Hydroboration of terminal alkynes

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... 

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]. 

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. 

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). 

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

The reactivity and selectivity of cycloaddition can be considerably increased in intramolecular versions. The protocol was first demonstrated in the Diels-Alder reaction between anthrone 253416,417 and 4-hydroxy-2-butenoate mediated by phenylboronic acid (Equation (73)).418,419 Another method developed for the intramolecular cycloaddition is the synthesis of trienylboranes 256 by hydroboration of terminal alkynes (Equation (74)).419-422... 

The synthesis of 9-alkenyl-9-BBN via hydroboration of terminal alkynes with 9-BBN suffered from the formation of 1,1-diborylalkanes via dihydroboration along with desired monohydroboration products. Alternatively, selective monohydroboration of terminal alkynes with dicyclohexylborane was followed by transmetallation with 9-MeO-9-BBN (Equation (193)).710 Treatment of the hydroboration intermediates with DIBAL-H in the presence of a borane-trapping reagent such as 1-hexene gave 1-alkenylaluminum compounds with complete retention of the (E)-stereochemistry (Equation (194)).711... 

A contraction for secondary isoamyl, abbreviated Sia. This is the 1,2-dimethylpropyl group. Disiamylborane is used for hydroboration of terminal alkynes because this bulky borane adds only once to the triple bond. (p. 412)... 

A )-and (Z)-alkenylboranes, or the corresponding boronic esters or potassium tri-fluoroborates, are conveniently available via hydroboration of terminal alkynes or 1-halo-1-alkynes using either disiamylborane, dicyclohexylborane, dibromoborane dimethyl sulfide, or catecholborane as hydroborating agents. ... 

Hydroboration is especially valuable for the synthesis of stereodefined 1- alkenyl-boronic acids. A general method is the hydroboration of terminal alkynes with cate-cholborane 7 [16, 17] (eq (14)). The reaction is generally carried out at 70 without solvent, but it is very slow in THF solvent. More recent results demonstrate that the hydroboration of alkenes or alkynes with catecholborane is strongly accelerated in the presence of palladium [18]. rhodium [19]. or nickel catalysts [20], thus allowing the reaction to proceed below room temperature. 

Hydroboration of terminal alkynes is complete within 1 h. but internal alkynes require approximately 4 h at 70 °C. Although the reaction is generally carried out without solvent, a similar result can be obtained in benzene, but it is very slow in THF solution. 

Diborylated organoboranes with a BCB backbone such as (17) are usually prepared by double hydroboration of terminal alkynes (equation 29). The treatment of the diborane B2pin2 with diazomethane derivatives provides an interesting alternative that allows for the synthesis of species pinB-CR2-Bpin (18, R=H, Ph equation 30), which are not accessible via hydroboration reactions. ... 

Hydroboration of terminal alkynes, e.g. 1 -hexyne, 1 -octyne or cyclohexylacetylene, with a dialkylborane, such as bis(l, 2-dimethylpropyl)borane, followed by copper(I)cyanide and copper(II) acetate in HMPA containing a trace of water, gives isomerically pure ( )-l-cyanoalk-l-enes (equation 29)133. Successive treatment of 1-bromo-l-alkynes with dialkylboranes and sodium methoxide results in the borinate esters 208, which are converted into ( )-alkenes of greater than 99% isomeric purity by protonolysis. The action of alkaline hydrogen peroxide on the borinates produces ketones (equation 30)134. 

Recently Miyaura and co-workers have reported a trans-hydroboration of terminal alkynes using [Rh(COD)Cl]2[PCPr)3]4 or [Ir(COD)Cl]2[P( Pr)3]4 (eq 13). Mechanistic studies via deuteriumlabeling show that after the oxidative addition of the alkyne to the metal, the acetylenic deuterium undergoes migration to the S-carbon resulting in the formation of a vinylidene metal complex. Oxidative addition of borane to the metal complex and 1,2-... 

Regiocontrolled net hydroboration of terminal alkynes can also be realized due to the nature of NHC-Cu complex employed. Thus, with an iV-arylated NHC (e.g., IMes or IPr), a-vinylboronates are formed, while with a bulky 7V-alkyI NHC-Cu, the p-isomer prevails. Substrates such as propargyl ethers, amines, and a variety of aryl-substituted alkynes have all been studied. ... 

Aldehydes have been converted to functionalized homoallylic alcohols in high de, using 1-alkenylboronates, acting as synthetic equivalents of y-substituted allyl-boronates. The 1-alkenylboronates are readily prepared by hydroboration of terminal alkynes. ... 

Table 5.23 Hydroboration of terminal alkynes with 9-BBN under neat conditions at 25 °C [14]...

Alkenylboronates can be converted into functionalized allylboronates such as 42 by way of a Matteson homologation (Equation 24) [63-65]. This strategy is the reverse of the addition of alkenylmetal intermediates to halomethylboronates described in Section 6.2.1.2. It is an attractive method because the requisite alkenylboronates are readily prepared by the hydroboration of terminal alkynes. Differentially substituted 3,3-dialkyl allylboronates, a class of reagents that would be difficult to access otherwise, were synthesized using this route [66]. The use of Cl2CHLi as reagent in this method... 

Scheme 15.34 Ligand-controlled regioselective hydroboration of terminal alkynes.

ObUgadon TV, Neely JM, Yazdani AN, Pappas I, Chirik PJ (2015) Cobalt cateilyzed Z-selective hydroboration of terminal alkynes and elucidation of the origin of selectivity. J Am Chem Soc 137(18) 5855-5858. doi 10.1021/jacs.5b00936... 

Finally, further developments of this methodology led to the report of a double hydroboration of terminal alkynes, yielding vicinal diboronates. The main features of this reaction are the use of a diboron reagent, instead of a borane, and its high chemo-, regio- and enantioselectivity (Equation (11.5)). ... 

Hydroboration of terminal alkynes catalyzed by NHC-Rh complexes 125 was found to be influenced by the electronic nature of the substituents on the NHC backbone, 7i-withdrawing groups affording even lower yields than... 

The transformation of vinylboron halides, obtained from the hydroboration of terminal alkynes using haloborane complexes, to air- and water-stable trifluoroborates is known in the literature. Recently, the conversion of halovinylboron dihalides into the corresponding bromovinylboronate esters and trifluoroborate derivatives has also been reported. Good yields are obtained in both cases (Scheme 23.43). 

Hydroboration of terminal alkynes 13 with 9-BBN typically leads to diborylated product 15. However, the vinylboranes 14 can be easily regenerated by reacting 15 with an aldehyde (Scheme 28.4). ° Internal alkynes 16 undergo clean hydroboration with 9-BBN and furnish the vinylboranes 17a from the less hindered side of the alkyne (Scheme 28.5). ... 

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