Metathesis of Olefin-Substituted Pyridines: The Metalated NCN-Pincer Complex in a Dual Role as Protecting Group and Scaffold
✍ Scribed by Harm P. Dijkstra; Aleksey Chuchuryukin; Bart M. J. M. Suijkerbuijk; Gerard P. M. van Klink; Allison M. Mills; Anthony L. Spek; Gerard van Koten
- Publisher
- John Wiley and Sons
- Year
- 2002
- Tongue
- English
- Weight
- 180 KB
- Volume
- 344
- Category
- Article
- ISSN
- 1615-4150
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✦ Synopsis
Pincer-palladium(II) and -platinum(II) cations, YCY-M (YCY [2,6-(YCH 2 ) 2 C 6 H 3 ] À ; 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.