Radical a-ketenyl

DFT and ab initio calculations have shown that oq/S-unsaturated acyl radicals and a-ketenyl radicals are not resonance forms but are rapidly interconverting isomers separated by a low but not negligible energy barrier of <22 kJmol-1.64 Bi-cyclo[2.2.2]octenones (60) bearing an endo vinyl substituent have been shown to give diquinane products (61) on triplet-sensitized irradiation the reaction has been described as occuring via an oxa-di-7t-methane rearrangement. 

Pattenden and co-workers recently succeeded in trapping an a-ketenyl radical by sequential 5-exolS-exo cyclizations (Scheme 4-11) [24]. They further developed a series of interesting reactions based on this key isomerization reaction of a,(f unsaturated acyl radicals [25]. 

The calculated C=CO—OH bond energy is very weak, 7.1 kcal mol in C=CO—OH and 7.8 kcal mol in the methyl substituted CC=CO—OH the bond strengths are only on the order of a hydrogen bond. The ethynyl hydroperoxide moiety is highly unstable. This molecule will rapidly dissociate to form a ketenyl radical + OH ... 

The C=CO—OC and CC CO—OC bond energies in ethynyl peroxides are 3.27 and 3.72 kcal mof respectively but they are too small to exist physically at high temperature the ethynyl peroxide moiety is unstable. The HC COOCHs and CH3C=COOCH3 molecules will immediately dissociate to form a ketenyl radical (C H=C=0 or CC =C=0 and CH3O) with bond energies less than 4 kcal mof. This instability is similar in the ethynyl hydroperoxide molecules. 

Scheme 4.10 a-Ketenyl radicals in vinyl radical carbonylation. 

The ketenyl radical (HCCO) is a key intermediate in the oxidation of acetylene in flames. It is mainly formed from the O + C2H2 HCCO + H reaction. In lean flames, the HCCO + O2 reaction is the main pathway for decay of HCCO, and this reaction has recently been shown to be the source of prompt CO2 [44, 45]. 

The reaction mechanism is presumed to rely on the ability of the a,/3-unsaturated acyl 151 to generate an cr-ketenyl radical 152, which reacted as an electrophile toward the imino functionality to produce the radical-zwitterion 153 (Scheme 63 <2003JA5632>). 

Recently, Pattenden, following the same D strategy, has developed the use of a-ketenyl cyclooctanyl radical 114 toward a new and concise formal synthesis of modhephene (Scheme 33). The (x,)5-unsaturated seleno ester 112, treated under usual BusSnH-AIBN conditions, generates the corresponding a,/9-unsaturated acyl radical 113, which transannularly cyclizes via its mesomeric radical counterpart 114. The resulting tertiary radical undergoes a 5-cxo-dig cyclization onto the ketene central carbon giving rise to a final enoxy radical, which is reduced to provide the tricyclic ketone 115 [49]. 

Reported errors for each of the standard species (App. A) + estimated error due to Values are from Group Additivity unless noted otherwise ketenyl radical Groups developed in this woik, not previously available Lee et al. Ref. % From parent alcohol in Table 2, and vinoxy (phenoxy) bond energy of 85 kcal mol Estimate does not account for radical on secondary or tertiary alkyl site Mebel et al. Ref 33 the method (App. C) -1. 9 ... 

Ketenyl radical (27) is a species involved both in ketene formation and in ketene reactions, and may be formed by hydrogen atom abstraction from ketene. Ethoxy-acetylene is a thermal source of ketene and forms ketenyl radical by 193 nm photolysis in helium, argon, or xenon, with time-resolved IR emission spectroscopy used to examine the lowest quartet state of the radical. The radical with 1785 36 cm for the fundamental transition of the V2 asym-CCO stretch was observed in helium while CO did not form in this medium, but in Ar or Xe vibrationally excited CO was observed, suggesting rapid coUision-induced intersystem crossing occurred due to a heavy-atom effect (Eqn (4.17)). The presumed product CH formed with CO was not however observed and the expected stretching frequencies would be quite weak. 

Ketenyl carbenes are proposed as the key intermediates in the photochemical decomposition of maleic and dichloromaleic anhydrides in cryogenic matrices. The former anhydride yields cyclopropenone and ethyne while the latter yields dichlorocyclopropenone and dichloroethyne. Dichloropropadienone is observed as a minor product. Maleimide (155) is excited into its triplet state on irradiation in solvents. In hydrogen-donating solvents the radical (156) is... 

---