Chiral allylic alcohols hydroboration

Hydroboration of Chiral Allyl Amines and Chiral Allyl Alcohols... 

Substrate-controlled diastereoselective hydroboration of protected chiral allylic alcohols [25-27] or amines [28, 29] with 9-BBN gives almost always anti selective products. On the other hand, catalyzed hydroboration in most of the cases using catecholborane as hydroborating agent tends to be syn selective [28-30] (Eq. 5.9). 

Burgess and Ohlmeyer [30] have reported that electronic effects are important in catalyzed hydroboration, e.g., allylic acetates are hydroborated with less syn selectivity than allylic trifluoroacetate is [25], and proposed the general model [A] (Fig 5.4) for catalyzed hydroboration of chiral allylic alcohols. The model predicts that the OCOCF, substituent (good a acceptors) will preferentially orientate anti to the approaching rhodium complex. The largest of the other two substituents on the chiral center will occupy the outside position, and the smallest will reside in the inside (crowded) site and thus, syn selectivity will result... 

Table 5.7 Catalyzed and uncatalyzed hydroboration of chiral allylic alcohols [35]...

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. 

The paper describes the first asym. hydroboration with a chiral catalyst. E A soln. of norbornene and catalytic amounts of chloro(l,5-cyclooctadiene)rhodium(I) dimer and DIOP (2,3-0-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane) in THF flushed with argon, stirred at 20° for 15 min, cooled to —78°, 1.2 eqs. catecholborane added, stirred for 5 min, allowed to warm to —40°, maintained at this temp, for 72 h, recooled to —78°, ethanol, 3 M NaOH, and 30% H2O2 added, the mixture allowed to warm to 25° during ca. 1 h, then stirred for 12h cjco-(lR, 2R)-norborneol. Y 99% (e.e. 55%). The method is generally applicable (even to 1,1-disubst. ethylene derivs. ) and expensive chiral boron-reagents are not necessary. F.e.s. K. Burgess, M.J. Ohlmeyer, J. Org. Chem. 53, 5178-9 (1988) from allyl alcohol O-derivs. s. Tetrahedron Letters 30, 395-8 (1989). 

Hydroboration of terminal alkynes followed by the B/Zn exchange and enantioselective catalytic addition to aldehydes serves as a simple and efficient method for the preparation of chiral nonracemic allylic alcohols. 

Chemistry-based kinetic resolution methods, which make use of the preferential reaction of one enantiomer with a chiral reagent (e.g. hydroboration of racemic alkenes with diisopinocampheylborane) or an achiral reagent in the presence of an appropriate chiral catalyst (e.g. Sharpless epoxidation of racemic allylic alcohols with t-BuOOH in the presence of (2R,3R)- or (25,35)-diisopropyl tartrate and Ti(Oi-Pr)4) have not been exploited so far for the isolation of e.p. labeled substances. In contrast, biochemical methods have been widely used, particularly for the resolution of racemic a-[ " C]amino acids and various [ C]carboxylic acids. Such methods, including ... 

---