Terminal alkynes hydroboration

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

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

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... 

Organoboranes undergo transmetallation. 1-Hexenylboronic acid (438) reacts with methyl acrylate via the transmetallation with Pd(OAc)2, giving methyl 2,4-nonadienoate (439)[399], The ( )-alkenylboranes 440, prepared by the hydroboration of terminal alkynes, are converted into the alkylated ( )-alkenes 441 by treatment with an equivalent amount of Pd(OAc)2 and triethylamine[400]. The ( )-octenylborane 442 reacts with CO in MeOH in the... 

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). 

The hydroboration/oxidation sequence is complementary to the direct, mercury(ll)-catalyzed hydration reaction of a terminal alkyne because different products result. Direct hydration with aqueous acid and mercury(IJ) sulfate leads to a methyl ketone, whereas hydroboration/oxidation of the same terminal alkyne leads to an aldehyde. 

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. 

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. 

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. 

A formal trans-hydroboration of terminal alkynes with catecholborane and pinacolborane to yield cis-l-alkenylboronates has also been carried out in the presence of Rh(I) and Ir(I). The dominant factors for reversing the conventional cis-hydroboration to the trans-hydroboration were the use of NEts and the use of bulk phosphines such as P Pr3 and PCy3, and an excess of alkyne in front of the boron reagent [46]. 

As would be expected, catalytic hydroboration is effective for alkynes as well as al-kenes, and prior examples have been reviewed [6]. An interesting development has been the diversion of the normal syn- to the anti-addition pathway for a terminal alkyne, with 99% (catechoborane) and 91% (pinacolborane) respectively (Fig. 2.5) [20]. The new pathway arises when basic alkylphosphines are employed in combination with [Rh(COD)Cl]2 as the catalyst in the presence of Et3N. Current thinking implies that this is driven by the initial addition of the rhodium catalyst into the alkynyl C-H bond, followed by [1,3]-migration of hydride and formal 1,1-addition of B-H to the resulting alkylidene complex. The reaction is general for terminal alkynes. 

Miyaura and co-workers have reported the rhodium- or iridium-catalyzed trans-hydroboration of terminal alkynes, where hydroboration ofthe vinybdene complexes is considered as a key reaction path Ohmura, T., Yamamoto, Y. and Miyaura, N. (2000) Journal of the American Chemical Society, 122, 4990. 

Hydroboration-oxidation of alkynes preparation of aldehydes and ketones Hydroboration-oxidation of terminal alkynes gives syn addition of water across the triple bond. The reaction is regioselective and follows anti-Markovnikov addition. Terminal alkynes are converted to aldehydes, and all other alkynes are converted to ketones. A sterically hindered dialkylborane must be used to prevent the addition of two borane molecules. A vinyl borane is produced with anU-Markovnikov orientation, which is oxidized by basic hydrogen peroxide to an enol. This enol tautomerizes readily to the more stable keto form. 

Laurel Schafer of the University of British Columbia reports (Organic Lett. 2003,5,4733-4736) that terminal alkynes undergo smooth hydroamination with a Ti catalyst. The intermediate imine 4 can be hydrolyzed to the aldehyde 5 or reduced directly to the amine 6. The alkyne to aldehyde conversion has previously been carried out by hydroboration/oxidation (J. Org. Chem. 1996, 61, 3224), hydrosilylation/oxidation (Tetrahedron Lett. 1984,25, 321), or Ru catalysis (J. Am. Chem. Soc. 2001, 123, 11917). There was no previous general procedure for the anti-Markownikov conversion of a terminal alkyne to the amine. 

New mechanistic studies with [Cp2Ti(CO)2] led to the observation that the tita-nocene bis(borane) complex [Cp2Ti(HBcat)2] (Hbcat = catecholborane) generated in situ is the active catalyst.603 It is highly active in the hydroboration of vinylarenes to afford anti-Markovnikov products exclusively, which is in contrast to that of most Rh(I)-catalyzed vinylarene hydroboration. Catecholborane and pinacolborane hydroborate various terminal alkynes in the presence of Rh(I) or Ir(I) complexes in situ generated from [Rh(COD)Cl2] or [Ir(COD)Cl2] and trialkylphosphines.604 The reaction yields (Z)-l-alkenylboron compounds [Eq. (6.107)] that is, anti addition of the B—H bond occurs, which is opposite to results found in catalyzed or uncatalyzed hydroboration of alkynes ... 

Hydroboration of a variety of alkenes and terminal alkynes with catecholborane in the fluorous solvent perfluoromethylcyclohexane was performed using fluorous analogs of the Wilkinson catalyst.135 136 Recycling of a rhodium-based alkene hydrosilylation catalyst was also successful.137 Activated aromatics and naphthalene showed satisfactory reactivity in Friedel-Crafts acylation with acid anhydrides in the presence of Yb tris(perfluoroalkanesulfonyl)methide catalysts.138... 

Step 1 Regioselective hydroboration of the terminal alkyne carbon. Step 2 Suzuki coupling. 

Steps a-c (hydroboration-protonolysis-oxidation) represent a method for the selective reduction of a terminal alkyne in the presence of an alkene. 

Pinacolborane 49 is a highly stable hydroborating agent. It can be easily prepared and stored without decomposition. Pinacolborane 49 reacts with alkenes and alkynes under relatively milder conditions unlike catecholborane 38. Alkenes 50 react slower than alkynes and usually undergo hydroboration in 2-3 days at 50 °C furnishing the terminal pinacol boronates 51 as the major regioisomer (>98%). Hydroboration of terminal alkynes 52 with pinacolborane proceeds at room temperature with an excellent level of regioselectivity to yield the terminal vinyl boronates 53 (Scheme 7). 

The rate of hydroboration with catecholborane and pinacolborane can be tremendously increased by the addition of transition metal catalysts. Hydroboration of pinacolborane 49 with alkenes 50 <1996JA909> and terminal alkynes 52 <19950M3127> proceeds with high regioselectivity in the presence of catalytic HZrCp2Cl furnishing the terminal boronates 51 and vinylboronates 53, respectively (Scheme 8). 

The hydrides 44b have been found to polymerize ethylene and react with a variety of protic reagents such as terminal alkynes and nitriles. Catalytic effects in the hydroboration of olefins have also been observed [27]. A well-defined /i-ethynyl complex of yttrium is formed by protolysis of the alkyl derivative 38b with acetylene (Eq. 18). Figure 14 shows the dimeric structure of 45b with bridging ethynyl ligands [27, 65]. 

BBN hydroborates internal alkynes cleanly (Equation B1.13) and thus for this reaction it is superior to borane which tends to give intractable polymers when added to alkynes. The reaction is less useful for terminal alkynes as monohydroboration can only be achieved if an excess of alkyne is used. 

Bisdiborane, B-C-B, compounds are usually prepared by double hydroboration of terminal alkynes with dialkylboranes.93 95 They are... 

The basicity of Gilman cuprates is so low that they do not undergo acid/base reactions with acetylene or higher terminal alkynes. Instead, Gilman cuprates effect the car-bocupration of the C=C triple bond (Figures 13.12 and 13.13). This reaction formally resembles the hydroboration of a C=C triple bond ( see example in Figure 13.10). The regioselectivity also is the same hence, the metal is connected to the Cl center of a terminal alkyne. Finally, the reaction shows the same stereoselectivity as in the case of the hydroboration of a C=C triple bond carbocupration occurs as a cis addition. 

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