Acrolein Lewis acid complexes

Enantioselective D-A reactions of acrolein are also catalyzed by 3-(2-hydroxyphenyl) derivatives of BINOL in the presence of an aromatic boronic acid. The optimum boronic acid is 3,5-di-(trifluoromethyl)benzeneboronic acid, with which more than 95% e.e. can be achieved. The TS is believed to involve Lewis acid complexation of the boronic acid at the carbonyl oxygen and hydrogen bonding with the hydroxy substituent. In this TS tt-tt interactions between the dienophile and the hydroxybiphenyl substituent can also help to align the dienophile.114... 

The effects of Lewis acids on the stereoselectivities can also be understood in terms of orbital interactions. The variation in charge at the respective basic centre gives rise to a change in the magnitude of the orbital coefficients of the entire interacting molecular orbital. These effects are visualized by the HOMO and LUMO representations of the Lewis acid-base complex of acrolein and trifluoroborane (Figure 3), and in an even more extreme case by the HOMO and LUMO representations of one of the simplest dienophile-Lewis acid complexes protonated acrolein92,93. 

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). 

Treatment of boracycle 13 [28] with [(MeCN)3Cr(CO)3] in THF gave air- and moisture-sensitive [( / -borabenzene-THF)Cr(CO)3] (14) that reacts with 3-(di-raethylamino)acrolein to afford [(t7 -borabenzene-(3-(dimethylamino)acrolein) Cr(CO)3] (15). The crystal structure of 15 displays the following features, each of which is typical for aldehyde-Lewis acid complexes (1) the Lewis acidic atom lies in the plane of the carbonyl group (B-04-C7-C8=-176 ) (2) the Lewis acid binds syn to the hydrogen of the aldehyde rather than anti, (3) the Lewis acid-oxygen-carbon angle is roughly 120° (B-04-C7 = 123°) (Scheme 1-4). 

In 1979, Koga and coworkers disclosed the first practical example of a catalytic enantioselective Diels-Alder reaction [44] promoted by a Lewis acidic complex, presumed to be menthoxyaluminum dichloride (1), derived from menthol and ethylaluminum di chloride, whose structure remains undefined [45]. This complex catalyzed the cycloaddition of cyclopentadiene with acrolein, methyl acrylate, and methacrolein with enantioselectivities as high as 72% ee. Oxidation of 2 (predominantly exo) followed by recrystallization actually lowered the ee ... 

The authors contend that the Lewis acid complex is heHcal, but characterization of the catalyst is Hmited to cryoscopic molecular weight measurements of a related complex in benzene. Two attributes of this system deserve attention 1) the tetraalkoxytitanium species still possesses sufficient Lewis acidity to catalyze the reactions of interest at low temperatures 2) the catalyst exhibits a fairly flat enantioselectivity-temperature profile (88% ee at 0 °C for the acrolein-cyclopen-tadiene reaction). The ligand was synthesized in five steps from (i )-(-l-)-3,3 -di-bromobinaphthol dimethyl ether, and while other groups may be used in lieu of the tri-o-tolylsilyl group, the highest levels of enantioselectivity were realized with Hgand 57. 

The mechanism of the carbo-Diels-Alder reaction has been a subject of controversy with respect to synchronicity or asynchronicity. With acrolein as the dieno-phile complexed to a Lewis acid, one would not expect a synchronous reaction. The C1-C6 and C4—C5 bond lengths in the NC-transition-state structure for the BF3-catalyzed reaction of acrolein with butadiene are calculated to be 2.96 A and 1.932 A, respectively [6]. The asynchronicity of the BF3-catalyzed carbo-Diels-Alder reaction is also apparent from the pyramidalization of the reacting centers C4 and C5 of NC (the short C-C bond) is pyramidalized by 11°, while Cl and C6 (the long C-C bond) are nearly planar. The lowest energy transition-state structure (NC) has the most pronounced asynchronicity, while the highest energy transition-state structure (XT) is more synchronous. 

Hersh et al. found that the cationic complex [CpFe(CO)2(THF)]BF4 (23) can accelerate the [4 + 2] cycloaddition of acrolein and cyclopentadiene [32]. However, the catalytic activity was higher than expected from rate constants determined in stoichiometric experiments, indicating that a Brpnsted or Lewis acid impurity might accelerate this process and generating doubts about the role of 23. 

Brown proposed a mechanism where the enolate radical resulting from the radical addition reacts with the trialkylborane to give a boron enolate and a new alkyl radical that can propagate the chain (Scheme 24) [61]. The formation of the intermediate boron enolate was confirmed by H NMR spectroscopy [66,67]. The role of water present in the system is to hydrolyze the boron enolate and to prevent its degradation by undesired free-radical processes. This hydrolysis step is essential when alkynones [68] and acrylonitrile [58] are used as radical traps since the resulting allenes or keteneimines respectively, react readily with radical species. Maillard and Walton have shown by nB NMR, ll NMR und IR spectroscopy, that tri-ethylborane does complex methyl vinyl ketone, acrolein and 3-methylbut-3-en-2-one. They proposed that the reaction of triethylborane with these traps involves complexation of the trap by the Lewis acidic borane prior to conjugate addition [69]. 

However, in the presence of Lewis acids such as AlBr3, without the presence of the CpFe(CO)2+ Lewis acid the [3 + 2[-cycloaddition process was more effective, giving the product in 45% yield. The application of a chiral cationic CpFe(diphosphine) complex as the catalyst (Scheme 9.37) for the asymmetric [3 + 2]-cycloaddition of nitrones to acrolein derivatives was described by Kiindig and coworkers [95],... 

To the extent that the enolate resulting from conjugate addition at the (3-carbon can be stabilized, the rate of this reaction pathway is enhanced. For example, (3-Michael additions are observed for MVK, acrolein, and acetylenic electrophiles even without the presence of a Lewis acid. Furthermore, MVK reacts with the 2,5-dimethylpyrrole complex (22) to form a considerable amount of (3-alkylation product, whereas only cycloaddition is observed for methyl acrylate. The use of a Lewis acid or protic solvent further enhances the reactivity at the (3-position relative to cycloaddition. While methyl acrylate forms a cycloadduct with the 2,5-dimethylpyrrole complex (22) in the absence of external Lewis acids, the addition of TBSOTf to the reaction mixture results in exclusive conjugate addition (Tables 3 and 4). 

The moderate Lewis acidity of ruthenium complexes was used to promote catalytic Diels-Alder reaction of dienes and acrolein derivatives [21-23]. The enantioselective Diels-Alder reaction of methacrolein with dienes was catalyzed with cationic ruthenium complexes containing an arene or cyclo-pentadienyl (Cp) ligand and a chiral ligand such as phosphinooxazoline, pyridyl-oxazoline, monoxidized 2,2 -bis(diphenylphosphino)-1, T-binaphthyl (BINPO)or l,2-bis[bis(pentafluorophenyl)phosphanyloxy]-l,2-diphenylethane (BIPHOP-F). The reaction gave the cycloadduct in high yields with excellent... 

The complexes are isolated, characterized and used as chiral Lewis acids. Dissociation of the labile ligand liberates a single coordination site at the metal center. These Lewis acids catalyze enantioselective Diels-Alder reactions. For instance, reaction of methacrolein with cyclopentadiene in the presence of the cationic iron complex (L = acrolein) occurs with exo selectivity and an enantiomeric excess of the same order of magnitude as those obtained with the successful boron and copper catalysts (eq 3). ... 

Two other Lewis acids were also included in these studies. Lithium cation coordination with acrolein gave a linear, s-trans complex as the most stable structure (Table 4). ° An increase of the s-cis-s-trans gap by 1.4 kcal mol is now difficult to rationalize on steric grounds since the C=0-Li bond is nearly linear. The researchers have suggested that this effect is due to an increase in the electron density on the carbonyl oxygen upon complexation, which in turn would increase nonbonded electron pair-hydrogen repulsion of the alkene termini in the s-cis conformer . A similar effect is observed with (Z)-acrylic acid, where the linear s-trans structure is once again more stable than the s-cis complex by 1.5 kcal mol". ... 

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). 

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