Structural Effects in Solvolytic Reactions. 49. Steric Effects as a Major Factor in the Exo:Endo Rate Ratios for the Solvolysis of 2,7,7-Trimethyl- and 2,6,6-Trimethyl-2-norbornyl p-Nitrobenzoates

Article abstract of DOI:10.1021/jo00208a004

The exo:endo rate ratio for solvolysis, in 80percent aqueous acetone at 25 deg C, decreases from 885 for 2-methyl-2-norbornyl p-nitrobenzoate to 6.1 for 2,7,7-trimethyl-2-norbornyl p-nitrobenzoate.On the other hand, it increases remarkably to 3 630 000 in the case of 2,6,6-trimethyl-2-norbornyl p-nitrobenzoate.These changes are clearly attributable to steric effects caused by the syn methyl group at the 7- and 6-positions, respectively.In the 2,7,7-trimethyl-2-norbornyl system, the very low exo:endo rate ratio arises primarily from an increased rate of solvolysis of the endo isomer, attributed to relief in steric strain, involving the syn 7-methyl and exo 2-methyl groups, during ionization.The extremely large exo:endo rate ratio in the 2,6,6-trimethyl-2-norbornyl system is attributed to the high rate of solvolysis of the exo isomer, caused by the relief in steric strain involving the endo 2-methyl and endo 6-methyl groups, as well as an especially slow rate for the endo isomer, caused by enhanced steric retardation of ionization.Thus, these results show clearly that exo:endo rate ratios can be strongly affected by steric effects in the rigid norbornyl system.