Pentane interaction

The minimization of gauche pentane interactions in the transition stales is also an important consideration in the reactions of substituted allylboronates and a-heteroatom-substituted aldehydes4,52d 54,56. Transition states 8 and 11 have been identified by Roush and Hoffmann as the least sterically hindered ones accessible in reactions with (E)- and (Z)-allylboronates. 

Fig. 2.34 The two conformations free of destabilizing syn-pentane interaction [215, 216] in 2,4-disubstituted y-amino acid derivatives with like and unlike configuration. According to the nomenclature proposed by Balaram...

Out of the two conformations of the like-y -am no acid backbone (Fig. 2.34) that do not suffer from syu-pentane interaction, conformation II is almost identical to that found into the 2.6-hehcal structure reported for y" -peptides. This suggest that avoidance of unfavorable sy -pentane interactions in y-amino acids substituted at both the 2 and 4 positions can be used as an additional element of de-... 

Optimal pre-organization of the y-peptide backbone towards the formation of open-chain turn-like motifs is promoted by unlike-y " -amino acid residues. This design principle can be rationalized by examination of the two conformers free of syn-pentane interaction (f and II", Fig. 2.34). Tetrapeptide 150 built from homo-chiral unlike-y -amino acid building blocks 128e has been shown by NMR experiments in pyridine to adopt a reverse turn-like structure stabilized by a 14-mem-bered H-bond pseudocycle [202] (Fig. 2.37 A). 

The observed selectivities are consistent with a model introduced by Danishefsky [53], which invokes a synclinal approach of the crotylsilane with the crotyl methyl group pointed away from the ring (Fig. 5). In the case of compound 148, such an approach results in a 5jyn-pentane interaction between the crotyl methyl group and the equatorial methyl group, leading to the turnover in selectivity that was observed. 

As can be seen, all the compounds have a strong solubility interaction. Nonetheless Figure 6 shows that they are also well separated, so the more subtle difference between the other four interaction terms are important for their discrimination, ft is worth noting that pentane interacts only via solubility, benzene has a very small hydrogen bonding term, while hydrogen bonding is present in methanol and acetic acid. 

The presence of a stereogenic center on the aldehyde can strongly inlinence the diastereoselectivity in allylboration reactions, especially if this center is in the a-position. Predictive rules for nucleophilic addition on snch a-snbstitnted carbonyl substrates such as the Felkin model are not always snitable for closed transition structures.For a-substituted aldehydes devoid of a polar substituent, Roush has established that the minimization of ganche-ganche ( syn-pentane ) interactions can overrule the influence of stereoelectronic effects. This model is valid for any 3-monosubstituted allylic boron reagent. For example, althongh crotylboronate (E)-7 adds to aldehyde 39 to afford as the major prodnct the diastereomer predicted by the Felkin model (Scheme 2), " it is proposed that the dominant factor is rather the minimization of syn-pentane interactions between the Y-snbstitnents of the allyl unit and the a-carbon of the aldehyde. With this... 

The stereoinductive effect of the a-substituent is often more powerful when this substituent is a polar group such as an alkoxy or amino gronp. In this instance as well, minimization of syn-pentane interactions with 3-monosnbstituted reagents is important, and the selectivity can be amplified if it acts in concert with other effects. Known models inclnde the stereoelectronic preference... 

In the absence of chelation, comparison of the destabilizing syw-pentane interactions recently encouraged Evans and coworkers to use the Comforth model to justify the exalted 3,4-anti selectivity observed for a series of chiral a-oxygenated aldehydes reacting with the Z(O) boron and lithium enolates of 2-methyl-3-pentanone (Scheme 117)568. Comple-mentarily, the corresponding E(O) isomers showed, as expected, a striking difference in their 2,3-selectivities, while the 3,4 anti-selectivity was lowered in both cases a finding inconsistent with the PFA model. 

The shapes of these isotherms are controlled by surface heterogeneity, and thus provide a means of deducing the various modes of interfacial attraction presented by the substrates. Isotherms of pentane on unsized IM6 before and after plasma treatment are congruent. Pentane interacts only by dispersion force attraction, as does methylene iodide the registry of these isotherms is predicted by the similarity in ft obtained from wetting measurements. Pentane isotherms serve as a reference with which to assess the effect of additional modes of solid/vapor interaction with acidic and basic probes. 

The estimated value of four gauche for the skew-pentane interaction was confirmed experimentally by Allinger and Miller (iO), who obtained 3.70 kcal/mole for the heat of isomerization of cm-1,1,3,5-tetramethyl-cyclohexane to the trans isomer ... 

The skew-pentane interaction is sometimes called the l,3-diaa ial-dimethyl interaction (31). 

Includes one skew-pentane interaction, figured as equivalent to four gauche-butane interactions. 

Includes two skew-pentane interactions of 4 kcal each. 

Not included in the average. The energy of this skew-pentane interaction is roughly 4 gauche - - 2, or about 6 kcal/mole. 

Includes 2 skew-pentane interactions. The skew-pentane interaction is equivalent to four gauche Interactions. (See text. Includes 1 skew-pentane Interaction. 

Includes four skew-pentane Interactions. Includes two skew-pentane Interactions. 

Table XXV). The sole exception, as far as the more stable conformation is concerned, is due to the skew-pentane interaction that occurred in cis-syw-l-5yw-6-dimethyldecalin but not in the equivalent equatorial-methyl conformation of cis-si/w-l-an

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