Email Discussion: 66 Carbonylation Catalysis / Crown Ether Cleavage Thomas Schmidt

Introduction

In the presence of fluoride ion, alkyl iodides RI are carbonylated by molybdenum hexacarbonyl to esters RCOOR, and diiodides lead to good yields of the corresponding lactones.

The reaction can be made catalytic in molybdenum with methyl formate as CO source, and catalytic in fluoride providing a base (pyridine) is present (1). The present investigation on the mechanism of this reaction is mainly concerned with the role of the water molecule, including :

- Isotopic labelling experiments using H218O

- Study of the influence of the order of introduction of the reagents

- Exclusion of water from the reaction mixture

Isotopic labelling experiments

The reaction was performed in the presence of H218O : molybdenum hexacarbonyl (1 eq), KF-(18-crown-6) (2 eq) (dried by azeotropic distillation with toluene), hexyl iodide (1 eq) and H218O (10 eq) in anhydrous THF were refluxed for 18 h. The resulting hexyl heptanoate RCOOR (R = C6H13; 89% yield) was labelled in the following way, as determined by mass spectrometry :

The reliability of the titration method was checked by comparison with the mass spectrum of authentic R-CO-18OR, synthesized by reaction of heptanoyl chloride R-CO-Cl on labelled hexanol R-18OH obtained by substitution of H218O on RI in HMPA.

A control experiment showed that no label was incorporated if H218O was added to the reaction mixture at the end of the reflux period instead of being added at the same time as the other reactants. This results shows that water is involved in the course of the reaction rather than at the final hydrolysis stage.

The presence of a small amount of ester labelled on both oxygen atoms indicate that some oxygen exchange between water and carbonyl ligands occurs during the reaction. However the esters bearing one label are found in significantly larger amount than would have been produced by a complete statistical redistribution. Comparison of the amounts of R-C18O-OR and R-CO-18OR shows practically equal distribution of the label on the two oxygen atoms of the ester molecule. This result is consistent with the intermediacy of a [Mo]-COOH species.

Influence of the order of introduction of the reagents

The two following reactions were compared :

Refluxing a mixture of molybdenum hexacarbonyl (1 eq), tetrabutylammonium fluoride trihydrate (2 eq) and hexyl iodide (1 eq) in anhydrous THF for 1 h led mainly to starting hexyl iodide (80%) along with RCOOR and a small amount of RCOOH.

A mixture of all the reagents but hexyl iodide in THF (same concentrations) was first refluxed for 3 h, then hexyl iodide was added and the mixture was refluxed for 1 h. In this case only 4% of the starting hexyl iodide remained unreacted; the major product was the ester RCOOR (81%) along with RCOOH.

Comparison of these results indicates that the first step of the reaction involves molybdenum hexacarbonyl, fluoride ion and water, to give an intermediate which reacts more rapidly upon addition of an alkyl iodide leading to the ester.

If fluoride ion is replaced with hydroxide ion (using tetrabutylammonium hydroxide), only a trace amount of ester is obtained under the same conditions.

Exclusion of water from the reaction mixture

Even if no water is deliberately added to the reaction mixture, the ester product is easily formed when the experimental procedure does not ensure drastically anhydrous conditions. This indicates the involvement of a very hygroscopic intermediate at some stage of the reaction.

When water was strictly excluded from the reaction mixture using KF-(18-crown-6) as fluoride source, the following product was obtained instead of the ester :

This product results from double cleavage of the crown ether molecule. The corresponding reaction is remarkably regioselective, since the analogous products R-CO-O-(CH2CH2-O)n-CO-R (n = 1, 2, 4, 5, 6) are detected in zero or trace amounts.

Conclusion

The experimental results favour the initial formation of a negatively charged [Mo-]-COOH species

formed by successive reactions of fluoride ion and water on molybdenum hexacarbonyl. This could occur through a [Mo-]-CO-F intermediate, similarly to the proposed mechanism for fluoride-promoted generation of iron carbonyl anions (2).

Addition of RI may lead to the following scheme :

The byproduct RCOOH would be formed by direct reductive elimination on the R-[Mo]-COOH intermediate.

Molybdenum complexes Cp-[Mo]-COOH (3) and Cp-[Mo]-COOR (4), which are stable analogs of the proposed intermediates R-[Mo]-COOH and R-[Mo]-COOR, have been described. A reductive elimination very similar to the last step of this tentative mechanism has been proposed in the case of a tungsten complex (5).

References

(1) M. Imbeaux, H. Mestdagh, K. Moughamir, C. Rolando, J. C. S., Chem. Comm. 1992, 1678.

(2) H. Alper, L. C. Damude, Organometallics 1982, 1, 579.

(3) D. H. Gibson, K. Owens, T. S. Ong, J. Am. Chem. Soc. 1984, 106, 1125.

(4) T. S. Coolbaugh, B. D. Santarsiero, R. H. Grubbs, J. Am. Chem. Soc. 1984, 106, 6310.

(5) P. B. Brian, M. A. Atta, G. R. Thomas, J. C. S., Chem. Comm. 1993, 1432.