Chair transition structure

Table 8 summarizes the results of the calculations of the C2h symmetric chair and the C2v symmetric boat transition structures [83]. The local spin density approximation predicts a tight transition structure, comparable to the one obtained by the MP2(fc)/6-31G method, but fails to reproduce the experimental activation energies [84], The use of the gradient-corrected BLYP functional yields loose, aromatic-type transition structures and improved activation energies for the chair transition structures. The activation energy for the boat transition structure is, however, too low by 9-10 kcal/mol as compared to the experimental value of 44.7 kcal/mol [85]. The activation energies for the chair and the boat transition structures obtained by the Becke 3LYP method are in... 

The cis-selectivity of these reactions can be rationalised by invoking a pseudo-chair transition structure. Transition structure A leading to the trans product suffers from a 1,3-diaxial interaction between the alkyl substituent and an axial hydrogen atom on the ring. This unfavourable interaction is... 

The boron-aldol reaction of the p-methoxyben-zyl(PMB)-protected methylketone 16 proceeds with excellent 1,5-anti-selectivity (Scheme 4). In cases where the asymmetric induction is lower it may be improved by a double stereodifferential aldol reaction with chiral boron ligands [7]. The reason for this high stereoselectivity is currently unknown. Ab initio calculations suggest the involvement of twisted boat structures rather than chair transition structures [6]. 

The intramolecular variant of the chrDmium(II) ion mediated Barbier-(jiignard-type addition reaction was first described by Still and Mobilio in an elegant synthesis of ( )-Aspeidiol (122 R = H). Chro-mium(II)-mediated cyclization of (121 R = Bn) provided a 4 1 mixture of (122) and (123) in 64% yield (equation 50). The relative topicity of the process is Ik. This result is in acco with a syncliiud chair transition structure (124) in which both hydrocarbon chains diverge from the reacting centers pseudo-... 

Only a few exceptions to this general pattern of diastereoselection have been observed. °- Some syn diastereomeric a-substituted 3-bromoacetoxy aldehydes and ketones provide diastereomeric mixtures of products or the opposite diastereomeric product than is anticipated on the basis of the transition structure proposed in Scheme 4 (equation 60). Steric factors which preclude access to chair transition structures may be responsible for the change in the sense of diastereoselectivity in these examples. 

Kuroda et al. have found that the diastereoselectivity of the Lewis acid-promoted cyclization of 132 is highly dependent on the geometry of the allylsilane moiety (Scheme 10.154) [429]. The stereochemical outcomes can be rationalized by chair transition structures involving a secondary orbital interaction wifhout severe steric repulsion. 

Organoyttrium catalysts have been utilized to effect the cyclization of dienes under reductive conditions [39]. Excellent selectivity is achieved in these reactions between two monosubstituted alkenes leading to a single regioisomeric product (Eq. 40), and the diastereoselectivity is consistent with the simple chair transition structure model (Fig. 7). Both acetals and thioacetals are tolerated (Eq.41), whereas nitriles, esters, and sulfones preclude product formation (Eq.42). 

If both the Z(0)- and the (0)-enolates can be made, and if both follow the Zimmerman-Traxler models (i.e., chair transition structures), then both syn and anti adducts should be available (Scheme 5.11, path a vs. b or c V5. d). Since both enantiomers of the auxiliary are available, any desired combination of relative and absolute configurations in the products would be available. 

Stabilized by a 2,3-P,3,4-M gauche pentane interaction (c/ Figure 5.5), as indicated in Figure 5.7c. Roush suggests that an anti Felkin-Anh (anti-Cram) chair transition structure more adequately explains the facts, as shown in Figure 5.7d [123]. 

Suprafacial-suprafacial chair transition structure for the Cope rearrangement. 

Products of the Cope rearrangement of racemic (top) and meso (bottom) diastereomers of 3,4-di-methyl-1,5-hexadiene through chair transition structures. 

The results of a computational study of the transition structures of the Claisen rearrangement were published by Vance et al. in 1988 [30]. The authors provided an analysis of the geometrical and electronic properties of the boat and chair transition structures for the Claisen rearrangement of the parent (unsubstituted) aUyl vinyl ether. The results were based on calculations on the RHF level of theory with correlation-corrected energies obtained by MP2/6-31G frequency analysis. It was found that larger basis sets in combination with polarization functions and... 

Because of the dispute over these questions, a large number of different methods have been used for the investigation of the Cope rearrangement the results from some of the most important aie summarized in Table 9. For the C2 -symmetric chair transition structure, most methods predict an aromatic character with C1-C6 bond lengths ranging from 1.93 to 2.19 A. The aromatic character of the transition structures was also confirmed by calculations of the magnetic properties of the transition structure. In contrast, the perturbation... 

Prins cyclizations, which proceed by intramolecular addition of alkenes to oxocarbenium ions, provide a simple, efficient method for the stereoselective synthesis of carbocycles and cyclic ethers [77]. Halosilanes and (la) have been used for Prins cyclizations not only as Lewis acids but also as heteroatom nucleophiles. For instance, in the presence of MesSil or MesSiBr, and lutidine, mixed acetals (26) are efficiently cyclized to 4-halotetrahydropyrans (27) with high diastereoselectivity [78]. The halide is introduced into the axial site of the C(4) position. The proposed mechanism for the MesSiBr-promoted reaction involves the initial formation of a-bromoethers (28) from (26). Solvolysis of (28) provides the intimate ion pair (29). Cyclization to the chair transition structure (30) and proximal addition of the bromide produces the observed axial adduct (27). The role of lutidine is to suppress a less selective HBr-promoted cyclization (Scheme 9.23). Acetals bearing an alkyne or allene moiety also undergo the halosilane-promoted cyclization to form haloalkenes [79, 80]. 

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