Kinetic studies carbonyls

The use of a reagent bearing a basic center or the addition of a base to the reaction mixture was recognized as necessary to prevent the acid-catalyzed elimination of the elements of water from the intermediates. Since the publication of this work, a number of similar intermediates have been isolated from thioamides and a-halogeno carbonyl compounds (608, 609, 619, 739, 754, 801), and as a result of kinetic studies, the exact mechanism of this reaction has been well established (739, 821). 

Kinetic studies on substitution reactions of carbonyl metal complexes. H. Werner, Angew. Chem., Int. Ed. Engl., 1968, 7,930-941 (106). 

No comparative kinetic study has been made on the same alkyl carbonyl system for two members of a given transition metal triad. Qualitative data show that the middle member is more reactive than the heaviest one e.g., CpMo(CO)jR > CpW(CO)jR (Section VI,B), Rh(III) > Ir(III) (Section VI,E), and Pd(II) > Pt(II) (Section VI,F). However, the extreme unreactivity of CpW(CO)jR and a considerable difference in lability between most alkyls of Rh(III) and Ir(III), as well as those of Pd(II) and Pt(II), have prevented detailed investigations. Surprisingly, no kinetic studies have been conducted on insertion reactions of RRe(CO)5, which would seem readily amenable to such investigations. 

The oxidation of alcohols to carbonyl compounds by permanganate proceeds most rapidly in basic solution and it is with this medium that the majority of kinetic studies have been performed. 

A detailed kinetic study of the reaction of trialkyl phosphites (44) with benzil has been carried out (see Chapter 2 for the reactions of a-diketones with trialkyl phosphites). The reaction is first-order in both phosphite and benzil and the rate constant increases with the dielectric constant of the solvent. The authors propose initial attack of phosphite at carbonyl carbon (45), in opposition to the original suggestion by Ramirez, who proposed initial attack at carbonyl oxygen. 

There have been three primary motives behind the study of metal carbonyl photochemistry in the gas phase first, to discover the shapes of metal carbonyl fragments in the absence of perturbing solvents or matrices second, to probe the effect of uv photolysis wavelength on product distribution and third, to measure the reaction kinetics of carbonyl fragments. All three areas have already proved fruitful. The photochemistry of two molecules, Fe(CO)5 and Cr(CO)6, has been studied in detail. 

Withers, H.P., Eliezer, K.F., and Mitchell, J.W. 1990. Slurry-phase Fischer-Tropsch synthesis and kinetic studies over supported cobalt carbonyl derived catalysts. Ind. Eng. Chem. Res. 29 1807-14. 

This system shows an induction period of about six hours before constant activity is attained during which the Ru3(C0)12 undergoes complete conversion to another ruthenium carbonyl complex. In situ nmr studies suggest this species to be the HRu2(C0)e ion. Kinetic studies show complex rate profiles however, a key observation is that the catalysis rate is first order in Pco at low pressures (Pcohigher pressures. A catalysis scheme consistent with these observations is proposed. 

The overall enthalpy change of the insertion process contains contributions from four bonds (M-CO, M-COR, M-R and CO-R). As there is no significant difference between (Mn-R) and Zs(Mn-COR) then, at least in the case of manganese and hydrocarbon groups, R, the dominant factor will be the difference between T (Mn-CO) and E R-COX) [for R = CH3, E = 339 kJ mop1 (X = H), 370 kJ mol"1 (X = Cl) (Ref.23 )] which suggests that the insertion reaction is thermodynamically favoured with respect to decarbonylation. Kinetic studies of the carbonyl insertion reaction in solution have shown87) that the enthalpy of activation is 62 kj mol-1 for inser-... 

Kinetic studies of the stoichiometric carbonylation of [Ir(CO)2l3Me] were conducted to model the rate-determining step of the catalytic cycle [73,85]. The reaction can form both fac,cis and mer,trans isomers of [Ir(CO)2l3 (COMe)] (Scheme 13), the product ratio varying with the solvent and temperature used. An X-ray crystal structure was obtained for the fac,cis isomer. Carbonylation of [Ir(CO)2l3Me] is rather slow and requires temperatures > 80 °C in chlorinated solvents (e.g. PhCl). However, the presence of protic solvents (e.g. methanol) has a dramatic accelerating effect. This is interpreted in terms of the protic solvent aiding iodide dissociation by solvation. 

Kinetic studies established that tetra-n-butylammonium borohydride in dichloromethane was a very effective reducing agent and that, by using stoichiometric amounts of the ammonium salt under homogeneous conditions, the relative case of reduction of various classes of carbonyl compounds was the same as that recorded for the sodium salt in a hydroxylic solvent, i.e. acid chlorides aldehydes > ketones esters. However, the reactivities, ranging from rapid reduction of acid chlorides at -780 C to incomplete reduction of esters at four days at 250 C, indicated the greater selectivity of the ammonium salts, compared with sodium borohydride [9], particularly as, under these conditions, conjugated C=C double bonds are not reduced. 

Kinetic studies of the unnatural 6-a -epimer of ampicillin, fi-ept-ampicillin (154), have revealed an intramolecular process not undergone by ampicillin (or other natural /3-substituted penicillins) At pH 6-9, intramolecular attack of the jS-lactam carbonyl group by the side-chain amino group of (154) yields a stable piperazine-2,5-dione derivative (155). Theoretical calculations show that the intramolecular aminolysis of 6-epi-ampicillin nucleophilic attack occurs from the a-face of the -lactam ring with an activation energy of 14.4kcalmor In other respects, the hydrolysis of the b-a-epimer is unexceptional. 

Snapper and Hoveyda reported a catalytic enantioselective Strecker reaction of aldimines using peptide-based chiral titanium complex [Eq. (13.11)]. Rapid and combinatorial tuning of the catalyst structure is possible in their approach. Based on kinetic studies, bifunctional transition state model 24 was proposed, in which titanium acts as a Lewis acid to activate an imine and an amide carbonyl oxygen acts as a Bronsted base to deprotonate HCN. Related catalyst is also effective in an enantioselective epoxide opening by cyanide "... 

---