Stereoselectivity hydroboration

Rationalizations of hydroboration stereoselectivity using models with dimeric boranes are thus not viable. Recent secondary isotope-effect measurements may suggest a model (Mann et al., 1986). Addition of a chiral dialkyl borane to 3-deuterio-cw-3-pentene, followed by alkaline oxidation, yields equal amounts of 3- and 4-deuterio-3-hexanols showing that the secondary isotope effects at these alkene sites are vanishingly small. Deuterium substitution at the allylic sites has a much larger effect. Thus similar treatment of 4,4-dideuterio-cw-5-decene yields a 2.86 1 mixture of 4,4-dideuterio- and 6,6-dideuterio-5-decanols. 

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-... 

Models accounting for hydroboration stereoselectivity have been advanced. ... 

Another possibility for asymmetric reduction is the use of chiral complex hydrides derived from LiAlH. and chiral alcohols, e.g. N-methylephedrine (I. Jacquet, 1974), or 1,4-bis(dimethylamino)butanediol (D. Seebach, 1974). But stereoselectivities are mostly below 50%. At the present time attempts to form chiral alcohols from ketones are less successful than the asymmetric reduction of C = C double bonds via hydroboration or hydrogenation with Wilkinson type catalysts (G. Zweifel, 1963 H.B. Kagan, 1978 see p. 102f.). 

The coupling of alkenylboranes with alkenyl halides is particularly useful for the stereoselective synthesis of conjugated dienes of the four possible double bond isomers[499]. The E and Z forms of vinylboron compounds can be prepared by hydroboration of alkynes and haloalkynes, and their reaction with ( ) or (Z)-vinyl iodides or bromides proceeds without isomerization, and the conjugated dienes of four possible isomeric forms can be prepared in high purity. 

Hydroboration-oxidation of a pinene (page 235) like catalytic IS stereoselective Addition takes place at the less hindered face and a single alcohol is produced in high yield (89%) Sug... 

In general, hydroboration—protonolysis is a stereoselective noncatalytic method of cis-hydrogenation providing access to alkanes, alkenes, dienes, and enynes from olefinic and acetylenic precursors (108,212). Procedures for the protonolysis of alkenylboranes containing acid-sensitive functional groups under neutral or basic conditions have been developed (213,214). 

Others. The stereoselective deuteration by deuterioboration or hydroboration—deuteriolysis is straightforward (207,459,460). Olefins monodeuterated in the aHyhc position are readily prepared by deuteriolysis of aHylic organoboranes (461). 

Hydroboration - regloseiective and stereoselective (syn) addition of BH3 (RBH2, R2BH) to olefins. Synthesis of alcohol including optically active alcohols from olefins. Also useful In synthesis of ketones by stitching ot olefins and CO... 

If R and R are different, the two faces of the double bond become nonequivalent, permitting stereoselective reactions at the double bond. These effects have been explored, for example, using 4-silyl-2-pentenes. Reactions such as epoxidation and hydroboration proceed by preferential addition fiom the face opposite the bulky silyl substituents. 

The overall result of the sequence hydroboration -I- oxidation is a regioselective and-Markownikoff-addition of water to an alkene. This reaction is an important method in organic synthesis, since it can be made stereoselective and even enantioselective. 

It would be difficult to overestimate the role that the polyether antibiotics have played in the development of organic synthesis, particularly in the area of acyclic stereocontrol. These molecules have inspired many spectacular achievements in organic synthesis, achievements that have dramatically expanded the power and scope of the science. In fact, it would not be inaccurate to attribute much of our understanding about the factors controlling acyclic stereoselectivity for such fundamental processes as hydroboration,5 epox-... 

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

The use of chiral ligands in catalysts can lead to enantioselective hydroboration. Rh-BINAP171 C and the related structure D172 have shown good stereoselectivity in the hydroboration of styrene and related compounds (see also Section 4.5.3). 

Analyze the data below concerning the effect of allylic and homoallylic benzyloxy substituents on the regio- and stereoselectivity of hydroboration-oxidation. Propose a TS that is consistent with the results. 

The syntheses in Schemes 13.45 and 13.46 illustrate the use of oxazolidinone chiral auxiliaries in enantioselective synthesis. Step A in Scheme 13.45 established the configuration at the carbon that becomes C(4) in the product. This is an enolate alkylation in which the steric effect of the oxazolidinone chiral auxiliary directs the approach of the alkylating group. Step C also used the oxazolidinone structure. In this case, the enol borinate is formed and condensed with an aldehyde intermediate. This stereoselective aldol addition established the configuration at C(2) and C(3). The configuration at the final stereocenter at C(6) was established by the hydroboration in Step D. The selectivity for the desired stereoisomer was 85 15. Stereoselectivity in the same sense has been observed for a number of other 2-methylalkenes in which the remainder of the alkene constitutes a relatively bulky group.28 A TS such as 45-A can rationalize this result. 

Prior literature indicated that olefins substituted with chiral sulfoxides could indeed be reduced by hydride or hydrogen with modest stereoselectivity, as summarized in Scheme 5.10. Ogura et al. reported that borane reduction of the unsaturated sulfoxide 42 gave product 43 in 87 13 diastereomer ratio and D20 quench of the borane reduction mixture gave the product 43 deuterated at the a-position to the sulfoxide, consistent with the hydroboration mechanism [10a]. In another paper, Price et al. reported diastereoselective hydrogenation of gem-disubstituted olefin rac-44 to 45 with excellent diastereoselectivity using a rhodium catalyst [10b],... 

Very recent investigations of styrene hydroboration with C2-symmetrical 1,2-diphosphines give promising regio- and stereoselectivities. As with the other ligands described above, the electronic properties of the phosphorus substituents prove paramount in determining the reactivity (Scheme 12). Hence, the combination of (li ,2i )-l,2-bis(diphenylphosphino)cyclohexane (19) with [Rh(COD)2]+... 

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