Brown’s catalyst

Although the asymmetric hydrogenation of itaconic acid derivatives is a potential synthetic approach to many useful product [105], lower enantioselectivities are often reported. In contrast with other catalysts, f-Bu-BisP, Ad-BisP, t-Bu-MiniPHOS, BIPNOR 27, and Brown s ligand 25 gave high to almost perfect ees in the hydrogenation of these substrates (Scheme 23) [101]. 

Brown-black, insoluble, anhydrous, platinum(IV) oxide is made by fusing hexachloroplatinic(IV) add with NaN03 at about 500VC. The alkali salts are washed out with H20 to free the fine insoluble residue of platinum(IV) oxide. This compound is known as Adam s catalyst. 

Nickel affords selective catalysts for the hydrogenation of alkenes, dienes, and alkynes. When catalyzed by C. A. Brown s P-2 nickel, prepared by the reduction of Ni(0Ac)2 with NaBH in ethanol, the individual rates as well as the competitive rates appear to be sensitive to the alkene structure as judged by the reported initial rates of hydrogen addition (ref. 23). Alkene isomerization is relatively slow. Except for the most reactive alkenes such as norbornene, the individual hydrogenations seem to be first order in alkene. This indicates that alkenes are more weakly bound to Ni than to Pt or Pd. Similar selectivities are reported by Brunet, Gallois, and Caubere for a catalyst prepared by the reduction of Ni(0Ac)2 with NaH and t-amyl alcohol in THF (ref. 27). 

An explanation for this difference in selectivity of the Ni catalysts is suggested by the studies of Okamoto et al. who correlated the difference in the X-ray photoelectron spectra of various nickel catalysts with their activity and selectivity in hydrogenations (ref. 28,29). They find that in individual as well as competitive hydrogenations of cyclohexene and cyclooctene on Ni-B, cyclooctene is the more reactive while the reverse situation occurs on nickel prepared by the decomposition of nickel formate (D-Ni). On all the nickel catalysts the kinetically derived relative association constant favors cyclooctene (ref. 29). The boron of Brown s P-2 nickel donates electrons to the nickel metal relative to the metal in D-Ni. The association of the alkene with the metal is diminished which indicates that, in these hydrocarbons, the electron donation from the HOMO of the alkene to an empty orbital of the metal is more important than the reverse transfer of electron density from an occupied d-orbital of the metal into the alkene s pi orbital. 

Lewis acid-promoted asymmetric addition of dialkylzincs to aldehydes is also an acceptable procedure for the preparation of chiral secondary alcohol. Various chiral titanium complexes are highly enantioselective catalysts [4]. C2-Symmet-ric disulfonamide, chiral diol (TADDOL) derived from tartaric acid, and chiral thiophosphoramidate are efficient chiral ligands. C2-Symmetric chiral diol 10, readily prepared from 1-indene by Brown s asymmetric hydroboration, is also a good chiral source (Scheme 2) [17], Even a simple a-hydroxycarboxylic acid 11 can achieve a good enantioselectivity [18]. 

The use of oxazaborohdines as asymmetric reduction catalysts and the enantioselectivity of diphenyloxazaborohdine reduction of ketones have been reviewed. Large-scale practical enantioselective reduction of prochiral ketones has been reviewed with particular emphasis on the Itsimo-Corey oxazaborolidine and Brown s 5-chlorodiisopinocampheylborane (Tpc2BCl) as reagents. Brown himself has also reviewed the use of Ipc2BCl. Indohnoalkylboranes in the form of dimers have been confirmed by B NMR as the products of the reduction of trifluoroacetylindoles by diborane. ... 

Homogeneous hydrogenation with Wilkinson s catalyst is selective for the less-hindered alkene. Therefore in this case the product is as shown below. See M. Brown and L. W. Piszkiewicz, J. Org. Chem., 32 (1967), 2013. 

Fig. 2 PES of the Brown mechanism for hydrogenation with Wilkinson s catalyst...

Three of many examples of directed hydrogenation are shown in Equations 15.15-15.17. Equation 15.15 shows the reduction of a homoaUylic alcohol, which was one of the substrates first used to demonstrate this effect. Equation 15.16 shows a more complex substrate in which the diastereoselective reduction by Crabtree s catalyst is directed by the amide function as part of tlie synthesis of pulmitoxins. Equation 15.17 shows that the addition of hydrogen can be directed to a hindered face of a bicyclic system. In this case, the cationic rhodium system qf Brown, as well as Crabtree s catalyst, led to hi selectivity. Many other reactions occur with high selectivity in the presence of Brown s cationic rhodium system. Diastereoselective additions to acyclic systems, along with a rationalization for the selectivity in these types of substrates, can be found in the review by Evans. ... 

To a 250 mL 3-neck round bottom flask fitted with an overhead stirrer, 5 wt% Karstedt s catalyst in xylenes (0.28 mL, 100 ppm H2O) and triethoxysilane, which had been distilled under nitrogen (6.12 mL, 33.14 mmol), was added. The solution turned brown upon which 5 ml of the bis-allyl copolymer was added directly with stirring. The remaining bis-allyl copolymer was added drop wise at 70-75°C over 60 minutes. 

A different approach is to add substituents to or manipulate substituents on an existing allyl- or propar-gyl-silane system. Thus propargyltrimethylsilane itself can be alkylated at the terminus and the triple bond reduced with Brown s nickel catalyst or by hydroalumination-protonation, both of which give the (Z)-allylsilane. Alternatively, zirconium-catalyzed carboalumination of propargylsilanes introduces more substituents onto the allyl framework.Allylsilane anions show some selectivity for alkylation a to the silyl group, and the geometry of the double bcmd can be controlled by using either the kinetic or the thermodynamic allyllithium intermediate (Scheme 78). ... 

Iodine monochloride [7790-99-0] ICl, mol wt 162.38, 78.16% I, is a black crystalline soHd or a reddish brown Hquid. SoHd ICl exists ia two crystalline modifications the a-form, as stable mby-red needles, d = 3.86 g/mL and mp 27.3°C and as metastable brownish red platelets, d = 3.66 g/mL, mp 13.9°C and bp 100°C (dec). Iodine monochloride is used as a halogenation catalyst and as an analytical reagent (Wij s solution) to determine iodine values of fats and oils (see Fats and fatty oils). ICl is prepared by direct reaction of iodine and Hquid chlorine. Aqueous solutions ate obtained by treating a suspension of iodine ia moderately strong hydrochloric acid with chlorine gas or iodic acid (118,119). 

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