Palladium Pincer Complex Catalyzed Cross‐Coupling of Vinyl Epoxides and Aziridines with Organoboronic Acids

Abstract

Palladium-catalyzed cross-coupling of vinyl epoxides and aziridines with organoboronic acids was performed by using 0.5-2.5 mol % pincer-complex catalyst. The reactions proceed under mild conditions affording allyl alcohols and amines with high regioselectivity and in good to excellent yields. Under the applied reaction conditions aromatic chloro-, bromo- and iodo substituents are tolerated. Our results indicate that the mechanism of the pincer complex catalyzed and the corresponding palladium(0) catalyzed process is substantially different. It was concluded that the transformations proceed via transmetalation of the organoboronic acids to the pincer-complex catalyst followed by an S(N)2'-type opening of the vinyl epoxide or aziridine substrate. In this process the palladium atom is kept in oxidation state +2 under the entire catalytic process, and therefore oxidative side reactions can be avoided.

Transition metals in organic synthesis: Highlights for the year 2005

“…Ring-opened arylated products were observed as side-products [629]. Palladium catalyzed the reaction of allylic oxiranes and aziridines with aromatic boron reagents to give allylic alcohols and amines, respectively [630]. Related reactions of alkynyl oxiranes with arylboronic acids gave 1-aryl-1,2-dien-4-ols [631].…”

Pd‐Mediated Reactions of Epoxides

“…In the presence of LiCl, Szabó and co-workers isolated the corresponding complex [Equation (49)]. [73] In the absence of other reactive species, 13A evolves to afford either an unsaturated carbonyl compound (paths a or b) or a dienic alcohol (path c). Reactions between 13A and added species can afford unsaturated alcohols (path d), alkenyl heterocycles (paths e and f), alkoxycarbonylation products (path g) or unsaturated diols (path h).…”

“…[43] These couplings were subsequently carried out with aryl or vinylic halides and triflates in place of the organic mercurials [Equations (84), (85), and (86)]. [123,124] It should be noted that similar compounds can be obtained with the aid of Pd catalysis and 1-alkenylboranes [73,125] or aryl-and vinylstannanes, [126] but these reactions usually firstly involve the formation of an η 3 -allylpalladium complex such as 13A (see Scheme 13, path d) [126][127][128] rather than the aryl-or vinylpalladation of the C=C unit (Scheme 16). According to Scheme 28. Scheme 16, allylic alcohols are obtained under Larock's conditions because intermediates such as 16B react with a formate salt.…”

“…[105,106] The method was then used for the synthesis of analogous cis-diols from syn-benzene dioxides [Equation (73)]. [107] In their first report, the authors wrote that the mechanism "may involve opening of one epoxide ring followed by formation of a π-allyl palladium complex and rearrangement, via an oxetane intermediate, with CO acting as a reducing agent".…”

The Trans Influence in Unsymmetrical Pincer Palladacycles: An Experimental and Computational Study

“…Pincer palladacycles are an interesting type of palladacycle, of which there are two different types. The majority of pincer palladacycles studied have been of the symmetrical YCY type, such as NCN [6], SCS [7], PCP [8], and SeCSe [9,10]. There are limited numbers of reported unsymmetrical pincer palladacycles owing to their more difficult synthesis.…”