Penultimate Group Steric Effects

Acrylamide 1 was synthesized [4] and telomerized with four different alkyl iodides and allyltributylstannane (Fig. 3). GC and HPLC were used to determine selectiv-ities in the = 1 (2 and 3) and n = 2 products (4-7). For the n = 1 products, both diastereomers separated in all cases. However, for the n = 2 products, only the neopentyl-derived telomers (4a-7a) would separate by GC for all four diastereomers in all of the other telomer mixtures, two of the four diastereomers co-eluted or incompletely separated. The results are summarized in Table 2. 

Analysis of the distribution of 4a-7a shows a definite preference for erythro arrangements in addition to the stereocontrol provided by the auxiliary compare n = 1 selectivity (Table 3) with selectivity in n = 2 telomers (Fig. 4). The temperature dependence of selectivity was studied and A(A//) and A(A5) were obtained. As expected, selectivity was greater at lower temperatures. Eyring plots for the three selectivities (addition of the second monomer to the n = 1 radical, allyl transfer 

A series of telomerizations were performed in which the auxiliaries were varied in an attempt to determine which factors influence the penultimate effect. Bulky auxiliary groups analogous to those shown in Fig. 3 were used, and mixed auxiliary telomerizations were performed in which methyl acrylate was co-telomerized with the acrylimides. In cases where auxiliaries were sterically bulky and especially when 

These results allow the formulation of a working hypothesis based on the precedents in methyl methacrylate polymerizations. Four reasonable conformations of the radical 8, leading to the major = 2 telomers, are presented in Fig. 6. The important features of this hypothesis are  

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Several methods have been developed for the incorporation of thioamide bonds at defined positions into linear peptides (for details see Vol. E 22c, Section 10.9.1) 498,499 The resulting linear precursors can be cyclized by standard procedures, although the thioamide bond should not be located in penultimate position, since activation of the carboxy group leads to thia-zolone formation as outlined in Scheme 31.1500-502 The cyclization yield of thioamide-containing peptides is generally rather low. This effect results from the additional local steric restrictions exerted by the thioamide bond, which leads to decreased conformational flexibility 488 ... 

Interestingly, Natta et al. (303) showed that an isopropyl group on the para position of styrene decreased reactivity because of steric rather than polar effects. Similarly, Overberger and Ang (314) found that para f-butylstyrene was difficult to polymerize. This suggests that the steric hindrance to complexing arises from interaction with the penultimate unit in the growing polymer chain (315). 

The incoming monomer unit would then be forced, either because of steric interactions, or by the interaction of its carboxyl group with lithium at the chain-end, to add in a specific manner to re-form the same loose ring structure present initially. One variant of this mechanism [192] involves a covalently bonded six membered ring formed by enolization of the active chain end followed by alkoxide ion attack on the penultimate carboxyl group. In polar solvents, or in the presence of moderate amounts of them, competition for solvation of the counter-ion would be produced and the intramolecular solvation producing the stereospecificity would be reduced in effectiveness as the ether concentration is increased. Replacing the lithium counter-ion with sodium or other alkali metal would be... 

On the other hand, for a reason why the intermolecular ht addition may be equilibrated, the electrostatic repulsion between the highly polar anhydride units may be considered similar to the well known case in the radical copolymerization of vinyl monomers carrying carbonyl(34) or nitrile group(35) in which the penultimate effect is involved. That is, the polymer(D) obtained via intramolecular hh and intermolecular tt additions in which five-membered anhydride units are separated by two methylene units is sterically and/or electrostatically favorable compared with the polymer(E) formed via intramolecular ht and intermolecular ht additions in which six-membered anhydride units are separated by only one methylene unit. 

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