Sleric Factors

Aryl radicals are produced in the decomposition of alkylazobenzenes and diazonium salts, and by f)-scission of aroyloxy radicals (Scheme 3.73). Aryl radicals have been reported to react by aromatic subsitution (e.g. of Sh) or abstract hydrogen (e.g. from MMA10) in competition with adding to a monomer double bond. However, these processes typically account for <1% of the total. The degree of specificity for tail vs head addition is also very high. Significant head addition has been observed only where tail addition is retarded by sleric factors e.g. methyl crotonate10 and -substituted methyl vinyl ketones 79, 84). 

Present evidence does not establish whether the direction of cleavage of the cyclopropane ring is determined by slereoeleclronic or sleric factors. 

On page 832 the statement is made that in most fluorides the reduction of sleric factors allows the lone pair to emerge to the surface of the molecule,..and yet there are two hexafluoro species mentioned in this chapter that are perfectly octahedral. What are they How can they be octahedral in light of the above statement ... 

Cydohexenones.2 This enone undergoes photochemical [2 + 2]cycloaddition to alkenes. The products, obtained in high yield, can be converted into cydohexenones in two steps. An example is formulated in equation (1). The regioselectivity of the cycloaddition depends on both sleric and electronic factors.1... 

Bond lengths and multiple bonomg were discussed in Chapter S, and a comparison of various types of atomic radii will be discussed in Chapter 8, but a short discussion of factors that affect the distance between two bonded atoms will be ven here to complement the previous discussion of sleric fectors. 

The relatively low reactivity of 13 compared to normal Grignard reagents is a characteristic and general feature. As indicated above, this is probably caused by both sleric and eleetronic factors, and the reactivity decreases with increasing substitution by silicon. This lack of reactivity sometimes limits the achievement of certain synlhetic goals the only electrophile which always reads instantaneously is water (or DyO) ... 

Sleric (spatial) factors exert the greatest effect on the hydrolytic stability of organoxysilanes [47]. Any complication of the alkoxy group retards the hydrolysis of alkoxysilanes, but the hydrolysis rate is lowered the most by... 

---