Diynes Pentadiynes

When the diyne is already conjugated as in 1,3-pentadiene 52, with 4 equivalents of nBuLi-TMEDA, an isomerization can also be achieved [91] (Scheme 1.22). The selectivity of this process is low starting material and 1,4-pentadiyne are also found. 

Phenyl-1,3-pentadiyne has been prepared by the dehydro-bromination of the corresponding butadiene tetrabromide.6 Other unsymmetrical 1,3-diynes have been prepared by scheme... 

Very high reactivity toward thiol addition is observed when the diyne system is attached ro a carbonyl group, but the stereoselectivity drops sharply, and mixtures of cis-trans isomers are obtained irrespective of solvent. Thus, the product from the addition of methanethiol to l-phenyl-2,4-pentadiyn-l-one (114) in either methanol or THE consists of the cis (115) and trans (116) isomers with the former predominat-ing s- . It was shown that 116 does not arise by isomerization of 115. Similarly, addition to 2,4-hexadiynoic acid (117) in DMSO yields a mixture of cis-trans isomers (118) . ... 

Addition of amines to carbonyl-activated diynes occurs with great ease, and often the initial adducts react further to give cyclic products. Syn addition occurs in the reaction of piperidine with l-phenyl-2,4-pentadiyne-l-one (151) in either ethanol or ether as solvent giving 152... 

With monosubstituted diynes, addition occurs predominantly at the terminal triple bond, although the selectivity depends on the nature of the halogen and the substituent . The dichloro derivatives 178a and 178b were obtained in a 3 1 ratio from the reaction of chlorine with excess 1,3-pentadiyne at —40 °C. In spite of the fact that a two-fold excess of pentadiyne was used, tetra- and higher polychlorides... 

Hydrogen adds exclusively to the terminal triple bond of monosubstituted conjugated diynes but the initially formed enynes react rapidly with hydrogen, and even from the beginning, products of over-hydrogenation appear - . After the absorption of one mole of hydrogen by 1,3-pentadiyne in the presence of Pd(CaCOs), the product contains 67% l-penten-3-yne. No products have been detected which correspond to the initial addition of hydrogen to the internal triple bond. 

Diynes can also participate in 1,3-dipolar cycloadditions. The monoadduct 225 is obtained when equimolar proportions of diazomethane and butadiyne are mixed and allowed to stand, while the diadduct 226 is obtained when a 2 1 ratio is used . Addition of diazomethane to 1,3-pentadiyne occurs principally at the unsubstituted triple bond to give 227 . The greater reactivity of the double bond... 

Attachment of two alkynyl groups to a CHj group confers an unusually high acidity on the protons, and as a consequence 1,4-diynes undergo basc-catalysed rearrangement under very mild conditions. Thus, l-phenyl-l,4-pentadiyne (269) gives l-phenyl-l,3-pentadiyne (271) in good yield upon treatment with ethanolic... 

Brandsma and De Jong have reported a one-pot synthesis of substituted as well as underivatized T23bT (Scheme 11.2) using derivatives of 1,3-pentadiynes in yields of roughly 50% [ 11 ]. This procedure relies on the reaction of the intermediate potassiated diyne through the allenic form. One of the major impurities found is the 2,5-disubstituted thiophene thiolate that is easily separated owing to its water solubility. 

Addition of primary amine to a 1,4- or 1,5-diyne could be accomplished using titanium complex Ti(NMe2)2(dpma), resulting in an imine-yne that can undergo cyclization to the pyrrole derivatives. The Markovnikov hydroamina-tion products of 1,4-pentadiyne could undergo 5-endo-dig cyclization to yield a 2-methylpyrrole. Meanwhile, Markovnikov hydroamination of 1,5-hexandiyne would yield an imine-yne that could undergo 5-exo-dig cyclization to a 2,5-dimethylpyrrole [314] (Scheme 14.135). 

A soln. of l,l-dimethyl-5-phenyl-2,4-pentadiyn-l-ol in tetrahydrofuran added slowly with ice-cooling and efficient stirring to a mixture of benzophenone, KOH, and tetrahydrofuran, the product isolated 15 min. after room temp, has been reached l,l,5-triphenyl-2,4-pentadiyn-l-ol. Y 91%. F. e., also 2,4-diyne-... 

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