Unravelling mechanistic insights in the platinum-catalysed dihydroalkoxylation of allenes

María Teresa Quirós,Enrique Gómez-Bengoa,María Paz Muñoz

doi:10.1515/pac-2019-0214

María Teresa Quirós, Enrique Gómez-Bengoa + Show 1 more

Open Access

https://doi.org/10.1515/pac-2019-0214

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Abstract

Abstract The mechanism of the platinum-catalysed dihydroalkoxylation of allenes to give acetals has been studied experimentally and by computational methods. Our findings further explain divergent reactivity encountered for platinum- and gold-vinyl intermediates after the first nucleophilic attack onto the coordinated allene, as well as provide new details on the catalytic cycle with platinum, uncovering enol ethers as resting states of the catalytic cycle, a SEOx process via Pt(IV)–H as the final protodemetallation step after the second nucleophilic attack when neutral platinum complexes are used, and a fast acid promoted addition of methanol to enol ethers when cationic platinum complexes are employed.

Highlights

We found a divergent behaviour of platinum-allene complexes when compared to gold regarding ligand exchange that occurs mainly via a ligand-independent dissociative mechanism, where coordinating solvents might be involved in the stabilisation of the intermediates
Kinetic analysis has to be done with caution due to the induction periods observed, we carried out the analysis of the reaction in the presence of methanol (2b) as nucleophile using the cationic platinum catalyst to give acetal 3ab [87]
Computational comparison of key vinyl-metal intermediates revealed that platinum and gold species are opposite and complementary, explaining the formation of allyl ethers from gold intermediates by protonation of the C–Au bond and formation of platinum carbenes by protonation of the internal carbon

Summary

Introduction

In recent years a lot of attention has been paid to gold-catalysis [1,2,3,4,5,6,7,8,9,10,11,12,13] and numerous mechanistic studies have been carried out to understand the remarkable reactivity and selectivity of gold complexes [14,15,16,17,18]. We have reported the platinum-catalysed dihydroalkoxylation of allenes [75] which give acetals by double addition of alcohol nucleophiles to the terminal double bond of the allene, and complete saturation of the second double bond (Scheme 1d) This reaction constitutes a remarkable example of different reactivity of platinum and gold complexes under similar conditions. The slipped structures resembling η1-coordination of the platinum complex to the central carbon of the allene found in our studies (I, Scheme 2), would initially support the formation of a platinum carbene (III, Scheme 2), as a new key intermediate proposed in the catalytic cycle for the formation of acetals from allenes catalysed by platinum This metallic carbene intermediate has not been proposed in similar gold-catalysed processes in which vinyl-gold intermediates (analogous to II, Scheme 2) give allyl alcohols as final products via protonolysis of the C–Au bond. Quirós et al.: Unravelling mechanistic insights in the platinum-catalysed 169

Results and discussion

Cl Pt Cl

H OR H VI

Conclusions