Photochemistry of cyclopropene derivatives. Ring-opening reaction of 3-heteroaryl substituted cyclopropenes

Abstract

The photochemical rearrangement of several 3heteroaryl substituted cyclopropenes has been studied. The rearrangements are derived from the '~t--'lt* singlet state of the cyclopropene. Ring opening occurs to give a vinylcarbene intermediate which undergoes a subsequent electrocycliration. The transient intermediate so produced can undergo either a 1,3-or a 1,5-sigmatropic hydrogen shift to give the observed products. One strong piece of evidence for carbene intervention is the observation of a 1,3-butadiene derivative as one of the photoproducts obtained from the photolysis of a 3-pyrrolyl substituted cyclopropene. The 1,3-diene is thought to be derived by insertion of the vinylcarbene into the neighbouring methyl group. The regioselectivity of the rearrangement can be accounted for in terms of competitive 1,3and 1,5-sigrnatropic hydrogen migrations.

Thermally Induced Cyclopropene−Carbene Rearrangements: An Overview

“…In the case of unsymmetrically 1,2-substituted cyclopropenes, selective formation of single indenes may occur as shown below. Cyclization also occurs into heterocycles at the 3-position of the cyclopropene: 98,[101][102][103][104] In other cases, the intermediate may react further, in the following example by an ene reaction: 101 Similar reactions have also been reported for 3-ferrocenyl-3-phenylcyclopropenes. [105][106][107] It is notice-able that, in almost all cases, the cyclopropenes described above are tetrasubstituted.…”

Synthesis of Heteroarene‐Fused Cyclopentadienes and Related Compounds Suitable for Metallocene Preparation

“…Photolysis of 3-methyl-3-(N-methylpyrrol-2yl)-1,2-diphenylcyclopropene (obtained by the reaction of 1-methyl-2,3-diphenylcyclopropenylium cation with 2-(N-methylpyrrolyl)lithium) resulted in compound 69 (Scheme 9). [97] Rhodium(III)-catalyzed oxidative annulation of cyclopentane-1,2-dione, 2-aminopyridine, and alkynes afforded 4,5-dihydrocyclopenta[b]pyrrol-6(1H)ones. [98] 2-Aminopyrimidine was also introduced (compound 78) as an easily installable and readily remov- [84][85][86][87] Scheme 5.…”

“…The starting compound was prepared by treating 2-furyllithium with the 3-methyl-1,2-diphenylcyclopropenyl cation. [97]…”

“…In line with the photochemically induced rearrangement of cyclopropenes to diphenyl derivatives of cyclopenta[b]pyrrole 69 and cyclopenta[b]furan 97, the product of the reaction of thienyllithium with 3methyl-1,2-diphenylcyclopropenylium cation was transformed into product 159 (Scheme 24A); the yield of 159 was the highest among the five-membered heterocycles. [97] Also, halogen-substituted cyclopenta [b]thiophenes were transformed into alkyl-and arylsubstituted compounds using catalytic cross-coupling methods (Scheme 24B). [55,63] Scheme 21.…”

“…[43,129] Scheme 24. Synthesis of cyclopenta[b]thiophenes by photochemical rearrangement of cyclopropene (A) [97] and use of cross-coupling reactions (B). [53,63] -"--"--"--"--"--A -"--"--"--"--"--C Thus, thiophene-annelated cyclopentenones and cyclopentadienes were the most intensively studied compounds among the heterocycles discussed in Section 3.…”