Hydroboration Kinetics of Alkynes

The reaction rate is governed by two electronic modes [8, 9], inductive (I) and conjugative (K), and by steric effect. In most cases increasing steric effect decreases the reaction rate (Chart 4.4). 

However, in P-substituted olefins, the steric effect is slightly overridden by the inductive and hyperconjugation of the methyl group to place more electron density [10] at C-1 than at C-2 (Chart 4.5). 

The effect of an alkyl group, a, on the site of hydroboration is different than if it is p. P Substituent (type A) increases the reaction rate by factor of-1.87, whereas alkyl substituent a (type B) lowers the rate of reaction by an average of -170. The calculation, e.g., for three alkyl substituents, two a and one p, to the site of hydroboration are done as = =1.56x10 (rate reduction). The estimate is in good agreement with the experimentally observed effect of ],1.64xl0 (rate reduction). 

The hydroboration of alkynes with (9-BBN)2 exhibits kinetics similar to those for the hydroboration of alkenes. The kinetics are studied (1) by pumping the reaction mixture through a sodium chloride IR cell and monitoring the disappearance of B-H bridges of (9-BBN)2 at 1,570 cm by quantitative IR spectrometry [Ij and (2) by quenching the aliquots of the reaction mixture periodically with excess methanol and analyzing by GLC for residual alkyne. For the more 

Factor by which Type Hyperconjugative effects of Inductive effects 

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