Ruthenium electrophilic addition

The synthesis and chemistry of metal complexes of thiophenes have been reported including the electrophilic additions to osmium-thiophene complexes <9902988> and nucleophilic additions to ruthenium-thiophene complexes <99JOMC242>. The selectivity for the insertion of ruthenium into 3-substituted thiophenes was studied <99CC1793>. For example, treatment of 3-acetylthiophene (84) with Ru(cod)(cot) led to a regioselective 1,2-insertion of ruthenium giving thiaruthenacycle 85. 

Grant and Krische have described a racemic protocol for the synthesis of allcarbon C3 quaternary centers from 3-hydroxy-3-tert-prenyloxindole 76 that was accessed via ruthenium catalyzed addition of 1,1-dimethylallene 75 to isatin 74 [45]. As outlined in Scheme 21, 76 was converted to the electrophilic 3-chloro derivative, which was trapped with indole under basic conditions to afford 78 in 60% yield. A mechanism has been proposed for the C-C bond-forming event that involved first-order irmizatirHi of chloride irm assisted by delocaUzatiOTi of oxindole... 

Nitrosyl complexes of ruthenium react in a similar manner. In this case, supposedly, nucleophilic attack of the coordinated NO molecule on the benzyl halide takes place, or electrophilic addition of PhCH to the coordinated NO group occurs with concomitant addition of X to the metal ... 

Fischer-type osmium alkoxycarbenes are scarce when compared to those of ruthenium. The addition of alcohols to the electrophilic Go -atom of vinylidene or allenylidene groups has also proved to be, as for the ruthenium complexes. 

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... 

Activation of the triple bond of enynes with electrophilic metal derivatives, especially cationic gold complexes, platinum salts such as PtCl2, and ruthenium derivatives, has been reviewed.117 These catalysts make possible nucleophilic addition of the double... 

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). 

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. 

Several ruthenium complexes are able to promote the classical Markovnikov addition of O nucleophiles to alkynes via Lewis-acid-type activation of triple bonds. Starting from terminal alkynes, the anti-Markovnikov addition to form vinyl derivatives of type 1 (Scheme 1) is less common and requires selected catalysts. This regioselectivity corresponding to the addition of the nucleophile at the less substituted carbon of the C=C triple bond is expected to result from the formation of a ruthenium vinylidene intermediate featuring a highly reactive electrophilic Ca atom. 

Homopropargylic alcohols (but-3-ynols) as well as propargylic epoxides are suitable products to form cyclic ruthenium alcoxycarbenes upon intramolecular nucleophilic addition of the OH group to the electrophilic a-carbon of ruthenium vinylidene species. The recovery of the organic ligand as a lactone... 

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