Indenes electrophilic addition

The regioselectivity of electrophilic addition is governed by the ability of an aromatic ring to stabilize an adjacent carbocation. This is clearly seen in the addition of hydrogen chloride to indene. Only a single chloride is formed. 

The silver(I) fluoride/bromine and silver(I) fluoride/iodine systems can be exploited for generation in situ and electrophilic addition of "bromine monofluoride and iodine monofluoride to cyclohexene, mcthylenecyclohcxane, styrene, indene, and acenaphthylene.55... 

Aromatic rings typically react by substitution, so that they can retain the aromatic sextet. By contrast, alkenes react by electrophilic addition—the classic test for an alkene is that they decolourize bromine water. So, how will our hydrocarbon (known as indene) react It contains an aromatic ring, but the five-membered ring is not aromatic—it contains a saturated carbon atom. So there is a choice of substitution on the six-membered ring or addition to the alkene in the five-membered ring. Alkenes are more reactive than benzene, so the alkene reacts first ... 

The transition state of singlet carbene cycloaddition to alkenes involves an electrophilic approach of the vacant p orbital to the n bond of alkenes. By contrast, the first step of the triplet addition process may involve the in-plane a orbital of the carbene. As in the case of C—H insertion (see Section 5.1), the difference in the transition structure between the singlet and triplet cycloaddition becomes important in the intramolecular process, especially when approach to a double bond is restricted by ring strain. Direct photolysis of ( )-2-(2-butenyl)phenyldiazomethane (99) in the presence of methanol gives l-ethenyl-l,la,6,6fl-tetrahydrocycloprop [fljindene [100, 29%, (E/Z)= 10 1] and l-(2-butenyl)-2-(methoxymethyl)benzene (101, 67%). Triplet-sensitized photolysis results in a marked increase in the indene (52%, EjZ) = 1.3.T) at the expense of the ether formation (4%) (Scheme 9.30). On the other hand, direct photolysis of phenyldiazomethane in an equimolar mixture of... 

When fluorine is passed through a mixture of methanol in acetonitrile at -45 °C or propionitrile at -75 °C a solution of methyl hypofluorite, stabilised by the solvent, is produced [200]. While methyl hypofluorite can be isolated, its chemistry has been investigated by treating various substrates with freshly prepared solutions rather than with isolated material. Unlike the hypofluorites which have been discussed so far, methyl hypofluorite behaves as a synthon for the rare methoxylium species, MeO+ , and adds to various alkenes electrophilically in accordance with Markovnikov s Rule [201]. In the reaction between methyl hypofluorite and indene, almost exclusive trans addition occurred. 

Cabaleiro and Johnson (1967) report that the addition of chlorine to -methyl cinnamate in chloroform or acetic acid is syn-selective, SS 0-75, in chloroform and acetic acid acetoxychloro derivatives are produced as well. Again, Dewar and Fahey (1964) argue that the normal course of addition of hydrogen halides onto olefins is a polar electrophilic process involving classical carbonium ions as intermediates and leading mostly but not exclusively to cis-adducts. A syn-preference was found in the additions of deuterium bromide to acenaphthylene, indene, and cis- and fraws-phenylpropene. In the case of indene, phenylpropene and methyl cinnamate, which are styrene analogs, concerted syn addition is symmetry-allowed (see bottom of p. 273). 

Similarly, alkane- and arenesulfonyl isocyanates gave 2-sulfonyl-2,3-dihydroisoindol-l-ones 11 with benzocyclopropene. 2,3-Dihydroisoindol-l-one was produced as a byproduct in 4% yield in the case of arenesulfonyl isocyanates. Cyclopropa[ ]naphthalene underwent addition to 4-phenyl-47/-l,2,4-triazole-3,5-dione at 20°C almost instantaneously to give an indazole 12 in 92% yield.Both reactions are believed to occur by electrophilic attack on the cyclo-proparene and may involve a zwittcrionic intermediate. When benzyne was generated from benzenediazonium-2-carboxylate in refluxing dioxane in the presence of cyclopropa[Z ]naph-thalenc, 5//-naphtho[2,5-a]indene was formed in 13% yield. 

The Wu group reported a palladium-catalyzed three-component approach to fluorinated indene-fused quinolines 301 using 2-alkynylbromobenzenes 299, 2-alkynylanilines 330, and A-fluorobenzenesulfonimide (NSFI) (Scheme 19.81) [151]. The reaction mechanism proposed includes (1) oxidative addition of 299 to paUa-dium(O) and coordination with the triple-bond of 300 to generate the intermediates 302, (2) formation of vinylpalladium(II) intermediates 303 by the first triple-bond insertion, (3) the second triple bond insertion and an intramolecular C—N bond formation to afford 5/f-indeno[l,2-c]quinoline 304, and (4) electrophilic reaction of NSFI in the presence of a base to give the fluorinated ll//-indeno[l,2-c]quinolines 301. 

A condensation of vinyl sulphides of synthetic value involves the reaction of a-ethylthio-styrenes with an aldehyde in the presence of a Lewis acid catalyst to give 3-ethylthio-indenes. Regiospecific and stereospecific pentannelation i.e, fusion of a cyclopentane ring) of an alicyclic ketone involves arylthiomethylen-ation followed by base-catalysed addition to an aj8-unsaturated ester. a-Phenyl-thiobutenolides are less reactive Michael acceptors than their sulphoxide counterparts, and /SjS-dichlorovinyl sulphides are less reactive in electrophilic reactions than their oxygen counterparts. Dimerization of aryl a-cyanovinyl sulphides occurs more readily than that of alkyl analogues, since the intermediate 1,4-diradical ArSC(CN)CH2CH2d(CN)SAr, which cyclizes to the cyclobutane, is more stable. Products obtained by photolysis of methyl vinyl sulphide are accounted for by homolysis of both vinyl—S and methyl—S bonds. The kinetics of catalysed cis-trans isomerization of aryl vinyl sulphides have been studied. ... 

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