5-Membered rings stable carbenes

Carbene lv is photolabile, and 400 nm irradiation produces a mixture of products.108 By comparison with calculated IR spectra the major product was identified as cyclopropene 3v. The formation of 3v is irreversible, and it cannot be thermally (by annealing the matrix) nor photochemically converted back to carbene lv. The lv -> 3v rearrangement is calculated (B3LYP/6-31G(d) + ZPE) to be endothermic by only 5.4 kcal/mol with an activation barrier of 18.2 kcal/mol. Due to the two Si-C bonds in the five-membered ring of 3v this cyclopropene is less strained than 3s, which is reflected by the smaller destabilization relative to carbene lv. The thermal energy available at temperatures below 40 K is much too low to overcome the calculated barrier of 12.8 kcal/mol for the rearrangement of 3v back to lv, and consequently 3v is stable under the conditions of matrix isolation. 

The p-functionalized 2-hydroxyphenyl isocyanide not only contains the isocyanide and the nucleophile within the same molecule, but both functional groups are also arranged in one plane for an intramolecular nucleophilic attack [176]. This arrangement, in addition to the aromaticity of the five-membered ring obtained after cyclization to the carbene ligand, particularly favors the intramolecular nucleophilic attack. In contrast to 2-hydroxyethyl isocyanide, free 2-hydroxyphenyl isocyanide is not stable [177]. The stable 2-trimethylsiloxyphenyl isocyanide [178] can serve as a synthon for 2-hydroxyphenyl isocyanide. Carbene complexes 63 with an NH,0-stabilized NHC ligand can be obtained from the complexes 62 with the... 

The carbene l,3-di-l-adamantylimidazol-2-ylidene is additionally stabilized by aromaticity of the 5-membered ring and by steric protection by the neighboring adamantyl substituents. Similar carbenes with the adamantyl groups replaced by aryl groups are also stable [162]. An alkyl group may be regarded as a weak X -type substituent. The So state of dime thy lcarbene, (CH3)2C, is predicted to be only 5.9 kJ/mol lower than T by ab initio calculations [163]. 

Stable cyclic (alkyl)(amino)carbenes (CAAC) have been developed by Bertrand et al. and can be readily prepared in a few steps starting from simple imines 16 (Fig. 4, Scheme 9) [64,65,82-84], A special feature of these 5-membered ring carbenes is their stabibzation by the help of a quarternary carbon next to the carbene. 

The decreasing A/I rate oi 2 e,f with cycloalkenes has been attributed to increasing steric hindrance in going from a 5-membered to an 8-membered ring. Steric effects mask the reactivity of the cycloalkenecarbenes in these reactions. In the case of the more stable (less reactive) cycloalkene carbenes the transition state is more product-like i.e. the carbenes are more selective. Steric effects can obscure this pattern. 

Starting from (silyl)(phosphino)diazomethane derivatives, stable pseudo-diazoalkenes and carbenes have been obtained. The diazomethylenephosphoranes are very reactive dipolarophiles but are also synthetic equivalents of naked carbon C . The stable carbenes react in a similar manner to their transient analogues, but also exhibit new reactions the synthesis of unsaturated three-membered heterocycles is particularly noteworthy. Ring expansion reactions involving the latter are of special interest since they afford novel non-antiaromatic four-7t-electron four-membered heterocycles. 

We had previously demonstrated that the stable (phosphino)(silyl)carbene 16 [31] reacts with trimethylsilyl trifhioromethanesulfonate to give the phosphino-substituted carbocation 17 [32], one of the very few stable phosphorus analogues of iminium salts [33]. The same synthetic strategy was adopted to prepare the diphosphino carbocation 17 or its valence isomer 2"Pa. A dichloro-methane solution of bis(diisopropylamino)phosphenium triflate was added at 0°C to a pentane solution of the carbene 16. The adduct 2 rPa [34] was isolated in 66% yield as extremely air sensitive white crystals (melting point 89-90°C) by recrystallization from tetrahydrofuran at -5°C (Scheme 13). The symmetrical three-membered ring structure was evident from the spectroscopic data (31P NMR s, S + 7.3 ppm, 13C NMR t, <549.6 ppm, ]PC 7.3 Hz, 29Si NMR t, <5-10.7 ppm Jpsz 10 Hz). 

---