Propargyl systems isomerization

This system was described in one report and has been synthesized by a copper-assisted cycloisomerization of alkynyl imines. The authors proposed the following mechanism at first, 372 could undergo a base-induced propargyl-allenyl isomerization to form 373 next, coordination of copper to the terminal double bond of the allene (intermediate 374) would make it subjected to intramolecular nucleophilic attack to produce a zwitterion 375. The latter would isomerize into the more stable zwitterionic intermediate 376, which would be transformed to the thiazole 377 (Scheme 55) <2001JA2074>. 

J 1 -HYDROGEN-HYDROGEN TRANSPOSITIONS ISOMERIZATION OF ALLYL AND PROPARGYL SYSTEMS... 

Allenylsilveifl) compounds are prepared in situ by deprotonation of the allenic hydrocarbon with n-butyllithium and subsequent treatment with silver bromide (equation 5). Allenylsilver compounds afford allenic derivatives when treated with a variety of electrophiles, without significant isomerization to the propargyl system (equation Reaction of the allenylsilver compound with carbon disulfide results in an interesting transformation to p, y-unsaturated-7-dithiolactones presumably by way of silver salts (Scheme 11). - ... 

Efforts to tune the reactivity of rhodium catalysts by altering structure, solvent, and other factors have been pursued.49,493 50 Although there is (justifiably) much attention given to catalysts which provide /raor-addition processes, it is probably underappreciated that appropriate rhodium complexes, especially cationic phosphine complexes, can be very good and reliable catalysts for the formation of ( )-/3-silane products from a air-addition process. The possibilities and range of substrate tolerance are demonstrated by the two examples in Scheme 9. A very bulky tertiary propargylic alcohol as well as a simple linear alkyne provide excellent access to the CE)-/3-vinylsilane products.4 a 1 In order to achieve clean air-addition, cationic complexes have provided consistent results, since vinylmetal isomerization becomes less competitive for a cationic intermediate. Thus, halide-free systems with... 

In general, acyl azides are too unstable to survive at the temperatures required for addition to acetylenes, although benzoyl azide adds readily to ynamines in toluene. Ethoxycarbonyl azide also gives triazoles in good yield with ynamines. The azide adds to propargylic alcohols in boiling ethanol, and to acetylene at 100° under pressure. Addition to phenylacetylene and to electron-deficient acetylenes has been carried out at 130°. Oxazoles are also formed at this temperature by competing thermal decomposition of the azide, and addition of ethoxycarbonylnitrene to the acetylenes. The triazole obtained from phenylacetylene is 2-ethoxycarbonyl-4-phenyltriazole the two 1-ethoxycarbonyltriazoles can be isolated if the addition is carried out at 50° over several weeks. Since the IH- to -triazole isomerization takes place readily in these systems, a IH-structure cannot be assumed for a triazole formed by addition of these azides. 

Ruthenium and platinum carbenoids, derived from tertiary propargyl carboxylates, also reacted with furans in a similar manner, leading to triene systems (as represented in Scheme 9) <20060L1741>. The initially formed mixture of (2Z,4E) and (2Z,47) isomers 20 and 21, respectively, could be isomerized completely to a single (2E,4E) isomer. 

In contrast to the reaction mode of the propargyl ethers, a,/3-unsaturated aldehydes and ketones are isolated in the catalytic isomerization of the propargyl alcohols. Trost et al. developed a new catalytic system, (IND)RuCl(PPh3)2/InCl3/ NH4PF13/THF (IND = -indenyl), that efficiently effected such isomerization [14]. The reaction is cocatalyzed by a mixture of NHEtBPFj and NH4PF6, and addition of indium trichloride accelerates the reaction (Eq. 12.9). 

Enynes which have the secondary or tertiary center at the propargylic position are isomerized to 2-alkenyl-methylenecyclopentane in moderate to high yield with a catalytic amount of [CpRu(CH3CN)3]PF6 in acetone or dimethylformamide (DMF) [25], This catalyst system is acidic, and an acid-labile group such as a dimethyl acetal 54 is hydrolyzed to aldehyde 55. 

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