Hydroboration of allyl alcohols

In contrast to the dihydroxylations, the hydroboration of allylic alcohols takes place without an inside alkoxy effect in the sense 5.132 with the oxygen atom treated as the medium-sized group. It is probably significant that the inside alkoxy effect is most noticeable with reagents which are relatively electrophilic in nature, and not with boranes, which are only mildly electrophilic. 

Stereoselective hydroboration of allylic alcohols. Acyclic secondary ally lie alcohols of the types 1 and (E)- and (Z)-2 undergo hydroboration stereoselectively to yield threo-, 3-dioh (3). Highest t/ireo-selectivity in hydroboration of 1 is obtained using 9-... 

Diastereoselective rhodium-catalyzed hydroborations of allylic alcohol derivatives give results complementary to those observed in the uncatalyzed reaction with 9-Bombicyclo[3.3.1]nonane. The syn selectivity of the catalyzed reaction increases as the bulk of the R group increases (syn anri = 79 21 for R = TBDPS) (eq 9) ... 

Diastereofacial selectivity in the uncatalyzed, intermolecular hydroboration of allylic alcohols and their derivatives is interpreted in terms of Houk s transition state model 53,65 (see Section D.2.3.5.2.). The smallest substituent at the pre-existing stereogenic center is oriented inside over the face of the transition state ring while the oxygen atom is oriented outside". Consequently, anti selectivity with respect to the pre-existing stereogenic center is observed. In contrast, the intramolecular hydroboration of allylic alcohol derivatives involves a transition state with the oxygen atom fixed inside and leads to syn selectivity. 

Scheme 1). Preparation of 1,3-diols by hydroboration of allylic alcohols with thexylborane has been shown to proceed with high stereoselectivity when the substrates are 1-(1-hydroxyalkyl)-cyclohexenes. Stereocontrol in the hydrosilation of allylic and homoaHylic alcohols has been studied jointly by the groups of Tamao... 

Still documented a classic study involving the diastereoselective hydroboration of allylic alcohols (Equation 1) [21]. For example, when allylic alcohol 20 was allowed to react with borane, a 1 1.4 mixture of diols 21 and 22 was isolated. However, the hydroboration of 20 with the bulkier 9-BBN resulted in a pronounced preference for 21 (dr=ll 1). 

Hydrogenation of olefins, enols, or enamines with chiral tVilkinson type catalysts, e.g., Noyort hydrogenation. Hydroboration of olefins with chiral boranes. Sharpless epoxi-dation of allylic alcohols. 

Wilkinson s catalyst has also been utilized for the hydroboration of other alkenes. Sulfone derivatives of allyl alcohol can be hydroborated with HBcat and subsequently oxidized to give the secondary rather than primary alcohol. This reactivity proves to be independent of substituents on the sulfur atom.36 Similarly, thioalkenes undergo anti-Markovnikoff addition to afford a-thioboronate esters.37 The benefits of metal-catalyzed reactions come to the fore in the hydroboration of bromoalkenes (higher yields, shorter reaction times), although the benefits were less clear for the corresponding chloroalkenes (Table 3).38,39 Dienes can be hydroborated using both rhodium and palladium catalysts [Pd(PPh3)4] reacts readily with 1,3-dienes, but cyclic dienes are more active towards [Rh4(CO)i2].40... 

The hydroboration of allylic amine or alcohol derivatives can be used for the preparation of alkylzinc reagents with excellent diastereoselectivity (Equation (34)). Rhodium-catalyzed hydroborations are also compatible... 

Straightforward is the stereoselective introduction of two oxygen functionalities onto carbon-carbon double bonds, as represented by the Sharpless epoxidation4 or osmium tetroxide oxidation of allyl alcohols.5 An alternative route may be envisioned by anti-Markovnikov hydration of enol ether derivatives, as in the present method, but such an approach has so far been rarely studied. The only other reported method is hydroboration which affords syn/anti ratios in the range of 83/17 to 3/97 in essentially the same systems as those examined in this procedure. The method described here is a highly stereoselective route to 2,3-syn isomers of 1,2,3-triol skeletons from a-hydroxy enol ethers. 

Hydroborations. Addition of Catecholborane to alkenes is accelerated by Wilkinson s catalyst, and other sources of rhodium-(I) complexes. Unfortunately, the reaction of Wilkinson s catalyst with catecholborane is complex hence if the conditions for these reactions are not carefully controlled, competing processes result. In the hydroboration of styrene, for instance, the secondary alcohol is formed almost exclusively (after oxidation of the intermediate boronate ester, eq 37) however, the primary alcohol also is formed if the catalyst is partially oxidized and this can be the major product in extreme cases. Conversely, hydroboration of the allylic ether (12) catalyzed by pure Wilkinson s catalyst gives the expected alcohol (13), hydrogenation product (14), and aldehyde (15), but alcohol (13) is the exclusive (>95%) product if the RhCl(PPh3)3 is briefly exposed to air before use. The 5yn-alcohol is generally the favored diastereomer in these and related reactions (eq 38), and the catalyzed reaction is therefore stereocomplementary to uncatalyzed hydroborations of allylic ether derivatives. ... 

Similarly, Evans et al [36] have also found that several classes of allylic alcohols on rhodium-catalyzed hydroboration afford allylic alcohols with high dia-stereoselectivity-sy product and isomer complementing to that furnished by uncatalyzed variant of the reaction (9-BBN)—the anti product. 

Hydroboration of allenes has long been known to give the corresponding allylbor-anes, which are useful for the preparation of allyl alcohols [346-348]. When 1-... 

Jung, M.E. and Yoo, D. (2008) Synthesis of the C]-Ci2 fragment of the tedanolides. Selective hydroboration-protonation of allylic alcohol approach. Tetrahedron Lett., 49,816-819. 

Houk has suggested that stereoelectronic effects have influence over the stereochemical course of hydroboration reactions of allylic alcohols [19]. Because borane is an electrophilic reagent, it exhibits a preference for electron-rich partners in hydroboration reactions. The more reactive conformer of an allylic alcohol is that in which the olefin avoids additional hyperconjugative interactions that would render it electron-deficient, such as jic=c ( c-x (allylic). Therefore, allylic hydroxy or alkoxy substituents tend to avoid the anti position with respect to the partially formed bonds (cf. transition structure 36). Altogether, both steric and electronic effects work in concert to support the predominance of transition structures 28 and 36,... 

Catalyzed hydroboration has proven to be valuable in controlling the stereoselectivity of hydroboration of functionalized alkenes.169 For example, allylic alcohols... 

As expected, allylic alcohols and ethers afford v/c-diol derivatives upon hydroboration. However, in the presence of Wilkinson s catalyst, a reverse regiochemistry for the hydroboration is observed [108]. There is the possibility that boron, rather than hydrogen, is transferred from the metal to the olefin. In other words, the boron acts as a donor. 

Albert S.C. Chun of the Hong Kong Polytechnic University reports (J. Org. Chem. 68 1589, 2003) two important transformations. The three-component (Mannich) condensation of 10 with 11 and 12 proceeds with high diastereoselectivity, to give the amino alcohol 13. Hydroboration of the alkyne 14 followed by transmetalation of the intermediate vinyl borane gives a zinc species, which under catalysis by the easily-prepared 3-naphthol 13 adds to aromatic and branched aldehydes with high . The product allylic alcohols are useful intermediates for organic synthesis. 

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