Hydroboration with chiral boranes

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. 

Thought Experiment I on the Hydroboration of Chiral Alkenes with Chiral Boranes Mutual Kinetic Resolution... 

There are two common ways to accomplish an asymmetric reaction. Either a second chiral center is created in a molecule under the influence of an existing chiral center in that molecule or a chiral reagent acts on a prochiral substrate to create a new chiral center. The conversion of chiral a-keto esters to di-, astereomeric a-hydroxy esters is an example of the first type of asymmetric reaction, and the asymmetric hydroboration of alkenes with chiral boranes is an example of the second type (Fig. 1). 

Thought Experiments II and III on the Hydroboration of Chiral Olefins with Chiral Boranes Reagent Control of Diastereoselectivity, Matched/Mismatched Pairs,... 

As mentioned in Section 9.12.2.1.1, the boron-zinc exchange can be performed stereoselectively if diisopropyl-zinc instead of diethylzinc is used. For example, hydroboration of the chiral, racemic endocyclic olefin 134 with diethylzinc, followed by twofold transmetallation and electrophilic capture of the resulting copper intermediate with allyl bromide was used for the highly diastereoselective formation of the stereotriad in product 136 (Scheme 35).35,35a 103 QorreSp0nding enantioselective transformations were carried out with chiral boranes and catalytic amounts of copper salts (see Section 9.12.2.2.2).36... 

With this spectrum of reagents, virtually all unsaturated hydrocarbons can be hydroborated. Moreover, many functional groups are tolerated in hydroboration with substituted borane derivatives, since double and triple C—C bonds react more rapidly than most other functionalities (see Table 1). Only aldehydes, ketones and carboxylic groups must be protected. Consequently, for the first time, functionalized organometall-ics are available. Chiral organoboranes of high optical purity can be prepared ... 

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],... 

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. 

The synthesis of enantiomerically pure compounds is the challenging problem for organic chemists. The synthesis becomes obsolete if the intermediates produce racemic mixtures. The problem is particularly acute when the asymmetric centers do not reside in a rigid cyclic or polycyclic framework. To be able to carry out efficient syntheses of complex molecules, chemists have to control the sense of chirality at each chiral center as it is introduced in the course of synthesis. Monoalkyl- or dialkyl-boranes exhibit a remarkable chemo-, stereo-, and regioselectivity for the hydroboration of unsaturated compounds. This property, coupled with the capability for asymmetric creation of chiral centers with chiral hydroboration agents, makes the reaction most valuable for asymmetric organic synthesis. In some of the cases, however, this has been achieved by diborane itself as shown in the synthesis of monensin by Kishi et al. A stereospecific synthesis of its seven carbon. component has been accomplished by two hydroboration reactions (Eq. 129) 209. ... 

Asymmetric hydroboration 2171 of prochiral alkenes with monoisopinocampheyl-borane in the molar ratio of 1 1, followed by a second hydroboration of non-prochiral alkenes with the intermediate dialkylboranes, provides the chiral mixed trialkylbo-ranes. Treatment of these trialkylboranes with acetaldehyde results in the selective, facile elimination of the 3-pinanyl group, providing the corresponding chiral borinic acid esters with high enantiomeric purities. The reaction of these intermediates with base and dichloromethyl methyl ether provides the chiral ketones (Eq. 130)2l8>. A simple synthesis of secondary homoallylic alcohols with excellent enantiomeric purities via B-allyldiisopinocampheylborane has been also reported 219),... 

Addition to C=C. Chiral boranes obtained from Rh-complex-catalyzed hydroboration with catecholborane are transformed into amines with retention of configuration on sequential treatment with MeMgCl and H NOSOjH. 

By hydroboration of natural products such as a-pmene, H. C. Brown and coworkers have prepared mono- 2.15 (R = H) and diisopinocampheylboranes 2.16 (R = H). These reagents promote highly enantioselective hydroborations [580, 583], The two a-pinene enantiomers are available, so both enantiomers of these reagents can be used. The intermediate di- or trialkylboranes formed in these hydroborations are treated with MeCHO. This forms a chiral boronate 2.17, and the a-pinene is freed for recovery and recycling. From 2.17, it is possible to obtain many functionalized compounds. Additionally, new chiral boranes 2.18 are available, and these are precursors of many chiral compounds bearing the R group [169, 580, 583, 585-588] (Figure 2.2). 

Asymmetric hydroboration of prochiral olefins can be performed either with a chiral borane in the absence of a catalyst or with catecholborane and a chiral rhodium catalyst [549]. The rich chemistry of the boranes generated by these methods opens the way to a variety of enantioenriched molecules bearing different functionalities [583],... 

Reduction of aldehydes labeled with deuterium or tritium leads to primary alcohols which are chiral by virtue of the label. Alternatively, the label can be introduced by the use of deuterated Alpine-Borane, which can be prepared by hydroboration with 9-borabicyclo[3.3.1]nonane-... 

Chiral boranes can be used as reducing agents for carbonyl compounds. 30 Brown prepared a variety of chiral boranes in connection with extensive studies of hydroboration reactions (sec. 5.4.B). Reaction of diborane and a-pinene, for example, gave (-)-( / ,25,3/ ,5/ )-diisopinocampheylborane (206), which reduced carbonyl derivatives to alcohols with high asymmetric induction.33 Trialkylboranes are also capable of... 

The excellent syn selectivity of chiral allylamine derivatives, using catalyzed hydroboration with 9-BBN and borane-THF complex, can be explained by the following reactive conformations (Fig. 5.3). 

An experimental study of the hydroboration of prochiral alkenes with chiral Lewis base-borane complexes has been undertaken. The study used N-isobornyl-N-methylaniline-borane... 

---