Alkynes catecholborane addition

Triple bonds can be monohydroborated to give vinylic boranes, which can be reduced with carboxylic acids to cis alkenes or oxidized and hydrolyzed to aldehydes or ketones. Terminal alkynes give aldehydes by this method, in contrast to the mercuric or acid-catalyzed addition of water discussed at 15-4. However, terminal alkynes give vinylic boranes (and hence aldehydes) only when treated with a hindered borane such as 47, 48, or catecholborane (p. 798)," or with BHBr2—SMe2. The reaction between terminal alkynes and BH3 produces 1,1-... 

The dominant factors reversing the conventional ds-hydroboration to the trans-hydroboration are the use of alkyne in excess of catecholborane or pinacolborane and the presence of more than 1 equiv. of EtsN. The P-hydrogen in the ris-product unexpectedly does not derive from the borane reagents because a deuterium label at the terminal carbon selectively migrates to the P-carbon (Scheme 1-5). A vinylidene complex (17) [45] generated by the oxidative addition of the terminal C-H bond to the catalyst is proposed as a key intermediate of the formal trans-hydroboration. 

One most important observation for the mechanistic discussion is the oxidative addition/insertion/reductive elimination processes of the iridium complex (31) (Scheme 1-10) [62]. The oxidative addition of catecholborane yields an octahedral iridium-boryl complex (32) which allows the anti-Markovnikov insertion of alkyne into the H-Ir bond giving a l-alkenyliridium(III) intermediate (34). The electron-... 

It took another decade however before the idea of developing a rhodium-catalyzed olefin hydroboration process came to fruition. This occurred in 1985 when Mannig and Noth reported the first examples of such a process.8 They discovered that Wilkinson s catalyst 2 was effective for the addition of catecholborane 1 to a range of alkenes and alkynes, as exemplified by cyclopentene 4 (Scheme 2). 

Alkenylcatecholborane 11 is a good reagent for the conjugate addition and is easily obtained by the hydroboration of an alkyne with catecholborane. One-pot asymmetric synthesis of the conjugate addition product, /9-alkenyl ketone, is possible starting from an alkyne and catecholborane without isolation of the alkenylcatecholborane [12]. 

Alkynes are reactive toward hydroboration reagents. The most useful procedures involve addition of a disubstituted borane to the alkyne. This avoids the complications which occur with borane that lead to polymeric structures. Catecholborane is a particularly useful reagent for hydroboration of alkynes.168 Protonolysis of the adduct with acetic acid results in reduction of the alkyne to the corresponding Z-alkene. Oxidative workup with hydrogen peroxide gives ketones via enol intermediates. 

This reaction occurs rapidly at room temperature using a small excess of alkyne and either pinacol- (HBpin) or catecholborane (HBcat). When an excess of borane was used, the Z/E ratio of the products was slowly eroded, eventually attaining a thermodynamic distribution of isomers. Equilibration presumably occurs via addition/elimination of excess Rh-H. Miyaura s method provides a useful synthetic complement to knovm cis-hydroboration methods. Under optimized conditions, good yields and high stereoselectivity (>90 10) were achieved for a variety of alkenylboronates (Table 9.8). The best selectivities were generally obtained with the use of catecholborane and Et3N as an additive. As in related reactions, the presence of base seems to suppress undesired reaction pathways. 

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 ... 

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). 

Although the B-H addition of borane etherates to simple alkenes occurs quite rapidly under ambient conditions, less electrophilic boranes are correspondingly less reactive. A classic example is given by catecholborane which reacts with alkynes only above 70 °C and with alkenes under somewhat more forcing conditions [1,85]. The reason for this is not hard to find, since the electrophilic character of the borane is substantially diminished by conjugation between boron and adjacent oxygens [86,87]. At the same time, the acidity of B-H is enhanced through an... 

Addition to multiple bonds. Hydroboration of 1-alkynes with catecholborane (when /-Pr3P and EtjN are also present) gives (Z)-l-alkenylboronates. On the other hand, dehydrogenative coupling between styrenes and pinacolborane is observed. 

Halogenation of vinylboranes generates vinyl halides. Both cis and trans halides are available by modification of the reaction conditions. When trans-alkenyl boronic acid (73, derived from 1-octyne) was treated with iodine and sodium hydroxide, trans-1-iodo-l-octene (74) was formed in 90%. When the boronic acid was treated with iodine and then with base, the (Z)-alkenyl iodide (75) was produced.Vinylboranes derived from internal alkynes lead to cis-trans mixtures with both of these procedures. Boronic acids derived from alkynes and catecholborane give the (Z)-bromide on addition of bromine followed by sodium hydroxide. ... 

The hydroboration of olefins is a classic reaction in organic synthesis. - Dialkylbo-ranes add rapidly to alkenes in the absence of catalyst. However, dialkoxyboranes, such as catecholborane and pinacolborane, add more slowly to olefins and alkynes. Thus, transition metal complexes could catalyze the addition of dialkoxyboranes to olefins and alkynes without interference from the background reaction. The potential to alter chemoselectivity, regioselectivity, enantioselectivity, and diastereoselectivity has led a munber of groups to develop metal-catalyzed versions of hydroboration. " Enantioselective hydroboration would alleviate the need to use boranes containing stoichiometric amounts of chiral substituents to generate optically active alkylboranes. 

Like other hydroborations, the reaction of alkynes with catecholborane is a syn addition and its regioselectivity is opposite to Markovnikov s rule. 

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