Palladium catalysts allylic amination

Palladium complexes are general and versatile catalysts for allylic amination.1,la lh The palladium-catalyzed allylic aminations of 1,3-symmetrically disubstituted substrates, including enantioselective versions, have been widely studied.1, a h It has been important to control the regioselectivity in allylic amination of unsymmetrical substrates 1 or 2 (Equation (1)). In general, palladium-catalyzed allylic amination gives the ( )-linear product 3Tla lh regiocontrol in amination has recently attracted much attention in approaches toward the branched product 4. 

The resulting peripheral phosphine-functionalized dendrimers were used as a ligand in the palladium-catalyzed allylic amination of crotyl acetate in a CFMR. The active catalysts were prepared by mixing all three components the dendrimer, the phosphine ligand and a suitable palladium precursor [(crotyl)PdCl]2. The catalytic activity and selectivity of 16 in a batch process... 

Table 2 Palladium-catalyzed allylic amination using catalysts that are noncovalently anchored in the core of a carbosilane dendrimer...

Allylic acetoxy groups can be substituted by amines in the presence of Pd(0) catalysts. At substituted cyclohexene derivatives the diastereoselectivity depends largely on the structure of the palladium catalyst. Polymer-bound palladium often leads to amination at the same face as the aoetoxy leaving group with regioselective attack at the sterically less hindered site of the intermediate ri -allyl complex (B.M. Trost, 1978). 

Reduction of iV-(3-bromopropyl) imines gives a bromo-amine in situ, which cyclizes to the aziridine. Five-membered ring amines (pyrrolidines) can be prepared from alkenyl amines via treatment with N-chlorosuccinimide (NCS) and then BusSnH. " Internal addition of amine to allylic acetates, catalyzed by Pd(PPh3)4, leads to cyclic products via a Sn2 reaction. Acyclic amines can be prepared by a closely related reaction using palladium catalysts. Three-membered cyclic amines (aziridines)... 

For the synthesis of heterocycles, an efficient strategy has been introduced utilizing the dual transition metal sequences (Scheme 6).11,lla The key issue is the compatibility of the two catalyst systems. Jeong et al. studied the one-pot preparation of bicyclopentenone 35 from propargylsulfonamide 33 and allylic acetate.11 This transformation includes two reactions the first palladium-catalyzed allylation of 33 generates an enyne 34 and the following Pauson-Khand type reaction (PKR) of 34 yields a bicyclopentenone 35. The success of this transformation reflects the right combination of catalysts which are compatible with each other because the allylic amination can be facilitated by the electron-rich palladium(O) catalyst and the PKR needs a Lewis-acidic catalyst. Trost et al. reported the one-pot enantioselective... 

After the initial demonstration of stoichiometric nucleophilic attack on 7i-allyl ligands, catalytic allylic substitution reactions were pursued. In 1970, groups from Union Carbide [3, 4], Shell Oil [5], and Toray Industries [6] published or patented examples of catalytic allylic substitution. All three groups reported allylic amination with palladium catalysts. The Toray Industries report also demonstrated the exchange of aryl ether and ester leaving groups, and the patent from Shell Oil includes catalysts based on rhodium and platinum. 

Table 3. Catalysis results for the Barbier reaction of benzaldehyde and allyl bromide in presence of homogeneous palladium catalyst (entry 1) and those prepared by complexing palladium to amine functionalized mesoporous silica (entry 2-4) [74]...

A regio- and stereoselective formation of homoallylic alcohols and amines has been realized by the reaction of easily available allylic acetates 213 and 217 with bispinacolatodiboron 149 in the presence of palladium catalysts followed by the addition of aldehydes/imines (Scheme 36) <2005EJO2539>. The mechanism involves the formation of an 7]3-allyl-Pd complex, which inserts between B-B bond, and subsequent reductive elimination to yield the allyl... 

Several catalytic systems have been investigated for hydroamination of unsaturated bonds [16]. Takahashi et al. reported the telomerization of 1,3-dienes in the presence of an amine leading to octadienylamine or allylic amines when palladium catalysts are used in association with monodentate or bidentate phosphine ligands, respectively [17]. Dieck et al. demonstrated the beneficial effect of addition of an amine hydroiodic salt in the hydroamination reaction of 1,3-dienes in which the allylic amines are produced via an intermediate Jt-allyl palladium complex [18]. Coulson reported the Pd-catalyzed addition of amines to allenes where dimerization is incorporated [4]. This reaction presumably proceeds via a cyclic palladium intermediate in which the Pd activates the olefinic bond for nucleophilic attack the reactions are therefore different from pronucleophilic additions. 

A remarkable increase in catalyst selectivity was observed as a function of the dendrimer generation. This steady increase in ee values for the allylic amination was less pronounced for the PPI-derived catalysts [40% ee for PPI(PyrphosPdCl2)64] than for the palladium-PAMAM dendrimer catalysts, for which an increase in selectivity from 9% ee for the mononuclear complex to 69% ee for PAMAM(PyrphosPdCl2)64 was found. The same general trend was observed in the asymmetric allylic alkylation of 1,3-diphenyl-l-... 

One problem with the metal-catalyzed Overman reaction is the basicity of the imidates. However, this problem has also been solved be Overman et al. by the introduction of the less basic allylic N-benzoylbenzimidates. The application of these allylic iV-benzoylbenzimidates and palladium(II) chloride as the catalyst improved both the yield, selectivity and rate for the formation of the allyl amines [9]. 

Vinyl epoxides can also be used as substrates for formation of optically active allyl amines catalyzed by the same type of chiral palladium complexes as in Eq. (10). By reaction of simple vinyl epoxides with phthalimide as the nitrogen source in the presence of the chiral palladium complexes as the catalyst, very high ee (> 98 %) and regioselectivity (> 97 %) were obtained [26]. A variety of different applications of the use of the palladium-catalyzed approach for the formation of allyl amines and the use of this in total synthesis has been pursued by several research groups, and further details can be obtained in a review by Trost et al. [19d]. 

With a combination of organic halide, alkene or alkyne, and palladium catalyst, the reaction may take a superficially similar, but different pathway other than a Heck-type reaction. In these, the halogen is retained in the products, as an alkyl or vinyl halide. These results occur when the starting organic halide is allyl, a- to a carbonyl or similar group, or attached to a perhalogenated carbon atom. In much of the early work, mixtures with Hecklike elimination products were obtained, due to elimination caused by the presence of amine bases in the reaction mixture (equation 165)323. 

FIGURE 24 Generation of supramolecular catalysts for allylic amination (89) (A) assembly of pseudoenantiomeric bisoxazoline derivatives. CAChe-minimized model of the palladium-containing supramolecular complex and (B) palladium-mediated enan-tioselective allylic amination. (See Color Insert in the back of this book.)... 

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