Carbenes, reaction with benzene

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]. 

Substituted benzenes are obtained from the reaction of carbenes with phosphorins. The phosphepin (123) is thought to be an intermediate because the related compound (124) decomposes to a substituted benzene. 

Since the most direct evidence for specihc solvation of a carbene would be a spectroscopic signature distinct from that of the free carbene and also from that of a fully formed ylide, TRIR spectroscopy has been used to search for such car-bene-solvent interactions. Chlorophenylcarbene (32) and fluorophenylcarbene (33) were recently examined by TRIR spectroscopy in the absence and presence of tetrahydrofuran (THF) or benzene. These carbenes possess IR bands near 1225 cm that largely involve stretching of the partial double bond between the carbene carbon and the aromatic ring. It was anticipated that electron pair donation from a coordinating solvent such as THF or benzene into the empty carbene p-orbital might reduce the partial double bond character to the carbene center, shifting this vibrational frequency to a lower value. However, such shifts were not observed, perhaps because these halophenylcarbenes are so well stabilized that interactions with solvent are too weak to be observed. The bimolecular rate constant for the reaction of carbenes 32 and 33 with tetramethylethylene (TME) was also unaffected by THF or benzene, consistent with the lack of solvent coordination in these cases. °... 

In addition to the reaction of vinylcarbene complexes with alkynes, further synthetic procedures have been developed in which Fischer-type carbene complexes are used for the preparation of benzenes. Most of these transformations are likely to be mechanistically related to the Dbtz benzannulation reaction, and can be rationalized as sequences of alkyne-insertions, CO-insertions, and electrocycli-zations. A selection of examples is given in Table 2.18. Entry 4 in Table 2.18 is an example of the Diels-Alder reaction (with inverse electron demand) of an enamine with a pyran-2-ylidene complex (see also Section 2.2.7 and Figure 2.36). In this example the adduct initially formed eliminates both chromium hexacarbonyl ([4 -I- 2] cycloreversion) and pyrrolidine to yield a substituted benzene. 

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>. 

The existence of the carbene intermediate has also been demonstrated by formation of dimers12,54, reaction with solvent (benzene) to form a norcaradiene [observed only with electron-deficient 3ZZ-pyrazoIes72,154,162 (Scheme 57)], by trapping with active alkenes55,56,162 and dienes55,72,162 (often... 

One approach, which has proved revealing in the case of 9-carbena-fluorene and which may be generally applicable, is to examine the influence of additives which can deactivate electronically excited carbenes without undergoing reaction themselves (e.g. hexafluoro-benzene) or which react preferentially with carbenes of a particular spin state (e.g. oxygen and butadiene, which are both thought to react preferentially with triplet carbenes). The technique is analogous to well-... 

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... 

Bis(trifluoromethyl)diazomethane is a reactive, electrophilic compound. It forms adducts with nucleophiles such as amines and phosphines5 and adds to olefins, acetylenes,5 and thiocar-bonyl compounds6 to form heterocycles. It has been used as a source of bis(trifluoromethyl) carbene in reactions with benzene,5 saturated hydrocarbons,7 carbon disulfide,8 and transition metal compounds,8 and it undergoes a unique radical chain reaction with saturated hydrocarbons to form adducts that are hydrazoncs and azines.7... 

Carbene adducts of the heavier pnictogen pentaflu-orides have been reported. Reaction of AsFs or SbFs with carbene (1) (R = Mes, R = Cl) in 1,3-bis(trifluoromethyl)benzene resulted in the isolation of(l) AsF5 (112) and (1) SbFs (113). The crystallographic analysis for (112) and (113) reveals the expected octahedral geometry at the pnictogen center with a long As C (2.009(5)A) and Sb-C (2.175(5)A) bond distance respectively. Recently, a novel hypervalent antimonide complex was reported. The nucleophihc addition of carbene (1) (R = Mes, R = H) to Sb(CF3)3 (equation 12) resulted in the... 

Some arsonium ylides have been reported to decompose when heated in solution. For example, when triphenylarsonium benzylide was heated in a boiling benzene-ether mixture it decomposed to give triphenylarsine and a mixture of stilbenes. Trimethylar-sonium methylide likewise undergoes thermal decomposition to give trimethylarsine and ethylene A likely mechanism for these reactions involves carbenic decomposition of the ylide followed by attack of the carbene on unchanged ylide with expulsion of an arsine fragment (equation 2). 

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. 

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