Hydroboration-protonation

Alkynes are also reduced to ds-alkenes by the hydroboration-protonation method.  

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Z-151, and hydroboration/protonation affords -cycloalkene. In 2002, Trost reported an alternative procedure for the synthesis of the -olefin from an alkyne by hydrosihlation... 

RCM of dienes to cycloalkenes provides a useful method for the syntheses of carbo- and heterocycles and thus has been proved to be extremely effective in total synthesis of various natural products. Usually, however, mixtures of (E)- and (Z)-olefms result. In contrast, ring-closing alkyne metathesis provides a reliable route for synthesis of both (E)- and (Z)-macrocycloalkenes in a stereoslective manner taking advantage of stereoselective partial reduction of resulting cycloalkynes. A Lindlar reduction gives (Z)-cycloalkenes, whereas a hydroboration/ protonation sequence afford ( )-cycloalkenes (Equation (23)). Recently, Trost reported an alternative procedure for the synthesis of (E)-olefins from alkynes through hydrosilylation by a ruthenium catalyst. This procedure converts cycloalkyne 130, for example, to vinylsilane 131 and then to (E)-cycloalkene 132 in a stereoselective manner (Scheme 46)7 ... 

Jung, M.E. and Yoo, D. (2008) Synthesis of the C]-Ci2 fragment of the tedanolides. Selective hydroboration-protonation of allylic alcohol approach. Tetrahedron Lett., 49,816-819. 

A synthetically useful virtue of enol triflates is that they are amenable to palladium-catalyzed carbon-carbon bond-forming reactions under mild conditions. When a solution of enol triflate 21 and tetrakis(triphenylphosphine)palladium(o) in benzene is treated with a mixture of terminal alkyne 17, n-propylamine, and cuprous iodide,17 intermediate 22 is formed in 76-84% yield. Although a partial hydrogenation of the alkyne in 22 could conceivably secure the formation of the cis C1-C2 olefin, a chemoselective hydrobora-tion/protonation sequence was found to be a much more reliable and suitable alternative. Thus, sequential hydroboration of the alkyne 22 with dicyclohexylborane, protonolysis, oxidative workup, and hydrolysis of the oxabicyclo[2.2.2]octyl ester protecting group gives dienic carboxylic acid 15 in a yield of 86% from 22. 

Replacement of the chloro substituents and hydroboration of the benzoannelated diborafulvene 21 with LiBH4 leads to an anionic product, which after protonation affords the l-carbapentaborane(lO) 22 in 36% yield (Scheme 3.2-12) [34],... 

Scheme 3.2-12. Exchange of chlorine for hydrogen with LiBH4 in the diborabenzofulvene 21 followed by hydroboration and coordination with BH3 as well as protonation to give 1-carba-arachno-pentaborane(lO) 22.

In the hydroboration of terminal alkenes, carrying a ketone or aldehyde group, with a variety of borane reagents, dicyclohexylborane has been identified as the most efficient reagent. Analogous hydroboration of alkynyl ketones and alkynyl aldehydes with dicyclohexylborane yields the corresponding olefinic carbonyl compounds after protonation, or dicarbonyl compounds after oxidation. ... 

The dynamism of these systems is further illustrated in the chemistry of the adduct of vinyl acetate (Entry 12), where reversible coordination of the C=0 function dominates, but competes with electrophilic attack of the vinyl group of the ester moiety (Scheme 18).132 Irreversible proton transfer from the OH tautomer of this minor olefin adduct results in the expulsion of CeFsH and formation of the internally coordinated adduct shown as the eventual thermodynamic product in the reaction. A related chelating RC(=0)0R species results from the hydroboration of H2C=C(H)CH2CH2C(=0)0Et using HB(C6F5)2.30... 

To avoid isolating the glycal, the reaction was quenched with an excess of borane in THF and, upon oxidative work-up under basic conditions, C-disaccharide 38 was obtained in 64% yield over two steps (Scheme 11) (77). The stereochemistry of the hydroboration step was verified on the corresponding acetate by examination of the Jy%T and J2 y coupling constants in the proton NMR spectra. 

There are several ways to perform the cis reduction of alkynes. For example, catalytic hydrogenation using a poisoned catalyst and hydroboration followed by protonation yields ds-alkenes. For examples and mechanism, see Chapter 6, section 6.2. 

The BH3 adduct of 2-aminopyridine seems to be very stable to internal hydroboration and polymerisation compared to borane pyridine. We attribute the extra stability to hydride-proton interactions these together with those of fluorine-proton were reported for other amine borane compounds16 (Figure 5). The MM models for the two boron complexes shown in Figure 6, present atomic distances... 

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