Rhodium-catalyzed C(sp²)–H Addition of Arylboronic Acids to Alkynes Using a Boron-based, Convertible ortho-Directing Group

Abstract

Temporary modification of a boronyl group with pyrazorylaniline allowed insertion of arylpropiolates and diphenylacetylenes into the o-CH bond of arylboronic acids in the presence of rhodium catalysts, giving 3,3-diarylacrylates and triarylethene containing aryl groups bearing an o-boryl group stereoselectively. The boronyl group in the 3,3-diarylacrylates was converted into various functional groups, including chlorine, hydrogen, hydroxy, and aromatic groups. Keywords: C-H activation | Rhodium catalyst | Convertible directing groupDirected or nondirected addition of an aromatic CH bond across carboncarbon multiple bonds is recognized as one of the most efficient synthetic methods for arylethene and styrene derivatives.1 In particular, directed CH alkenylation has attracted much attention because it allows the highly regioselective introduction of organic groups into the o-position of the directing group. Since the establishment of Murai's Rucatalyzed, carbonyl-directed alkenylation, 2 a variety of directing groups in combination with various transition metals, including Ru, 3 Rh, 4 Ir, 5 and Pd 6 have been developed for o-CH alkenylation. However, this strategy significantly limits the substrate scope, when the directing group is not removable or convertible. To overcome this limitation, much attention has been focused on the development of convertible or traceless directing groups, 7 such as triazene 4f and carboxylic acid 3o groups. However, those convertible directing groups often lack high versatility in the subsequent transformations. Therefore, it is highly desirable to establish directing groups that can be converted into a variety of functional groups. We have established pyrazorylaniline (PZA) 8 and anthranilamide (AAM) 9 as easily attachable and detachable directing groups for directed CH functionalization reactions. In these studies, the PZA-modified boronyl group (Bpza) served as an o-directing group, which could be utilized for further transformations by the versatile reactivity of the boronyl group. However, the use of PZA has so far been limited to Ru-catalyzed o-CH silylation 8a and Ir-catalyzed o-CH borylation. 8c Our current focus is on the utilization of temporary directing groups such as PZA and AAM in o-CH addition to carboncarbon multiple bonds. In this work, we demonstrate the addition of o-CH bonds of arylboronic acids to arylpropiolates, which provides 3,3-diarylacrylates bearing two different aryl groups, one of which has an o-boryl group. Unsymmetrical 3,3-diarylacrylates are utilized as key starting materials for asymmetric reactions for the synthesis of enantioenriched 3,3-diarylpropanoates.10 Although stereoselective synthesis of unsymmetrical 3,3-diarylacrylates has been accomplished through several reactions, 11 such as MizorokiHeck-type reactions of cinnamate 12 and the Cu-catalyzed conjugate addition of arylboronic acid to alkynoate, 13 only a few examples of regioselective synthesis of o-functionalized 3,3-diarylacrylate have been reported. Herein, we report Rh-catalyzed o-C...

Boryl-Directed, Ir-Catalyzed C(sp3)–H Borylation of Alkylboronic Acids Leading to Site-Selective Synthesis of Polyborylalkanes

“…We have developed pyrazorylaniline (PZA) and anthranilamide (AAM) as easily attachable and detachable ortho -directing boron modifiers in C­(sp 2 )–H functionalization of arylboronic acids such as Ru-catalyzed silylation, , Ir-catalyzed borylation, and Rh-catalyzed alkenylation . PZA also works as a directing group in Ru-catalyzed C­(sp 3 )–H silylation of methyl and ethylboronic acids .…”