Disulfonimide, chiral

Chiral disulfonimides of the general structure 236 have been used as highly efficient catalysts for reactions such as an asymmetric Mannich reaction of silyl ketene acetals with A/-Boc-amino sulfones (13JA15334) as well as an asymmetric three-component synthesis of homoaUylic amines (13AGE2573). Analogs of pyrrolo[l,2,5]benzothiadiazepine 237a and the... 

A similar approach, even though initiated by a Brpnsted acid, is the organocatalytic approach put forward by List et al. [50]. They used their chiral disulfonimide 36 to induce VMARs stereoselectively. With aromatic aldehydes as the substrates, selectivities up to 98 2 er can be obtained. Additionally, they were able to perform bis-VMARs with triene 40. As mentioned earlier, even though a chiral Brpnsted acid is used, the transition state also includes a hypervalent silicon species (42), which contains the disulfonimide anion and the coordinated aldehyde in a pentavalent, trigonal bipyiamidal geometry (Scheme 2.130). 

Besides chiral phosphoric acids, also chiral imidodiphos-phoric acids 161 [72], disulfonimides 162 [73], or trifly-limides 163 [74] have emerged as powerful (Brpnsted or Lewis) acid organocatalysts for challenging asymmetric transformations like spiroacetalizations, Mukaiyama aldol reactions, or Nazarov cyclizations (Scheme 6.27). 

In 2009, List introduced a binaphthyl-derived, chiral disulfonimide (28) as a new structural motif of a powerful chiral Brensted acid that could activate simple aldehydes [80, 81]. Evaluation of the catalytic activity and stereocontrolling abihty of 28 in the Mukaiyama aldol reaction of sUyl ketene acetal with naphthaldehyde revealed that 28 was not only far more reactive than phosphoric acid 29 and phos-phoramide 30 but also capable of affording the aldol product with high enantiose-lectivity (Scheme 7.53). 

Scheme 7.53 Chiral disulfonimide as catalyst for the Mukaiyama aldol reaction.

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