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The Shvo catalyst is an
In the presence of a suitable hydrogen donor or hydrogen gas, Shvo's catalyst effects the hydrogenation of several polar functional groups, e.g. aldehydes, ketones, imines, and iminium ions. Many alkenes and ketones undergo hydrogenation, although conditions are forcing: 145 °C (500 psi). One obstacle to the use of Shvo's catalyst in the hydrogenation of alkynes is its propensity to bind the alkyne quite tightly, forming a stable complex that gradually poisons the catalyst. Intramolecular reactions proceed as well, illustrated by the conversion of allylic alcohols to ketones. Shvo's catalyst also catalyzes dehydrogenations.
Addition of the amine is facilitated through oxidation to the 
Another case of "hydrogen borrowing", the alkylation of amines using other amines is also promoted by Shvo's catalyst. The reaction proceeds through oxidation to an imine, which allows nucleophilic attack, followed by an elimination step and reduction of the double bond.Hollmann, D.; Bahn, S.; Tillack, A.; Beller, M. Angew. Chem. Int. Ed. 2007, 46, 8291.
organoruthenium compound Organoruthenium chemistry is the chemistry of organometallic compounds containing a carbon to ruthenium chemical bond. Several organoruthenium catalysts are of commercial interest and organoruthenium compounds have been considered for cancer thera ...
that catalyzes the hydrogenation
Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a catalyst such as nickel, palladium or platinum. The process is commonly employed to reduce or saturate org ...
of polar functional groups including aldehydes, ketones and imines. The compound is of academic interest as an early example of a catalyst for transfer hydrogenation that operates by an "outer sphere mechanism". Related derivatives are known where p-tolyl
In organic chemistry, tolyl groups are functional groups related to toluene. They have the general formula , the change of the relative position of the methyl and the R substituent on the aromatic ring can generate three possible structural ...
replaces some of the phenyl groups. Shvo's catalyst represents a subset of homogeneous hydrogenation catalysts that involves both metal and ligand in its mechanism.
Synthesis and structure
The catalyst is named after Youval Shvo, who uncovered it through studies on the effect ofdiphenylacetylene
Diphenylacetylene is the chemical compound C6H5C≡CC6H5. The molecule consists of two phenyl groups attached to a C2 unit. A colorless solid, it is used as a building block in organic synthesis and as a ligand in organometallic chemistry.
Prepa ...
on the catalytic properties of triruthenium dodecacarbonyl
Triruthenium dodecacarbonyl is the chemical compound with the formula Ru3(CO)12. Classified as metal carbonyl cluster, it is a dark orange-colored solid that is soluble in nonpolar organic solvents. The compound serves as a precursor to other o ...
. The reaction of diphenylacetylene and Ru3(CO)12 gives the piano stool complex
Half sandwich compounds, also known as piano stool complexes, are organometallic complexes that feature a cyclic polyhapto ligand bound to an MLn center, where L is a unidentate ligand. Thousands of such complexes are known. Well-known examples in ...
. Subsequent hydrogenation of this tricarbonyl affords Shvo's catalyst. Y. Blum, D. Reshef, and Y. Shvo. H-transfer catalysis with Ru3(CO)12. Tetrahedron Lett. 22(16) 1981, pp. 1541-1544. Blum, Y.; Shvo, Y. Isr. J. Chem. 1984, 24, 144. The iron analogue is also known, see Knölker complex.
The compound contains a pair of equivalent Ru centres that are bridged by a strong hydrogen bond and a bridging hydride. In solution, the complex dissociates unsymmetrically:
:(C5Ph4O)2HRu2H(CO)4 C5Ph4OH)RuH(CO)2 + (C5Ph4O)Ru(CO)2
Hydrogenation catalysis
In the presence of a suitable hydrogen donor or hydrogen gas, Shvo's catalyst effects the hydrogenation of several polar functional groups, e.g. aldehydes, ketones, imines, and iminium ions. Many alkenes and ketones undergo hydrogenation, although conditions are forcing: 145 °C (500 psi). One obstacle to the use of Shvo's catalyst in the hydrogenation of alkynes is its propensity to bind the alkyne quite tightly, forming a stable complex that gradually poisons the catalyst. Intramolecular reactions proceed as well, illustrated by the conversion of allylic alcohols to ketones. Shvo's catalyst also catalyzes dehydrogenations.
Mechanism
The mechanism of hydrogenation catalyzed by Shvo's catalyst has been a matter of debate, broadly between two alternative descriptions of the double bond's interaction with the complex at the rate-determining step. The proposed alternatives are an inner-sphere mechanism, where the transition state involves interaction with the metal only, and an outer-sphere mechanism, in which the cyclopentadienol proton also interacts with the substrate. Kinetic isotope studies provide evidence of a concerted transfer due to strong rate influence from both the ligand -OH and the metal hydride.Other reactions
Shvo's catalyst facilitates theTishchenko reaction The Tishchenko reaction is an organic chemical reaction that involves disproportionation of an aldehyde in the presence of an alkoxide. The reaction is named after Russian organic chemist Vyacheslav Tishchenko, who discovered that aluminium alko ...
, i.e., the formation of esters from alcohols. The early step in this reaction is the conversion of the primary alcohol to the aldehyde.
Addition of the amine is facilitated through oxidation to the ynone
In organic chemistry, an ynone is an organic compound containing a ketone () functional group and a triple bond. Many ynones are α,β-ynones, where the carbonyl and alkyne groups are conjugated. Capillin is a naturally occurring example. Som ...
, followed by reduction of the product.
Another case of "hydrogen borrowing", the alkylation of amines using other amines is also promoted by Shvo's catalyst. The reaction proceeds through oxidation to an imine, which allows nucleophilic attack, followed by an elimination step and reduction of the double bond.Hollmann, D.; Bahn, S.; Tillack, A.; Beller, M. Angew. Chem. Int. Ed. 2007, 46, 8291.
References