Thermal C1-C5 diradical cyclization of enediynes.
Computational studies at the BLYP/6-31G(d) level (supplemented by BCCD(T)/cc-pVDZ calculations) suggest that in aryl-substituted 1,2-diethynylbenzenes, steric effects disfavor the thermal C1-C6 diradical cyclization reaction (Bergman) and electronic effects favor the regiovariant C1-C5 cyclization to the extent that the C1-C5 process should become an important reaction pathway in the thermolyses of such compounds. Experimentally, thermolyses of 1,2-bis(2,4,6-trichlorophenylethynyl)benzene, a particularly favorable case, yields only products derived from C1-C5 cyclization [specifically, 1-(2,4,6-trichlorobenzylidene)-2-(2,4,6-trichlorophenyl)-1H-indene and its hydrogenation product 3-(2,4,6-trichlorobenzyl)-2-(2,4,6-trichlorophenyl)-1H-indene], and even for the parent hydrocarbon 1,2-bis(phenylethynyl)benzene, the formation of C1-C5 cyclization products is competitive with the major Bergman reaction. Although some C1-C5 cyclization products are probably formed by transfer hydrogenation from 1,4-cyclohexadiene (commonly included in such reactions), thermolyses in the absence of 1,4-CHD as well as deuterium labeling studies confirm the existence of direct C1-C5 diradical cyclizations for diaryl-substituted enediynes.
Vavilala
Chandrasekhar
C
Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA.
Byrne
Neal
N
Kraml
Christina M
CM
Ho
Douglas M
DM
Pascal
Robert A
RA
D000466:Alkadienes / Q000737:chemistry; D002244:Carbon / Q000737:chemistry*; D018360:Crystallography, X-Ray; D003500:Cyclization; D053281:Enediynes / Q000737:chemistry*; D005609:Free Radicals / Q000737:chemistry; D008958:Models, Molecular; D015394:Molecular Structure; D013696:Temperature
D000466:Alkadienes; D053281:Enediynes; D005609:Free Radicals; C025947:propadiene; D002244:Carbon