Catalyzed hydroboration of alkenes and alkynes

The catalyzed hydroboration did not attract much attention until Sneddon in 1980 and Noth in 1985 reported that rhodium complexes significantly accelerate the addition of B-H bond to alkenes or alkynes. The protocol was proved to be an interesting strategy to realize the different chemo-, regio-, diastereo-, and enantioselectivities, relative to the uncatalyzed reaction. The reaction has been reviewed.132-135 

Many metal complexes catalyze the hydroboration. A neutral rhodium-phosphine complex RhCl(PPh3)3 is the most studied catalyst for the hydroboration of alkenes, but the complex is unfortunately highly sensitive to air. Thus, handling the catalyst under argon or in air resulted in different regioselectivity. The in situ preparation of the catalyst 

HBcat/Rh(PPh3)3CI 93, HBpin/Rh(PPh3)3CI 94, HBcat/Rh(PPh3)3CI 95, HBcat, Rh(PPh3)3CI 

Entry Alkyne Rl R2 HBXZ Catalyst/solvent/temp. Yield (%) 101/102 

The kinetic resolution of racemic alkenes 112 was demonstrated in asymmetric hydroboration with Rh-QUINAP catalyst. A 78% yield with 98% ee was achieved when using 0.6 equiv. of HBcat compared to the alkene.180 

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Scheme 4.S8. Zirconium- and rhodium-catalyzed hydroborations of alkenes and alkynes with pinacolborane.

We note that there are NMR-based kinetic studies on zirconocene-catalyzed pro-pene polymerization [32], Rh-catalyzed asymmetric hydrogenation of olefins [33], titanocene-catalyzed hydroboration of alkenes and alkynes [34], Pd-catalyzed olefin polymerizations [35], ethylene and CO copolymerization [36] and phosphine dissociation from a Ru-carbene metathesis catalyst [37], just to mention a few. 

The mechanisms of these reactions are varied, but can still be categorized. The hydrocyanations, hydrosilylations, and many of the hydroborations, occur with late-metal catalysts. These reactions occur by oxidative addition of the H-X bond, followed by migratory insertion of the olefin into the M-H or M-X bond, and reductive elimination to form the final product. Hydrocyanation occurs by insertion of the unsaturated reagent into the M-H bond, while hydrosilylation and hydroboration have been shown to occur by insertion of the olefin into the M-H bond in some cases and into the M-X bond in others. HydrosUy-lations and hydroborations of alkenes and alkynes catalyzed by (P transition metal complexes and by lanthanides follow a different pathway because these complexes caimot undergo oxidative addition. The mechanism of the reactions catalyzed by these complexes involves u-bond metatheses. 

It took another decade however before the idea of developing a rhodium-catalyzed olefin hydroboration process came to fruition. This occurred in 1985 when Mannig and Noth reported the first examples of such a process.8 They discovered that Wilkinson s catalyst 2 was effective for the addition of catecholborane 1 to a range of alkenes and alkynes, as exemplified by cyclopentene 4 (Scheme 2). 

Hartwig65 reported that dimethyltitanocene is an efficient catalyst for the hydroboration of alkenes and examined the mechanism of titanocene dicarbonyl-catalyzed the hydroboration of alkynes (Scheme 5).66... 

This interest in catalytic hydroboration led to the development of the transition-metal-catalyzed diboration of alkenes and alk5mes. The diboration of alkenes and alkynes generates bifunctional products, and additions to alkenes have now been conducted with high enantioselectivity. The following sections describe the t es of catalysts used for catalytic hydroboration and diboration of alkenes, alkynes, and dienes, as well as catalytic cycles that accoimt for selectivities and side products formed during these processes. 

Hydroboration of alkynes is easier to achieve than B-H addition to alkenes. A mixture of alcohol, ketone, and diol products is produced upon [Rh(PPh3)3Cl] catalyzed hydroboration of... 

Hartwig, J. F., Muhoro, C. N. Mechanistic Studies of Titanocene-Catalyzed Alkene and Alkyne Hydroboration Borane Complexes as Catalytic Intermediates. Organometattics 2000,19, 30-38. 

Functionalized alkenes and alkynes can be hydroborated . Directive effects are discussed in 5.3.2.5.1(i). As follows from Table 2, hydroboration of vinylic and allylic derivatives leads to the placement of boron in the a., fi or y position to the substituent. The -substituted organoboranes are prone to uncatalyzed cis elimination and acid- or base-catalyzed trans- elimination The rate of elimination depends on the substituent at... 

The elements of HjO can be added to the carbon-carbon triple bond of an alkyne by the same two reactions used for the hydration of alkenes, namely hydroboration-oxidation and acid-catalyzed hydration. Even though the reagents are similar, the products from hydration of alkenes and allgmes are quite different. 

Iridium-catalyzed hydrogenation of alkynes, hydrosilylation of alkenes, and hydroboration of alkenes ... 

In comparison with the hydroboration and diborafion reactions, thioboration reactions are relatively limited. In 1993, Suzuki and co-workers reported the Pd(0)-catalyzed addition of 9-(alkylthio)-9-BBN (BBN = borabicyclo [3.3.1] nonane) derivatives to terminal alkynes to produce (alkylthio)boranes, which are known as versatile reagents to introduce alkylthio groups into organic molecules [21], Experimental results indicate that the thioboration reactions, specific to terminal alkynes, are preferentially catalyzed by Pd(0) complexes, e.g. Pd(PPh3)4, producing (thioboryl)alkene products, in which the Z-isomers are dominant. A mechanism proposed by Suzuki and co-workers for the reactions involves an oxidative addition of the B-S bond to the Pd(0) complex, the insertion of an alkyne into the Pd-B or Pd-S bond, and the reductive elimination of the (thioboryl)alkene product. 

Hydroboration. Although hydroboration seldom requires a catalyst, hydrobora-tion with electron-deficient boron compounds, such as boric esters, may be greatly accelerated by using transition-metal catalysts. In addition, the chemo-, regio- and stereoslectivity of hydroboration could all be affected. Furthemore, catalyzed hydroboration may offer the possibility to carry out chiral hydroboration by the use of catalysts with chiral ligands. Since the hydroboration of alkynes is more facile than that of alkenes the main advantage of the catalytic process for alkynes may be to achieve better selectivities. Hydroboration catalyzed by transition-metal complexes has become the most intensively studied area of the field.599... 

Like the double bond, the carbon-carbon triple bond is susceptible to many of the common addition reactions. In some cases, such as reduction, hydroboration and acid-catalyzed hydration, it is even more reactive. A very efficient method for the protection of the triple bond is found in the alkynedicobalt hexacarbonyl complexes (.e.g. 117 and 118), readily formed by the reaction of the respective alkyne with dicobalt octacarbonyl. In eneynes this complexation is specific for the triple bond. The remaining alkenes can be reduced with diimide or borane as is illustrated for the ethynylation product (116) of 5-dehydro androsterone in Scheme 107. Alkynic alkenes and alcohols complexed in this way show an increased structural stability. This has been used for the construction of a variety of substituted alkynic compounds uncontaminated by allenic isomers (Scheme 107) and in syntheses of insect pheromones. From the protecting cobalt clusters, the parent alkynes can easily be regenerated by treatment with iron(III) nitrate, ammonium cerium nitrate or trimethylamine A -oxide. ° ... 

Like hydroalumination and hydrozirconation, hydroboration of alkynes also provides a convenient and Stereospecific route to alkenyl metal reagents. However, initial attempts to achieve palladium-catalyzed cross-coupling of alkenylboranes with alkenyl halides were unsuccessful, due to the poor carbanionic character of these reagents. Later, Suzuki discovered that the desired transformation could be effected in the presence of an alkoxide or hydroxide base weaker bases, such as sodium acetate or triethylamine, were not generally effective. The reaction is suitable for the preparation of ( , )-, ( ,Z)- and (Z,Z)-dienes. Since reactions of alkenylboronates are higher yielding than those of alkenylboranes, the recent availability of (Z)-l-alkenylboronates " substantially improves the Suzuki method for the preparation of (Z)-alkenes. An extension of the methodology to the synthesis of trisubstituted alkenes has also been reported. " ... 

The hydroboration of olefins is a classic reaction in organic synthesis. - Dialkylbo-ranes add rapidly to alkenes in the absence of catalyst. However, dialkoxyboranes, such as catecholborane and pinacolborane, add more slowly to olefins and alkynes. Thus, transition metal complexes could catalyze the addition of dialkoxyboranes to olefins and alkynes without interference from the background reaction. The potential to alter chemoselectivity, regioselectivity, enantioselectivity, and diastereoselectivity has led a munber of groups to develop metal-catalyzed versions of hydroboration. " Enantioselective hydroboration would alleviate the need to use boranes containing stoichiometric amounts of chiral substituents to generate optically active alkylboranes. 

Reactions catalyzed by titanocene and lanthanocene complexes occur by different mechanisms (Schemes 16.13 and 16.14). The mechanism of the hydroboration of vinylarenes catalyzed by titanium complexes is shown in Scheme 16.13. In this mechanism, a titanocene bis-borane complex dissociates borane to generate a 16-electron complex that coordinates the alkene or alkyne. Coupling of one carbon of the resulting metallacycle with the boron of the coordinated borane forms the final product. The mechanism of the lanthanocene-catalyzed reactions, shown in Scheme 16.14, relates to the mechanism of... 

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