ISSN:
0009-2940
Keywords:
Bond formation, C - C, stereoselectivity of
;
Bond cleavage, C - C, kinetics of
;
Calculations, force field
;
Chemistry
;
Inorganic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
Thermolabile Hydrocarbons, 31. - Stereoselective Formation and Cleavage of the Dimers of the 1-(5,6,7,8-Tetrahydro-1-naphthyl)neopentyl RadicalThe 1,2-diaryl-1,2-di-tert-butylethanes meso- and DL-6 are synthesised by partial catalytic hydrogenation of the parent dinaphthylethanes 5. The crystal structure of meso-6 was obtained experimentally and calculated by molecular mechanics methods (MM2). It is shown, that 1-(5,6,7,8-tetrahydro-1-naphthyl)neopentyl radical 8 forms its dimers 6 with high stereoselectivity, e.g. DL-6: meso-6 = 45 (at -20°C) and 7.07 (at 100°C). The selectivity was measured over a range of 300 K by using several radical precursors. The difference of the enthalpy of activation has been derived for the two dimerisation reactions: δH†dim(DL - meso) = -2.8 ± 0.2 kcal/mol. The cleavage of DL-6 and of meso-6 into 8 was measured kinetically, and the enthalpies of activation δH†dis = 46.2 ± 0.6 (DL-6) and 52.6 ± 1.3 kcal/mol (meso-6) and the entropies of activation δ†dis = 8.4 ± 0.6 (DL-6) and 20.0 ± 2.5 e.u. (meso-6) have been obtained. A complete thermodynamic cycle is constructed by using the calculated (MM2) heats of formation δH°f = -27.6 (DL-6) and -30.0 kcal/mol (meso-6). Thus, the diastereomer (DL-6), which is formed preferentially, appears to be the thermodynamically and kinetically less stable one. It turns out, that the high stereoselectivity of the dimerisation of 8, compared to the parent 1-phenyl-neopentyl radical (2a), is mainly caused by the steric repulsions between the approaching radicals.
Additional Material:
2 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/cber.19911240918
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