Isomerization alkynes

For a number of other pharmacologically active unsaturated compounds, it is assumed that a reactive allene is formed in situ by an alkyne isomerization [160] or an elimination reaction [161]. The prime example of the formation of such a highly reactive allene through chemical activation of an unsaturated precursor is the ene-diyne antibiotic neocarzinostatin (Scheme 18.57) [162],... 

Of further significance is the fact that no 1,3-pentadiene is formed This behavior is similar to that of the butynes, where also no 1,3-butadiene was observed. Furthermore, this is in complete accordance with the proposed mechanism of the potassium 3-aminopropylamide-mediated isomerization of internal alkynes to terminal alkynes by repetitive alkyne-allene-alkyne isomerizations [24]. 

In some examples, two alkynes isomerized, leading to cross-conjugated systems such as 35 [47, 48] (Scheme 1.13). 

Other examples reported with a terminal and an internal alkyne as in 50, which do not like 49 have an additional possibility for conjugation, show that then the terminal alkyne isomerizes [62, 87, 88] (Scheme 1.21). 

Zinc, chloro(2-methylphenyl)-, (84109-17-1), 66, 74 Zipper reaction, alkyne Isomerization, 66, 129... 

Abrams, S.R. (1984). Alkyne isomerization reagents mixed alkali metal amides. Can. J. Chem., 62, 1333-1334. 

Reaction of HqFeRu3(C0)i -t With Alkynes. Isomeric products also result from the reaction of H2FeRu3(C0) 3 with a series of internal alkynes, eq. 10 (20). 

Hydrozirconation occurs with yyn-addition of the Zr-H bond across a C=C or C=C bond (equation 8.16). Due to lower steric hindrance, the addition also tends to be regiospecific, with the zirconium attached to the less substituted position (just as in hydroboration). Internal alkenes and alkynes isomerize to 1-alkyl and 1-alkenyl complexes, respectively—presumably by alternating reactions of insertion and deinsertion—until the complex with the least steric hindrance is formed. 

The enantioselective version of the relay transformation by organic and metallic catalyses was successfully demonstrated by Gong and coworkers (Scheme 3.39) [83]. They accomplished the direct transformation of o propargylaniline derivatives into tetrahydroquinolines in a highly enantioselective manner through the hydroamina tion of alkynes/isomerization/enantioselective transfer hydrogenation (see Sec tion 3.3 for details) sequence under the relay catalysis of an achiral Au complex/ chhal phosphoric acid binary system. 

Finally, as far as alkyne isomerization is concerned. Lavalley and coworkers j 147] used the isomerization of but-I-yne (yielding but-2-ync and. in some cases, buta-1.2-diene) as a probe to characterize basicity of several alkali-exchanged zeolites. 

Garcia-Yebra et reported on the synthesis of alkynyl- and vinylidene-niobocene complexes and an unexpected / -vinylidene- / -alkyne isomerization. They used AIM to locate CPs. For example, the existence of the metal-CH interaction was confirmed via the presence of two BCPs, one between Nb and H and the other between Nb and C. A RCP between Nb, C and H was also found. 

Isomerization of Alkenes and Alkynes. Isomerization of alkenes proceeds through anion intermediates by abstraction of an allylic proton from alkene molecules by solid bases. In the case of 1-butene isomerization, high cisitrans ratio of 2-butene is characteristic of the base-catalyzed isomerization. On the... 

As an alternative to keto-alkyne isomerization, aDenylketones themselves can undergo cycloisomerization, providing access to furans and dimeric products, depending upon the palladium catalyst and allene employed (Scheme 6.24) [33]. 

Aside from the allylic rearrangements, however, relatively little has been reported on Pd-catalyzed alkene and alkyne isomerization per se, even though many scattered examples of isomerization have been discussed as side reactions, which are mostly un-wanted. " ... 

In this section, attention is focused primarily on selective alkene and alkyne isomerization reactions of synthetic interest. A limited number of studies on the Pd-catalyzed isomerization of monoenes including its mechanistic aspects will also be discussed. 

Palladium catalysis has been shown to be effective in inducing thermodynamically favorable and selective alkene and alkyne isomerization reactions. Aside from the formation of stereoisomeric mixtures, these reactions can be both selective and high-yielding and hence synthetically useful. 

Singh VK, Bustelo E, de Los RI, et al. Internal alkyne isomerization to vinyhdene versus stable t-alkyne theoretical and experimental study on the divergence of analogous Cp Ru and TpRu systems. Organometallics. 2011 30 4014-4031. 

Isomerization of Allylic Alcohols and Formation of 1-Halo-1-alkynes. Isomerization of primary and secondary allylic alcohols to tertiary isomers proceeds in CH2CI2 at 25 °C in the presence of a catalyst that is prepared in situ by activation of vanadyl bis(acetylacetonate) or Mo02(acac)2 with (1) (eq 3). On the other hand, terminal alkynes react with (1) in the presence of copper or zinc halides in THF at —15 °C to afford terminal 1-halo-1-alkynes in 40-85% yields. By the same method, 1-cyano-l-alkynes are obtained in 65% yield by use of copper cyanide. 

A related type b cyanoamidation leading to 3-pyrrohn-2-ones was reported by the Takemoto group (Scheme 19 20060L2711). Treatment of the alkyne 116 with catalytic Pd(PPh3)4 in xylene gives an equal mixture of 3-pyrrolin-2-one 117 and alkyhdene lactam 118. The mechanism presumably involves a 5-cxo-trig oxidative addition of cyanide and the amide carbonyl across the alkyne isomerization of 118 under the reaction conditions then leads to the formation 117. 

Sodium amide is a strong base before the alkyne isomerization occurs, the alcohol group in 1 will be deprotonated to form an aUcoxide ion (a negative charge on an oxygen atom) which is carried through until the final reprotonation event (when H2O is introduced, which takes place after the transformation below is complete). 

As shown in Eq. (8), the reaction is able to proceed through either rj -coordination or -coordination pathway, and it depends on the catalyst species used in the reaction. In the reaction directed to left, r] -coordination of alkyne followed by nucleophilic attack to carbon-a occurs [22, 23]. On the other hand, in the reaction directed to right, alkyne isomerization to aUene takes place first, followed by q -allyl coordination, and subsequent nucleophilic attack to carbon-I> (the terminal carbon of q -allyl metal species) takes place [48]. 

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