Premium
Unraveling the Mechanism of 1,3‐Diyne Cross‐Metathesis Catalyzed by Silanolate‐Supported Tungsten Alkylidyne Complexes
Author(s) -
Schnabel Tobias M.,
Melcher Daniel,
Brandhorst Kai,
Bockfeld Dirk,
Tamm Matthias
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.201801651
Subject(s) - metathesis , chemistry , catalysis , trimethylsilyl , dimer , carbyne , alkyne , salt metathesis reaction , stereochemistry , disproportionation , medicinal chemistry , carbene , polymerization , organic chemistry , polymer
The benzylidyne complex [PhC≡W{OSi(O t Bu) 3 } 3 ] ( 1 ) catalyzed the cross‐metathesis between 1,4‐bis(trimethylsilyl)‐1,3‐butadiyne ( 2 ) and symmetrical 1,3‐diynes ( 3 ) efficiently, which gave access to TMS‐capped 1,3‐diynes RC≡C−C≡CSiMe 3 ( 4 ). Diyne cross‐metathesis (DYCM) studies with 13 C‐labeled diyne PhC≡ 13 C− 13 C≡CPh ( 3* ) revealed that this reaction proceeds through reversible carbon–carbon triple‐bond cleavage and formation according to the conventional mechanism of alkyne metathesis. The reaction between 1 and 3* afforded the 3‐phenylpropynylidyne complex PhC≡ 13 C− 13 C≡W{OSi(O t Bu) 3 } 3 ] ( 5* ), indicating that alkynylalkylidyne complexes are likely to act as catalytically active species. Attempts to isolate 5* from mixtures of 1 and 3* afforded crystals of the ditungsten 2‐butyne‐1,4‐diylidyne complex [( t BuO) 3 SiO} 3 W≡ 13 C− 13 C≡ 13 C− 13 C≡W{OSi(O t Bu) 3 } 3 ] ( 6* ), which was additionally characterized by X‐ray diffraction analysis. Depolymerization‐macrocyclization of a carbazole‐butadiyne polymer, obtained from 3,6‐diethynyl‐9‐dodecylcarbazole ( 7 ) under copper‐catalyzed Hay coupling conditions, was also efficiently catalyzed by 1 and afforded a mixture of mono‐, diyne‐ and triyne‐containing tetrameric macrocycles, revealing that diyne disproportionation into monoynes and triynes occurs as a slow side reaction that interferes with a high diyne metathesis selectivity. Potential catalytic pathways were studied by means of quantum‐chemical calculations, and kinetic studies were performed to substantiate an α,α‐mechanism for the catalytic diyne metathesis reaction, which involves intermediate alkynylalkylidyne and α,α′‐dialkynylmetallacyclobutadiene intermediates.