Back-side approach

There are two simple ways in which the SN2 reaction of methyl chloride could occur with hydroxide ion. These differ in the direction of approach of the reagents (Figure 8-1). The hydroxide ion could attack chloromethane at the front side of the carbon where the chlorine is attached or, alternatively, the hydroxide ion could approach the carbon on the side opposite from the chlorine in what is called the back-side approach. In either case, the making of the C-O bond is essentially simultaneous with the breaking of the C-Cl bond. The difference is that for the back-side mechanism the carbon and the attached hydrogens become planar in the transition state. 

The stereochemical consequences of front- and back-side displacements are different. With cyclic compounds, the two types of displacement lead to different products. For example, an SN2 reaction between cis-3-methylcyclopentyl chloride and hydroxide ion would give the cis alcohol by front-side approach but the trans alcohol by back-side approach. The actual product is the trans alcohol, from which we know that reaction occurs by backside displacement ... 

If the back-side approach is hindered, the acetylide ion may abstract a proton, giving elimination by the E2 mechanism. 

The highly hindered alkene adamantylideneadamantane forms a bromonium ion that crystallizes as a tribromide salt. This particular bromonium ion does not react further because of extreme steric hindrance to back-side approach by bromide ion. " Other very hindered alkenes allow observation of both the initial complex with Br2 and the bromonium ion. An X-ray crystal structure has confirmed the cyclic nature of the bromonium ion species (Figure 5.2). ... 

Back side approach of Nu- is impeded by the axial hydrogens on the same side of the ring. Each causes steric hindrance similar to that shown in the attack of a nucleophile on a halopropane in an anti conformation (Figure 6-9C). 

If there is steric hindrance to back-side approaches due to the large sizes of the penultimate substituents, front-side attacks take place. This occurs even in nonpolar medium. The incoming monomers can therefore attack the cation either from the front side or from the back side. All depends upon the tightness or the Coulombic interaction of the ion pair and on the difference in the steric hindrance between the two modes of attack. 

SCHEME 20.22 Partly stereoregular high-cis polymers obtained with an achiral Mo alkyhdene complex hearing electron deficient alkoxide ligands (Ar = 2,6-i-Pr2C6H3, P = polymer chain R = CF3, C02Me, or chiral R arrow indicates back side approach of the monomer). When successive monomers approach the same CNO face, isotactic sequences are obtained (Reference 85). 

Highly selective back side approach of the monomer... 

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