Olefin—carbene

Low-coordinate species of the main group elements of the second row such as carbenes, olefins, carbonyl compounds (ketones, aldehydes, esters, amides, etc.), aromatic compounds, and azo compounds play very important roles in organic chemistry. Although extensive studies have been devoted to these species not only from the physical organic point of view but also from the standpoints of synthetic chemistry and materials science, the heavier element homologues of these low-coordinate species have been postulated in many reactions only as reactive intermediates, and their chemistry has been undeveloped most probably due to... 

In order to rationalize the catalyst-dependent selectivity of cyclopropanation reaction with respect to the alkene, the ability of a transition metal for olefin coordination has been considered to be a key factor (see Sect. 2.2.1 and 2.2.2). It was proposed that palladium and certain copper catalysts promote cyclopropanation through intramolecular carbene transfer from a metal carbene to an alkene molecule coordinated to the same metal atom25,64. The preferential cyclopropanation of terminal olefins and the less hindered double bond in dienes spoke in favor of metal-olefin coordination. Furthermore, stable and metastable metal-carbene-olefin complexes are known, some of which undergo intramolecular cyclopropane formation, e.g. 426 - 427 415). 

Detection of propagating metal-carbene-olefin complexes. 1508... 

It has generally been assumed that in olefin metathesis reactions the olefin first coordinates to the metal carbene complex, en route to the formation of the intermediate metallacyclobutane complex, and that after cleavage of this intermediate the newly formed double bond is temporarily coordinated to the metal centre. A number of stable metal-carbene-olefin complexes are known see elsewhere116,117 for earlier references. They are mostly stabilized by chelation of the olefin and/or by heteroatom substituents on the carbene, although some have been prepared which enjoy neither of these modes of stabilization118,119. 

The only direct evidence for the presence of metal-carbene-olefin intermediates in catalytic metathesis systems comes from a study of the interaction of the tungsten cyclopentylidene complex 27 with cycloalkenes such as cycloheptene 28 in CD2CI2. When these are mixed at —96 °C and the temperature raised to between —53 and —28 °C, no polymerization occurs but the 13C NMR spectrum contains additional resonances which may be assigned to the metal-carbene-olefin complex 29. The line intensities show that the equilibrium 7 moves to the right as the temperature is lowered120. 

On the question of the transitory existence of metal-carbene-olefin intermediates, for which there is kinetic evidence in one system144 and spectroscopic evidence in another (see Section III.B.4)120, MO calculations do not reveal a potential-energy-well intermediate between the reactants Ti(=CH2)(Cl)2 + CH2=CH2 and the product metallacyclobutane, although the metal-carbene-olefin configuration does have an intermediate energy in the overall exothermic reaction134,145 similarly for the reaction of Mo(=CH2)(C1)4 with CH2=CH2133. 

Keywords Homogeneous catalysis N-heterocyclic carbenes Olefin metathesis ... 

Scheme 6.6 Coupling of chlorobenzene and styrene catalysed by Pd carbene-olefin complexes...

Takahashi and coworkers [170] synthesized the antifungal diterpene casbene (287) by using carbene-olefin coupling for ring closure. As shown in Scheme 96, the... 

Figure 4.22 Synthesis of a chiral ruthenium(ll) carbene olefin metathesis catalyst on a 1,V-biphenyl scaffold.

Casey and co-workers 178, 179) also studied the photochemistry of olefin-substituted carbene complexes and observed CO dissociation. For example, irradiation of a 1.38 1 mixture of the Z and E isomers of 145 led to CO substitution by the allyl-carbene group to give the isolable carbene-olefin complex 146 [Eq. (138)], a reaction which demonstrates CO dissociation. (Z)-145 has the proper stereochemistry to yield 146 upon photoinduced CO loss. However, the E isomer does not. Yet, the authors observed simultaneous disappearance of the E isomer upon irradiation of... 

On the other hand, late transition metal-based catalyst systems that had been identified by the early 1990s were characterized by low activity but high functional group tolerance, especially toward water and other protic solvents. These features led to reinvestigations of ruthenium systems and, ultimately, to the preparation of the first well-defined, ruthenium-carbene olefin metathesis catalyst (PPh3)2(Cl)2Ru=CHCH=Ph2 (Ru-1) in 1992 [5]. 

Doyle has put forward arguments against the intermediacy of such complexes in catalytic cyclopropanation . Firstly, metal coordination activates the alkene to nucleophilic attack. Hence, an electrophilic metal carbene would add only reluctantly or not at all. Secondly, the stable PdCl2 complexes of dienes 8 and 428 do not react with ethyl diazoacetate, even if Rh fOAc) or PdCljfPhCbOj is added. The diazoester is decomposed only when it is added to a mixture of the Pd complex and excess diene. These results exclude the metal-carbene-olefin intermediate, but they leave open the possibility of metal carbene interaction with an uncomplexed olefin molecule. The preferred formation of exo-cyclopropanes in the PdCyPhCN) -catalyzed reactions between 8 and N2CHCOOEt or N2CPh2, with exo. endo ratios virtually identical to those observed upon cyclopropanation of monoolefin 429, also rule out coordination of a palladium carbene to the exocyclic double bond of 8 prior to cyclopropanation of the endocyclic double bond. 

While the first process is likely in the case of iridium, nickel, and cobalt, it should not be so easy on platinum, because of its competition with carbene-olefin isomerization (see Section III, Scheme 29). We believe that the only way of explaining why 1,2-dicarbenes may account for the hydrogenolysis of cyclic hydrocarbons (Scheme 34), but only for a minor part for the hydrocracking of acyclic hydrocarbons, is the competition, for the latter, between carbene-dicarbene formation and carbene-olefin isomerization. Carbene-olefin interconversions are unlikely in the case of cyclic hydrocarbons, since a dicarbene species cannot transform into a 1,1,2,3-tetraadsorbed species (l-carbene-2,3-olefin) and further into a 1,1,3-triadsorbed species without C-C rupturing. 

The metallocyclobutane mechanism, already invoked to account for some aspects of the bond shift and cyclic mechanisms, allows one to rationalize the mechanism of 1-5 ring closure-ring enlargement. This mechanism is best represented by Schemes 61 and 62 for the aromatization of 1,1,3-trimethylcyclopentane and 2,2,4-trimethylpentane, respectively. It is emphasized that in the former case carbene-olefin recombination must be favored over carbene isomerization to di-Tt-adsorbed olefin, since xylenes are the major reaction products while 2,4-dimethylhexane is not detected (69). 

Schemes 61 and 62, which include ring opening by a metallocyclobutane mechanism followed by carbene-olefin addition, account for the following results ...

On platinum films, gemdisubstituted cyclopentanes initially produce much larger amounts of aromatics than methyl- and ethylcyclopentanes. Relative rates of aromatization are given in Scheme 63. This is readily explained by either methyl stabilization of the n-adsorbed olefin in the carbene-olefin species, or by the greater propensity for a,y,-bonding, that is, metallocyclobutane formation, across a quaternary center. 

The initial formation, from isopropylcyclopentane, of ethylcyclopentane, 1-methyl-l-ethylcyclopentane, and ethylbenzene on platinum films or Pt/Al2O3 catalysts 68, 131), illustrates these points concerning the metallocyclobutane mechanism and carbene-olefin addition (Scheme 64). 

The formation of wetn-labeled toluene can be explained neither by direct 1-6 ring closure, nor by cyclic-type isomerization of n-heptane to 3-methylhexane followed by 1-6 ring closure of the latter (94). We suggest that the abnormal aromatization process responsible for the formation of meta-labeled toluene is initiated by a dicarbene as in the nonselective mechanism A (see Section IV, Scheme 47). Aromatization is not influenced by the dispersion of the platinum on the support (758), so that it may be assumed that aromatization involves a single metal atom. Isomerization of the dicarbenes (7) to the dicarbenes (8) via rt-adsorbed cyclopentanes, followed by isomerization to the suitable carbene-olefin species (9), would result in 1-6 ring closure and aromatization (Scheme 69). 

However, metallocarbenes may isomerize in different ways to carbene-olefin species and further to adsorbed diolefins (Scheme 70). The latter may readily be desorbed as paraffins, and variations of the experimental con-... 

This reaction probably involves the participation of more than two carbon atoms and the extension of a delocalized n-electron system. The results are readily explained by the unequal stabilities of the two possible intermediates, involving carbene formation on the methyl group and on the alkyl group, respectively (Scheme 74). In the first case, demethylation easily occurs by metallocarbyne formation, while in the second case, stabilization by carbene-olefin isomerization (very fast compared to hydrocracking) prevents dealkylation. However, when the a-carbon atom in the alkyl group cannot dehydrogenate to give a metallocarbene, as for 1-methyl-1-terf-butylcyclohexane, dealkylation prevails over demethylation. 

In any event, all these results suggest a mechanism in which the reactive species are formed by the removal of two intraannular hydrogen atoms and are attached to the metal by two c carbon-metal bonds in the 1-5 position. Because of conformational strain, indeed, the dicarbenes or carbene-olefin precursors invoked for the 1-5 ring closures of type A and C cannot be formed with medium-sized compounds. 

Hot on the heels of the recent isolation of sesquicarene (63) five independent syntheses have been reported. In essence, all these syntheses have depended upon an intramolecular carbene-olefin cyclisation, viz. of (64, R = Me), (64, R = H), and (65). Only those syntheses which ensured the trans-A double bond in the precursors were stereospecific. Recently, Corey and Achiwa" have shown that mercuric iodide not only catalyses the diazo decomposition of (65) (i.e. cis,trans) but also isomerises the A double bond of the trans,trans analogue of (65). Thus, sesquicarene can now be obtained from commercially available farnesol trans,trans cis,trans, 1.5 1) in approximately 35%... 

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