Decarbonylation annulations

Similar five-membered ring formation was observed in decarbonylative annulation of benzothiophenedione 117 (Scheme 2.75) [128]. The [4-Hl]-type annulation reaction produced 3-methyl-3-alkenylbenzo[c]thiophen-l(3//)-one 118. 

Scheme 2.75 Ni-catalyzed decarbonylative annulation of benzothiophenedione 117 and MCP 83. (MAD methylaluminum bis(2,6-di-tert-butyl-4-methylphenoxide)...

Decarbonylative annulation of cyclobutenone 89 with norbornene proceeded in the presence of [RhCl(CO)2]2 under argon to afford cyclopentenone 91, while the reaction under 30 atm CO yielded the [4-1-2] annulation product 92 (Scheme 3.52) [61]. 

An intramolecular version of decarbonylative annulation was catalyzed by rhodium(l) - PPhj catalysts to provide bicyclic cyclopentenones (Scheme 3.55) [64]. 

Scheme 3.55 Rh(l)-catalyzed intramolecular decarbonylative annulation of cyclobutene-diones.

If rapid bond formation at the stage of the Norrish I fragments competes with decarbonylation, 1,2-acyl shift products are formed as the major components from direct photolysis. In case of the cyclobuteno-annulated bicyclo[2.2.2]octenone 21, 1,2-acyl shift results in the cyclo-butanone 22, whereas ODPM rearrangement (photolysis in acetone) results in the cyclobuteno-diquinane 23 in 44% yield (Sch. 21) [49]. 

The facile decarbonylation of pentacarbonyl complexes 23a and 23b results in tetra-carbonyl carbene complex intermediates 24a and 24b, respectively. Their annulation can produce benzene and/or cyclopentadiene derivatives 25 and/or 26. In the case of aryl acyla-mino complex 23a, the reaction course is shifted towards the benzannulation reaction (25a 26a = 84 16). With the vinyl carbene complex 23b, the benzannulation product 25b (25b 26b = 100 0) is produced exclusively. 

The competition of benzannulation and pentannulation significantly depends on the donor ability of the carbene ligand. Substitution of alkoxy for amino groups in the carbene ligand increases the thermal stability of the metal-carbonyl bond which hampers both the primary decarbonylation and the CO incorporation into the final product. As a consequence, annulation of chromium amino(aryl)carbenes requires elevated temperatures (> 90°C) and affords cyclopentannulation products (indenes). A-Acylation e.g. by Boc) reduces the donor properties of the amino substituent and thus favours again the... 

The use of both LIU and HIU has been shown to increase the efficiency of the P-K reaction, which involves the formation of cyclopentenone from the annulation of a cobalt alkynyl carbonyl complex and an alkene. The use of low-power ultrasound, as for example, from a cleaning bath, although capable of producing intramolecular P-K reactions, generated relatively low cyclization yields. The motivation for the use of high intensity came from its ability, as previously described, to effectively decarbonylate metal carbonyl and substituted metal carbonyl complexes. Indeed, HIU produced by a classic horn-type sonicator has been shown to be capable of facile annulation of norbornene and norbornadiene in under 10 min in the presence of a trimethylamine or trimethylamine N-oxidc dihydrate (TMANO) promoter, with the latter promoter producing cleaner product mixtures. This methodology also proved effective in the enhancement of the P-K reaction with less strained alkenes such as 2,5-dihydrofuran and cyclopentene, as well as the less reactive alkenes -fluorostyrene and cycloheptene. The mechanism has been postulated to involve decarbo-nylation of the cobalt carbonyl alkyne, followed by coordination by the amine to the vacant coordination sites on the cobalt. 

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