Catecholborane with alkynes

During the reaction of pinacol/catecholborane with alkynes in the presence of catalysts such as [Rh(COD)Cl]2[PCPr)3]4 and [Ir(COD)Cl]2[P( Pr)3]4, Miyauraand coworkers noticed the formation of (Z)-boronates as opposed to the expected (E)-isomer as shown in Scheme 28.16. ... 

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... 

The organoborane used in a Suzuki reaction is prepared by the reaction of catecholborane with an alkene or an alkyne. 

Catecholborane (34) reacts with alkynes but requires reaction temperatures that are higher than those used for reaction of alkynes and alkylboranes, as noted earlier. Phenylethyne, for example, reacts with 34 to give excellent yields of vinyl borane, but the reaction must be done in refluxing THF. Alkenylborinates derived from catecholborane are stable and can be isolated by distillation or recrystallization,... 

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-... 

Hydroboration of alkenes or alkynes followed by cross-coupling with organic electrophiles provides a straightforward method for the carbon-carbon bond formation (Scheme 1-19). The hydroboration of thioalkynes with catecholborane in the presence of a nickel or palladium catalyst yields P-(aLkylthio)-l-alkenylboronates (72a)... 

Alkenylmercury compounds can be prepared by hydroboration of an alkyne with catecholborane, followed by reaction with mercuric acetate.185... 

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]. 

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 ... 

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... 

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). 

Fortunately, though, there is one borane, catecholborane (A in Figure 16.15), that adds to C=C triple bonds but not to C=C double bonds (in the absence of transition metal catalysts). This reagent adds to alkynes with cw-selectivity, so that the reaction stops at the stage of trans-alkenylboronic acid esters (B in Figure 16.15). 

Alkenylboronic acids and esters have been prepared by thermal or catalyzed hydroboration of 1-alkynes with catecholborane (HBcat), pinacolborane (HBpin), or dihaloboranes 41-43, followed by hydrolysis to boronic acids or alcoholysis to boronic esters. A convenient alternative to improve chemo- and regioselectivity is the hydroboration of alkynes with dialkylboranes. For selective removal of dummy groups, the oxidation of two cyclohexyl groups was conduced by treatment of l-alkenyl(dicyclohexyl)borane intermediates with Me3N-0 (Equation (7)).116 The... 

On the other hand, it is well known that stereodefined 1-alkyenyldialkylboranes are readily prepared by the monohydroboration of alkynes, i.e. dialkylboranes such as disiamylborane and catecholborane permit the monohydroboration of terminal alkynes, thus making readily available the corresponding (E)-l-alkenyldialkyl-boranes with high stereoselectivity, more than 99% (Eq. 97) . Highly pure (Z)-l-alkenyldialkylboranes (purity, more than 98%) are prepared without any difficulty via the monohydroboration of 1-halo-1-alkynes with disiamylborane or dicyclohexylborane, followed by treatment with t-butyllithium (Eq. 98) Consequently, if such 1-alkenyldialkylboranes react with 1-alkenyl halides or 1-alkynyl... 

Catecholborane is somewhat more reactive than (1), though still far less reactive than dialkylbo-ranes. 27 28 It is readily prepared by the reaction of catechol with borane-THF, is stable at 0 "C, and hydroborates alkenes slowly and alkynes (equation 49) more rapidly in refluxing The rate of... 

---