Isomerisation versus carbonylative pathways in the hydroxy-carbonylation, methoxy-carbonylation, and amino-carbonylation of N-tosyl-3-pyrroline
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The reactivity of N-tosyl-3-pyrroline is significantly lower than that of mono-substituted alkenes in Pd catalysed methoxycarbonylation reactions. For example, most bulky diphosphine/ Pd catalysts, including the well-known Pd catalyst derived from 1,2-bis(di-tert-butylphosphino)xylene (DTBPX), were found to give no product at all in the methoxycarbonylation of N-tosyl-3-pyrroline. The competing pathways in methoxycarbonylation of N-methane-sulfonyl-3-pyrroline using Pd/ DTBPX were studied using DFT calculations; these show that the coordination of the alkene is unfavourable, and once coordinated, isomerisation is a lower energy pathway that ultimately leads to an alternative product. Experimentally a side product resulting from alkene isomerisation and addition of methanol is formed slowly (if CO is present), and rapidly if CO is not. A less bulky derivative of DTBPX forms the required alkene complex with much lower barriers. A study has been made of the enantioselective carbonylation of N-tosyl-3-pyrroline using water, methanol or aniline as nucleophile. This study revealed that there is a range of possible products with most of these initiated by a Pd-catalysed isomerisation of the alkene. Using less bulky members of the Pd/Phanephos family of catalysts, it is possible to produce the methoxycarbonylation product from this poorly reactive alkene with reasonably good chemoselectivity and around 80% e.e. at higher pressures of CO.
Fuentes García , J A , Durrani , J , Leckie , S M , Crawford , L , Buehl , M & Clarke , M 2016 , ' Isomerisation versus carbonylative pathways in the hydroxy-carbonylation, methoxy-carbonylation, and amino-carbonylation of N -tosyl-3-pyrroline ' Catalysis Science & Technology , vol 6 , no. 20 , pp. 7477-7485 . DOI: 10.1039/C6CY01268J
Catalysis Science & Technology
Copyright 2016 the Authors. This work is made available online in accordance with the publisher’s policies. This is the author created, accepted version manuscript following peer review and may differ slightly from the final published version. The final published version of this work is available at https://dx.doi.org/10.1039/C6CY01268J
The authors thank the EPSRC for funding, primarily from grant number EP/M003868/1.
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