Alkenylgermanes

Alkenylgermanes have been synthesized highly stereoselectively by addition of triethylgermyl lithium to cyclobutyl ketones and subsequent Lewis acid-promoted stereospecific ring opening of the resulting cyclobutylethanol derivatives (Scheme 11.43) [67]. 

Ikenaga et reported that trimethyl(styryl)germanes coupled with arenediazouium tetrafluoroborates in the presence of a palladium catalyst (Scheme 36). Although the reaction proceeds smoothly under mild conditions, stereochemistry of the alkenylgermanes is not retained as was observed with alkenylsilanes shown in Scheme 2. The mechanism is ascribed to a carbopalladation route as discussed with alkenylsilanes. 

Acylsilanes, acylgermanes, and acylstannanes are potential precursors of alkenylsilanes, alkenylgermanes, and alkenylstannanes, which are important and powerful synthetic tools [73, 74]. However, practical methodology for their preparation via olefination had not been reported. Olefination of the acylsilanes 87 with ynolates provides the p-silyl-a,p-unsaturated esters 88 in high yields with excellent Z-selectivity (Fig. 31). In most cases, the E isomers cannot be detected by NMR and HPLC at all. This is the first general method for the stereoselective synthesis of tetrasubstituted olefins [75]. 

In the same way, acylgermanes 89 and acylstannanes 90 are olefinated with ynolates to give Z-alkenylgermanes 91 and Z-alkenylstannanes 92 (Fig. 33). Although the E/Z ratios are a little lower than that of acylsilanes 87, satisfactory ratios are obtained [80]. 

Properties and Reactions of Intramolecularly Activated Alkenylsilanes, Alkenylgermanes... 

The intrinsically activated alkenylgermanes 106 are more reactive in the cross coupling reaction (Fig. 39). The reaction is dependent on the palladium catalyst and, as a ligand, Pd(Pt-Bu3)2 works very efficiently in NMP to afford the desired coupling product 107 in better yield without the generation of the protonated byproducts, which generally cause poor yields in this type of coupling reaction. 

This coupling reaction of alkenylgermanes can be applied to the synthesis of estrogen receptor modulator tamoxifen (112), which is used in the clinical treatment of estrogen-dependent breast cancer (Fig. 40). The benzoyl triethylgermane (108) is olefinated with ynolate to give the aUcenylgermane 109, which is coupled... 

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