Acrolein protonated

FIGURE 3. Frontier molecular orbital energies (eV) and representations of the coefficients of acrolein, protonated acrolein and the acrolein-trifluoroborane complex101. Geometry optimizations were performed at the AMI1 °2,103 level of theory orbital energies and electronic distributions were determined at HF/3-21G104-107... 

Some acrolein protons overlap with the polybutadiene bands for example, the protons (-CH2 CH CH-0-) and (-CH9-CH-)... 

For the catalytic reaction, it was assumed that adsorbed acrolein reacts on free dihydrop)T an, as suggested by the thermodynamical results. In fact, the complexation of acrolein with a Lewis acid reduces the LUMO-HOMO gap to 6.9 eV and thus stabilizes the transition state of the reaction. The acrolein protonation lowers again the orbital level of the diene and in this case the gap is lowered to 2.2 eV. In all three cases (uncatalyzed, Lewis and Bronsted acid-catalyzed) the major orbital interaction leading to the transition state occurs between the LUMO of acrolein and the HOMO of dihydropyran. Moreover the maximum overlap principle predicts the same major regioisomer, in agreement with experimental results. 

Figure 1.3. Frontier orbital energies (eV) and confidents for acrolein and protonated acrolein. In the latter case the upper numbers refer to the situation where bond lengths and angles correspond to those of acrolein. The lower numbers are more suitable for a hydroxyallyl cation. The actual situation is assumed to be intermediate. The data are taken from ref. 104.

The chemical reactivity of these two substituted ethylenes is in agreement with the ideas encompassed by both the MO and resonance descriptions. Enamines, as amino-substituted alkenes are called, are vety reactive toward electrophilic species, and it is the p carbon that is the site of attack. For example, enamines are protonated on the carbon. Acrolein is an electrophilic alkene, as predicted, and the nucleophile attacks the P carbon. 

The FMOs of acrolein to the left in Fig. 8.2 are basically slightly perturbed butadiene orbitals, while the FMOs of protonated acrolein resemble those of an allyl cation mixed in with a lone-pair orbital on the oxygen atom (Fig. 8.2, right). Based on the FMOs of protonated acrolein, Houk et al. [2] argued that the predominant interaction in a normal electron-demand carbo-Diels-Alder reaction is between the dienophile LUMO and diene HOMO (Fig. 8.1, left). This interaction is greatly... 

The coordination of the dienophile to a Lewis acid (in the calculations a proton was used as the Lewis acid) leads also to an increase in regioselectivity. The re-gioselectivity of reactions of electron-rich, or conjugated dienes, with electron-deficient dienophiles is also controlled hy the diene HOMO-dienophile LUMO interaction. From Fig. 8.2 it appears that the difference in magnitudes of the LUMO coefficients at carhon atoms 1 and 2 of acrolein (Ci -C2 = 0.20) is smaller than the same difference for protonated acrolein (Ci -C2 = 0.30-0.43) so that the reaction of the latter should he considerable more regioselective than the former in accordance with the experimental results [3]. 

Fig. 8.2 Frontier-orbital coefficients and energies (eV) for acrolein and protonated acrolein [2 ...

Fig. 8.3 Diene HOMO-dienophile LUMO interaction of endo transition state for the reaction of cyclopen-tadiene with acrolein (a) and protonated acrolein (b)...

The chiral catalyst 142 achieves selectivities through a double effect of intramolecular hydrogen binding interaction and attractive tt-tt donor-acceptor interactions in the transition state by a hydroxy aromatic group [88]. The exceptional results of some Diels-Alder reactions of cyclopentadiene with substituted acroleins catalyzed by (R)-142 are reported in Table 4.21. High enantio- and exo selectivity were always obtained. The coordination of a proton to the 2-hydroxyphenyl group with an oxygen of the adjacent B-0 bond in the nonhelical transition state should play an important role both in the exo-endo approach and in the si-re face differentiation of dienophile. 

Regioselectivities [7] and endo selectivity [8, 9] increase upon Lewis acid catalysis of Diels-Alder reactions (Scheme 9). Houk and Strozier [10] found that protonation on the carbonyl oxygen of acrolein amplifies the LUMO at the terminal and... 

A number of examples have been reported documenting the use of palladium phosphine complexes as catalysts. The dialkyl species [PtL2R2] (L2 = dmpe, dppe, (PMe3)2 R = Me, CH2SiMe3) catalyze the reaction of [PhNH3]+ with activated alkenes (acrylonitrile, methyl acrylate, acrolein).176 Unfunctionalized alkenes prove unreactive. The reaction mechanism is believed to proceed via protonation of Pt-R by the ammonium salt (generating PhNH2 in turn) and the subsequent release of alkane to afford a vacant coordination site on the metal. Coordination of alkene then allows access into route A of the mechanism shown in Scheme 34. Protonation is also... 

The benzotriazolyl derivative of acrolein acetal, compound 882, is lithiated, treated with chlorodiphenylphosphine, and the obtained intermediate is oxidized with hydrogen peroxide to phosphine oxide 883 (Scheme 145). The relatively acidic proton in derivative 883 is easily removed by a base, and the obtained anion adds to a carbonyl group of aldehyde or ketone. Subsequent rearrangement and elimination of the phosphorane group generates diene 884. For the derivatives of aldehydes (884, R2 = H), (E)-(E) stereoselectivity of the elimination is observed. Acidic alcoholysis of dienes 884 affords esters of P,y-unsaturated carboxylic acids 885 < 1997JOC4131>. 

In fact, a mechanism for this reaction can be drawn that does not involve Pd at all, but let s assume that Pd is required for it to proceed. Cl- must be nucleophilic. It can add to Cl of the alkyne if the alkyne is activated by coordination to Pd(II). (Compare Hg-catalyzed addition of water to alkynes.) Addition of Cl- to an alkyne-Pd(II) complex gives a o-bound Pd(II) complex. Coordination and insertion of acrolein into the C2-Pd bond gives a new a-bound Pd(II) complex. In the Heck reaction, this complex would undergo P-hydride elimination, but in this case the Pd enolate simply is protonated to give the enol of the saturated aldehyde. 

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. 

In open-chain systems nmr indications of predominant O- (or C-) protonation are the restricted rotation around the Q-N bond and the slowness of NH-exchange. The nmr spectrum of j3-dimethylamino-acrolein in aqueous perchloric acid shows that the non-equivalence of the two N-methyl groups is retained (Kramer and Gompper, 1964). S-Benzylaminoacrolein is also mainly 0-protonated (Kramer, 1966). 

The cationic chiral Lewis acids 10, generated from the corresponding oxazaboroli-dines by protonation by trifluoromethanesulfonic acid, are excellent catalysts for the enantioselective reaction of 2-substituted acroleins, a-unsaturated a,p-enones, a-unsaturated acrylic acid esters, and a-unsaturated acrylic acids with a variety... 

The conjugate addition of nitroalkanes to a,P-unsaturated aldehydes (Sect. 2.2.2) has been investigated by Uggerud, who compared the uncatalysed, proton catalysed and iminium ion catalysed additions [232]. The results suggested that protonated acrolein was more activated towards addition than the iminium ion catalysed process and also indicated that an intermediate oxazolidin structure 183, unobserved experimentally, may be involved in the reaction pathway (Fig. 17) with the transition state resembling that of a [3+2] cycloaddition process. 

CH2 CH2 CH0 which corresponds neither to (1,2) unit nor to (1,4) unit. Moreover, Ha and He protons belong to the aliphatic NMR bands and Involve a % (1,2) overestimate. For block polymers, the living end concentration Is very high so that this phenomenon Is very Intense and perhaps can explain the differences between the acrolein block mlcrostruc-tures observed with the two counter-lone. 

The proton originates from acidic OH-groups of the catalyst surface. The importance of acidic sites is stressed by several authors [11,141,312] of whom Takita et al. [312] have demonstrated a linear relationship between activity and the concentration of acidic sites in the case of a Sn—Mo—O catalyst. Regarding the attachment of oxygen to the carbon formed, Moro-oka et al. proved by the use of H2180 together with 1602 that the introduced oxygen mainly stems from water. (It is noted that acrolein, formed as a by-product, did not contain ]80.) On this basis, the mechanism proposed by Buiten is... 

For attack of F on acrylic acid, first a hydrogen bonded complex is formed which then proceeds to the transition state and then to a stable carbanion. The methyl in the methacrylic acid reduces stabilization of the carbanion as predicted. Subsequent studies using ammonia as the nucleophile indicated that attack proceeded by a rate-determining intramolecular proton transfer from the nucleophile to the ligand, assisted by a discrete water molecule that acts as a catalyst17. They predicted that acrolein underwent 1,4-addition, acrylic acid either 1,2- or 1,4-addition and acrylonitrile 1,2-addition. 

The use of isoxazole derivatives in organic synthesis is of great interest, but little has been done on the utilization of such compounds as a part of a diene system in [4 + 2]-cycloadditions. 3-Methyl-5-vinylisoxazole 236 gave cycloaddition reactions in a sealed tube in benzene solution at 120°C for 3 days. With the dienophiles acrolein and methyl acrylate, aromatiza-tion of the isoxazole ring via a 1,3-proton shift occurs readily under the reaction conditions, allowing the direct isolation of compounds 237, which are also detected in the mass spectrum of the raw reaction material. The reactions are regioselective (85H2019). 

Protonation of glycerol 6.4 catalyses dehydration via secondary carbonium ion 6.5 to give enol 6.6. Acid catalysed elimination of a second water molecule affords acrolein 6.7. Thus glycerol acts essentially as a protected form of acrolein, slowly releasing this unstable a,p unsaturated aldehyde into the reaction medium. Better yields are realised with this approach than if acrolein itself is present from the start. The reaction proceeds with a Michael addition of aniline 6.3 to acrolein, producing saturated aldehyde 6.8 which cyclises via an aromatic substitution reaction to alcohol 6.9. Acid-catalysed dehydration to 6.10 then oxidation yields quinoline 6.1. Nitrobenzene can be used as a mild oxidant, as can iodine and ferric salts. 

The more a carbonyl group is like that of protonated acrolein (Fig. 4.12), the more likely it is that all nucleophiles will attack directly at the carbonyl carbon atom. In agreement with this perception, and in contrast to its behaviour with methyl acrylate, ammonia reacts with acryloyl chloride at the carbonyl carbon atom to give acrylamide. 

All values in eV. Taken from ref. 55. Taken from ref. 56. Taken from ref. 57. Protonated acrolein taken from ref. 58 CINDO/2 values are reported. 

This reaction is the acid-catalyzed counterpart of a 1,4-addition reaction to an a,l3-unsaturated carbonyl compound. Chloride ion, without an acid present, will not add to acrolein. That is, chloride ion is not a strong enough nucleophile to drive the reaction to the right. However, if the carbonyl is protonated, the intermediate cation is a stronger electrophile and will react with chloride ion. 

None of the cations produced by protonation at the carbons of acrolein is as stable as the cation produced by protonation at oxygen. We will consider the possibilities. 

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