Thermal C2-C6 cyclization of enyne-allenes. Experimental evidence for a stepwise mechanism and for an unusual thermal silyl shift.
Enyne-allenes 4a-c bearing various cyclopropyl systems as radical clock reporter groups at the allene terminus have been synthesized and subjected to thermal C2-C6 cyclization. The ratio of ene versus formal Diels-Alder products could be rationalized on the basis of steric effects. Only the thermolysis of 4c, equipped with the fast diphenylcyclopropylcarbinyl radical clock, afforded a 1,3-butadienyl benzofulvene clearly formed via cyclopropyl ring opening. This finding provides unambiguous evidence for a stepwise mechanism of the C2-C6 cyclization making it possible to suggest a lifetime for the intermediate diradical of >1x10(-10) s (at 170 degrees C). An interesting corollary was the isolation of an unexpected silyl shift product in the thermolysis of all three enyne-allenes that allows explanation of the loss of the TIPS group in some of the Diels-Alder products. For a full understanding of the mechanism, silyl and hydrogen shift processes were interrogated using DFT.
Schmittel
Michael
M
Organische Chemie I, FB 8 (Chemie-Biologie), Universität Siegen, Adolf-Reichwein-Str., D-57068 Siegen, Germany. schmittel@chemie.uni-siegen.de
Mahajan
Atul A
AA
Bucher
Götz
G
Bats
Jan W
JW
D000466:Alkadienes / Q000737:chemistry*; D003500:Cyclization; D005609:Free Radicals; D006358:Hot Temperature; D012821:Silanes / Q000737:chemistry*
D000466:Alkadienes; D005609:Free Radicals; D012821:Silanes; C025947:propadiene