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Efficient endo Cycloisomerization of Terminal Alkynols Catalyzed by a New Ruthenium Complex with 8‐(Diphenylphosphino)quinoline Ligand and Mechanistic Investigation
Author(s) -
Cai Tao,
Yang Yu,
Li WeiWei,
Qin WenBing,
Wen TingBin
Publication year - 2018
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.201703971
Subject(s) - cycloisomerization , ruthenium , chemistry , quinoline , catalysis , ligand (biochemistry) , reactivity (psychology) , medicinal chemistry , stereochemistry , stoichiometry , enol , catalytic cycle , organic chemistry , receptor , pathology , medicine , biochemistry , alternative medicine
Several new ruthenium complexes supported by the P,N‐donor ligand 8‐(diphenylphosphino)quinoline (DPPQ) were synthesized, including [RuCl 2 (DPPQ) 2 ] ( 1 ), [Ru(μ‐Cl)(DPPQ) 2 ] 2 (BPh 4 ) 2 ( 2 ), and [RuCl(DPPQ) 2 Py](BF 4 ) ( 3 ). Complex 2 , with only 1 mol % loading, was found to be catalytically active for the endo cycloisomerization of various terminal alkynols to endo ‐cyclic enol ethers in moderate to excellent yields. In particular, the 7‐ and 8‐ endo heterocyclization can be achieved efficiently to give the seven‐membered 3‐benzoxepine and eight‐membered 3‐benzo[ d ]oxocine derivatives. The stoichiometric reactions of 2 with various alkynol substrates have been carried out to investigate the mechanism, which led to a series of seven‐, six‐, and five‐membered oxacyclocarbene ruthenium complexes including [RuCl(DPPQ) 2 {=CCH 2 C 6 H 4 CH 2 CH 2 O}](BPh 4 ) ( 12 ) and [RuCl(DPPQ) 2 {=CCH 2 (CH 2 ) n CH 2 O}](BPh 4 ) ( n =3, 12′ ; n =2, 13 ; n =1, 14 ). The quantitative transformation of oxacyclocarbene 12 into catalyst 2 and 3‐benzoxepine 5 a as well as the efficient catalytic activity of 12 for the endo ‐cyclization of 2‐(2‐ethynylphenyl)ethanol ( 4 a ) demonstrated that 12 is a key intermediate involved in the catalytic cycle. Moreover, comparative studies on the modeling reactions and catalytic activity of the series of oxacyclocarbene complexes indicated that the different catalytic activity of 2 for the endo ‐cycloisomerization of different types of alkynols can be related to the reactivity of the respective ruthenium oxacyclocarbene intermediates.