Stereochemistry steric hindrance

In this case the relationship between stability and stereochemistry is easily explained on the basis of van der Waals strain The methyl groups on the same side of the ring m cis 1 2 dimethylcyclopropane crowd each other and increase the potential energy of this stereoisomer Steric hindrance between methyl groups is absent m trans 1 2 dimethylcyclopropane... 

This contrary stereochemistry in the Bucherer - Bergs reaction of camphor has been attributed to steric hindrance of e.w-attack of the cyanide ion on the intermediate imine. Normally, equatorial approach of the cyanide ion is preferred, giving the axial (t>Mr/o)-amino nitrile by kinetic control. This isomer is trapped under Bucherer-Bergs conditions via urea and hydan-toin formation. In the Strecker reaction, thermodynamic control of the amino nitrile formation leads to an excess of the more stable compound with an equatorial (e.w)-amino and an axial (endo)-cyano (or carboxylic) function13-17. 

Results are not easily rationalized. On one hand, the trend of the product s stereochemistry is comparable with that observed over Cu/Si02, but the reaction rate seems to depend on the steric hindrance of the donor. Moreover, as already mentioned, MgO play a significant role in this reaction. Such observations make it impossible to draw any conclusion about the mechanism. 

The fact that Schrock s proposed metallocyclobutanes decomposed to propylene derivatives rather than cyclopropanes was fortunate in that further information resulted regarding the stereochemistry of the olefin reaction with the carbene carbon, as now the /3-carbon from the metal-locycle precursor retained its identity. The reaction course was consistent with nucleophilic attack of the carbene carbon on the complexed olefin, despite potential steric hindrance from the bulky carbene. Decomposition via pathways f-h in Eq. (26) was clearly confirmed in studies utilizing deuterated olefins (67). 

Steric hindrance may hamper the correct stereochemistry required for 13-elimination, and perhaps this can be used to stabilise our metal alkyl complex. In the modem polymerisation catalysts for polypropene this feature has actually been observed, which leads to higher molecular weight polymers. This now forms part of the design of new catalysts. 

Many studies demonstrated that in the approach of diimide to the double bond, steric hindrance is the decisive factor in determining stereochemistry. Highly selective additions of hydrogen (deuterium) from the sterically less hindered side189,190 are illustrated in Eqs. (11.48) and (11.49) ... 

Scheme 77 illustrates some examples of the enantioselective reactions induced by chiral Ni catalysts (187). Atropisomeric binaphthyl or ter-naphthyl compounds can be obtained in high ee with these catalysts. The stereochemistry of the reactions may be determined kinetically by the diastereomeric diorganonickel(II) intermediates. These intermediates have a chiral propeller structure that undergoes little epimerization because of steric hindrance. 

One of the most interesting developments in the stereochemistry of organic compounds in recent years has been the demonstration that trans-cyclooctene (but not the cis isomer) can be resolved into stable chiral isomers (enantiomers, Section 5-IB). In general, a Wa/w-cycloalkene would not be expected to be resolvable because of the possibility for formation of achiral conformations with a plane of symmetry. Any conformation with all of the carbons in a plane is such an achiral conformation (Figure 12-20a). However, when the chain connecting the ends of the double bond is short, as in trans-cyclooctene, steric hindrance and steric strain prevent easy formation of planar conformations, and both mirror-image forms (Figure 12-20b) are stable and thus resolvable. 

The mechanism of epoxidation has been studied in detail both with P450 enzymes [68] and synthetic metal porphyrins [69], The problem finding a conclusive answer on how the enzymatic reaction proceeds is due to the fact that intermediates have not been detected but inferred by investigating the stereochemistry of product formation. By and large it is safe to say that the reaction depends on the steric hindrance imposed by the olefin s substitutents, the electron donating character of the olefin, and the electron demand of the oxo-iron(IV) porphyrin used. In particular the last aspect makes it difficult to draw conclusions from reactions with model compounds, since these metal porphyrins behave quite differently from native P450 due to the distinct electronic nature of both the metal and the porphyrin. 

The use of silylallyl anion in organic synthesis has been extensive24,29. The regio-and stereochemistry of these reactions can be controlled. For example, alkylation of the anion generated from the corresponding allylsilane with an electrophile E+ takes place selectively at the /-position due to steric hindrance (equation 197)352. 

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