Diarylcarbene complexes

Heteroatom-substituted carbene complexes are less electrophilic than the corresponding methylene, dialkylcarbene, or diarylcarbene complexes. For this reason cyclopropanation of electron-rich alkenes with the former does not proceed as readily as with the latter. Usually high reaction temperatures are necessary, with radical scavengers being used to supress side-reactions (Table 2.16). Also acceptor-substituted alkenes can be cyclopropanated by Fischer-type carbene complexes, but with this type of substrate also heating is generally required. 

The diarylcarbene complexes 39 (R = H, Me, OMe, Br, CF3) also reacted with various organyl isothiocyanates R NCS (R = Ph, Et, Me) to give the corresponding diarylthioketone complexes 40.50,173 On the basis of a kinetic study of these reactions and the dependence of the reaction rate on the... 

The route could be extended to the syntheses of selenoketone complexes of chromium and tungsten (43) from the corresponding diarylcarbene complexes (e.g., 39 R = H, Me, OMe, Br, CF3) and PhNCSe. Later on, the use of tetraethylammonium or potassium salts of [ECN]- as the heteroatom sources turned out to be superior to R NCE.175 Thus it was even possible to synthesize a pentacarbonyl tellurobenzophenone complex of tungsten (44) from K+[TeCN]" and 39 (R = H).176... 

A series of other polycyclic hydrocarbons has been synthesized in moderate to good yields by benzannulation of tricyclic diarylcarbene complexes [36b]. 

Diarylcarbene complexes of transition metals are considerably less reactive toward alkenes than the analogous monoarylcarbene complexes and, therefore, have been little used for preparation of gew-diarylcyclopropanes. Some examples, however, have been reported. When a solution of pentacarbonyldiphenylmethylidenetungsten in ethyl vinyl ether was heated to 37 °C for 3 hours, l-ethoxy-2,2-diphenylcyclopropane (2) was obtained in 61% yield together with 1,1-diphenylethene (3) and 2-ethoxy-l,l-diphenylethene If the reaction is carried in... 

The addition of carbon nucleophiles to the carbene carbon atom generates a new C-C bond. Organolithium reagents add to alkoxycarbene complexes to give alkyl metalates which undergo alkoxide elimination upon addition of acid or exposure to silica gel and thus may serve as precursors for non-heteroatom-stabilized diarylcarbene complexes (Scheme 1). [14b]... 

Arene(alkoxy)carbene chromium complexes react with aryl-, alkyl-, terminal, or internal alkynes in ethers or acetonitrile to yield 4-alkoxy-1-naphthols, with the sterically more demanding substituent of the alkyne (Rl Figure 2.24) ortho to the hydroxy group. Acceptor-substituted alkynes can also be used in this reaction (Entry 4, Table 2.17) [331]. Donor-substituted alkynes can however lead to the formation of other products [191,192]. Also (diarylcarbene)pentacarbonyl chromium complexes can react with alkynes to yield phenols [332]. 

The first examples of a chalcogen insertion into a transition metal-carbene bond were reported in 1981. Stirring solutions of the pentacarbonyl-(diarylcarbene)tungsten complexes 39 in pentane in the presence of solid sulfur afforded the thioketone complexes 40 [Eq. (8)].171 The reaction rate increased considerably when pentane was replaced by CS2 as the solvent due to the higher solubility of elemental sulfur in CS2. 

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