygen, and unsaturated carbon-carbon bonds were given.…”