Metallo-carbene

Some of these intermediates are analogous to those proposed by Chauvin in olefin metathesis ( Chauvin s mechanism ) [36]. They can be transformed into new olefins and new carbene-hydrides. The subsequent step of the catalytic cycle is then hydride reinsertion into the carbene as well as olefin hydrogenation. The final alkane liberation proceeds via a cleavage of the Ta-alkyl compounds by hydrogen, a process already observed in the hydrogenolysis [10] or possibly via a displacement by the entering alkane by o-bond metathesis [11]. Notably, the catalyst has a triple functionality (i) C-H bond activation to produce a metallo-carbene and an olefin, (ii) olefin metathesis and (iii) hydrogenolysis of the metal-alkyl. 

We use the term carbene transfer to represent the net chemical transformation and not necessarily to imply that this is a reactive intermediate (either a free carbene or a metallo-carbene). 

Besides the conventional methods, the metallo-carbene route to access cyclic compounds has become a versatile tool in sugar chemistry. Synthesis of stavudine 112, an antiviral nucleoside, from an allyl alcohol [101] is realized by a Mo(CO)5-mediated cyclization reaction (O Scheme 26). Molybdenum hexacarbonyl smoothly reacts with the triple bond of 113 to generate the intermediate Mo-carbene, which undergoes a clean cyclorearrangement to yield the furanoid glycal 114. Alkynol isomerization is effected by group-6 transition metal carbonyl complexes [102]. 

According to the nature of alkene metathesis, this general reaction can be divided into different subgroups cross-metathesis (CM), ring-closing metathesis (RCM), ringopening metathesis polymerization (ROMP), and acyclic diene metathesis polymerization (ADMET). " From an industrial perspective, a more cost-effective method for alkene metathesis is to generate the metallo-carbene in situP This reaction has been extensively reviewed. ... 

With regard to 1,4-eyelohexadiene (Table 1.8), all these above groups realized the intermoleeular C—H bond insertion reaction with the Ir-carbenoids derived from a-diazo esters 88 to achieve the corresponding adducts 89 in excellent yields and enantioselectivity. It is worth noting that, in Katsuki s work, the methyl a-diazopropionate (88, R = Me) was recognized as a feasible metallo-carbene precursor in this reaction. Historically, intermolecular reactions with... 

Probably, PhsP being in the coordinating sphere of metallo-carbene active centre, prevents a reverse ring-closing reaction of Z-isomer retaining 1,6Z-undecadiene unchanged in the reaction mixture. 

The metallo-carbene catalysts for ring-closing metathesis (RCM) reactions are ideal for this purpose. As a result of the mild reaction conditions and lack of competition with the coordination chemistry of the intermediate helical complex, the RCM strategy is particularly well adapted to the synthesis of copper(I)-complexed catenanes. A natural extension of this work was the preparation of a trefoil knot following the strategy depicted in Figure 11. 

In general, cyclooleflns having substituents connected directly with double bond are not active in metathesis because of higher stability of metallo-carbene intermediates [153], However, probably in the case of the cyclobutene derivative, a high thermodynamic driving force became a predominate factor. 

In the same line, Mansny showed that CCI4 is a sonrce of dichlorocarbene ligand, which also shows how CCI4 conld be metabolized by cytochrome P450 and revealed for the first time the biochemical dimension of metallo-carbenes - ... 

We should emphasize that Fischer-Tropsch and Schrock type of carbenes are extreme representations of metal carbenoids [17-21]. Furthermore, representation of the metal-carbene bonds in terms of singlet and triplet carbenes is just a convenient way to describe these bonds. For example, both types of metallo-carbenes can be explained on the basis of a singlet carbene fragment very strong it-back donation would lead to the Schrock-type of metallocarbene, while no back donation or a weak n-back donation corresponds to the Fischer-Tropsch type of metallocarbene. Of course, it is also possible to understand both types of metallo-carbenes solely on the basis of the triplet carbene fragment. Once again, the use of either both models or only one of them to understand the nature of the M-carbene bond is just a matter of preference. 

Scheme 2.6 Schematic presentation of the whole spectrum of metal-carbene bonds in metallocarbenes. Here, we based our presentation on the Fischer-Tropsch type of metallo-carbene.

When alkenes are allowed to react with certain catalysts (mostly tungsten and molybdenum complexes), they are converted to other alkenes in a reaction in which the substituents on the alkenes formally interchange. This interconversion is called metathesis 126>. For some time its mechanism was believed to involve a cyclobutane intermediate (Eq. (16)). Although this has since been proven wrong and found that the catalytic metathesis rather proceeds via metal carbene complexes and metallo-cyclobutanes as discrete intermediates, reactions of olefins forming cyclobutanes,... 

For reviews involving carbenes and metallo carbenoids, see (a) Padwa, A. Hombuckle, S. 

Propagation involves coordination of the -olefin to the surface carbene giving rise to a metallo-cyclobutane transition state followed by (i-H transfer. Similar reaction has been observed recently by Pettit (20) who selectively obtained propylene from the reaction of ethylene with an octacarbonyl-f -methylene-diiron complex. 

J. Amer. Chem. Soc., 101, 5451 (1979) metallo-cyclopen-tane may lead to metallo-cyclobutane+carbene by ring contraction. The opposite path which involves insertion of a carbene in a metallo-cyclobutane is not unreasonable. 

Carbenes ( CR2) are the most reactive organic intermediates. The Simmons-Smith reaction (see chapter 6) tamed the extraordinarily reactive methylene ( CH2) in the form of a metal-carbenoid ( ICH2ZnI ). In 1964, Ernest Otto Fischer made the first metallocarbenes [e.g., (C0)5W=CCH3(0CH3)] and they were stable enough for X-ray diffraction studies on crystalline samples. Although unsuspected at the time, metallo-carhenes would hold the secret to the magic of olefin metathesis. 

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