Computational understanding of catalyst-controlled borylation of fluoroarenes: directed vs. undirected pathway.

Yu-Hua Liu,Zhong-Jie Jiang

doi:10.1039/d0ra03428b

Abstract

In this work, density functional theory (DFT) calculations are performed to understand the origin of the regioselective C–H borylation of aromatics catalyzed by Co(i)/iPrPNP and Ir(iii)/dtbpy (4,4-di-tert-butyl bipyridine). The calculation results indicate that for the Co(i)/iPrPNP catalytic system, the undirected pathway is 2.9 kcal mol−1 more favoured over the directed pathway leading to ortho-to-fluorine selectivity. In contrast, for the Ir(iii)/dtbpy catalytic system, the directed pathway is 1.2 kcal mol−1 more favoured over the undirected pathway bringing about ortho-to-silyl selectivity. For Co(i)/iPrPNP catalyzed borylation, the undirected pathway which involves steps of ortho-to-fluorine C–H oxidative addition, C–B reductive elimination, B–B oxidative addition, and B–H reductive elimination is favorable due to the electron deficient character of the ortho-to-fluorine C–H bond. For Ir(iii)/dtbpy catalyzed borylation, the directed pathway consisting of Si–H oxidative addition, B–H reductive elimination, C–H oxidative addition, B–B oxidative addition, C–B reductive elimination, Si–H reductive elimination is favored over the undirected pathway attributed to the directing effect of the hydrosilyl group. The favourable undirected pathway (ortho-to-fluorine selectivity) for Co(i)/iPrPNP catalyzed borylation and the favourable directed pathway (ortho-to-silyl selectivity) for Ir(iii)/dtbpy catalyzed borylation could explain well the experimentally observed ortho-to-fluorine borylation of hydrosilyl substituted fluoroarenes with cobalt catalyst (J. V. Obligacion, M. J. Bezdek and P. J. Chirik, J. Am. Chem. Soc., 2017, 139, 2825–2832) and ortho-to-silyl selectivity with iridium catalyst (T. A. Boebel and J. F. Hartwig, J. Am. Chem. Soc., 2008, 130, 7534–7535).

Highlights

Transition metal catalyzed C–H borylation of aromatics has attracted considerable attention as it offers an alternative method to standard organic synthesis.[1]
According to the probable mechanisms of Ir(III) and Co(I) catalyzed borylation of aromatics proposed in literature,12a,13 the directing group controlled borylation catalyzed by iridium may proceed in a way that the iridium centre is brought close to the ortho-to-silyl C–H bond by the silyl group and induces the activation of ortho-to-silyl C–H bond to form a 5-membered cycle.12a In contrast, cobalt-catalyzed borylation starts with ortho-to- uorine C–H oxidative addition[13] and no participation of directing group
C–H bond is favorable over the borylation of the aromatic C–H bond.9a Comparing the rate-determining step in the directed path I0 and undirected path II0 for the iridium catalyzed C–H borylation (TS9a0 vs. TS2b0), the directed pathway leading to ortho-to-silyl selectivity is 1.2 kcal molÀ1 more favorable over undirected pathway leading to ortho-to- uorine selectivity (24.6 kcal molÀ1 barrier of TS9a0 in Fig. 4a vs. 25.8 kcal molÀ1 barrier of TS2b0 in Fig. 4a).With Ir(III)/dtbpy catalytic system, the calculated favorable directed pathway is consistent with experimental observed ortho-to-silyl selectivity[11] (82%, Scheme 2)

Summary

Introduction

Transition metal catalyzed C–H borylation of aromatics has attracted considerable attention as it offers an alternative method to standard organic synthesis.[1]. 1 in Fig. 3b) We believe that the lower overall barrier of the undirected pathway than directed pathway can be ascribed to the electron-withdrawing of the uorine atom ortho to the activated C–H bond, because the electron de ciency property of the transition state accelerates the C–B reductive elimination (18.0 kcal molÀ1 of the barrier for TS4b vs 29.2 kcal molÀ1 of that for TS4a, Fig. 3a).

Results

Conclusion