Minimal steric interaction

For acyclic fragments and molecules, the principle of longest pathways has been implemented in CORINA (sec Figure 2-95) i.e., since no ch configuration is specified, all torsions arc set to anti in order to minimize steric interactions. 

Because of the same preference for coplanarity in the enamine system, a alkyl substituents adopt an axial conformation to minimize steric interaction with the amino groi. ... 

Aimulene offers a particularly significant test of the Hiickel rule. The internal cavity in [18]annulene is large enough to minimize steric interactions between the internal hydrogens in a geometry that is free of angle strain. Most MO calculations find the delocalized structure to be more stable than the polyene. ... 

The Cope rearrangement usually proceeds through a chairlike transition state. The stereochemical features of the reaction can usually be predicted and analyzed on the basis of a chair transition state that minimizes steric interactions between the substituents. Thus, compound 26 reacts primarily ttuough transition state 27a to give 28 as the major product. Minor product 29 is formed flirough the less sterically favorable transition state 27b. 

On page 132, atropisomerism was possible when ortho substituents on biphenyl derivatives and certain other aromatic compounds prevented rotation about the bond. The presence of ortho-substituents can also influence the conformation of certain groups. In 88, R= alkyl, the carbonyl unit is planar with the trans C=0 - F conformer more stable when X=F. When X=CF3, the cis and trans are planar and the trans predominates. When R = alkyl there is one orthogonal conformation but there are two interconverting nonplanar conformations when R=0-alkyl. In 1,2-diacylbenzenes, the carbonyl units tend to adopt a twisted conformation to minimize steric interactions. " ... 

There are also steric effects. In some cases the direction of elimination is determined by the need to minimize steric interactions in the transition state or to relieve steric interactions in the ground state. 

The introduction of an alkyl substituent at the a-carbon in the enolate enhances stereoselectivity somewhat. This is attributed to a steric effect in the enolate minimize steric interaction with the solvated oxygen, the alkyl group is d t °... 

Ipc)2BH adopts a conformation that minimizes steric interactions. This conformation can be represented schematically as in H and I, where the S, M, and L substituents are, respectively, the 3-H, 4-CH2, and 2-CHCH3 groups of the carbocyclic structure. The steric environment at boron in this conformation is such that Z-alkenes encounter less steric encumbrance in TS I than in H. 

Alkyl groups can also migrate from one position to another on the ring.36 Such migrations are also thermodynamically controlled and proceed in the direction of minimizing steric interactions between substituents. 

Figure 33 The two coordination planes lie parallel to each other but the overall binuclear dication adopts a stepped conformation in order to minimize steric interactions.

The rationalization for the outcome of these cyclization reactions was based on minimizing steric interactions in the transition state leading to benzothiazine formation. For example, at least two possible transition states 101 and 102 could give rise to product 92 (or a diastereomer). Transition state 102 has gauche steric interactions that appear to be absent in the staggered transition state 101. This favors the latter and leads to the observed products 92 (Figure 8). 

Substituent effect on the stability of the transition state for deprotonation (D, Figure 6). The addition of two ortho-methyl groups to Me-[8+] to give Me-[10+] results in an increase in kp for proton transfer to solvent from 1.4 x 106 s 1 to 8.3 x 107 s 1 (Table 1). There should be relatively little steric hindrance to the reaction of solvent with the /1-hydrogens of Me-[10+] because these are relatively distant from the ortho-methyl groups. However, the twisting about the CAr-Ca bond that minimizes steric interactions between the methyl... 

MSI principle The principle of minimal steric interaction which states that the preferred conformation of a group is that which results in the smallest possible steric effect. 

A nonsymmetric substituent prefers that conformation which minimizes steric interactions. 

This model is rationalized by a combination of steric and stereoelectronic effects. From a purely steric standpoint, an approach from the direction of the smallest substituent, involving minimal steric interaction with the groups L and M, is favorable. The stereoelectronic effect involves the interaction between the approaching hydride ion and the LUMO of the carbonyl group. This orbital, which accepts the electrons of the mcommg... 

This reversal of diastereoselectivity i.e., high selectivity in favor of the. mt-alkylation product 19b has been explained by the authors by an allylic strain effect65. If R3 is bulky, then in A and B the preferred entry is syn to R1. In these conformations the C—H bond in the exn allylic position is coplanar with the enolate n-system and R2 is antiperiplanar to R1 to minimize steric interaction. 

Similar double deprotonation and alkylation of the -substituted diastereomeric homologs of complex 15, i.e., 22 and 23, occurs with a high degree of stereocontrol and each homolog follows the reaction path that minimizes steric interactions of the /1-substituent with the metal ligands27,70. 

A conformational assignment to benzoylaziridines from IR spectra concluded (74SA(A)1471)that the c/sderivative(63, R = Ar) should have an s-cis conformation in order to be able to minimize steric interactions, while the s-trans conformation better applies to the trans derivative 64 in order to... 

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