Interesting Alkynes

S3ITiple ProblGfTl 11.1 Give the lUPAC name for the following alkyne. 

Give the structure corresponding to each of the following names, a. frans-2-ethynylcyclopentanol b. 4-fert-butyl-5-decyne 

The physical properties of alkynes resemble those of hydrocarbons having a similar shape and molecular weight. 

Explain why an alkyne often has a slightly higher boiling point than an alkene of similar molecular weight. For example, the bp of 1 -pentyne is 39 °C, and the bp of 1 -pentene is 30 °C. 

Acetylene, HC=CH, is a colorless gas with an ethereal odor that bums in oxygen to form CO2 and H2O. Because the combustion of acetylene releases more energy per mole of product formed than other hydrocarbons, it bums with a very hot flame, making it an excellent fuel for welding torches. 

Monthly cycles of hormones from the pituitary gland cause ovulation, the release of an egg from an ovary. To prevent pregnancy, the two synthetic hormones in many oral contraceptives have different effects on the female reproductive system. 

A The elevated level of ethynylestradiol, a synthetic estrogen, fools the pituitary gland into thinking a woman is pregnant, so ovulation does not occur. 

B The elevated level of norethindrone, a synthetic progesterone, stimulates the formation of a thick layer of mucus in the cervix, 

Two other synthetic hormones with alkynyl appendages are RU 486 and levonorgestrel. RU 486 blocks the effects of progesterone, and because of this, prevents implantation of a fertilized egg. RU 486 is used to induce abortions within the first few weeks of pregnancy. Levonorgestrel interferes with ovulation, and so it prevents pregnancy if taken within a few days of unprotected sex. 

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The benzannulation approach has been applied to aromatic molecules of biological interest. Alkynes bearing porphyrin (for example 37) [51a, 51b], ferrocene (for example 39) [51c], and carbohydrate functionalities [51d, 51e] have been incorporated into the (hydro)quinone skeleton, as demonstrated for porphyrinyl and ferrocenyl naphthoquinones 38 and 40 (Scheme 18). 

While regioselectivity is not catalyst controlled, as illustrated in Table 15.14, reactivity is observed across a broad range of substrates, although widely varying reaction concbtions must be used. Notably, only bulky primary amines have been reported here. Interestingly, alkyne hydroamination with secondary amines gives enamines as useful reactive intermediates for the synthesis of allenes (Scheme 15.32) [189]. 

The catalytic reduction of alkynes converts the carbon-carbon triple bond to a carbon-carbon single bond with the addition of two equivalents of hydrogen (H2) across the triple bond (Equation 6.64, where R and R may be hydrogen or alkyl or aryl). Presumably this reaction proceeds via addition of one equivalent of H2 to produce the alkene and then, in a separate step, a second equivalent of H2 adds. Interestingly, alkynes are more easily reduced than the corresponding alkenes, in concert with the bond strengths as discussed above and also because, apparently, alkynes are more easily adsorbed on the catalyst than are alkenes. 

Interesting formation of the fulvene 422 takes place by the reaction of the alkenyl bromide 421 with a disubstituted alkyne[288]. The indenone 425 is prepared by the reaction of o-iodobenzaldehyde (423) with internal alkyne. The intermediate 424 is formed by oxidative addition of the C—H bond of the aldehyde and its reductive elimination affords the enone 425(289,290]. 

Dimethyl acetylenedicarboxylate (DMAD) (125) is a very special alkyne and undergoes interesting cyclotrimerization and co-cyclization reactions of its own using the poorly soluble polymeric palladacyclopentadiene complex (TCPC) 75 and its diazadiene stabilized complex 123 as precursors of Pd(0) catalysts, Cyclotrimerization of DMAD is catalyzed by 123[60], In addition to the hexa-substituted benzene 126, the cyclooctatetraene derivative 127 was obtained by the co-cyclization of trimethylsilylpropargyl alcohol with an excess of DMAD (125)[6l], Co-cyclization is possible with various alkenes. The naphthalene-tetracarboxylate 129 was obtained by the reaction of methoxyallene (128) with an excess of DMAD using the catalyst 123[62],... 

Interestingly. 4-hydroxythiazoles (11) react like the 4-hydroxy-THISs with alkynes and alkenes (Scheme 12) (20). further demonstrating the usefulness of 4-hydroxythiazole derivatives for the preparation of 2-pyridones and thiophenes. 

The direct combination of selenium and acetylene provides the most convenient source of selenophene (76JHC1319). Lesser amounts of many other compounds are formed concurrently and include 2- and 3-alkylselenophenes, benzo[6]selenophene and isomeric selenoloselenophenes (76CS(10)159). The commercial availability of thiophene makes comparable reactions of little interest for the obtention of the parent heterocycle in the laboratory. However, the reaction of substituted acetylenes with morpholinyl disulfide is of some synthetic value. The process, which appears to entail the initial formation of thionitroxyl radicals, converts phenylacetylene into a 3 1 mixture of 2,4- and 2,5-diphenylthiophene, methyl propiolate into dimethyl thiophene-2,5-dicarboxylate, and ethyl phenylpropiolate into diethyl 3,4-diphenylthiophene-2,5-dicarboxylate (Scheme 83a) (77TL3413). Dimethyl thiophene-2,4-dicarboxylate is obtained from methyl propiolate by treatment with dimethyl sulfoxide and thionyl chloride (Scheme 83b) (66CB1558). The rhodium carbonyl catalyzed carbonylation of alkynes in alcohols provides 5-alkoxy-2(5//)-furanones (Scheme 83c) (81CL993). The inclusion of ethylene provides 5-ethyl-2(5//)-furanones instead (82NKK242). The nickel acetate catalyzed addition of r-butyl isocyanide to alkynes provides access to 2-aminopyrroles (Scheme 83d) (70S593). 

Electrophilic additions to allenes represent an interesting reaction type which is related to additions to both alkenes and alkynes. An allene could, for example, conceivably be protonated at either a terminal s[p- carbon or the central sp carbon. 

The derivative 171 is of interest in this series of complexes since it undergoes isomerization of the alkyne substituent in position 1 to yield 172 and 173. The allyl complex 172 reacts with diphenylarsine to yield predominantly the chelate 174 (X = AsPh2) together with 175 and 176 (98IC1105). Complexes of the types 174 (X = PPh2) and 177 are known (93BSCF673 97BSCF471). 

Interestingly, the product actually isolated from alkyne hydration is not the vinylic alcohol, or enol (ene + ol), but is instead a ketone. Although the enol is an intermediate in the reaction, it immediately rearranges to a ketone by a process called keto-enol tautomerisni. The individual keto and enol forms are said to be tautomers, a word used to describe constitutional isomers that interconvert rapidly. With few exceptions, the keto-enol tautomeric equilibrium lies on the side of the ketone enols are almost never isolated. We ll look more closely... 

Since then, the metathesis reaction has been extended to other types of alkenes, viz. substituted alkenes, dienes and polyenes, and to alkynes. Of special interest is the metathesis of cycloalkenes. This gives rise to a ring enlargement resulting in macrocyclic compounds and eventually poly-... 

Alkynes can also be arylated by the Meerwein procedure, as shown by Muller in 1949. The reaction of buta-l-en-3-yne (Scheme 10-47) was studied by Kheruze and Petrov (1960). Arylation at an sp2-hybridized carbon is obviously considerably faster than the analogous reaction at an sp-hybridized carbon. A mechanistically interesting case of a Meerwein reaction with phenylacetylene will be discussed later in this section. 

The novel highly substituted spiro[4.4]nonatrienes 98 and 99 are produced by a [3+2+2+2] cocyclization with participation of three alkyne molecules and the (2 -dimethylamino-2 -trimethylsilyl)ethenylcarbene complex 96 (Scheme 20). This transformation is the first one ever observed involving threefold insertion of an alkyne and was first reported in 1999 by de Meijere et al. [81]. The structure of the product was eventually determined by X-ray crystal structure analysis of the quaternary ammonium iodide prepared from the regioisomer 98 (Ar=Ph) with methyl iodide. Interestingly, these formal [3+2+2+2] cycloaddition products are formed only from terminal arylacetylenes. In a control experiment with the complex 96 13C-labeled at the carbene carbon, the 13C label was found only at the spiro carbon atom of the products 98 and 99 [42]. 

Peterson and co-workers have carried out a careful investigation of the electrophilic addition of trifluoroacetic acid to a series of aliphatic alkynes (38) and alkenes (39,40). Of particular interest is the behavior of 3-hexyne. At. 1 M concentrations of 3-hexyne, nearly equal amounts of the cis- and trans-3-hexen-3-yl trifluoroacetates are formed in 98% yield, together with about 2%... 

Particularly interesting are the results obtained with the phosphonium ylides including an acyl rest derived from aminoacid if the N-H bond reactivity is blocked by an amide protection, the alkyne formation takes place [25,27], but if the N-H bond is not deactivated, an intramolecular cyclization occurs to give a new stabilized ylide [27,28]. 

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