Hydroboration kinetics: Unusual kinetics for the reaction of 9-borabicyclo[3.3.1]nonane with representative alkenes

Abstract

The kinetics of hydroboration of alkenes with 9-borabicyclo[3.3.lnonane dimer, (9-BBN)2, exhibit interesting characteristics. With more reactive alkenes, such as 1-hexene, 2-methyl-1-pentene, 3,3-dimethyl-1-butene, and cyclopentene, the reaction exhibits first-order kinetics, first order in (9-BBN)2 and zero order in alkene. On the other hand, with less reactive alkenes, such as cyclohexene, 1-methylcyclohexene, and 2,3-dimethyl-2-butene, the reaction exhibits three-halves-order kinetics, first order in alkene and one-half order in (9-BBN)2. These kinetics can be accounted for in terms of the following mechanism:(9-BBN)2 -2 9-BBN 9-BBN + alkene --B-alkyl-9-BBN In the case of the more reactive alkenes, the rate-determining step is the dissociation of the dimer. With less reactive alkenes, the reaction of the alkene with the monomer becomes the rate-determining step. For certain alkenes, such as 2-methyl-2-butene and cis-3-hexene, neither of the two steps is a decisive rate-determining step. Therefore, the reaction exhibits kinetic behavior between that of first-and three-halves-order kinetics.The hydroboration reaction is one of the cleanest, most versatile reactions known in organic chemistry (2-5). It makes organoboranes readily available for synthetic purposes [1].

Formation of Breslow Intermediates from N‐Heterocyclic Carbenes and Aldehydes Involves Autocatalysis by the Breslow Intermediate, and a Hemiacetal

“…The observation of a reaction order of 0.4 for the BI autocatalyst indicates that some of it is tied up in an inactive form that has to dissociate first to liberate the free BI, similar to the case observed in some hydroborations involving borane dimers [35] . In fact, our NMR monitoring of the stoichiometric reaction of SIPr ( 1 a ) with anisaldehyde (2 b ) revealed that the Breslow intermediate BI 1a,2b forms an H‐bonded aggregate BI 1a,2b ⋅ 1 a (see Scheme 5, bottom left) with SIPr ( 1 a ).…”

Kinetics and Mechanism of the Arase-Hoshi R2BH-Catalyzed Alkyne Hydroboration: Alkenylboronate Generation via B–H/C–B Metathesis

“…In situ 19 F-NMR spectroscopic analysis of the reaction of alkyne 2 with 1pin catalyzed by (9-BBN)2, revealed a similar but not identical set of intermediates to those found for Cy2BH. In situ 11 B NMR spectroscopic analysis also confirmed that the free 9-BBN is predominantly dimeric (K0 <1.4´10 -4 M; k0 ~1 ´10 -3 s -1 ) 40 under the reaction conditions. Independent reaction of alkyne 2 with 9-BBN generated E-49BBN and the double hydroboration 28 product 5(9BBN)2, which was also detected (0.8%) during catalytic turnover.…”

Philicity of Acetonyl and Benzoyl Radicals: A Comparative Experimental and Computational Study

“…[5a] The reactions were monitored by 1 HNMR spectroscopyu ntil high conversionsw ere achieved. The relative rates of reaction for each substituted styrene were then calculated from the ratio of logarithmic fractional conversions, [23] also knowna st he Ingold-Shaw expression [24] (see Section3.2 in the Supporting Information). Linear free-energy relationships were constructed by plotting the decadic logarithms of the relative rates against various parameters of the styrene substituents.…”