Tandem reactions palladium-catalyzed cyclization, carbonyl

Fig. 46 Light-stimulated palladium-catalyzed tandem radical cyclization/carbonylation/ Suzuki-Miyaura coupling reactions...

An indium-mediated radical cyclization sequence has been used to synthesize stereoselectively 3-alkylideneoxindoles [65, 66]. The generation of predominantly the i -isomer, such as seen with 96 below, is attributed to the strong coordination of the indium to the carbonyl of the oxindole intermediate, and the transformation of various iodo-ynamides to the cyclized oxindoles occurred in good yield. Selective approaches to the E- and disubstituted 3-alkylideneoxindoles involving a tandem palladium-catalyzed cross coupling reaction were also highlighted in this report. 

The carbonylative cyclization of 2-iodophenols with norbornadiene or norbornene was also carried out. In 1989, Catellani s group studied the annulation of 2-iodophenol with norbornadiene to coumarin (Scheme 3.11a). ° In this method, the elimination of cyclopentadiene via retro-Diels-Alder reaction was involved in the formation of the terminal product. Fiaud and co-workers reported the palladium-catalyzed carbonylative cyclization of 2-iodophenol with norbornene in 1997 (Scheme 3.11b). They reported that the selectivity for the production of the two regioisomers can be controlled. Catellani and co-workers studied the intramolecular cyclization of ort/zo-iodophenyl 3-butenoate to 4-methylcoumarin with a palladium catalyst. 85% of 4-methylcoumarin was produced in the presence of benzonitrile and carbon monoxide, which is compulsory (Scheme 3.12a). Silva, Costa and their co-workers reported a tandem process for the synthesis of coumarins from 2-iodophenols and enoates with the assistance of a palladium catalyst (Scheme 3.12b). The general scope of this Heck-lactonization involves E- and Z-enoates as substrates. It was shown that this reaction is sensitive to steric hindrance around the double bound in... 

Although only a few examples have been reported, palladium-catalyzed C—O bond formation has proved useful in the synthesis of isocoumarins. Wang and Shen utilized an intramolecular C—O bond formation in tandem with an intermolecular Stille reaction to produce isocoumarin products such as 66 in excellent yields from gem-dibromovinylarene substrates such as 65 (Scheme 24.37) [129], The Stille reaction was postulated to occur first with the E-alkenyl bromide before the intramolecular C—O bond-forming cyclization and ensuing elimination of methyl bromide. Tadd et al. reported a palladium-catalyzed carbonylative isocoumarin synthesis, beginning with the same a-(o-haloaryl) ketone substrates as those used previously in benzofuran synthesis (Scheme 24.30) [130]. 

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