Ketenes complexes with alkenes

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],... 

Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. 

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-... 

The transition metal-catalyzed cyclopropanation of alkenes is one of the most efficient methods for the preparation of cyclopropanes. In 1959 Dull and Abend reported [617] their finding that treatment of ketene diethylacetal with diazomethane in the presence of catalytic amounts of copper(I) bromide leads to the formation of cyclopropanone diethylacetal. The same year Wittig described the cyclopropanation of cyclohexene with diazomethane and zinc(II) iodide [494]. Since then many variations and improvements of this reaction have been reported. Today a large number of transition metal complexes are known which react with diazoalkanes or other carbene precursors to yield intermediates capable of cyclopropanating olefins (Figure 3.32). However, from the commonly used catalysts of this type (rhodium(II) or palladium(II) carboxylates, copper salts) no carbene complexes have yet been identified spectroscopically. 

Similar to the thermal reaction of ketenes with alkenes, the photolysis of alkoxycarbenes 18 in the presence of (electron-rich) olefins 19 leads to cyclobutanones (Scheme 6) [9]. In these reactions the sterically more strained [2 + 2] cycloadducts of type rac-17 are generally formed with good regio- and diastereoselectivity. Starting from complexes of type 21, the intramolecular version of this reaction affords bicyclic products of type rac-20 (Scheme 7) [9]. 

Titanocene methylidene phosphine complexes Cp2Ti= CH2L (L = PMes, PMe2Ph, PEts) have been generated by the thermolysis of titanocene metallacyclobutanes in the presence of excess phosphine (equation 9). These titanocene methylidene phosphine complexes have been found to react with alkenes, alkynes, and CO to form metallacyclobutanes, metallacyclobutenes, and a ketene complex, respectively. The phosphine ligand is labile, and an equilibrium mixture is rapidly established if a second phosphine is added. ... 

Butadienoate esters undergo AICI3 and EtAlCh catalyzed stereospecific [2 + 2] cycloadditions with a wide variety of alkenes to give alkyl cyclobutylideneacetates in good yield. The stereospecificity and ratios of ( )- and (Z)-isomers suggest a [ 2 + v2a] cycloaddition of the ester-Lewis acid complex to the alkene analogous to the cycloaddition of ketenes with alkenes. Similar results are obtained with methyl 2,3-pentadienoate, methyl 4-methyl-2,3-pentadienoate and methyl 2-methyl-2,3-butadi-... 

Complex reactions involving insertion of a P-X bond, O transfer from C to P, and ring closure occur when PhPCla reacts with jS-hydroxyketones . In contrast, ketene reacts with PBra to yield only a single product involving insertion into the alkene linkage . This result parallels the addition of SF5CI into ketene . 

In the presence of alkynes and PPhs, no ReCp 02( -RCsCR) complex is formed, as in the ReMeOs case. Cycloaddition of ketenes to (126) and (127) gives the Re species (128) and (129). No glycolates are formed in similar reactions with alkenes, in contrast to Tc analogs and OSO4,... 

The reaction of carbene complexes with enamines has been found to give cyclopropanes in low yield (Dorter et at., 1974). Reaction of carbene complexes with l-vinyl-2-pyrrolidone in the absence of added CO pressure gives alkene scission products (Dorrer and Fischer, 1974a), whereas at high CO pressure products possibly derived from ketenes are obtained (Dorrer and Fischer, 1974b) (see Scheme 11). 

Ketenes and isocyanates also undergo facile [6 + 2]-photocycloaddition with metal complexed cyclic polyenes. Irradiation of 232 in the presence of diphenylketene gave 256 in good yield (Scheme 58)120. This should be contrasted with the normal behavior of ketenes toward alkenes, which typically involves [2 + 2]-cycloaddition. Isocyanates such as 257 work as well. The adducts are produced in high yields and have considerable potential in synthesis. 

In 1994, Thomas reported146,147 that alkenes also underwent an addition reaction with vinylketene complexes that differed crucially in the loss of the ketene carbonyl fragment. Complexes 252.a-252.d were isolated as yellow crystalline solids. Clearly this suggests that the process occurs by a mechanism different from the alkyne insertion, and this will be discussed... 

A wide range of olefins can be cyclopropanated with acceptor-substituted carbene complexes. These include acyclic or cyclic alkenes, styrenes [1015], 1,3-dienes [1002], vinyl iodides [1347,1348], arenes [1349], fullerenes [1350], heteroare-nes, enol ethers or esters [1351-1354], ketene acetals, and A-alkoxycarbonyl-[1355,1356] or A-silyl enamines [1357], Electron-rich alkenes are usually cyclopropanated faster than electron-poor alkenes [626,1015],... 

The reactivity of the produced complexes was also examined [30a,b]. Since the benzopyranylidene complex 106 has an electron-deficient diene moiety due to the strong electron-withdrawing nature of W(CO)5 group, 106 is expected to undergo inverse electron-demand Diels-Alder reaction with electron-rich alkenes. In fact, naphthalenes 116 variously substituted at the 1-, 2-, and 3-positions were prepared by the reaction of benzopyranylidene complexes 106 and typical electron-rich alkenes such as vinyl ethers, ketene acetals, and enamines through the Diels-Alder adducts 115, which simultaneously eliminated W(CO)6 and an alcohol or an amine at rt (Scheme 5.35). 

Mixed ketene silyt methyl acetals 46 react with electron-deficient alkenes 47 in the presence of zinc(II) or titaniurn(IV) salts giving cycloadducts 48.1 y 20 These reactions are of limited use as complex mixtures of stereoisomers are formed.19... 

The advantage of using the photocycloaddition of pentacarbonylcarbenechromium complexes over the ketene cycloaddition method is the absence of ketene dimerization and the avoidance of use of excess alkene in the former method. Also, the mild reaction conditions associated with the use of chromium carbene complexes avoids epimerization and thermodynamic equilibration of 2-monosubstituted cyclobutanones. 

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