Steric model catalysts

For the model catalysts 1 of Scheme 2 the olefin n-complexes with branched alkyls are more stable than with the linear ones [13d]. This electronic preference is strongly affected by the steric bulk on the catalyst due to an interaction b etween the alkyl group and the catalyst substituents [13d]. Thus, for the most bulky real catalysts the n-complexes with linear aUcyl have lower energy. This is true for both ethylene and propylene complexes. A similar effect was observed for both, the diimine and anilinotropone systems [13c,d, 28]. The presence of the steric bulk also affects the olefin complexation energies, and the relative stability of ethylene and propylene complexes. This has been discussed in details in [13d]. 

Sequential Iminium-Enamine Catalysis. Directed Electrostatic Activation. A comparison of the standard catalytic cycles for enamine activation (Scheme 2.1) and for iminium ion activation (Scheme 2.12) show that iminium catalysis proceeds, after the addition of the nucleophile, via an ( )-enamine. In the presence of a suitable electrophile, this enamine gives rise to an iminium ion that after hydrolysis can give rise to an a,p-diftmctionalyzed carbonyl (Scheme 2.13) [85]. Scheme 2.13 also shows that when using a chiral 2-substituted pyrrohdine or an imidazolidinone as the catalyst, the sequential apphcation of the steric model for Michael addition to iminium ions (Figure 2.15) and of the steric model for electrophilic attack to enamines (Figure 2.IB) predicts the absolute stereochemistry of the major isomer obtained in the reaction. 

Since regular helices with the inner layer matching the catalyst particle size have been observed[4,5], we propose a steric hindrance model to explain the possible formation of regular and tightly wound helices. 

A comparison of several of the PY and IM types of catalysts in intramolecular reactions of allylic diazoacetates led to a consistent model for the enantioselectivity. The highest e.e. values are observed for ds-substituted allylic esters. Both R and R1 are directed toward the catalyst and introduce steric interactions that detract from enantioselectivity.208... 

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