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- W2135368304 abstract "Pincer-palladium(II) and -platinum(II) cations, YCY-M (YCY=[2,6-(YCH2)2C6H3]; Y=NMe2, SPh; M=PdII, PtII), 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-PtII-center (5a) gave a fast metathesis reaction, while the same reaction with the PdII 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-MII species, was observed as a competing reaction. 1H NMR spectroscopy experiments revealed that pyridines are stronger bound to a cationic NCN-PtII-center than to its PdII-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-PdII-(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-PtII)-template. In such a system, intrapyridine metathesis (steric hindrance) as well as isomerization reactions (strong M-N(py) bond) are suppressed." @default.
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- W2135368304 date "2002-08-01" @default.
- W2135368304 modified "2023-10-16" @default.
- W2135368304 title "Metathesis of Olefin-Substituted Pyridines: The Metalated NCN-Pincer Complex in a Dual Role as Protecting Group and Scaffold Part of this work was presented at the 14th International Symposium on Olefin Metathesis, Boston, August 2001" @default.
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- W2135368304 doi "https://doi.org/10.1002/1615-4169(200208)344:6/7<771::aid-adsc771>3.0.co;2-c" @default.
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