Cobalt‐Catalyzed Addition of Arylzinc Reagents to Alkynes to Form ortho‐Alkenylarylzinc Species through 1,4‐Cobalt Migration

Abstract

Metal migration reactions through C À H bond activation, 1,4-migrations in particular, have attracted significant attention in organorhodium and organopalladium chemistry not only as mechanistically intriguing processes but also as entries to novel remote CÀH bond functionalization reactions. [1][2][3] After the first observation of 1,4-rhodium migration in the reaction of an arylboronic acid with norbornene by Miura et al., [4] Hayashi and co-workers reported that a rhodium-catalyzed hydroarylation reaction of an alkyne with an arylboronic acid involves insertion of the alkyne into an arylrhodium species, vinyl-to-aryl 1,4-rhodium migration, and protonation of the resulting ortho-alkenylarylrhodium species (Scheme 1 a). [5] These seminal studies set the stage for the subsequent development of domino cyclization reactions involving 1,4-rhodium migration followed by trapping of the arylrhodium species with an internal electrophile (e.g., carbonyl group).[2]Herein, we report on a cobalt-catalyzed reaction of an arylzinc reagent with an alkyne that involves a similar vinylto-aryl 1,4-cobalt migration but is unique in that the 1,4-migration is followed by transmetalation with the arylzinc reagent (Scheme 1 b). Trapping of the resulting ortho-alkenylarylzinc species with external electrophiles allows access to a variety of 1-alkenyl-arenes functionalized in the 2-position, many of which are not readily accessible by existing synthetic methods.Recently, we reported the use of a CoCl 2 -Xantphos [6] complex for the catalysis of a zinc insertion reaction into aryl iodides, bromides, and chlorides that allows efficient and convenient preparation of arylzinc reagents. [7] During this study, we found that the Co-Xantphos catalyst promoted an addition reaction of the thus-prepared arylzinc reagent to an unactivated internal alkyne. Thus, a 4-methoxyphenylzinc reagent 1 a prepared from 4-iodoanisole and Zn·LiCl with the cobalt catalyst in THF (denoted as type A reagent) reacted with 4-octyne (2 a) at 608C, and on quenching with water afforded the syn-addition product 3 aa in 70 % yield (Scheme 2 and entry 1 in Table 1). Despite the recent report of Oshima and co-workers on an arylzincation reaction of an unactivated alkyne catalyzed by CoBr 2 alone, [8] this initial observation attracted our attention because their reaction proceeded in acetonitrile but not at all in THF.A subsequent study revealed that Xantphos is a uniquely effective ligand for the present reaction (Table 1). Not Scheme 1. a) Rhodium-catalyzed alkyne hydroarylation involving 1,4-rhodium migration, and b) cobalt-catalyzed alkyne arylzincation involving 1,4-cobalt migration.Scheme 2. Cobalt-catalyzed addition of arylzinc reagents to 4-octyne and types of arylzinc reagent used in this study. For type A and D reagents, the CoCl 2 -Xantphos catalyst that was used to promote the zinc insertion was also used for the addition reaction.

Synthesis of triphenylene-fused phosphole oxides via C–H functionalizations

“…The present synthetic study commenced with the recently reported preparation of 7-hydroxybenzo[b]phosphole derivative 3 from 3-(methoxymethoxy)phenylzinc (1), 5-decyne (2), and PhPCl 2 in the presence of a cobalt-diphosphine catalyst (Scheme 2). This one-pot construction of the benzo[b]phosphole core ensured the preferential phosphole ring closure in proximity of the alkoxy group of the arylzinc reagent 1 (regio- selectivity of ≈3:1), presumably due to a secondary interaction between the MOM group and the cobalt catalyst during the key C-H activation step, i.e., 1,4-cobalt migration in the alkenylcobalt intermediate [29]. The oxidation of the benzo[b]phosphole phosphorous atom and cleavage of the MOM group took place simultaneously, and thus afforded compound 3 in 33% yield on a 5 mmol scale [16].…”

Development of Cobalt-Catalyzed C–H Bond Functionalization Reactions

“…58 The reaction of 4-methoxyphenylzinc reagent with 5-decyne, when quenched with D 2 O, resulted in predominant deuteration of the ortho position of the alkenyl group, indicating the formation of an ortho-alkenylarylzinc species rather than the expected alkenylzinc species. Consistent with this observation, in the reaction of pentadeuteriophenylzinc reagent and 5-decyne, one of the ortho-deuterium atoms was almost completely transferred to the vinylic position of the addition product.…”

Studies on the Nature of Active Cobalt Species for the Production of Methane and Propylene in Catalytic Dehydrogenation of Propane

“…Low-valent cobalt catalysts are known to be efficient in C-H functionalization, such as hydroarylation of olefins [7,8], C-H/electrophiles coupling [9,10], addition of arylzinc reagents to alkynes [11,12]. For these reactions, it is speculated that the mechanism proceeds through a Co 1? intermediate and an oxidative addition/reductive elimination pathway.…”