Carbene complexes with alkenes

Reaction of ayfi-Unsaturated Fischer Carbene Complexes with Alkenes,... 

Metal carbene complexes are also involved in metathesis (described in chapter 15). Exchange of carbene complexes with alkenes via a metallacyclobutane releases volatile alkenes such as ethylene with the formation of new alkenes. Ring closing metathesis is particularly favoured but normally leads to no new chiral centres. The simple Mo and Ru carbene catalysts described in chapter 15 cannot of course be used to induce asymmetry but a new generation of asymmetric Schrock 152 and Grubbs 153 catalysts can create asymmetry if a choice between two enantiotopic alkenes is offered.36... 

In 1982, Casey and co-workers reported the first reactions that could be considered hydrocarbations because they involved the direct C-H bond addition across the C-C double bond of alkenes. They showed that the cationic bridging iron methylidyne complex undergoes this type of reaction with alkenes with anti-Markovnikov regioselectivity. No other hydrocarbation reactions had been reported until recently, when Kubiak and co-workers reported hydrocarbation reactions of a nickel carbene complex with alkenes. Thus, the dicationic aminocarbene complex 31 reacts with ethylene, resulting in a complete conversion to the ethylcarbene complex (Scheme 1). 

The reactions of electrophilic carbene complexes with alkenes have attracted particular attention. For example, enantioselective synthesis of cyclopropanes has been achieved via the reaction of (155)-(2) with styrene... 

These carbene (or alkylidene) complexes are used for various transformations. Known reactions of these complexes are (a) alkene metathesis, (b) alkene cyclopropanation, (c) carbonyl alkenation, (d) insertion into C-H, N-H and O-H bonds, (e) ylide formation and (f) dimerization. The reactivity of these complexes can be tuned by varying the metal, oxidation state or ligands. Nowadays carbene complexes with cumulated double bonds have also been synthesized and investigated [45-49] as well as carbene cluster compounds, which will not be discussed here [50]. 

The currently known carbometallation chemistry of the group 6 metals is dominated by the reactions of metal-carbene and metal-carbyne complexes with alkenes and alkynes leading to the formation of four-membered metallacycles, shown in Scheme 1. Many different fates of such species have been reported, and the readers are referred to reviews discussing these reactions.253 An especially noteworthy reaction of this class is the Dotz reaction,254 which is stoichiometric in Cr in essentially all cases. Beyond the formation of the four-membered metallacycles via carbometallation, metathesis and other processes that may not involve carbometallation appear to dominate. It is, however, of interest to note that metallacyclobutadienes containing group 6 metals can undergo the second carbometallation with alkynes to produce metallabenzenes, as shown in Scheme 53.255 As the observed conversion of metallacyclobutadienes to metallabenzenes can also proceed via a Diels-Alder-like... 

Calculations [28] on the formation of cyclopropanes from electrophilic Fischer-type carbene complexes and alkenes suggest that this reaction does not generally proceed via metallacyclobutane intermediates. The least-energy pathway for this process starts with electrophilic addition of the carbene carbon atom to the alkene (Figure 1.9). Ring closure occurs by electrophilic attack of the second carbon atom... 

Cycloreversion of four-membered metallacycles is the most common method for the preparation of high-valent titanium [26,27,31,407,599-606] and zirconium [599,601] carbene complexes. These are usually very reactive, nucleophilic carbene complexes, with a strong tendency to undergo C-H insertion reactions or [2 -F 2] cycloadditions to alkenes or carbonyl compounds (see Section 3.2.3). Figure 3.31 shows examples of the generation of titanium and zirconium carbene complexes by [2 + 2] cycloreversion. 

Most electrophilic carbene complexes with hydrogen at Cjj will undergo fast 1,2-proton migration with subsequent elimination of the metal and formation of an alkene. For this reason, transition metal-catalyzed cyclopropanations with non-acceptor-substituted diazoalkanes have mainly been limited to the use of diazomethane, aryl-, and diaryldiazomethanes (Tables 3.4 and 3.5). 

However, the reaction was shown to be catalyzed by a methylidene tungsten-carbene complex rather than the Fischer tungsten carbene complex. They proposed that the reaction would proceed by [2 + 2] cycloaddition of the tungsten carbene complex with the alkyne in Equation (3), ring opening, and another [2 + 2] cycloaddition with the alkene moiety to finally give the cyclized product. 

The use of alkenes with chiral auxiliary groups leads to chiral cyclobutanones 4. Reaction yields of 50 67% and diastereomeric excesses of 86-97% were obtained for the 3-amidocy-clobutanones which were obtained from cycloaddition of the chromium carbene complexes with chiral ene carbamates (see also Section 1.3.4.3.3.).11... 

Various cyclic compounds are prepared by the reaction of these carbene complexes with various unsaturated compounds [78-80]. The metallacyclobutane 246 is generated by [2+2] cycloaddition with electron-rich alkenes, and its reductive elimination affords the cyclopropanes 247. 

Alkylidene complexes are of two types. The ones in which the metal is in a low oxidation state, like the chromium complex shown in Fig. 2.4, are often referred to as Fischer carbenes. The other type of alkylidene complexes has the metal ion in a high oxidation state. The tantalum complex is one such example. For both the types of alkylidene complexes direct experimental evidence of the presence of double bonds between the metal and the carbon atom comes from X-ray measurements. Alkylidene complexes are also formed by a-hydride elimination. An interaction between the metal and the a-hydrogen atom of the alkyl group that only weakens the C-H bond but does not break it completely is called an agostic interaction (see Fig. 2.5). An important reaction of alkylidene complexes with alkenes is the formation of a metallocycle. 

Whereas Fischer-type chromium carbenes react with alkenes, dienes, and alkynes to afford cyclopropanes, vinylcyclopropanes, and aromatic compounds, the iron Fischer-type carbene (47, e.g. R = Ph) reacts with alkenes and dienes to afford primarily coupled products (58) and (59) (Scheme 21). The mechanism proposed involves a [2 -F 2] cycloaddition of the alkene the carbene to form a metallacyclobutane see Metallacycle) (60). This intermediate undergoes jS-hydride elimination followed by reductive elimination to generate the coupled products. Carbenes (47) also react with alkynes under CO pressure (ca. 3.7 atm) to afford 6-ethoxy-o -pyrone complexes (61). The unstable metallacyclobutene (62) is produced by the reaction of (47) with 2-butyne in the absence of CO. Complex (62) decomposes to the pyrone complex (61). It has been suggested that the intermediate (62) is transformed into the vinylketene complex... 

Equations 10.47 to 10.51 demonstrate several examples of cyclopropanation reactions. As shown in equation 10.47, reaction of Group 6 Fischer carbene complexes with an electron-poor alkene—at a temperature high enough to promote CO dissociation—gives diastereomeric mixtures of cyclopropanes.71 Path a (Scheme 10.6) is the likely mechanism for this reaction, and the first of the two diastereomeric products tends to be favored. 

E. O. Fischer and K. H. Dotz, Chem. Ber., 1972, 105, 3966 for a more recent paper on reactions of Fischer carbene complexes with electron-rich alkenes, see W. D. Wulff, D. C. Yang, and C. K. Murray, PureAppl. Chem., 1988, 60, 137. 

Coupling of a Fischer carbene complex with an alkene can generate a vinylcarbene intermediate 12 via an insertion-rearrangement reaction, which can then further react with a double bond. For intramolecular reactions of tethered enynes 10, the products formed are bicyclic cyclopropanes 14 intermolecular reactions lead to cycloalkenylcyclopropanes. 

Fischer-type carbenes are known as potential carbene transfer reagents to electron-rich and electron-deficient alkenes. Little is known about the chemistry of carbene complexes with silicon substituents at the carbene C-atom, whereas complexes with germanium, tin, or lead have not yet been prepared. The tungsten-carbene complexes 6 react with an excess of ethyl vinyl ether to give l,2-diethoxy-l-(trialkylsilyl)cyclopropanes 7." Only the f-isomers were formed and similar results can be achieved by using the corresponding molybdenum or chromium complexes. On the other hand, no reaction takes place with 2,3-dihydrofuran or ethyl ( )-but-2-enoate. ... 

The catalytic cycle begins with a metal carbene complex (96), which may be added directly to the reaction mixture or is afforded rapidly upon displacement of a suitable ligand on the metal center by the alkene. Subsequent addition to this carbene complex by another alkene (97) forms a metallacyclobutane intermediate, which can readily dissociate to a metaUacarbene complex and an alkene. In some catalysts, the metal bound carbene species has a high rotation barrier, which allows interaction of an empty pz orbital of the carbene complex with the incoming alkene (Scheme 22). 

Carbamoyl complexes from metal carbonyls and amines 5.8.2.12.4 Carbanions reactions with alkene complexes 5.8.2.3,4 metal carbonyls 5.8.2.S.5 Carbene complexes by alkene metathesis 5.8.2.3.11 formation 5.8.2.8.5 Carbides alkali metal formation 5.10.2.1 bonding 5.10.2 formation 5.10.2 industrial uses 5.10.2 interstitial formation 5.10.2 Carbometallacycle formation 5.S.2.2.2 Carbometallacycles from n-allyl complexes 5.S.2.3.9 Carbon reaction with alkali metals 5.10.2.1 Carbon dioxide complexes formation 5.8.2.14.1 Carbon monoxide displacement by alkenes 5.8.2.3.1 Carbonyl complexes by ligand exchange 5.8.2.12.2 from carbon monoxide 5.8.2.12.1, 5.8.2.12.2... 

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