Carbene complexes reactivity patterns

See, for example, the following sources and references therein a) M. P. Doyle, M. A. McKervey, T. Ye, Modern Catalytic Methods for Organic Synthesis with Diazo Compounds, Wiley, New York (1998). b) J. L. Poise, A. W. Kaplan, R. A. Andersen, R. G. Bergman, Synthesis ofan n2-N2-titanium diazoalkane complex with both imido-like and metal carbene-like reactivity patterns, J. Am. 

Organometallic complexes of copper, silver, and gold are ideal precursors for carbene complexes along with some C- and N-coordinated species. Their reactivity pattern, in particular in oxidative addition reactions, was the most comprehensively studied. 

BONDING MODELS AND REACTIVITY PATTERNS FOR TRANSITION METAL CARBENE AND CARBYNE COMPLEXES... 

The wealth of empirical information collected for transition metal carbene and carbyne complexes may be best interpreted within the framework of sound theoretical models for these compounds. Perhaps the most significant contribution made by the theoretical studies of carbene and carbyne complexes concerns an understanding of the reactivity patterns they display. In this section the relationship between bonding and reactivity is examined, with particular emphasis being given to the ways in which studies of Ru, Os, and Ir compounds have helped unify the bonding models applied to seemingly diverse types of carbene and carbyne complexes. 

The effect of metal basicity on the mode of reactivity of the metal-carbon bond in carbene complexes toward electrophilic and nucleophilic reagents was emphasized in Section II above. Reactivity studies of alkylidene ligands in d8 and d6 Ru, Os, and Ir complexes reinforce the notion that electrophilic additions to electron-rich compounds and nucleophilic additions to electron-deficient compounds are the expected patterns. Notable exceptions include addition of CO and CNR to the osmium methylene complex 47. These latter reactions can be interpreted in terms of non-innocent participation of the nitrosyl ligand. 

The olefin binding site is presumed to be cis to the carbene and trans to one of the chlorides. Subsequent dissociation of a phosphine paves the way for the formation of a 14-electron metallacycle G which upon cycloreversion generates a pro ductive intermediate [ 11 ]. The metallacycle formation is the rate determining step. The observed reactivity pattern of the pre-catalyst outlined above and the kinetic data presently available support this mechanistic picture. The fact that the catalytic activity of ruthenium carbene complexes 1 maybe significantly enhanced on addition of CuCl to the reaction mixture is also very well in line with this dissociative mechanism [11] Cu(I) is known to trap phosphines and its presence may therefore lead to a higher concentration of the catalytically active monophosphine metal fragments F and G in solution. 

When the development of carbene-complex chemistry began in the mid seventies, two different patterns of reactivity emerged and led to a, maybe overemphasized, division of these compounds into (electrophilic) Fischer-type and (nucleophilic) Schrock-type carbene complexes (Figure 1.1). 

This reactivity pattern is certainly unexpected. Why should low-valent complexes react as electrophiles and highly oxidized complexes be nucleophilic Numerous calculations on model compounds have provided possible explanations for the observed chemical behavior of both Fischer-type [3-8] and Schrock-type [9-17] carbene complexes. In simplified terms, a rationalization of the reactivity of carbene complexes could be as follows. The reactivity of non-heteroatom-stabilized carbene complexes is mainly frontier-orbital-controlled. The energies of the HOMO and LUMO of carbene complexes, which are critical for the reactivity of a given complex, are determined by the amount of orbital overlap and by the energy-difference between the empty carbene 2p orbital and a d orbital (of suitable symmetry) of the group L M. 

Abstract Allenylidene complexes have gained considerable significance in the context of transition-metal carbene chemistry due to their potential applications in organic synthesis. The aim of this chapter is to draw together a general presentation of the most efficient synthetic routes, the main structural features and reactivity patterns, as well as current applications in homogeneous catalysis, of aU-carbon-substituted allenylidenes and related cumulenylidene complexes containing an odd number of carbon atoms. 

Alkynes coordinated to platinum(O) are susceptible to electrophilic attack. The reaction which has been most fully studied is the protonation of complexes Pt(alkyne)(PPh3)2 to give vinyl platinum(Il) complexes then alkenes. The reaction has been discussed in Section 52. The vinyl complexes formed undergo isomerization in the final step, since the cis vinyl complex yields some tracts-alkene. Carbene intermediates have been proposed in the pathway for this isomerization.848 Platinum(II) alkyne complexes can be converted into carbene complexes, and this reaction has been discussed in Section 52.4,6. This pattern of differential reactivity is apparent in the IR spectra of the two sets of complexes. For alkyne complexes of platinum(O) the C==C stretching frequency is lowered by some 450 cm-1 upon coordination, but with the platinum(II) analogs the difference is only in the region of 200 cm-1. 

The analogy developed above between pentacoordinated complexes and organic free radicals is capable of meaningful extension to coordination compounds of other electron configurations and coordination numbers see Table III). Thus, similar reasoning leads to the expectation of similarities between the reactivity patterns of tetracoordi-nated d complexes and carbenes, pentacoordinated d complexes and carbanions, and pentacoordinated d complexes and carbonium ions. In each case the stoichiometries of the reactions which restore the stable closed-shell configurations are the same for both species hence the similarity of reactivity patterns. 

At the beginning of Section 10-3, we commented that metal-carbene complexes exhibit a spectrum of reactivities with nucleophiles and electrophiles, especially at Qarbene- Carbene complexes of mid-transition metals (Groups 7-9) without heteroatomic substituents at Ccarbene may show electrophilic behavior depending upon the nature of other ligands, oxidation state of the metal, and overall charge on the complex. From some observations listed below, we may be able to discern a pattern of reactivity.63... 

When phosphane-free nickel complexes, such as bis(cycloocta-l,5-diene)nickel(0) or te-tracarbonylnickel, are employed in the codimerization reaction of acrylic esters, the codimer arising from [2-1-1] addition to the electron-deficient double bond is the main product. The exo-isomer is the only product in these cyclopropanation reactions. This is opposite to the carbene and carbenoid addition reactions to alkenes catalyzed by copper complexes (see previous section) where the thermodynamically less favored e Jo-isomers are formed. This finding indicates that the reaction proceeds via organonickel intermediates rather than carbenoids or carbenes. The introduction of alkyl substituents in the /I-position of the electron-deficient alkenes favors isomerization and/or homo-cyclodimerization of the cyclopropenes. Thus, with methyl crotonate and 3,3-diphenylcyclopropene only 16% of the corresponding ethenylcyc-lopropane was obtained. Methyl 3,3-dimethylacrylate does not react at all with 3,3-dimethyl-cyclopropene, so that the methylester of tra 5-chrysanthemic acid cannot be prepared in this way. This reactivity pattern can be rationalized in terms of a different tendency of the alkenes to coordinate to nickel(O). This tendency decreases in the order un-, mono- < di-< tri- < tet-... 

Consider now the bond activation reactions in Fig. 23.6a and b in both cases, the insertion into the bond, C-H or C-X, requires singlet-triplet unpairing on the two reactants. These reactions have been treated extensively by Su et al. [57-59] using the VBSCD model. In all cases, an excellent correlation was obtained between the computed barriers of the reaction and the AEsj quantity, including the relative reactivity of carbene and its heavier analogs, and of PtL2 vi. PdL2 [57-59]. A similar treatment led to the same reactivity patterns for C-F bond activation reactions by fra 5-Rh(PR3)2X and trans-Ir(PR3)2X d -complexes [58]. 

Figure 2. Reactivity pattern of carbonyl(carbene) complexes.

During the past few decades, a wide variety of molecules with transition metal-carhon mulhple bonds have been studied. The chemistry of doubly bonded species - carbenes - is particularly interesting because it leads to several synthetically important transformations, and for this reason, metal carbenes are the main subject of this chapter. Our discussion begins with a classification of metal-carbene complexes based on electronic structure, which provides a way to understand their reactivity patterns. Next, we summarize the mechanistic highlights of three metal-carbene-mediated reactions carbonyl olefinafion, olefin cyclopropanafion, and olefin metathesis. Throughout the second half of the chapter, we focus mainly on ruthenium-carbene olefin metathesis catalysts, in part because of widespread interest in the applications of these catalysts, and in part because of our expertise in this area. We conclude with some perspectives on the chemistry of metal carbenes and on future developments in catalysis. 

The nucleophilic-electrophilic/Schrock-Fischer distinctions have been extremely useful throughout the development of metal-carbene chemistry because they provide a way to categorize metal carbenes and rationalize their reactivity patterns [6]. Yet, as an increasing variety of complexes are studied, it is becoming clear that these classifications represent only the prototypical complexes that were inihally discovered. We now know of many examples with intermediate characteristics and reactivity profiles, such as electrophilic species that lack heteroatom stahilizahon and even complexes like (Cp)(CO)2Re=CHR that display ambiphilic reachvity, meaning that this rhenium carbene reacts with both nucleophiles and electrophiles (Eq. 4.1) [7]. 

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