Alkynes, reaction with catecholborane

Benzylic boronic esters [52]can be obtained from bromomethyl boronic esters and aryl or vinyl stannanes [53]. Alkyl and vinyl boronic esters are accessible through hydroboration of alkynes with dialkylboranes followed by oxidation with acetaldehyde [43]. Alternatively, vinylboronates can be obtained directly from alkynes in a reaction -with catecholborane without a subsequent oxidation step [53]. Further diversification in the boronic acid derivatization is achieved by binding the boronic acids bearing another derivatizable functional group (for instance an amine) to suitable polymeric supports. The modified boronic acid may then be... 

Allylamines. Alkeneboronic acids are converted to allylamines of defined configuration on treatment with conventional Mannich reaction components (R2NH, paraformaldehyde). The required vinylboronic acids are available from 1-alkynes by reaction with catecholborane followed by hydrolysis. 

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

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. 

The synthesis of the E-alkenyl derivatives was accomplished by reaction of the corresponding alkynes with catecholborane followed by cross-coupling reaction of this product with the protected 2-iodoNECA. Deprotection in acidic medium gave the desired isomers (route a and b). 

The procedure described here is an example of a general method for preparing conjugated alkadienes by the palladium-catalyzed reaction of 1-alkenylboranes or boronates with vinylic halides. Hydroboratlon of 1-alkynes with catecholborane Is a standard method for obtaining (E)-l-alkenylboronates (1).2 Several different types of alkenylboranes and boronates (2-4) are now available as reagents for the cross-coupling reaction with vinyl halides. 

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

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

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-... 

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 ... 

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). 

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... 

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