Hydroborations alkynes

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 alternate strategy to access cyclopropanol derivatives from an acyclic precursor has been reported by Imai and coworkers (equation 79). Alkyne hydroboration followed by treatment of the resulting boronic acid with tetramethyltartramide produced the cyclopropanation precursor. A subsequent diastereofacial cyclopropanation afforded the... 

Oxidation of alkenylboranes by alkaline hydrogen peroxide gives aldehydes or ketones depending on the substituent pattern of the alkenyl group thus, when alkaline hydrogen peroxide oxidation is combined with alkyne hydroboration, the resulting two-step process is a procedure for converting alkynes to carbonyl compounds (Equations B2.7 and B2.8). 

Hydroboration-Oxidation of Alkynes Hydroboration-oxidation of an alkyne gives anti-Markovnikov addition of water across the triple bond. Di(secondary isoamyl)borane, called disiamylborane, is used, since this bulky borane cannot add twice across the triple bond. On oxidation of the borane, the unstable enol quickly tautomerizes to an aldehyde. (See Section 9-9F.)... 

With terminal alkynes, hydroboration always places the boron atom on the less substituted terminal carbon. 

Dichloroborane and monochloroborane etherates or their methyl sulfide complexes have been prepared by the reaction of borane and boron trichloride [44]. The hydroboration of alkenes with these borane reagents is, however, usually very slow because of the slow dissociation of the complex. Dichloroborane prepared in pentane from BCI3 and trimethylsilane is unusually highly reactive with alkenes and alkynes hydroboration is instantaneous at -78 °C (Eq. 20) [45]. 

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. 

Hydro stannylation of terminal alkynes Hydroboration and Hydroalumination of Alkynes Hydroboration of alkynes Hydroalumination of alkynes Hydroalumination ofpropargyl alcohols Hydrosilylation... 

All trisubstituted alkenes were obtained >98% isomerically pure) Scheme 3.9 1-Halo-l-alkyne hydroboration routes to trisubstituted alkenes. 

The Lindlar catalyst is a deactivated hydrogenation catalyst that stops alkyne hydrogenation at the alkene stage, allowing conversion of allies to ds-alkenes. Hydroboration followed by an acid workup (instead of basic peroxide) also gives cfs-alkenes from alkynes. Hydroboration-protonolysis is an alternative way to prepare a ds-alkene from an alkyne. 

In the intermolecular process, cross-metathesis offers a usefid and versatile alternative to alkyne hydroboration for the synthesis of functionalized alkenyl pinacol boronates with a moderate to high -selectivity. Styrenes, allylsilanes, protected alcohols and amines are good partners as 1,1-disubstituted olefins [105-107]. This reaction has been exploited recently as part of a macrocyclization strategy in the epothilone series (Scheme 9.52) [108]. 

Vinylboranes, which have a boron atom bonded to a carbon-carbon double bond, are prepared by hydroboration of an alkyne using catecholborane, a commercially available reagent. Hydroboration adds the elements of H and B in a syn fashion to form a trans vinylborane. With terminal alkynes, hydroboration always places the boron atom on the... 

When you know what functional group you want to create, you can try to remember the various ways it can be synthesized. For example, a ketone can be synthesized by the acid-catalyzed addition of water to an alkyne, hydroboration-oxidation of an alkyne, oxidation of a secondary alcohol, and ozonolysis of an alkene. Notice that ozonolysis decreases the number of carbons in a molecule. 

Also, it is possible to tune and change the properties of the metal center by careful changes to the structure of the corresponding cyclopentadienyl moiety. Various degrees of substitution on the cyclopentadienyl ring may change both electronic as well as steric properties of the metal complex. This provides delicate means to control the properties and behavior of the whole complex and also the environment in the close vicinity of the metal atom [3,4,5]. Both steric as well as electronic properties have a profound effect on the course of the reaction. These effects have been observed, e.g., in carboalumination of alkenes, [2-l-2-l-2]-cyclotrimerization of alkynes, hydroboration, etc. A number of other examples can be found as well. 

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