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

Abstract

Fast kinetic studies of the reactions of isoprene and 2,3-dimethyl-1,3-butadiene (DMB) with diphenylgermylene (GePh2, 2a) and of isoprene with a series of diarylgermylenes bearing polar ring substituents (GeAr2, 2b−g) have been carried out in hexanes solution. Though the major stable products of the reactions with isoprene are the corresponding 1,1-diarylgermacyclopent-3-ene derivatives, the results indicate that the major initial products are the corresponding transient 1,1-diaryl-2-vinylgermiranes (6a−g) resulting from formal (1+2)-cycloaddition to the less-substituted CC bond of the diene. These compounds are formed reversibly and with rate constants in excess of 109 M−1 s−1, and appear as discrete reaction intermediates exhibiting λmax = 285 nm and lifetimes of 2−670 μs depending on the identity of the germylene and the diene. The variations in the lifetimes with aryl substituents are shown to be most consistent with a stepwise mechanism for vinylgermirane → germacyclopent-3-ene isomerization, involving (reversible) dissociation to the free germylene and diene followed by (irreversible) (1+4)-cycloaddition. The bimolecular rate constants for (1+4)-cycloaddition to isoprene, calculated from the data on the basis of this model, vary over the range of 5 × 106 to 3 × 108 M−1 s−1 depending on the aryl ring substituent(s). The variation in the rate and equilibrium constants for reaction of 2a−g with isoprene indicates that the germylene plays the role of an electrophile in both the (1+2)- and (1+4)-cycloaddition processes and demands the involvement of a polarized steady-state intermediate in the (1+2)-addition reaction. The temperature dependence of the experimental rate and equilibrium constants for reaction of GePh2 with isoprene, and of the rate coefficient for decay of the corresponding vinylgermirane, allows most aspects of the potential energy surface to be defined quantitatively.

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

“…Arrhenius parameters have been determined, and the role of electronic factors in the transition states for the rate-determining steps of these reactions have been assessed through an examination of the effect of polar ring substituents on the absolute rate constants. As in earlier kinetic studies, 27,30,48 we employed the substituted 1,1-diaryl-3,4-dimethylgermacyclopent-3-enes 6a-6f as photochemical precursors to the corresponding diarylgermylenes (8a-8f) and tetraaryldigermenes (9a-9f; eq. [4]), each pair of which can be studied in the same experiment.…”

The Addition of Nitriles to a Molecular Digermene: Reversible Addition and Comparison to Surface Reactivity

“…The chemistry observed here is comparable to the reversible addition of ethylene to a distannyne 1. The reversible insertion of an isonitrile to a disilene9a and the reversible addition of a germylene to an alkene9c are notable related examples. The reversible complexation of unsaturated molecules, such as alkenes, is a key feature in the reactivity of transition metals which has led to the postulation that unsaturated main group compounds, noted primarily for their challenging syntheses and novel bonding, now have the potential of being developed into catalysts which offer several key advantages (low cost and toxicity) over their transition metal counterparts 2.…”

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

“…Within each family, transition states differ in the direction Table 5. Total a (E 0 /au) and relative energies (ΔE 0 /kJ mol -1 ), and the Gibbs free energies b (ΔG/kJ mol -1 ) of the conformers of hexa 1,3Z(E),5 trienes C 6 of attack of the GeCl 2 and orientaton of the free vinyl group. In accordance with this, their notations include the carbon atom being the major target and the orientation of the vinyl group.…”

“…3 and Table 7) located on the PES of the system GeCl 2 + C 6 H 8 correspond to concerted [6+1] cycloaddi tion of GeCl 2 to hexa 1,3Z,5 triene in its cis,cis 10 con formation, which ensures the spatial proximity of the ter minal carbon atoms C(1) and С (6). As for the [4+1] cy cloaddition reaction in the system GeCl 2 + C 4 H 6 (see above), the cyclization can proceed upon supra and ant arafacial approach of the reactants.…”