Benzene derivatives reaction with carbenes

There are several examples of intramolecular reactions of monocyclic /3-lactams with carbenes or carbenoids most of these involve formation of olivanic acid or clavulanic acid derivatives. Thus treatment of the diazo compound (106) with rhodium(II) acetate in benzene under reflux gives (107), an intermediate in the synthesis of thienamycin (80H(14)1305, 80TL2783). 

The reaction of alkoxyarylcarbene complexes with alkynes mainly affords Dotz benzannulated [3C+2S+1C0] cycloadducts. However, uncommon reaction pathways of some alkoxyarylcarbene complexes in their reaction with alkynes leading to indene derivatives in a formal [3C+2S] cycloaddition process have been reported. For example, the reaction of methoxy(2,6-dimethylphenyl)chromium carbene complex with 1,2-diphenylacetylene at 100 °C gives rise to an unusual indene derivative where a sigmatropic 1,5-methyl shift is observed [60]. Moreover, a related (4-hydroxy-2,6-dimethylphenyl)carbene complex reacts in benzene at 100 °C with 3-hexyne to produce an indene derivative. However, the expected Dotz cycloadduct is obtained when the solvent is changed to acetonitrile [61] (Scheme 19). Also, Dotz et al. have shown that the introduction of an isocyanide ligand into the coordination sphere of the metal induces the preferential formation of indene derivatives [62]. 

Benzene derivatives such as m-methylanisole (40) can be converted to distonic carbene ions. Reaction of 40 with O occurs with loss of H2, generating the conventional carbene anion 41 this anion reacts with molecular fluorine by dissociative ET, followed by nucleophilic attack of F on the methyl group, forming 42. In contrast to phenyhnethylene, 42 has a singlet ground state however, upon protonation it gives rise to the triplet state of m-hydroxyphenyl-methylene. This interesting reaction can be viewed as a spin-forbidden proton-transfer reaction. 

Ring expansion of aromatic compounds by carbene, carbethoxycarbene, chlorocarbene, and carbenoid is well known 256, 336, 351-356). Muller and co-workers reported the reaction of aromatic compounds with carbene generated from a catalytic decomposition of diazomethane with copper salts, and proposed a bimolecular two-step mechanism involving an inverse ylid for the reaction. Miller (336) proposed another bimolecular two-step mechanism for the reaction of benzene with alkylcarbenoids of aluminum. Baldwin and Smith (25) proposed a concerted mechanism for the reaction of aromatic compounds with carbethoxycarbene. Reaction of alkylbenzene with diethylzinc and ethylidene iodide gives 7-methylcyclohepta-l,3,5-triene derivatives in yield 369). The... 

The electrophilic carbene, 4H-imidazolylidene (63), has been prepared by photodecomposition of 4-diazo-4H-imidazole (64). Reaction with cyclohexane affords the C-H insertion product, 4(5)-cyclohexylimidazole (65), whereas reaction with benzene yields 4(5)-phenylimidazole (66). Analogous addition of 4H-imidazolylid-ene to derivatives of benzene is influenced by the presence and nature of the substituents. 

It will be interesting to see how general this behaviour is. Enders [39,40] reported that triazol-2-ylidene 37 reacted with oxygen, sulfur and selenium to give the expected urea derivatives in 54%, 98%, and 88% yields respectively (Scheme 12). The reaction with sulfur in benzene to give thiourea derivatives is rapid and is often used as a test of carbene formation. It is presumably initiated by nucleophilic attack by the carbene on Sg. 

In addition to the carboxylate-like reactivity of carbenes, these complexes also display a rich and quite remarkable metal-based chemistry. This can be divided into two types thermal and photochemical. The principle thermal reaction is the Dotz reaction with alkynes (Scheme 8.15). This involves heating an a,p-unsaturated carbene 8.40 with an alkyne and results in the formation of a phenol 8.41. The phenolic carbon is derived from CO. The a,p-unsaturation may be a part of a benzene ring, in which case a naphthol will be formed. If the reaction is run in the presence of acetic anhydride and a base, the corresponding acetate is... 

Reaction of unsaturated chromium and tungsten carbene complexes 407 with piperidazine provides amino-carbene complexes 408 and 409. Although various solvents such as CH2C12 benzene, ether, and THF can be used in the reaction, the yields of the desired products 410 are 32-59%, and considerable amounts of by-products 409 are formed. The carbene complexes 408 are rather stable and can be stored in a refrigerator. Oxidation of both complexes with iodoso-benzene affords oxo derivative 410 in 70% (M = Cr) and 41% (M =W) yields (Scheme 64) <1994CL777>. 

The reaction of BA with isopropyl alcohol is particularly revealing. Direct irradiation of DABA in neat isopropyl alcohol gives the ether (17%) expected from the reaction of the singlet carbene, and the radical coupling products characteristic of the triplet spin state (21). The ratio of the yields of radical-derived products to ether depends inversely on the concentration of isopropyl alcohol in a benzene solution. This behavior is particularly meaningful when compared with that of FL, XA and DPM. For these... 

The vapor-phase pyrolysis of 4-hydroxy-1,2,3-triazole and its iV-methyl derivative affords methan-imine and its A-methyl analog. Analysis of the reaction path by the MNDO method shows the presence of two stable or metastable isomers, (liif)-4-hydroxy-l,2,3-triazole and its ketone protomer <89NJC551>. 4-Diazo-1,2,3-triazoles (122) thermolyze or photolyze in benzene to 4//-l,2,3-tri-azolylidenes (123) which convert benzene to 4-phenyl-1,2,3-triazoles and/or isomerize to a-diazo-nitriles (124). Intermediates (124) react with benzene via a carbene to give addition, ring expansion or substitution products (Scheme 17) <82TL5115>. The similar thermolysis of diazotriazoles in substituted benzene gives complex mixtures in which all of the components are sometimes impossible to isolate and identify <90AHC(48)65>. 

Benzannulation reactions of Fischer carbene complexes have proved a useful route for the synthesis of benzene rings fused to one furan ring and one additional heteroaromatic ring (Equation 96) <2005TL2211>. Thus, reaction of chromium carbene complex 168 with the conjugated dienyne systems produces benzofuran derivatives (Table 7). 

The facile decarbonylation of pentacarbonyl complexes 23a and 23b results in tetra-carbonyl carbene complex intermediates 24a and 24b, respectively. Their annulation can produce benzene and/or cyclopentadiene derivatives 25 and/or 26. In the case of aryl acyla-mino complex 23a, the reaction course is shifted towards the benzannulation reaction (25a 26a = 84 16). With the vinyl carbene complex 23b, the benzannulation product 25b (25b 26b = 100 0) is produced exclusively. 

A variety of annulations leading to ring systems other than benzenes can be isolated from reactions of Fischer carbenes. Cyclopentenone derivatives are formed from a reaction of Z-6-amino-Q ,jS-unsaturated chromium carbenes with alkynes (Scheme 66). In contrast, -B-amino-Q, /3-unsaturated chromium carbenes reacts with alkynes to give cyclopentadienes (Scheme 67). 

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