Alkynes with boranes

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... 

Hydroboration of 1-bromo-l-alkynes with chloro(thexyl)-borane leads to the synthesis of alkynyl ketones in 61-63% yields by the sequence of reactions shown in eq 5. Sequential treatment of alkenylchloro(thexyl)boranes, which are formed by the reaction of chloro(thexyl)borane with alkynes, with lithium chloro-propargylide and aldehydes affords 1,3-enynols or 1,2,4-trienols depending on the reaction conditions. ... 

A route to non-racemic P-stereogenic vinylphosphine-boranes, e.g., (100), is afforded by the addition of methylphenylphosphine-borane with alkynes in the presence of a chiral diphosphine-palladium catalyst. The ehiral diphosphine-platinum complex-catalysed addition of diethylphosphine to the diene cis,cis-muco-nitrile has given the new diphosphine (101) as a 3 2 mixture of diastereoisomers. Further work has been reported on the use of cyclopentadienyliron complexes that act as metal templates for the intramolecular hydrophosphination of coordinated vinylphosphines with 1,2-diphosphino-alkanes and -benzenes, leading to 1,4,7-triphosphacyclononanes, e.g., (102), capable of further elaboration to form new cyclic phosphines. 

Diborane or aUcylboranes are used for reduaion of alkenes and alkynes via hydrobora-tion (see pp. 37f., 47f., 130f.) followed by hydrolysis of the borane with acetic acid (H.C. Brown, 1975). 

Palladium-catalyzed coupling of the borane derived from A with 1-bromo-1-pentyne to give B as well as the coupling of iodoalkene C with alkyne D were the two key-steps. 

In particular, iminoboranes (XBNR) are isoelectronic with alkynes (XCCR). Well-known comparable pairs of isoelectronic species are aminoboranes (X2BNR2) and alkenes (X2CCR2), amine-boranes (X3BNR3) and alkanes (X3CCR3), borazines [(XBNR)3] and benzenes [(XCCR)3], etc. The structure of aminoboranes, amine-boranes, and borazines is well known from many examples. It has turned out that these BN species are not only isoelectronic, but also have structures comparable with the corresponding CC species. In the case of borazines, the aromatic character was widely discussed on the basis of theoretical and experimental arguments. The structural and physical properties of... 

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. 

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... 

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. 

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... 

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)." ... 

Carbon has one more electron than boron, so the C—H moiety is isoelectronic with the B—H or BH2 moieties. Note that an isoelectronic relationship also exists between C and BH or B . In a formal sense it should be possible to replace a boron atom in a borane with a carbon atom (with an increase of one in positive charge) and retain an isoelectronic system. The best-studied system. CiBmHij, is isoelectronic with [BijHijJ" and may be synthesized readily from decaborane and alkynes and diethyl. sulfide as solvent. 

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. 

Alkynes monohydroborate with H2ClB OEt2 to give symmetrical dialkenylchloro-boranes. Internal alkynes react in a stoichiometric ratio, whereas 30-40 % xs of the terminal alkyne must be used to avoid competing dihydroboration ... 

Mixtures of alkynes, cyclopentadiene and boranes with Co atoms yield metallo-carborane clusters in small amounts e.g., with 2-butyne and BjHg ... 

Carbonylation An enone that bears chirality at both a and 3 carbon atoms can be synthesized through successive reactions of a chiral borane with an alkene and an alkyne, followed by exposure to acetaldehyde. Further treatment of the resulting alkenylborane with dichloromethyl methyl ether and EtjCOLi, and finally with HjO affords the product. 

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