Communication pubs.acs.org/JACS
Transition-Metal-Free, Visible-Light-Enabled Decarboxylative Borylation of Aryl N‑Hydroxyphthalimide Esters Lisa Candish, Michael Teders, and Frank Glorius* Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster, Corrensstrasse 40, 48149 Münster, Germany S Supporting Information *
ABSTRACT: Herein, we report a conceptually novel borylation reaction proceeding via a mild photoinduced decarboxylation of redox-activated aromatic carboxylic acids. This work constitutes the first application of cheap and easily prepared N-hydroxyphthalimide esters as aryl radical precursors and does not require the use of expensive transition metals or ligands. The reaction is operationally simple, scalable, and displays broad scope and functional group tolerance.
C
arboxylic acids are cheap, stable, and abundant, making them ideal synthetic precursors. In particular, the decarboxylative functionalization of carboxylic acids enables their site-specific derivatization to provide valuable materials. Despite its potential utility, decarboxylative functionalization of aromatic carboxylic acids is an underdeveloped class of transformation. It is generally achieved via transition-metalcatalyzed extrusion of CO2 to form nucleophilic organometallic intermediates.1 These processes require high reaction temperatures and are often limited to aromatic acids containing orthosubstituents.1h Alternatively, the oxidative decarboxylation of aromatic acids to afford aryl radicals has been reported using catalytic silver and superstoichiometric persulfate oxidants at high temperatures. Although this approach is not limited to carboxylic acids containing ortho-substitution, to date, it has only been successfully applied to hydrodecarboxylation and decarboxylative arylation protocols, potentially due to the harsh reaction conditions and the requirement for large excesses of the radical trapping species.2 Aryl radicals have also been generated with varying levels of success from the corresponding N-hydroxy-2-thiopyridone (Barton) esters in the presence of a radical initiator at high temperatures. However, as Barton esters are difficult to handle and store due to light sensitivity, this protocol is seldom applied.3 N-Hydroxyphthalimide (NHPI) esters 1 are stable, readily accessible, alternatives to Barton esters. Since the early work by Okada,4 generation of alkyl radicals from these redox-activated acids, by either a photocatalyzed5 or thermally promoted6 single electron reduction, has been widely explored and proven a versatile tool for the late stage functionalization of complex molecules (Figure 1a). However, to date, NHPI esters have never been exploited as a cheap, stable and readily accessible source of aryl radicals. Building on our recent work exploring the first photoredoxcatalyzed decarboxylation of aryl carboxylic acids,7 we became interested in employing NHPI esters 2 as aryl radical precursors © 2017 American Chemical Society
Figure 1. (a) Reductive decarboxylation of alkyl carboxylic acids, and (b) strategy for the decarboxylative borylation of aryl carboxylic acids.
(Figure 1b). The photophysical and electrochemical properties of the phthalimide unit have been extensively studied. Although these compounds are poor oxidants (E1/2 = −1.4 V) in the ground state, photoexcitation affords a good single electron oxidant (E1/2* = +1.6 V vs SCE), with a sufficiently long triplet state lifetime to partake in bimolecular electron transfer processes.8 We therefore hypothesized that in the presence of a suitable donor, light-promoted single electron reduction of phthalimide ester 2 might result in decarboxylation to yield the aryl radical, thus avoiding the need for added transition metal reductants. We were inspired by Jiao’s recent work describing the pyridine-catalyzed radical borylation of aryl halides,9 and reports that Lewis adducts of boronic esters and pyridines are readily oxidized.10 Thus, it was reasoned that the Lewis adduct of diboronate ester 3 and pyridine (i.e., I)11 should provide an electron donor species capable of reducing the excited state phthalimide (acceptor*) (Figure 1b). Borylation of the resultant aryl radical would provide an aryl boronic ester, an important synthetic building block.12 This strategy would enable the mild, transition-metal-free13,14 decarboxylative borylation of aryl N-hydroxyphthalimide esters,15 which would complement known transition metal-catalyzed decarbonylative borylation protocols.16 Studies commenced with the irradiation of ester 2A in the presence of 3, pyridine and a base using cheap, commercially Received: March 28, 2017 Published: May 17, 2017 7440
DOI: 10.1021/jacs.7b03127 J. Am. Chem. Soc. 2017, 139, 7440−7443
Communication
Journal of the American Chemical Society Table 1. Reaction Optimization
Scheme 1. Synthesis of NHPI Esters*
entry
[X]
LEDs (λmax, nm)
yield (%)
1 2 3 4 5 6 7 8
A A A A A A B C, D, or E
455 (5 W) 420 (3 W) 400 (3 W) 400a − − 400 400
