Palladium complexes iminophosphine

As shown above, the activity measured for the dendrimers was comparable or slightly higher than that of the monomers in many cases, but no general rules can be deduced concerning the efficiency of a dendritic catalyst. We have synthesized complexed iminophosphine derivatives, such as the monomeric palladium complex 4 and the first generation dendrimer 4-Gi. Comparison of their efficiency for various Stille couplings was really puzzling (Fig. 9.5). Measurements of the rate of conversion by NMR show that in some cases the monomer is more efficient than the dendrimer, in other cases they have the same efficiency, and in still other cases the dendrimer is more efficient. However, the real difference between the monomer 4 and the dendrimer 4-Gi is that the latter can be recovered and reused, but not the former [37]. 

Koprowski M, Sebastian RM, Maraval V, Zablocka M, Cadierno V, Donnadieu B, Igau A, Caminade AM, Majoral JP (2002) Iminophosphine palladium complexes in catalytic Stille coupling reactions from monomers to dendrimers. Organometallics 21 4680 687... 

An example that used this protocol the substrate of which contains a sensitive functionality is depicted in Scheme 45 [202]. 1,3-Diene monoepoxide is easily attacked by a palladium(O) complex to form the corresponding 7r-allylpalladium species. Thus such a process could be banished from the desired selective transformation as depicted in Scheme 46 by the employment of the alkynyltin reagent with an aid of triphenylarsine ligand [203]. Organotin protocol is also convenient for introduction of a small alkynyl moiety such as C2 or C3 or preparation of symmetrical diarylethynes (Scheme 47) [204]. Shirakawa et al. reported recently that iminophosphine 4 is much more effective ligand for palladium than tris(2-furyl)phosphine in this reaction (Scheme 48) [32,205]. 

In a similar manner to alkynes, arynes accept the addition of alkynylstannanes in the presence of a catalytic amount of a palladium-iminophosphine complex (Scheme 5.7.15). Here, a characteristic feature is the participation of a vinylstannane. The usual catalytic cycle (Scheme 5.7.16, left), starting with oxidative addition of a carbon-tin bond, is possible, but rather improbable, because there are no signs of oxidative addition of alkenylstannanes that can participate in the present carbostannylation. Consequently, the palladium(O) complex is likely to undergo oxidative cyclization with an aryne to form a palladacyclopropene (Scheme 5.7.16, right), which then reacts with an organostannane. 

Some palladium-catalyzed reactions of organotins, such as carbostannylations, are not related to the Stille cross-coupling. The history of the transition-metal-cata-lyzed carbostarmylation [148] began with alkynylstannylation of alkynes catalyzed by a palladium-iminophosphine complex [149]. Thus, alkynylstannanes added to a carbon-carbon triple bond of various acetylenes, conjugated ynoates and propar-gyl amines and ethers in the presence of a catalytic amount of a palladium-iminophosphine complex [150]. The reaction also proceeded with arynes to afford ortho-substituted arylstannanes, which could further be converted into 1,2-substituted arenes via carbon-carbon bond-forming reactions [151]. 

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