Ruthenium porphyrins carbene complexes

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]. 

Diazoalkanes are u.seful is precursors to ruthenium and osmium alkylidene porphyrin complexes, and have also been investigated in iron porphyrin chemistry. In an attempt to prepare iron porphyrin carbene complexes containing an oxygen atom on the /(-carbon atom of the carbene, the reaction of the diazoketone PhC(0)C(Ni)CH3 with Fe(TpCIPP) was undertaken. A low spin, diamagnetic carbene complex formulated as Fe(TpCIPP)(=C(CH3)C(0)Ph) was identified by U V-visible and fI NMR spectroscopy and elemental analysis. Addition of CF3CO2H to this rapidly produced the protonated N-alkyl porphyrin, and Bit oxidation in the presence of sodium dithionitc gave the iron(II) N-alkyl porphyrin, both reactions evidence for Fe-to-N migration processes. ... 

The most significant and widely studied reactivity of the ruthenium and osmium porphyrin carbene complexes is their role in catalyzing both the decomposition of diazoesters to produce alkenes and the cyclopropanation of alkenes by diazoesters. Ethyl diazoacetate is used to prepare the carbene complex 0s(TTP)(=CHC02Et)... 

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. 

Synthesis and structure of ruthenium and osmium porphyrin carbene complexes and their role in the metal-mediated cyclopropanation of alkenes 02CCR(231)151. 

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. ... 

Over the last decade a number of high oxidation state ruthenium porphyrin complexes containing 0x0 or imido ligands have been reported and have been thoroughly studied for their role in oxidation and atom-transfer chemistry. Although comparisons can be drawn with organometallic species (carbene, imido. and 0x0 ligands are formally isolobal) the chemistry of the 0x0 and imido complexes is beyond the scope of the review and will not be covered here. 

The porphyrin ligands in the diamagnetic ruthenium and osmium carbene complexes generally exhibit four-fold symmetry by NMR, indicating that the barrier to rotation about the M=C bond is low. The carbenoid protons appear shifted down-field in the H NMR spectra, for example appearing for Ru(TTP)=CHC02Et and Ru(TTP)=CHSiMc3 at 13.43 and 19.44 ppm, respectively, and for the osmium... 

Ruthenium porphyrin complexes are also active in cyclopropanation reactions, with both stoichiometric and catalytic carbene transfer reactions observed for Ru(TPP)(=C(C02Et)2> with styrene. Ru(Por)(CO)orRu(TMP)(=0)2 catalyzed the cyclopropanation of styrene with ethyidiazoacetate, with aiiti.syn ratios of 13 1... 

While metalloporphyrin carbene complexes are well established for ruthenium and osmium, they are less well known for rhodium. Cationic rhodium porphyrin carbene intermediates were implicated in a report by Callot et al. in w- hich... 

The preparation of cyclopropanes by intermolecular cyclopropanation with acceptor-substituted carbene complexes is one of the most important C-C-bond-forming reactions. Several reviews [995,1072-1074,1076,1077,1081] and monographs have appeared. In recent decades chemists have focused on stereoselective intermolecular cyclopropanations, and several useful catalyst have been developed for this purpose. Complexes which catalyze intermolecular cyclopropanations with high enantiose-lectivity include copper complexes [1025,1026,1028,1029,1031,1373,1398-1400], cobalt complexes [1033-1035], ruthenium porphyrin complexes [1041,1042,1230], C2-symmetric ruthenium complexes [948,1044,1045], and different types of rhodium complexes [955,998,999,1002-1004,1010,1062,1353,1401-1405], Particularly efficient catalysts for intermolecular cyclopropanation are C2-symmetric cop-per(I) complexes, as those shown in Figure 4.20. These complexes enable the formation of enantiomerically enriched cyclopropanes with enantiomeric excesses greater than 99%. Illustrative examples of intermolecular cyclopropanations are listed in Table 4.24. 

In addition to copper and rhodium catalysts commonly used in the generation of metal carbene complexes, other transition metals have also been explored in the diazo decomposition and subsequent ylide generation.Che and co-workers have recently studied ruthenium porphyrin-catalyzed diazo decomposition and demonstrated a three-component coupling reaction of a-diazo ester with a series of iV-benzylidene imines and alkenes to form functionalized pyrrolidines in excellent diastereoselectivities (Scheme 20). ... 

A review has appeared on the synthesis of enantiomerically enriched aziridines by the addition of nitrenes to alkenes and of carbenes to imines.45 A study of the metal-catalysed aziridination of imines by ethyl diazoacetate found that mam group complexes, early and late transition metal complexes, and rare-earth metal complexes can catalyse the reaction.46 The proposed mechanism did not involve carbene intermediates, the role of the metal being as a Lewis acid to complex the imine lone pah. Ruthenium porphyrins were found to be efficient catalysts for the cyclopropana-tion of styrenes 47 High diastereoselectivities in favour of the //-product were seen but the use of chiral porphyrins gave only low ees. 

Carbene and silylene complexes - Carbene complexes of ruthenium [312] and osmium [313] porphyrins were formed from the neutral dimers [M(P)]2 with diazoalkanes (Eq. 26). 

Ruthenium vinylidene and carbene complexes containing cyclopenta-dienyl ligands such as [(i75-C5H5)(PPh3)2Ru=C=CH2]+ are known and an excellent review describing the structures and reactivities of these classes of complexes is available (103). Treatment of the -vinyl-bridged porphyrin with Ru3(CO)12 yielded (TPP)Ru C=C(p-Cl-C6H4)2, ... 

Here, I focus on application of ruthenium complexes as catalysts for the cyclopropanation of olefins with diazoesters to describe their catalytic activity, stereoselectivity, and enantioselectivity together with structural analysis of intermediary carbene complexes, especially with nitrogen-based ligands including porphyrin derivatives [4,5]. 

In order to clarify the mechanism of cyclopropanation, several carbene-complexes of ruthenium have been isolated by reaction with diazocompounds. In the case of Pybox, the corresponding ruthenium-carbene complexes 38 were isolated and characterized using either NMR or X-ray analysis [32]. Similar ruthenium-carbene complexes, such as porphyrin-ruthenium carbene complex 39 [33] and pyridine-diimine-ruthenium complex 40 [34] were isolated and characterized (Chart 7.6). 

The carbene complexes were also prepared by interaction of geminal dihalides with zero-valent ruthenium porphyrins such as K2[(TTP)Ru] by the same... 

Table 3 X-ray structural data for carbene and carbide complexes of ruthenium and osmium porphyrin carbenes...

The catalytic production of olefins, diethyl maleate and fumarate, from ethyl diazoacetate has been reported with osmium [ 149] and ruthenium [ 128] porphyrins. Despite the periodic relationship of ruthenium to iron and osmium and the syntheses of different carbene complexes of ruthenium porphyrins, developed by Collman et al. [125-127], it is only very recently that cyclopropanation [135,171] and ethyl diazoacetate insertion into heteroatom bond reactions [172] were observed using ruthenium porphyrins as catalysts. The details of the catalytic reaction of diazo esters with simple olefins catalyzed with ruthenium porphyrins have been reported [173]. Product yields. 

There has also been a renewal of interest in reactions catalyzed by ru-thenium(II) porphyrin complexes, simultaneously with the development of new chiral ruthenium porphyrins [175-178]. Although these reactions focus mainly on asymmetric epoxidation of olefins [179,180], in some cases asymmetric cyclopropanations were very successful As a recent example, the intermolecular cyclopropanation of styrene and its derivatives with ethyl diazoacetate afforded the corresponding cyclopropyl esters in up to 98% ee with high trans/cis ratios of up to 36 and extremely high catalyst turnovers of up to 1.1 X 10 [140]. The structure of the metalloporphyrin is given in Fig. 2. Asymmetric intramolecular cyclopropanations were also reported with the same catalyst [140]. hi this case, the decomposition of a series of aUyhc diazoacetates afforded the cyclopropyl lactones in up to 85% ee. Both the inter-and intramolecular cyclopropanation were proposed to proceed via a reactive chiral ruthenium carbene intermediate. The enantioselectivities in these processes were rationahzed on the basis of the X-ray crystal structures of closely related stable chiral carbene complexes obtained from the reaction of the chiral complex with N2CPh2 and N2C(Ph)C02CH2CH = CH2. 

The ruthenium porphyrins, (TPP)RuCO and (TMP)RuCO catalyze carbene insertion into S - H bonds, leading to dialkyl and alkyl aryl sulfides using ethyl diazo acetate under mild conditions. The insertion process is regiospe-cific since dithiothreitol reacts to give the S - H insertion product without any trace of the ether compound (Scheme 18) [172]. With a homochiral porphyrin ruthenium complex, asymmetric insertions were obtained but with low enantioselectivities [191]. 

The ruthenium porphyrins, (TPP)Ru(CO) and (TMP)Ru(CO) also catalyze carbene insertion into N-H bonds [172]. Thus, the complex (TMP)Ru(CO)... 

Not considered within this review are the wide range of porphyrin and other nitrogen macrocycle-stabilized ruthenium and osmium hydrocarbyl complexes. Readers should see the following as a starting point for information on this type of alkyl/aryl, alkynyl, vinylidene, allenylidene, and carbene complexes. 

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