Conformational analysis acyclic systems

Careful conformational analysis of acyclic systems is needed. Hornoallylie Systems... 

Surprisingly, the MP2/6-31G model is not as satisfactory as any of the density functional models, both insofar as mean absolute error and in terms of individual errors. Use of the 6-311+G basis set in place of 6-3IG leads to marked improvement, and the results are now of comparable quality to those of the best density functional models. Given the large difference in cost between density functional and MP2 models, and given the apparent need for basis sets larger than 6-3IG for the latter, it seems difficult to recommend use of MP2 models for the purpose of conformational analysis involving acyclic systems. 

Because of the number of conformations that need to be considered for acyclic systems, cyclohexanones are somewhat simpler for analysis. However, even for these systems the situation is not easily amenable to isolating specific components of selectivity. Several explanations have been proposed over the years to account for the preference of axial attack of cyclohexanones by sterically unhindered nucleophiles (L1A1H4, NaBH4, AIH3) [9]. Equatorial attack is favoured for sterically hindered cyclohexanones or reducing agents (Fig. 6-7). 

We also covered the fact that molecular structures are dynamic, not static. Multiple degrees of vibrational freedom exist—namely, stretches, bends, torsions, etc. Each is quantized and the motions are constrained by a potential surface. For most organic chemistry purposes, only the quantization of bond stretches becomes relevant. The torsional degrees of freedom, when possessing enough energy, lead to the interconversion of conformers, both in acyclic and cyclic systems. The study of these interconversions is called conformational analysis. 

The Kishi synthesis ot monensin teatures allylic conformational analysis to predict stereochemistry of hydroboratlon-oxidations in acyclic systems. The Still synthesis features acyclic diastereoselection in carbonyl addition reactions (chelation control and Felkin-Ahn control). 

A particular advantage of the low-mode search is that it can be applied to botli cyclic ajic acyclic molecules without any need for special ring closure treatments. As the low-mod> search proceeds a series of conformations is generated which themselves can act as starting points for normal mode analysis and deformation. In a sense, the approach is a system ati( one, bounded by the number of low-frequency modes that are selected. An extension of th( technique involves searching random mixtures of the low-frequency eigenvectors using Monte Carlo procedure. 

Epoxidation of acyclic allyl alcohols with peracid and Mo/TBHP displays an opposite stereospecificity to that for the V/TBHP system. Trimethylsilyl-substituted allylic alcohols give t/zreo-epoxyalcohols with MCPBA and erythro-alcohols with VO(acac)a-TBHP, with high stereoselectivity. In the stereospecific epoxidation of cis- and trans-allyl alcohols, formation of a transition state is assumed with the development of two H bonds between the hydrogen atom of the hydroxy group of the allyl alcohol and the oxygen of the peracid, and between the hydrogen of the peracid OH and the oxygen of the ether 10. An analysis of the diastereometric transition-state interactions for stereoselective epoxidation of acyclic allylic alcohols has been published. A conformational effect may be responsible for the unexpected cis major product in Eq. 2. 

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