Evidence for cyclopropene intermediates in the rearrangement of aromatic carbenes to arylcarbenes

Abstract

A convenient, high-yield synthesis of 5//-dibenzo[a,c]cyclohepten-5-one is reported and the properties of dibenzo[fl,c]cycloheptatrienylidene are examined. As previously reported2 for 4,5-benzocycloheptatrienylidene, dibenzo [ti,c]cycloheptatrienylidene rearranges rapidly in solution to an arylcarbene (9-phenanthrylcarbene). When the annelated cycloheptatrienylidenes were generated in the presence of dienes, Diels-Alder adducts of the cyclopropene intermediates were obtained. The molecular geometry of the single furan adduct of dibenzo[a,c]cycloheptatrienylidene was determined from an analysis of lanthanide-induced proton nmr shifts. The geometry of the adducts of both of the carbenes with cyclopentadiene is shown to be the endo-anti isomers. These are the products expected from cycloaddition of 3-monosubstituted cyclopropenes with this diene. The furan adduct was obtained under both thermal (125°) and photochemical (30 and -60°) conditions. Tetracyclone adducts were obtained in thermal reactions and cyclopentadiene and butadiene adducts were obtained from low-temperature ( -60°) reactions. The adducts were shown not to be secondary photoproducts, and a two-step thermal process has been ruled out. Also, irreversible cyclopropene formation competitive with rearrangement is shown to be an unsatisfactory explanation of the experimental results.

Excited state carbon acid dissociation and competing photorearrangements of 5H-dibenzo[a,c]cycloheptene derivatives

“…The 'H NMR of the material obtained was Coburn and Jones (7). The 'H NMR of the product was idenidentical to the literature spectrum (6).…”

“…Conversion of phenanthrene (13) to dibenzo[a,c]cycloheptane (16) followed literature methods (6)(7)(8). Bromination of 16 with NBS gave 17, which was hydrolyzed in H20-CH3CN to give 18.…”

The Dibenzosuberenone Scaffold as a Privileged Substructure: From Synthesis to Application

“…A convenient, high-yield synthesis of 5H-dibenzo[a,c]cyclohepten-5-one (2) was achieved in five steps starting from phenanthrene (441) (Scheme 82). 243 The cyclopropanation of phenanthrene (441) with dichlorocarbene gave a seven-membered cycloheptatriene product 462 through ring expansion of the norcaradiene structure 461a. The synthesis was completed after oxidation, reduction, and elimination.…”

“…Gold-vinylidene and synthesis of 5H-dibenzo[a,c]cyclohepten-5-one3.2 Reactions of 5H-Dibenzo[a,c]cyclohepten-5oneCoburn and Jones generated dibenzo[a,c]cyclohepten-5ylidene (477) from 5H-dibenzo[a,c]cyclohepten-5-one(2) and examined its properties (Scheme 87) 243. The formation of cyclopropene intermediate 478 in the rearrangement of aromatic carbene 477 prior to the formation of phenanthrylcarbene 479 was proven by trapping it in the presence of dienes such as cyclopentadiene, furan, and tetraphenylcyclopentadienone to yield cycloadducts 480 and 481.…”

Application of Aryl Siloxane Cross‐Coupling to the Synthesis of Allocolchicinoids.

“…98.0-98.8 °C). 3 sodium acetate (79.3 mg, 0.967 mmol), and palladium on carbon (5%, 65.8 mg, 16.8 µmol) were stirred in 10 mL of EtOH under 1 ATM of hydrogen for 1 h. The reaction mixture was filtered, and poured into 50 mL of ether. The organic layer was washed with water (50 mL x2) and brine (50 mL).…”