Catalytic 1,2-dihydronaphthalene and E-aryl-diene synthesis via Co^III–Carbene radical and o-quinodimethane intermediates

Abstract

Catalytic synthesis of substituted 1,2-dihydronaphthalenes via metalloradical activation of o-styryl N-tosyl hydrazones is presented. Substrates with an alkyl substituent on the allylic position reacted to form E-aryl-dienes rather than the expected 1,2-dihydronaphthalenes.

Catalytic Dibenzocyclooctene Synthesis via Cobalt(III)–Carbene Radical and ortho‐Quinodimethane Intermediates

“…The EPR signal (g = 2.0066, A N = 14.5 G, A H = 2.8 G) is characteristic for a PBN-trapped carboncentered radical, [12,14] suggestive of the trapping of intermediate C or D. Mass spectrometry confirmed the presence of such PBN-trapped intermediates (see the Supporting Information). The combined data clearly support the mechanism depicted in Scheme 2.…”

“…This behavior is somewhat similar to observations in our previous studies regarding dihydronaphthalene and butadiene formation via related o-QDM intermediates. [14] Metalcatalyzed radical-rebound ring closure from D to form dihydronaphthalene 3 a (Scheme 1) is kinetically disfavored compared to the (slightly endergonic) dissociation of o-QDM intermediate E from D (see the Supporting Information, Scheme S1 for details). Intermediate E readily undergoes an 8p cyclization reaction, [22] which has a surprisingly low barrier (TS3: + 6.2 kcal mol À1 ), producing dearomatized intermediate F. The final product 2 a is readily produced from intermediate F by a low-barrier [1,5]-hydride shift reaction (TS4: + 10.7 kcal mol À1 ) to regain aromaticity in both rings, which provides a sufficiently large thermodynamic driving force for the entire process.…”

“…The p orbital of the carbene moiety is dominant in the SOMO of these reactive intermediates, effecting their typical radical-type behavior towards, for example, C À H, C = C, and C C bonds ( Figure 2). Such metalloradical approaches have previously been employed to synthesize cyclopropanes, [9] chromenes, [10] furans, [11] indenes, [12] ketenes, [13] butadienes, [14] and dihydronaphthalenes. [14] We now report the synthesis of unique dibenzocyclooctenes in high yields by cobalt-based metalloradical one-electron catalysis.…”

“…Such metalloradical approaches have previously been employed to synthesize cyclopropanes, [9] chromenes, [10] furans, [11] indenes, [12] ketenes, [13] butadienes, [14] and dihydronaphthalenes. [14] We now report the synthesis of unique dibenzocyclooctenes in high yields by cobalt-based metalloradical one-electron catalysis. [15] The formation of dibenzocyclooctenes (Scheme 1) is a relevant addition to the known reactivity of carbene radicals because of their potential relevance in total synthesis and the synthetic difficulties associated with the formation of eight-membered rings in general.…”

[Co(TPP)]‐Catalyzed Formation of Substituted Piperidines

“…This type of reactivity belongs to a more general class of catalytic reactions involving single‐electron elementary steps, called metalloradical catalysis . Cobalt(III)‐carbene radical chemistry has been successfully applied in the synthesis of carbo‐ and heterocyclic structures, including cyclopropanes, chromenes, furans, indenes, indolines, ketenes, butadienes and dihydronaphthalenes, dibenzocyclooctenes, as well as phenylindolizines . Recently, the group of Zhang reported the synthesis of chiral pyrrolidines and related five‐membered ring compounds, starting from N‐tosylhydrazone derivatives and catalyzed by cobalt(II) complexes of D 2 ‐symmetric chiral amidoporphyrins (Scheme A) .…”