Deuterium abstraction

Maybury, C. P., and J. C. Larrabee Kinetics of the Exchange Reaction of Aluminium Borohydride with Deuterium Abstracts of Papers, 135th Meet. Amer. chem. Soc. 1959, 28 M. 

The photochemistry of benzaldehyde (90% 13C=0), 519, deoxybenzoin (99%) 13C=0), 521, and / -chloro benzoin (99% 13C=0), 522, in cyclohexane-Dn solution has been studied633 by spectroscopic techniques, such as XH chemically induced dynamic nuclear634 or electron polarization635 (CIDNP/CIDEP) or dynamic nuclear polarization636 (DNP). In all these cases the formation of benzaldehyde-D with emissive 13C=0 polarization has been observed and the results rationalized by intermolecular hydrogen (deuterium) abstraction by the photoexcited ketones from the solvent molecules and by reactions of cage-escaped radicals (equations 303-308), Benzoin, 520, is formed also. 

It has been documented that the photoexcited deoxybenzoin can abstract H as well as D from appropriate solvents. Hydrogen abstraction by triplet 521 from cyclohexane-Hi2 (rate constant 2.1 x 105 mol-1 s 1) is faster than w-cleavage whereas deuterium abstraction (3.9 x 104 mol-1 s 1) is competitive with a-cleavage (deuterium isotope effect of 5). 

Stereochemical studies have brought an important contribution to the radical mechanism of the boro-hydride reduction of organomercurials. A recent H NMR study of the reduction of norbomyl-type mercurials with NaBD4 confirmed the previous finding of Gray and Jackson about the composition of the product mixture. The major components were clearly nortricyclanol and anti-7-norbomeol. H NMR data provided accurate measures of the preferred directions of deuterium abstraction by the radical intermediate. ... 

A tentative structure for 44, incorporating one chemical defect and possessing exchange coupling of a symmetric spin trimer, was proposed.105 It was speculated that the annelation of calix[4]arene-based macrocycles leads to substantial strain, especially in the center macrocycle.105 Such strain could have two consequences (1) increased basicity of the arylmethyl carbanion sites, which would facilitate deuterium abstraction from the deuterated solvent, and (2) increased out-of-plane twisting of the 7r-system, which would weaken the exchange coupling.105... 

Hydrogen abstractions by deoxybenzoin from the solvent, undeuterated and perdeuterated cyclohexane, were studied by using stationary l3C CIDNP [96]. As was inferred from the polarizations, the products are formed to a large degree by secondary encounters of escaped radicals. These reactions are accompanied by a-cleavage of the ketone, which is of comparable rate as deuterium abstraction but significantly slower than hydrogen abstraction. 

CIDNP spectroscopy has been employed to unravel the reactions following photoexcitation of PtMe4(bipyridyl) [129], From the polarization, it was inferred that the precursor multiplicity is triplet and that the primary photochemical step is cleavage of a Pt—Me-bond. Escape products are formed by deuterium abstraction from the solvent and geminate products by disproportionation. 

The enone (92) is reactive in its triplet state and when irradiated in methylene chloride solution is converted into the tetracyclic compound (93). The reaction involves a step-wise process in which the biradical (94) is involved. This process is reminiscent of (2-i-2)-cycloaddition reactions where bonding occurs at the P-atom of the enone, and rather than completing the cyclization, hydrogen (or deuterium) abstraction occurs. A detailed stereochemical analysis of the system was carried out and proof of the stereochemistry of the final product (93) has been presented. 

Timmons, R. B. de Guzman, Vamerin, R. E. /. Am. Chem. Soc. 1968,90,5996. Product ratios were determined from ratios of H2, HD, and D2 produced from the HBr and DBr by-products of the reaction. Because C6H5CH2D contains two abstractable hydrogen atoms and one deuterium atom, the ratio of the rate constant for hydrogen abstraction to that for deuterium abstraction was divided by 2 to give the h/ d value for the reaction. 

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