Metal carbene complexes 18-electron

The value of -NMR and 13C-NMR spectroscopy in characterizing transition metal carbene complexes was noted in Section III,B,2. The carbene carbon resonance is invariably found at low field (200-400 ppm) in the 13C-NMR spectrum, while protons attached to Ca in 18-electron primary and secondary carbene complexes also resonate at low fields. NMR data for some Ru, Os, and Ir alkylidene complexes and related compounds are given in Table V. 

Transition metal carbene complexes have broadly been classified into Fischer-type and Schrock-type carbene complexes. The former, typically low-valent, 18-electron complexes with strong 7t-acceptors at the metal, are electrophilic at the carbene carbon atom (C ). On the other hand, Schrock-type carbene complexes are usually high-valent complexes with fewer than 18 valence electrons, and without n-accepting ligands. Schrock-type carbene complexes generally behave as carbon nucleophiles (Figure 1.4). 

Electron-deficient (having less than 18 electrons) and coordinatively unsaturated (containing less than 6, usually only 4 or 5 ligands) metal carbene complexes are highly effective in metathesis.191-194... 

Fig. 15.20 Resonance forms for a transition metal carbene complex. Form (a) shows metal-carbon double bond character which results from donation of metal d electron density to an empty p orbital of carbon. Form (b) shows oxygen-carbon double bond character which results from donation of oxygen p electron density to an empty p orbital of carbon Form (W provides the dominant contribution.

Over the past 15 years the understanding of the mechanism of these reactions has been greatly enhanced through the preparation of metal carbene complexes, particularly of Mo, W and Ru, that are both electronically unsaturated (<18e) and coordinatively unsaturated (usually <6 ligands), and which can act directly as initiators of olefin metathesis reactions. The intermediate metallacyclobutane complexes can also occasionally be observed. Furthermore, certain metallacyclobutane complexes can be used as initiators. 

Among the first 18-electron (18e) Fischer-type metal carbene complexes to be used as part of an olefin metathesis catalyst system were W[=C(OMe)Et](CO)5 with BU4NCI (for pent-l-ene)79, and W[=C(OEt)Bu](CO)5 with TiCLt (for cyclopentene)80. These complexes may also be activated thermally, e.g. for the polymerization of alkynes81, or photochemically, e.g. for the ROMP of cycloocta-1,5-diene82. The essential requirement is that a vacancy be created at the metal centre to allow the substrate to enter the coordination sphere. Occasionally the substrate may itself be able to displace one of the CO ligands. 

TABLE 2. Examples of metal carbene complexes with a count of less than 18 electrons and their effectiveness as initiators of olefin metathesis0... 

Another example for methoxy functionalised imidazolium salts comes from the group of Cetinkaya [185,186] featuring an -alkyl tether. Cetinkaya etal.me the traditional route to transition metal carbene complexes employing the electron-rich olefins as carbene source [57-59], Thermal cleavage of the olefinic double bond in the presaice of the metal precursor complex yields the desired transition metal carbene complex (see Figure 3.66). Using this method, Cetinkaya et al. synthesised rhodium(l) [185,186] and ruthenium(ll) [185] complexes. 

The same authors then proceeded to synthesise the corresponding azolium salt based on the l,r-binaphthyl scaffold (see Figure 5.23) [79]. The same failure to synthesise either the free carbene or transition metal carbene complexes from it was estabUshed experimentally. Electronic destabilisation by a double annulation effect from the two phenyl (naphthyl) rings on the seven-membered azolium ring systan is indeed the most likely explanation. 

The reaction occurs well below the temperature at which most of the parent metal carbonyls exchange with free CO and so is a direct nucleophilic attack on coordinated CO, although it may alternatively proceed via a prior electron path. The resulting acyl anions can be isolated as their [R4N] " or [ (C6H5)3P 2N] salts but are reactive and are used directly in subsequent alkylations with organic halides, acetylenes, a-/i-unsaturated carbonyls and alkyloxonium salts to form organic condensation products or metal-carbene complexes. 

However, yields in the intermolecular cycloaddition reactions of vinylcarbene complexes, formed by intramolecular insertion of an alkynyl tethered metal carbene complex, are higher when molybdenum rather than chromium or tungsten carbene complexes are employed. Mild thermolysis (THF, 65 °C, 1 h) in the presence of ten equivalents of an electronically undemanding alkene directly leads to the 2-alkyl-2-(2-methoxycyclopentenyl)cyclopropanes 31. ... 

New evidence as to the nature of the intermediates in catalytic diazoalkane decomposition comes from a comparison of olefin cyclopropanation with the electrophilic metal carbene complex (CO)jW—CHPh on one hand and Rh COAc) / NjCHCOOEt or Rh2(OAc)4 /NjCHPh on the other . For the same set of monosubstituted alkenes, a linear log-log relationship between the relative reactivities for the stoichiometric reaction with (CO)5W=CHPh and the catalytic reaction with RhjfOAc) was found (reactivity difference of 2.2 10 in the former case and 14 in the latter). No such correlation holds for di- and trisubstituted olefins, which has been attributed to steric and/or electronic differences in olefin interaction with the reactive electrophile . A linear relationship was also found between the relative reactivities of (CO)jW=CHPh and Rh2(OAc) NjCHPh. These results lead to the conclusion that the intermediates in the Rh(II)-catalyzed reaction are very similar to stable electrophilic carbenes in terms of electron demand. As far as cisjtrans stereoselectivity of cyclopropanation is concerned, no obvious relationship between Rh2(OAc) /N2CHCOOEt and Rh2(OAc),/N2CHPh was found, but the log-log plot displays an excellent linear relationship between (CO)jW=CHPh and Rh2(OAc) / N2CHPh, including mono-, 1,1-di-, 1,2-di- and trisubstituted alkenes In the phenyl-carbene transfer reactions, cis- syn-) cyclopropanes are formed preferentially, whereas trans- anti-) cyclopropanes dominate when the diazoester is involved. 

The metal-carbene complexes postulated as intermediates in transition metal-catalyzed reactions of diazo compounds are electrophilic species (especially if they are derived from a-diazocarbonyl compounds). Accordingly, electron-rich olefins are the most suitable substrates for copper-catalyzed cyclopropanations, whereas electron-poor substrates such as a,P-unsaturated carbonyl compounds in general are not sufficiently reactive. 

Actual metal carbene complex catalysts can be divided into two broad classes, Fischer-type and Schrock-type . The Fischer-type carbene complexes are low-valent and generally characterized by the presence of one or two heteroatoms (O, N, or S) bonded to the carbene carbon. Such complexes do not normally initiate the chain metathesis of olefins, since they are both coordinatively and electronically (18e) saturated. However, they can sometimes be activated for metathesis by heating, or by reaction with a cocatalyst, or photochemically. Some examples are listed in Table 2.1. 

Stable metal carbene complexes, such as W[=C(OMe)Me](CO)5, were first prepared by Fischer, E.O. (1964). These 18-electron complexes can be activated as catalysts for the metathesis of pent-l-ene or the ROMP of cycloalkenes by the use of a cocatalyst, or by heat or UV irradiation see Table 2.1. For such complexes to become active as initiators of olefin metathesis it is necessary for a CO ligand to be displaced, allowing the substrate to enter the coordination shell and react with the metal carbene bond. For the ROMP of 1-methyl-rrans-cyclooctene initiated by W(=CPh2)(CO)5 at 50°C the Ph2C= end groups may be detected in the polymer by the UV absorption at 245 nm (Lee, S.J. 1976). 

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