Boat transition structure

The overall barrier to ring inversion is well established experimentally, and is believed to correspond to the energy difference between chair and half-chair structures. Less certain are the relative energies of chair and twist-boat conformers and the energy of the boat transition structure, although a small range of values for each of these quantities has been established experimentally. Comparison of the experimental data with the results of calculations is provided in Table 8-5. The... 

A successive refinement (MNDO-PM3) of the reaction of the Reformatsky derivative of methyl bromoacetate with both formaldehyde and methanimine was carried out including coordinated THF molecules. It identified a twist-boat transition structure as the most stable TS for the transformation of G into H7. 

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... 

To explain the stereochemical outcome of the reaction of allylic boron reagents with carbonyl compounds, Houk and Li carried out calculations on the transition structures of the model reaction of formaldehyde and allylboronic acid6 (Scheme 3.V). The bimolecular complex formed initially between allylboronic acid and formaldehyde would rearrange via a six-membered transition state to form an intermediate. Calculations show that chair transition state A is 8.2kcal/ mol more stable than twist-boat transition structure B, clearly confirming that the six-membered chairlike transition-state model is a legitimate scheme to predict the stereochemical outcome of the boron allylation reaction. 

Figure 5.3. (a) Chair and boat transition structures for ZfO]-enolates. (b) Chair and boat transition structures for fOj-enolates. (c) ul and Ik open transition structures. Note that in all cases, the topicity is such that ul -> syn Ik anti. 

Figure 5.7. Analysis of possible transition structures for the aldol addition in Scheme 5.26 (a) The observed topicity (b) boat transition structure postulated by Masamune [127] (c) gauche pentane interaction that destabilizes the Cram (or Felkin-Anh) selectivity of the aldehyde (d) anti-Cram (anti Felkin-Anh) addition via a chelated chair [123].

Goodman and Paterson studied this reaction using a force field based on MM2 [Bemardi et d. 1990]. The force field was parametrised to reproduce the geometries and relative energies of the chair and twist-boat transition structures with unsubstituted reactants, previously determined using ah initio methods (see Figure 11.33). It was assumed that the stereoselectivity was determined by the relative energies of the various possible transition structures (i.e. the reaction is assumed to be kinetically controlled). 

Suprafacial-suprafacial boat transition structure for the Cope rearrangement. 

Products of Cope rearrangement of racemic (top) and meso (bottom) diastereomers of 3,4-dimethyl-1,5-hexadiene through boat transition structures. 

Both of the pathways that allow out-of-plane relaxation of the a frame are [3s -f 3s], and are therefore allowed by the Rules. So are the other two, which are [3a + 3a], but these can be rejected as sterically unfavorable. Instead of setting up two separate orbital correlation diagrams, between the reactant and the chair and boat transition structures respectively, we make do with a single correspondence diagram in D2/1. For this purpose, the syn and anti conformers of the reactant are anasymmetrized to D2/1, in analogy to the superposition... 

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