Anti-Lewis acid interaction

These results led to a separation of the observed Diels-Alder reactivities into three categories (a) increase of the rate constants on increasing the Lewis acid character of the solvent as quantified by the AN parameter this behaviour reflects the interactions between the LUMO of the solvent and the HOMO of the reactants and is similar to Lewis acid catalysis (vide supra) (b) reaction retardation by electron donation, as quantified by the D-ji parameter the HOMOsoivent-LUMOreactant interactions are held responsible for this effect, representing an anti-Lewis acid interaction which increases the HOMO-LUMO gap and hence hampers the reaction (c) the Diels-Alder reactions show very small solvent effects and are relatively insensitive to specific reactant-solvent interactions, and... 

Lewis acids are important catalysts for promoting organic reactions because they coordinate heteroatoms of functional groups. Lewis acids interact with carbonyl oxygen in its plane in either syn or anti fashion 248). Such perturbation of acceptor molecules lowers the LUMO level... 

The reaction of nitrostyrene with cyclopentadiene gives the normal Diels-Alder adduct. However, the Lewis acid-catalyzed cycloaddition affords two isomeric nitronates, syn and anti in an 80-to-20 ratio. The major isomer is derived from an endo transition state. The preference of yy/i-fused cycloadducts can be understood by considering secondary orbital interactions (Eq. 8.95).152... 

Lewis acid sites have empty orbitals able to accept electron density from the occupied orbitals of a Lewis base, in parallel with back-donation from the catalyst to the empty anti-bonding orbitals of the base [33]. This interaction leads to the formation of an activated acid-base adduct. In the case of alkanes activation may proceed by hydride abstraction [38]. Y and Beta are good examples of zeolites with Lewis acidity, often quite significant for catalysis [39, 40]. 

As shown in Fig. 3, Lewis acids (i.e., metal ions and hydrogen bond donors) display syn or anti stereochemistry as they interact with the carboxylate anion. However, in a study of enzyme active sites. Candour (1981) first noticed that hydrogen bond donors to the carboxylates of aspartate and glutamate residues preferentially occur with syn stereochemistry. As a carboxylate-hydrogen bond donor interaction COg-H... 

The active center formed by the interaction of butadiene with bis( r-crotylnickel chloride) or the products of the reaction of (C4H7NiX)2 + Lewis acids does not contain halides, and there is no steric hindrance for syn complex to be isomerized into an anti complex, which is more stable thermodynamically according to Bank (2). 

Figure 7.13 First stage of hydrocyanation. Conversion of butadiene to 3PN. Under the reaction conditions 2M3BN is isomerized to 3PN. Interaction of Lewis acid with coordinated nitrile is not shown for clarity. The left and right side involve CN addition in an anti-Markovnikov and Markovnikov manner. L = P(OEt)3 or P(0-o-tolyl)3.

Finally, the mechanism of syn-anti isomerization remains elusive. Is ir-bonding truly higher in energy than the linear, planar arrangement as predicted by theory The answers to these questions will im-doubtedly help to clarify our emerging view of the interactions of the carbonyl group with Lewis acids. 

An alternate model using the transition states (V)-(XII) can be used to rationalize the selectivities in these reactions (Figure 16). This model requires synclinal orientation of the reacting double bonds. The transition states (V) or (VI) and (K) or (X) are favored over (VII), (VIII), (XI) and (XII) because steric interactions between the Lewis acid bonded to the oxygen lone pair anti to the R substituent of the aldehyde and the vinyl methyl group are minimized. 

The results obtained from the cyclization of model 5 indicated that the size of the Lewis acid-aldehyde complex influences the selectivity of the reaction. For model system 10 it appears that the steric bulk of the Lewis acid does not play a significant role in determining the stereochemical outcome of the reaction. In model system 10 no external methylene unit exists which could interact with the Lewis acid-aldehyde complex. In fact, the silane is fixed in an anti orientation with respect to the approaching aldehyde (anti Se ). The cyclization of model system 10 with fluoride ion affords primarily the distal product resulting from cyclization through an antiperiplanar transition structure. Thus, the antiperiplanar transition structure is accessible, but is not favored in reactions with the Lewis acids. 

Yamamoto found that, in general, the (Z)-stannanes 170 favored formation of the syn adducts 173 under thermal conditions, presumably through transition state 174 (Fig. 11-16). Under thermal conditions, however, the (F)-stannanes 171 generally favor formation of the anti adducts 172, where the anti stereochemistry is rationalized by reaction occurring through transition state 175. Also different from Keck s observations, Yamamoto found that the anri-adducts 172 were formed preferentially (via transition state 176) in the Lewis acid-promoted reactions of both the (Z)-170 and the ( )-171 substrates. Thus, as noted by Yamamoto, the secondary orbital interactions or the dipole effects experienced in the ring-closing transition states with the (y-alkoxyallyl)stannanes 170 and 171 are apparently not as important in the reactions of 158 and 159 [106]. 

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