Carbenes with benzene

Carbenes are so reactive that they add to the double bonds of aromatic rings. The products are usually not stable and rearrange to give ring expansion. Carbene reacts with benzene to give cycloheptatriene ... 

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. 

The reactions of benzyne and carbenes with phosphabenzenes and of carbenes with phospholes have been investigated. Whereas benzyne reacted in the anticipated manner with (27) to give (28), carbenes, or their precursors, reacted in a curious manner to give substituted benzenes (30). Good evidence for the intermediacy of the phosphepin (29) came from the finding that closely related compounds, such as (31), also decomposed to give substituted benzenes. The nature of the eliminated phosphorus entity has still to be determined. 

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>. 

P-Lactams,2 Decomposition of N-benzyl-N-r-butyldiazoacetoacetamide (1) with Rh2(OAc)4 in refluxing benzene results iti the P-lactam 2 in 98% yield. The acetyl group on the diazo carbon is necessary for this insertion into the p-C—H bond rather than reaction of the carbene with the aromatic ring. The /-butyl group... 

At first, N-heterocyclic carbenes 20 look bizarre to the organic chemist, since they are organic/inorganic hybrid compounds. However, borazines, sometimes called inorganic benzene , are isoelectronic with benzene and are therefore extraordinarily stable heterocycles. Exchange of a borane... 

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... 

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). 

The reactivity of nucleophilic carbenes with 2-bromo-2,3-dihydro-l//-l,3,2-diazaborole has also been described. Reaction of a benzene solution of (1) (R = Me, i-Pr R = H) I-----------------------1... 

Upon UV irradiation of dicyanofpyridinio- or pyridazinio )methanide, dicyanocarbene was generated, which then reacted with benzene to give bicyclo[4.1.0]hepta-2,4-diene-7,7-dicarboni-trile (2) or with (Z)-4-methylpent-2-ene nonstereospecifically to give cis- and trans-2-iso-propyl-3-methylcyclopropane-l,l-dicarbonitrile. However, due to competing photoreactions of the ylide, the carbene-transfer reaction is not efficient and the cyclopropanes were obtained in poor yield only. ... 

Although nonaromatic cyclic alkaoligoenes always react with transient phosphorus-substituted carbenes to give 1 1 adducts, aromatic derivatives sometimes give rise to a mixture of 1 1 and 1 2 adducts. Numerous bicyclo[4.1.0]hepta-2,4-dienes (norcaradienes) have been prepared by reacting transient phosphorus-substituted carbenes with aromatic compounds in order to study the influence of the substituents on the norcaradiene-cycloheptatriene equilibrium. Benzene itself and its functionalized derivatives, in which the three bonds are not equivalent, will be reviewed in separate sections. 

The reaction of transient dimethoxyphosphoryl-substituted carbenes with 1,4-disubstituted benzenes yields selectively cjcu-7-dimethoxyphosphorylbicyclo[4.1.0]hepta-2,4-dienes which can be in equilibrium with the cycloheptatriene isomers (Table 5) 5.35,36.38... 

The direct synthesis of benzocyclopropene (1) from benzyne and a carbene or from synthetic equivalents thereof has not yet been realized, but arc-generated carbon reacted with benzene at — 196°C to give benzocyclopropene (1) in 11% yield, together with toluene (24%), cyclo-heptatriene (17%), 7-phenylcycloheptatriene (21%) and heptafulvene (17%). Phenylcarbene in an excited state is a possible intermediate in the formation of benzocyclopropene. ... 

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. 

Dicyanocarbene. Dicyanodiazomethane when heated to about 70° or irradiated with ultraviolet light loses nitrogen with formation of dicyanocarbene.2 The most interesting reaction of this carbene is with benzene to give 7,7-dicyanonorcaradiene... 

Methylene is produced from diazomethane by the photodissociation reaction. This carbene reacts with benzene to form toluene and cylohepta-triene (Scheme 4). 

In a third method, unsaturated carbenes with a free double bond, Xni, may be directly synthesized from W(CO)g and the corresponding unsaturated Li derivative. The TT-complexation of the double bond is achieved in boiling benzene, affording thermostable XIV ... 

The reaction of carbenes with aromatic hydrocarbons is related to that with alkenes. Doering and Knox (1950, 1953) investigated this reaction using benzene as substrate even before their work with cyclohexene. They observed, however, ring expansion to give cycloheptatriene besides toluene (8-3, X = H). Norcaradiene as an intermediate was isolated only much later in the addition of dicyanomethylene... 

The formation of toluene in the reaction of diazomethane with benzene (8-3) brings us to the second characteristic reaction of carbenes, namely insertion into single bonds, in this case into an sp -C —H bond. Here again prehistoric examples (i.e., investigations before 1950) of Buchner and Meerwein are known (Buchner and Schulze, 1910 Buchner and Scholtenhammer, 1920 Meerwein et al., 1942), as well as early systematic work by Doering s group after 1950. 

In Section 8.1 it was briefly discussed that carbenes react with benzene leading to various products (8-3). With photolytically generated methylene, toluene and cycloheptatriene are obtained in the ratio 1 3.5 (Doering et al., 1953 products with... 

Reaction with benzene follows a similar pathway, yielding a bicyclo[3.2.2]nonatriene structure. Vinylcarbenoids also react with pyrroles to give tropanes via a cyclopropanation-Cope rearrangement route. The direct addition of carbenes to acetylenes does not give satisfactory yields of cyclopropanes, but the rhodium carboxylate catalysed reaction of diazo compounds with acetylenes is a useful source of cyclopro-panes. Carbenoids can also attack a carbonyl oxygen atom, giving rise to a zwitterion (249). An excellent review of intramolecular carbenoid reactions has appeared. ... 

Air-sensitive carbene adducts of chlorine (92-94) have been isolated from the reaction of carbenes with 1,2-dichloroethane as shown below (68). The Cl-Cl bonds of these adducts are reactive. Thus, benzene is chlorinated by 92 and 93 to give chlorobenzene under mild... 

In benzene (which is inert towards 3) Closs and Rabinow measured the absolute rate constant of reaction of triplet carbene with itself, with dienes and with methanol. 

The solvent may also influence the product distribution. [43] The reaction of chromium carbenes with alkynes in donors solvents like ethers or in benzene reveals a strong preference for benzannulation whereas non-coordinating solvents such as hexane or polar solvents such as DMF increase the yields of cyclo-pentannulation products. A strongly coordinating solvent like acetonitrile leads to the formation of moderate yields of cyclobutenones. 

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