Synthesis of Stereodefined Borylated Dendralenes through Copper‐Catalyzed Allylboration of Alkynes

Abstract

An efficient method to access diversely substituted borylated dendralenes from simple and readily available materials is reported. This method is based on a multicomponent copper‐catalyzed allylboration of alkynes with diboron and a 1,4‐dibromo‐2‐butene which provides bromo‐ and boron‐substituted skipped dienes with a remarkable chemo‐, stereo‐, and regioselectivity. These products can be easily transformed into dendralenic organoboronates, which display an extremely versatile reactivity as demonstrated by novel selective transformations.

Site‐Selective (Z)‐α‐Borylalkenyl Copper Systems for Nucleophilic Stereodefined Allylic Coupling

“…[8] It has been described that copper-catalyzed allylboration of alkynes with 1,4-dibromo-2-butenes and diboron reagents, generate borylated dendralenes via β-borylalkenyl copper intermediates. [9] However, the trapping of the allylic partners with β-borylalkenyl copper complexes usually requires a Pd co-catalysts, to generate skipped dienes with Bpin moiety at the terminal position (Scheme 3a), [10] in sharp contrast to the straightforward allylic coupling with α-Scheme 1. Chemoselective activation of geminal BÀ C(sp 2 )À B sites towards selective coupling reactions.…”

[n]Dendralenes as a Platform for Selective Catalysis: Ligand-Controlled Cu-Catalyzed Chemo-, Regio-, and Enantioselective Borylations

“…Dendralenes are acyclic cross-conjugated polyenes which have been much less investigated than linear polyenes or annulenes, for instance (Figure A). , For a long time, these structures were difficult to prepare and were believed to be unstable entities. Owing to 20 years of remarkable synthetic efforts, the Sherburn laboratory has streamlined access to a great variety of dendralenes on a practical scale, with derivatives containing up to 12 olefinic units. , The same group has investigated the specific physicochemical properties of dendralenes as well as their ability to engage into diene-transmissive Diels–Alder cycloadditions (DTDA) (Figure B). ,− Selective functionalization of dendralenes is challenging. Ru-catalyzed cross-metathesis of [3]­dendralenes was realized by resorting to a protection/deprotection strategy. , Lipshutz showed that only judiciously designed [3]­dendralenic Michael acceptors undergo selective Cu-catalyzed 1,4- or 1,6-conjugate addition (Figure B) .…”