Rhodium-amine complexes

Rhodium complexes with oxygen ligands, not nearly as numerous as those with amine and phosphine complexes, do, however, exist. A variety of compounds are known, iucluding [Rh(ox)3] [18307-26-1], [Rh(acac)3] [14284-92-5], the hexaaqua ion [Rh(OH2)3] [16920-31 -3], and Schiff base complexes. Soluble rhodium sulfate, Rh2(804 )3-a H2 0, exists iu a yellow form [15274-75-6], which probably coutaius [Rh(H20)3], and a red form [15274-78-9], which contains coordinated sulfate (125). The stmcture of the soluble nitrate [Rh(N03)3 2H20 [10139-58-9] is also complex (126). Another... 

When the substrate is an allylic alcohol or amine, the addition is generally anti," though the stereoselectivity can be changed to syn by the use of catecholborane and the rhodium complexes mentioned above. Because the mechanism is different, use of this procedure can result in a change in regioselectivity as well, [e.g., styrene PhCH=CH2 gave PhCH(OH)CH3]. ... 

The anionic rhodium complexes [(PR3)2Rh(NHPh)2] Li (see above) have been shown to catalyze (TON = 21) the condensation of aniline with styrene to give the expected amine and the corresponding imine (oxidative amination) (Eq. 4.37, TOP 0-07 h- ) [161]. 

Prochiral imines can be hydrogenated to the corresponding amines with extremely high enan-tioselectivities in H20/ethyl ethanoate biphasic systems, using Rh1 complexes of sulfonated phosphines 342 The cationic rhodium complex [Rh(NBD)(131)]+ was an active catalyst for hydrogenation of 2-ethanamido-propenoic acid in aqueous solution.343... 

Some of the hydroarylation product is also observed substituted anilines afford the two products to varying degrees (Equation (15)). The closely related rhodium complexes [Rh(PCy3)2Cl]2, [Rh(dmpe)Cl]2 (where dmpe= l,2-bis(dimethylphosphino)ethane), and [Rh(C8H14)Cl]2 show essentially no catalytic activity.166 Application of [Rh(PEt3)2Cl]2 to the reaction of aniline with styrene gives a mixture of hydroamination and oxidative amination products, the latter predominating.167 Other related rhodium-catalyzed amination reactions (oxidative amination) have been reported.168 169... 

The first example of anti-Markovnikoff hydroamination of aromatic alkenes has been demonstrated with cationic rhodium complexes.170 A combination of [Rh(COD)2]+/2PPh3 in THF under reflux yields the N-H addition product as the minor species alongside that resulting from oxidative amination (Scheme 37). Hydrogenation products are also detected. 

Takasago A catalytic process for the enantioselective isomerization of allylic amines. The catalyst is a chiral rhodium complex. Used in the manufacture of (-)menthol. Named after Takasago International Corporation, the Japanese company which commercialized the process in 1983. 

As shown in the previous two sections, rhodium(n) dimers are superior catalysts for metal carbene C-H insertion reactions. For nitrene C-H insertion reactions, many catalysts found to be effective for carbene transfer are also effective for these reactions. Particularly, Rh2(OAc)4 has demonstrated great effectiveness in the inter- and intramolecular nitrene C-H insertions. The exploration of enantioselective C-H amination using chiral rhodium catalysts has been reported by several groups.225,244,253-255 Hashimoto s dirhodium tetrakis[A-tetrachlorophthaloyl-(A)-/ r/-leuci-nate], Rh2(derived rhodium complex, Rh2(i -BNP)4 48,244 afforded moderate enantiomeric excess for amidation of benzylic C-H bonds with NsN=IPh. 

A novel route was examined for the synthesis of the u,y9-unsaturated amide 59 vio the intermolecular couphng of four components an alkyne, a hydrosilane, an amine, and CO (Eq. 11). All of these components are assembled in an ordered manner with the assistance of a rhodium complex. The use of pyrrolidine as the nucleophile gave the optimum results, whereas alcohols were not reactive nucleophiles under similar reaction conditions [19 ej. 

Alternatively, the rhodium dimer 30 may be cleaved by an amine nucleophile to give 34. Since amine-rhodium complexes are known to be stable, this interaction may sequester the catalyst from the productive catalytic cycle. Amine-rhodium complexes are also known to undergo a-oxidation to give hydridorhodium imine complexes 35, which may also be a source of catalyst poisoning. However, in the presence of protic and halide additives, the amine-rhodium complex 34 could react to give the dihalorhodate complex 36. This could occur by associative nucleophilic displacement of the amine by a halide anion. Dihalorhodate 36 could then reform the dimeric complex 30 by reaction with another rhodium monomer, or go on to react directly with another substrate molecule with loss of one of the halide ligands. It is important to note that the dihalorhodate 36 may become a new resting state for the catalyst under these conditions, in addition to or in place of the dimeric complex. 

Muller has explored enantioselective C-H insertion using optically active rhodium complexes, NsN=IPh as the oxidant, and indane 7 as a test substrate (Scheme 17.8) [35]. Chiral rhodium catalysts have been described by several groups and enjoy extensive application for asymmetric reactions with diazoalkanes ]46—48]. In C-H amination experiments, Pirrung s binaphthyl phosphate-derived rhodium system was found to afford the highest enantiomeric excess (31%) of the product sulfonamide 8 (20equiv indane 7, 71% yield). 

Oxidative amination of carbamates, sulfamates, and sulfonamides has broad utility for the preparation of value-added heterocyclic structures. Both dimeric rhodium complexes and ruthenium porphyrins are effective catalysts for saturated C-H bond functionalization, affording products in high yields and with excellent chemo-, regio-, and diastereocontrol. Initial efforts to develop these methods into practical asymmetric processes give promise that such achievements will someday be realized. Alkene aziridina-tion using sulfamates and sulfonamides has witnessed dramatic improvement with the advent of protocols that obviate use of capricious iminoiodinanes. Complexes of rhodium, ruthenium, and copper all enjoy application in this context and will continue to evolve as both achiral and chiral catalysts for aziridine synthesis. The invention of new methods for the selective and efficient intermolecular amination of saturated C-H bonds still stands, however, as one of the great challenges. 

Kollner et al. (29) prepared a Josiphos derivative containing an amine functionality that was reacted with benzene-1,3,5-tricarboxylic acid trichloride (11) and adamantane-l,3,5,7-tetracarboxylic acid tetrachloride (12). The second generation of these two types of dendrimers (13 and 14) were synthesized convergently through esterification of benzene-1,3,5-tricarboxylic acid trichloride and adamantane-1,3,5,7-tetracarboxylic acid with a phenol bearing the Josiphos derivative in the 1,3 positions. The rhodium complexes of the dendrimers were used as chiral dendritic catalysts in the asymmetric hydrogenation of dimethyl itaconate in methanol (1 mol% catalyst, 1 bar H2 partial pressure). The enantioselectivities were only... 

Counterions for the anionic rhodium complexes present in catalyst solutions may also be provided by the addition of salts. A salt may be used as the sole promoter, but it appears that under many conditions a combination of salt and amine provides the best results. Table XI indicates that... 

Both the rhodium and the cobalt complexes catalyze olefin isomerization as well as olefin hydroformylation. In the case of the rhodium(I) catalysts, the amount of isomerization decreases as the ligands are altered in the order CO > NR3 > S > PR3. When homogeneous and supported amine-rhodium complexes were compared, it was found that they both gave similar amounts of isomerization, whereas with the tertiary phosphine complexes the supported catalysts gave rather less olefin isomerization than their homogeneous counterparts (44, 45). 

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