Propargyl alkyne hydrogenation

C-H bond fission and the production of ethynyl radicals. Butadiyne and vinyl acetate are formed when the T -shaped ethyne dimer is irradiated at 193 nm in argon or xenon. The dynamics of the photodissociation of propyne and allene have been studied. The H2 elimination from propyne is a minor route for propyne dissociation and the major path identified in this study is loss of the alkyne hydrogen. A study of the photodissociation dynamics of allene and propyne has been reported and this work has demonstrated that allene gives rise to a propargyl radical while propyne yields the propynyl radical. Other research has examined the photodissociation of propyne and allene by irradiation at 193 nm. ... 

Dehydration of propargyl alcohols occurs commonly on mononuclear ruthenium complexes.[ l Water is formed from the terminal alkynic hydrogen and by the alcoholic OH this is the more common dehydration process (it is denoted Route A). These reactions afford organic intermediates leading to cumulene complexes useful for the synthesis of doped polyacetylenes or of non-linear optic materials. Dehydration of rhodium-coordinated propargyl alcohols " leads to free or coordinated cumulenes and can be catalyzed by alumina and chloride ions, Fig. 16. 

Different reactivity trends on M3(CO)i2 clusters (M = Fe, Ru, Os) have been observed, depending on the nature of the cluster metals. Two dehydration routes have been observed for cluster-bound propargyl alcohols - the less common is similar to the main process observed for mononuclear complexes and involves loss of the terminal alkynic hydrogen and of the alcoholic OH (Route A). The more common, when CH3 groups are available, involves loss of one methyl hydrogen and of the adjacent OH (Route B). The former process leads to allenylidene and the second to ene-yne (or vinylacetylide) derivatives. Examples are given in Fig. 21. 

Among several propargylic derivatives, the propargylic carbonates 3 were found to be the most reactive and they have been used most extensively because of their high reactivity[2,2a]. The allenylpalladium methoxide 4, formed as an intermediate in catalytic reactions of the methyl propargylic carbonate 3, undergoes two types of transformations. One is substitution of cr-bonded Pd. which proceeds by either insertion or transmetallation. The insertion of an alkene, for example, into the Pd—C cr-bond and elimination of/i-hydrogen affords the allenyl compound 5 (1.2,4-triene). Alkene and CO insertions are typical. The substitution of Pd methoxide with hard carbon nucleophiles or terminal alkynes in the presence of Cul takes place via transmetallation to yield the allenyl compound 6. By these reactions, various allenyl derivatives can be prepared. 

If the alkene is an alkyne instead, we are dealing with a propargylic alcohol and now the thermodynamics are more favourable and the product is an enone. Commercial application is found in the production of citral from dehydrolinalool via vanadium-catalysed isomerisation (Figure 5.9). Note that the last step involves a transfer of hydrogen as well when the enol rearranges to the aldehyde ... 

The organozinc prepared from propargyl bromide did not react with terminal alkynes because, under the reaction conditions, abstraction of the acidic terminal acetylenic hydrogen from the propargylic metabotropic form presumably occurred (equation 154)185. 

As catalytic semihydrogenation of alkynes to Cis-alkenes is not only a very important synthetic operation (ref. 1) but also of industrial interest, it is a challenging task for both synthetic and catalytic chemists. For instance, the importance of the problem is illustrated by numerous recent publications on different aspects of the selective hydrogenation of many compounds related to the propargyl alcohol structure (refs. 2-7). In this respect, 1,4-butenediol, obtained by the liquid-phase semihydrogenation of 1,4-butynediol, is a raw material for insecticides and Vitamin Bg (refs. 2,8,9). Furthermore, the total and selective liquid-phase hydrogenation of this compound is one of the procedure for making butanediol, the top 95 chemical produced in the United States (refs. 10,11), whose major use is in the manufacture of polyesters. 

Arynes, generated in situ from o-silylaryl triflates (229), undergo ene reaction with alkynes (230) possessing propargylic hydrogen in the presence of KF-18-crown-6 in... 

First, 7 is converted into its dianion 23 by two equivalents of n-butyllithium. Not only the terminal hydrogen of alkyne 7 but also the propargylic one is acidic. 23 then undergoes intramolecular nucleophilic attack of the terminal chlorine. NH4C1 work-up yields the desired cyclopropyl acetylene 25.8... 

Enantiopure propargylic sec-alcohols are valuable synthetic intermediates that can be prepared by a number of efficient asymmetric transformations [131]. KR of the racemic propargylic sec-alcohols is a valuable approach, particularly as subsequent exhaustive hydrogenation of the alkyne provides an indirect approach to enantio-enriched alkyl alkyl sec-alcohols which themselves are challenging substrates for both enzymatic and non-enzymatic KR. 

Conversion of a terminal alkyne to its alkynylsilane prevents loss of the relatively acidic terminal hydrogen (pKa of ethyne c. 25) during later synthetic steps. For example, the terminal hydrogen of propyne was masked whilst its propargylic anion was used in a synthesis of Cecropia juvenile hormone, a chemical which plays ail important role in insect development (Figure Si5.2). 

Controlled reduction of alkynyl silanes produces the corresponding vinyl silanes and the method of reduction dictates the stereochemistry. Lindlar hydrogenation adds a molecule of hydrogen across the alkyne in a cis fashion to produce the 2-vinyl silane. Red Al reduction of a propargylic alcohol leads instead to the E-isomer. 

Terminal alkynes couple with propargyl-type halides (45) in the presence of copper(i) chloride and ammonia or an amine . Two types of coupling products have been observed, allenynes 46 and 1,4-diynes 47. When R is hydrogen, the... 

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