Cycloaddition reactions palladium-catalysed

Palladium-catalysed processes typically utilise only 1-5 mol% of the catalyst and proceed through small concentrations of transient palladium species there is a sequence of steps, each with an organopalladium intermediate, and it is important to become familiar with these basic organopalladium processes in order to rationalise the overall conversion. Concerted, rather than ionic, mechanisms are the rule, so it is misleading to compare them too closely with apparently similar classical organic mechanisms, however curly arrows can be used as a memory aid (in the same way as one may use them for cycloaddition reactions), and this is the way in which palladium-catalysed reactions are explained in the following discussion. (For convenience, an organometallic component can be referred to as the nucleophilic partner and the halide as the electrophilic partner, but this should not necessarily be taken to imply reactivity as defined in classical chemistry. Also, references to the halide should be understood to include all related substrates, such as triflates.)... 

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. 

C. W. Holzapfel and T. L. van der Merwe, Palladium-catalysed [3 + 2]-cycloaddition reactions employing sulphonyl-activated unsaturated carbohydrate derivatives, Tetrahedron Lett., 37 (1996) 2303-2306. 

In 2003, Trost and Frandrick developed a novel type of DYKAT based on the asymmetric palladium-catalysed cycloaddition between isocyanates and vinylaziridines. Performed in the presence of a chiral diphosphine as the ligand, the reaction allowed the access to a broad array of imidazolidin-2-ones with high yields and enantioselectivities of up to 95% ee, as shown in Scheme 2.58. In 2005, the same authors applied this methodology to the total synthesis of (-I- )-pseudodistomin D. ... 

Grigg, R Liu, A., Shaw, D Suganthan, S Washington, M.L., Woodall, D.E. and Yoganathan, G. (2000) Palladium-catalysed cascade molecular queuing-cycloaddition, cyclocondensation and Diels-Alder reactions. Tetrahedron Letters, 41,... 

The presence of five-membered rings such as cyclopentanes, cyclopentenes, and dihydrofurans in a wide range of target molecules has led to a variety of methods for their preparation. One of the most successful of these is the use of trimethylenemethane [3 + 2] cycloaddition, catalysed by pal-ladium(O) complexes. The trimethylenemethane unit in these reactions is derived from 2-[ (trimethylsilyl)methyl]-2-propen- 1-yl acetate which is at the same time an allyl silane and an allylic acetate. This makes it a weak nucleophile and an electrophile in the presence of palladium(0). Formation of the palladium 7t-allyl complex is followed by removal of the trimethylsilyl group by nucleophilic attack of the resulting acetate ion, thus producing a zwitterionic palladium complex that can undergo cycloaddition reactions. 

Diene-l,6-diones have been synthesized from dienetricarbonyliron complexes in good yield via a route involving two Friedel-Crafts acylation steps (Scheme 41).61 via Coupling and Cycloaddition Reactions - Optically active allysilanes have been prepared by a cross-coupling reaction catalysed by an optically active ferrocenylphosphine-palladium complex (Scheme 42). 6 ... 

Activated acetylenic compounds, for example esters RC=CC02R" undergo cycloadditions to adenosine derivatives to yield two possible products (3, 4), depending on the conditions or reaction. The derivatives of 2-alkynoic acids undergo a regiospecific and stereospecific hydroacetoxylation reaction, catalysed by palladium acetate, as shown in reaction 2. ... 

In 2006, Lu and coworkers [53] described a method for the synthesis of carbazoles using the palladium(ll)-catalysed oxidative Heck reaction (Scheme 9.25). Indole 177 was subjected to catalytic Pd(OAc)2 and 2.1equiv of benzoquinone to afford carbazole 178 in 88% yield. Because the alkene is 1,1-disubstituted, a 5-exo cyclization mode (as seen in Ferreira and Stoltz s [48] indole annulation) is unproductive, and a 6-endo cyclization ultimately occurs. The putative intermediate is then believed to be oxidized to the aromatic carbazole by the excess benzoquinone. When indole 179, featuring a terminal alkene, was treated with these oxidative conditions, a mixture of products from 6-endo cyclization (180) and 5-exo cyclization (and subsequent isomerization and [3+2] cycloaddition, 181) was observed. A variety of substituted carbazoles were obtained by this palladium(n)-catalysed oxidative cyclization. 

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