Ruthenium with carbenes

If ROM-RCM of cycloalkene-yne 123, which has a substituent at the 2-position of cycloalkene, is carried out under ethylene gas, what compound is formed In this reaction, ruthenium carbene XIX would be formed via [2-1-2] cycloaddition of ruthenium methylidene carbene and alkyne as shown in Eq. (6.91). If XIX reacts with an olefin intramolecularly or ethylene, bicyclic compound 124 or triene 125... 

In 1998, Galardon et al. [58] reported the crystal structure of tetraphenyl-porphyrinate-ruthenium-(diethoxycarbonyl)carbene complex 25, which exhibited catalytic activity for cyclopropanation of EDA and styrene, giving 85% of the product with 93 7 trans-to-cis ratio. The Ru-C distance is 1.829 A and the carbon resonance is at <5C=271.3 ppm. In 2000, Bianchini and Lee [59] isolated the similar ruthenium carbene complexes 26 with tridentate imine ligands and EDA Ru=CH-, <5H=20.44 ppm (s), <5C=299.9 ppm [59]. Simonneaux et al. [60] isolated the phosphonate carbene complexes of ruthenium porphyrins. 

Figure 3.66 Rhodlum(l) and ruthenium(ll) carbene complexes with methoxyalkyl functionalised NHC ligands.

Numerous highly enantioselective ring-forming reactions have also been discovered with the assistance of 1. Cyclopropanation with the rhodium complex of 1 furnishes rranr-cyclopropanes selectively (eq l)y A discrete ruthenium vinyl carbene was similarly successful in the stoichiometric cyclopropanation, whereas enantioselection in the copper(I)-catalyzed variant was nonselec-... 

Later, Grubbs and co-workers showed that water-stable ruthenium II) carbenes can be synthesized by allowing 3,3-diphenylcyclopropene to react with RuCl2(PPh3)4 (eq. (7)) [30]. 

Ulman, M., Grubbs, R. H. Relative Reaction Rates of Olefin Substrates with Ruthenium(ll) Carbene Metathesis Initiators. Organometallics 1998, 17, 2484-2489. 

The chiral ruthenium(II) carbene complex 8, prepared from diazo(trimethylsilyl)methane, (p-cymene)2ruthenium(II) chloride, and 2,6-bis(4-isopropyloxazolinyl)pyridine, has been introduced as catalyst for the enantioselective cyclopropanation of alkenes with ethyl diazoacetate. The carbene complex 8 also serves as a transfer reagent for trimethylsilylcarbene and cyclopro-panates styrene in 34% yield. This reaction demonstrates the similarities between catalytic and stoichiometric cyclopropanations and between in situ generated and isolated transition metal carbenes. 

Ring-opening metathesis polymerization (ROMP) of substituted bicyclo octa-dienes or paracyclophane-enes initiated by Gmbbs molybdenum, tungsten-based carbenes have been used to prepare PPV s [178—181]. The living character of ROMP has been exploited to prepare soluble well-defined precursors, which can be converted into XI. Yu and Turner have used ROMP of tetra octyloxy-substituted paracyclo-phanedienes initiated by reactive ruthenium-based carbenes to prepare monodisperse, soluble yellow fluorescent PPV with an alternating cis-trans microstructure and molecular weights as calculated [178] (Fig. 9.21). 

The first ruthenium porphyrin carbene complex was reported by Balch and coworkers [ 120] by metallation of an N,N -vinyl-bridged porphyrin [ 105,121 ] with Ru3(CO)i2 (Scheme 12). In this reaction, both of the C - N bonds (vinyl) were broken. Surprisingly, this reaction also yields two ruthenium(Il) dicarbonyl complexes in which the N,N -vinyl bridge remains intact, but the ruthenium has been inserted into a pyrrole C-N bond [122,123]. Upon heating, these two complexes are converted to the axial ruthenium carbene complex. 

The first X-ray structure of a ruthenium porphyrin carbene complex was reported by Simonneaux and coworkers (Fig. 1) [133]. To stabilize the ruthenium carbene complex, ethyl diazomalonate was used instead of ethyl diazomethyl acetate, as was previously reported in the bis(oxazolinyl)pyridine (pybox) series [134]. The presence of this complex as an intermediate in cy-clopropanation was also discussed in relation with stoichiometric transfer to alkenes (vide infra) [135]. 

Another synthetically useful isomerization that can be combined with a metathesis event in a domino transformation is a metallotropic [l,3]-shift of a transient ruthenium aUdnyl carbene complex [33]. Treatment of the polyunsaturated substrate 162 with Grubbs catalyst 2 afforded the oligoenyne 163 in 86% yield... 

The two examples shown in Scheme 2.57 illustrate that the metaUotropic shift depends on the nature of the substituent on the 1,3-diyne [33bj. When the substrates 164 and 166 were subjected to 1-octene in the presence of catalyst 2, an initial enyne CM occurred followed by cychzation to give a ruthenium alldnyl carbene intermediate. Termination by CM with the co-olefin after metaUotropic [l,3]-shift was observed for the benzyloxymethyl substituent with production of 165, whereas termination at the sterically less substituted carbon deUvered the complementary regioisomer 167 in case of triisopropylsilyl substitution. 

Enyne metathesis/metallotropic [l,3]-shift domino processes are also valuable for natural product synthesis [33c,d]. Reaction of substrate 168 with cis-l,4-diacetoxy-2-butene in the presence of Grubbs catalyst 2 generated the intermediate ruthenium alkinyl carbene through a relay RCM with the hberation of 2,5-dihydrofuran followed by metallotropic [l,3]-shift and terminating (Z)-selective CM with the co-olefin to yield the conjugated enediyne 169 (Scheme 2.58) [33c]. The antitumor active Panax ginseng constituent (3R,9R,10R)-panaxytriol was readily synthesized from 169 in six steps. 

Ruthenium]IV)-carbene complexes can be employed for the direct arylation of alkenes with chloroarenes [130]. Phosphine oxides have also been used as preU-gands in ruthenium-catalyzed direct arylations via C—H bond functionalization using aryl chlorides or aryl tosylates [131-133]. A noteworthy recent finding shows... 

In conclusion, the disappearance of the benzylidene fragment during the ATRP of methyl methacrylate could be explained by the reaction of the ruthenium benzylidene with the monomer, giving rise to highly unstable ruthenium ester-carbene complexes, and it is possible that these species then quickly decompose. In addition, the absence of [Ru=CH2] is also most probably indicative of the decomposition of these ruthenium carbene species, since [Ru=CH2] are presumed to be the propagating species in RCM and related ruthenium methylidene derivatives have a quite long lifetime in olefin metathesis. Until now, the exact nature of the inorganic decomposition products is not known. 

The second step involved the ROMP of the functionalised PEO macromer using the well-defined Schrock (hexa-fluorinated) or Grubbs initiating complexes. Each initiator had a metal core (molybdenum or ruthenium) with bulky ligands that controlled the approach and co-ordination of the reactive carbene bond to the double bond of the norbomene group... 

Initiation is generally determined by a kinetic measurement made under standard, pseudo-order conditions with a large excess of a reactive alkene. Ethyl vinyl ether (EVE) or butyl vinyl ether (BuVE) are widely used and acceptable for this purpose because there is no significant metathesis from the resultant stable, ruthenium Fischer-carbene complexes [13]. The initiation rate can be easily determined by NMR spectroscopy or by monitoring changes in the UV-vis spectrum. If analysis... 

Scheme 5.10 Ruthenium(ll)-carbene complex with low catalytic activity R, R = Ph, Cy,...

Some general reviews relating to the chemistry of Ru/Os-r hydrocarbon complexes appear in the literature the reactivity of Ru-H bonds with alkenes and alkynes/ aspects of ruthenium/osmium vinylidene/allenylidene/cumul-enylidene complexes,equilibria of M-R/M=CR2/M=CR complexes, the organometallic chemistry of metal porphyrin complexes, and the reactions of [Os(P Pr3)2(CO)HGl], ruthenium pyrazoly I borate complexes,and metallabenzynes. Other reviews relate more to applications of some of the complexes outlined in this chapter. See, for example, metal vinylidenes in catalysis,the development of Grubbs-type alkene metathesis catalysts, applications of ruthenium/osmium carbene complexes in metathesis polymerization, and the role of Ru /V-hetero-cyclic carbene complexes in metathesis polymerization. ... 

RCM reaction and concomitant metallotropic shift, and they applied this strategy for the synthesis of panaxytriol (145, Scheme 24.37). Panaxytriol 145 was a component of Red Ginseng and exhibits an antitumor activity. The key process involves the generation of ruthenium carbene species 146 by the relay RCM of 142 and the subsequent CM reaction of alkene 143 with carbene 147, which is formed by two consecutive metallotropic [1,3]-shifts of the carbene... 

---