Carbenes theoretical studies

Fischer-type carbene complexes, generally characterized by the formula (CO)5M=C(X)R (M=Cr, Mo, W X=7r-donor substitutent, R=alkyl, aryl or unsaturated alkenyl and alkynyl), have been known now for about 40 years. They have been widely used in synthetic reactions [37,51-58] and show a very good reactivity especially in cycloaddition reactions [59-64]. As described above, Fischer-type carbene complexes are characterized by a formal metal-carbon double bond to a low-valent transition metal which is usually stabilized by 7r-acceptor substituents such as CO, PPh3 or Cp. The electronic structure of the metal-carbene bond is of great interest because it determines the reactivity of the complex [65-68]. Several theoretical studies have addressed this problem by means of semiempirical [69-73], Hartree-Fock (HF) [74-79] and post-HF [80-83] calculations and lately also by density functional theory (DFT) calculations [67, 84-94]. Often these studies also compared Fischer-type and... 

The synthesis and characterization of the monomeric amidinato-indium(I) and thallium(I) complexes [Bu C(NAr)2]M[But(NAr(NHAr)] (M = In, Tl Ar = 2,6-Pr2CgH3) have been reported. Both compounds were isolated as pale yellow crystals in 72-74% yield. These complexes, in which the metal center is chelated by the amidinate ligand in an N, j -arene-fashion (Scheme 33), can be considered as isomers of four-membered Group 13 metal(I) carbene analogs. Theoretical studies have compared the relative energies of both isomeric forms of a model compound, In[HC(NPh)2]. ... 

One-electron oxidation of the vinylidene complex transforms it from an Fe=C axially symmetric Fe(ll) carbene to an Fe(lll) complex where the vinylidene carbon bridges between iron and a pyrrole nitrogen. Cobalt and nickel porphyrin carbene complexes adopt this latter structure, with the carbene fragment formally inserted into the metal-nitrogen bond. The difference between the two types of metalloporphyrin carbene, and the conversion of one type to the other by oxidation in the case of iron, has been considered in a theoretical study. The comparison is especially interesting for the iron(ll) and cobalt(lll) carbene complexes Fe(Por)CR2 and Co(Por)(CR2) which both contain metal centers yet adopt... 

Pyykkd, P. and Runeberg, N. (2006) Comparative Theoretical Study of N-Heterocyclic Carbenes and Other Ligands Bound to Au. Chemistry - An Asian Journal, 1, 623-628. 

Recently, Schleyer and co-workers reported a theoretical study on the stabilities and geometries of a series of carbonyl type compounds, R2M=0 (M = C, Si, Ge, Sn, Pb R = H, CH3), and those of carbene type isomers [R(RO)M ] as shown in Table II.15 These results show that in (CH3)2 M=0 series the carbene type structure is more stable than the double bond type structure in the case of germanium, tin, and lead, unlike the case of carbon in which the double bond structure is preferred. 

Only limited number of neutral monocyclic 11,3,2 diazaphosphole representatives have been reported, which have mostly been prepared by [4+1] cyclocondensation of diaminomaleodinitrile (DAMN) with P(III) reagent and the alkylation of the initially formed 1,3,2-diazaphospholide [2, 4, 7], During recent times, 67t-aromatic [l,3,2]diazaphospholium ions of type 46 [45], more often represented as cyclic phosphenium cation 47 [46,47], have attracted more attention due to their isoelec-tronic nature with Arduengo carbenes . Nature of bonding and aromaticity of these cations have been the subject of several experimental and theoretical studies (Structure 2) [48-52],... 

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. 

Theoretical studies of metal carbene complexes have been undertaken by several groups. Rather than attempting to analyze the complexes as a whole, it is more convenient to consider the carbene and metal fragments separately and then to analyze the effects of combining them. 

The chemistry of transition metal-carbyne complexes is rather less developed than the chemistry of carbene complexes. This is almost certainly because reactions which form new carbyne complexes are relatively rare when compared with those forming metal carbenes. The few theoretical studies of carbyne complexes which are available indicate that close parallels exist between the bonding in carbene and carbyne compounds. These parallels also extend to chemical reactivity, and studies of Group 8 complexes again prove instructive. 

We theoretically studied the reactions of stable West silylenes 32 and 73 with phosphorus ylide H2C=PMe3.74 Similarly to the simplest analogs of carbenes, these compounds can form betaines in which the negative charge is localized on the silicon atom and the positive charge is localized on the phosphorus atom. These betaines can thermally decompose to form silenes (direction A, Scheme 39) or be isomerized to ylides via direction B. 

The simplest carbene capable of a 1,2-H shift, methylcarbene (78) is very difficult to study experimentally. Theoretical studies suggest that the carbene is a ground state triplet lying 5 kcal/mol below the singlet.97... 

These isomerization reactions are of great interest to theoreticians because the role of many factors (metal, substituents on the organic fragment, ancillary ligands) on the outcome of the reaction can be studied through computations. The purpose of this chapter is to describe the theoretical studies carried out on the isomerization of alkyne to vinylidene and alkene to carbene in the presence of transition metal fragments. 

Dotz reaction is proposed. According to our calculations the addition of the alkyne molecule to the carbene complex takes place before CO loss in the initial steps of the reaction. Further, our study shows that a novel proposal involving a chromahexatriene intermediate entails lower energy barriers and more stable intermediates than the previous reaction mechanisms postulated by Dotz and Casey. The novel findings query revision of the classically assumed paths and put forward that additional experimental and theoretical studies are necessary to definitely unravel the reaction mechanism of this intringuing reaction. 

Despite the undeniable synthetic value of the benzannulation reaction of aryl and alkenyl Fischer carbene complexes, the details of its mechanism at the molecular level remain to be ascertained. Indeed, although a relatively large number of theoretical studies have been directed to the study of the molecular and electronic structure of Fischer carbene complexes [22], few studies have been devoted to the analysis of the reaction mechanisms of processes involving this kind of complexes [23-30]. The aim of this work is to present a summary of our theoretical research on the reaction mechanism of the Dotz reaction between ethyne and vinyl-substituted hydroxycarbene species to yield p-hydroxyphenol. 

The Dotz reaction mechanism has received further support from kinetic and theoretical studies. An early kinetic investigation [37] and the observation that the reaction of the metal carbene with the alkyne is supressed in the presence of external carbon monoxide [38] indicated that the rate-determining step is a reversible decarbonylation of the original carbene complex. Additional evidence for the Dotz mechanistic hyphotesis has been provided by extended Hiickel molecular orbital [23, 24] and quantum chemical calculations [25],... 

The most recent theoretical study, by Alhrichs and co-workers, deals with the di(phosphino)carbene Id and (phosphino) (phosphonio)carbenes Ie,f.16 The optimized geometry of the di(phosphino)carbene Id is weakly bent (PCP angle 160.5°) and highly unsymmetrical Only one of the phosphorus centers (P1) is in a planar environment, and it is much more closely bonded to the carbenic center than the other one (P1C 1.533 and P2C 1.765 A). The atomic charges (P1 +1.0, C -0.8, P2 +0.6) indicate that the short P bond is a double bond reinforced by Coulombic attraction, while the nature of the molecular orbitals revealed a slight delocalization of the carbene lone pair into the low-lying a (P-N) orbitals of the two phosphino substituents. The distortion from the symmetrical structure can be viewed as a second-order Jahn-Teller effect. 

Theoretical studies on carbodiphosphoranes and related divalent carbon(O) compounds (carbones) of the type CL2 and the collection of forgotten CL2 compounds show that there is a class of molecules which has a unique bonding situation that is clearly different from divalent carbon(II) compounds (carbenes). 

NHCs are often compared to phosphines and theoretical studies by Frenking et al. [211-213] showed that the substitution of the phosphines in 89 for two NHCs leads to compounds with a carbon(O) atom that is formally stabilized by two NHC C donor-acceptor bonds. The first carbodicarbene 90 was prepared by Bertrand et al. who selected benzimidazolin-2-ylidenes as carbene donors and the deprotonation of the conjugated acid of an allene as the synthetic strategy (Fig. 29)... 

A proposed mechanism for this transformation, provided in Scheme 42, is based on the identification of alcohol-carbene complexes by Movassaghi and Schmidt. Mesityl substituted imidazolinylidine carbene acts as a Brpnsted base as transesterification occurs to produce LXVII. Upon O N acyl transfer, the observed product is formed. The evidence provided for this mechanism includes the control experiment in which LXVII is resubjected to the reaction conditions and proceeds with amide formation. A similar mechanism has recently been reported in a theoretical study of transesterification by Hu and co-workers [139], In light of this work, it seems reasonable to suggest a similar that mechanism is operative in the transesterification reactions discussed throughout this section. 

Species (A) and (B) constitute the main class of unsaturated carbenes and play important roles as reactive intermediates due to the very electron-deficient carbon Cl [1]. Once they are coordinated with an electron-rich transition metal, metal vinylidene (C) and allenylidene (D) complexes are formed (Scheme 4.1). Since the first example of mononuclear vinylidene complexes was reported by King and Saran in 1972 [2] and isolated and structurally characterized by Ibers and Kirchner in 1974 [3], transition metal vinylidene and allenylidene complexes have attracted considerable interest because of their role in carbon-heteroatom and carbon-carbon bond-forming reactions as well as alkene and enyne metathesis [4]. Over the last three decades, many reviews [4—18] have been contributed on various aspects of the chemistry of metal vinylidene and allenylidene complexes. A number of theoretical studies have also been carried out [19-43]. However, a review of the theoretical aspects of the metal vinylidene and allenylidene complexes is very limited [44]. This chapter will cover theoretical aspects of metal vinylidene and allenylidene complexes. The following aspects vdll be reviewed ... 

The electronic structure of these carbenes was investigated by early theoretical studies [36,40-48] to elucidate the reasons for the surprising stability, which came to different conclusions concerning the importance of the sta-bihzing effect of the /r-delocalization. While early studies predicted that the C-N 7r-interaction does not play a major role [33], others found that the pM population at the carbene carbon atom is 30% higher for the unsaturated case, indicating that cychc delocalization is clearly enhanced in the unsaturated carbene [48]. 

K. N. Houk, N. G. Rondan, and J. Mareda, Theoretical Studies of Halocarbene Cycloaddition Selectivities. A New Interpretation of Negative Activation Energies and Entropy Control of Selectivities, Tetrahedron 1985, 41, 1555. Calculations on carbene addition reactions led to a general explanation of why it is possible for very exothermic, bimolecular reactions to have negative activation enthalpies. 

For monographs, see Jones Moss Carbenes, 2 vols. Wiley New York, 1973-1975 Kirmse Carbene Chemistry, 2nd ed. Academic Press New York, 1971 Rees Gilchrist Carbenes, Nitrenes, and Arynes Nelson London, 1969. For reviews, see Minkin Simkin Glukhovtsev Russ. Chem. Rev. 1989, 58, 622-635 Moss Jones React. Intermed. (Wiley) 1985, 3, 45-108,1981,2, 59-133,1978, /, 69-115 Isaacs, Ref. 1, pp. 375-407 Bethell Adv. Phys. Org. Chem. 1969, 7, 153-209 Bethell, in McManus, Ref. 1, pp, 61-126 Closs Top. Stereochem. 1968, 3, 193-235 Herold Caspar Fortschr. Chem. Forsch. 1966, 5, 89-146 Rozantsev Fainzil berg Novikov Russ. Chem. Rev. 1965,34, 69-88. For a theoretical study, see Liebman Simons Mol. Struct. Energ. 1986, /, 51-99. 

Coordinated nitrenes RN2- have been thoroughly investigated in recent years285,286 owing to their formal similarity to carbenes and to their potential importance, e.g. as catalysts in the Haber ammonia process. Theoretical studies have suggested that imido ligands may also prove effective in promoting alkene metathesis.287... 

---