Electronic structure carbenes

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... 

Strassner T (2004) Electronic Structure and Reactivity of Metal Carbenes. 13 1-20 Strong LE, see Kiessling LL (1998) 1 199-231... 

Bettinger, H. F., Schleyer, P. v. R., Schreiner, P. R., Schaefer, III, H. F., 1997, Computational Analyses of Prototype Carbene Structures and Reactions in Modem Electronic Structure Theory and Applications in Organic Chemistry, Davidson, E. R. (ed.), World Scientific, Singapore. 

Interaction of a carbonyl group with an electrophilic metal carbene would be expected to lead to a carbonyl ylide. In fact, such compounds have been isolated in recent years 14) the strategy comprises intramolecular generation of a carbonyl ylide whose substituent pattern guarantees efficient stabilization of the dipolar electronic structure. The highly reactive 1,3-dipolar species are usually characterized by [3 + 2] cycloaddition to alkynes and activated alkenes. Furthermore, cycloaddition to ketones and aldehydes has been reported for l-methoxy-2-benzopyrylium-4-olate 286, which was generated by Cu(acac)2-catalyzed decomposition of o-methoxycarbonyl-m-diazoacetophenone 285 2681... 

The identification of unknown chemical compounds isolated in inert gas matrices is nowadays facilitated by comparison of the measured IR spectra with those computed at reliable levels of ab initio or density functional theory (DFT). Furthermore, the observed reactivity of matrix isolated species can in some instances be explained with the help of computed reaction energies and barriers for intramolecular rearrangements. Hence, electronic structure methods developed into a useful tool for the matrix isolation community. In this chapter, we will give an overview of the various theoretical methods and their limitations when employed in carbene chemistry. For a more detailed qualitative description of the merits and drawbacks of commonly used electronic structure methods, especially for open-shell systems, the reader is referred to the introductory guide of Bally and Borden.29... 

It is well known that double bonds have a different effect on the singlet-triplet energy separation and thus on the reactivity of the carbene than alkyl groups. The description of the electronic structure of such carbenes is rendered more difficult by the fact that several low-lying electronic states are possible. For... 

The matrix IR spectra of la and several isotopomers (cU-la, l80-la) reveal details of the electronic structure of the carbene.23 In particular the red-shift of the C=0 stretching vibration (compared to p-benzoquinone) below 1500 cm-1 indicates a substantial contribution of the phenoxyl/phenyl resonance structure to the wave function of la. The C2V symmetry of the carbene was experimentally revealed by measuring the IR dichroism of partially oriented samples of matrix-isolated la. The orientation of la in an argon matrix was achieved by irradiation with linearly polarized light. 

Carbenes play important roles both as reactive intermediates and also as ligands consequently, considerable effort has been devoted to understanding their molecular and electronic structures. Special interest is associated with carbenes that feature the attachment of donor groups to the carbenic carbon since they behave as nucleophiles and, in some instances, can be isolated. Pioneering work on nucleo-... 

This report begins with a brief review of the electronic and structural features that underlie all of carbene chemistry. Next, we introduce the set of related aromatic carbenes that are the basis for our dissection of the effects of structure on carbene properties. The chemical and spectroscopic techniques and procedures used to probe these carbenes are described and explained briefly in the succeeding section. Then, the results of the application of these probes to the chosen carbenes are presented. Finally, the revealed relation of a carbene s structure to its chemical and physical properties is placed within the predictive framework of molecular orbital theory. Our objective in this report is to present sufficient information to permit us to forecast the properties of an aromatic carbene directly and reliably from its structure. 

The product is exclusively carbon monoxide, and good turnover numbers are found in preparative-scale electrolysis. Analysis of the reaction orders in CO2 and AH suggests the mechanism depicted in Scheme 4.6. After generation of the iron(O) complex, the first step in the catalytic reaction is the formation of an adduct with one molecule of CO2. Only one form of the resulting complex is shown in the scheme. Other forms may result from the attack of CO2 on the porphyrin, since all the electronic density is not necessarily concentrated on the iron atom [an iron(I) anion radical and an iron(II) di-anion mesomeric forms may mix to some extent with the form shown in the scheme, in which all the electronic density is located on iron]. Addition of a weak Bronsted acid stabilizes the iron(II) carbene-like structure of the adduct, which then produces the carbon monoxide complex after elimination of a water molecule. The formation of carbon monoxide, which is the only electrolysis product, also appears in the cyclic voltammogram. The anodic peak 2a, corresponding to the reoxidation of iron(II) into iron(III) is indeed shifted toward a more negative value, 2a, as it is when CO is added to the solution. 

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. 

Isonitrile complexes, having a similar electronic structure to carbonyl complexes, can also react with nucleophiles. Amino-substituted carbene complexes can be prepared in this way (Figure 2.6) [109-112]. Complexes of acceptor-substituted isonitriles can undergo 1,3-dipolar cycloaddition reactions with aldehydes, electron-poor olefins [113], isocyanates [114,115], carbon disulfide [115], etc., to yield heterocycloalkylidene complexes (Figure 2.6). 

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