1-hexyne-, lithium

The product i n this case is a cis-disubstituted alkene, so the fi rst question is, " What is an immediate precursor of a cis-disubstituted alkene " We know that an alkene can be prepared from an alkyne by reduction and that the right choice of experimental conditions will allow us to prepare either a trans-disubstituted alkene (using lithium in liquid ammonia) ora cis-disubstituted alkene (using catalytic hydrogenation over the Lindlar catalyst). Thus, reduction of 2-hexyne by catalytic hydrogenation using the Lindlar catalyst should yield cis-2-hexene. 

The /3-trimethyltin-substituted alkenylborane 36 was prepared in situ by treatment of triethylborane with 1-lithio-l-hexyne to form the lithium 1-alkynyltriethyl-borate 35 followed by trimethyltin chloride (Scheme 20.9) [37]. The trimethyltin chloride-induced migration of an ethyl group from boron to the adjacent acetylenic carbon is stereoselective with the boron and the tin substituents in 36 cis to each... 

Another hydride, magnesium hydride prepared in situ from lithium aluminum hydride and diethylmagnesium, reduced terminal alkynes to 1-alkenes in 78-98% yields in the presence of cuprous iodide or cuprous r rt-butoxide, and 2-hexyne to pure cij-2-hexene in 80-81% yields [///]. Reduction of alkynes by lithium aluminum hydride in the presence of transition metals gave alkenes with small amounts of alkanes. Internal acetylenes were reduced predominantly but not exclusively to cis alkenes [377,378]. 

The kinetics of the cyclization reaction of 5-hexynyllithiums 19 was also studied showing that (6-phenyl-5-hexynyl)lithium (19, R = Ph) has a half-life of ca 6 min at —50.6 °C, whereas cyclization reaction of 5-decynyllithium (19, R = Bu) is some 106 times slower. The rather dramatic increase in the rate of cyclization on going from an alkyl-substituted 5-hexyn-l-yllithium to a phenyl-substituted substrate is most likely a consequence of a reduction in A// due to stabilization of the incipient vinyllithium product by the phenyl group19. It is also interesting to note that the 5-exo-dig cyclization reaction of 19 (R = Bu) is slower than the corresponding 5-exo-trig carbolithiation reaction of 5-hexenyllithium. 

Cis dimethylation of diphenylacetylene is reported for a stoichiometric magnesium dimethylcuprate reagent in THF (186). Lithium dimethylcuprate reacted with 1-hexyne in ether, but the products were not identified (241). 

The lithium organosilylcuprate 265 adds to hex-3-yne to yield a copper derivative 266, which can be converted into a variety of alkenylsilanes by treatment with electrophiles (e.g. equation 38). 1-Hexyne forms the analogue 267 regioselectively . 

In this way trans-hept-2-ene nitrile was prepared in 87% yield from 1-hexyne. Cis-aj8-unsaturated nitriles may be prepared by use of lithium di-isobutylmethyl-aluminium hydride which is prepared from di-isobutyl-aluminium hydride and methyl lithium in monoglyme. This reagent adds trans to alkynes ... 

The product of the reaction between the l-bromo-3-hexyne and lithium dieth-ylcuprate is an alkyne. Draw it. Is there anything in the IR that would allow one to distinguish the starting alkyne and the product alkyne How about the proton NMR How about information that can be obtained from the mass spectrum ... 

Hydroboration of 1-hexyne with dicyclohexylborane produced the alkenylborane 55, which on treatment with the lithium acetylide 56 furnished the organoborate complex 57 (Scheme 14) (24). Trimethyltin chloride then induced a stereoselective migration of the 1-hexenyl group to the adjacent acetylenic carbon to afford the diene-allene 58 and subsequently the o-isotoluenes 59 and 60. A variety of other o-isotoluenes were likewise synthesized. 

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