ONIO-ASSISTED S(N)2-REACTIONS - GENERAL A CCESS TO SYMMETRICAL AND UNSYMMETRICAL GEMINALLY BISONIO SUBSTITUTED METHANE DERIVATIVES

The arsonium salt [Ph3As-CH2-OTf](+) OTf- 11a contains a 1,1-biselectr ophilic C-sp(3) center which permits to synthesize a wide range of sym metrical and unsymmetrical geminally bisonio-substituted methane deriv atives. With neutral nucleophiles INu under mild conditions a series o f 1,1-bisonium salts [Ph3As-CH2-Nu](2+) 2OTf(-) 12-23 is obtained in g ood yields. Under more stringent conditions the triphenylarsonio funct ion in these salts can also be mobilized as a nucleofuge in a subseque nt SN-reaction with a second nucleophile /Nu', yielding a series of no vel unsymmetrical 1,1-bisonium salts [Nu-CH2-Nu'](2+) 2OTf(-) 24-27. S tructures 18, 23 and 26b were confirmed by X-ray analysis. For certain nucleophiles a discrete stepwise substitution of both nucleofuges in 11a cannot be realized and the corresponding symmetrical 1,1-bisonium salts (e.g. 28 and 30-32) are obtained directly. The experimental mate rial can be rationalized on the basis of a simple MO model. The conclu sion is that stabilization of a hypervalent S(N)2 transition state by an equatorial oniosubstituent parallels the stability of the underlyin g ylids. This is corroborated by model calculations. The geminal bison ium salts reported are potential precursors to mono- and 1,1-bisylids and, in special cases, to functional nucleophilic carbenes.