Indole-benzoquinone coupling

In addition to examining the vinylation of l-methyl-2-indolecarboxaldehyde with methyl acrylate (Pd(OAc>2/HOAc/AgOAc) to give methyl ( )-3-(2-formyl-l-methyl-3-indoiyl)-acrylate in 60% yield, Pindur found that similar reactions of methyl 3-(l-methyl-2-indolyl)-acrylate afford bis(carbomethoxy)carbazoles albeit in low yield [85]. Fujiwara discovered that the combination of catalytic Pd(OAc)2 with benzoquinone and t-butylhydroperoxide serves to couple indole with methyl acrylate to give methyl ( )-3-(3-indolyl)acrylate in 52% yield [86]. 

The first step is to make an indole by Pd(II)-catalysed cyclization in the presence of benzoquinone as reoxidant. The nucleophilic nature of the 3-position of the indole (Chapter 43) was exploited to introduce the required iodine functionality. Rather than direct iodination, a high yielding two-step procedure involving mercuration followed by iodination was employed. The more reactive iodide was then involved in a Heck coupling with an unsaturated side chain in the absence of phosphine... 

An interesting variant of the Nenitzescu indole synthesis, involving the Lewis acid-directed coupling of enol ethers with benzoquinone mono- and b/s-imides, was developed by T.A. Engler et al. for the synthesis of substituted - and -tetrahydrocarbolines. ... 

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