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Metathesis of Olefin‐Substituted Pyridines: The Metalated NCN‐Pincer Complex in a Dual Role as Protecting Group and Scaffold
Author(s) -
Dijkstra Harm P.,
Chuchuryukin Aleksey,
Suijkerbuijk Bart M. J. M.,
van Klink Gerard P. M.,
Mills Allison M.,
Spek Anthony L.,
van Koten Gerard
Publication year - 2002
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 - 10.1002/1615-4169(200208)344:6/7<771::aid-adsc771>3.0.co;2-c
Subject(s) - pincer movement , chemistry , metathesis , pyridine , isomerization , salt metathesis reaction , medicinal chemistry , cationic polymerization , ring opening metathesis polymerisation , palladium , steric effects , catalysis , combinatorial chemistry , organic chemistry , polymerization , polymer
Pincer‐palladium(II) and ‐platinum(II) cations, YCY‐M (YCY=[2,6‐(YCH 2 ) 2 C 6 H 3 ] ; Y=NMe 2 , SPh; M=Pd II , Pt II ), bound to diolefin‐substituted pyridines (3,5‐ or 2,6‐substitution) were successfully synthesized, and subsequently used in olefin metathesis (RCM) as a model study for template‐directed synthesis of macrocycles. Especially a 3,5‐disubstituted pyridine bound to a NCN‐Pt II ‐center ( 5a ) gave a fast metathesis reaction, while the same reaction with the Pd II analogue ( 4a ) was much slower and less selective (isomerization products were formed). Furthermore, it was found that 2,6‐diolefin‐substituted pyridines ( 4b, 5b, 5c ) gave slow metathesis reactions, which is mainly ascribed to steric hindrance during the ring‐closing step. In all cases where prolonged reaction times were required an isomerization process, most likely assisted by cationic pincer‐M II species, was observed as a competing reaction. 1 H NMR spectroscopy experiments revealed that pyridines are stronger bound to a cationic NCN‐Pt II ‐center than to its Pd II ‐analogue. This aspect is of crucial importance when these pincer‐pyridine complexes are applied in metathesis, since free pyridine in solution deactivates the Ru‐metathesis catalyst. For the templated construction of macrocycles, a strong M‐N(py) bond is also important since it determines the selectivity for the desired product. In addition, these results open a new research field in which organometallic (pincer) complexes are used as protecting groups for strong Lewis‐basic groups in catalysis. From failed attempts to prepare macrocycles using hexakis[SCS‐Pd II ‐( 1a )] complex 14 , and from the results obtained with the monometallic pincer complexes in RCM, it can be concluded that the most suitable candidate for constructing macrocycles should comprise 2,6‐diolefin‐substituted pyridines bound to a multi‐(NCN‐Pt II )‐template. In such a system, intrapyridine metathesis (steric hindrance) as well as isomerization reactions (strong M‐N(py) bond) are suppressed.