Boranes, reaction with alkynes

Terminal alkynes also generally give complicated mixtures of products in reactions with borane-THF.However, internal alkynes react in a more controlled manner and it is possible to obtain reasonable yields of the corresponding (Z)-trialkenylboranes from such reactions (equation 10). ... 

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-... 

Both alkynes and alkenes can be obtained from adducts of terminal alkynes and boranes. Reaction with iodine induces migration and results in the formation of the alkylated alkyne.32... 

Synthesis of 12- and 13-membered sulfur-containing lactones by homolytic macrocyclization of mercaptoacetic esters and alkynes has been investigated [95S307]. Reaction of the mercaptoester 245 with alkynes using triethyl borane radical initiation gave the macrolactones 247 and 248 in low yield [95TL2293], Remote asymmetric induction is observed during the cyclization. 

If the hydroboration reaction is to be used to convert 1-alkynes into aldehydes, some way to stop the addition at the vinylborane stage is needed. The problem is that there is not enough steric hindrance at the end carbon of the vinylborane. The solution is to build extra steric hindrance into the other alkyl groups attached to the boron of the vinylborane. A borane, R2BH, with two bulky R groups already attached to the boron is used as the hydroboration reagent. One such reagent is prepared by the reaction of two equivalents of 2-methyl-2-butene (also known by the common name of isoamylene) with borane to produce a dialkylborane called di si amyl borane (a shortened version of diisoamylborane) ... 

The stereoselective synthesis of the title compounds has been achieved. Thexylborane on reaction with 2 molar equivalents of 1-iodo-l-alkyne at 0 °C proceeds to near completion (88% for 1-iodo-l-hexyne) to form fully substituted organoborane (33), which upon treatment with 2 molar equivalents of sodium methoxide at 0 °C readily produces frunx-1,2,3-butatrienes (Eq. 102) The same reaction, however, with either 1-chloro or 1-bromo-l-alkynes is sluggish to form the thexyl-l-halo-l-alkenyl-borane . 

Dichloroborane and monochloroborane etherates or their methyl sulfide complexes have been prepared by the reaction of borane and boron trichloride [44]. The hydroboration of alkenes with these borane reagents is, however, usually very slow because of the slow dissociation of the complex. Dichloroborane prepared in pentane from BCI3 and trimethylsilane is unusually highly reactive with alkenes and alkynes hydroboration is instantaneous at -78 °C (Eq. 20) [45]. 

If alkyldichloroboranes are specifically required, dichloroborane-dimethyl sulfide is the reagent of choice. It is more stable and more convenient than the dichloroborane-diethyl ether complex, but its hydroborating properties are very similar." Dichloroborane complexes ethers even more strongly than monochloroborane, and its reactions with alkenes in this solvent are slow and lead to mixtures. Therefore, it is generally used in pentane and trichloroborane is added to liberate uncomplexed dichloroborane. Under these conditions it readily gives alkyldichloroboranes on reaction with alkenes or alkenyldichloro-boranes on reaction with alkynes. " " The latter reaction has been applied to alkynylsilanes (equation 44)." ... 

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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