Norbomenes reactions with electrophiles

Based on a transformation described by Catellani and coworkers [80], Lautens s group [81] developed a series of syntheses of carbocycles and heterocycles from aryl iodide, alkyl halides and Mizoroki-Heck acceptors. In an early example, the authors described a three-component domino reaction catalysed by palladium for the synthesis of benzo-annulated oxacycles 144 (Scheme 8.37). To do so, they used an m-iodoaryl iodoalkyl ether 143, an alkene substimted with an electron-withdrawing group, such as t-butyl acrylate and an iodoalkane such as -BuI in the presence of norbomene. It is proposed that, after the oxidative addition of the aryliodide, a Mizoroki-Heck-type reaction with nor-bornene and a C—H activation first takes place to form a palladacycle PdCl, which is then alkylated with the iodoalkane (Scheme 8.37). A second C—H activation occurs and then, via the formation of the oxacycle OCl, norbomene is eliminated. Finally, the aryl-palladium species obtained reacts with the acrylate. The alkylation step of palladacycles of the type PdCl and PdCl was studied in more detail by Echavarren and coworkers [82] using computational methods. They concluded that, after a C—H activation, the formation of a C(sp )—C(sp ) bond between the palladacycle PdCl and an iodoalkane presumably proceeds by oxidative addition to form a palladium(IV) species to give PdC2. This stays, in contrast with the reaction between a C(sp )—X electrophile (vinyl or aromatic halide) and PdCl, to form a new C(sp )—C(sp ) bond which takes place through a transmetallation. 

In contrast to the reactions of electrophiles la and lb with alkenes 2-6, interactions of the same reagents with norbomene 7 in CH2CI2 at 20°C led to the formation of 1,2-chloroselenide 13a and 1,2-chlorotelluride 13b in nearly quantitative yields (Scheme 2). 

The reaction proceeds smoothly and gives 326 in 43% yield. The stereochemistry of the addition is exo with respect to the norbomene moiety and in line with the usual cycloaddition behavior of quadricyclane [358]. The norbomene double bond in 326 is easily accessible by electrophiles, and, for example, the anti-addition of benzenesulfenyl chloride proceeds quantitatively at room temperature (Scheme 4.66). 

An essential feature of this process is the reaction of a nucleophile with a fluorocarbon radical. It is important to emphasise that radicals, being electron-deficient, are electrophilic and therefore that fluorocarbon radicals are even more electrophilic. These processes are, of course, aided by ultraviolet irradiation and inhibited by radical traps, or the radicals may be intercepted, e.g. by norbomene as in the example shown in Figure 5.7a [15], Further examples are given in Table 5.2. 

Brown notes that the formation in solvolysis of tight ion pairs rather than of free carbocations turns the rate of reactions of ions with nucleophiles to be lower than that of reaching an equilibrium between epimers. Therefore in order to trap the classical 2-norbornyl cation it is necessary not to use solvolysis, but other reactions in which more free carbocations are formed. The electrophilic addition of acids to norbomene belong to these reactions. Brown considers results obtained to be due to the capture of the classical 2-norbomyl cation before a complete equilibrium is established ... 

Examples of the insertions of alkenes or alk5mes into metal-amido bonds are also rare. Examples of the insertions of alkenes into tihe M-N bonds of isolated amido complexes include the reaction of a rhodium anilide complex with alkenes to form imines witii kinetic behavior that is consistent with migratory insertion,and the formal insertion of the strongly electrophilic acrylonitrile into a platinum anilide. Additional examples include reactions of a lanthanide-amido complex generated in situ, a catalytic carboamination process in which the stereochemistry implies insertions of olefins into amides, and a catalytic hydroamination that appears to occur through an aminoalkyl complex generated by S3m addition of the iridium and amido groups across the C=C bond of norbomene. 

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