Cycloadditions tetrakis palladium

Cyclopentene annelation by (3 + 2] cycloaddition.1 A key step in a synthesis of the alkene 6 is the cycloaddition of I to 2,3-dicarbomethoxynorbornene (2) catalyzed by tetrakis(triisopropyl phosphite)palladium(0). The product (3) was converted to the... 

Intramolecular [3-I-2]-cycloaddition reactions have also been performed using alkyne moieties as reactants. An early example of this methodology is the high-yield formation of 27 from alkyne 26. As in the case of the (Z)-3-benzyloxybut-2-enoate derivatives (vide supra), the presence of the 3-siloxy group is essential for the reaction to proceed. A diastereomeric mixture of 26 leads to the formation of the two diastereomeric bicyclic adducts 27 in a combined yield of 88%. ° Interestingly, albeit with only 60 /o conversion after 100 hours, the same products are also formed at room temperature under the influence of ultrasound and 6 mol /o of tetrakis(triphenylphosphane)palladium(0) as catalyst. In a purely thermal reaction in the absence of a metal catalyst, a totally different reaction, presumably involving a Diels —Alder type addition of the alkyne to a vinylbenzene moiety, takes place. ... 

These cycloadditions are more sensitive to the quality of the catalyst, the major side reaction being protodesilylation of the allylsilane subunit. Since this can not be measured readily either in the case of the tetrakis(triphenylphosphane)palladium(0) or the palladium acetate/triisopropyl phosphite methods, an improved method for generating the palladium(O) species has been developed22. This involves in situ preparation of tetrakis(triisopropyl phos-phite)palladium(O) by direct reduction of palladium acetate with butyl lithium. This method is illustrated by the addition of the methyl-substituted TMM-Pd complex to eyelopentenone. 

In the case of cycloadditions to the c/.s-acrylate employing tetrakis(triphenylphos-phane)palladium (2mol%) in refluxing toluene, complete diastereoselectivity was observed. The product formed in 69% yield was, once again, that derived from formal Felkin Anh addition to the double bond and the fact that the product was exclusively cw-substituted has direct implications concerning the concertedness of the reaction55. 

The disylanylstannane 635 adds terminal alkynes regio- and stereoselectively to form (Z)-alkenes 636, which undergo a tetrakis(triphenylphosphine)palladium-catalysed reaction with phenylacetylene to give 1 -sila-4-stannacyclohexa-2,5-dienes 637. 1,4-Dithianes 639 ( = 1) and 1,4-dithiepanes 639 n = 2) are obtained by the azoisobutyronitrile - induced homolytic cycloaddition of alkynes R C=CR (R = Pr, Bu or CMe20H R = H or Pr) to the dithols 63836 7a b. 

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