Hexene from 3-hexyne

Hydrogenation of 2,5-diacetoxy-2,5-dimethyl-3-hexyne 10 over 0% palladium-on-carbon is exceptionally complex. Seven different products are formed together with acetic acid. All are hydrogenolysis products arising from the initially formed 2,5-diacetoxy-2,5-dimethyl-3-hexene 11. One of these, 2,5-dimethyl-2-acetoxy-4-hexene 12 forms in as much as 4S yield. 

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. 

What kinds of information can we get from a mass spectrum Certainly the most obvious information is the molecular weight, which in itself can be invaluable. For example, if we were given samples of hexane (MW = 86), 1-hexene (MW = 84), and 1-hexyne (MW = 82), mass spectrometry would easily distinguish them. 

In contrast to the behavior of 3-hexyne in trifluoroacetic acid, addition of HCl in acetic acid yields essentially rra s-3-chloro-3-hexene (48%) and 3-hexanone (52%) as products, with less than 1% of the cis chloride (31,42,43). The 3-hexanone has been shown to arise from an intermediate vinyl acetate. The kinetics are complicated, but they seem to be of first order in substrate and second order in HCl. Added tetramethylammonium chloride increases the rate of product formation and changes the product composition to >95% trans-3-chloro-3-hexene and <5% 3-hexanone. A termolecular electrophilic addition via an intermediate such as 14 has been proposed (31,42) to account for these data. 

Dinuclear palladium complexes catalyze m-hydroarylation of alkynes with arenes.56 The reaction of 3-hexyne with benzene in the presence of a dinulear palladium complex Pd2R2(M-OH)(//-dpfam) [dpfam = j/V,Ar -bis[2-(diphenyl-phosphino)phenyl]formamidinate, R=/>-Tol] and tri(/z-butyl)borane at 100 °C for 4h affords ( )-3-phenyl-3-hexene quantitatively (Equation (53)). The hydroarylation of 3-hexyne with monosubstituted benzenes ( )-3-aryl-3-hexenes with a 2 1 ratio of the meta- and ra -isomers. This regioselectivity is different from that of the hydroarylation of diphenylacetylene catalyzed by Rh4(GO)12.57... 

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]. 

An independent proof of this finding was recently obtained by Ciardelli, Benedetti, Pieroni and Pino (23, 24) who prepared an atactic poly-(S)-4-xnethyl-1-hexene having [M] >5 = +190 from a poly-(S)-4-methyl-l-hexyne having an optical purity of about 89.5% as demonstrated by the optical purity of (S)-3-methyl-pentanoic acid obtained by ozonization of the unsaturated polymer. The rotatory power of the polymer thus prepared is very near to the rotatory power of the non crystallizable poly-(S)-4-methyl-l-hexene obtained from a monomer having a 93% optical purity (see Table 8). 

Also from this study the difference in the rate of bromination of alkenes and alkynes is quite evident. Thus the bimolecular coefficient (k2) for styrene is 2 x 103 times that for phenylacetylene and k2 for 3-hexene is 1-4 x 105 times the value for 3-hexyne. Since the first two compounds are believed to react via carbonium ions and the latter two via bromonium ions there seems to be an extra factor of 102 in the stability of bromonium ions from alkenes relative to those from alkynes. 

The hydrosilation of 1-alkynes can give either Z-or E-isomers (see (E) (Z) Isomers) of the linear vinylsilane or the Markownikow product (equation 5). Reaction of 1-hexyne with trichlorosilane under platinum catalysis conditions gives a mixture of 1- and 2-silyl-l-hexene (78 22). From the E-stereochemistry of the 1-silyl isomer, a yyn-addition mechanism can be assumed. 

Silver(I) compounds are often used to generate cationic metal complexes from the corresponding metal halides. Suzuki and coworkers found that -hexylzirconocene chloride (61), derived from 1-hexene and Schwartz reagent 60, can react with aldehydes in the presence of a catalytic amount of AgAsFs to give secondary alcohols [27]. The reaction with hydrocinnamaldehyde, for example, provides the alcohol 62 in 95 % yield (Sch. 14). Allylic alcohols are also obtainable by a similar procedure using 1-hexyne as a starting material. 

Hexyne is best suited to study activity and selectivity of a hydrogenation catalyst. A mixture of 1,10-phenanthroline-stabilized seven- and eight-shell Pd clusters (Pd 7/8) on Ti02, dispersed in ethanol, catalyzes the transformation of 2-hexyne to cA-2-hexene with ca. 95 % selectivity and a turnover frequency (TOF) of 35 min . As can be seen from Figure 2, tranx-2-hexene, n-hexane, and other hexene isomers are only formed in consecutive reactions. If the phenan-... 

The nickel-catalyzed hydrogenation of 2- and 3-hexyne showed a selectivity of about 0.97 and the olefins were principally of the cis-configuration (92). Using alumina-supported palladium (96) and no solvent, the initial olefin distribution from 3-hexyene was cis-3-hexene, 92% [Pg.182]

A mixture of Na, hexamethylphosphoramide, 1-hexene, and cyclohexane as internal glpc-standard stirred at room temp, until a blue color appears, 3 portions of tert-butanol added at 1.5 hrs. intervals, and the product isolated after 6 hrs. when the blue color has vanished -> hexane. Y 98%. - Similarly 3-Hexyne -> hexane. Y 79%. - The above reducing mixture provides a convenient and general method of saturating unactivated carbon-carbon multiple bonds. F. e., also with tert-butanol-O-d, s. G. M. Whitesides and W. J. Ehmann, J. Org. Chem. 35, 3565 (1970) alcohols from 0x0 compds., also with K in place of Na, s. C. r. 272 (C), 794 (1971) also with Li in hexamethylphosphoramide-tetrahydrofuran, steric effect of metal and medium, s. R. Lapouyade, P. Labandibar, and H. Bouas-Lau-rent, Tetrah. Let. 1971, 979 reduction of anthracenes s. C. r. 272 (C), 1257 (1971). 

The boiling points of 1-hexene (64°C) and 1-hexyne (71°C) are sufficiently close that it is difficult to achieve a clean separation by distillation. Show how you might use the acidity of 1-hexyne to remove the last trace of it from a sample of 1-hexene. 

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