Catalysis of linear alkene metathesis by Grubbs-type ruthenium alkylidene complexes containing hemilabile α,α-diphenyl-(monosubstituted-pyridin-2-yl)methanolato ligands.

Tegene T Tole,Hermanus C M Vosloo,Johan H L Jordaan

doi:10.3762/bjoc.15.19

Abstract

Four new Grubbs-type precatalysts [RuCl(H2IMes)(O^N)(=CHPh)], where [O^N = α,α-diphenyl-(3-methylpyridin-2-yl)methanolato, α,α-diphenyl-(4-methylpyridin-2-yl)methanolato, α,α-diphenyl-(5-methylpyridin-2-yl)methanolato and α,α-diphenyl-(3-methoxypyridin-2-yl)methanolato] were synthesized and tested for their activity, stability and selectivity in the 1-octene metathesis reaction. Overall the precatalysts showed good activity and high stability for the metathesis of 1-octene at temperatures above 80 °C and up to 110 °C. Selectivities towards the primary metathesis products, i.e., 7-tetradecene and ethene, above 85% were obtained with all the precatalysts at 80 and 90 °C. High selectivities were also observed at 100 °C for the 4-Me- and 3-OMe-substituted precatalysts. With an increase in temperature an increase in isomerisation products and secondary metathesis products were observed with the latter reaching values >20% for the 3-OMe- and 3-Me-substituted precatalysts at 110 and 100 °C, respectively. All the precatalysts exhibits first-order kinetics at 80 °C with the 3-substituted precatalysts the slowest. The behaviour of the 3-substituted precatalysts can be attributed to electronic and steric effects associated with the adjacent bulky phenyl groups.

Highlights

The alkene metathesis reaction is well established as a powerful synthetic tool in organic and polymer chemistry [1,2]
The development of metal alkylidene precatalysts based on ruthenium, starting with the so-called Grubbs 1 (1) and 2 (2) metal carbenes, played a major role to extend the versatility of the reaction including the application of these in industrial processes (Figure 1)
We investigated a number of Grubbs 1- and Grubbs 2-type (5) metal carbenes with pyridinyl alcoholato ligands for the 1-octene metathesis reaction (Figure 3) [10,11,12,13,14]

Summary

Introduction

The alkene metathesis reaction is well established as a powerful synthetic tool in organic and polymer chemistry [1,2]. Denk et al [9] synthesised N-heterocyclic (NHC) ruthenium alkylidene complexes containing pyridinyl-alcoholato ligands (4) These complexes were tested as precatalysts at different temperatures in the ROMP of norbornene and cyclooctene. We investigated a number of Grubbs 1- and Grubbs 2-type (5) metal carbenes with pyridinyl alcoholato ligands for the 1-octene metathesis reaction (Figure 3) [10,11,12,13,14]. An interesting result was the very high affinity (“stickiness”) to untreated, unmodified and commercially available chromatography-grade silica This was exploited further by Cabrera et al [17,18] when 5b and related complexes were investigated as heterogeneous precatalysts in biphasic RO-RCM and CM reactions. The stability, selectivity and turnover frequency (TOF) of 2 increased upon substituting Me and OMe groups on the various

Results and Discussion

Conclusion

Experimental procedures