ISSN:
0947-3440
Keywords:
Diradicals
;
Gas-phase kinetics
;
NO and O2, trapping
;
Racemization barrier
;
Chemistry
;
Organic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
The Racemization of Allene, a Two-Step ReactionFor four vinyl- and phenyl-substituted allenes racemization enthalpies have been determined, which are 2-6 kcal · mol-1 below the estimate derived from the parent system by correction for the additional radical stabilization of the intermediate allyldiradical 2(I), indicating the growing importance of the carbene type structure 2(II) for the intermediate. - The racemization of 1,3-dimethylallene 1b at temperatures 〉300°C in the presence of NO allows the trapping of an intermediate, which is considered to be the orthogonal allyl diradical 2b. From the trapping kinetics (307-388°C), the reversible bimolecular addition of NO at low temperatures (225-285°C) and the strength of the C—NO bond an enthalpy well for the singlet diradical of 2.7 kcal · mol-1 and a singlet-triplet splitting of 5.6 kcal · mol-1 is derived with the triplet being the groundstate. - The postulated intermediate diradical 2 can be stabilized electronically by phenyl and vinyl groups or by strain when incorporated in an eight-membered ring. Thermolyses of the optically active 1,2,5-cyclooctatriene 18 in the presence of oxygen allows to determine the enthalpy well of the corresponding diradical 25 to be 13.1 kcal · mol-1.
Additional Material:
6 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/jlac.199619961231
