Catalytic Dehydrogenative C–C Coupling by a Pincer-Ligated Iridium Complex

Abstract

The pincer-iridium fragment (PCP)Ir (PCP = κ-2,6-CH(CHPR)) has been found to catalyze the dehydrogenative coupling of vinyl arenes to afford predominantly (E,E)-1,4-diaryl-1,3-butadienes. The eliminated hydrogen can undergo addition to another molecule of vinyl arene, resulting in an overall disproportionation reaction with 1 equiv of ethyl arene formed for each equivalent of diarylbutadiene produced. Alternatively, sacrificial hydrogen acceptors (e.g., tert-butylethylene) can be added to the solution for this purpose. Diarylbutadienes are isolated in moderate to good yields, up to ca. 90% based on the disproportionation reaction. The results of DFT calculations and experiments with substituted styrenes indicate that the coupling proceeds via double C-H addition of a styrene molecule, at β-vinyl and ortho-aryl positions, to give an iridium(III) metalloindene intermediate; this intermediate then adds a β-vinyl C-H bond of a second styrene molecule before reductively eliminating product. Several metalloindene complexes have been isolated and crystallographically characterized. In accord with the proposed mechanism, substitution at the ortho-aryl positions of the styrene precludes dehydrogenative homocoupling. In the case of 2,4,6-trimethylstyrene, dehydrogenative coupling of β-vinyl and ortho-methyl C-H bonds affords dimethylindene, demonstrating that the dehydrogenative coupling is not limited to C(sp)-H bonds.

One‐pot Synthesis of 1,3‐Butadiene and 1,6‐Hexanediol Derivatives from Cyclopentadiene (CPD) via Tandem Olefin Metathesis Reactions

“…The brown solution was transferred into a screw‐capped NMR tube under nitrogen and 1 H NMR spectra were collected over six hours (t[min]=14, 26, 40, 49,69, 102, 129, 192, 257, 308) at room temperature. 1 H NMR of the products 8 and 9 were in agreement with literature data (Figure ) . After 24 hours, part of the reaction mixture (0.1 mL) was hydrogenated over Pd/C in EtOH and the reaction mixture was analyzed by GC‐MS measurements.…”

DFT Studies on the Mechanism of the Iridium-Catalyzed Formal [4 + 1] Cycloaddition of Biphenylene with Alkenes

“…7 Int0 shows a catalytic active species generated from [Ir(cod)Cl] 2 and the diphosphine ligand along with the liberation of cycloocta-1,5-diene (thermodynamic consideration of the generation of Int0 is shown in the Supporting Information). 8 The first step is the coordination of biphenylene ( 1 ) to Int0 to give a η 2 -coordinated intermediate Int1 , which was judged by the natural bond orbital (NBO) analysis. 9 Oxidative addition of the iridium center to the C–C bond of biphenylene gave Int2 via the transition state TS1 (activation energy: Δ G = 19.1 kcal/mol).…”

Regioselective Gas‐Phase n‐Butane Transfer Dehydrogenation via Silica‐Supported Pincer‐Iridium Complexes

“…Pincer‐iridium complexes of the form [( R4 PCP)Ir] ( R4 PCP=κ 3 ‐C 6 H 3 ‐2,6‐(XPR 2 ) 2; X=CH 2 , O; R= t Bu, i Pr) have received great attention in the context of hydrocarbon functionalization and have found application in reactions such as alkane dehydrogenation and alkane metathesis among other C−H activation reactions, [2] dehydroaromatization, [3] and dehydrogenative C−C coupling [4] . Thermal (acceptorless) dehydrogenation can be performed using these species at rates of >700 hr −1 at 200 °C [2a–f,h−n] .…”