Carbene stability

For the most electronegative ligand, fluorine, we expect a relativistic destabilization in the Au—F bond, which was indeed determined to be —0.36eV at the coupled cluster level [182,183], Nevertheless, AuF has a sufficiently high dissociation energy of about 3.17 eV and has been identified recently in the gas phase [184]. In solution or in the solid state it would disproportionate to metallic Au and compounds of Au (AuF3 for the solid). However, a carbene-stabilized Au(I) fluoride was synthesized only very recently (see discussion in the next section) [185]. 

Figure 7.11. Singlet carbene stabilizing substituents D = electron donating groups, W electron withdrawing groups.

Testing the limits of carbene stabilization by substituents, Bertrand and coworkers reported the synthesis and analysis of (amino)(aryl)carbenes where only one substituent can contribute to the stability of the carbene center. Carbene 30 was designed to have the amino substituent as a jr-donor and the 2,6-bis(trifluoromethyl)... 

Due to their isoelectronic relationship with Arduengo carbenes , stabilization of cyclic phosphenium ions by Ti-electron delocalization has been a matter of debate. 

For 1, X=F, however, A ° is independent of the bond dissociation energies. The greater carbene-stabilizing ability of F28 (vs. Cl or Br) drives the partition of excited alkylfluorodiazirines mainly to carbene, thus minimizing the RIES pathway. In the case of the cyclobutylhalodiazirine 21-X photolyses (Scheme 4), RIES accounted for 68% of reaction when X=C1, but only 12% when X=F the balance of reaction was due to carbene in each case.28... 

It is clear that the variation in the carbene stabilization modes dramatically modify the carbene properties in terms of steric and electronic factors, which is very essential in the design of new ligands for transition metals. In comparison to classical NHCs, non-NHC carbenes offer a large range of possible structural variations, which should rapidly enhance the interest in non-NHC-based complexes. We can anticipate that the fine tuning of the properties of stable non-NHC carbenes makes these ligands very promising for the development of novel and efficient catalysts. 

Albert, K., Gisdakis, P., Roesch, N. On C-C Coupling by Carbene-Stabilized Palladium Catalysts A Density Functional Study of the Heck... 

Plentiful examples of [C2 + C J cycloreversions of cyclopropanes have been reviewed. Most examples of this transformation are initiated photochemically, "" thermally initiated Ci-ex-trusions have only been observed for carbenes stabilized with electron-donating or fluorine substituents as well as with the participation of transition metal catalysts. ... 

Irradiation of the diazo compounds at 5 K in a rigid matrix gave stable triplet vinylcarbenes. The rates of vinylcarbene formation in the thermolysis of vinyldiazomethancs have been used to determine relative carbene stabilities. 

The basic nature of N-heterocyclic carbenes, stabilized by the presence of two adjacent N atoms, has been emphasized by several authors [38], Therefore, the yield and the selectivity of any organic synthesis (driven by the reactivity of a substrate vs. appropriate bases and carried out in imidazohum salts as solvents) can be affected by side reactions related to the acidity of C2-H unit in 1,3-dialkylimidazo-lium cation and to the basicity and nucleophilicity of the resulting N-heterocychc carbene. On this subject, noticeable work (related to the Baylis-Hillman, Knoevenagel and Claisen condensation, Homer reaction, etc.) carried out in RTlLs has been reported and discussed [37]. 

A range of (phosphino)(aryl)carbenes (20 R = i-Pr, c-hexyl V, W, X, Y, Z = various combinations of H, F, Me, CF3, NMe2) have been generated by photolysis of their diazo precursors. The substituent effects on carbene stability were investigated both experimentally and theoretically. The presence of an acceptor and also a spectator aryl substituent resulted in stable or at least persistent carbenes, which could be characterized by standard spectroscopic techniques, whereas a donor aryl ring produced a very fleeting carbene. 

Ito, Kawakami and Sawamura recently described the borylation of al-lylic carbonates by B2pin2, catalyzed by bis(phosphine)copper(I) alkox-ides. It was proposed that bis(phosphine)copper(I) boryl species formed by alkoxide/boryl a-bond metathesis are key intermediates in the catalytic cycle [231]. Making use of related N-heterocyclic carbene stabilized precursors, Sadighi and co-workers have very recently isolated the thermally labile copper boryl complex (IPr)CuBpin (11.1) together with the products of oxygen atom, styrene and aldehyde insertion into the Cu-B bond (11.2-11.5 Scheme 24) [232,233,237]. The structure of 11.1 in the solid state reveals an approximately linear Cu(I) coordination geometry [ZB-Cu-C 168.1(2)°] and a Cu-B distance [2.002(3) A] which is somewhat shorter than the sum of the expected covalent radii [2.05 A] [106]. Yet further evidence for the... 

Clearly, by the appropriate choice of substituents, carbenic stability, reactivity, and philicity can be simultaneously varied, while the delicate interrelations of these properties can be understood in empirical and, more precisely, in theoretical terms. The kinetic range of the carbene reactions that we have considered is enormous the rate constants span 9 or more orders of magnitude. From this perspective, it is remarkable that the classical tools of physical organic chemistry, resonance and inductive effects, and Hammett relationships, provide such a satisfactory qualitative rationalization of the entire picture. Augmented by modem experimental methods such as LFP, and theoretical tools (FMO and computational methods), we are now able to understand and manipulate carbenic philicity in an intellectually satisfying and synthetically useful manner. 

Bromotiiazolopyridine 87a at 1.7 atm and 100 °C decomposed to form a pyridylcarbene intermediate by nitrogen expulsion. Carbene stabilization gave vinylpyridine 124, alcohol 125, ketone 126, cyclopropyl derivatives 127 and 128 (Scheme 27) (07ARK297). 

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