Ruthenium electrophilic addition reactions

The metal-carbon triple bond chemistry of ruthenium and osmium described in this article bears a close resemblance to the metal-carbon double bond chemistry of these elements as exemplified by the methylene complexes [26]. In both systems two structural classes are found, five coordinate (trigonal bipyramidal, formally zero oxidation state) and six coordinate (octahedral, formally +2 oxidation state). In both systems the five coordinate compounds exhibit multiple metal-carbon bonds which are rather non-polar and typically undergo addition reactions with electrophilic reagents. On the other hand the six coordinate compounds, both M=C and M=C, begin to show electrophilic character at the carbon centres especially in cationic complexes. Further development of the carbyne chemistry of ruthenium and osmium will depend upon the discovery of new synthetic methods allowing the preparation of a broader range of compounds with widely differing carbyne substituents. 

Unlike the above example, the majority of five-coordinate complexes appear to undergo oxidative-addition reactions in two separate steps. Additions to the bis phosphine complexes of ruthenium(O) (36) and osmium(O) (39) (XIV) are the most thoroughly studied examples of this generalization (see Section IV). The configurations of these complexes have been established by infrared spectroscopy and, in the case of the osmium complex, by X-ray diffraction (72), Addition of an electrophile A" " (for example H+, HgX+, or Br+ from Br2) to the five-coordinate complexes... 

This ready access to 77 -arene ruthenium(O) complexes has allowed the entry to other functionalized derivatives. Thus, electrophilic substitution reactions have been performed starting from [Ru(77" -GOD)(r7 -haloarene)] via sequential addition of LiBu and a suitable electrophile at low temperature. A wide range of electrophiles such as acyl chlorides, chloroformates, chlorophosphines, epoxides, ketones, 7-lactones, etc., have been involved in this transformation. As an example, lithiation of [Ru(77" -GOD)(77 -l,2-MeG6H4Br)] and further reaction with... 

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). 

Grignard additions, 9, 59, 9, 64 indium-mediated allylation, 9, 687 in nickel complexes, 8, 150 ruthenium carbonyl reactions, 7, 142 ruthenium half-sandwiches, 6, 478 and selenium electrophiles, 9, W11 4( > 2 in vanadocene reactions, 5, 39 Nitrites, with trinuclear Os clusters, 6, 733 Nitroalkenes, Grignard additions, 9, 59-60 Nitroarenes, and Grignard reactivity, 9, 70 Nitrobenzenes, reductive aminocarbonylation, 11, 543... 

Finally, ruthenium-catalyzed carbocyclization by intramolecular reaction of allylsilanes and allylstannanes with alkynes also led to the formation of vinyl-alkylidenecyclopentanes [81] (Eq. 60). This reaction is catalyzed by RuC13 or CpRuCl(PPh3)2/NH4PF6 in methanol. The postulated mechanism involves the coordination of the alkyne on the ruthenium center to form an electrophilic /f-alkyne complex. This complex can thus promote the nucleophilic addition of the allylsilane or stannane double bond. 

The electron-rich ruthenium center can render the bound X ligand nucleophilic as has been demonstrated by the reactions of (rj5-C5H5)-(PPh3)2Ru—C=N (30) with a variety of electrophiles. Baird and Davies have shown, for example, that addition of alkyl halides to 30 gives the corresponding isocyanides in moderate yield [Eq. (32)] (29). Other... 

The electron-rich ruthenium center of (t)5-C5H5)(PR3)2RuX combines with certain electrophiles E+ to generate highly reactive cationic addition complexes susceptible to elimination-substitution reactions, as shown in Eq. (50). 

Hydrogenation of propionaldehyde, catalyzed by various ruthenium-TPPTS complexes, was dramatically influenced by the addition of certain salts [122, 123]. Whereas in the absence of salts there was no reaction at 35 °C and 50 bar H2, in the presence of Nal TOFs of more than 2000 h 1 were determined. This was lowered to 300 h-1 when the sodium cation was selectively sequestered by a cryptant (4,7,13,16,21-pentoxa-l,10-diazabicyclo[5.8.8]tricosane). Obviously, the larger part of the salt effect belonged to the cation. It was concluded that electrophilic assistance by Na+ facilitated C-coordination of the aldehyde and formation of a hydroxy-alkyl intermediate. 

A one-step chemical procedure (i, in the scheme) has proved valuable. Thus cholesterol and its oxidation product, dehydroisoandrosterone have been selectively aromatised by reaction with the electrophilic ruthenium complex (Cp Ru ), t -cyclopentadienyl Ru. obtained by protonation in THF of [Cp Ru(OMe)j2 (1 mol) with triflic acid, CF3SO3H (2 mol). The addition of cholesterol (2 mol) in THF during 40 hours at 120°C (or in dichloromethane at 90 C) afforded estrone in 48% yield with evolution of methane (ref. 114). 

The reactions collected in Schemes 56 and 57 contain overwhelming evidence proving that the direction of the addition to alkynyl and alkenyl acetate complexes of osmium and ruthenium is determined by the electronic nature of the metallic center (Os or Ru), by the electronic properties of the ancillary ligands of the complexes, and also by the source of the electrophile. 

The electrophilic activation of terminal alkynes by arene-ruthenium(II) catalysts has provided selective access to enol esters. Enol esters are much more reactive than alkyl esters and have been used in a variety of reactions. In the past decade, Dixneuf and co-workers have developed selective approaches to the Markovnikov and antz-Markovnikov addition of carboxylic acids across alkynes by employing different arene-ruthenium(II) catalysts [48,53,54]. Of special interest is the synthesis of AT-Boc-protected 1-alanine isopropenyl ester 110 from N-Boc-l-alanine 108 and propyne 109 mediated by (Ty -cymene)RuCl2(PPh3) complex 107 (Scheme 30) [53]. Addition of the amino acid 108 to the propyne 109 proceeded exclusively in the Markovnikov sense and without accompanying racemization of the substrate. 

As carboxylic acid additives increased the efficiency of palladium catalysts in direct arylations through a cooperative deprotonation/metallation mechanism (see Chapter 11) [45], their application to ruthenium catalysis was tested. Thus, it was found that a ruthenium complex modified with carboxylic acid MesC02H (96) displayed a broad scope and allowed for the efficient directed arylation of triazoles, pyridines, pyrazoles or oxazolines [44, 46). With respect to the electrophile, aryl bromides, chlorides and tosylates, including ortho-substituted derivatives, were found to be viable substrates. It should be noted here that these direct arylations could be performed at a lower reaction temperatures of 80 °C (Scheme 9.34). 

In addition, as discussed above, oxidation reactions and reactions which use CO2 as a reagent as well as a solvent are worth investigating. Examples of both are discussed below. Finally, electrophilic processes may be advantageously transferred to supercritical CO2, as demonstrated by the improved isomerization of C4-C12 paraffins catalyzed by aluminum bromide. 2,44) Below, we describe three catalytic reactions which appear promising by these criteria asymmetric catalytic hydrogenation of enamides, ruthenium-catalyzed two-phase oxidation of cyclohexene, and the catalytic copolymerization of CO2 with epoxides. 

Reactions Involving sp -CH Activation. The insertion of ruthenium complexes into alkane C—H bonds is quite limited most synthetic routes require an adjacent nitrile group to first coordinate to the metal center. Oxidative addition of the C—H bond to the ruthenium center gives the hydrido ruthenium intermediate. An aldehyde or an a.jS-unsaturated carbonyl acts as the electrophile, which after reductive elimination from the metal affords the corresponding alcohol. This reaction is typically catalyzed by either CpRuCl(PPh3)2 or RuH2(PPh3)4 (58). 

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