Onio-assistierte SN2-Reaktionen: Allgemeiner Zugang zu symmetrischen und unsymmetrischen geminal

G em in a l B ison ium Salts, SN-R eac tio n s, E le c tro s ta tic Effects, Synthesis, X -R ay D a ta T he a rson ium sa lt [Ph3A s C H 2O T f ] + O T f11a con ta ins a 1 ,1-biselectrophilic C sp3 c en ­ te r w hich p e rm its to syn thesize a w ide ran g e o f sym m etrical an d unsym m etrica l gem inally b iso n io -su b stitu ted m eth an e derivatives. W ith n e u tra l n u c leoph iles INu u n d e r m ild co n d i­ tio n s a series o f 1,1-bisonium sa lts [Ph3A s C H 2N u ] 2+ 2 0 T f~ 12-23 is o b ta in ed in good yields. U n d e r m o re strin g en t co n d itio n s th e trip h en y la rso n io fu n ction in these sa lts can also be m obilized as a nucleofuge in a su b seq u e n t SN-reac tio n w ith a second n u c leoph ile IN u ’, y ield ing a series o f novel u n sy m m etrica l 1 ,1 -b ison ium salts [ N u C H 2N u ’]2+ 2 0 T f _ 24-27. S tru c tu re s 18, 23 an d 26 b w ere c o n firm ed by X -ray analysis. F o r certa in n u c leoph iles a d is­ c re te stepw ise su b s titu tio n o f b o th n u cleo fuges in 1 1 a can n o t be realized and the c o rre sp o n d ­ ing sym m etrica l 1 ,1-bisonium sa lts (e.g. 28 an d 30-32) are o b ta in e d directly. T he e x p e rim e n ­ tal m ate ria l can be ra tio n a lized o n th e basis o f a sim ple M O m odel. T he conclusion is th a t s tab iliza tio n o f a h y p e rv a len t SN2 tran s itio n s ta te by an eq u a to ria l o n io su b stitu en t paralle ls th e stab ility o f th e underly ing ylids. This is c o rro b o ra te d by m o d el calculations. T he gem inal b ison ium salts re p o rte d are p o te n tia l p re cu rso rs to m onoand 1,1 -bisylids and , in special cases, to fu nc tional nuc leophilic carbenes.