C5DT00062A.pdf (755.64 kB)
O,N,N-pincer ligand effects on oxidatively induced carbon-chlorine coupling reactions at palladium
journal contribution
posted on 2015-12-08, 12:13 authored by Luka A. Wright, Eric G. Hope, Gregory A. Solan, Warren B. Cross, Kuldip SinghThe syntheses of two families of sterically tuneable O,N,N pro-ligands are reported, namely the 2-(phenyl-2'-ol)-6-imine-pyridines, 2-(C6H4-2'-OH),6-(CMe=NAr)C5H3N [Ar = 4-i-PrC6H4 (HL1(a)), 2,6-i-Pr2C6H3 (HL1(b))] and the 2-(phenyl-2'-ol)-6-(amino-prop-2-yl)pyridines, 2-(C6H4-2'-OH),6-(CMe2NHAr)C5H3N [Ar = 4-i-PrC6H4 (HL2(a)), 2,6-i-Pr2C6H3 (HL2(b))], using straightforward synthetic approaches and in reasonable overall yields. Interaction of HL1(a/c) and HL2(a/b) with palladium(II) acetate affords the O,N,N-pincer complexes, [{2-(C6H4-2'-O)-6-(CMe=NAr)C5H3N}Pd(OAc)] (Ar = 4-i-PrC6H4 (1a), 2,6-i-Pr2C6H3 (1b)) and [{2-(C6H4-2'-O)-6-(CMe2NHAr)C5H3N}Pd(OAc)] (Ar = 4-i-PrC6H4 (2a), 2,6-i-Pr2C6H3 (2b)), which can be readily converted to their chloride derivatives, [{2-(C6H4-2'-O)-6-(CMe=NAr)C5H3N}PdCl] (Ar = 4-i-PrC6H4 (3a), 2,6-i-Pr2C6H3 (3b)) and [{2-(C6H4-2'-O)-6-(CMe2NHAr)C5H3N}PdCl] (Ar = 4-i-PrC6H4 (4a), 2,6-i-Pr2C6H3 (4b)), respectively, on reaction with an aqueous sodium chloride solution. Treating each of 3a, 3b, 4a and 4b with two equivalents of di-p-tolyliodonium triflate at 100 °C in a toluene/acetonitrile mixture affords varying amounts of 4-chlorotoluene along with the 4-iodotoluene by-product with the conversions highly dependent on the steric and backbone properties of the pincer complex employed (viz. 4a > 3a > 4b > 3b); notably, the least sterically bulky and most flexible amine-containing 4a reaches 90% conversion to 4-chlorotoluene in 15 h as opposed to 17% for imine-containing 3b. In the case of 3a, the inorganic palladium species recovered from the reaction has been identified as the Pd(II) salt [{2-(C6H4-2'-O)-6-(CMe=N(4-i-PrC6H4)C5H3N}Pd(NCMe)][O3SCF3] (5a), which was independently prepared by the reaction of 3a with silver triflate in acetonitrile. Single crystal X-ray structures are reported for HL1(a), HL2(a), 1a, 1b, 2a, 2b, 3a and 5a.
Funding
We thank the University of Leicester for financial assistance. Johnson Matthey PLC are thanked for their generous loan of palladium salts.
History
Citation
Dalton Transactions, 2015, 44 (13), pp. 6040-6051Author affiliation
/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of ChemistryVersion
- VoR (Version of Record)