Intermediate acetylenic bond

Without other details, it appears that enamines are formed only with chloro compounds able to give a cyclohexyne. This also explains the formation of two isomers corresponding to nucleophilic condensation on both carbons of the intermediate acetylenic bond. Of course, steric effects may eventually play a role on the relative ratios of the two products. 

Both terminal and nonterminal acetylenes have been used. Activating groups oL to the acetylenic bond have included sulfone (131-135), sulfoxide (134), ester (28,133-139), and ketone (134,140). Whether adduct 183 Is designated as cis or trans depends on the investigators and the particular compound. If the addition reaction is carried out in aprotic solvents, the major isomer is 183 formed by cis addition (135,138,139). For example, the addition of aziridine to dimethyl acetylenedicarboxylate (182, X, Y = CO2CH3) in dimethyl sulfoxide (135) gave 75 % of a mixture containing 95 % of the Chester 185. Collapse of the intermediate zwitterion intermediate 186... 

Scheme 43 shows the details of the different steps involved in the equilibrium. The nucleophilic attack of the P(III) derivative on the acetylenic bond yields a 1,3-dipole which, after a fast protonation, frees aZ ion. If the subsequent addition of this ion occurs on the P atom (reaction a), a P(V) phosphorane is formed, but the addition of Z on the ethylenic C atom (reaction b) results in the formation of an ylide. Both of these reactions occur under kinetic control and, in both cases, X is always an OR group from the initial acetylene dicarboxylic ester. When the acetylenic compound is a diketone and X is an alkyl or aryl moiety, the C=0 group is much more electrophilic and the attack by the Z ion produces an alcoholate (reaction c), a new intermediate which can cyclize on to the P+ to form a phosphorane, or attack the a-C atom to form an ylide as in Scheme 42. Hence, reactions a and c can coexist, and are strongly dependent on the nature of the trapping reagent and of the P compound, but reaction b is blocked, whatever the reagent. This is well illustrated by the reaction of the 2-methoxytetramethylphospholane 147 on diben-zoylacetylene in the presence of methanol as trapping reagent. The proportions of the vinylphosphorane 157 and spirophosphorane 158 formed (Figure 24) are 13% and 84%, respectively.

The electron transfer to the acetylenic bond forms the frans-sodiovinyl radical 20 that, after protonation, produces tram radical 21. At low temperature (—33°C) in the presence of excess sodium, the conversion of the trans radical to sodiovinyl intermediate 22 is slightly more rapid than the conversion of the tram radical to the cis radical 23 (21 —> 22 > 22 —> 23). As a result, protonation yields predominantly the trans alkene. However, low sodium concentration and increased temperature lead to increasing proportion of the cis alkene. Although other dissolving-metal reductions are less thoroughly studied, a similar mechanism is believed to be operative.34 Another synthetically useful method for conversion of alkynes to trans alkenes in excellent yields is the reduction with CrS04 in aqueous dimethylforma-mide.198... 

The acetylenic product is a common intermediate for the general synthesis of naturally occurring PGs of series 1 and 2 (Scheme 6) (10). The controlled hydrogenation of the 5,6-acetylenic bond, which leaves the... 

JCS(C)2510 69JOU621 78JCS(P1)1428]. The first step is a nucleophilic addition of a sulfur atom to the acetylenic bond. In some cases intermediates 44 were isolated (74CS35) (Scheme 10). 

The red R group may seem to get in the way of the reaction but, of course, the dienophile is not approaching in the plane of the diene but from underneath. Itis difficult to find a convincing example of this stereochemistry as there are so few known, partly because of the difficulty of making E,2-dienes. One good approach uses two reactions you met in Chapter 31 for the control of double bond geometry. The cis double bond is put in first by the addition of methanol to butadiyne and the trans double bond then comes from LIAIH4 reduction of the intermediate acetylenic alcohol. 

Bridged species such as 76 are well documented in rhodium porphyrin chemistry.240-241 An acetylene bonded to one metal-centered radical is presumed to be trapped by addition of a second metal-centered radical. Lower bond dissociation energies of cobalt relative to rhodium would disfavor species such as 76 and facilitate the reaction with metal—hydride intermediates to form a trans product. 

Probably, the most important conclusion of the present study is that the triple acetylenic bond is preserved in the reaction products and this in spite of the fact that the initial attack by the unpaired electron of the CN radical destroys it. The three possible reaction intermediates have, in fact, lost the triple bond, but when, because of the high internal energy with which they are formed, they dissociate to products, the triple bond is retrieved in the new molecular products. This observation can be generalized to the case of the reactions of CN with polyynes and is quite important for the speculated routes of formation of cyanopolyynes in various environments. 

A mechanism is proposed to accommodate the observed stereoselectivity. The mechanism includes a first silicon shift to an acetylenic bond and a carbene-type zwit-terionic rhodium complex (7) as the key intermediate, which undergoes isomerization from a higher energy form (Z-complex, 6) to a lower energy form ( -complex, 8) followed by reductive elimination to cis-isomer (3) as the kinetic product. 

Therefore, a practical retrosynthetic analysis was made for the intermediate [B], which was divided into three main segments, A (Q-C23), B (C1-C7, C24, C25, C26), and C (C27-C30 or C3]). We chose an acetylenic compound as segment A, because its acetylide would act as a good nucleophile for an aldehyde (segment B), and the acetylenic bond would be selectively reduced to a (Z)-olefin at an appropriate step after coupling. The chiral pool method was applied during the synthesis of each segment. 

Apparently, aminobutenyne A, the intermediate of the pyrrole synthesis, is fixed in an advantageous configuration by coordination to the Cu" " cation, whereas the absence of catalyst may result in the formation of imine B having an active methylene group which attacks the acetylene bond to form dihydropyridine C and then pyridine 2 (by dehydrogenation). 

Barton and coworkers used free radical cyclization in the synthesis of tetracyclines (Scheme 105). Photolysis of 254 (X,Y = SR or OR) gives the corresponding radical, which cyclizes to the (Cy6) compound 255 in 80% yield when X,Y = SCH2CH2O. Quite remarkably, 255 is formed only in the cis form. Another completely stereoselective reaction toward the cis compound involving intramolecular addition to an acetylenic bond has been described by Pradhan and was discussed in Section IX.2 (Scheme 70). An analogous reductive cyclization (K, NH3) of ethynyl ketones has been used by Stork in the construction of a tricyclic intermediate for the synthesis of gibberellic acid. ... 

Contrary to acetylene, which does not undergo isomerization of intermediate acetylene metal complexes, in the presence of Ni(CO)4 olefins isomerize in many cases. In turn products are obtained with a carboxyl group linked at a C-atom which does not belong to the double bond of the starting material [146], table 43. 

Alkynes undergo stoichiometric oxidative reactions with Pd(II). A useful reaction is oxidative carboiiyiation. Two types of the oxidative carbonyla-tion of alkynes are known. The first is a synthesis of the alkynic carbox-ylates 524 by oxidative carbonylation of terminal alkynes using PdCN and CuCh in the presence of a base[469], Dropwise addition of alkynes is recommended as a preparative-scale procedure of this reation in order to minimize the oxidative dimerization of alkynes as a competitive reaction[470]. Also efficient carbonylation of terminal alkynes using PdCU, CuCI and LiCi under CO-O2 (1 I) was reported[471]. The reaction has been applied to the synthesis of the carbapenem intermediate 525[472], The steroidal acetylenic ester 526 formed by this reaction undergoes the hydroarylalion of the triple bond (see Chapter 4, Section 1) with aryl iodide and formic acid to give the lactone 527(473],... 

Rea.ctlons, Propargyl alcohol has three reactive sites—a primary hydroxyl group, a triple bond, and an acetylenic hydrogen—making it an extremely versatile chemical intermediate. 

When aiomatics aie present, they can capture the intermediate vinyl cation to give P-aryl-a,P-unsatutated ketones (182). Thus acylation of alkyl or aryl acetylenes with acyhum salts in the presence of aromatics gives a,P-unsaturated ketones with a trisubstituted double bond. The mild reaction conditions employed do not cause direct acylation of aromatics. 

Halogenation and Hydrohalogenation. Halogens add to the triple bond of acetylene. FeCl catalyzes the addition of CI2 to acetylene to form 1,1,2,2-tetrachloroethane which is an intermediate in the production of the industrial solvents 1,2-dichloroethylene, trichloroethylene, and perchloroethylene (see Chlorocarbons and chlorohydrocarbons). Acetylene can be chlorinated to 1,2-dichloroethylene directiy using FeCl as a catalyst... 

The above cycloaddition process consists of two separate [3-1-2] cycloaddition steps and represents a 1,3-2,4 addition of a multiple bond system to a hetero-1,3-diene [7S7]. The structure ot the azomethine imine intermediate has been proved unequivocally by X-ray analysis [195] Ethylene [194], acetylene [/iS2] . many alkyl- and aryl- as well sgemmal dialkyl- and diaryl-substituted alkenes [196,197, 198, 199], dienes [200], and alkynes [182, 201], certain cyclic alkenes [198, 199,... 

Hydrazides of vicinal acetylene-substituted derivatives of benzoic and azole carboxylic acids are important intermediate compounds because they can be used for cyclization via both a- and /3-carbon atoms of a multiple bond involving both amine and amide nitrogen atoms (Scheme 131). Besides, the hydrazides of aromatic and heteroaromatic acids are convenient substrates for testing the proposed easy formation of a five-membered ring condensed with a benzene nucleus and the six-membered one condensed with five-membered azoles. 

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