Pd catalysis

Some particular features should be mentioned. Instead of Michael additions, a-nitroolefins are reported to yield allyl sulfones under Pd catalysis (equation 21). Halogenated acceptor-olefins can substitute halogen P to the acceptor by ipso-substitution with sulfinate (equation 22 , equation 23 ) or can lose halogen a to the acceptor in the course of a secondary elimination occurring P to the introduced sulfonyl groups (equation 24). On the other hand, the use of hydrated sodium sulfinates can lead to cleavage at the C=C double bond (equation 25). 

As an explanation, if was suggested that the degree of charge development in the transition state determines the preferred site of cyclopropanation A transition state with little charge development should prefer the endocyclic double bond (Pd catalysis), whereas one with much charge development should favor the exocyclic bond (Rh catalysis). 

The ideal ligand for the cross-coupling reactions should satisfy the requirements of all stages of the catalytic cycle. In practice this is unlikely, as these requirements are not parallel. A careful compromise between various factors is necessary for any particular realization of cross-coupling chemistry. The path of reasoning should be similar for both Pd- and Ni-catalyzed processes however, since very little information is available concerning Ni catalysis, only Pd catalysis will be discussed here. 

A few additional Pd-catalyzed schemes have been employed for Ilac type cyclization chemistry. Palladium-phenanthroline complexes were used by the Ragaini group to prepare indoles via the intermolecular cyclization of nitroarenes and alkynes in the presence of carbon monoxide <06JOC3748>. Jia and Zhu employed Pd-catalysis for the annulation of o-haloanilines with aldehydes <06JOC7826>. A one-pot Ugi/Heck reaction was employed in the preparation of polysubstituted indoles from a four-component reaction system of acrylic aldehydes, bromoanilines, acids, and isocyanides <06TL4683>. 

Direct C-Harylation of purines in position 8 by diverse aryl iodides has been achieved with Pd catalysis in the presence of Cul and CS2CO3. The methodology is general and efficient and was applied in the consecutive regioselective synthesis of 2,6,8-trisubstituted purines bearing three different C-substituents in combination with two cross-coupling reactions... 

Cooperative Ru/Pd catalysis enabled the coupling of aldehydes to aryl iodides (Equation (124)). 

Although the majority of allylic etherification reactions have primarily utilized allylic carboxylates or carbonates as electrophiles (and occasionally allylic chlorides), the use of allylic alcohols for this transformation would be more desirable from a practical standpoint. Reported strategies involving Pd catalysis include the use of P(OPh)3 as the ligand197 and Ti(OPf)4198 as an additive for the in situ activation of the hydoxyl group (Equation (49)).199... 

In addition to alkoxides, carbonyl oxygens have occasionally been recruited to function as nucleophiles in allylic etherification processes. The cyclization reactions of ketones containing internal allylic systems occur through O-allylation under Pd catalysis to give rise to vinyl dihydrofurans203 or vinyl dihydropyrans (Equation (51))204,205 in good yields. 

Although unusual, a nucleophile has occasionally been observed to add to the central carbon of a metal 7r-allyl species to generate a metallacyclobutane intermediate, which can then undergo further transformations. In the reaction between 2,3-dibromopropene and sodium phenoxide under Pd catalysis (Equation (56)), the central carbon of the initially formed 7r-allyl intermediate is attacked by phenoxide to furnish a palladacyclobutane. Displacement of the... 

The carbonyl groups that participate in the alkyne-addition process have not been limited to those that can form enol tautomers. For example, amides have been used as nucleophiles in a one-pot reaction sequence for the preparation of 2,3-disubstituted furanopyridones using Pd catalysis (Equation (96)).343 Furopyridines have also been obtained from the reaction of iodopyridones with alkynes under Pd catalysis,344 and alkynyl pyrimidones have been converted into 2-substituted furanopyrimidones under the influence of an AgN03 catalyst.345... 

In addition to /3-H elimination, olefin insertion, and protonolysis, the cr-metal intermediate has also proved to be capable of undergoing a reductive elimination to bring about an alkylative alkoxylation. Under Pd catalysis, the reaction of 4-alkenols with aryl halides affords aryl-substituted THF rings instead of the aryl ethers that would be produced by a simple cross-coupling mechanism (Equation (126)).452 It has been suggested that G-O bond formation occurs in this case by yy/z-insertion of a coordinated alcohol rather than anti-attack onto a 7r-alkene complex.453... 

Several reactions and syntheses of pyrroles, which involve Pd catalysis, do not fall into the previous categories in this chapter, and therefore are presented here. 

Use of aryl, vinyl and alkynyl iodides as electrophiles is possible using Pd° catalysis. Dieter and Li have evaluated the reaction between Al-Boc-pyrrolidine and Af-Boc-piperidine with several aryl and heteroaryl iodides, 1- and 2-iodo-l-hexene, and 1-iodohexyne. The yields range from about 10-80%, with typical yields in the 40-60% range (Scheme 32). 

Cross-coupling reaction of alkenyl iodides with Ar3Bi were efficient under Pd catalysis [59] (Schemes 41 —4-3). Various alkenyl iodides furnished cross-coupling products in good yields in short reaction times under mild conditions. A bromine substituent did not affect the reaction and afforded the corresponding bromine-substituted products. 

Functionalized isoflavones can be easily obtained from the cross-coupling reaction of 3-iodochromones with Ar3Bi under Pd catalysis (Scheme 44). The coupling reaction of a variety of substituted 3-iodochromones and Ar3Bi afforded substituted isoflavones in high yields [60]. 

Allylic arylations of substituted cinnamyl acetates can be carried out with Ar3Bi under Pd catalysis. The couplings afforded 1,3-diarylpropenes in good yields in short reaction times (Scheme 52) [66],... 

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