Trichlorogermane radicals

Formation of the anti-Markovnikov adduct points to the intermediacy of GeCl3 radicals. Formation of the radicals is explained by one electron transfer in different ionic pairs and separation of the formed radical pairs without recombination. This one-electron transfer at the stage of trichlorogermane ionization is expected because of the low potential of GeCl3 oxidation ( 1/2 = 0.48 V) and was confirmed by examination of CIDNP in the reactions (see equation 19 below). 

Reaction of trichlorogermane with 1-heptene (equation 15) to form 12 was studied most carefully57. The authors believe that 1-heptene serves as an effective mediator in electron transfers and thus assists in the radical mechanism. 

The stereochemistry of the radical addition was studied in the reaction of trichlorogermane with 1-methylcyclohexene, which gives both 14 and 15 (equation 17). 

CIDNP effects were found in the reactions of trichlorogermane with styrene, 2-methyl-2-butene and 2,3-dimethyl-2-butene. Analysis of the CIDNP effects was carried out by the Kaptein rules61 and testifies for the radical participation in these reactions59. 

It should be noted that addition of trichlorogermane to MA in inert atmosphere leads to high yield of 9-trichlorogermyl-9-methyl-9,10-dihydroanthracene82. Moreover, hidden radical steps of the reaction were revealed by studying CIDNP effects82 (see Section VIII.F). 

Some evidence of the reaction mechanism was obtained from the UV spectra. Dissolving MA in dry trichlorogermane at low temperature leads to an intensively blue [MA]+" cation-radical. Generation of the intensively colored cation-radical MA+ was ascribed to a secondary reaction of the weakly colored a-complex [MAH]+ with [MA]94,95 (equations 58 and 59). 

The absorption maximum at 640 nm in the UV spectrum of the dark-blue solution of MA in trichlorogermane at —70 to — 90 °C belongs to the cation-radical [MA]+ and disappears on raising the temperature when the reaction is over. 

Nevertheless, a CIDNP effect was observed in the reaction of MA with trichlorogermane when it was carried out in a resonance cell of an NMR spectrometer. The intensive positive integral CIDNP effect of the aromatic protons surely testifies in favor of hidden radical stages, some of which were observed experimentally97. 

The spin-spin coupling constants of 7(H He) = 3.8 Hz and 7(H Ha) = 10.3 Hz strongly testify in favor of an axial position of tf. Having in mind the above mentioned constants, the third coupling constant 7(H tf) = 3.8 Hz was obtained from the ordinary NMR spectrum. This value serves as evidence for the equatorial position of H, i.e. the structure corresponds to stereoisomer 14a. Thus, the radical addition of trichlorogermane to 1-methylcyclohexene proceeds solely as an awfi-addition with a diequatorial disposition of the entering groups. 

The well-known literature ESR spectrum of the cation-radical was not observed for the blue solutions of MA in trichlorogermane, probably due to the existence of [MA]+" and [MAH]+" in the poor electrolyte HGeCl3 as a singlet pair. Attempts to observe absorption bands of the CH2 group of the a-complex by means of a diffused hght version of IR spectroscopy had faUed. ... 

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