Abstract
Optimisation, scope and mechanism of the platinum‐catalysed addition of indoles to indolylallenes is reported here to give 2,3′‐BIMs with a novel core structure very relevant for pharmaceutical industry. The reaction is modulated by the electronic properties of the substituents on both indoles, with the 2,3′‐BIMs favoured when electron donating groups are present. Although simple at first, a complex mechanism has been uncovered that explains the different behaviour of these systems with platinum when compared with other metals (e.g. gold). Detailed labelling studies have shown Pt‐catalysed 6‐endo‐trig cyclisation of the indollylallene as the first step of the reaction and the involvement of two cyclic vinyl‐platinum intermediates in equilibrium through a platinum carbene, as the key intermediates of the catalytic cycle towards the second nucleophilic attack and formation of the BIMs.
Highlights
The indole moiety is regarded as one of the most ubiquitous compound constituents in nature, for example, as part of the essential amino acid tryptophan
Results and Discussion the Au-catalysed reaction of indoles with cyclopropenes for the synthesis of 3,3’-BIMs has been reported (Scheme c).[6c]. Metal-catalysed reactions of indoles with allenes has been widely explored in recent years.[7]
We have optimised the platinum-catalysed reaction of indolylallenes with external indoles to give substituted
Summary
The indole moiety is regarded as one of the most ubiquitous compound constituents in nature, for example, as part of the essential amino acid tryptophan. 3 xy depending on the nucleophilicity of the indolylallene and the external indole.[18] The desired 2,3’-BIM 3 xy was formed preferentially when position 5 was substituted with an electron-donating group (entries 2 and 3, Table 1), while forma-
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