Chromium‐Catalyzed Selective Dimerization/Hydroboration of Allenes to Access Boryl‐Functionalized Skipped ( E , Z )‐Dienes

Abstract

A chromium-catalyzed dimerization/hydroboration of allenes is developed to access synthetically versatile borylfunctionalized skipped dienes with a catalyst generated in situ from CrCl 2 and a pyridine-2,6-diimine ligand mes PDI. A variety of allenes reacted with pinacolborane (HBpin) to afford the corresponding boryl-functionalized (E,Z)-1,4-dienes in high yields and with excellent selectivity. Electron paramagnetic resonance (EPR) spectroscopic studies suggest that this chromium-catalyzed reaction probably proceeds through a chromium(I) hydride intermediate. Catalysts based on first-row transition metals, such as iron, cobalt, nickel, and copper, have been extensively studied in reaction discovery and development over past decades partially because of their low price, high abundance, and low toxicity. [1] Chromium, a first-row early transition metal in Group 6, is an abundant element in the Earths crust. Although Cr VI compounds are highly toxic, Cr II and Cr III compounds, such as CrCl 2 and CrCl 3 , have low toxicity. [2] Chromium compounds have shown unique catalytic reactivities in Nozaki-Hiyama-Kishi reactions, [3] various crosscoupling reactions, [4] and the polymerization and oligomerization of olefins. [5] However, chromium-catalyzed hydroboration reactions of unsaturated hydrocarbons that can afford synthetically versatile organoboron compounds [6] have remained unknown. Skipped dienes are valuable structural motifs in numerous bioactive natural products and synthetic compounds (Scheme 1 A). [7] Several synthetic approaches, such as hydroallylation of alkynes, [8] hydroalkenylation of 1,3-dienes, [9] and coupling of vinyl metallic reagents with various allylic electrophiles, [10] have been developed to prepare skipped dienes. However, systematic approaches to access borylfunctionalized skipped dienes from readily accessible starting materials are still lacking. Considering broad applications of organoboron compounds in chemical synthesis, [6] we are interested in developing selective approaches to synthesize boryl-functionalized skipped dienes because they can be further converted to other unsaturated hydrocarbons via selective C À B bond transformations. Scheme 1. Boryl-functionalized skipped dienes generated by coupling of two allenes with boron reagents.

First-Row d-Block Element-Catalyzed Carbon–Boron Bond Formation and Related Processes

“…Recently, the CrCl 2 -catalyzed dimerization/hydroboration of allenes employing a pyridine-2,6-diimine ligand ( Mes PDI) was developed which affords boryl-funtionalized ( E , Z )-1,4-dienes with excellent regio- and stereoselectivity (Scheme ). A series of substituted allenes were reacted with HBpin, and the corresponding boryl containing skipped dienes were obtained in good to high yields. Electron paramagnetic resonance spectroscopic analysis showed that the reaction possibly proceeds via a Cr­(I)-H species as a key intermediate.…”

Regio- and Stereoselective Diarylation of 1,3-Dienes via Ni/Cr Cocatalysis

“…The replacing of NiCl 2 with CrCl 2 showed a singlet resonance with a g value of 1.98 (Scheme a-2). This characteristic signal matched with that of Cr species (Scheme b) . Then, upon the addition of NiCl 2 to the mixture of CrCl 2 / L3 , a new singlet resonance appeared with a g value of 2.16 (Scheme a-3), which is consistent with the value reported for Ni­(I) species .…”

Silaborative Assembly of Allenamides and Alkynes: Highly Regio‐ and Stereocontrolled Access to Bi‐ or Trimetallic Skipped Dienes

“…In terms of three‐component coupling, Fañanás‐Mastral et al discovered a copper/palladium catalytic system for combining allyl carbonates and alkynes with B 2 Pin 2 to synthesize 1,4‐dienes that feature a single boryl group (Scheme 1A(ii)) [13e] . Last year, Ge and Zhao reported an elegant catalytic method to access borylated ( E , Z )‐1,4‐dienes [13g] through a chromium(I) hydride intermediate.…”