Ketenylation

Ketenes can react in several ways with organometaUic compounds and complexes. They can add as ligands to coordinated metals forming stable ketene, ketenyl, and ketenyfldene complexes. Ketenes can be inserted into metal—hydride, metal—alkyl, metal—OR, and metal—NR2 bonds, react with metal—oxide complexes, and with coordinated Hgands. This chemistry has been reviewed (9,51). 

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]. 

More detailed information about the dynamics of ketenyl formation was obtained at the higher Ec of 12.6kcal mol-1, which was reached by crossing the same reactant beams at 135°. 

At the HF/6-31G level, ketenyl carbenes (1) were calculated to be intermediates in the decarbonylation of 1,2-bisketenes (2) to form cyclopropenones." At the MP2/6-31G and B3LYP levels, however, decarbonylation was predicted to form the cyclopropenones directly. The anfi-ketenyl carbenes were found to be 2.2-5.4kcalmoP higher in energy than the syn isomers (1). The mechanism of reaction of [l.l.ljpropellane with singlet dihalocarbene has been reported. ... 

CO)9Cp (184) which decarbonylates slowly in refluxing toluene to OS3W (/i.4-C2)(/r3-CPh)(CO)9Cp (185). This sequence corresponds to oxidative decar-bonation of alkynyl ketenyl alkylidyne. ... 

The ketenyl radical (HCCO) is subsequently oxidized to CO and CO2, mainly by the fast reaction with O2,... 

Ketenyl complexes, in Ru-Os mixed-metal clusters, 6, 1080 Ketenylidenes, in iron cluster compounds, 6, 301 Ketenylidenetriphenylphosphoranes, with platinum, 8, 632 Ketimines... 

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. 

Addition of the electron-rich amino alkyne, Et2NC=CMe, to the 172-ketenyl tungsten complex, (T 2-RC=C=0)CpW(PMe3)(C0), results in trimethylphosphine substitution and opening of the Tj2-ketenyl ligand to an V-ketenyl [Eq. (18)] (71). 

Stepwise formation of hydroxy and alkoxy alkynes from carbonyl-car-byne coupling reactions has been studied by Kreissl and co-workers over the past decade (102). These coupling reactions typically form ketenyl complexes prior to electrophilic addition at oxygen to yield alkyne pro-... 

Angelici and co-workers have recently reported that PEt3 adds to [HB(pz)3](CO)2W=CSMe (pz = C3H3N2) to form an T 2-ketenyl product which can be methylated with MeOS02F at room temperature to yield a cationic complex with an unusual alkyne ligand, MeOC=CSMe [Eqs. (45) and (46)] (109). 

Anionic T -ketenyl complexes were first reported in 1983 from the reaction of (Ws=CPh)(CO)2Br(phen) with KCN to form [W(CO)(rj2-PhCCO)-(CN)2(phen)]- (112). 

Kreissl has pointed out that the rj -ketenyl ligand in CpW(CO)(Et2-NC=CMe)[i71-C(CO)C6H4Me] acts as a multiple electron donor with resonance form ii below an important contributor to the description of this ligand (71). The impact of this 7r-donor role for the V-ketenyl ligand on the cis-alkyne geometry is minimal as the aminoalkyne ligand structural parameters fall in the range typical of four-electron donor alkynes. 

The deprotonation of (trimethylsilyl)diazomethane with n-butyllithium afforded the lithium silyldiazomethane 37, which reacts with CO at — 78 °C to give an acyllithium 3855,56. This intermediate underwent nitrogen extrusion generating the silylynolate 39 used as ketenylating reagent (Scheme 9). 

Figure 5.42 Ketenyl formation via carbyne-carbonyl coupling Wl 2 = W(CO)Cp...

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>). 

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