Isomerism alkynes

Problem 8.2 There are seven isomeric alkynes with the formula Draw and name them. 

In another reduction, the propargylic phosphate 64 is reduced with samarium(II) iodide in the presence of tetrakis(triphenylphosphine)palladium and tert-butanol as a proton source the allene 65 is produced almost exclusively, <1% of the isomeric alkyne 66 being present in the product mixture [19]. 

It can be inferred from additional examples (Table 3) that the stereoselectivity and stereochemical outcome of the reaction strongly depend on the type of metal hydride and the leaving group. Furthermore, the reaction temperature is important in several cases67. The scope of the method is broad, however, varying amounts of isomeric alkyne isomers are formed as byproducts, sometimes accompanied by the corresponding alkenes. 

For each molecular formula, draw all the isomeric alkynes, and give their IUPAC names. Circle the acetylenic hydrogen of each terminal alkyne. 

Reaction of Fe3(CO)12 with 3-pentyn-l-ol gives the hydrido cluster 4, formed by coupling of an allenylidene unit with a coordinated carbonyl ligand and methoxy group. In contrast, reaction with the isomeric alkyne 2-methyl-3-butyn-2-ol affords a binuclear iron complex.18 Treatment of Fe3(p3-S)2(CO)9 with Me3NO affords the vinylferrocene complex 5, whereas the similar reaction with Fe3(p3-Te)2(CO)9 gives no cluster products.19... 

Draw the seven isomeric alkynes having molecular formula CeHio, and give the lUPAC name for each compound. Consider constitutional isomers only. 

The base-induced dehydrohalogenation of vinyl halides and allyl halides often gives low yields of allenes because of the competing reaction to alkynes alkynes can either be formed by direct elimination from vinyl halides or by isomerization of the allene first formed to the isomeric alkyne. Since it has been established that anti elimination of hydrogen halide from vinyl halides to yield alkynes is much faster than syn elimination, the proper choice of the starting material is often important for a successful allene synthesis. When ( )-4-bromo-4-octene was treated with NaOMe, the sole product was 3,4-octadiene, whereas the conesponding Z-educt yielded 4-octyne (Scheme 66). ... 

Especially in THF, methylphenylacetylene was found to undergo a base-catalyzed rearrangement to its isomeric alkyne and allene ... 

Moving the triple bond towards the center of the alkyne substantially increases the relative abundance and hence stability of the corresponding molecular ion (Figure 2). With the isomeric octynes the fragmentation yields (total fragment ion current divided by [M ]) are 125 for 1-, 6.3 for 2-, 2.1 for 3- and 0.77 for 4-octyne. Analogous, albeit not as profound, trends were observed at 70 eVk Thus, determination of the fragmentation yield of isomeric alkynes suffices to ascertain the position of the triple bond". ... 

Isomeric alkynes can be distinguished from their El mass spectra. Although partial isomerizations take place during ionization and/or decomposition, differences remain and allow for definitive structural assignments in combination with reference spectra. 

Write the structural formulas and lUPAC names for all the isomeric alkynes with the formula CsHg. 

Most allenes are less stable than their isomeric alkynes. For example, allene itself is less stable by 6.7 kj (1.6 kcal)/mol than its constitutional isomer propyne, and 1,2-butadiene is less stable than 2-butyne by 16.7 kJ (4.0 kcal)/mol. 

Because of their lower stability relative to isomeric alkynes, allenes are generally only minor products of alkyne-forming dehydrohalogenation reactions. 

FIGURE 12.11 Reprotonation either re-forms the allene or gives the isomerized alkyne, 1-butyne. 

Kirss, H. Kuus, M. Siimer, E. Kudryavtseva, L. Measurement, correlation and prediction of binary vapour-liquid equilibria and enthalpies of mixing for systems containing isomeric alkynes. Thermochim. Acta 1992, 195, 85-94. 

In alkene and alkyne tritioborations, the boron adds preferentially to the less hindered side of the multiple bond, so that the label should be located mostly at the adj acent atom this is evident in the isotope distribution in [ HJsafrole (264) and [ HJisosafrole (265). resulting from reactions with the respective isomeric alkyne precursors. In a reaction that nicely exploited the differing reactivity of functional groups towards B H3 THF, the a,/3-unsaturated carbonyl function of 7-triethylsilyl-13-oxo-baccatin III (266) was selectively reduced to 7-triethylsilyl[13- H]baccatin HI (267). a key intermediate in the synthesis of tritiated paclitaxel (268). an efficacious and widely used anticancer dmg . 

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