Rhodium-Catalyzed Allylic Aminations

The transition metal-catalyzed allylic amination is particularly pertinent, because of the synthetic utihty of aUyhc amines in the construction of biologically important natural and urmatural products [34]. The allylic amination of cyclic and acycHc substrates. 

70 Rhodium(l)-CatalYzed Allylic Substitution Reactions and their Applications 

Evans and co-workers demonstrated that rhodium-catalyzed allylic amination of enantiomerically enriched acyclic unsymmetrical allylic carbonates occurs with excellent regio- and enantiospedfidty (Tab. 10.5) [35]. Interestingly, while the classical nitrogen nucleophiles furnished allylic amination products in poor yield and with modest regioselectivity, the lithium anion of N-toluenesulfonyl-N-alkylamines proved optimal, in terms of nucleophilicity and basicity. 

The stereoselective construction of nitrogen heterocycles remains a topic of intense synthetic interest [39]. Evans and Robinson described the combination of the stereospecific aUylic amination with ring-closing metathesis as a strategy for the constmction of mono- and disubstituted azacycles, which they demonstrated with the stereospecific construction of cis- and tra s-2,5-disubstituted pyrrolines [40]. Furthermore, this approach provided an ideal system for the determination of whether the enantiospecific rhodium-catalyzed aUyhc amination with an enantiomerically enriched nucleophile experiences a matched and a mismatched reaction manifold. 

The relative importance of N-substituted arylamines for the construction of biologically significant molecules, particularly pharmaceuticals and agrochemicals, prompted the extension of rhodium-catalyzed allylic amination to aniline nucleophiles (Tab. 10.6) [41, 42]. The N-arylsulfonyl-protected anihnes were again optimal for high selectivity, analogous to that observed with the N-toluenesulfonyl-N-aLkylamines. 

---

Scheme 10.8 outlines the application of rhodium-catalyzed allyhc amination to the preparation of (il)-homophenylalanine (J )-38, a component of numerous biologically active agents [36]. The enantiospecific rhodium-catalyzed allylic amination of (l )-35 with the lithium anion of N-benzyl-2-nitrobenzenesulfonamide furmshed aUylamine (R)-36 in 87% yield (2° 1° = 55 1 >99% cee) [37]. The N-2-nitrobenzenesulfonamide was employed to facilitate its removal under mild reaction conditions. Hence, oxidative cleavage of the alkene (R)-36 followed by deprotection furnished the amino ester R)-37 [37, 38]. Hydrogenation of the hydrochloride salt of (l )-37 followed by acid-catalyzed hydrolysis of the ester afforded (i )-homophenylalanine (R)-3S in 97% overall yield. 

Tab. 10.5 Enantiospecific rhodium-catalyzed allylic amination of enantiomerically enriched allylic carbonates.

Tab. 10.6 The TolN(Li)Mbs. scope of regioselective rhodium-catalyzed allylic amination of carbonates with...

In the Rh-BINAP-catalyzed allyl amine isomerization step used in Takasago s Menthol process, the catalyst is inhibited by water through the formation of a hydroxyl-bridged rhodium trinuclear complex [ Rh(BINAP) i(/<2-0H)2]C104 [61]. 

The combination of allylic amination, ring-closing metathesis, and a free radical cyclization provides a convenient approach to the dihydrobenzo[b]indoline skeleton, as illustrated in Scheme 10.10. The rhodium-catalyzed aUylic amination of 43 with the lithium anion of 2-iodo-(N-4-methoxybenzenesulfonyl)arrihne furnished the corresponding N-(arylsulfonyl)aniline 44. The diene 44 was then subjected to ring-closing metathesis and subsequently treated with tris(trimethylsilyl)silane and triethylborane to afford the dihydrobenzojhjindole derivative 46a in 85% yield [14, 43]. 

Rhodium-Catalyzed Nucleophilic Ring Cleaving Reactions of Allylic Ethers and Amines... 

Intermolecular amination experiments described by Muller using 02NC,5H4S02N=IPh (NsN=IPh) as the nitrene source underscore the value of certain rhodium(II) catalysts for C-H insertion (Scheme 17.5) [12, 34—36]. In accord with Breslow s finding, dirhodium carboxylates were demonstrated to catalyze the amination of allylic, benzylic, and adamantyl substrates. Notably, structurally related tetracarboxamide dimers fail to give... 

RHODIUM-CATALYZED ENANTIOSELECTIVE ISOMERIZATION OF ALLYLIC AMINES... 

Allylic amide isomerization, 117 Allylic amine isomerization ab initio calculations, 110 catalytic cycle, 104 cobalt-catalyzed, 98 double-bond migration, 104 isotope-labeling experiments, 103 kinetics, 103 mechanism, 103 model system, 110 NMR study, 104 rhodium-catalyzed, 9, 98 Allylnickel halides, 170 Allylpalladium intermediates, 193 Allylsilane protodesilylation, 305 Aluminum, chiral catalysts, 216, 234, 310 Amide dimers, NMR spectra, 282, 284 Amines ... 

RHODIUM-CATALYZED enantioselective isomerization of ALLYLIC AMINES 103... 

Some other rhodium-catalyzed reactions of CO are collected here. The cyclization of allyl amine to y-butyrolactone (equation 97) is catalyzed by the complexes RhCl3, [Rh(acac)3], [Rh2Cl2(C2H4)4], [RhCl(PPh3)3] (23) and [RhCl(CO)(PPh3)2] (71). If allyl chloride or iodide is used together with ammonia or a primary amine, analogous cyclizations occur.461... 

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... 

In 1965, Tsuji et al. observed that palladium could catalyze the allylic alkylation reaction [18]. This discovery, which is a very attractive way to expand the scope of the allylic amination reactions mentioned above, has stimulated an intense research in this field, and even though complexes of nickel, platinum, rhodium, iron, ruthenium, molybdenum, cobalt, and tungsten have been found also to catalyze the alkylation, palladium complexes have received by far the greatest attention [19]. 

---