Carbene complexes vinylic

In contrast to aryl carbene complexes, vinyl carbene complexes are known to yield only the benzannulation products [37]. For instance, carbohydrates [38], tetramethyl ketals of qui-nones [39], heterocycles, and oxacycloalkenylidene carbene complexes [40] have been used as part of a (cyclic) vinyl carbene complex. For example, complex 29 and diphenylethyne were converted to the acyl hydroquinone 30. Thus, 29 serves as a synthon for the (electron-poor) benzoyl vinyl carbene complex (Scheme 14) [40]. 

Seven-membered carbocycles are also available from the reaction of alkenylcarbene complexes of chromium and lithium enolates derived from methyl vinyl ketones [79b] (Scheme 65). In this case, the reaction is initiated by the 1,2-addition of the enolate to the carbene complex. Cyclisation induced by a [1,2]-migration of the pentacarbonylchromium group and subsequent elimination of the metal fragment followed by hydrolysis leads to the final cyclo-heptenone derivatives (Scheme 65). 

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]. 

CM products from vinylhalides are highly desirable especially because of the possible use in metal catalysed coupling reactions. Johnson and co-workers, performed detailed studies of the possible deactivation pathways [161]. The Fischer-carbene complexes of the vinyl halides have an increased stabihty compared to their alkylidene counterparts and the Fischer carbenes may be deactivated either by migration of the phosphine or by elimination of HX leading to a carbide. 

Diels-Alder additions to 17-19 parallel the known Diels-Alder additions to vinyl substituted Fischer carbene complexes (34). It appears that both M(CO)5 carbene moieties, where M is Cr or W, and (ferra-/3-diketonato)BF2 substituents can activate C=C double bonds toward Diels-Alder additions. 

Anionic alkoxy Fischer-type carbene complexes were shown to react with (Ph3P)AuGl to give a unique vinyl ether complex, which is bound in a quasi-771 fashion to the Cr/Mo centers. Upon treatment with triphenylphosphine, the gold-vinyl ether ligand can be liberated and isolated. This reaction thus gives access for the first time to aurated vinyl ethers (Scheme 57).38... 

Z,Z)-l,4-Dialkoxy-l,3-dienes can be readily prepared from propargyl ethers and molybdenum carbene complexes (equation 185)307. High stereoselectivity in this reaction may be due to the formation of stable vinyl hydride complex with the enol ether. 

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-... 

Intermolecular enyne metathesis has recently been developed using ethylene gas as the alkene [20]. The plan is shown in Scheme 10. In this reaction,benzyli-dene carbene complex 52b, which is commercially available [16b], reacts with ethylene to give ruthenacyclobutane 73. This then converts into methylene ruthenium complex 57, which is the real catalyst in this reaction. It reacts with the alkyne intermolecularly to produce ruthenacyclobutene 74, which is converted into vinyl ruthenium carbene complex 75. It must react with ethylene, not with the alkyne, to produce ruthenacyclobutane 76 via [2+2] cycloaddition. Then it gives diene 72, and methylene ruthenium complex 57 would be regenerated. If the methylene ruthenium complex 57 reacts with ethylene, ruthenacyclobutane 77 would be formed. However, this process is a so-called non-productive process, and it returns to ethylene and 57. The reaction was carried out in CH2Cl2 un-... 

Despite the undeniable synthetic value of the benzannulation reaction of aryl and alkenyl Fischer carbene complexes, the details of its mechanism at the molecular level remain to be ascertained. Indeed, although a relatively large number of theoretical studies have been directed to the study of the molecular and electronic structure of Fischer carbene complexes [22], few studies have been devoted to the analysis of the reaction mechanisms of processes involving this kind of complexes [23-30]. The aim of this work is to present a summary of our theoretical research on the reaction mechanism of the Dotz reaction between ethyne and vinyl-substituted hydroxycarbene species to yield p-hydroxyphenol. 

Hegedus and co-workers8 reported the synthesis of (4S,5R)-4,5-diphenyl-3-vinyl-2-oxazolidinone (the enantiomer of the compound prepared here) via the chromium carbene complex in a fair yield. This is an interesting method, but the procedure is complicated (e.g., low temperature, argon atomsphere) and the chromium waste must be disposed of in an appropriate way. On the other hand, this procedure, consisting of transacetalization9 and pyrolysis,10 is simple and safe. Optically active 3-vinyl-2-oxazolidinone is also used for the synthesis of (1 R,2S)-2-fluorocyclopropylamine11a b that is the key intermediate for novel antibacterial quinolonecarboxylic acids. 

In addition to the ring opening of cyclopropenes noted above, vinylketene complexes 103 have been prepared by (1) ligand initiated carbonyl insertion of vinyl carbene complexes 104 and (2) benzoylation of ,/3-unsaturalcd acyl ferrates 105 (Scheme 20)114. X-ray diffraction analysis of these vinylketene complexes indicates that the structure may be best represented as a hybrid between an /j4-dicnc type complex (103) and an jj3-allyl r/1 acyl complex (106). The Fe-Cl distance (ca 1.92 A) is shorter than the Fe-C2, Fe-C3, or Fe-C4 distances (ca 2.1-2.2 A)113a-C. In addition, the C—C—O ketene array is not linear (bend angle ca 135°). 

The heteroatom-substituted carbene complexes most frequently used in organic synthesis are carbonyl complexes of the type (CO)5M=C(X)R (M Cr, Mo, W X OR, NR2 R H, alkyl, aryl, vinyl, alkynyl, etc.). To some extent such complexes behave as carboxylic esters or amides, the (CO)5M=C group having electronic properties similar to those of a carbonyl group (Figure 2.16). 

Fig. 2.17. Typical reactions of Fischer-type vinyl- and alkynylcarbene carbene complexes [e.g. L (CO)5Cr],...

Aryl(dialkylamino)carbene chromium complexes do not yield aminonaphthols upon treatment with alkynes, but form indene derivatives. Vinyl(dialkylamino)car-bene complexes, however, react with alkynes to yield aminophenols as the main products if solvents of low nucleophilicity are used [335]. (2-Amino-1-vinyl)carbene complexes do not undergo benzannulation when treated with alkynes, but form cyclopentadienes or heterocycles instead [251]. 

The formation of cycloheptadienones from alkoxy(cyclopropyl)carbene complexes and alkynes (Entry 5, Table 2.24) [388,389] proceeds essentially by the same mechanism as the Dotz benzannulation reaction (see Figure 2.32). The cyclopropyl group participates in the electrocyclic rearrangement as the equivalent of a vinyl group. 

Particularly interesting is the reaction of enynes with catalytic amounts of carbene complexes (Figure 3.50). If the chain-length between olefin and alkyne enables the formation of a five-membered or larger ring, then RCM can lead to the formation of vinyl-substituted cycloalkenes [866] or heterocycles. Examples of such reactions are given in Tables 3.18-3.20. It should, though, be taken into account that this reaction can also proceed by non-carbene-mediated pathways. Also Fischer-type carbene complexes and other complexes [867] can catalyze enyne cyclizations [267]. Trost [868] proposed that palladium-catalyzed enyne cyclizations proceed via metallacyclopentenes, which upon reductive elimination yield an intermediate cyclobutene. Also a Lewis acid-catalyzed, intramolecular [2 + 2] cycloaddition of, e.g., acceptor-substituted alkynes to an alkene to yield a cyclobutene can be considered as a possible mechanism of enyne cyclization. 

C-H Insertions into vinylic C-H bonds are also a common reaction of electrophilic carbene complexes. Insertions into aromatic or heteroaromatic C-H bonds can proceed via cyclopropanation and rearrangement (Figure 4.6). 

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],... 

Experimental Procedure 2.2.8. [4 + 3] Cycloaddition of a Chromium Vinyl-carbene Complex to a 1-Azadiene rra/w-4-(2-Furyl)-2-methoxy-5-methyl-4,5-dihydro-3H-azepine... 

Cr(CO)3-coordinated hydroquinone from vinylic alkoxy pentacarbonyl chromium carbene (Fischer carbene) complex and alkynes. 

Addition of neutral nucleophiles to C affords the corresponding vinyl complexes, whereas anionic nucleophiles usually give carbene complexes. 

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