Carbenes structure, stability, reactivity

The effect of fluorination on the structure, stability, and reactivity of carbenes has been the subject of several reviews (also sec Houben-Weyl. Vol. E19b, p 1461).13,119 121 a-Fluorinated carbenes are ground-state singlet rather than triplet species as a consequence of resonance stabilization.122,123... 

Two resonance-contributing structures (3a and 3b), in the formalism of ylide structures, can be used to describe metal carbene intermediates. The highly electrophilic character of those derived from Cu and Rh catalysts suggests that the contribution from the metal-stabilized carbocation 3b is important in the overall evaluation of the reactivities and selectivities of these metal carbene intermediates. Emphasis on the metal carbene structure 3a has led to the subsequently discounted proposal that cyclopropane formation from reactions with alkenes occurs through the intervention of a metallocyclobutane intermediate [18]. The metal-stabilized carbocation structure 3b is consistent with the cyclopropanation mechanism in which LnM dissociates from the carbene as bond-formation occurs between the carbene and the reacting alkene (Eq. 5.4) [7,15]. 

Several excellent reviews covering stable carbene chemistry have been published since the first one by Herrmann and Kocher in 1997 [3]. They include the influence of the substituents on the stability of carbenes, the synthetic methods available, structural data, reactivity, coordination behavior, and the catalytic properties of the corresponding complexes. 

Ever since the discovery of "d " alkylidene complexes, we have been looking for complexes that contain a heteroatom directly bound to the alkylidene a carbon atom, a situation that appears to be necessary to ensure the stability of many "low oxidation state" carbene complexes. We have now prepared several examples. So far we can say that such species do not appear to have structures or reactivities that clearly set them apart from their hydrocarbon analogs, with the notable exception of possible bonding of the heteroatom to the metal in an anti rotamer of the Re=CHSPh complex. It is interesting to note that we have not yet found any interpretable reactions of complexes of the type... 

Figure 13 Calix[4]arene-derived carcerand used by Warmuth et al. to stabilize reactive intermediates, such as o-benzyne and a singlet carbene. Note the butylene spacers that account for the larger size of the inner void space, compared to Cram s original structure.

In Chapter 11 we will discuss the structure and reactivity of carbenes. These are traditionally extremely unstable structures, where carbon only has six electrons. However, there are cases of stable carbenes, typically possessing resonance structures with stabilizing features such as zwitter-ionic and aromatic character. For example, for moderately large R, carbene A can be isolated and does not dimerize to a tetraaminoethylene derivative. Yet, carbene B dimerizes irreversibly, presumably due to the lack of additional aromatic stability. 

Blom B, Tan G, Enthaler S, Inoue S, EppingJD, Driess M. Bis-N-heterocychc carbene (NHC) stabilized ti -arene iron(0) complexes synthesis, structure, reactivity, and catalytic activity.. Tm Chem Soc. 2013 135 18108-18120. 

Orzechowsld L, Jansen G, Harder S (2006) Synthesis, structure, and reactivity of a stabilized calcium carbene R2CCa. J Am Chem Soc 128 14676-14684. doi 10.1021/ja065000z... 

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... 

In this chapter, we will consider examples of RIs characterized by a hypervalent or valency-deficient carbon, such as carbocations, carbenes, carbanions, and carbon radicals. In the first part, we will consider examples that take advantage of stabilization and persistence to determine their structures by single crystal X-ray diffraction. In the second part we will describe several examples of transient reactive intermediates in crystals. ... 

Studies have shown that carbene reactivity toward a wide variety of substrates is dramatically affected by the nature and multiplicity of the electronic state. - Similarly, the structure, electronic state, thermochemical stability, and reaction kinetics of both singlet and triplet carbenes can be significantly affected by the R-substituents. If R provides steric hindrance, the carbene center can be shielded to slow down inter-molecular reactions (kinetic stabilization). Additionally, bulky and/or geometrically... 

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... 

Indeed, such donation is calculated to stabilize singlet dimethoxycarbene by 76 kcal/mol relative to the corresponding triplet.93 The electron donation also modulates carbenic reactivity 78 a strong electron donor on Q raises both the carbene s HOMO and LUMO energies, thereby increasing the carbene s nucle-ophilicity while rendering its LUMO less accessible to nucleophiles (decreasing its electrophilicity).94 These consequences are illustrated by 69 and the related structures in Scheme 6. 

CF2 is unique among carbenes because of its high stability and low reactivity. Investigations of the ultraviolet absorption spectrum of CF2 have led to estimates of roughly 10 milliseconds to one minute for the half-life of CF2 at pressures in the region of one atmosphere. The gas phase molecule does not react with BF3, N20, S02, CS2 or CF3I at 120 °C5 K The nature of CF2 is perhaps best presented in separate sections discussing its preparation, structure and physical properties, reaction chemistry, and reaction kinetics. 

However, another study concluded that the changes of the hydrogen-bond stability may be important in biological processes. For these, the influence of local electric fields created by Li+, Na+, and Mg2+ ions on the properties and reactivity of hydrogen bonds in HF and HC1 dimer has been carried out by means of ab initio self-consistent field (SCF) method [33]. A few years later, the effect of intensity and vector direction of the external electric field on activation barriers of unimole-cular reactions were studied using the semiempirical MINDO/3 method [34]. However, both semiempirical and ab initio calculations were performed to study the multiplicity change for carbene-like systems in external electric fields of different configurations (carbene and silylene) and the factor that determines the multiplicity and hence the reactivity of carbene-like structures is the nonuniformity of the field [35]. 

---