A Comment on the Gurjar Mechanism for Alkene Isomerization Using the Grubbs Olefin Metathesis Catalysts

Chris D. Edlin; James Faulkner; David Fengas; Christopher K. Knight; Jeremy Parker; Ian Preece; Peter Quayle; Stuart N. Richards

doi:10.1055/s-2005-862384

LETTER

A Comment on the Gurjar Mechanism for Alkene Isomerization Using the Grubbs Olefin Metathesis Catalysts

Chris D. Edlin^a, James Faulkner^b, David Fengas^b, Christopher K. Knight^b, Jeremy Parker^c, Ian Preece^b, Peter Quayle*^b, Stuart N. Richards^d
^a Medicinal Chemistry 1, GlaxoSmithKline Medicines Research Centre, Gunnels Wood Road, Stevenage, Herts, SG1 2NY, UK
^b Department of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, UK
Fax: +44(161)2754598; e-Mail: peter.quayle@manchester.ac.uk;
^c AstraZeneca Process R&D, Avlon Works, Severn Road, Hallen, Bristol BS10 7ZE, UK
^d Avecia, P.O. Box 42, Hexagon House, Blackley, Manchester M9 8ZS, UK

Abstract

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Abstract

The mechanism of allylic alcohol isomerization in the presence of the Grubbs metathesis catalysts is discussed.

Key words

Grubbs - atom transfer - isomerization - mechanism - allylic alcohol - metathesis

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References

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Whilst metallacyclobutanes are universally accepted as intermediates [9] (or possibly transition states [10] ) in metathesis reactions their isolation in the case of Ru is hampered by their lability. [11]

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15c

We have yet to rule out the possibility that this ‘isomerization’ reaction might proceed via an elimination-intermolecular Kharasch sequence. Experiments are in hand to differentiate between these alternate pathways.

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19b

in this study Grubbs reported the 1,2-addition of hydride (rather than conjugate reduction) to an α,β-unsaturated ketone when treated to propan-2-ol, H₂ and NaOH in the presence of post-metathesis catalyst.

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20c Grée has reported that various hydrido ruthenium complexes effect the same isomerization reaction albeit with much higher levels of deuterium incorporation (up to 90% retention). The degree of d-incorporation is ligand dependent. We thank Prof. Grée for spectral data of 12a. See: Uma R. Davies MK. Crévisy C. Grée R. Eur. J. Org. Chem. 2001, 3141

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23a The exact nature of the decomposition products arising from the Grubbs’ catalysts has been the subject of some debate. A by-product, 20, formed in the preparation of ruthenium carbene complexes (originally misassigned by Fürstner et al. has now been fully characterized by Grubbs and Mol: Dinger MB. Mol JC. Eur. J. Inorg. Chem. 2003, 2827

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Similar suspicions have been raised in the area of polymer chemistry. See, for example:

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26 See also: Wu Z. Nguyen ST. Grubbs RH. Ziller JW. J. Am. Chem. Soc. 1995, 117: 5503

27 Schmidt has recently reported the partitioning of reactivity between the complexes 1a and 1b in RCM/ATRC reactions. In keeping with our results these workers found that the more robust second generation catalyst, 1b, did not catalyze effectively ATRC reactions. See: Schmidt B. Pohler M. Costisella B. J. Org. Chem. 2004, 69: 1421

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29 Alcaide A. Almendros P. Alonso JM. Tetrahedron Lett. 2003, 44: 8693 ; see ref. 16

Mechanistic studies:

30a For Ru see: McGrath DM. Grubbs RH. Organometallics 1994, 13: 224

30b See also: Trost BM. Kulawiec RJ. J. Am. Chem. Soc. 1993, 115: 2027

30c For Rh see: Bianchini C. Meli A. Oberhauser W. New. J. Chem. 2001, 25: 11

30d See further: Bergens SH. Bosnich B. J. Am. Chem. Soc. 1991, 113: 958

For recent discussions see:

31a Schmidt B. Eur. J. Org. Chem. 2004, 1865

31b Alcaide B. Almendros P. Chem.-Eur. J. 2003, 9: 1259