Abstract
Indenyl (Ind) rhodium N‐heterocyclic carbene (NHC) complexes [Rh(η 5‐Ind)(NHC)(L)] were synthesised for 1,3‐bis(2,6‐diisopropylphenyl)‐4,5‐dihydroimidazol‐2‐ylidene (SIPr) with L=C2H4 (1), CO (2 a) and cyclooctene (COE; 3), for 1,3‐bis(2,4,6‐trimethylphenyl)‐4,5‐dihydroimidazol‐2‐ylidene (SIMes) with L=CO (2 b) and COE (4), and 1,3‐bis(2,4,6‐trimethylphenyl)imidazol‐2‐ylidene (IMes) with L=CO (2 c) and COE (5). Reaction of SIPr with [Rh(Cp*)(C2H4)2] did not give the desired SIPr complex, thus demonstrating the “indenyl effect” in the synthesis of 1. Oxidative addition of HSi(OEt)3 to 3 proceeded under mild conditions to give the Rh silyl hydride complex [Rh(Ind){Si(OEt)3}(H)(SIPr)] (6) with loss of COE. Tethered‐fluorenyl NHC rhodium complexes [Rh{(η 5‐C13H8)C2H4N(C)C2H x NR}(L)] (x=4, R=Dipp, L=C2H4: 11; L=COE: 12; L=CO: 13; R=Mes, L=COE: 14; L=CO: 15; x=2, R=Me, L=COE: 16; L=CO: 17) were synthesised in low yields (5–31 %) in comparison to good yields for the monodentate complexes (49–79 %). Compounds 3 and 1, which contain labile alkene ligands, were successful catalysts for the catalytic borylation of benzene with B2pin2 (Bpin=pinacolboronate, 97 and 93 % PhBpin respectively with 5 mol % catalyst, 24 h, 80 °C), with SIPr giving a more active catalyst than SIMes or IMes. Fluorenyl‐tethered NHC complexes were much less active as borylation catalysts, and the carbonyl complexes were inactive. The borylation of toluene, biphenyl, anisole and diphenyl ether proceeded to give meta substitutions as the major product, with smaller amounts of para substitution and almost no ortho product. The borylation of octane and decane with B2pin2 at 120 and 140 °C, respectively, was monitored by 11B NMR spectroscopy, which showed high conversions into octyl and decylBpin over 4–7 days, thus demonstrating catalysed sp3 C−H borylation with new piano stool rhodium indenyl complexes. Irradiation of the monodentate complexes with 400 or 420 nm light confirmed the ready dissociation of C2H4 and COE ligands, whereas CO complexes were inert. Evidence for C−H bond activation in the alkyl groups of the NHC ligands was obtained.
Highlights
CÀ H activation is typically defined as the weakening or cleavage of a CÀ H bond at a metal centre.[1]
We describe the synthesis of indenyl Rh N-heterocyclic carbene (NHC) and fluorenyl-tethered NHC Rh complexes, their use in the catalytic CÀ H borylation of alkanes and arenes as well as describe their photochemistry and stoichiometric reactivity
Only partial consumption of the starting materials was evident along with the generation of a complex mixture of products. This demonstrated that [Rh(Ind)(C2H4)2] reacts much more readily with SIPr than the Cp* analogue, a manifestation of the “indenyl effect” where auxiliary ligand substitution is accelerated as a result of η3 coordination of the indenyl ligand driven by recovery of benzene ring aromaticity.[33a]. A single crystal X-ray diffraction study identified 10 as one of products from the reaction and provided evidence for one of the pathways involving loss of one ethylene ligand, CÀ H activation of the other and loss of two H atoms to generate a RhÀ Rh bonded dimer with bridging σ,π-vinyl ligands
Summary
“directed” CÀ H activation, where an additional group or ligand on the substrate helps enable the reaction,[2c,4] “undirected” CÀ H bond activation still remains a challenge.[1e,f,2e,5] This includes achieving the high reactivity required to functionalise strong, nonpolar CÀ H bonds, while still controlling the selectivity in order to give the desired product. Substituted arylhydroxamates and cyclopropenes compared to the Cp* analogue.[34] Westcott, Marder and co-workers have used Rh indenyl complexes in hydroboration catalysis,[35] which gave products from the dehydrogenative borylation of alkenes, and demonstrated that [Ir(Ind)(COD)] was a precursor to [Ir(η6-arene)(Bcat)3].[36] The analogous pinacolateboryl complex [Ir(η6-arene)(Bpin)3], in combination with phosphine ligands such as PMe3, Me2PCH2CH2PMe2 and Ph2PCH2CH2PPh2, functioned as an arene CÀ H borylation catalyst in reactions of HBpin at high temperatures (100– 150 °C).[28b] a diversity of reactivity is already evident for indenyl, including the relative importance of η1, η3 and η5 coordination modes as well as potential loss of the indenyl ligand. We describe the synthesis of indenyl Rh NHC and fluorenyl-tethered NHC Rh complexes, their use in the catalytic CÀ H borylation of alkanes and arenes as well as describe their photochemistry and stoichiometric reactivity
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