Premium
Tungsten and Molybdenum 2,4,6‐Trimethylbenzylidyne Complexes as Robust Pre‐Catalysts for Alkyne Metathesis
Author(s) -
Haberlag Birte,
Freytag Matthias,
Jones Peter G.,
Tamm Matthias
Publication year - 2014
Publication title -
advanced synthesis and catalysis
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.541
H-Index - 155
eISSN - 1615-4169
pISSN - 1615-4150
DOI - 10.1002/adsc.201400051
Subject(s) - chemistry , molybdenum , alkyne , catalysis , metathesis , steric effects , tungsten , medicinal chemistry , molecular sieve , substrate (aquarium) , lithium (medication) , stereochemistry , inorganic chemistry , organic chemistry , polymerization , medicine , polymer , endocrinology , oceanography , geology
A series of 2,4,6‐trimethylbenzylidyne tungsten and molybdenum complexes was prepared from the tribromides mer ‐[MesCMBr 3 (dme)] ( 9a , M=W; 9b , M=Mo, dme=1,2‐dimethoxyethane). Successive reaction of complexes 9 with lithium or potassium hexafluoro‐ tert ‐butoxide and lithium 1,3‐di‐ tert ‐butylimidazolin‐2‐imide, (Im t ‐Bu N)Li, afforded the imidazolin‐2‐iminato complexes [MesCM{OC(CF 3 ) 2 Me} 2 (Im t ‐Bu N)] ( 10a , M=W; 10b , M=Mo), whereas the reactions of 9 with three equivalents of LiOSi(O‐ t‐ Bu) 3 or KOC(CF 3 ) 3 gave [MesCM{OSi(O‐ t‐ Bu) 3 } 3 ] ( 11a , M=W; 11b , M=Mo) and [MesCM{OC(CF 3 ) 3 } 3 ] ( 12a , M=W; 12b , M=Mo), respectively. For comparison, the benzylidyne complex [PhCMo{OSi(O‐ t‐ Bu) 3 } 3 ] ( 7b ) was also prepared, and its molecular structure together with those of 10a , 10b , 11b , 12a and 12b were established by X‐ray diffraction analysis. Complexes 10 and 11 were employed as pre‐catalysts for the alkyne metathesis of the test substrate 3‐pentynyl benzyl ether ( 13 ) at low catalyst loadings (1 mol%) in the presence of molecular sieve (5 Å). Comparative studies of these 2,4,6‐trimethylbenzylidyne species (MesCM) with their benzylidyne analogues (PhCM) revealed that the increased steric bulk renders the former more stable and manageable in air in solid form for shorter periods of time, but at the expense of a slower initiation, which requires higher temperatures or longer reaction times.