Hydroboration olefin substrates

There have been several reviews of asymmetric synthesis via chiral organoboranes (6,8,378,382,467—472). Asymmetric induction in the hydroboration reaction may result from the chiraHty present in the olefin (asymmetric substrate), in the reagent (asymmetric hydroboration), or in the catalyst (catalytic asymmetric hydroboration). 

EinaHy, kinetic resolution of racemic olefins and aHenes can be achieved by hydroboration. The reaction of an olefin or aHene racemate with a deficient amount of an asymmetric hydroborating agent results in the preferential conversion of the more reactive enantiomer into the organoborane. The remaining unreacted substrate is enriched in the less reactive enantiomer. Optical purities in the range of 1—65% have been reported (471). 

The retrosynthetic elimination of olefinic stereocenters (E or Z) was illustrated above by the conversion 147 => 148 under substrate spatial control. It is also possible to remove olefinic stereocenters under transform mechanism control. Examples of such processes are the retrosynthetie generation of acetylenes from olefins by transforms such as trans-hydroalumination (LiAlH4), ci5-hydroboration (R2BH), or ci -carbometallation... 

In this chapter, theoretical studies on various transition metal catalyzed boration reactions have been summarized. The hydroboration of olefins catalyzed by the Wilkinson catalyst was studied most. The oxidative addition of borane to the Rh metal center is commonly believed to be the first step followed by the coordination of olefin. The extensive calculations on the experimentally proposed associative and dissociative reaction pathways do not yield a definitive conclusion on which pathway is preferred. Clearly, the reaction mechanism is a complicated one. It is believed that the properties of the substrate and the nature of ligands in the catalyst together with temperature and solvent affect the reaction pathways significantly. Early transition metal catalyzed hydroboration is believed to involve a G-bond metathesis process because of the difficulty in having an oxidative addition reaction due to less available metal d electrons. 

We thus summarize the yield ratios of the conceivable hydroboration products E, F, G, and H should be much little little none. One enantiomer of E comes from the reaction of the S-olefin with the. S, .S -borane the other enantiomer of E comes from the reaction of the /f-olefin with the f ,f -borane. Thus each enantiomer of the reagent has preferentially reacted with one enantiomer of the substrate. The diastereoselectivity of this reaction thus corresponds to a mutual kinetic resolution. 

Glycals can also be obtained from suitable substrates, by ring-closing olefin metathesis reactions. In a general approach to variously linked C-disaccharides, illustrated in O Scheme 48, nonreducing-end glycals are typical intermediates, finally subjected to hydroboration or dihy-droxylation to afford the desired C-analogs of O-disaccharides [238,239]. 

The reduction thus resembles the hydroboration of a trisubstituted olefin, a substrate which undergoes asymmetric hydroboration with high efficiency20. 

Thus, detailed experimental and theoretical studies are highly desirable on the mechanism of the transition-metal-catalyzed olefin hydroboration reactions, as well as on the role of the transition-metal center, substrates, and electronic and steric factors in the mechanism. MMM [67] have presented the first detailed ab initio molecular orbital (MO) study of possible reaction pathways illustrated in Fig. 22 for the reaction of C2H4 with the boranes HB(0H)2 and HB02(CH2)3 catalyzed by the model Wilkinson catalyst RhCl(PH3)2. The reaction of BH3 with C2H4 catalyzed by the Rh(PH3)2Cl have been studied by MMM [68] and DS [69]. 

Asymmetric hydroboration. Diisopinocampheylborane (1, 262-263 4, 161) is a useful reagent for asymmetric hydroboration of disubstituted olefins, but reacts only slowly with hindered trisubstituted olefins. For such substrates, the less hindered borane 1 is a useful reagent. Thus it reacts with 1-methyIcyclo-pentene at - 25° oxidation leads to 7ronj-2-methylcyclopentanol with an optical purity of 55.4%, with the new asymmetric center having the (S)-configuration. The paper reports asymmetric hydroboration of two other alkenes. 

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