*[35138-22-8]*
· C_{16}H_{24}BF_{4}Rh
· Bis(1,5-cyclooctadiene)rhodium Tetrafluoroborate-(*R*)-2,2´-Bis(diphenylphosphino)-1,1´-binaphthyl
· (MW 406.07)
*R*)-BINAP)

*[76189-55-4]*
· C_{44}H_{32}P_{2}
· Bis(1,5-cyclooctadiene)rhodium Tetrafluoroborate-(*R*)-2,2´-Bis(diphenylphosphino)-1,1´-binaphthyl
· (MW 622.68)

(catalyst for asymmetric hydrogenation,^{2} isomerization,^{3} hydroboration,^{4} and intramolecular hydrosilation^{5} of alkenes)

*Physical Data:* [Rh(cod)_{2}]BF_{4}: mp 206-8 °C; (*R*)-BINAP: mp 240-241 °C; [a]*c* = 0.32, benzene).

*Form Supplied in:* [Rh(cod)_{2}]BF_{4}: orange-red crystals; (*R*)-BINAP: colorless crystals.

*Analysis of Reagent Purity:* (*R*)-BINAP: ^{31}P NMR (4:1 C_{6}D_{6}-CD_{3}OD): d -12.8 (s); mp and optical rotation shown above are also useful for analysis of the purity.

*Purification:* [Rh(cod)_{2}]BF_{4}: recrystallization from CH_{2}Cl_{2} and ether; (*R*)-BINAP: recrystallization from a mixture of toluene and EtOH.

*Handling, Storage, and Precautions:* [Rh(cod)_{2}]BF_{4}: hygroscopic; corrosive.

The diene-free cationic rhodium complex of (*R*)-BINAP catalyzes the enantioselective hydrogenation of dehydroamino acids. *S*)-*N*-benzoylphenylalanine in 100% ee (eq 1).^{2} 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).

Optically active homoallylic alcohols are hydrogenated with differentiation of the diastereofaces (eq 2).^{6} Use of the matched ligand, i.e. (*R*)-BINAP, gives a product of 96% de, while the mismatched (*S*)-ligand affords low selectivity.

Cationic Rh^{I} diphosphine complexes are very active catalysts for *E*)-enamines and *R*)-BINAP as a diphosphine ligand.^{3} Diethylnerylamine, which has (*Z*) geometry, gives (*R*)-(*E*)-diethylcitronellenamine in 95% ee in the presence of 1 mol % of [Rh{(*R*)-BINAP}(cod)]ClO_{4}, while the isomeric diethylgeranylamine gives (*S*)-(*E*)-diethylcitronellenamine in 96% ee (eq 3). Thus the method presented above offers a desired enantiomer by proper choice of alkene geometry and chirality of BINAP.

Cationic Rh-(*R*)-BINAP complexes also catalyze the *R*)-4-hydroxy-2-cyclopentenone of 91% ee at 72% conversion at 0 °C (eq 4).^{7}

Rhodium complexes are known to catalyze hydroboration of alkenes with unreactive borane derivatives, e.g. ^{8} This reaction proceeds enantioselectively by use of BINAP as a ligand for neutral^{9-11} or cationic^{4,12} rhodium complexes. Reaction of styrene with catecholborane followed by oxidation affords (*R*)-1-phenylethanol in 96% ee in the presence of (*R*)-BINAP and [Rh(cod)_{2}]BF_{4} (eq 5).^{4}

Intramolecular hydrosilation of allylic alcohols followed by oxidation is a convenient method for the stereoselective preparation of ^{13} An enantioselective version is achieved by use of diene-free BINAP-Rh^{+} (eq 6).^{5} Both silyl ethers derived from cinnamyl alcohol and its *cis* isomer give (*R*)-1-phenylpropane-1,3-diol in high ee regardless of alkene geometry.

** Bis(bicyclo[2.2.1]hepta-2,5-diene)rhodium Perchlorate**;

- 1. (a) Takaya, H.; Noyori, R.
*COS***1991**,*8*, Chapter 3.2. (b) Smith, K.; Pelter, A.*COS***1991**,*8*, Chapter 3.10. (c) Hiyama, T.; Kusumoto, T.*COS***1991**,*8*, Chapter 3.12. (d) Noyori, R.; Kitamura, M. In*Modern Synthetic Methods*, Sheffold, R., Ed.; Springer: Berlin, 1989; Vol. 5, p. 115. (e)*Comprehensive Organometallic Chemistry*; Wilkinson, G., Ed.; Pergamon: Oxford, 1982; Vol. 8. - 2. (a) Miyashita, A.; Yasuda, A.; Takaya, H.; Toriumi, K.; Ito, T.; Souchi, T.; Noyori, R.
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Yoshihiko Ito & Michinori Suginome

*Kyoto University, Japan*

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