BINAP-based rhodium complexes

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... 

An important application of an isomerisation is found in the Takasago process for the commercial production of (-)menthol from myreene. The catalyst used is a rhodium complex of BINAP, an asymmetric ligand based on the atropisomerism of substituted dinaphthyl (Figure 5.5). It was introduced by Noyori [1],... 

Substituted tetraphenylenes are known as interesting biaryl-based chiral cyclic scaffolds. The cationic rhodium(l)/Cy-BINAP or QuinoxP complex-catalyzed enantioselective double homo-[2+2+2] cycloaddition of triynes afforded chiral tetraphenylenes with high enantioselec-tivity (Scheme 21.20) [24]. 

In the 1980s, the main focus in the field shifted from rhodium-based catalysts to the corresponding homogeneous ruthenium-based systems [88]. These were attractive because they overcome a number of undesirable limitations in the scope of the olefin substrates. After initial studies [89], Noyori reported a number of landmark advances based on the use of BINAP-Ru catalysts, providing a process that displays a wide substrate range [5, 90-93]. As an example, the reduction of 115 in the presence of (S)-BINAP-Ru acetate complex 116 gives 117 in > 99.5 % ee and quantitative yield (Equation 32) [90]. Noyori also found that acrylic acids served as excellent substrates, as showcased in a synthesis of the anti-inflammatoiy drug (S)-naproxen (119, Equation 33) and the reduction of 120 (Equation 34) [92]. 

BINAP was introduced by Noyori [18], It has been particularly explored for reduction with ruthenium catalysts. While the first generation rhodium catalysts exhibited excellent performance with dehydroamino acids (or esters), the second generation of hydrogenation catalysts, those based on ruthenium /BINAP complexes, are also highly enantioselective for other prochiral alkenes. An impressive list of rather complex organic molecules has been hydrogenated with high e.e. s. 

The first catalysts used were Co(I)-DIOP (DIOP, see Figure 11b) based systems. Up to 32% ee was achieved with 39% yield in the isomerization of (A, AO-diethylgeranylamine to (i )-citronellal. Subsequently, some rhodium (I)-DIOP or BINAP (BINAP = 2,2 -bis(diphenylphosphino)-1,1 -binaphthyl) systems proved to be very active. In particular, cationic rhodium(I)-BINAP complexes show very high selectivities and catalytic activities for this isomerization. BINAP is an atropoisomeric diphosphine (Figure 20) which was first synthesized by Noyori and Takaya and since then its metal complexes have been extensively used as catalysts in a variety of asymmetric syntheses. 

The proposed mechanism, based on detailed studies [S.-I. Inoue, H. Takaya, K. Tani, S. Otsuka, T. Sato and R. Noyori, J. Am. Chem. Soc., 1990,112, 4897] is schematically represented in Figure 22. Dissociation of one of the BINAP ligands gives the solvated species 1 which coordinates an allylamine molecule affording 11. Complex 11 undergoes (3-elimination to form the transient imi-nium-rhodium hydrido n complex 111. This complex isomerizes to the aza-allyl... 

The other major producer of synthetic L-menthol is the Japanese company Takasago. They produce about 1000 tonnes per annum using elegant chemistry developed by Noyori (Scheme 4.22). Pyrolysis of / -pinene gives myrcene, to which diethylamine can be added in the presence of a catalytic amount of strong base. This produces N,N-diethylgeranylamine. Isomerization of this with the rhodium 2,2 -(diphenylphosphino)-1,1-binaphthyl (BINAP) complex produces the enamine of citronellal. The elegance of this route stems from the fact... 

A small number of enantiomerically pure Lewis acid catalysts have been investigated in an effort to develop a catalytic asymmetric process. Initial work in this area was carried out by Narasaka and coworkers using the titanium complex derived from diol (8.216) in the cycloaddition of electron-deficient oxazolidinones such as (8.217) with ketene dithioacetal (8.218), alkenyl sulfides and alkynyl sulfides. Cyclic alkenes can be used in this reaction and up to 73% ee has been obtained in the [2- -2] cycloaddition ofthioacetylene (8.220) and derivatives with2-methoxycarbonyl-2-cyclopenten-l-one (8.221) usingthe copper catalyst generated with bis-pyridine (8.222). Furthermore, up to 99% ee has been obtained in the [2-1-2] cycloaddition of norbornene with alkynyl esters using rhodium/Hs-BINAP catalysts. This reaction is not restricted to the use of transition metal-based Lewis... 

Based on the initial Alder-ene cycloisomerization of alkyl and (hetero)aryl-substituted alkynyl allyl alcohols 13, the rhodium(I)-BINAP complex subsequently can be employed in the reduction of the primary products with hydrogen, furnishing 2,7-dioxabicyclo[3.2.1]octanes 14 in the sense of a sequentially Rh-catalyzed one-pot process (Scheme 12.7) [22]. 

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