Gauche 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. 

Figure 5.5. The illustrated conformation of cw-2-ethyl-1-isopropylcyclohexane is the only one that has no destabilizing gauche pentane interactions [98] similar interactions restrict the conformational motion of a boron ligand available from menthone [97].

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].

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]. 

Scheme 5.39. Allyl sulfoxide additions , fa 1,4-mechanism [198]. (b) Tandem 1,2-addition / 3,3-rearrangement mechanism [148] (see also ref. [199]). (c,d) Transition structures for allyl phosphine oxides [196,197]. Inset Gauche pentane interaction between lithium and the N/je methyl.

The generation of the cis-enolate 176 was accomplished through the utilization of Sn(OTf)2 (Scheme 4.18) [90]. Importantly, substrate 170 and Sn(II) form a chelated adduct 175 in which the ethyl group orients itself in a manner that avoids a double gauche pentane interaction while at the same time... 

Stereoelectronic effects and nonbonded interactions are non-cooperative in the reactions of (E)-allylboronates and x-heteroatom-substituted aldehydes. Thus, while transition state 8 experiences the fewest nonbonded interactions (gauche pentane type, to the extent that X has a lower steric requirement than R3), transition state 9 is expected to benefit from favorable stereoelectronic activation (Felkin-type)58f. This perhaps explains why the reaction of 2,3-[iso-propylidenebis(oxy)]propanal and ( >2-butenylboronate proceeds with a modest preference (55%) by way ol transition state 9. This result is probably a special case, how ever, since C-3 of 2.3-[isopropylidenebis(oxy)]propanal is not very stcrically demanding in 9 owing to the acetonide unit that ties back the oxygen substituent, thereby minimizing interactions with the... 

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 ... 

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

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 one locked skew-pentane interaction equal to six gauche interactions. 8Includes two skew-pentane Interactions. 

A subtle, but important conformational effect is the g+g- pentane interaction, which gives rise to what is commonly called si/n-pentane strain. Recall that there are two possible gauche forms around an internal bond in a linear alkane, and the + / -designation of Figure 2.7 is used to differentiate them. For ri-pentane the all-anti conformer is the most stable. However, in some situations gauche interactions will develop. A common example is in a reaction transition state, when reactants are held in a cyclic array. 

Muller et al. focused on polybead molecules in the united atom approximation as a test system these are chains formed by spherical methylene beads connected by rigid bonds of length 1.53 A. The angle between successive bonds of a chain is also fixed at 112°. The torsion angles around the chain backbone are restricted to three rotational isomeric states, the trans (t) and gauche states (g+ and g ). The three-fold torsional potential energy function introduced [142] in a study of butane was used to calculate the RIS correlation matrix. Second order interactions , reflected in the so-called pentane effect, which almost excludes the consecutive combination of g+g- states (and vice-versa) are taken into account. In analogy to the polyethylene molecule, a standard RIS-model [143] was used to account for the pentane effect. 

The columns are indexed by the three states of bond i, the rows are indexed by the three states of bond i—1, and the order of indexing is t, g+, g. The statistical weight denoted by cr, which is approximately exp (-250/T), is the first-order weight of a gauche state relative to a trans state. The second-order interaction, which arises from the pentane effect where a bond pair is g gT, is weighted by to, which is approximately exp (-1000/T). 

Fig. 7.5 Illustration of how dispersion forces affect gauche (G) conformations. Compared to structures with gauche forms devoid of dispersion forces (i.e., HF-optimized), structures with gauche forms subject to dispersion forces (MP2 optimized) contract in such a way that the 1. ..5 nonbonded interactions in an attractive part of the van der Waals potential are shortened. Thus, in GG-pentane (shown above), MP2-optimized torsional angles are contracted by several degrees compared to the HF-optimized geometry, causing a reduction in the 1...5 nonbonded distances by several tenths of an A. For additional details and the numerical values see R. F. Frey, M. Cao, S. Q. Newton, and L. Schafer, J. Mol. Struct. 285 (1993) 99.

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