Borane reaction with alkynes

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... 

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)." ... 

Similar to the addition of secondary phosphine-borane complexes to alkynes described in Scheme 6.137, the same hydrophosphination agents can also be added to alkenes under broadly similar reaction conditions, leading to alkylarylphosphines (Scheme 6.138) [274], Again, the expected anti-Markovnikov addition products were obtained exclusively. In some cases, the additions also proceeded at room temperature, but required much longer reaction times (2 days). Treatment of the phosphine-borane complexes with a chiral alkene such as (-)-/ -pinene led to chiral cyclohexene derivatives through a radical-initiated ring-opening mechanism. In related work, Ackerman and coworkers described microwave-assisted Lewis acid-mediated inter-molecular hydroamination reactions of norbornene [275]. 

Tanaka and have independently developed effective protocols for the catalytic cyclization/borylsilylation of diynes to form bis(functionalized) dialkylidene cyclopentanes. As an example of the Tanaka procedure, reaction of 1,6-heptadiyne with borylsilane 99 catalyzed by a 1 2 mixture of Pd2(DBA)3 and ETPO (ETPO = 4-ethyl-2,6,7-trioxa-l-phosphabicyclo[2.2.2]octane) at 110°G for 2h gave 100 in 81% yield (Equation (64)). As an example of the Ito procedure, reaction of 1,7-octadiyne with dimethylphenylsilylpinacolborane, catalyzed by the Ni(0) complex generated in situ from a 1 2 2 mixture of Ni(acac)2, DIBAL-H, and P(/ -Bu)3 in toluene at 110°C, gave the bis(functionalized) dialkylidene cyclohexane 101 in 55% yield (Equation (65)). " Tanaka s protocol was also effective for the cyclization/borylsilylation of 1,6-enynes. As an example, reaction of dimethyl allylpropargylmalonate with 99 catalyzed by Pd2(DBA)3/ETPO gave the bis(functionalized) alkylidene cyclopentane 102 in 84% yield with exclusive delivery of the borane to the alkyne moiety of the enyne (Equation (66)). ... 

Alkynylboration has been achieved in the reaction of alkynyl(pinacol)boranes with alkynes in the presence of nickel catalysts, giving d.v-1 -borylbul-1 -en-3-yne derivatives. 1-Arylalk-l-ynes underwent the alkynylboration regioselectively with the selective introduction of the alkynyl groups at their 1-positions, where the aryl groups were attached.45... 

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) ... 

Hydroboration-Oxidation In Section 8-7 we saw that hydroboration-oxidation adds water across the double bonds of alkenes with anti-Markovnikov orientation. A similar reaction takes place with alkynes, except that a hindered dialkylborane must be used to prevent addition of two molecules of borane across the triple bond. Di(second-ary isoamyl)borane, called disiamylborane, adds to the triple bond only once to give a vinylborane. (Amyl is an older common name for pentyl.) In a terminal alkyne, the boron atom bonds to the terminal carbon atom. 

The larger nido and arachno cages are usually obtained via degradation (in one or more steps) of icosahedral carboranes, as previously mentioned, but the smaller members are typically prepared in reactions of alkynes with neutral boranes, as in the synthesis of 2,3-RR C2B4H8 (Fig. 5-14). However, the nido-2,4-QBJHf carbons apart ion can be obtained by removal of an apex boron from closo-2,4-QB5B7 (Fig. 5-10d).8... 

An important method for the synthesis of carbaboranes is the reaction of a borane cluster with an alkyne. However, an analogous reaction of a metallaborane cluster with an alkyne is not commonly used to prepare metallacarbaborane clusters. Equation (21) illustrates one example. 

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 . 

Tris(methylseleno)borane and tris(phenylseleno)borane react with terminal alkynes in the presence of oxygen or in the presence of AIBN as radical initiator [97,98]. The mechanism of this reaction is not completely elucidated, however, the generation of phenylselenyl radical intermediates is assumed by the authors. When applied to enynes, this reaction furnish cyclic compounds. An approach to the frameworks of kainoids is depicted in Eq. (39). Treatment of the enyne... 

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). ... 

Conjugated enynes are of importance for themselves, as well as for the synthesis of conjugated dienes. The cross-coupling reaction of 1-alkenyl(disiamyl)boranes (3c) with 1-bromo-l-alkynes (Scheme 2-34) provides conjugated enynes in high yields [45]. 

Internal alkynes induce olefin loss from alkylboranes to form alkenyl boranes. Thus, (C HsjaB reacts rapidly and reversibly with alkynes at 160-200°C to form trial-kenylborane and C2H4. With (r-C4H9)3B, the reaction proceeds under milder conditions. It is not known whether such reactions proceed via B-H intermediates produced by -hydride elimination, or by intramolecular hydride transfer from the alkyl group to a coordinated acetylene. This reaction is useful for reduction of acetylenic hydrocarbons. 

Borane adds to alkynes in the same way it adds to alkenes. That is, one mole of BH3 reacts with three moles of alkyne to form one mole of boron-substituted alkene (Section 4.9). When the addition reaction is over, aqueous sodium hydroxide and hydrogen peroxide are added to the reaction mixture. The end result, as in the case of alkenes, is replacement of the boron by an OH group. The enol product immediately rearranges to a ketone. 

---