Charles P. Casey

Cyclopentadienone Iron Alcohol Complexes: Synthesis, Reactivity, and Implications for the Mechanism of Iron Catalyzed Hydrogenation of Aldehydes

Cyclopentadienone iron alcohol complexes generated from the reactions of [2,5-(SiMe(3))(2)-3,4-(CH(2))(4)(eta(5)-C(4)COH)]Fe(CO)(2)H (3) and aldehydes were characterized by (1)H NMR, (13)C NMR, and IR spectroscopy. The benzyl alcohol complex [2,5-(SiMe(3))(2)-3,4-(CH(2))(4)(eta(4)-C(4)CO)]Fe(CO)(2)(HOCH(2)C(6)H(5)) (6-H) was also characterized by X-ray crystallography. These alcohol complexes are thermally unstable and prone to dissociate the coordinated alcohols. The alcohol ligand is easily replaced by other ligands such as PhCN, pyridine, and PPh(3). Dissociation of the alcohol ligand in the presence of H(2) leads to the formation of iron hydride 3. The reduction of aldehydes by 3 was carried out in the presence of both potential intermolecular and intramolecular traps. The reaction of 3 with PhCHO in the presence of 4-methylbenzyl alcohol as a potential intermolecular trapping agent initially produced only iron complex 6-H of the newly formed benzyl alcohol. However, the reaction of 3 with 4-(HOCD(2))C(6)H(4)CHO, which possesses a potential intramolecular alcohol trapping agent, afforded two alcohol complexes, one with the newly formed alcohol coordinated to iron and the other with the trapping alcohol coordinated. The intramolecular trapping experiments support a mechanism involving concerted transfer of a proton from OH and hydride from Fe of 3 to aldehydes. The kinetics and mechanism of the hydrogenation of benzaldehyde catalyzed by 3 are presented.

Spectroscopic determination of hydrogenation rates and intermediates during carbonyl hydrogenation catalyzed by Shvo's hydroxycyclopentadienyl diruthenium hydride agrees with kinetic modeling based on independently measured rates of elementary reactions.

The catalytic hydrogenation of benzaldehyde and acetophenone with the Shvo hydrogenation catalysts were monitored by in situ IR spectroscopy in both toluene and THF. The disappearance of organic carbonyl compound and the concentrations of the ruthenium species present throughout the hydrogenation reaction were observed. The dependence of the hydrogenation rate on substrate, H2 pressure, total ruthenium concentration, and solvent were measured. In toluene, bridging diruthenium hydride 1 was the only observable ruthenium species until nearly all of the substrate was consumed. In THF, both 1 and some monoruthenium hydride 2 were observed during the course of the hydrogenation. A full kinetic model of the hydrogenation based on rate constants for individual steps in the catalysis was developed. This kinetic model simulates the rate of carbonyl compound hydrogenation and of the amounts of ruthenium species 1 and 2 present during hydrogenations.

Reaction of metal-Carbene complexes with vinyllithium reagents☆

Abstract The reaction of vinyllithium with (CO) 5 CrC(OCH 3 )C 6 H 5 (Ic) at−78° followed by the treatment with HCl at −78° gave 43% ( Z )-1-methoxy-1-phenylpropene (IIIa) and 21% 1,4-dimethoxy-1,4-diphenyl-1,3-butadiene (IVa) and no trace of a vinylphenylcarbene complex or its expected decomposition products. IIIa and IVa are proposed to arise from electrophilic attack at the carbon-carbon double bond of a σ-allychromium intermediate. The reaction of phenyllithium with (CO) 5 CrC(OCH 3 )CH=CHC 6 H 5 (V) gave 9% (CO) 5 CrC(OCH 3 )CH 2 CH- (C 6 H 5 ) 2 (VI) and 17% ( E )-1-methoxy-1,3-diphenylpropene (VII). Reaction of V with lithium diphenylcuprate gave 30% of the conjugate addition product VI.

Synthesis of α-methylene-γ-butyrolactone via transition metal carbene complexes

Abstract The anion of (2-oxacyclopentylidene)pentacarbonylchromium(0), (I), is alkylated by addition to ClCH2OCH3 to give (-5methoxymethyl-2-oxacyclopentylidene)pentacarbonylchromium(0), (VI), and (5,5-dimethoxymethyl-2-oxacyclopentylidene)pentacarbonylchromium(0), VII, in 45 and 13% yields, respectively, as well as (5-hydroxymethyl-2-oxacyclopentylidene)pentacarbonylchromium(0), (VIII), and (5-hydroxymethyl-5-methoxymethyl-2-oxacyclopentylidene)pentacarbonylchromium(0), (IX), in a combined yield of 18%. Methanol and water are eliminated from VI and VIII, respectively, to give 64% of (5-exo-methylene-2-oxacyclopentylidene)pentacarbonylchromium(0), (IV), which affords 76% α-methylene-γ-butyrolactone, (X), upon oxidation by ceric ion. Direct addition of one-half equivalent of ClCH2OCH3 to the anion of I gave a 74% yield of the methylene bridged dimer V. Addition of excess ClCH2OCH3 to the bis(triphenylphosphine)iminium salt of the anion of I gave primarily VI (38%).

Shvo’s Catalyst in Hydrogen Transfer Reactions

This chapter reviews the use of ShvoB1;apos;s catalyst in various hydrogen transfer reactions and also discusses the mechanism of the hydrogen transfer. The Shvo catalyst is very mild to use since no activation by base is required in the transfer hydrogenation of ketones or imines or in the transfer dehydrogenation of alcohols and amines. The Shvo catalyst has also been used as an efficient racemization catalyst for alcohols and amines. Many applications of the racemization reaction are found in the combination with enzymatic resolution leading to a dynamic kinetic resolution (DKR). In these dynamic resolutions, the yield based on the starting material can theoretically reach 100%. The mechanism of the hydrogen transfer from the Shvo catalyst to ketones (aldehydes) and imines as well as the dehydrogenation of alcohols and amines has been studied in detail over the past decade. It has been found that for ketones (aldehydes) and alcohols, there is a concerted transfer of the two hydrogens involved, whereas for typical amines and imines, there is a stepwise transfer of the two hydrogens. One important question is whether the substrate is coordinated to the metal or not in the hydrogen transfer step(s). The pathway involving coordination to activate the substrate is called the inner-sphere mechanism, whereas transfer of hydrogen without coordination is called the outer-sphere mechanism. These mechanistic proposals together with experimental and theoretical studies are discussed.

Thermolysis of (2-oxacyclopentylidene)pentacarbonylchromium(0): evidence against free carbenes in thermal decomposition of metal–carbene complexes

Thermolysis of (2-oxacyclopentylidene)pentacarbonylchromium(0)(I) gives mainly the dimer and no cyclobutanone, a product characteristic of a free carbene; a bimolecular mechanism for decomposition of (I) is proposed based on kinetic studies.

Synthesis and reactions of directly bonded zirconiumruthenium heterobimetallic complexes

Abstract The directly bonded zirconiumruthenium heterobimetallic compounds Cp2(X)ZrRu(CO)2Cp ( X = Cl, OCMe3, CH3, CH2CH3) were synthesized by reaction of Cp2(X)ZrCl with K+ Cp(CO)2Ru−. The zirconiumdiruthenium compound Cp2Zr[Ru(CO)2Cp]2 (21) was synthesized by reaction of Cp2ZrI2 with K+Cp(CO)2Ru−. Reaction of 21 with a variety of ligands led to expulsion of Cp(CO)2RuH and formation of C5H4-Zr products or intermediates. Reaction of 21 with CO produced Cp2(CO)Zr(μ-η1,η5- C5H4)Ru(CO)2 (22), with PMe3 produced Cp2Zr(μ-CO)(μ-η1,η5-C5H4)Ru(CO)(PMe3) (23), and with CH2CH2 produced Cp2Zr(μ-CH2CH2C5H4)Ru(CO)2 (26). All three of these reactions proceeded at the same rate which was independent of incoming ligand concentration. All three reactions are proposed to involve rate determining formation of the reactive intermediate Cp2Zr(μ-η1,η5-C5H4)Ru(CO)2 (I). The reaction of CO adduct 22 and H2 led to hydrogenolysis of the ZrC5H4 bond and to the formation of Cp2Zr(μ-CO)(μ-OCH)Ru(CO)Cp (32). Labelling studies demonstrated that reversible formation of both zirconium formyl and ruthenium formyl intermediates occurs during the reduction of 22 to 32.

Interconversions of .eta.5-cyclopentadienyl, .eta.1-cyclopentadienyl, and ionic ".eta.0"-cyclopentadienyl rhenium compounds - x-ray crystal structure of tetrakis(trimethylphosphine)methylnitrosylrhenium cyclopentadienide

The reaction of (eta/sup 5/-C/sub 5/H/sub 5/)Re(NO)(CH/sub 3/)(PMe/sub 3/) and PMe/sub 3/ (trimethylphosphine) produced (eta/sup 1/C/sub 5/H/sub 5/)Re(NO)(CH/sub 3/)(PMe/sub 3/)/sub 3/. The reaction is reversible with K/sub eq/ = 0.4 M/sup -2/ at 4/sup 0/C in THF-d/sub 8/. Upon heating at 48/sup 0/C in THF in the presence of high concentrations of PMe/sub 3/, the equilibrium mixture was converted to (Re(NO)(CH/sub 3/)(PMe/sub 3/)/sub 4/)/sup +/(C/sub 5/H/sub 5/)/sup -/, which precipitates from solution. The structure was determined by X-ray crystallography: monoclinic space group P2/sub 1//c, with unit cell constants a = 12.893 (2) A, b = 13.622 (2) A, c = 15.068 (2) A, ..beta.. = 97.47 (2)/sup 0/, and Z = 4. 20 references, 2 figures, 2 tables.La reaction de (η 5 −C 5 H 5 )Re(NO)(CH 3 )(PMe 3 ) et PMe 3 donne (η 1 −C 5 H 5 ) Re(NO)(CH 3 )(PMe 3 ) 3 . Par chauffage a 48°C dans THF, le melange des 2 complexes est converti en [Re(NO)(CH 3 )(PMe 3 ) 4 ] 4 [C 5 H 5 ] − , dont on etudie la structure cristalline

The effect of alkyl substitution on the thermodynamic stability and kinetic reactivity of α-anions of metal—carbene complexes

Abstract Alkyl substitution was found to have only small effects on the thermodynamic and kinetic acidity and on the reactivity of anions generated α to the carbene carbon atom in metal—carbene complexes. The thermodynamic acidities of (2-oxacyclopentylidene)pentacarbonylchromium(0), I, and of (5-methyl-2-oxacyclopentylidene)pentacarbonylchromium(0), II, were found to be nearly equal. The rate of the base catalyzed deuterium exchange of the less substituted complex I, is 1.57 times faster than that of II. The reactivity of the anions generated from the less substituted complex I is 2.6–5.2 times less than that of anions generated from II.

Metal formyl and hydroxymethyl metal compounds

Abstract Metal formyl anions are produced by reaction of borohydrides with metal carbonyls. The resulting metal formyl anions have moderate kinetic stability but are thermodynamically less stable than the corresponding metal hydrides. Anionic metal formyl compounds can react as hydride donors. Reaction of (C5H5)Re(CO)2(NO)+ with HB (OR)−3 produces the neutral metal formyl complex (C5H5)Re(CO)(NO)(CHO) which undergoes disproportionation as a neat oil to give the metallo ester (C5H5)(CO)(NO)ReCO2CH2(CO)-(NO) (C5H5). Indirect hydrolysis of this metallo ester produced (C5H5Re-(CO) (NO)(CH2OH), the first authentic hydroxymethyl metal compound. Reduction of (C5H5)Re(CO)2(NO)+ with H2Al(CH2CH3−2 provides a more convenient synthesis of (C5H5Re(CO)(NO)(CH2OH). (C5H5)Re(CO)(NO)-(CH2OH) does not react with CO below 90 °C, the temperature at which the hydroxymethyl compound loses H2O to form an ether dimer [(C5H5)Re-(CO) NO)CH2]2O. At room temperature, (C5H5)Re(CO)(NO)CH3 reacts with two equivalents of P(CH3)3 to produce (η1-C5H5)Re(CO)-(NO) [P(CH3)3]2CH3.