Isotope effects with boron

Both sets of results may also be discussed in terms of inductive differences between hydrogen and deuterium (see Halevi, 1963). Brown et al. (1966) jDoint out that both the inductive and steric explanations qualitatively predict isotope effects in the same direction, but that an inductive effect would be expected to operate from the 3 and 4 positions nearly as effectively as from the 2 position . Furthermore, there is no observable isotope effect on the heat of reaction of 2,6-(dimethyl-de)-pyridine with the relatively small molecule diborane A AH = —20 18 cal mol ), but a significant effect is obtained with the larger molecule boron trifluoride AAH = 230 + 150 cal mol ). 

Fig. 17. Field cooled magnetization measured for increasing temperature at hqH = 2.5 mT on single crystalline YNi2B2C with the two isotopes l0B (solid lines) and 11B (dotted lines), clearly indicating a boron isotope effect...

Decomposition of H3B,PF3 by irradiation with a CO2 laser (10.6 pm band) is due to vibrational excitation, and not to net rise in temperature. No boron isotope effect was detected. The rate-determining step seems to be the reaction ... 

Even a boron isotope effect has been determined (B vs B ) for the cleavage of some boronic acids by mercuric chloride (Matte-son et al., 1964). The boron-carbon bonds cleaved with isotope effects of 2-3%. A noteworthy point is the unusual method used to determine the isotope ratios. The method took advantage of the widely differing cross sections of B " and B to neutron bombard-... 

The effects of structural modifications on the mechanism of substitution at tetrahedral boron have been probed by a series of kinetic investigations in which the determination of rate laws and activation parameters, of deuterium isotope effects, and of Hammett correlations, have featured. The reactions were of 7V-alkyl and of B-alkyl and J -aryl derivatives of HsBjNHa with tri-n-butyl phosphine or with 2,2 -diphenylethylamine. The results can all be accommodated by the two-term rate law ... 

Dilsopinocampheylborane derivatives continue to be attractive as reagents for enantloselective processes. Studies on the secondary kinetic isotope effect of deuterium on enantioselective hydroborations with (+)(Ipc)2BH have provided significant experimental evidence which provides a test for any detailed explanation of the process. The reagent has been used in a systematic study with representative heterocycles bearing an endocycllc double bond in order to establish the asymmetric induction achieved. It turns out that the reaction provides a simple and efficient method of synthesising heterocyclic boronates... 

The hydrolysis of pyridine-diphenylborane follows second-order kinetics, first-order both in water and amine-borane 175). An observed kinetic isotope effect is small and temperature-independent for substitution of deuterium for protium on boron, whereas, the effect is large and temperature-dependent for a similar isotopic substitution in water 175, 176), A mechanism consistent with the isotope effect involves an initial attack of water on the boron-hydrogen bond... 

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