Pentynes, hydrogenation

Figure 5. Model of the palladium surface during 1-pentyne hydrogenation. (Reprinted from Reference [65], 2006, with permission from Elsevier).

The alkane formed by hydrogenation of (S) 4 methyl 1 hexyne is optically active but the one formed by hydrogenation of (S) 3 methyl 1 pentyne is not Explain Would you expect the products of hydrogenation of these two compounds in the presence of Lindlar palladium to be optically active" ... 

D,L-Arahinitol can be prepared by the action of hydrogen peroxide in the presence of formic acid on divinyl carbinol (36) and, together with ribitol (Fig. ld),fromD,L-erythron-4-pentyne-l,2,3-triol,HOCH2CHOHCHOHC=CH (37). 

Strategy Compare the product with the starting material, and catalog the differences. In this case, we need to add three carbons to the chain and reduce the triple bond. Since the starling material is a terminal alkyne that can be alkylated, we might first prepare the acetylide anion ol 1-pentyne, let it react with 1-bromopropane, and then reduce the product using catalytic hydrogenation. 

The synthesis of enantiomerically pure propargylic alcohols is possible using the same methodology 43b. Thus, addition of (—)-[(l-chloro-2-phenylethyl)sulfinyl]-4-methylbenzene (14) to propan-al led to a mixture of the diastereomers 15A/15B (d.r. 44 56) which are easily separated by column chromatography. After thermal elimination of the sulfinyl group the vinyl chlorides 16A/16B were obtained as a mixture of E- and Z-oleftns. Elimination of hydrogen chloride was carried out with three equivalents of butyllithium, leading to enantiomerically pure 1 -phenyl-1-pentyn-3-ol. 

Figure 11.2. Hydrogenation of 1-pentyne following hydrogenation of trans-2-pentene for Ih.

This suggests that the iran5 -2-pentene is more strongly bound than the pentyne and that the slow rate of hydrogenation may be due to strong adsorption rather than the thermodynamic stability of the trans-isovaer. 

A very efficient, stereospecific synthesis of DL-ribose was based26 on the use of l,l-diethoxy-5-(tetrahydropyran-2-yloxy)-2-pentyn-3-ol as the substrate. Catalytic hydrogenation of this alkyne to the cts-alkene was accompanied by cyclization, to give 2-ethoxy-2,5-dihydro-5-(tetra-hydropyran-2-yloxy)furan (35). cis-Hydroxylation of the double bond in 35 was effected with potassium permanganate, yielding the ethyl DL-ribofuranoside derivative 36, which was hydrolyzed to DL-ribose. 

Similarly, Petrov1 has repented addition of hydrogen bromide to l,2-epoxy-3-pentyne (Eq. 712), which gives rise to 2-bromo-3-pentyn-l-ol exclusively, Thu and related results are doubtless attributable to the ability of double and triple bonds to stabilize partial oarboniuin ion character development in the borderline 8 i2 transition state.1 ... 

The alkane formed by hydrogenation of (5 )-3-methyl-l-pentyne is achiral it cannot be optically active. 

Terminal alkynes react rapidly with borane to produce an intermediate compound that is easily oxidized to an aldehyde. For example, you can produce pentenal by reacting pentyne with borane and oxidizing the resulting intermediate with aqueous hydrogen peroxide. 

In many cases the use of mono-substituted alkynes in reactions with M3(CO)l2 (M = Fe, Ru, Os) gives products very similar to those obtained with disubstituted alkynes (43-48). Nevertheless, the hydrogen atoms a to the triple bond may undergo a transfer from the ligand to the metal framework (49-52). Another interesting chemical transformation occurs in the reaction of Fe3(CO)12 with 1-pentyne (53). One of the products obtained, in very low yield, is the pentanuclear carbide Fe5C(CO)15 [Eq. (2)]. 

FIGURE 42 Comparison of mean initial activities per unit silica volume (ry jio ) for the selective hydrogenation of 3-methyl-l-pentyn-3-ol (A) and mean maximum yields of the desired intermediate product 3-methyl-l-penten-3-ol in the hydrogenation of 3-methyl-l-pentyn-3-ol (B) averaged over several runs for the conversion catalyzed by acicular mullite with large pores, acicular mullite with small pores, and cordierite. Conditions hydrogen pressure 2.3 bar alkyne concentration 20 mol/m solvent, ethanol and temperature, 297.4 K. 

The so-called ROT AC AT reacfor was tested in the hydrogenation of 3-mefhyl-l-pentyn-3-ol (47). The performance was similar fo fhat of a... 

Michael additions. Gerlach and Kiinzler report that the lithium enolate of S-t-butyl thioacetate undergoes 1,4-addition to cyclopentenone. They have extended this Michael reaction to a synthesis of methyl jasmonate (5), based on the similar conjugate addition of the trimethylsilyl enolate 1 promoted by tetra-n-butylam-monium fluoride. The adduct 2 was alkylated by l-bromo-2-pentyne in the presence of tetra-n-butylammonium fluoride to give 3 in rather low yield. Remaining steps to 5 were methanolysis and partial hydrogenation of the triple bond. 

Alkoxyacetylenes are obtained by alcohol and hydrogen halide elimination from dialkylacetals of a-chloroaldehydes, induced by NaNH, in liquid ammonia (equation 81)108. 4-Pentyn-l-ol is obtained in similar fashion (equation 82) . 

Pentyne and 1-hexyne are hydrogenated with [Ti(Cp)2(CO)2] to 1-pentene and 1-hexene, respectively, but phenylacetylene gives ethylbenzene. Reduced compounds of [Ti(Cp)2Cl2] with Na, Mg, Ca, sodium naphthalenide, or butyllithium catalyze the hydrogenation of a variety of alkynes to alkenes. 

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