Hydroboration, alkene alkyne

Dibromoborane—dimethyl sulfide is a more convenient reagent. It reacts directly with alkenes and alkynes to give the corresponding alkyl- and alkenyldibromoboranes (120—123). Dibromoborane differentiates between alkenes and alkynes hydroborating internal alkynes preferentially to terminal double and triple bonds (123). Unlike other substituted boranes it is more reactive toward 1,1-disubstituted than monosubstituted alkenes (124). 

An efficient general synthesis of a-chiral (Z)- and (H)-a1kenes ia high enantiomeric purity is based on the hydroboration of alkynes and 1-bromoaIkynes, respectively, with enantiomericaHy pure IpcR BH readily available by the hydroboration of prochiral alkenes with monoisopiaocampheylborane, followed by crystallization (519). 

The chemistry of alkynes is dominated by electrophilic addition reactions, similar to those of alkenes. Alkynes react with HBr and HC1 to yield vinylic halides and with Br2 and Cl2 to yield 1,2-dihalides (vicinal dihalides). Alkynes can be hydrated by reaction with aqueous sulfuric acid in the presence of mercury(ll) catalyst. The reaction leads to an intermediate enol that immediately isomerizes to yield a ketone tautomer. Since the addition reaction occurs with Markovnikov regiochemistry, a methyl ketone is produced from a terminal alkyne. Alternatively, hydroboration/oxidation of a terminal alkyne yields an aldehyde. 

Alkynes are reactive toward hydroboration reagents. The most useful procedures involve addition of a disubstituted borane to the alkyne, which avoids complications that occur with borane and lead to polymeric structures. Catechol borane is a particularly useful reagent for hydroboration of alkynes.212 Protonolysis of the adduct with acetic acid results in reduction of the alkyne to the corresponding cw-alkene. Oxidative workup with hydrogen peroxide gives ketones via enol intermediates. 

Other disubstituted boranes have also been used for selective hydroboration of alkynes. 9-BBN can be used to hydroborate internal alkynes. Protonolysis can be carried out with methanol and this provides a convenient method for formation of a disubstituted Z-alkene.217... 

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... 

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... 

The hydroboration of alkynes gives predominantly cis isomer, as in the following example. Diborane adds as BH3 to alkynes in a cis isomer and forms vinyl boranes which react with acetic acid at low temperature to give cis-alkenes. 

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 alkenes, alkynes do not react readily with trialkylboranes. Under severe reaction conditions, only hydroboration products are obtained.116 Mikhailov113 showed that triallylboranes react with various alkynes (20 C) to afford syn addition products of type (78) which rapidly cyclize (40-60 C) to give the cyclic boranes of type (79 Scheme 42). In the case of trimethylsilylacetylene and ethoxyacetylene, the reaction affords compounds of type (78 R = MesSi or OEt) which do not cyclize further.113 Recently, a transition metal silylboration reaction has been described by Oshima and cowoikers.117 A formal syn carboboration reaction leading to a variety of alkenylboranes has been reported by Suzuki and cowoikers118 (Scheme 43). Hexamethyldistannylacetylene (80) reacts readily with various trialkylboranes119 to afford syn addition products of type (81 equation 25). 

Dicarbonyltitanocene [Cp2Ti(CO)2] is also active for the hydroboration of alkynes at room temperature. Dimethyltitanocene [Cp2TiMe2] catalyses the hydroboration of alkene with CBH at room temperature [203],... 

Hydroboration is not restricted to alkenes alkynes also react well to give vinyl boranes. These may be used directly in synthesis or oxidized to the corresponding enol, which immediately tautomerizes to the aldehyde. An example of this transformation is the conversion of 1-octyne into octanal by hydroboration with disiamylborane and oxidation with sodium perborate under very mild conditions. 

CH,),QH,]2BH (1). Mol. wt. 250.20, m.p. 68°, air stable, in 70% yield by reduction of fluorodimesitylborane with LiAlHj. This borane is recommended for hydroboration of alkynes, par-e hydroboration of unsymmetrical alkynes (equation I). 1-Alkynes khydes in high yield. Since alkcnes react only slowly with this iwration of alkynes in the presence of alkenes is possible. 

The term hydrosilation (or hydrosilylation) refers to the addition of a molecule containing a Si-H bond across the multiple bond of a substrate, usually an alkene, alkyne, or carbonyl compound (equation 1). The reaction can be promoted by UV-light, radiation (y- and X rays), radical initiators, Lewis acids, nucleophiles, or, most importantly, transition metal catalysts. Hydrosilation is related to the important processes of hydrogenation (see Hydrogenation) and hydroboration (see Hydroboration), all of which belong to the general reaction class of hydroelementation. 

Part of the intense interest in M-B compounds derives from the participation of these species in various catalytic processes.1,2 Scheme 6 depicts proposed catalytic cycles for metal-catalysed alkene hydroboration and alkyne diborylation. Key steps involve the insertion of unsaturated organic substrates into M-B bonds and a key intermediate involved in the formation of product is molecule A in which there are ad jacent M-C and M-B bonds. The ruthenium and osmium boryl complexes described in this section provide models for these steps and intermediates. 

Catecholborane is somewhat more reactive than (1), though still far less reactive than dialkylbo-ranes. 27 28 It is readily prepared by the reaction of catechol with borane-THF, is stable at 0 "C, and hydroborates alkenes slowly and alkynes (equation 49) more rapidly in refluxing The rate of... 

Advantage may be taken of the selective hydroboration of alkynes as compared to alkenes by dicy-clohexylborane or disiamylborane (Section 3.10.4.3) to provide syntheses of dienes and their deuteriated derivatives e.g. equation 62). Dibromoborane-dimethyl sulUde may be used similarly (Section 3.10.4.4)." The approach has been used for the stereospecific synthesis of an allylic hydroxydiene (equation 63). ... 

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. " ... 

Monoalkylbromoboranes hydroborate terminal alkynes to yield alkylalkenylbromoboranes, which then rearrange to yield (Z)-alkenes after protonation this useful procedure has been applied to the preparation of muscalure (35 Scheme 35). ( -Alkenes are available by a similar sequence if 1-bromoalkynes are employed at the hydroboration stage. Similarly, dialkenylthexylboranes yield dienes on rearrangement and protonation. ... 

Reversing the Markovnikov regioselectivity calls for hydroboration.13 We discussed hydroboration of alkynes in the last chapter and many of the same principles apply here. The reaction is a syn addition of R2B H to an alkene in which the boron bonds to the less substituted end of the alkene. The same sort of hydroborating agents are used - such as 9-BBN -and the mechanism is similar. The most important interaction is between the full n orbital of the alkene (HOMO) and the empty p-orbital on boron (LUMO) 80, but the reverse interaction between o(B-H) and Ji (alkene) also comes into play as the reaction proceeds. The result is syn addition via a transition state 81 with some positive charge on carbon and some negative charge on boron. 

Organoboranes possessing one B—C bond in the molecule can also be obtained by hydroborating alkenes and alkynes with heterosubstituted borane derivatives, especially dihalogenoboranes, which can be stabilized by complexation with amines, phosphines, ethers and sulfides. The last two are useful complexing agents for both the preparation and the reactions of dihalogenoboranes . The most convenient synthesis of dichloroborane etherate is the reaction of LiBH. with BCl, in Et,0 ... 

Two equivalents 1-alkyne with thexylborane produces the thexyldialkenylboranes . Thexylalkylalkenylboranes can be obtained either by sequential hydroboration of an alkene-alkyne pair with thexylborane [Eq. (ct)] or via hydroboration of an alkene with thexylchloroborane, followed by reduction and hydroboration of an alkyne [Eq. (cw)]. Cyclic trialkylboranes are synthesized via hydroboration. Thus, HjB-THF reacts 2 3 with simple acyclic dienes to give predominantly the dumbbell-shaped organoboranes. When heated, they undergo isomerization to the more thermodynamically stable isomers. These empirical rules predict their thermal behavior (a) the thermal... 

Borabicyclo[3.3.1]nonane (9-BBN) has found use in the selective hydro-boration of alkenes in the presence of other reducible functional groups and its reaction with alkynylstannates has been studied. o-Stannyl- and a-silyl-substituted crotyl-9-BBN show promise as reagents for the stereo-regulated synthesis of acyclic systems. A series of papers covers the question of olefin-alkyl exchange in. 5-alkyl-9-BBN s, " the kinetics of reduction of substituted benzaldehydes with 9-BBN, and the kinetics and mechanism of hydroboration of alkynes with 9-BBN dimers. Selective dehalogenation of tertiary alkyl, benzyl, and allyl halides in the presence of secondary or primary alkyl or aryl halides is possible with (165). The... 

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