Pentane, gauche

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. 

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... 

This is a 2,3-P-3,4-M gauche pentane conformation, which is equivalent to 1,3-diaxial substituents on a cyclohexane. Note that - because the carbonyl substituent is a rcrt-butyl - it cannot be avoided by rotation around the terf-butyl-carbonyl bond. For further elaboration of this effect, see Figure 5.5 and the accompanying discussion. For an explanation of the P,M terminology, see the glossary. Section 1.6. 

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.

Figure 7.1. Postulated transition structures for the asymmetric reduction of unsaturated ketones by BINAL-H [12]. Structures (a) and (b) differ in the orientation of Rjat and Run, the saturated and unsaturated ketone ligands, respectively, (a) UI topicity P reagent attacking Re face of ketone, (b) Lk topicity P reagent attacking Si face of ketone, (c) Alternate chair that is destabilized by the gauche pentane conformation accented by the bold lines (c/. Figure 5.5). Transition structures containing this conformation were considered by Noyori to be unimportant [12].

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... 

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