Nitro-Grela-type complexes containing iodides – robust and selective catalysts for olefin metathesis under challenging conditions

Summary Iodide-containing nitro-Grela-type catalysts have been synthesized and applied to ring closing metathesis (RCM) and cross metathesis (CM) reactions. These new catalysts have exhibited improved efficiency in the transformation of sterically, non-demanding alkenes. Additional steric hindrance in the vicinity of ruthenium related to the presence of iodides ensures enhanced catalyst stability. The benefits are most apparent under challenging conditions, such as very low reaction concentrations, protic solvents or with the occurrence of impurities.


General
Toluene was washed with citric acid (1 M in water) and deionized water, dried with sodium sulfate and by distillation over Na. Dry solvent was transferred under argon and stored over 4 Å MS. MeOH, iPrOH, and 2-MeTHF (all ACS grade) were purchased from Sigma, toluene (pure p.a.) was purchased from POCH SA (Poland); all low quality solvents were used for metathesis reactions without further purification.

MS (ESI): Micromass LCT mass spectrometer and LCT PremierXE Waters mass spectrometer.
All metathesis substrates and products are known compounds. The identity of metathesis products previously reported by us and authenticated by NMR spectroscopy was confirmed by comparison of GC retention times.
s10 General procedure for metathesis in dry, degassed Toluene.
Substrate (1 mmol) was placed in a flame-dried Schlenk tube and dry, degassed toluene (C = 0.2 M) was added. The reaction vessel was placed in an oil bath and was thermostated at 70 °C for 15 minutes prior to addition of the solution of the catalyst in dry, degassed toluene. After 2 h a sample for GC analysis was taken and it was quenched with ethyl vinyl ether.

General procedure for macrocyclizations (in dry, degassed toluene).
A stock solution of substrate 17 or 18 in dry, degassed toluene (0.01 M, 10 ml) was placed in a flame-dried Schlenk tube and dry, degassed toluene (10 ml) was added. The reaction vessel was placed in an oil bath and it was thermostated at 70 °C for 15 minutes prior to addition of the first portion of catalyst. The catalyst was added (as a stock solution in toluene) in 10 portions in 7-minute-intervals. After 2 h a sample for GC analysis was taken and it was quenched with ethyl vinyl ether. The GC yield and selectivity was calculated using an external standard.

General procedure for metathesis in ACS grade solvents.
Substrate (1 mmol) was placed in a round-bottom flask and diluted with an appropriate solvent (C = 0.2 M). The reaction vessel was placed in an oil bath and it was thermostated at requested temperature for 15 minutes prior to addition of a solution of the catalyst in DCM. After 2 h a sample for GC analysis was taken and it was quenched with ethyl vinyl ether. External standards were used to calculate the GC yield and selectivity.