Ruthenium-vinylidene complex

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). 

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)). 

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... 

Highly reactive organic vinylidene and allenylidene species can be stabilized upon coordination to a metal center [1]. In 1979, Bruce et al. [2] reported the first ruthenium vinylidene complex from phenylacetylene and [RuCpCl(PPh3)2] in the presence of NH4PF6. Following this report, various mthenium vinylidene complexes have been isolated and their physical and chemical properties have been extensively elucidated [3]. As the a-carbon of ruthenium vinylidenes and the a and y-carbon of ruthenium allenylidenes are electrophilic in nature [4], the direct formation of ruthenium vinylidene and ruthenium allenylidene species, respectively, from terminal alkynes and propargylic alcohols provides easy access to numerous catalytic reactions since nucleophilic addition at these carbons is a viable route for new catalysis (Scheme 6.1). 

Ruthenium vinylidene species can be transformed into small carbocyclic rings via carbocyclization reactions. Ruthenium vinylidene complex 2, generated from the electrophilic reaction of alkyne complex 1 with haloalkanes, was deprotonated with "BU4NOH to give the unprecedented neutral cyclopropenyl complex 3 (Scheme 6.2) [5]. Gimeno and Bassetti prepared ruthenium vinylidene species 4a and 4b bearing a pendent vinyl group when these complexes were heated in chloroform for a brief period, cyclobutylidene products 5a and Sb formed via a [2 + 2] cycloaddition between the vinylidene Ca=Cp bond and olefin (Scheme 6.3) [6]. 

Finn et al. reported the first instance of a metal-catalyzed aromatization of enediynes via vinylidene intermediates [7]. Aromatization of unstrained enediynes is knovm as Bergman cyclization and occurs at 200-250 °C via diradical intermediates [8]. Ruthenium-vinylidene complex 7 was formed when 1,2-benzodiyne 6 was treated with RuCp(PMe3)2Cl and NH4PF6 at 100 °C, ultimately giving good naphthalene product 8 ingood yields (Scheme 6.4). This process mimics Myers-Saitocyclizationof5-allene-3-... 

The preparation of optically active ruthenium vinylidene complexes with the objective of performing the asymmetric version of this reaction has been attempted but has led to moderate enantioselectivities [50, 51]. 

The formation of metal vinylidene complexes directly from terminal alkynes is an elegant way to perform anti-Markovnikov addition of nucleophiles to triple bonds [1, 2], The electrophilic a-carbon of ruthenium vinylidene complexes reacts with nucleophiles to form ruthenium alkenyl species, which liberate this organic fragment on protonolysis (Scheme 1). 

Coupling of C=C andC=C Bonds via Ruthenium Vinylidene Complexes... 

From ruthenium vinylidene complexes, three main chemical processes leading to catalytic reactions are involved ... 

Well-defined ruthenium vinylidene complexes are efficient catalyst precursors for the ring-opening metathesis polymerization (ROMP) of cyclic olefins (Fig. 9). Most of them are neutral 16-electron complexes of the type RuC12(L)2(=C=CHR) (LI) [65-68] and the more active precursors contain... 

The neutral 18-electron RuCl(Tp)(PPh3)(=C=CHPh) and RuCl(Cp )(PPh3)-(=C=CHPh) complexes also catalyse the ROMP of norbornene and their catalytic activity is enhanced by addition of a Lewis acid [66]. Other ruthenium vinylidene complexes featuring a bidentate (N,0) (M) [71] or a tridentate (N,N,N) (N) ligand [73] are also precatalysts of metathesis polymerisation. 

In the presence of a base (Et3N) the ruthenium vinylidene complex RuCl(Cp )(PPh3)(=C=CHPh) promotes the selective cross-coupling of a bulky terminal alkyne with internal alkynes at room temperature to yield functionalized enynes (Eq. 10) [77]. 

Although the reaction of copper acetylides with transition metal halides has been successfully applied to the preparation of a variety of transition metal acetylides (64), the generation of copper-complexed derivatives is not unprecedented (65). A simpler and more general route to ruthenium acetylide complexes involves the deprotonation of ruthenium vinylidene complexes as described in Section VI,C. 

The interaction of an alkyne with (tj5-C5H5)(PR3)2RuX can result in the formation of a wide variety of ruthenium complexes. The nature of the products formed depends on the conditions used and the type of alkyne reacted. Reactions between I and terminal alkynes in the presence of ammonium hexafluorophosphate lead to the formation of cationic monosubstituted ruthenium vinylidene complexes in high yield, as shown for phenylacetylene in Eq. (61) (4,67,68). 

The formation of complexes 109 has been shown to proceed via a vinylidene ruthenium intermediate (112), which has been indirectly isolated by protonation of an acetylide-ruthenium complex (112). Arene ruthenium vinylidene complexes 113 appear to be much more reactive than their isoelectronic (C5H5)(R3P)2Ru=C=CHR+ complexes (63,66). 

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