Dienes cationic rhodium complexes

The synthesis of cationic rhodium complexes constitutes another important contribution of the late 1960s. The preparation of cationic complexes of formula [Rh(diene)(PR3)2]+ was reported by several laboratories in the period 1968-1970 [17, 18]. Osborn and coworkers made the important discovery that these complexes, when treated with molecular hydrogen, yield [RhH2(PR3)2(S)2]+ (S = sol-vent). These rhodium(III) complexes function as homogeneous hydrogenation catalysts under mild conditions for the reduction of alkenes, dienes, alkynes, and ketones [17, 19]. Related complexes with chiral diphosphines have been very important in modern enantioselective catalytic hydrogenations (see Section 1.1.6). 

Scheme 14.18 Proposed 1,4-hydrogenation mechanism for the hydrogenation of dienes by cationic rhodium complexes [P2Rh(diene)]+A (54).

A cationic rhodium complex-catalyzed codimerization of 1,3-dienes with alkynes gives the corresponding cyclohexadienes in good yields with high regioselectively, as exemplified in the reaction of 2-methyl-l,3-butadiene with phenylacetylene (Eq. 12) [31]. 

Aryltrimethylstaimanes arylate aldehydes in the presence of a cationic rhodium complex, [Rh(cycloocta-l,5-diene)(MeCN)2]+, yielding secondary alcohols.114... 

Probably the most important developments in this field over the past 10 years, however, have been in the area of enantioselective see Enantioselectivity) hydroborations using cationic rhodium complexes of the type [Rh(diene)L ]+ (L = chiral ligand). An excellent review on this topic has recently been published. New chiral see Chiral) catalyst systems are typically tested in hydroborations of vinyl arenes. Although catalyzed hydroboration of vinyl arenes can be used as a mild and efficient route to preparing 1-arylethanol... 

Asymmetric Hydrogenation. The diene-free cationic rhodium complex of (R)-BINAP catalyzes the enantioselective hydrogenation of dehydroamino acids. a-(Benzoylamino)acrylic acid is hydrogenated at rt to afford (S)-lV-benzoylphenylalanine in 100% ee (eq 1). To obtain maximal stereoselectivity the reaction should be carried out under a low concentration of substrate (100% in 0.013 M vs. 62% in 0.15 M) and low initial hydrogen pressure (100% at 1 atm, but 71% at 50 atm). 

The diene (2.32) undergoes selective hydrogenation of the enamide double bond to give an amino acid derivative (2.33) containing an alkene functionality. The most enantioselective catalyst for this reaction was found to be the cationic rhodium complex of DUPHOS (2.05), which afforded less than 1% of the fully hydrogenated product as a by-product. 

Asymmetric cycloisomerization of nitrogen-bridged 1,6-enynes occurs in the presence of a cationic rhodium complex coordinated with a chiral diene/phosphine tridentate ligand to give chiral 3-azabicyclo[4.1.0]heptenes with high enantioselectivity (Scheme 149). " ... 

In general, the most efficient catalysts for alkene and ketone hydrogenations have been those derived from rhodium complexes bearing chiral C2 diphosphane ligands. In particular, cationic rhodium complexes of the type [Rh(diene)(ligand)]+X (where X is noncoordinating anion as BF4 , PFe , and OTf ) have proven to be excellent catalyst for enamide hydrogenations. 

A hydrosilylation/cyclization process forming a vinylsilane product need not begin with a diyne, and other unsaturation has been examined in a similar reaction. Alkynyl olefins and dienes have been employed,97 and since unlike diynes, enyne substrates generally produce a chiral center, these substrates have recently proved amenable to asymmetric synthesis (Scheme 27). The BINAP-based catalyst employed in the diyne work did not function in enyne systems, but the close relative 6,6 -dimethylbiphenyl-2,2 -diyl-bis(diphenylphosphine) (BIPHEMP) afforded modest yields of enantio-enriched methylene cyclopentane products.104 Other reported catalysts for silylative cyclization include cationic palladium complexes.105 10511 A report has also appeared employing cobalt-rhodium nanoparticles for a similar reaction to produce racemic product.46... 

The published quantification of the rate of hydrogenation of the dienes COD and NBD of a large number of cationic rhodium(I) chelate complexes allows a good estimation of expected effects on the rate of enantioselective hydrogenation of prochiral alkenes. From the first-order pseudo-rate constants the time needed for complete hydrogenation of the diene introduced as part of the rhodium precursor can be easily calculated as six- to seven-fold the half life. It is recommended that the transfer into the solvent complex be followed by NMR spectroscopy. 

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