- Iridium-Catalyzed Borylation of Primary Benzylic C-H Bonds without a Directing Group: Scope, Mechanism, and Origins of Selectivity
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Primary benzylic boronate esters are useful intermediates in organic synthesis, but these reagents cannot be prepared by hydroboration. The benzylic C-H borylation of methylarenes would be a method to form these products, but such reactions without neat methylarene or a directing group are unknown. We report an approach to divert the borylation of methylarenes from aromatic positions to benzylic positions with a silylborane as reagent and a new iridium catalyst containing an electron-deficient phenanthroline as ligand. This system forms benzylic boronate esters selectively over the corresponding aryl boronate esters. An Ir diboryl monosilyl complex ligated by the phenanthroline was isolated and determined to be the resting state of the catalyst. Mechanistic studies show that this complex is kinetically competent to be an intermediate in the catalytic process. Kinetic studies of benzylic and aryl C-H borylation catalyzed by various Ir complexes show that the rate of aryl C-H borylation decreases with decreasing electron density at the metal center of the Ir catalyst, but that the rate of benzylic C-H borylation is less sensitive to the degree of electron density at the metal center of the Ir catalyst. Kinetic and computational studies suggest that the two borylation reactions respond differently to the degree of electron density at the metal center because they occur with different turnover-limiting steps. The turnover-limiting step in the borylation of aryl C-H bonds is known to be C-H oxidative addition, but the turnover-limiting step of the borylation of benzylic C-H bonds appears to be an isomerization prior to C-B reductive elimination.
- Larsen, Matthew A.,Wilson, Conner V.,Hartwig, John F.
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p. 8633 - 8643
(2015/07/15)
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- Synthesis and photophysical properties of 3,8-disubstituted 1,10-phenanthrolines and their ruthenium (II) complexes
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The palladium-catalysed cross-coupling reaction between 3,8-dibromo-1,10-phenanthroline with phenylacetylene or 3, 5-bis (trifluoromethyl) phenylboronic acid gives good yields of the 3, 8-disubstituted products. These 1,10-phenanthroline derivatives are used for the formation of novel ruthenium complexes of the type [(tbbpy)2Ru(phenR 2)]2+ [where tbbpy = 4, 4′-di-tert-butyl-2,2′- bipyridine, phen = 1, 10-phenanthroline, R represents the substituents at the 3, 8 positions with bromine, phenylacetylene or 3, 5-bis (trifluoromethyl)phenyl]. All compounds are completely characterised by NMR and UV/Vis spectroscopy, MS, electrochemical measurements and Raman and resonance Raman spectroscopy. The photophysical properties indicate a strong influence of the substitution of the phenanthroline ligand on the absorption, emission and Raman properties. With resonance Raman spectroscopy the localisation of the singlet metal-to-ligand charge-transfer (1MLCT) excited state is determined. The solid-state structures of 3, 8-dibromo-1, 10-phenanthroline (phenBr2) and the corresponding ruthenium complex [(tbbpy)2Ru(phenBr2)] 2+ and a structural motif of f(tbbpy)2- Ru[phen-3,8-bis[3, 5-bis (trifluoromethyl)phenyl]}]2+ are also reported.
- Karnahl, Michael,Krieck, Sven,Goerls, Helmar,Tschierlei, Stefanie,Schmitt, Michael,Popp, Juergen,Chartrand, Daniel,Hanan, Garry S.,Groarke, Robert,Vos, Johannes G.,Rau, Sven
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experimental part
p. 4962 - 4971
(2010/05/01)
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