Addition of boranes

Since borane BH3 reacts with only one or two equivalents of a sterically hindered alkene, it is possible to prepare less reactive and more selective borane reagents R2BH and RBH2 respectively. In addition to disiamylborane 8 and thexylbo-rane 10, the 9-borabicyclo[3.3.1]nonane (9-BBN) 14 is an important reagent for hydroboration, since it is stable to air it is prepared by addition of borane 2 to cycloocta-1,5-diene 13 ... 

Hydration of an alkene—the addition of water—is carried out by either of two procedures, depending on the product desired. Oxymercuration involves electrophilic addition of Hg2+ to an alkene, followed by trapping of the cation intermediate with water and subsequent treatment with NaBH4. Hydroboration involves addition of borane (BH3) followed by oxidation of the intermediate organoborane with alkaline H202- The two hydration methods are complementary oxymercuration gives the product of Markovnikov addition, whereas hydroboration/oxidation gives the product with non-Markovnikov syn stereochemistry. 

Hydrolioration (Section 7.5) Addition of borane (BH i) or an alkylborane to an alkene. The resultant trialkyliiorane products are useful synthetic intermediates that can be oxidized to yield alcohols. 

A catalytic cycle proposed for the metal-phosphine complexes involves the oxidative addition of borane to a low-valent metal yielding a boryl complex (35), the coordination of alkene to the vacant orbital of the metal or by displacing a phosphine ligand (35 —> 36) leads to the insertion of the double bond into the M-H bond (36 —> 37) and finally the reductive elimination to afford a hydroboration product (Scheme 1-11) [1]. A variety of transition metal-boryl complexes have been synthesized via oxidative addition of the B-H bond to low-valent metals to investigate their role in cat-... 

Figure 11.1 The hydroboration-oxidation of 1-methylcyclopentene. The first reaction is a syn addition of borane. (In this illustration we have shown the boron and hydrogen both entering from the bottom side of 1-methylcyclopentene. The reaction also takes place from the top side at an equal rate to produce the enantiomer.) In the second reaction the boron atom is replaced by a hydroxyl group with retention of configuration. The product is a trans compound (trans-2-methyl-cyclopentanol), and the overall result is the syn addition of -H and -OH.

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. 

Borane-THF complex was obtained from Toso-Akzo Chemical Company, Ltd. in Japan and should be titrated before use. Vigorous evolution of hydrogen is observed during addition of borane-THF solution to the reaction mixture. 

Another example of great synthetic interest, involves the hydroboration reaction of alkenes [62], In general, the addition of borane to alkenes proceeds stepwise, the final product being the trialkylborane. However, hindered alkenes react slowly, especially when the dialkylborane precipitates from the medium. It was found that trialkyl bor-anes could be obtained rapidly under sonication, even with highly hindered substrates (Eq. 3.5). Applications of this useful modification were published, among which were the reduction-hydroxylation of vinyl groups by 9-BBN [63,64]. 

Addition of boranes to olefins, followed by basic oxidation of the organoborane adducts, resulting in alcohols. 

The addition of borane to the double bond takes place in such a manner that the boron atom gets attached to the sp carbon carrying greater number of hydrogen atoms. The alcohol so formed looks as if it has been formed by the addition of water to the alkene in a way opposite to the Markovnikov s rxrle. In this reaction, alcohol is obtained in excellent yield. 

Fig. 2.8 A d irect route to vinylboranes in rhodium-catalyzed hydroborations with phos-phine-free catalysts (including oxidative degradation of a rhodium phosphine). The key intermediate is a rhodium hydride, capable of reversible insertion into the alkene (step A), followed by addition of borane in step B. This leads to reductive elimination of RH in step C followed by boryl migration in step D. A further...

The potential vigor of this exotherm cannot be overemphasized. It occurs later and is correspondingly stronger if vigorous reflux is not maintained during the addition. The reaction mixture should be watched closely throughout the addition of borane, and addition should be temporarily suspended at the onset of the exotherm. 

The Addition of Boranes to Activated Double Bonds Hydro-alkyl-addition (overall transformation)... 

Addition of organometallic compounds to unsaturated ketones 5-19 Addition of boranes to unsaturated... 

Isopinocampheol has been prepared by hydrogenation of nickel catalyst at 70-100°. The hydroboration reaction provides a convenient procedure for the conversion of olefins to alcohols without rearrangement and with a predictable stereochemistry. The reaction has been applied to a large number of olefins of widely different structures.4,6 The results obtained support the proposed generalization that hydroboration involves an awfi-Markownikoff, cia-addition of borane from the less hindered side of the double bond. ... 

Another method for the preparation of acetylenic carboxylates involves the reaction of 1-alkenylboranes with CO and alcohols (equation 135). The alkenyl boranes are readily prepared from alkynes by addition of boranes. PdCl2 was again the catalyst, with NaOAc as base and benzoquinone as oxidant.541... 

Addition of borane to 2,3-dimethylbut-2-ene gives 2,3-dimethyl-2-butylborane (thexylborane) which is also a synthetically useful substituted borane. 

An important consequence of the mechanism of this oxidation is that the stereo-isomeric features, resulting from the cis-addition of borane formulated previously (above), are retained in the final product, i.e. the overall reaction is highly stereospecific. Thus reaction of 1-methylcyclohexene with borane followed by oxidation gives frans-2-methylcyclohexan-l-ol.66... 

Sterically demanding boranes offer enhanced selectivity. One example of a sterically demanding borane (9-BBN) is generated by the double addition of borane to 1,5-cyclooctadiene ... 

The anti-cancer compound coriolin 23 has three fused five-membered rings and two epoxides. Notice that the 3/5 and both 5/5 ring fusions are cis. There have been many syntheses of coriolin, most using stereochemistry from folded precursors.7 We shall feature a couple of examples. Matsomoto s synthesis involves the hydroboration of alkene 24. The addition of borane is cis 25 and the boron is replaced by OH with retention of configuration to give 26. The hydroboration occurred on the outside of the molecule, on the same face as the ring junction hydrogens.8... 

Borane transforms a wide range of alkenes into trialkylboranes under mild conditions but the trifunctional nature of borane and its trialkylborane products imposes some limitations on its use. Many of the synthetically useful reactions of the trialkylboranes (see Chapters B.2 and B.3) use all three alkyl substituents, but some reactions only utilize either two or even one of the alkyl substituents. This sets a maximum yield (based on the alkene starting material) for these latter transformations of 66% and 33% respectively which is clearly undesirable especially if the alkene involved is the product of a multi-step synthetic sequence. To overcome this problem, and others such as the production of intractable polymers on addition of borane to dienes and alkynes, monoalkylborane and dialkylborane hydroborating reagents were introduced. Some commonly used reagents are depicted in Figure B 1.2 and two are described in more detail below. 

Transfers of hydride from boron or lithium to carbon usually occur in the context of addition of the complete M—H moiety to polar or non-polar unsaturation. Additions of boranes to alkenes have been extensively reviewed (Brown et al., 1983a), but the experimental characterization of the hydroboration transition state remains problematic. Dialkylboranes, including 9-borabicyclo[3.3.1]nonane (Wang and Brown, 1980), borinane (Brown et al., 1984), and disiamylborane (Chandrasekharan and Brown, 1985) have now been shown to be dimeric in hydrocarbon and ethereal solvents. With unreactive alkenes, their additions are first order in alkene and half order in the dimer. With reactive terminal alkenes, the reactions are first order only in dimer, with intermediate behaviour between these extremes. A reaction scheme (10) involving reaction of monomeric borane with the alkene satisfies the data, with the observed order depending on the ratio k i/k2. 

The course of these additions of lithium hydride resembles that found for the addition of borane (Nagase et al., 1980 Graham et al., 1981). With ethylene, the initial step is exothermic formation of a Jt-complex without barrier, then rate-determining transformation to the borane via a four-centre transition structure. In both the borane and lithium hydride additions, there is relatively little development of the new C—H bond with distances of 1.692 and 1.736 A respectively in the transition structures. When a carbanionic product is not formed, for example in the reaction of lithium hydride with cyclopropenyl cation yielding cyclopropene and lithium cation (Tapia et al., 1985), reaction again occurs via a hydride-bridged complex, but the C- H- -Li array remains nearly linear throughout the reaction. 

The addition of borane to alkenes was first reported by H. C. Brown et al. [3] in 1956. The anti-Markovnikov insertion of an unsaturated moiety into a B-H bond of the borane (R2BH, RBH2 and BH3) proved to be the initial step for introduction of a very wide variety of functional groups. Within the following decade, the same author described the replacement of a boron atom by an amino group, affording a synthetic route from alkenes to amines [4] (Scheme 1). 

Although the B-H addition of borane etherates to simple alkenes occurs quite rapidly under ambient conditions, less electrophilic boranes are correspondingly less reactive. A classic example is given by catecholborane which reacts with alkynes only above 70 °C and with alkenes under somewhat more forcing conditions [1,85]. The reason for this is not hard to find, since the electrophilic character of the borane is substantially diminished by conjugation between boron and adjacent oxygens [86,87]. At the same time, the acidity of B-H is enhanced through an... 

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