Vinylidene complexes reactions

An extension of this synthetic methodology leads to /x-vinylidene complexes. Reaction of H2C=CBr2 with 4 leads to complex 106 (Fig. 19) (253,254), containing only terminal carbonyls, together with 4. The vinylic hydrogens are nearly coplanar with the Co—Co bond in the solid state. Successive reactions of 106 with acid and hydride give the alkylidene 107. 

In 1998, Wakatsuki et al. reported the first anti-Markonikov hydration of 1-alkynes to aldehydes by an Ru(II)/phosphine catalyst. Heating 1-alkynes in the presence of a catalytic amount of [RuCljlCgHs) (phosphine)] phosphine = PPh2(QF5) or P(3-C6H4S03Na)3 in 2-propanol at 60-100°C leads to predominantly anti-Markovnikov addition of water and yields aldehydes with only a small amount of methyl ketones (Eq. 6.47) [95]. They proposed the attack of water on an intermediate ruthenium vinylidene complex. The C-C bond cleavage or decarbonylation is expected to occur as a side reaction together with the main reaction leading to aldehyde formation. Indeed, olefins with one carbon atom less were always detected in the reaction mixtures (Scheme 6-21). 

Reaction of the carbonyl complex 26 with the mercury diazomethane 27 gives the highly reactive 17e intermediate carbyne complex 28 which dimerizes to form the / -biscarbyne complex 30. In this case, the intermediate terminal carbyne complex 28 has been trapped by reaction with the mercury diazomethane 29 to form the cyclic vinylidene complex 31. 31 was also characterized by a single crystal X-ray structure analysis. 

Clark and co-workers have reported reactions of Ir(III) cations with terminal alkynes in methanol in which alkoxycarbene complexes are formed (60). By analogy with a more extensively studied Pt(II) system (61), it has been concluded that cationic vinylidene complexes, e.g., 35, are reaction intermediates, e.g.,... 

Pyridines can be functionalized by a range of metal complexes, notably ruthenium analogs. Ruthenium vinylidene complexes promote the reaction of pyridines with silylalkynes in both a regio- and stereoselective manner, affording 2-styrylpyridines (Equation (78)). 

Ru-vinylidene complexes can be easily prepared by reaction of low-valent ruthenium complexes with terminal acetylenes. Treatment of the Ru(ii) complex 117 with phenylacetylene gave the Ru(iv)-vinylidene complex 118 in 88% yield (Scheme 41 ).60 The reaction of 118 with C02 in the presence of Et3N afforded selectively the Ru-carboxylate complex 120, probably via the terminal alkynide intermediate 119. 

A most significant advance in the alkyne hydration area during the past decade has been the development of Ru(n) catalyst systems that have enabled the anti-Markovnikov hydration of terminal alkynes (entries 6 and 7). These reactions involve the addition of water to the a-carbon of a ruthenium vinylidene complex, followed by reductive elimination of the resulting hydridoruthenium acyl intermediate (path C).392-395 While the use of GpRuGl(dppm) in aqueous dioxane (entry 6)393-396 and an indenylruthenium catalyst in an aqueous medium including surfactants has proved to be effective (entry 7),397 an Ru(n)/P,N-ligand system (entry 8) has recently been reported that displays enzyme-like rate acceleration (>2.4 x 1011) (dppm = bis(diphenylphosphino)methane).398... 

The alternative building scheme C2 + Q was used by Petasis and Hu [89], who reacted various aldehydes and ketones with alkenyltitanocene derivatives 172 to obtain the corresponding allenes 173 in high chemical yields (Scheme 2.54). The reaction probably proceeds via titanocene vinylidene complexes, which can also be trapped with alkynes and isocyanides to afford allenylketene imines [90],... 

Although the transformation of a primary alkyne into a vinylidene complex, 2, in presence of a number of transition metal systems is well reported [2, 3], only rare examples are known for the transformation of an alkene into a carbene complex [4, 5]. Given the increased role played by vinylidene and carbene complexes as key partners in metathesis reactions and related catalytic processes [6, 7], opening up new efficient and easy synthetic routes to such complexes is an important challenge. 

Obviously, the first intermediates in the syntheses with terminal alkynols are the vinylidene complexes [Ru(bdmpza)Cl(=C= CH(CH2) +iOH)(PPhg)] (n = 1, 2), which then react further via an intramolecular addition of the alcohol functionality to the a-carbon (Scheme 22), although in none of our experiments we were able to observe or isolate any intermediate vinylidene complexes. The subsequent intramolecular ring closure provides the cyclic carbene complexes with a five-membered ring in case of the reaction with but-3-yn-l-ol and with a six-membered ring in case of pent-4-yn-l-ol. For both products type A and type B isomers 35a-I/35a-II and 35b-I/ 35b-II are observed (Scheme 22, Fig. 22). The molecular structure shows a type A isomer 35b-I with the carbene ligand and the triphenylphosphine ligand in the two trans positions to the pyrazoles and was obtained from an X-ray structure determination (Fig. 25). 

Acyl complexes can also result from the reaction of terminal alkynes with cationic, hydrated complexes of iron (Entry 4, Table 2.7) [47]. An electrophilic vinylidene complex is probably formed as intermediate this then reacts with water and tautomerizes to the acyl complex. 

Electrophilic vinylidene complexes, which can be easily generated by a number of different methods [128], can react with non-carbon nucleophiles to yield carbene complexes (Figure 2.9 for reactions with carbon nucleophiles, see Section 3.1). 

Protonation of alkenyl complexes has been used [56,534,544,545] for generating cationic, electrophilic carbene complexes similar to those obtained by a-abstraction of alkoxide or other leaving groups from alkyl complexes (Section 3.1.2). Some representative examples are sketched in Figure 3.27. Similarly, electron-rich alkynyl complexes can react with electrophiles at the P-position to yield vinylidene complexes [144,546-551]. This approach is one of the most appropriate for the preparation of vinylidene complexes [128]. Figure 3.27 shows illustrative examples of such reactions. 

Although terminal alkynes can easily be converted into vinylidene complexes, vinylidene complexes have not yet been extensively used as intermediates in organic synthesis [150,546,576-578,944]. Some cyclization reactions, which might proceed by transient formation of vinylidenes, are listed in Table 3.23 (see also Sections 2.1.5.1 and 2.2.2). 

Water-soluble mthenium vinyUdene and aUenylidene complexes were also synthetized in the reaction of [ RuCl2(TPPMS)2 2] and phenylacetylene or diphenylpropargyl alcohol [29]. The mononuclear Ru-vinylidene complex [RuCl2 C=C(H)Ph)(TPPMS)2] and the dinuclear Ru-aUylidene derivative [ RuCl(p,-Cl)(C=C=CPh2)(TPPMS)2 2] both catalyzed the cross-olefin metathesis of cyclopentene with methyl acrylate to give polyunsaturated esters under mild conditions (Scheme 7.10). 

Attempts to produce vinylidene in the free state result in rapid reversion to ethyne, with a lifetime of 10 ° s [1]. As with many reactive organic intermediates, however, vinylidene can be stabilized by complexation to a metal center, using the lone pair for coordination and thus preventing the reversion to ethyne. Most 1-aIkynes can be converted into the analogous vinylidene complexes by simple reactions with appropriate transition metal substrates (Equation 1.2) ... 

Much of the chemistry of metal-vinylidene complexes has been summarized in several reviews [11-14] and the following will merely summarize the main preparative methods and survey the reactions of many of the metal complexes so obtained. Complexes of most transition metals have been described, although most work has been developed using electron-rich ruthenium derivatives, which have been used in... 

There is not sufficient space to discuss all vinylidene complexes which have been reported, for example over 200 crystal structures are listed in the CCDC. Consequently, this article largely concentrates on the chemistry of metal vinylidene complexes which has been described since 1995. Vinylidene complexes are generally available for the metals of Groups 4—9, with several reactions of Group 10 alkynyls being supposed to proceed via intermediate vinylidenes. However, few of the latter compounds have yet been isolated. This chapter contains a summary of various preparative methods available, followed by a survey of stoichiometric reactions of vinylidene-metal complexes. A short section covers several non-catalytic reactions which are considered to proceed via vinylidene complexes. The latter, however, have been neither isolated nor detected under the prevailing conditions. 

This is the most common route to vinylidene complexes and occurs in reactions of the 1 -alkynes with metal complexes, preferably with labile neutral or anionic ligands, which give neutral or cationic complexes, respectively. In the latter case, halide is commonly extracted, either by spontaneous displacement by a polar solvent, or by using sodium, silver or thallium salts. 

The reactions on Rh/Ir usually proceed via oxidative addition to give hydrido (alkynyl) complexes, which then undergo 1,3-H shifts to form the vinylidene complexes. In general, a unimolecular mechanism has been considered to be operative. Recent studies of RhCl(PPr 2R)2 (R = C=NCBu =CHNMe) complexes have shown a remarkable acceleration of the isomerization, with the =C=CHBu complex being formed within seconds [32]. Suitable cross-over experiments showed that a bimolecular mechanism, earlier suggested by some experimental and computational results [33], did not operate. 

The following are some examples of reactions which have produced vinylidene complexes but are either not of general application or have not been further developed. Oxygen atom transfer occurs in reactions of NbH (t] — OC — CPh2) Cp 2 with nitriles or isonitriles to give isocyanates and Nb(=C=CPh2)Cp 2 [260]. Metathesis of Ph(R) C=C=NPh (R = Me, aryl) with W(=CHPh)(CO)5 proceeds via W C(NPh=CHPh)=C (R)Ph (CO)s, which is converted to W =C=C(R)Ph (CO)5 by treatment with BF3. OMe2 [261]. 

Unusual iron-porphyrin vinylidene complexes were obtained from DDT [l,l-bis(4-chlorophenyl)-2,2,2-tricMoroethane] and Fe(tpp) [tpp = meso-tetraphenylporphinato (2-)] in the presence ofa reducing agent [10a, 264]. The derived N,N -vinylene-bridged porphyrin reacts with metal carbonyls [Fe3(CO)i2, Ru3(CO)i2] to break one or both N—C bonds with insertion of the vinylidene into an M—N bond. While the iron complex was formed in 90% yield, the reaction with Ru3(CO)i2 afforded three products, the vinylidene being formed in only 40% yield [265]. 

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