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Rhodium Indenyl NHC and Fluorenyl‐Tethered NHC Half‐Sandwich Complexes: Synthesis, Structures and Applications in the Catalytic C−H Borylation of Arenes and Alkanes
Author(s) -
Evans Kieren J.,
Morton Paul A.,
Luz Christian,
Miller Callum,
Raine Olivia,
Lynam Jason M.,
Mansell Stephen M.
Publication year - 2021
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.202102961
Subject(s) - borylation , chemistry , carbene , rhodium , catalysis , medicinal chemistry , denticity , hydride , stereochemistry , imes , alkene , iridium , cyclooctene , oxidative addition , benzene , alkyl , crystal structure , hydrogen , organic chemistry , aryl
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=C 2 H 4 ( 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*)(C 2 H 4 ) 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 ‐C 13 H 8 )C 2 H 4 N(C)C 2 H x NR}(L)] ( x =4, R=Dipp, L=C 2 H 4 : 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 B 2 pin 2 (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 B 2 pin 2 at 120 and 140 °C, respectively, was monitored by 11 B NMR spectroscopy, which showed high conversions into octyl and decylBpin over 4–7 days, thus demonstrating catalysed sp 3 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 C 2 H 4 and COE ligands, whereas CO complexes were inert. Evidence for C−H bond activation in the alkyl groups of the NHC ligands was obtained.