Methyl acrylate borane complexes

Both experimental [7] and theoretical [8] investigations have shown that the anti complexes of acrolein and boranes are the most stable and the transition states were located only for these four anti complexes. The most stable transition-state structure was calculated (RHF/3-21G) to be NC, while XT is the least stable of the four located. The activation energy has been calculated to be 21.6 kcal mol for the catalyzed reaction, which is substantially above the experimental value of 10.4 1.9 kcal mol for the AlCl3-catalyzed addition of methyl acrylate to butadiene [4a]. The transition-state structure NC is shown in Fig. 8.5. 

The borane complexes of acrylic acid and methyl acrylate were studied next (Table 5). The syn, s-trans conformers are again preferred. Notably, the steric arguments discussed previously can nicely rationalize these results. Single point calculations on a linearly restricted BH3 complex (a = 3 = 180°) revealed only a 0.1 kcal mol preference for the s-trans structure, thereby supporting the notion that, in contrast to the Li case, the steric gearing effect is dominant in determining the structure here. 

Ab initio calculations for the borane-methyl acrylate complex indicate that complexation of the lone pair anti the OMe group E complex) is favored by... 

The general preference for the s-trans conformer carries over to some extent for carboxylic esters. Ab initio calculations for the borane-methyl acrylate complex show a 1.4 kcal/mol preference for the s-trans conformer (Fig. 10), presumably due to reduced steric interactions (B-H for the s-trans vs. B-CH2 for the s-cis) [33]. Solid state structures, however, show that both conformers can be observed for esters (Figs. 11 and 12). 

The X-ray structure of the indicated borane-methyl acrylate complex (Fig. 19) unequivocally confirms the design concept. The solid state structure shows a close contact (3.40 A from the center of the substituted phenyl ring to the carbonyl carbon) between the electron-rich arene and boron-bound methoxycarbo-nyl group, an arrangement which also exists in solution. As the polarizability of the aryl group is increased, the dienophile is drawn closer to the arene, suggestive of a dipole-induced attractive interaction. The air-sensitive catalyst 13 was synthesized by way of a resolution in 5 steps. 

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