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Thermolabile Kohlenwasserstoffe, 31 Stereoselektive Bildung und Spaltung der Dimeren des 1‐(5,6,7,8‐Tetrahydro‐1‐naphyl)neopentyl‐Radikals
Author(s) -
Peyman Anuschirwan,
Beckhaus HansDieter,
Kramer Dirk,
Peters Karl,
Schnering Hans Georg Von,
Rüchardt Christoph
Publication year - 1991
Publication title -
chemische berichte
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 0009-2940
DOI - 10.1002/cber.19911240918
Subject(s) - chemistry , stereoselectivity , thermolabile , diastereomer , enthalpy , stereochemistry , medicinal chemistry , standard enthalpy of formation , catalysis , organic chemistry , physics , quantum mechanics , enzyme
Thermolabile Hydrocarbons, 31. – Stereoselective Formation and Cleavage of the Dimers of the 1‐(5,6,7,8‐Tetrahydro‐1‐naphthyl)neopentyl Radical The 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: δ H † 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 δ H † 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 δ H ° 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.