Chemoselective reactions hydrogenation

Hydroformylation of a range of 1,1-di- and 1,1,2-trisubstituted unsatur-ated esters yields quaternary aldehydes (Table 1, entries 1-8). Hence, the regiochemistry-directing influence of the electron-withdrawing ester function overcompensates Keuleman s rifle. Furthermore, hydroformylation of 1,2-disubstituted unsaturated esters occurred with high a-selectivity and chemoselectivity (Table 1, entries 9 and 10). As a side reaction hydrogenation of the alkene has been observed [41]. 

Highly Chemoselective Catalytic Hydrogenation of Polar Unsaturation Using Cu(I) Complexes and Hydrogen, in Catalysis of Organic Reactions, Scaros, M. and Prunier, M. L. (eds.), Chem. Ind. 62) Marcel Dekker, New York, 1995, 235-248. 

These compounds are unique alkylating or arylating reagents and their reactivity has been reviewed [25,26], Some chemoselective reactions are presented. The addition reaction of aldehydes to 10 proceeds faster than that of ketones [23,25,27,28], Compound 8 reacts chemoselectively with an acetal, and not with an ester group [29], The structure of TiMeCl3(dmpe) (12) has been determined both by X-ray and neutron diffraction analysis. The latter confirms that there is a unique agostic interaction of the a-hydrogen atom with the titanium atom [30,31], This weak interaction stabi-... 

Baeyer-Villiger oxidation (p. 853) catalytic hydrogenation (p. 844) chemoselective reaction (p. 848) dissolving-metal reduction (p. 846) enantioselective reaction (p. 857) epoxidation (p. 855) functional group interconversion (p. glycol (p. 858)... 

Ru/tppms catalysts exhibited excellent yields (98%) in the transfer hydrogenation of unsaturated aldehydes such as cinnamaldehyde or crotonaldehyde, to the corresponding unsaturated alcohols under mild reaction conditions (30-80°C), with HCOONa in an aqueous/organic two phase system. Similarly, ruthenium modified with the water soluble, air stable phosphine 100 (pta Table 5) is an effective catalyst for the chemoselective transfer hydrogenation of a,P-unsaturated aldehydes to unsaturated alcohols using formate in an aqueous/ organic two phase system. In contrast, Rh/pta afforded the cor-... 

This chemoselectivity stands in contrast to that of 2,6-disubstituted pyridines. For example, 2,6-dimethylpyridine 35 was reacted with hydrogen peroxide and acetic anhydride to produce the expected acetoxy derivative 36. A second iteration of the previous reaction conditions did not afford an aldehyde, as in the previous example, but 2,6-bis-acetoxy derivative 37. 

A synthetically useful virtue of enol triflates is that they are amenable to palladium-catalyzed carbon-carbon bond-forming reactions under mild conditions. When a solution of enol triflate 21 and tetrakis(triphenylphosphine)palladium(o) in benzene is treated with a mixture of terminal alkyne 17, n-propylamine, and cuprous iodide,17 intermediate 22 is formed in 76-84% yield. Although a partial hydrogenation of the alkyne in 22 could conceivably secure the formation of the cis C1-C2 olefin, a chemoselective hydrobora-tion/protonation sequence was found to be a much more reliable and suitable alternative. Thus, sequential hydroboration of the alkyne 22 with dicyclohexylborane, protonolysis, oxidative workup, and hydrolysis of the oxabicyclo[2.2.2]octyl ester protecting group gives dienic carboxylic acid 15 in a yield of 86% from 22. 

Until recently, iron-catalyzed hydrogenation reactions of alkenes and alkynes required high pressure of hydrogen (250-300 atm) and high temperature (around 200°C) [21-23], which were unacceptable for industrial processes [24, 25]. In addition, these reactions showed low or no chemoselectivity presumably due to the harsh reaction conditions. Therefore, modifications of the iron catalysts were desired. 

Scheme 24) [38]. Chemoselective enolization of the less substituted enone moiety under hydrogenation conditions accompanied by subsequent aldol reaction provided the corresponding hydroxyl-enones, such as 87-89, which could be converted to various building blocks for polypropionate synthesis. p-Me2N styryl vinyl enone also was employed successfully as an enolate precursor, as demonstrated by the formation of hydroxy enone 90. 

The solvent is very important for the hydrogenation of ketones. One of the most important factors in the liquid-phase hydrogenation of ketones is whether the medium is acidic, neutral, or basic, and a great deal of work has gone into attempting to understand chemoselectivity and stereoselectivity based on combinations of the metal catalyst and the reaction medium. 

The synthesis of 2,3,5-trialkylpyrroles can be easily achieved by conjugate addition of nitroalkanes to 2-alken-l,4-dione (prepared by oxidative cleavage of 2,5-dialkylfuran) with DBU in acetonitrile, followed by chemoselective hydrogenation (10% Pd/C as catalyst) of the C-C- double bond of the enones obtained by elimination of HN02 from the Michael adduct. The Paal-Knorr reaction (Chapter 10) gives 2,3,5-trialkylpyrroles (Eq. 4.124).171... 

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