Synthesis and Properties of Bimetallic Hoveyda–Grubbs Metathesis Catalysts

Abstract

The catalytic activity of ruthenium Hoveyda–Grubbs complexes in olefin metathesis is a function of complex steric and electronic effects acting on initiation and propagation steps. In order to study the π-electron factors influencing the initiation process, we attempted syntheses of bimetallic complexes with common organic ligands bearing two chelate rings. While most of the studied ligand exchange reactions of the isomeric bis-chelating benzene derivatives gave mixtures of unstable complexes, a homodinuclear derivative of 1,4-dimethoxy-2,5-divinylbenzene was sparingly soluble and precipitated from the reaction mixture in a pure form. The complex was studied with spectroscopic and X-ray methods, which confirmed the symmetrical bimetallic structure. However, in model metathesis reactions the catalyst displayed activity very comparable to the related monometallic complexes. This suggests that in the bimetallic system two consecutive initiation processes of the metathesis catalyst (first, bimetallic complex + olefin → monometallic complex + propagating species; second, monometallic complex + olefin → styrene + propagating species) proceed at similar rates and, thus, no cooperativity between the two steps is displayed. Properties of the family of bimetallic complexes were probed with NMR studies, and π-electronic effects operating in the systems were discussed.

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“…Following the General Procedure described for 1‐ethenyl‐2,3‐dimethoxybenzene ( 23 ), Wittig reaction of 2,3‐dimethoxybenzaldehyde ( 22 , 20.0 g, 120 mmol) with ethyltriphenylphosphonium bromide (49.2 g, 132 mmol) and t BuOK (14.9 g, 132 mmol) in THF (400 ml), afforded after purification by flash chromatography (600 g silica gel, pentane/Et 2 O, 19:1; R f 0.54) compound 28 (19.3 g, 90% yield) as a yellowish oil consisting of a mixture of (1′ Z / E ) isomers in the ratio of 72:28 (GC). The spectroscopic data were identical to those reported for a (1′ Z / E ) mixture of 83:17 in ref …”

“…Flash chromatographic isolation provided the corresponding prop-1-en-1-yl benzene 28 in 90% yield as a 72:28 (GC) mixture of (1 0 Z/E) isomers, with spectroscopic data identical to those reported in the literature for a (1 0 Z/E) mixture of 83:17. [22] Simmons-Smith cyclopropanation in analogy to the synthesis of compound 24 afforded the corresponding cyclopropane 29 in 84% yield as a diastereomeric mixture of 73% unlike-and 27% like-configured isomer, corresponding very well with the (1 0 Z/E) ratio of the starting material 28. The relative configuration of the cyclopropane moiety is apparent from the vicinal coupling constants 3 J HH of the benzylic hydrogen H-C(1 0 ), which in the case of the major unlike-diastereomer appears as a triplet of doublets at d(H) 2.26 ppm with two cisoid coupling partners of 3 J HH = 9.0 Hz and one transoid coupling partner of 3 J HH = 6.5 Hz, while it appears as a doublet of triplets at d(H) 1.97 ppm with one cisoid coupling partner of 3 J HH = 8.5 Hz and two transoid coupling partners of 3 J HH = 5.0 Hz in the minor like-diastereomer.…”

“…As summarized in Scheme , the synthesis of the 6‐(2′‐methylcyclopropyl)‐substituted target compound 32 proceeded on essentially the same route as for the first target 27 , commencing with a Fitjer–Quabeck modified Wittig reaction utilizing ethyltriphenylphosphonium bromide. Flash chromatographic isolation provided the corresponding prop‐1‐en‐1‐yl benzene 28 in 90% yield as a 72:28 (GC) mixture of (1′ Z / E ) isomers, with spectroscopic data identical to those reported in the literature for a (1′ Z / E ) mixture of 83:17 . Simmons–Smith cyclopropanation in analogy to the synthesis of compound 24 afforded the corresponding cyclopropane 29 in 84% yield as a diastereomeric mixture of 73% unlike ‐ and 27% like ‐configured isomer, corresponding very well with the (1′ Z / E ) ratio of the starting material 28 .…”

“…In this article, the syntheses and olfactory properties of the three possible C 4 6-cyclopropylalkyl-substituted 2H-benzo[b] [1,4]dioxepin-3(4H)-ones is reported. All syntheses commence from 2,3-dimethoxybenzaldehyde (22) as central building block.…”

“…As delineated in Scheme 1, for the synthesis of the lowest member, the 6-cyclopropyl-2H-benzo[b] [1,4]dioxepin-3(4H)-one (27), 2,3-dimethoxybenzaldehyde (22) was first subjected to a Wittig reaction with methyltriphenylphosphonium bromide employing potassium tert-butoxide as a base in THF according to the classical conditions of Fitjer and Quabeck. [16] The corresponding vinyl arene 23, the spectroscopic data of which corresponded to the literature, [17] was isolated in 91% yield by flash chromatography and then subjected to a standard Simmons-Smith cyclopropanation with the organozinc carbenoid generated from diiodomethane and diethylzinc in toluene between 5°a nd room temp.…”