Carbon monoxide stereochemistry

A complete description of stereochemistry of the carbon monoxide insertion and decarbonylation requires knowledge of configurational changes at the metal and a-carbon. Calderazzo and Noack (54) showed that the optical activity of the equilibrium mixture... 

Insertion of carbon monoxide into Csp2—Zr bonds occurs readily at ambient temperatures or below to produce a,(5-unsaturated, reactive acyl zirconocene derivatives [27—29]. Early work by Schwartz demonstrated the potential of such intermediates in synthesis [5d], as they are highly susceptible to further conversions to a variety of carbonyl compounds depending upon manipulation. More recently, Huang has shown that HC1 converts 16 to an enal, that addition of a diaryl diselenide leads to selenoesters, and that exposure to a sulfenyl chloride gives thioesters (Scheme 4.11) [27,28]. All are obtained with (F)-stereochemistry, indicative of CO insertion with the expected retention of alkene geometry. 

T he epoxidation of olefins using organic hydroperoxides has been studied in detail in this laboratory for a number of years. This general reaction has also recently been reported by other workers (6,7). We now report on the effects of five reaction variables and propose a mechanism for this reaction. The variables are catalyst, solvent, temperature, olefin structure, and hydroperoxide structure. Besides these variables, the effect of oxygen and carbon monoxide, the stereochemistry, and the kinetics were studied. This work allows us to postulate a possible mechanism for the reaction. 

In the hydroformylation reaction, V, the situation is even worse. Here there is no definite stereochemistry between the phosphine ligand and the metal. One of the reactants, carbon monoxide, competes so well with the phosphine for sites on the metal that it is difficult to insure that the chiral agent is present when the new asymmetric center is formed. 

Formation of the 2-olefin was then conveniently measured by the percent racemization. Under conventional Oxo conditions the products were 4-methylhexanal (V), 3-ethylpentanal (VI), and 2,3-dimethyl-valeraldehyde (VII) in the ratio 93 4 3 and there was only 1.8% racemization of (V). At higher temperature the racemization of (V) increased to 32.2% at 145° and 93.9% at 180° C. The yield of (V) also dropped off to 61.3% at 180° C. The carbon monoxide pressures in these cases were in the range 71-95 atm. Decreasing the carbon monoxide pressure from 95 to 16 atm at 90° C increased the racemization from 1.8% to 8.7%. These results clearly show that under normal Oxo conditions (V) was mainly formed by a mechanism which did not change the stereochemistry at C3. The degree of racemization can be explained by the reversible formation of a 2-olefin intermediate. Pino et al. concluded that under conventional Oxo conditions, little isomerization occurred. 

Propynyl alcohols have been converted into (Z)-a-(alkoxycarbonyl)methylene /3-lactones by dialkoxycarbonylation in alcoholic media, under a carbon monoxide-air (3 1) atmosphere, using a PdVKI catalyst (Equation 37) <1997J(P1)147>. The (Z)-stereochemistry of the products was attributed to the syn nature of the carbon monoxide insertion. Substitution at the ct-alkyl position was essential to generate the lactone products in good yields. When the propynyl alcohols were cr-alkyl-unsubstituted, no /3-lactone formation was observed instead, a maleic diester and its cyclic isomer were the predominant products. Where substrates were mono-a-alkyl-substituted, yields of the /3-lactone were low, unless the alkyl group was sufficiently sterically bulky. 

Very simple experimental conditions (palladium(II) chloride (catalyst, 0.1 equivalent) copper(II) chloride (oxidant, 3 equivalents) and sodium acetate (buffer, 3 equivalents) in acetic acid under carbon monoxide at normal pressure and temperature) are necessary for this asymmetric route to saturated fused heterocycles with defined stereochemistry. 

Give a mechanism for this carbonylation reaction. Comment on the stereochemistry and explain why the yield is higher if the reaction is carried ojut under a carbon monoxide atmosphere. 

Their retro synthetic study was based around the Pauson-Khand cyclization (6), which couples an alkene, an alkyne, and a carbon monoxide source (typically dicobalt octacarbonyl) to give a cyclopentenone ring (Fig. 3.5, top). This reaction has been widely used for synthetic purposes, and some excellent reviews (7,8) have covered its principal features and the recent improvements to its experimental conditions. This reaction, in its intramolecular version, is ideal for the assembly of the l//-[2]pyrindi-none scaffold in two distinct versions, differing in the stereochemistry of the ring junction (Fig. 3.5, bottom). Hence, the readily available unsaturated amino acid derivatives 3.1a,b undergo intramolecular Pauson-Khand reaction to produce the two unsamrated scaffolds 3.2a,b. 

Dibutylbis(cyclopentadienyl)zirconium was added to A-allyl-Ai-(benzyl)cyclo-hex-2-enylamine (282) and the generated zirkonacycle was treated with carbon monoxide (Scheme 32). The metal was removed with acid and the tricyclic ketone 285 was isolated in excellent yield. The relative configuration of the newly formed stereogenic centers could not be determined at this point, which turned out to be advantageous. The authors assumed according to kinetic preference that the tricycle 286 with all cis stereochemistry had been formed and started the... 

A mixture of 1.0 mmol of the 5-hydroxyalkene, 2.2 mmol of copper(I) chloride and 0.10 mmol of bis(ace-tonitrile)dichloropalladium in 3 mL of CH3OH under carbon monoxide (1.11 bar) is stirred at 23 °C until the reaction is complete (3.5 10 h, TLC monitoring). After removal of the methanol, the residue is triturated with pentane and the pentane solution is concentrated to give the crude organic products. Purification by short-path vacuum distillation provides the cyclization products. The stereochemistry is determined by an analysis of the H-NMR chemical shift of the hydrogen at C-5 this signal is 0.1 -0.2 ppm further downfield when the proton is cis to the ester side chain compared to the trans arrangement. 

II. 3.2.1.1. Mechanistic Studies and Stereochemistry of Carbon Monoxide Insertion Reactions. 

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