Electrophilic allylic substitution of allylstannanes with aldehyde and imine substrates could be achieved by employment of palladium pincer complex catalysts. It was found that the catalytic activity of the pincer complexes is highly dependent on the ligand effects. The best results were obtained by employment of PCP pincer complexes with weakly coordinating counterions. In contrast to previous applications for electrophilic allylic substitutions via bisallylpalladium complexes, the presented reactions involve monoallylpalladium intermediates. Thus, employment of pincer complex catalysts extends the synthetic scope of the palladium-catalyzed allylic substitution reactions. Moreover, use of these catalysts eliminates the side reactions occurring in transformations via bisallylpalladium intermediates. The key intermediate of the electrophilic substitution reaction was observed by (1)H NMR spectroscopy. This intermediate was characterized as an eta(1)-allyl-coordinated pincer complex. Density functional theory (DFT) modeling shows that the electrophilic attack can be accomplished with a low activation barrier at the gamma-position of the eta(1)-allyl moiety. According to the DFT calculations, this reaction takes place via a six-membered cyclic transition-state (TS) structure, in which the tridentate coordination state of the pincer ligand is preserved. The stereoselectivity of the reaction could be explained on the basis of the six-membered cyclic TS model.

Palladium-catalyzed cross-coupling of vinyl epoxides and aziridines with organoboronic acids was performed by using 0.5-2.5 mol % pincer-complex catalyst. The reactions proceed under mild conditions affording allyl alcohols and amines with high regioselectivity and in good to excellent yields. Under the applied reaction conditions aromatic chloro-, bromo- and iodo substituents are tolerated. Our results indicate that the mechanism of the pincer complex catalyzed and the corresponding palladium(0) catalyzed process is substantially different. It was concluded that the transformations proceed via transmetalation of the organoboronic acids to the pincer-complex catalyst followed by an S(N)2'-type opening of the vinyl epoxide or aziridine substrate. In this process the palladium atom is kept in oxidation state +2 under the entire catalytic process, and therefore oxidative side reactions can be avoided.

Pincer complex catalyzed substitution of various propargylic substrates could be achieved using tin- and silicon-based dimetallic reagents to obtain propargyl- and allenylstannanes and silanes. These reactions involving chloride, mesylate, and epoxide substrates could be carried out under mild conditions, and therefore many functionalities (such as COOEt, OR, OH, NR, and NAc) are tolerated. It was shown that pincer catalysts with electron-supplying ligands, such as NCN, SCS, and SeCSe complexes, display the highest catalytic activity. The catalytic substitution of secondary propargyl chlorides and primary propargyl chlorides with electron-withdrawing substituents proceeds with high regioselectivity providing the allenyl product. Opening of the propargyl epoxides takes place with an excellent stereo- and regioselectivity to give stereodefined allenylstannanes. Silylstannanes as dimetallic reagents undergo an exclusive silyl transfer to the propargylic substrate affording allenylsilanes with high regioselectivity. According to our mechanistic studies, the key intermediate of the reaction is an organostannane (or silane)-coordinated pincer complex, which is formed from the dimetallic reagent and the corresponding pincer complex catalyst. DFT modeling studies have shown that the trimethylstannyl functionality is transferred to the propargylic substrate in a single reaction step with high allenyl selectivity. Inspection of the TS structures reveals that the trimethylstannyl group transfer is initiated by the attack of the palladium-tin sigma-bond electrons on the propargylic substrate. This is a novel mechanism in palladium chemistry, which is based on the unique topology of the pincer complex catalysts.

Nucleophilic rather than electrophilic reactivity is ascribed to the allyl moiety in (η1‐allyl)palladium complexes such as 1, which are generated catalytically from allylstannanes and a palladium pincer complex and react smoothly with aldehydes.

Palladium pincer complex-catalyzed reaction of functionalized propargyl chloride (and mesylate) derivatives with hexamethylditin gives allenyl- and propargyl-stannane products. This catalytic activity is in sharp contrast with the reactivity of commonly used palladium(0) catalysts inducing addition of hexamethylditin to the triple bond. The product distribution of the pincer complex-catalyzed reaction is controlled by the substituent effects of the propargylic substrate: electron-withdrawing functionalities give mainly allenyl stannane products, while with electron-donating groups the main product is propargyl stannane. The catalytic reaction proceeds under very mild conditions tolerating many functionalities such as OH, OAc, NR3, and NR2Ac groups. Our mechanistic studies indicate that the key intermediate of the reaction is a monotrimethylstannane palladium pincer complex. A remarkable feature of the studied catalytic process is that the palladium catalyst does not undergo redox reactions, but its oxidation state is restricted to palladium(II). Since palladium(0) intermediates does not occur in this process, the catalyst is very stable and highly chemoselective.