Alkenylation, carbon nucleophile

Allenes also react with aryl and alkenyl halides, or triflates, and the 7r-allyl-palladium intermediates are trapped with carbon nucleophiles. The formation of 283 with malonate is an example[186]. The steroid skeleton 287 has been constructed by two-step reactions of allene with the enol trillate 284, followed by trapping with 2-methyl-l,3-cyclopentanedione (285) to give 286[187]. The inter- and intramolecular reactions of dimethyl 2,3-butenylmalonate (288) with iodobenzene afford the 3-cyclopentenedicarboxylate 289 as a main product) 188]. 

Arylation or alkenylation of soft carbon nucleophiles such as malonate is carried out by using a copper catalyst, but it is not a smooth reaction. The reaction of malononitrile, cyanoacetate, and phenylsulfonylacetonitrile with aryl iodide is possible by using a Pd catalyst to give the coupling products. 

Dienes and allylarcncs can be prepared by the Pd-catalyzcd coupling of allylic compounds with hard carbon nucleophiles derived from alkenyl and aryl compounds of main group metals. Allylic compounds with various leaving groups can be used. Some of them are unreactive with soft nucleophiles, but... 

As shown in the previous sections, a (cr-allenyl)palladium species, which is formed from a propargyl electrophile and a Pd(0) catalyst, reacts with a hard carbon nucleophile in a manner analogous to the Pd-catalyzed cross-coupling reaction to give a substituted allene. The results indicate that the reactivity of the (cj-allenyl)palladium species is similar to that of an alkenylpalladium intermediate. Indeed, it was found that the (cr-allenyl)palladium species reacted with olefins to give vinylallenes, a reaction process that is similar to that of the Heck reaction of alkenyl halides [54]. 

In contrast, soft carbon nucleophiles attack at C5. The reaction of 23 with diethylaminopropyne yields alkenyl(amino)pentatetraenylidene complexes (34) by insertion of the C = C bond of the alkyne into the C4=C5 bond of the pentatetrae-nylidene ligand [9]. The reaction is initiated by a nucleophilic attack of the ynamine at C5 followed by ring closure and electrocyclic ring opening (Scheme 3.34). Complexes 34 are obtained as mixtures of s-cis/s-trans isomers. 

Alkenyl(phenyl)iodonium salts are highly reactive in vinylic nucleophilic substitution reactions because of the excellent leaving group ability of the phenyliodonium moiety. Only a few examples of non-catalytic alkenylation of carbon nucleophiles are known [50,51]. In most cases these reactions proceed with predominant retention of configuration via the addition-elimination mechanism or ligand coupling on the iodine [42,50]. 

The selectivity of the alkenylation reactions and the yields of products can be dramatically improved by carrying out the reaction of alkenyliodonium salts with carbon nucleophiles in the presence of transition metal compounds in stoichiometric or catalytic amounts. Thus, the reactions of bicycloalkenyldiiodo-nium salts 62 with cyanide anion or with alkynyllithium in the absence of transition metals are non selective and lead to a wide spectrum of products, while the same reactions in the presence of the equimolecular amount of copper(I) cyanide afford the respective products of vinylic nucleophilic substitution in good yields (Scheme 29) [52,53]. 

Reactions of halides with 1,2-, 1,3- and 1,4-dienes generate 7i-allylpalladium intermediates, which react further with nucleophiles. The reaction of 1,3-dienes with aryl and alkenyl halides is explained by the following mechanism. The insertion of 1,3-diene to the aryl or alkenylpalladium bond generates 7r-allylpalladium complexes 54, which react further in several ways. As expected, nucleophiles such as carbon nucleophiles, amines, and alcohols attack the 7i-allylpalladium 54 to form the 1,4-... 

A particularly attractive approach is the deracemization reaction of cycHc alkenyl carbonates since easily prepared racemic starting materials can be converted into synthetically useful enantiomerically emiched products [55]. For example, indenyl carbonate and a cyclohexenyl carbonate have been deracemized to give the products in 98% ee using the enantiomer of ligand 49. In both cases the nucleophile was a carboxylate salt (propionate and pivalate, respectively), Eqs. (5) and (6). 

Interestingly, Widenhoefer reported a similar palladium(II) catalyzed cycliza-tion of indoles onto alkenes (Scheme 58) [72]. This mild protocol for cyclization/ carboxylation of 2-alkenyl indoles makes possible catalytic addition of a carbon-nucleophile and carbonyl group across a C-C bond. The mechanism, however, is thought to involve outer-sphere attack of indole onto a palladium-olefin complex rather than the electrophilic C-H activation of the indole C(3)-H bond, exhibited by the Stoltz carbocyclization. 

Displacements. A rather unusual displacement of alkenyl chlorides with carbon nucleophiles has been reported. ... 

Few examples of non-catalytic alkenylations of carbon nucleophiles have been published. For example, enolate anions derived from various 1,3-dicarbonyl compounds can be vinylated with cyclohexenyl or cyclopentenyl iodonium salts 715 to afford products 716 (Scheme 3.286) [964]. 

More recently, a cascade process that presumably consists of (i) oxidative addition of Pd to an alkenyl iodide, (ii) Pd-alkene 7r-complex formation, (iii) anti-attack by a carbon nucleophile, and (iv) reductive elimination has been devised. This cascade process has been used for the synthesis of ( )-A -capnellene (Scheme 22). 

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