1,4-Enynes

FORMATION OF DIENYL COMPLEXES BY REACTION OF OsHCI(CO)(P Pr3)2 WITH ENYNES 

The complex OsHCl(CO)(P Pr3)2 reacts with 2-methyl-l-buten-3-yne to give the dienyl derivative Os ( )-CH=CHC(Me)=CH2 Cl(CO)(P,Pr3)235 in 86%, which is a result of the selective addition of the Os—H bond of the starting complex to the carbon-carbon triple bond of the enyne (Eq. 4). 

An important contribution of the resonance form b requires the donation of electron density form the metal to the dienyl ligand [M(dM) - C(pn-) contribution], The presence of a carbonyl group (a strong TT-acceptor ligand) trans to the dienyl reduces the M(dM) - C(ptt) contribution and, therefore, the nucleo-philicity of the unsaturated ii -carbon ligand. Then the nucleophilic center of the molecule is not the alkenyl ligand but the metallic center, and the protonation at the metal leads to the olefin via reductive elimination from a hydride-dienyl intermediate.24 

The cis-trans isomerization of alkenyl ligands in transition metal alkenyl compounds is proposed to occur via zwitterionic carbene intermediates.46 According to this, the low contribution of the form b to the metal-dienyl bond in Os (Z)-CH=CHC(Me)=CH2 K1-0C(0)Me (C0)2(P,Pr3)2 could explain why this compound does not evolve into its (if )-isomer. 

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Vigreux column gave the enyne alcohol, b.p. 75 C/15 mmHg, n 1.4955, in 55-7451 yield. 

The enyne system in the amines 828=88-8 8-882 can be reversed by potassium amide in liquid ammonia. Addition of the enyne amines to an equivalent amount of this reagent gives the potassium acetylides, K-8e8-88=88-8R2, from which the ynene" amines can be obtained in excellent yields by addition of solid ammonium chloride. 

This isomerization, which must proceed through a 1,2,3-trienylanine, is not "contra-thermodynamic", since with a catalytic amount of potassium tert.-butoxide the same result is obtained. Enyne ethers, H2C=CH-CsC-0R, undergo a similar conversion into HCeC-CH=CH-OR upon interaction with alkali metal amides in liquid NH3, followed by hydrolysis . Enyne sulphides, H2C=CH-CsC-SR, and the hydrocarbons H2C=CH-CsC-R (R = or phenyl) give only tars or polymeric products under... 

A mixture of 1 g of KO-terfc.0.6 g of HO-tert.-CgHg and 40 ml of dry, pure HHPT was warmed at 40-50°C until all of the solid material had disappeared. The solution obtained was cooled to 15°C and 15 g of the enyne ether (note 1) were added in 2 min. The temperature of the mixture was kept at aa. 25°C and after the addition this temperature was maintained for an additional 10 min. 

As indicated in the general scheme below, butatrienes are the first products from base-induced 1,4-elinination of hydrogen and a suitable leaving group. The butatriene in general very readily undergoes isomerization into enynes, if sufficiently "acidic" protons are available (see Chapter 11 in Ref. 3a). In aprotic media cumulenic ethers are fixed as their lithio derivatives if an excess of alkyllithium is applied... 

Other 1,4-eliminations of synthetic interest are the formation of enyne amines 48... 

Treatment of conjugated enyne systems, H C-C=C-CeC-CH-OR, with a suitable base in principle may give rise to 1,6-elimination of HOR with primary formation of the system C=C-C=C=C=C, which subsequently may undergo isomerization to a dienyne. ... 

Jote I. If more CuBr is used, the yields of the enyne are lower and more residue... 

Vote 2. A large excess of R MgCl is required, as the subsequent metallation of the enyne is rapid. In the case of C2H5 the Grignard reagent was prepared from ethyl bromide. 

To a solution of 0.10 mol of the enyne alcohol (see Chapter III, Exp. 41) in 140 ml of dry diethyl ether was added in 15 min with cooling at about -30°C a solution of 0.22 mol of ethyllithium in about 180 ml of diethyl ether (see Chapter II, Exp. 1). The clear solution was then warmed to 25 C and after standing for 10 min at that temperature, it was cautiously poured into 200 ml of an aqueous solution of 30 g of NH,C1. The layers formed after standing were separated, and the aqueous layer was extracted three times with diethyl ether. The combined... 

The alkynyl iodide 150 undergoes the oxidative addition to form an alky-nylpalladium iodide, and subsequent insertion of an alkene gives the conjugated enyne 151 under phase-transfer conditions[120]. 

Terminal alkynes undergo the above-mentioned substitution reaction with aryl and alkenyl groups to form arylalkynes and enynes in the presence of Cul as described in Section 1.1.2.1. In addition, the insertion of terminal alkynes also takes place in the absence of Cul, and the alkenylpalladium complex 362 is formed as an intermediate, which cannot terminate by itself and must undergo further reactions such as alkene insertion or anion capture. These reactions of terminal alkynes are also treated in this section. 

In the alkylative cyclization of the 1,6-enyne 372 with vinyl bromide, formation of both the five-membered ring 373 by exn mode carbopalladation and isomerization of the double bonds and the six-membered ring 374 by endo mode carbopalladation are observed[269]. Their ratio depends on the catalytic species. Also, the cyclization of the 1,6-enyne 375 with /i-bromostyrene (376) affords the endo product 377. The exo mode cyclization is commonly observed in many cases, and there are two possible mechanistic explanations for that observed in these examples. One is direct endo mode carbopalladation. The other is the exo mode carbopalladation to give 378 followed by cyclopropana-tion to form 379, and the subsequent cyclopropylcarbinyl-homoallyl rearrangement affords the six-membered ring 380. Careful determination of the E or Z structure of the double bond in the cyclized product 380 is crucial for the mechanistic discussion. 

The benzene derivative 401 by the intermolecular insertion of acrylate[278], A formal [2 + 2+2] cycloaddition takes place by the reaction of 2-iodonitroben-zene with the 1,6-enyne 402. The neopentylpalladium intermediate 403 undergoes 6-endo-lrig cyclization on to the aromatic ring to give 404[279],... 

The cyclic 2,4-dienoate 184, formed by the Pd-catalyzed cyclization of the 1,6-enyne 183, reacted with 154 to form the azulene derivative 185[118], The 3-methylenepyrrolidine 188 is formed by the reaction of the Zn reagent 186 with the chiral imine 187 with high diastereomeric excess. The structure of the allylic ethers is important for obtaining high diastereoselectivity[l 19],... 

In the coupling of the allenyl ester 7 with a terminal alkyne, an electron-deficient phosphine (Ph3P) gave the enyne-conjugated ester 8 as the major product, while an electron-rich phosphine (TDMPP or TTMPP) yielded the non-conjugated enyne esters ( )- and (Z)-9[4],... 

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