Vinylcarbene from carbonyl complexes

As described in the previous section, a-elimination of dialkyltitanocenes is of limited use for the preparation of titanocene-alkylidenes. Since thioacetals are readily available from carbonyl compounds or through alkylation of bis(phenylthio)methane and related organosulfur compounds, a thioacetal-titanocene(II) system enables the use of different types of carbene complexes. Use of appropriate thioacetals is of crucial importance in this system for the preparation of alkylidene complexes 48, the corresponding diphenyl thioacetals are the starting materials of choice. No carbene complexes are formed from dialkylthio-acetals. To generate vinylcarbene complexes 49, trimethylene thioacetals of aji-unsaturated aldehydes or l,3-bis(phenylthio)propene derivatives are employed (Scheme 4.41). The low-valent titanium species 44 is also easily prepared by the reduction of titanocene dichloride with magnesium in the presence of triethyl phosphite at room temperature. The presence of molecular sieves 4A is essential for the reproducibility of this preparation. In relatively large-scale preparations, care should be taken to control the reaction temperature [92, 93g]. 

Weiss studied68a the reactivity of both new complexes, and found that a variety of phosphines and phosphites would also convert the vinylcarbene complex 139 into the corresponding vinylketene complex (140), capturing one of the carbonyl ligands from the coordination sphere of the metal to become the ketene carbonyl. Only in the case of triphenylphosphine was the dicarbonyl(phosphine)vinylcarbene complex (141) isolated, which then required addition of carbon monoxide to convert it to the dicarbonyl(triphe-nylphosphine)vinylketene complex 140.a. This interconversion was reversible and proceeded quantitatively. 

The vinylketene complex 221.a was shown3 to undergo ligand replacement when treated with triphenylphosphine to yield the dicarbonyl(triphe-nylphosphine)vinylketene complex 234. This complex was also isolated from the reaction of vinylketone 222.a with methyllithium in the presence of triphenylphosphine, indicating that a phosphine, as well as carbon monoxide, could induce the migration of a coordinated carbonyl ligand to form a ketene carbonyl. Mitsudo, Watanabe, and Weiss have all shown that this process occurs in the reaction between phosphines and isolated rf-vinylcarbene complexes (see Sections VI,B and VI,C).48,68,106... 

The methoxyketene 297, coordinated to Cr carbonyl, is formed from methoxy-carbene easily by insertion of CO under irradiation [90]. An ester is formed by the reaction of ketene with alcohol. The aminocarbene complex 298 was prepared from benzamide and converted to phenylalanine ester 300 under irradiation of sunlight in alcohol via ketene 299 [91]. The eight-membered lactone 304 was prepared in high yield by the reaction of the alkyne 301 having the OH group in a tether with Cr carbene without irradiation. The vinylcarbene 302 is formed at first and converted to the vinylketene intermediate 303 as expected. The keto lactone 304 is formed from 303 by intramolecular reaction with the OH group and hydrolysis [92],... 

The mechanism of the Dotz benzannulation reaction has not been fully elucidated. The first step is the ratedetermining dissociation of one carbonyl ligand from the Fischer carbene complex, which is cis to the carbene moiety. Subsequently, the alkyne component coordinates to the coordinatively unsaturated carbene complex, and then it inserts into the metal-carbon bond. After the alkyne insertion, a vinylcarbene is formed that can lead to the product by two different pathways (Path A or Path b). ... 

The subsequent steps of the reaction are too fast in order to allow further kinetic investigation. A previously proposed 16-electron chromacyclobutene intermediate arising from a formal [2-i-2]-cycloaddition of the alkyne ligand aross the metal-carbene bond was later discarded as a result of theoretical studies which support a direct insertion of the alkyne into the metal-carbene bond to generate an 18-electron valence-isomer, the rj -vinylcarbene complex D. [48] A related species III has been isolated from the reaction of an aminocarbene complex. [46] Subsequent insertion of a carbonyl ligand leads to an rj -yinylketene complex E, of which structural analogues as enaminoketene complex IV have been synthesized. 

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