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Onio-assistierte S N 2-Reaktionen: Allgemeiner Zugang zu symmetrischen und unsymmetrischen geminal bisoniosubstituierten Methanderivaten / Onio-Assisted S N 2-Reactions: General Access to Symmetrical and Unsymmetrical Geminally Bisonio Substituted Methane Derivatives

1998; De Gruyter; Volume: 53; Issue: 5-6 Linguagem: Inglês

10.1515/znb-1998-5-618

1865-7117

Robert Weiß, Matthias Handke, Silvia Reichel, Frank Hampel,

Synthetic Organic Chemistry Methods

Abstract The arsonium salt [Ph 3 As-CH 2 -OTf] + OTf - 11a contains a 1,1-biselectrophilic Csp 3 center which permits to synthesize a wide range of symmetrical and unsymmetrical geminally bisonio-substituted methane derivatives. With neutral nucleophiles INu under mild condi- tions a series of 1.1-bisonium salts [Ph 3 As-CH 2 -Nu] 2+ 2OTf - 12-23 is obtained in good yields. Under more stringent conditions the triphenylarsonio function in these salts can also be mobilized as a nucleofuge in a subsequent S N -reaction with a second nucleophile | Nu’, yielding a series of novel unsymmetrical 1,1-bisonium salts [Nu-CH 2 -Nu’] 2+ 2OTf - 24-27. Structures 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 material can be rationalized on the basis of a simple MO model. The conclusion is that stabilization of a hypervalent S N 2 transition state by an equatorial oniosubstituent parallels the stability of the underlying ylids. This is corroborated by model calculations. The geminal bisonium salts reported are potential precursors to mono- and 1,1-bisylids and, in special cases, to functional nucleophilic carbenes.