Hydroxy-directive hydrogenation

In these reactions, the major diastereomer is formed by the addition of hydrogen syn to the hydroxyl group in the substrate. The cationic iridium catalyst [Ir(PCy3)(py)(nbd)]+ is very effective in hydroxy-directive hydrogenation of cyclic alcohols to afford high diastereoselectivity, even in the case of bishomoallyl alcohols (Table 21.4, entries 10-13) [5, 34, 35]. An intermediary dihydride species is not observed in the case of rhodium complexes, but iridium dihydride species are observed and the interaction of the hydroxyl unit of an unsaturated alcohol with iridium is detected spectrometrically through the presence of diastereotopic hydrides using NMR spectroscopy [21]. 

In the case of cyclopentenyl carbamate in which a directive group is present at the homoallyl position, the cationic rhodium [Rh(diphos-4)]+ or iridium [Ir(PCy3)(py)(nbd)]+ catalyst cannot interact with the carbamate carbonyl, and thus approaches the double bond from the less-hindered side. This affords a cis-product preferentially, whereas with the chiral rhodium-duphos catalyst, directivity of the carbamate unit is observed (Table 21.7, entry 7). The presence of a hydroxyl group at the allyl position induced hydroxy-directive hydrogenation, and higher diastereoselectivity was obtained (entry 8) [44]. 

Table 21.8 Hydroxy-directed hydrogenation of acyclic allyl aclohols. 

Table 21.15 Hydroxy-directed hydrogenation of /Eketo esters. 

Hydroxy-directed hydrogenation of (phenyldimethylsilyl)allyl alcohols with a cationic rhodium complex provides a highly diastereoselective route to /3-silyl alcohols201,202. 

Ward and Mason157) explain these effects by direct hydrogen bonds between the hydroxy group of hydroxyproline in position 3 and the carboxylic side chain of glutamic acid in the adjacent chains. From the sequence we know that in the case of parallel and... 

This is a strong directing effect that can exert stereochemical control even when steric effects are opposed. Entries 4 and 5 in Scheme 12.11 illustrate the hydroxy-directing effect. Other substituents capable of hydrogen bonding, in particular amides, also can exert a. vyn-directing effect.79... 

Stereoselective hydrogenation of 0-hydroxy acrylatesThese esters can be prepared by condensation of methyl acrylate with aldehydes catalyzed by DABCO. On hydrogenation catalyzed by 1, these esters are converted with high selectivity into a/ift-a-methyl-0-hydroxy esters (equation I). Similar directed hydrogenation... 

A method for the preparation of 1-hydroxypyrrolizidine was published by Adams et al.7i Cyclization of 1 -(/ -cyanoethyl)pyrrole under the conditions of the Houben-Hoesch reaction gave rise to l-oxo-3/f-l,2-dihydropyrrolo(l,2-a)pyrrole (123) (cf. refe. 48 and 73), which can be converted into 1-hydroxypyrrolizidine by either direct hydrogenation over rhodium on alumina or hydrogenation of the corresponding hydroxy derivative 124. This route has some interest as a potential... 

The diastereoselectivity dropped drastically in presence of protic methanol and totally disappeared for the corresponding silyl ethers. These data are in agreement with the presence of a hydroxy directing effect in the Patemo-Bilchi reaction. Threo stereoisomer can be favored through the formation of an hydrogen bond between triplet excited benzophenone and the substrate in the exciplex, while the formation of the erythro stereoisomer would be less favored due to allylic strain (Scheme 3.41). 

The direct formation of disubstituted l,2,5-thiadiazole-l,l-dioxides (57) by condensation of a-diketones with sulfamide was reported by Wright and by Vorreither and Ziegler. The 1,2,5-thiadiazoline-1,1-dioxides (58) were prepared by a similar method employing a-hydroxy ketones. Hydrogenation of both 57 and 58 over Adams catalyst provided the corresponding disubstituted 1,2,5-thiadiazolidine-1,1 -dioxides (60). 2-Alkyl-l, 2,5-thiadiazoline-l, 1-... 

Moderate selectivity has been observed in the epoxidation of the homoallylic alcohol (64 equation 25). In the nonsteroidal conformation of (64), the axial hydroxy is suitably oriented for forming a hydrogen bond with the peroxy acid in the transition state. That the epoxidation of (64 R = OH) is indeed hydroxy-directed is supported by the observation that the epoxidation of the acetate (64 R = OAc) furnishes exclusively the a-epoxide (66 R = OAc). 

The X-ray-determined structure of the complex of 16 and 19 with quaternary salt 15 revealed that the primary discriminative forces leading to an efficient resolution are the formation of directional hydrogen bonds of hydroxy groups of cinchonidine and BINOL with the halide anion as well as aryl-aryl interaction between the naphthyl and the quinoline rings [40]. 

Hydroxy groups exert a directive effect on epoxidation and favor approach from the side of the double bond closest to the hydroxy group. Hydrogen bonding between the hydroxy group and the peroxidic reagent evidently stabilize the TS. 

Machado AS, Oleskta- A, Castillon S, Lukacs G (1985) Hydroxy group directed hydrogenation with rhodium and iridium catalysts. Synthesis of protected chiral carbocyclic analog of daunosamine. J Chem Soc Chem Commim 530-532... 

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