Regio- and stereoselective synthesis of tetra- and triarylethenes by N-methylimidodiacetyl boron-directed palladium-catalysed three-component coupling

E Emily Lin,Ji-Lin Li,Xiao-Xuan Su,Frank Glorius,Jia-Qiang Wu,Tian-Miao Ou,Xin Zhao,Honggen Wang,Ke-Feng Wang,Felix Schäfers,Yunyun Chen,Qingjiang Li,Huihui Ti

doi:10.1038/s42004-019-0137-0

Abstract

The flexible synthesis of tetra- and triarylethenes bearing different aryl groups has been a long-standing challenge in organic synthesis. Here we report a palladium-catalysed syn-diarylation of arylethynyl N-methyliminodiacetyl (MIDA) boronates. The products, triarylalkenyl N-methyliminodiacetyl boronates, allow a step-economic and modular synthesis of tetra- or triarylethenes via a subsequent stereospecific Suzuki-Miyaura coupling reaction or base-promoted protodeborylation, respectively. Use of the sp3-B(MIDA) masked aryl alkyne is the key factor for success by offering an exceptionally good regioselectivity for the boron-retentive coupling. The unusual regioselectivity is believed to arise from the stabilization due to the strong electron donation from the C−Pd σ bond to the p-orbital of boron in the transition state of migratory insertion. A broad range of differently substituted tetra- and triarylethenes are constructed in good yields and geometrical control. Synthetic manipulation of the C-B bond also enables the facile construction of several other types of tetra-substituted alkenes.

Highlights

We report a palladium-catalysed regio- and stereoselective diarylation of arylethynyl MIDA boronates with aryl halides and arylboronic acids (Fig. 1c)
We first investigated the feasibility of the palladium-catalysed coupling reaction of phenylethynyl MIDA boronate 1a with iodide 2a (2.0 equiv.) and boronic acid 3a (2.0 equiv.)
Structural elucidation revealed that the phenyl and the 3,5-dichlorophenyl group resides trans to each other across the resulting carbon–carbon double bond, which is in contrast to the previous observations that the aryl group derived from the boronic acid preferentially locates at the position proximal to the aryl substituent of the alkyne, indicating the unique directing effect of MIDA boron

Summary

Introduction

We report a palladium-catalysed regio- and stereoselective diarylation of arylethynyl MIDA boronates with aryl halides and arylboronic acids (Fig. 1c). The regioselectivity, guided by MIDA boron, is distinct from the previous observations when aryl alkyl acetylenes were used as substrates[27,28]. The starting arylethynyl MIDA boronates are readily accessible via a Sonogashira coupling of the corresponding aryl halides with commercially available ethynyl MIDA boronate. Synthetic manipulations of the MIDA boron moiety in the formed diarylated products via a stereospecific Suzuki–Miyaura coupling or base-promoted protodeborylation lead to a truly flexible and stereodefined synthesis of tetra- and triarylethenes

Methods

Results

Conclusion