Palladium-Catalyzed Oxidative Coupling of the Allenic C–H Bond with α-Diazo Esters: Synthesis of [3]Dendralenes

Abstract

A palladium-catalyzed highly regio-and stereoselective allenic C−H oxidative coupling with α-diazo esters is developed. The reaction pathway involves allylic palladium carbene as the key intermediate, which is followed by a carbene migratory insertion process. The reaction proceeds efficiently under mild conditions without external base, providing substituted [3]dendralenes bearing various functional groups.

Recent advances in transition-metal-catalyzed carbene insertion to C–H bonds

“…202 A palladium-catalyzed formal allenic C-H functionalization was also realized by means of donor/acceptor a-diazo esters (Scheme 43). 203 With 10 mol% Pd(OAc) 2 as the precatalyst and 2,6-dimethylbenzoquinone (DMBQ) as the oxidant, ethyl 5-methylhexa-3,4-dienoate (119a) and its ester or amide analogs reacted with the stated diazo compounds 120 to give the desired [3] Intramolecular decarboxylative carbene insertion to benzylic C-H bond was observed as the side reaction in Rh(II)-catalyzed cyclopropanation of methallyl diazoacetates (31). 204 In the case of 122 using Rh 2 (4S-IBAZ) 4 as the catalyst, the target cyclopropanation product 123 was obtained with formation of the decarboxylative carbene insertion product 124 (14.4%) as the side product.…”

“…202 A palladium-catalyzed formal allenic C–H functionalization was also realized by means of donor/acceptor α-diazo esters (Scheme 43). 203 With 10 mol% Pd(OAc) 2 as the precatalyst and 2,6-dimethylbenzoquinone (DMBQ) as the oxidant, ethyl 5-methylhexa-3,4-dienoate ( 119a ) and its ester or amide analogs reacted with the stated diazo compounds 120 to give the desired [3]dendralene derivatives 121 in up to 87% yields. The plausible mechanism suggests that Pd(OAc) 2 initially reacts with the allene substrate to generate species int-121a .…”

Substituents Regulate the Cyclization of Conjugated Alkynes to Accurately Construct Cyclo-(E)-[3]dendralenes

“…A noticeable breakthrough in the practical synthesis of dendranlenes was reported by Sherburn and co-workers in 2009, finding that dendralenes are actually stable and also have surprising reactivities [4]. Subsequently, dendralenes attracted increased interest because of their unusual structure and electronic phenomenon, with widespread applications of dendralenes in polymer chemistry [5,6], theoretical chemistry [7][8][9][10], materials chemistry [11,12], electrochemistry [13], and synthetic chemistry [14][15][16][17][18][19][20][21][22][23][24][25][26], among which, cyclo- [3]dendralene, as the primary member of the family, has attracted the greatest attention owing to its wide existence in naturally occurring products and engagement in synthetic compounds [27]. For linear dendralenes, the physical and chemical properties of the annulenes are dominated by this parity-dependent behavior.…”