Crotonaldehyde Lewis acid complexes

Once the acid is complexed to the aldehyde, the aldehyde becomes an excellent electrophile. The electrophilicity depends directly on how low is the LUMO energy of the complex. To establish a scale of Lewis acidity, the LUMO energy of crotonaldehyde Lewis acid complexes was calculated by MNDO (gas phase. Table 4). The calculations allow one to rank order the acids, and BCI3 proved to be the strongest acid, and SnCLj the mildest one ... 

Lewis acid complexes of -substituted a, 3-unsaturated ketones and aldehydes are unreactive toward alkenes. Crotonaldehyde and 3-penten-2-one cannot be induced to undergo ene reactions like acrolein and methyl vinyl ketone. The presence of a substituent on the -carbon stabilizes the enal- or enone-Lewis acid complex and stericdly retards the approach of an alkene to the -carbon. However, Snider et al. have found that a complex of these ketones and aldehydes with 2 equiv. of EtAlCk reacts reversibly with alkenes to give a zwitterion (22). This zwitterion, which is formed in the absence of a nucleophile, reacts reversibly to give a cyclobutane (23) or undergoes two 1,2-hydride or alkyl shifts to generate irreversibly a p, -disubstituted-a,P-unsaturated carbon compound (24). 

Recent work has focused on developing catalytically controlled asymmetric 1,3-dipolar cycloadditions of cyclic nitrones such as 2,3,4,5-tetrahydropyridine IV-oxide 174. The Lewis acid iron complex 181 catalyzes the cycloaddition of 2,3,4,5-tetrahydropyridine jV-oxide 174 with methacrolein to give (3A,5A)-isoxazolidine 182 in good yield and high enantiomeric selectivity (Scheme 48) <2002JA4968>. The same catalyst 181 however gave (3R,4A,5R)-isoxazolidine 183 with much lower selectivity when crotonaldehyde was used. 

Taking advantage of the Lewis acidic character of the cationic ruthenium complex 107, catalytic asymmetric 1,3-dipolar cycloadditions of nitrones have been developed [113]. In the presence of 5 mol % of 107, the cycloaddition of 108 with crotonaldehyde afforded an isoxazolidine 109 in 80% yield with 66% ee (Eq. 47). 

The first polymer containing fluorinated arylborane substituents has recently been reported. The highly functionalized soluble polymer (195) was obtained in good yield by treatment of poly(4-dibromoborylstyrene) with equimolar amounts of pentafluorophenylcopper at low temperature. Polymer (195) shows a slightly lower Lewis acidity than the perfluorinated borane B(C6F5)3 as determined by complex-ation with crotonaldehyde (equation 41). However, in this... 

Uncomplexed acrolein, methacrolein, and crotonaldehyde all favor the s-trans conformer, and this preference is enhanced upon complexation to a Lewis acid. For example, Corey showed that the BFj-methacrolein complex adopts the s-trans conformation in the solid state as well as in solution by crystallographic and NMR spectroscopic methods (Fig. 8B) [27], while Denmark and Almstead found that methacrolein adopts the s-trans geometry upon complexation with SnCl4 (Fig. 8C) [28]. Yamamoto demonstrated that methacrolein is also observed in the s-trans conformation upon complexation to his chiral acyloxybo-rane (CAB) catalyst (Fig. 14A and Sect. 3.1.2) [40]. Interestingly, with the same CAB system, crotonaldehyde exhibited varying preferences for the two possible conformers depending on the exact substituents on the boron. On the basis of NOE enhancements, the s-trans conformer was observed exclusively with a hydrogen substituent on boron (Fig. 14B) the s-cis conformer was the only one detected in the case of the aryl-substituted acyloxyborane (Fig. 14C). 

Table 3 The enthalpy of complexation (AH) of crotonaldehyde with various Lewis Acids in dichioromethane at 295...

Several other acidity scales have been proposed. One of them consists in analyzing the complexation of a Lewis acid to crotonaldehyde by H or NMR spectroscopy. This approach is attractive because it is experimentally simple and takes into account solvent effects. Addition of a Lewis acid to a 0.3M solution of crotonaldehyde in CD2CI2 at -20 °C causes a downfield shift (A5) of the 2-, 3-, and 4-proton resonances (and an erratic shift of the 1-proton). The shift is commensurate with the electron withdrawing effect of the acid, and therefore to the energy of the LUMO (ji ). This gives rise to another scale of Lewis acidity, based on NMR shifts. The NMR-based scale parallels very closely the LUMO-based scale, and the... 

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