Transition-metal carbene complex

Finally, if one considers molecules with a carbon-carbon triple bond of the type that exists in alkynes, one realizes that there also are three possible paths to metal-containing derivatives (Fig. lc). One may construct r-compounds as discussed in the previous cases or one may use both 7r-bonds to synthesize complexes (11) in which the two metal-ligand bonds exist at a right angles to each other. Let us finally consider a cleaved triple bond in which one-half is replaced by a metal complex fragment thus we come to the carbyne complexes about which I shall report in the second part of this review. 

We quickly established that these complexes are not particularly stable. They tend to split off the carbene ligand with a simultaneous hydrogen shift thereby liberating aldehydes, a fact that Japanese investigators also discovered (13). Very recently, we learned how to prepare these hydroxy-carbene complexes analytically pure (14) Previously, these complexes, without isolation, had been successfully converted to the substantially more stable methoxycarbene compounds by treatment with diazomethane (12). 

The first X-ray crystal structure determination, carried out by Mills in cooperation with us 31) on pentacarbonyl[methoxy (phenyl) carbene ]-chromium(0), confirmed our originally postulated bonding concept. According to this concept, the carbene carbon atom is sp1 hybridized. It should therefore possess an empty p-orbital and be electron-deficient. 

Substantial compensation for this strong electron deficiency is provided by a pir—pir bond between one of the free electron pairs on the oxygen atom of the methoxy group and the unused p-orbital of the carbene 

Further important insights into the bonding relationships of the carbene complexes are made possible by a consideration of the vco bands of vibrational spectra (20, 35-37). As we know, the carbonyl ligands in metal-carbonyl complexes may be considered as very weak donor systems. They donate electron density from the carbon s free electron pair to unused orbitals on the metal atom, a process that formally leads to a negative charge on the metal. This is reduced primarily by a back donation 

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MII Transition Metal Carbene Complexes (F. R. Kreissel, ed.), VCH,... 

A decade after Fischer s synthesis of [(CO)5W=C(CH3)(OCH3)] the first example of another class of transition metal carbene complexes was introduced by Schrock, which subsequently have been named after him. His synthesis of [((CH3)3CCH2)3Ta=CHC(CH3)3] [11] was described above and unlike the Fischer-type carbenes it did not have a stabilizing substituent at the carbene ligand, which leads to a completely different behaviour of these complexes compared to the Fischer-type complexes. While the reactions of Fischer-type carbenes can be described as electrophilic, Schrock-type carbene complexes (or transition metal alkylidenes) show nucleophilicity. Also the oxidation state of the metal is generally different, as Schrock-type carbene complexes usually consist of a transition metal in a high oxidation state. 

Free carbenes can also be avoided by using transition metal-carbene complexes L M—CRR (L = a ligand, M = a metal),which add the group CRR to double bonds.An example is ... 

C. G. Kreiter and E. O. Fischer Transition metal carbene complexes new spectroscopic and preparative results, pp. 151-168 (45). 

Given the success of the Grubbs-type NHC-Ru catalysts in metathesis polymerisation (Chapter 3), it is somewhat surprising that more research has not been done on mid-transition metal carbene complexes for coordination-insertion polymerisation. At this stage however, there are only a few reported attempts with the metals Co, Fe and Ir. 

The importance of transition metal carbene complexes (compounds with formal M=C bonds) and of transition metal carbyne complexes (compounds with formal M=C bonds) is now well appreciated. Carbene complexes are involved in olefin metathesis (7) and have many applications in organic synthesis (2), while carbyne complexes have similar relevance to... 

Carbene Complex Geometries. Molecular orbital studies of the various conformations of several transition metal-carbene complexes have been undertaken by the groups of Fenske and Hoffmann (8,13). Of the two... 

Thus the reactivity of transition metal-carbene complexes, that is, whether they behave as electrophiles or nucleophiles, is well explained on the basis of the frontier orbital theory. Studies of carbene complexes of ruthenium and osmium, by providing examples with the metal in either of two oxidation states [Ru(II), Os(II) Ru(0), Os(O)], help clarify this picture, and further illustrations of this will be found in the following sections. 

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. 

In spite of the fact that silver(i) X-heterocyclic carbene complexes were widely employed as carbene-transfer reagents for the synthesis of other transition metal carbene complexes, their synthesis could also be achieved by the reaction of silver salts with relatively more labile carbene metal complexes, albeit rare. Complexes 71a-71c were reported to be synthesized from the reaction of the corresponding pentacarbonyl(carbene)chromium(i) complexes with silver(i) hexafluorophosphate in CDC13 under inert atmosphere (Scheme 17).117... 

Enyne metathesis is unique and interesting in synthetic organic chemistry. Since it is difficult to control intermolecular enyne metathesis, this reaction is used as intramolecular enyne metathesis. There are two types of enyne metathesis one is caused by [2+2] cycloaddition of a multiple bond and transition metal carbene complex, and the other is an oxidative cyclization reaction caused by low-valent transition metals. In these cases, the alkyli-dene part migrates from alkene to alkyne carbon. Thus, this reaction is called an alkylidene migration reaction or a skeletal reorganization reaction. Many cyclized products having a diene moiety were obtained using intramolecular enyne metathesis. Very recently, intermolecular enyne metathesis has been developed between alkyne and ethylene as novel diene synthesis. 

More recently, the catalytic activities of a large pool of transition-metal carbene complexes have been screened by means of ion-molecule reactions in tandem-MS experiments. [156-158] Different from the concepts and methods discussed so far, the latter experiments are not designed to study the fundamentals of mass spectrometry. Instead, sophisticated methods of modem mass spectrometry are now employed to reveal the secrets of other complex chemical systems. 

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). 

As will be discussed more thoroughly in Section 3.2.5, transition metal carbene complexes can mediate olefin metathesis. Because heteroatom-substituted carbene complexes are usually less reactive towards olefins than the corresponding nonheteroatom-substituted complexes, it is, e.g., possible to use enol ethers to terminate living polymerization or other types of metathesis reaction catalyzed by a non-heteroatom-substituted carbene complex. Olefin metathesis can also be used to prepare new heteroatom-substituted carbene complexes (Figure 2.15, Table 2.11). 

In addition to catalytically active transition metal complexes, several stable, electrophilic carbene complexes have been prepared, which can be used to cyclopropanate alkenes (Figure 3.32). These complexes have to be used in stoichiometric quantities to achieve complete conversion of the substrate. Not surprisingly, this type of carbene complex has not attained such broad acceptance by organic chemists as have catalytic cyclopropanations. However, for certain applications the use of stoichiometric amounts of a transition metal carbene complex offers practical advantages such as mild reaction conditions or safer handling. 

However, with substrates prone to form carbocations, complete hydride abstraction from the alkane, followed by electrophilic attack of the carbocation on the metal-bound, newly formed alkyl ligand might be a more realistic picture of this process (Figure 3.38). The regioselectivity of C-H insertion reactions of electrophilic transition metal carbene complexes also supports the idea of a carbocation-like transition state or intermediate. 

Carbenes and transition metal carbene complexes are among the few reagents available for the direct derivatization of simple, unactivated alkanes. Free carbenes, generated, e.g., by photolysis of diazoalkanes, are poorly selective in inter- or intramolecular C-H insertion reactions. Unlike free carbenes, acceptor-substituted carbene complexes often undergo highly regio- and stereoselective intramolecular C-H insertions into aliphatic and aromatic C-H bonds [995,1072-1074,1076,1085,1086],... 

A variety of transition metal-carbene complexes have been prepared and characterized. None of these are known to efficiently effect intermolecular C-H insertion. An electrophilic iron carbcne complex can, however, participate in intramolecular C-H insertions (Section I.2.2.3.2.I.). More commonly, transition metal complexes are used to catalyze intramolecular C-H insertion starting with a diazo precursor. In these cases, the intermediate metal carbene complexes are not isolated. 

In contrast to transition metal carbene complexes generated catalytically, those of the early transition metals are generally stable (Fischer-type carbenes) and undergo... 

W. D. Wulff, Transition Metal Carbene Complexes Alkyne and Vinyl Ketene Chemistry, in Comprehensive Organometallic Chemistry II, Vol. 12, L. S. Hegedus, Ed., Pergamon, Tarrytown, NY, 1995, Chapter 5.3, pp. 469/. 

For a survey see K. H. Dotz, H. Fischer, P. Hoffmann, F. R. Kreissl, U. Schubert and K. Weiss Transition Metal Carbene Complexes, Verlag Chemie, Weinheim, 1983, p, 176... 

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.

While the first transition metal carbene complex was reported in 1964134, the first cyclopropylcarbene complex salt [(CO)5Cr=C(chromium hexacarbonyl, followed by tetramethylammonium bromide135. Subsequent reaction with trimethyloxonium fluoro-borate gave methoxycarbene complex (CO)5Cr=C(OMe)(c-Pr) (equation 62)136. 

Table 7 Cyclopropanations Using Transition Metal-Carbene Complexes... 

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