Supermesityl group

The remarkably stable silaallene 133 described recently by West213,214 showed related behavior. Photolysis of 133 is reported to result in the C—H bond addition, across the Si=C bond, of a methyl group on the aromatic ring attached to the carbon end of the silaallene, resulting in the polycyclic compound 134. Alternatively, treatment of the silaallene with acid yields compound 135 by the nominal addition that occurs when one of the C—H bonds of an ortho r-Bu group of the supermesityl group attached to silicon adds to the ends of the Si=C bond with the opposite regiochemistry. 

As shown in Fig. 1 there are two chances. First, the stability can be reached thermodynamically by lowering the energy of the PC double bond via mesomeric 7t-electron delocalization. Second, the stability can be kinetically obtained by introducing voluminous and bulky bridgehead atoms to shield the reactive center. Substituents for the latter process are 2-butyl, mesityl, or especially tri-2-butylphenyl, the so-called supermesityl group. 

Many isolable R2Si=SiR2 compounds are now known, most containing even larger R groups, such as 2-adamantyl, Me3C and Tip, which is a sort of supermesityl, 1-XIV. 

Compounds la/Ib add to the sparse number of air-stable primary phosphines previously reported (Fig. 2). Within this elass, supermesityl phosphine triptycylphosphine 3 and various related eompounds " have had their stability attributed to steric faetors. However the resistance of the ferrocene 4a to air oxidation is as yet unaeeounted for the ethyl group is crucial in rendering the molecule air-stable, as the related compound 4b is somewhat sensitive to oxidation.Similarly the diprimary phosphines and 6 also display surprising air-stabilities that have... 

Bickelhaupt published an improved route for the synthesis of phosphasUenes 836 in a one-pot reaction via the dilithiated phosphine 835 obtained from the primary phosphine 834 (equation 289). The drawback is that the substituent at phosphorous may not be smaller than the supermesityl (Sup) group. ... 

Under the same experimental conditions (5)-6 also demonstrates the same level of resistance to air-oxidation as (R)-5, supporting the earlier hypothesis that the methoxy group plays no role in stabilizing the latter phosphine. A small number of achiral, air-stable primary phosphines have been reported (Fig. 1.4) but these are stabilized on account of high steric hindrance such as supermesityl phosphine 7 [4, 5], triptycylphosphine 8 [6] and various related compounds [1], or their stability is not understood. Examples of the latter class are the air-stable ferrocene derivative 9 [7], and the heteroatom-containing diprimary phosphines... 

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