Bystander assistance factors

Using literature data, bystander assistance factors, B[Y], could be calculated for a number of substituents. For example, the thermolysis of certain ketone tosylhydrazone salts afforded dialkylcarbenes which gave competitive 1,2-H shifts, Eq. 23, where either Ha or Hb migrated. 

The product ratio a /b (statistically corrected for the number of competing H migrants), gives the relative migration rate of Ha vs. Hb, or k /kub- The rate constant for the migration of Ha corresponds to the intrinsic migratory aptitude of Ha (M[H]) multiplied by the bystander assistance factor for Y, B[Y], The carbon atom that bears Hb has no bystander substituent, so that km, is simply A/[H], We thus obtain Eq. 24. 

Both Af[H]s cancel, so that the bystander assistance factor, B[Y], for any substituent, Y, is simply the corrected product ratio a /b. 8 In Eqs. 23 and 24, bystanders Y were found to enhance H-shifts in the order MeO > alkyl > Ph quantitative data appear in Table 3.8... 

Table 3. Bystander Assistance Factors for Hydrogen Migration in Dialkylcarbenes°...

Using this type of evaluation, Nickon (1993) calculated the bystander assistance factors B[X] given in Table 8-1 for aprotic Bamford-Stevens reactions at 150 10°C (experimental data from various sources). 

Table 8-1. Some bystander assistance factors for hydrogen migration in thermally generated carbenes of type 8.43-8.44 (after Nickon, 1993).

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