Isomerization and decomposition of germiranes: a density functional study

Abstract

The formation and decomposition pathways of germiranes (germacyclopropanes), i.e., products of reactions of the GeH 2 and GeMe 2 germylenes with ethylene, tetramethylethylene, buta 1,2,3 triene, and tetramethylbuta 1,2,3 triene, were studied using the density functional approach (PBE/TZ2P approximation). The thermodynamic stabilities of the structures under consideration were evaluated by calculating the Gibbs free energies under normal condi tions (∆G°2 98 ). Addition of germylenes to the C=C bond can proceed as a single step process without a barrier or involve the formation of a π complex (the barrier to this process is lower than the sum of the energies of isolated reactants). Stability of the germiranes formed is determined by their stability to retrodecomposition into the initial germylene and olefin and to the three membered ring opening followed by simultaneous 1,2 migration of the substituent at the Ge atom and formation of the secondary germylene. Alkyl substituents can efficiently block the opening of the three membered ring and transformation of the cyclic structure into the secondary germylene, simultaneously decreasing the germirane stability to retrodecomposition. Decomposition into germylene and olefin under normal conditions is thermally favorable for hexamethylgermirane (∆G°2 98 = -5.7 kcal mol -1 ), being thermally forbidden for the other germiranes studied in this work (∆G°2 98 > 0). The activation energy (E a ) for the germirane ring opening depends on the substituents at the germanium atom, namely, E a ≤ 10 kcal mol -1 for unsubstituted germiranes and E a > 30 kcal mol -1 for methyl substituted germiranes. Taking the experimentally isolated germirane as an example, it was shown how the introduction of sub stituents and modification of the carbon skeleton make it possible to stabilize the germa cyclopropane system.

Kinetic and Mechanistic Studies of the Formal (1+2)- and (1+4)-Cycloadditions of Germylenes to Conjugated Dienes

“…5.7 kcal mol −1 higher in free energy than the isolated reactants; the complex is more stable than the free reactants by 1.2 kcal mol −1 at a higher level of theory (CCSD(T)/cc-pVTZ//B3LYP/6-31G(d,p)-6-311G(d,p)), but the (free energy) barrier for its conversion to the vinylgermirane disappears. The results of the higher level calculations are in better agreement with experimental kinetic data for the reactions of GeMe 2 with aliphatic dienes in the gas phase and in solution, which are consistent with a very small but significant barrier to reaction; they proceed with rate constants close to, but clearly less than, the diffusional (or collisional) limit at 25 °C. ,− The involvement of reactive π-complexes as intermediates in the addition of GeH 2 and GeMe 2 to ethylene and other alkenes is a common feature of all of the various theoretical studies of these reactions that have been reported − and is consistent with experimental kinetic studies of the reaction of GeH 2 with ethylene in the gas phase. , …”

“…The results of the higher level calculations are in better agreement with experimental kinetic data for the reactions of GeMe 2 with aliphatic dienes in the gas phase and in solution, which are consistent with a very small but significant barrier to reaction; they proceed with rate constants close to, but clearly less than, the diffusional (or collisional) limit at 25 °C. 24,[27][28][29] The involvement of reactive π-complexes as intermediates in the addition of GeH 2 and GeMe 2 to ethylene and other alkenes is a common feature of all of the various theoretical studies of these reactions that have been reported [39][40][41][42][43][44][45] and is consistent with experimental kinetic studies of the reaction of GeH 2 with ethylene in the gas phase. 27,41 Our data also provide information on the factors affecting the lifetime of vinylgermirane 6a, which was monitored in the presence of sufficiently high concentrations of isoprene (50-60 mM) to make it the only species detectable by laser photolysis methods.…”

GeCl2 cycloaddition reactions to unsaturated organic compounds taking ethylene, buta-1,3-diene, and hexa-1,3,5-triene as examples: a quantum chemical study

“…16,17,20 In the system GeH 2 + C 2 H 4 , transfer of a hydrogen atom from GeH 2 occurs with relative ease and is thermodynamically favor able. 17 However, in the reaction of GeCl 2 with ethylene a similar isomerization of 3 to 5 via TS2 (Scheme 1 and Table 1) is not accompanied by a decrease in the energy of the system; rather, it leads to a considerable increase in the energy. The key factor determining the endothermici ty of this reaction is that the dissociation energies of the ethylene π bond and a strong Ge-Cl bond cannot be compensated by the energy of formation of a weaker С-Cl bond and the С-Ge bond.…”

Reactions of germylenes with alkynes: A quantum chemical study. Factors determining the stabilities of germacyclopropenes

“…Data in Table 2 show that accumulation of Me groups causes the exother micity of addition to linear alkynes to decrease, although less noticeably compared to germylene reactions with olefins. 20 It is important that tetramethylgermirene 1d remains thermodynamically stable to retrocycloaddi tion into initial GeMe 2 and MeC≡CMe (Gibbs free en ergy of retrocycloaddition ∆G 0 298 = 10.6 kcal mol -1 ), whereas hexamethylgermirane is unstable (∆G 0 298 = -5.7 kcal mol -1 ). 20 Applying a correction for possible overestimation of the exothermicity of cycloaddition by the PBE method (see above) does not change this conclu sion.…”

“…The addition of GeH 2 to the triple bond of acetylene occurs barrierlessly in one step unlike the reactions of germylenes with olefins which usu ally involve formation of a π complex. [18][19][20] Recently, 20 we reported a PBE 21 density functional study of the reactions of GeH 2 and GeMe 2 with olefins and buta 1,2,3 trienes and showed that the overall stabil ity of the product, germacyclopropane (germirane), is de termined by its stability to retrocycloaddition into initial reactants and to a rearrangement similar to rearrange ment (b) in Scheme 1. Taking the stable germirane 10 as an example, we determined the effects of the olefin structure and the nature of the substituents at the Ge atom on the stability of the germacyclopropane system.…”