Aromatic compounds addition reactions with carbenes

D.ii. Addition to Aromatic Derivatives. Aromatic compounds also react with carbenes, but ring expansion usually follows the initial cyclopropanation. In a typical example, 2-methoxynaphthalene (373) reacted with dichlorocarbene to give 374, and subsequent ring expansion gave 375, 99 which is a general reaction of enol ethers, which give either unsaturated acetals or unsaturated carbonyls. oo... 

The high energy and reactivity of carbenes is also essential to another characteristic addition reaction of carbenes which has a specific synthetic use. This is the reaction which occurs when carbenes are generated in the presence of aromatic compounds. " The resulting adducts are in thermal equilibrium with the corresponding cycloheptatriene. The position of this equilibrium is a function of the substituents introduced via the carbene. 

Dimethyl-4-silacyclohexadienylidene (lv) is of interest as a potential source of silaxylene 24, however, all attempts to convert the carbene into an aromatic compound failed.107 The only isolated product from gas phase reactions is the dimer 25. In solution, carbene lv was found to add stereospecifically to cis-2-butene. With butadiene as trapping reagent both the products of the 1,2- and 1,4-addition 26 and 27, respectively, are observed (Scheme 21).107 In addition, silacyclopentene 28 is formed, which is the trapping product of cyclo-... 

Addition of carbenes to Jt-electron excessive aromatic compounds, or those which possess a high degree of bond fixation, is well established. Dihalocarbenes react with naphthalenes with ring expansion to produce benztropylium systems (Scheme 7.8). Loss of hydrogen halide from the initially formed product leads to an alkene which reacts with a second equivalent of the carbene to yield the spirocyclopropyl derivatives in high yield (>95%) [14, 50]. Insertion into the alkyl side chain (see Section 7.2) also occurs, but to a lesser extent [14]. Not unexpectedly, dichlorocarbene adds to phenanthrenes across the 9,10-bond [9, 10, 14], but it is remarkable that the three possible isomeric spiro compounds could be isolated (in an overall yield of 0.05% ) from the corresponding reaction with toluene [14]. 

Fullerenes, among which the representative and most abundant is the 4 symmetrical Cgg with 30 double bonds and 60 single bonds, are known to behave as electron-deficient polyenes rather than aromatic compounds [7]. The energy level of the triply degenerate LUMO of Cgg is almost as low as those of p-benzoquinone or tetracyanoethylene. Thus, a wide variety of reactions have been reported for Cgg such as nucleophilic addition, [4-1-2] cycloaddition, 1,3-dipolar addition, radical and carbene additions, metal complexation, and so on [7]. Fullerene Cgg also undergoes supramolecular complexation with various host molecules having electron-donating ability and an adequate cavity size [8]. 

Cyclopropanation reactions are one set in an array of C-C bond-forming transformations attributable to metal carbenes (Scheme 5.1) and are often mistakenly referred to by the nonspecific term carbenoid. Both cyclopropanation and cyclopropenation reactions, as well as the related aromatic cycloaddition process, occur by addition. Ylide formation is an association transformation, and insertion requires no further definition. All of these reactions occur with diazo compounds, preferably those with at least one attached carbonyl group. Several general reviews of diazo compounds and their reactions have been published recently and serve as valuable references to this rapidly expanding field [7-10]. The book by Doyle, McKervey, and Ye [7] provides an intensive and thorough overview of the field through 19% and part of 1997. 

Although the reactions between alkynes or alkenes and metal clusters are the main source of alkyne-substituted complexes, there are other reagents which can produce similar products. Two such reagents are tetraphenylcyclopentadienone, which in the reaction with Ru3(CO)i2 produces Ru3(CO)10(PhCCPh) (167), and dimethyl-vinylarsine, which has been made to react with several carbonyl clusters [Eq. (8)] (168, 169). In the reaction of M3(CO)12 (M = Ru, Os) with a number of tertiary phosphines and aromatic alcohols, an oxidative addition takes place and benzyne-triosmium compounds are obtained (170-176). The fact that Os3(CO)uPEt3 can be converted into an alkyne compound (177) suggests that the conversion goes through substituted intermediates. Carbene derivatives of clusters have also... 

The reaction of dichlorocarbene with aromatic compounds, with a few exceptions, lacks preparative value the adducts rearrange undergoing addition of another molecule of carbene and the final yield of complex product mixture rarely exceeds more than a few percent. Of synthetic importance are the reactions with indenes, phenanthrene (its 9-substituted derivatives 9-ethoxy-,9-trimethylsilyl-, " 9-arylthio-, 9-arylseleno- ), pyrene and various alkyl-naphthalenes. 120 Reactions with aromatic compounds are discussed in Houben-Weyl, Vol. E19b, pp 1551-1553. 

The high reactivity of carbenes is also essential to the addition reactions that occur with aromatic compounds. The resulting adducts are in thermal equihbrium with the corresponding cycloheptatrienes. The position of the equilibrium depends on the nature of the substituent (Scheme 5.27). 

Various carbene complexes Cr(CO)5 =C(OMe)CR =CHR2 react with HC=C(CH2)4CN by benzannulation and subsequent intramolecular addition processes to form spirocyclic or fused aromatic compounds depending upon how the reaction is quenched. The reactions of various p-aminovinyl carbene complexes M(CO)5 =C(OEt)CH=CR(NHRi) (M = Cr, W) with alkynes R CaCH results in loss of ethanol and the formation of pyiidinylidene derivatives 53. These undergo protonation which results in replacement of the metal by a proton and the formation of the corresponding pyridinium salts. i ... 

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