Trichlorogermane reactions

Trichlorogermane reactions involving participation of dichlorogermylene are not oxidation-reduction reactions in the usual sense of this term, though they are described by Gen/GeIV transfers. At the same time, the reductive properties of trichlorogermane can be illustrated by the reduction of nitrobenzenes to anilines. 

Trichlorogermane reactions with olefins and acetylenes, including functionally substituted compounds, are one of the basic methods of obtaining organogermanium derivatives. Trichlorogermane adds easily in the absence of any catalyst to olefinic and acetylenic... 

Equation (133)) and subsequently can be converted into a pentacoordinate 1,2-oxagermetanide (Scheme 23).173 Reaction of trichlorogermanes with mercaptoacetic acid or 2-mercaptoethanol also yields pentacoordinate organo-germanium compounds (Equations (134)174 and (135)).175... 

The redistribution and cleavage reaction of mixtures of tetramethylgermane and tetrabutylgermane with excess of aluminum chloride give di- as well as trichlorogermanes having mixed alkyl groups. 

Finally, the reaction of double germylation, such as formation of Cl3GeCH2CH2GeCl3 from ethylene and the etherate of trichlorogermane, is of great importance. 

Experimental data of reactions of trichlorogermane are reviewed elsewhere7,11. The main aim of this chapter is to gather investigations of the mechanistic aspects of reactions of trichlorogermane and to apply these data for providing a rational explanation of the experimental facts. 

The superacidic properties of trichlorogermane are clearly manifested in the properties of its etherates (vide infra) and the ability of DGeCl3 to participate in a deuterium-hydrogen exchange reaction with methylbenzenes32 (Section VIII.E). 

The ability of trichlorogermane as an acid to form stable organic salts having the GeCl3 anion was first observed in the study of the reaction of HGeCl3 with triphenylamine. It was found that at room temperature their interaction lead to the formation of an unusual iminium salt, i.e. trichlorogermanate 3,5-fe(trichlorogermyl)cyclohexylidenediphenyliminium 1 in 92% yield (equation 2)35,36. 

Reaction of trichlorogermane with excess of styrene at — 5 °C leads to the formation of both isomers with a 2.3 1 predominance of the anti-Markovnikov product. This ratio can be modified by varying the reaction conditions. Thus, if the reaction is carried out in concentrated HC1, the yield of 7 rises up to give an opposite regiochemistry with a ratio 6 7 = 1 9. The role of concentrated HC1 consists in the ionization of trichlorogermane and shift of equilibrium GeCl3 GeCl2 + Cl- to the left in the presence of excess of chloride anion. 

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. 

Reaction of trichlorogermane with butadiene (equation 20) and isoprene leads to formation of cyclic and oligomeric compounds, together with the usual products of... 

Reactions of trichlorogermane with -unsaturated carbonyl compounds are widely used for synthesis of biologically active germanium sesquioxides8,68,69, for example, for the synthesis of carboxyethylgermanium sesquioxide 24 via the adduct 23 (equation 25). 

A new type of trichlorogermyl group/hydrogen replacement was found in the reaction of trichlorogermane with -thienylacrylic acid (equation 26). 

The protodegermylation of adduct 25 to form 26 occurs under thermolysis. In the case of 27, hydrogermylation of the thiophene double bonds by action of excess of trichlorogermane occurs. The hydrogenative function of trichlorogermane was also marked in the reaction of alkoxybenzenes as shown below in Section VIII. 

In other cases such selectivity is absent. Two isomeric octyltrichlorogermanes are obtained in reaction of trichlorogermane with w-amylcyclopropane and 1-methyl-2 -butylcyclopropane and three isomeric methyltrichlorogermylcyclohexanes are obtained in the reaction with norcarane73. 

It is known that ketones, which are capable of aldol condensation, react with trichlorogermane in a particular way. First, an aldol condensation occurs and then trichlorogermane adds to the C=C double bond of the product of condensation74. However, in the case of methyl cyclopropyl ketone, the condensation does not occur and a single isomer derived from the ring opening reaction is formed with 65% yield (equation 28)75. 

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). 

An additional aim of this section is to draw attention to the worthwhile reexamination of many reactions of trichlorogermane at conditions where small amounts of oxygen were added carefully to the reaction mixture. Another interesting point is to reexamine the classical superacid reactions under similar conditions. 

The uncatalyzed hydrogermylation reaction of aromatic multiple bonds by trichlorogermane was discovered in 1965 by two of the authors of the present review. The reactions between anisole, naphthalene and thiophene and HGeCl3 were investigated in their works32,85. 

The reactions of trichlorogermane with phenoxy- and alkoxybenzenes, methylbenzenes and condensed aromatics86,87 have been studied under different conditions and additional data concerning their mechanisms have been obtained. 

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). 

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