Supported allylic substitution

The supported aqueous phase methodology was applied to the system Pd(OAc)2/5 TPPTS, a catalytic precursor for the Trost-Tsuji reaction. The characterization of the solid by 31P MAS NMR confirms the presence of Pd°(TPPTS)3 as the main surface species. The catalytic properties of the solid were tested for the allylic substitution of E-cinnamylethylcarbonate by different nucleophiles such as ethyl acetoacetate, dimethyl malonate, morpholine, phenol, and 2-mercapto-pyridine. The absence of palladium leaching was demonstrated, and having solved the problem of water leaching from the solid to the organic phase, the SAP-Pd catalyst was successfully recycled several times without loss in its activity. It was used in a continuous flow experiment which... 

Some typical results are shown in Table 2. The table shows that oxidation of conjugated dienes such as isoprene, piperylene (1,3-pentadiene), cyclopentadiene and 1,3-cyclohexadiene with a carbon anode in methanol or in acetic acid containing tetraethylammonium p-toluenesulfonate (EtjNOTs) as the supporting electrolyte yields mainly 1,4-addition products2. 1,3-Cyclooctadiene yields a considerable amount of the allylically substituted product. 

Blechert reported a skillful method of cross-enyne metathesis. Solid-supported alkyne 139 is reacted with alkene in the presence of Ic to give 140. For cleavage of 1,3-diene from solid-supported product 140 having an allyl acetate moiety, palladium-catalyzed allylic substitution is used. Thus, 140 is treated with Pd(PPh3)4 in the presence of methyl malonate to afford three-component coupling product 141 in good yield ... 

Glos and co-workers introduced the aza-bis(oxazolines) 258 and 259 (Fig. 9.78) as a new class of chiral C2-symmetric bis(oxazoline) ligands.These catalysts were used in various reactions such as enantioselective allylic substitution and cyclopropanation it was also shown that these new catalysts could easily be tethered to a polymeric support, as shown in structure 259, allowing for facile recovery of the catalyst. There have been other examples of bis(oxazoline) ligands immobilized on solid supports and their use in catalysis.These methods have shown mixed results. 

Tietze, L. F. Hippe, T. Steinmetz, A. Palladium-Catalysed Allylic Substitution on Solid Support, J. Chem. Soc. Chem. Commun. 1998, 793-794. 

Uozumi, Y. Danjo, H. Hayashi, T. New Amphiphilic Palladium-Phosphine Complexes Bound to Solid Supports Preparation and Use for Catalytic Allylic Substitution in Aqueous Media, Tetrahedron Lett. 1997, 38, 3557. 

For example, POPAM dendrimers of 1,3-diaminopropane type have been used in membrane reactors as supports for palladium-phosphine complexes serving as catalysts for allylic substitution in a continuously operated chemical membrane reactor. Good recovery of the dendritic catalyst support is of advantage in the case of expensive catalyst components [9]. It is accomplished here by ultra-or nanofiltration (Fig. 8.2). 

The group of Van Leeuwen has reported the synthesis of a series of functionalized diphenylphosphines using carbosilane dendrimers as supports. These were applied as ligands for palladium-catalyzed allylic substitution and amination, as well as for rhodium-catalyzed hydroformylation reactions [20,21,44,45]. Carbosilane dendrimers containing two and three carbon atoms between the silicon branching points were used as models in order to investigate the effect of compactness and flexibility of the dendritic ligands on the catalytic performance of their metal complexes. Peripherally phosphine-functionalized carbosilane dendrimers (with both monodentate... 

Investigation of the kinetics of the reaction of 4-chloro-2-pentene, an allylic chloride model for the unstable moiety of polyfvinyl chloride), with several thermal stabilizers for the polymer has led to a better understanding of the stabilization mechanism. One general feature of the mechanism is complexing of the labile chlorine atom by the metal atom of the stabilizer. A second general feature is substitution of the complexed chlorine atom by a ligand (either carboxylate or mercaptide) bound to the metal. Stabilization requires that the new allylic substituent (ester or sulfide) be more thermally stable than the allylic chlorine. The isolation of products from stabilizer-model compound reactions supports the substitution hypothesis of poly(vinyl chloride) stabilization. 

Multinuclear metal complexes that may act as active catalysts or off-cycle species can also be easily identified and studied via ESl-MS. For example, analysis of a simple Pd-catalyzed allylic substitution reaction lead to the discovery of two reversibly formed binuclear bridged palladium complexes (Fig. 6) that act as a reservoir for the active mononuclear catalyst [21], The observation of dimers when using ESl-MS is common and it is crucial to confirm that they truly exist in solution and are not just formed during the ESI process, in this case the detection was supported by P and H NMR studies of stoichiometric reaction mixtures and in situ XAFS experiments [49]. 

The Tsuji-Trost ally lie substitution catalyzed by Pd complexes using CH-acidic nucleophiles can be performed in an ionic liquid of type 1 alone [30] as well as in a biphasic system [31]. In the latter case the use of trisulfonated triphenylphosphine (TPPTS) prevents the catalyst from leaching into the organic phase. In comparison with water as the catalyst-supporting phase, the ionic liquid system exhibits higher activity and selectivity. The enantio-selective version of the allylic substitution with dimethyl malonate can also be performed in ionic liquids with a homochiral ferrocenylphosphine as the ligand [32]. 

Dos Santos, S., Quignard, F., Sinou, D., Choplin, A. Allylic substitution catalyzed by silica-supported aqueous phase palladium(O) catalysts. Top. in Cat. 2000, 13, 311-318. 

MeO-PEG-supported soluble polymeric chiral ligands 204 were synthesized and utilized in various asymmetric allylic substitution reactions (Scheme 3.66) [129]. 

An S—P-type chiral phosphinooxathiane was developed as an effective ligand for palladium-catalyzed allylic substitution reactions [131]. A polymer-supported chiral phosphinooxathiane 208 was also prepared and applied to asymmetric alkylations and aminations of acetate 201 [132]. Enanhoselechvihes of up to 99% ee were obtained in asymmetric Pd-catalyzed allylic amination of acetate 201 using the polymeric catalyst prepared from a PS-diethylsilyl support (Scheme 3.68). 

Heterogeneous aquacatalytic palladium-catalyzed allylic substitution with nitromethane as the Cl nucleophile has been developed by Uozumi (Scheme 3.71). By using an amphiphilic PS-PEG polymer-supported chiral palladium complex, the asymmetric allyhc nitromethylation of cycloalkenyl esters proceeded smoothly in water. For example, when polymer-supported palladium complex 214 was employed in the asymmetric nitromethylation of cycloheptenyl carbonate 215... 

Kragl et al.100 described the retention of diaminopropyl-type metallodendrimers bearing palladium phosphine complexes on ultra- or nanofiltration membranes and their use as catalysts for allylic substitution in a continuously operating chemical membrane reactor. Their results demonstrated a viable procedure for catalyst recovery, because these metallodendrimers acting as catalyst supports offered an advantage in that the intrinsic viscosity of the solution is smaller, facilitating filtration. 

The stereochemical preferences suggested above are supported by Ni-catalyzed allylic substitution reactions of cyclic ethers 88 and 90 (Scheme 4.1h). Whereas reaction of 88 affords 89 in 7() /i yield in one hour with > 950 diastereo- and regio-selectivity (5 mol % (PPhiJiNiCl.-. 22 C. TIIF). subjection of 90 to the same conditions results in the complete recovery of the starting material. 

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