Ketene complexes

Keywords Metal carbenes Photochemical reactions Metal-ketene complexes... 

No CO-insertion products (metal-ketene complexes) were observed, even when specifically sought [9,10]. 

Several stable Group 6 metal-ketene complexes are known [14], and photo-driven insertion of CO into a tungsten-carbyne-carbon triple bond has been demonstrated [15]. In addition, thermal decomposition of the nonheteroatom-stabilized carbene complexes (CO)5M=CPh2 (M=Cr, W) produces diphenylke-tene [16]. Thus, the intermediacy of transient metal-ketene complexes in the photodriven reactions of Group 6 Fischer carbenes seems at least possible. 

For a review on metal-ketene complexes see Geoffroy GL, Bassner SL (1988) Adv Organomet Chem 28 1... 

Carbonylation of the manganese carbene complex (tj5-C5Hs) (CO)2Mn=CPh2 to an analogous ketene complex has been reported (107). Compound 76 is also formed directly in the reaction of ketene with 45 (39). 

The cyclobutenone 70 is transformed to the r/4-vinylketene complex 72 with (t/5-indenyl)Co(PPh3)2 71. The vinylketene complex 72 undergoes cyclization with alkynes to produce the corresponding phenols 73. FeCl3 oxidation of the (2-phenylvinyl)ketene complex, however, leads to the naphthol 74. A catalytic synthesis of phenols via the vinylketene intermediates 72 is achieved by the use of Ni(COD)2 as a catalyst [36]. (Scheme 26)... 

A stoichiometric double C-H functionalization mediated by an iridium ketene complex leads to l,4-dien-3-ones,... 

The reaction course is shown in Scheme 4. Enyne 12 reacts with 2 to give vinyl carbene complex 17, which is in a state of equilibrium with vinyl ketene complex 21. [2+2] Cycloaddition of the ketene moiety and alkene part in 21 gives cyclob-utanone 22. On the other hand, the vinyl carbene complex 17 reacts with the alkene intramolecularly to produce metalacyclobutane 18. From metalacyclob-utane 18, reductive elimination occurs to give cyclopropane derivative 23. Ret-... 

Clear formation of ketene—zirconocene complexes upon treatment of acylzirconocene chlorides with a hindered amide base indicates that the carbonyl group of the acylzirconocene chloride possesses usual carbonyl polarization (Scheme 5.10). However, these zirconocene—ketene complexes are exceptionally inert due to the formation of strongly bound dimers [13a], Conversion of the dimer to zirconocene—ketene—alkylaluminum complexes by treating with alkylaluminum and reaction with excess acetylene in toluene at 25 °C has been reported to give a cyclic enolate in quantitative yield. Although the ketene—zirconocene—alkylaluminum complex reacts cleanly with acetylene, it does not react with ethylene or substituted acetylenes [13b]. Thus, the complex has met with limited success as a reagent in organic synthesis. 

As part of a study of the reactions of metallacyclic y-ketovinyl complexes of molybdenum and tungsten with acetylenes, directed toward the synthesis of complexed -/-lactones, Stone has reported92 the isolation of several vinyl-ketene complexes. When complex 72 was heated with 2-butyne, one molecule of the alkyne was incorporated into the complex with concomitant carbonylation. X-ray analysis of the product (73) has shown unequivocally that the C-l to C-4 vinylketene fragment is bonded in a planar, rj4-configu-ration. In contrast to the thermal reaction, ultraviolet irradiation of 72 or 74 in the presence of 2-butyne affords the complexes 75 and 76, respectively, where the lone carbonyl remaining after alkyne insertion had been replaced by a third molecule of the alkyne. 

Fig. 2.21. Photolytic generation and. synthetic applications of chromium ketene complexes.

Intramolecular acylation reactions with ketene complexes, generated, for instance, by thermolysis or photolysis of carbene complexes, can also be used for the preparation of six-membered rings. Illustrative examples are shown in Table 2.23. 

Further methods for preparing seven-membered rings with the aid of heteroatom-substituted carbene complexes include the intramolecular acylation reactions with ketene complexes discussed in Section I.2.6.2. 

This section deals with alkylidene complexes L M=CR2 and vinylidene complexes LnM=(C)n,=CR2 in which the metal-bound carbon atom bears only hydrogen, alkyl, or aryl groups, but neither heteroatoms (halogen, nitrogen, oxygen, or sulfur) nor electron-withdrawing groups. Dimetallacyclopropanes and ketene complexes will not be discussed. 

Recently, double insertion of CO into the reactive thorium-silicon bond of Cp2 ThSiR3(Cl) allowed the isolation of the first metalloxy ketene complex 14. Reactivity studies of complex 14 are described in Scheme 939. 

Hegedus et al. discovered that irradiation of chromium-carbene complexes resulted in a photoinsertion of CO into the Cr-carbene bond to form Cr-ketene complexes [96, 97]. This opened novel routes to the preparation of valuable compounds via Cr-ketene chemistry. Among them, the reaction of metallated ketenes with imines was intensively explored [98-100]. Within this context, the reaction between several model Cr-ketenes (120) and imines was explored at the B3LYP/6-31G ECP level of theory [101, 102], The mechanisms thus obtained are reported in Scheme 31. 

From the qualitative point of view, the structure of the ethylene/ketene complex is similar to the geometry of the TS of the same system in cycloaddition reaction58. In 22, R (= 3.46 A) is the distance between the centre of mass of the ethylene and the carbon of the carbonyl group of ketene this carbon is the most electrophilic centre of the ketene. In the case of the complex between acetylene and ketene, the same distance between the centre of mass of acetylene and the carbon of ketene was evaluated (by the same method) at 3.60 A. 

Cycloaddition of a ketene complex with unsaturated bonds other than alkenes and alkynes is also possible. The ketene 312, formed from 311, adds to imine 313 to give the /1-lactam 314 under sunlight photolysis. The optically active /1-lactam 314 was prepared from the optically active carbene complex 311 with 99% ee, and converted to 315 [95]. Irradiation of carbene complex 316 generates ketene 317, which cyclizes to the o-hydroquinone derivative 318 [96],... 

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