Lewis basic groups

Other Lewis basic groups can direct the hydrogenation. (Ir seems to be superior to Rh for these cases)... 

Chelation of Lewis-basic groups to ruthenium during catalysis can also strongly influence selectivity [39]. 

The addition of achiral additives lacking chiral conformations to asymmetric catalysts can enhance catalysts enantioselectivity. The exact nature of the interaction between the additive and the chiral catalyst, however, is often not clear. The additives usually contain Lewis basic groups, and can behave as ligands by coordinating to catalysts and changing the metal geometry and/or catalyst aggregation state. 

The reaction proceeds with good chemoselectivity and tolerates the presence of other electrophilic and/or Lewis basic groups (bromides, esters, azides. Table 24.18 Eq. 24.19). 

An additional benefit of using a Lewis acid cocatalyst in the carbocyanation reaction is that it enables the use of compounds with less reactive C(sp )-CN bonds. Aliphatic nitriles that bear no P-hydrogens, such as (substituted) acetonitrile [52], benzyl [53], and allyl cyanides [54], can be used successfully. Moreover, P-hydrogen-containing aliphatic nitriles also effectively furnish the carbo-cyanated product when the substrate has a suitable Lewis basic moiety at the y-position, as in 79 (Scheme 6.17) [55]. Intramolecular coordination of the Lewis basic group effectively suppresses undesired P-hydrogen elimination from intermediate 80. 

As other examples in this review demonstrate, addition of an extra equivalent of a Lewis acid is often sufficient to allow for a [4 + 2] cycloaddition of nitroalkenes in the presence of Lewis-basic groups, such as enol ether, ester, or nitrile. 

The great advantage of the Crabtree catalyst is that it is able to coordinate to Lewis basic groups in the substrate, thus directing the reduction to a specific face of the double bond. The diastereospecificity achieved is generally very high (Table 23.1), and such directed reductions have proved synthetically useful (Figure 23.30). 

Scheme 2 Different modes of stabilization for silylium ions (LB Lewis Basic Group)...

In cases in which the high Lewis acidity of the cationic silicon is decreased by an intramolecular Lewis basic group (LB), as in the tetracoordinated cation 9 (and likewise in the siliconium ion 10 Scheme 2), interactions between the positively charged silicon atom and the solvent and/or anion are of minor importance. The structures, spectroscopic properties, and reactivities of these sUyl cations are greatly determined by the electron-donating ability of the LB groups. As a consequence, the structural and spectroscopic features of silyl cations 21 and 22, which are stabilized by intramolecular electron donation from an aryl substituent, closely resemble those of benzenium ions 11 (Scheme 5) [33, 34]. The intramolecular stabilization operative in silylium ions 9 offers the intriguing possibility of... 

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