Cyclopropane ketenes

CycloadditionJ Reaction of cyclopropenone ketals with alkenes bearing only one electron-withdrawing group results in unstable cyclopropane ketene ketals, which are not isolated, but rather are converted into m-disubstituted cyclopropanes by acid hydrolysis. 

Cyclopropane ketenes undergo trimerization to give symmetrical trimers resulting from cycloaddition across the C=C bond under some conditions (see Section 4.1.2). The symmetrical trimer 84 is also obtained by heating the symmetric ketene dimer 83 in the presence of sodium methoxide . 

Allylic acetates react with ketene silyl acetals. In this reaction, in addition to the allylated ester 468, the cyclopropane derivative 469. which is formed by the use of bidentate ligands, is obtained[303]. Formation of a cyclopropane derivative 471 has been observed by the stoichiometric reaction of the 7r-allylpal-... 

There has been new information on the products of photolysis of derivatives of compound 1. Low temperature irradiation of the ester 254 gives a ketene (93JACS8621) the isolation of an isomeric ketene from a 3-pyridyldiazo ester suggests the involvement of the open chain form 255. Photolysis of the 3-phenyl derivative 256 in the presence of cyclopentadiene gives exo and endo cyclopropanes and a dipyridylstilhene, suggesting the intermediacy of the carhene 257 (99JOC6635). 

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. 

Alkoxycarbene complexes with unsaturation in the alkyl side chain rather than the alkoxy chain underwent similar intramolecular photoreactions (Eqs. 10 and 11) [60]. Cyclopropyl carbene complexes underwent a facile vinyl-cyclopropane rearrangement, presumably from the metal-bound ketene intermediate (Eqs. 12 and 13) [61]. A cycloheptatriene carbene complex underwent a related [6+2] cycloaddition (Eq. 14) [62]. 

Whereas metal-catalyzed decomposition of simple diazoketones in the presence of ketene acetals yields dihydrofurans 121,124,134), cyclopropanes usually result from reaction with enol ethers, enol acetates and silyl enol ethers, just as with unactivated alkenes 13). l-Acyl-2-alkoxycyclopropanes were thus obtained by copper-catalyzed reactions between diazoacetone and enol ethers 79 105,135), enol acetates 79,135 and... 

Considering the above-mentioned facts, according to which simple diazoketones yield dihydrofurans with ketene acetals but cyclopropanes with enol ethers, one exports an interlink between these clear-cut alternatives to exist, i.e. substrates from which both cyclopropanes and dihydrofurans result. In fact, providing an enol ether with a cation-stabilizing substituent in the a-position creates such a situation The Rh2(OAc)4-catalyzed decomposition of -diazoacetophenone in the presence of ethyl vinyl ether produces mainly cyclopropane 82 (R=H), but a small amount of dihydro-... 

Claisen rearrangement. As for the mechanism, the reaction begins with intramolecular cyclopropanation the resulting bicyclo[2.1.0]pentan-2-one then undergoes fragmentation to a p,y-unsaturated ketene which finally is trapped by the added alcohol to afford a p,y-unsaturated ester (Scheme 41). The intermediates could be observed in selected cases. 

In a noteworthy series of studies, Herndon has shown that cyclopropylcarbenes can be used as four-carbon components in molybdenum- and tungsten-mediated [4 + 2 + l]-reactions with alkynes and carbon monoxide (CO). These reactions give cycloheptadienones in moderate yields and with moderate selectivity (Equations (26)—(28)). The mechanism of this reaction is proposed to proceed through a series of steps involving metathesis, GO insertion, ketene formation, cyclopropane cleavage, and finally reductive elimination (Scheme 43).133... 

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-... 

Chiral dirhodium(II) catalysts with carboxylate or carboxamidate ligands have recently been developed to take advantage of their versatility in metal carbene transformation, and these have now become the catalysts of choice for cyclopropanation. Chiral carboxylate ligands 195,103 196,104 and 197105 have been used for tetrasubstitution around a dirhodium(II) core. However, the enantioselectivity in intermolecular reactions with simple ketenes is marginal. 

Cyelobutanone has been prepared by (1) reaction of diazomethane with ketene,4 (2) treatment of methylenecyclobutane with performic acid, followed by cleavage of the resulting glycol with lead tetraacetate,s (3) ozonolysis of methylenecyclobutane, (4) epoxidation of methylene-cyclopropane followed by acid-catalyzed ring expansion,7 and (5) oxidative cleavage of cyclobutane trimethylene thioketal, which in turn is prepared from 2-(co-chloropropyl)-l,3-dithiane.8... 

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. 

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 reaction of Cjq with silylated nucleophiles [47] requires compounds such as silyl ketene acetals, silylketene thioacetals or silyl enol ethers. It proceeds smoothly and in good yields in the presence of fluoride ions (KF/18-crown-6) (Scheme 3.10). The advantage of the latter synthesis is the realization of the cyclopropanation under nearly neutral conditions, which complements the basic conditions that are mandatory for Bingel reactions. Reaction with similar silyl ketene acetals under photochemical conditions and without the use of F does not lead to methanofullerenes but to dihydrofullerene acetate [48]. 

Das aus l-tert.-Butyl-2-methoxycarbonyl-l-trimethylsiloxy-cyclopropan mit Titan(IV)-chlorid in situ gebildete Keten-Derivat reagiert mit N,N-Dimethyl-methaniminium-chlorid in Gegenwart von katalytischen Mengen Trifluormethansulfonsaure-trimethylsilylester unter Bildung von 5,5-Dimethyl-2-dimethylaminomethyl-4-oxo-hexansaure-methylester (89%) ... 

Carbonylcarbene reacts with oxetane in a complex manner, giving cyclopropane and propene by a deoxygenation process and ketene plus ethylene as apparent insertion-fragmentation products (equation 66). The reaction is carried out by irradiation of a solution of carbon suboxide in oxetane under these conditions, photodecomposition of intermediates is possible. The initial attack is believed to be at the ring oxygen atom to give a zwitterionic species (78JA6425). 

The carbenoid from Et2Zn/CH2I2 [17], particularly when generated in the presence of oxygen [18], is more reactive than the conventional Simmons-Smith reagents. The milder conditions required are suitable for the preparation of 1-[16, 19] or 2-alkoxy-l-siloxycyclopropanes [20], which are generally more sensitive than the parent alkyl substituted siloxycyclopropanes (Table 2). Cyclopropanation of silyl ketene acetals is not completely stereospecific, since isomerization of the double bond in the starting material competes with the cyclopropanation [19]. 

The diverse chemistry of carbenes is beyond the scope of this account, but a few typical reactions are shown here to illustrate the usefulness of the photochemical generation of these reactive species. A carbene can insert into a C—H bond, and this finds application in the reaction of an a-diazoamide to produce a P-lactam (5.29). Carbenes derived from o-diazoketones can rearrange to ketenes, and thus a route is opened up to ring-contraction for making more highly strained systems <5.301. Carbenes also react with alkenes, often by cycloaddition to yield cyclopropanes in a process that can be very efficient (5.31) and highly stereoselective (5.321. 

An added complication in the interpretation of long-wavelength ketene photolysis is demonstrated by Cundall s discovery of ketene sensitized cis-trans isomerization of the 2-butenes.33 As the pressure of olefin increases, the rate of ketene decomposition decreases and the rate of olefin isomerization increases. At high olefin concentrations part of the apparent nonstereospecificity of cyclopropane formation can thus result from stereospecific singlet addition to already isomerized olefin. 

In addition to the alkylations discussed above, some special reactions have been reported that enable the solid-phase synthesis of cycloalkanes. These include the intramolecular ene reaction and the cyclopropanation of alkenes (Figure 5.5 see also [44]). Cyclobutanes have been prepared by the reaction of polystyrene-bound carbanions with epichlorohydrin, and by [2 + 2] cycloadditions of ketenes to resin-bound alkenes. 

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