Bonding acetylene

The following acid-catalyzed cyclizations leading to steroid hormone precursors exemplify some important facts an acetylenic bond is less nucleophilic than an olelinic bond acetylenic bonds tend to form cyclopentane rather than cyclohexane derivatives, if there is a choice in proton-catalyzed olefin cyclizations the thermodynamically most stable Irons connection of cyclohexane rings is obtained selectively electroneutral nucleophilic agents such as ethylene carbonate can be used to terminate the cationic cyclization process forming stable enol derivatives which can be hydrolyzed to carbonyl compounds without this nucleophile and with trifluoroacetic acid the corresponding enol ester may be obtained (M.B. Gravestock, 1978, A,B P.E. Peterson, 1969). 

The direction of the long-wavelength maximum shift caused by the heterosubstitution or the introduction of substituents is deterrnined by the Forster-Dewar-Knott rule (40—42). Spatial hindrances within the symmetrical PMDs cause bathochromic effects (39,43), whereas the introduction of an acetylenic bond is accompanied by the maximum shift to the short-wavelength spectral region (44). 

Since both complete hydrogenation of acetylene or any hydrogenation of the ethylene results in the production of a less valuable product such as ethane, conditions must be chosen carefiiUy and a catalyst must be used that is both sufficiently active for acetylene hydrogenation and extremely selective to avoid ethylene hydrogenation. Since hydrogenation of acetylenic bonds proceeds stepwise and since acetylene is more strongly adsorbed on the catalytic... 

Hydrogen cyanide adds to an olefinic double bond most readily when an adjacent activating group is present in the molecule, eg, carbonyl or cyano groups. In these cases, a Michael addition proceeds readily under basic catalysis, as with acrylonitrile (qv) to yield succinonitnle [110-61-2], C4H4N2, iu high yield (13). Formation of acrylonitrile by addition across the acetylenic bond can be accompHshed under catalytic conditions (see Acetylene-DERIVED chemicals). 

Dicyanoacetylene, 2-hiitynedinitri1e, is obtained from dimethyl acetylenedicarboxylate by ammonolysis to the diamide, which is dehydrated with phosphoms pentoxide (44). It bums in oxygen to give a flame with a temperature of 5260 K, the hottest flame temperature known (45). Alcohols and amines add readily to its acetylenic bond (46). It is a powerhil dienophile in the Diels-Alder reaction it adds to many dienes at room temperature, and at 180°C actually adds 1,4- to benzene to give the bicyclo adduct (7) [18341 -68-9] C QHgN2 (47). 

A very significant recent development in the field of catalytic hydrogenation has been the discovery that certain transition metal coordination complexes catalyze the hydrogenation of olefinic and acetylenic bonds in homogeneous solution.Of these catalysts tris-(triphenylphosphine)-chloror-hodium (131) has been studied most extensively.The mechanism of the deuteration of olefins with this catalyst is indicated by the following scheme (131 -> 135) ... 

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... 

Trialkylsilanes are usually formed by the addition of a lithium or Grignard reagent to the silyl chloride, and thus, discussions related to the formation of the silyl acetylene bond will be kept to a minimum. Silyl acetylenes are prepared from the alkynylcopper(I) reagents in the presence of PPh3, Zn or TMEDA in CH3CN at 100°, 36-98% yield. It is interesting to note that the... 

The ring closure seems to involve intramolecular attack of the deprotonated methylene group of primary adduct at the second acetylene bond (75DIS). 

The addition of 1,3-dipoles to alka-l,3-diynes has been studied in less detail than that to conjugated alkadienes and alkenynes (80UK1801). Conjugated diynes get involved in [2- -3]cycloaddition at the unsubstituted acetylene bond. 

Diacetylene homologs are involved in this reaction by terminal acetylene bond to form monoadducts 5-alkynylpyrazoles 82 (65ZOR610). 

The addition of benzyl azide to monosubstituted diacetylenes initially proceeds at the terminal acetylene bond to form two regioisomeric 4- and 5-ethynyl-1,2,3-triazoles 98 and 99 along with minor amounts of the corresponding diadducts (81ZOR741 82ZOR1619). 

For methoxybutenyne the 127 128 ratio is 1 2.6 (GLC). With lengthening of the alkyl radical at the acetylene bond the ratio of isomers is changed in favor of... 

The effect of the nature of the substituent at the acetylene bond is not so noticeable. Substitution reduces the C-3 activity due to polarization effects and steric factors. As aresult, in the cyclization with hydrazines and hydroxylamines an increase in the content of 5-substituted pyrazoles and isoxazoles is observed (81UK1252). As mentioned above, nonsymmetiic nitrogen-containing binucleophiles H2N—YH (Y = O, NMe, NPh) react with l-heteroalk-l-en-3-ynes in two alternative pathways by functions H2N and YH. 

Alkylpyrimidines were obtained in 59-70% yield from higher 1-dimethyl-amino- and l-methoxyalk-l-en-3-ynes (R = Me, Et, -Pr) by their reaction with formamide (70ZOR2374). The exception was l-methoxy-5,5-dimethylhex-l-en-3-yne, from which the pyrimidine was obtained in 45% yield only, which is related to steiic hindrance for the attack at the acetylene bond. 

The interaction of l-methoxybut-l-en-3-yne with aromatic azides proceeds at the unsubstituted acetylenic bond to furnish two structural isomeric triazoles, 166 and 167 (4 1 ratio), due to the different 1,3-dipole orientations (83DIS). 

Thus, unlike enyne amines, l-alkoxy-l,3-enynes react with 1,3-dipoles by their acetylenic bond. 

A synthesis of 2-alkyl-2,3-dihydro-y-pyrones (187) from methoxybutenyne and aldehydes has been described (83TL4551). The condensation of lithiomethoxy-butenyne (184) with aldehydes at -78°C leads to the secondary alcohols 185, which form the dihydropyrones 187 via hydration of the acetylenic bond and hydrolysis of the methoxyethenyl group to the ketoenol 186 (0°C, p-TSA, THF, H2O or 30% HCIO4, 20 min) folowed by intramolecular cycloaddition. 

Thus, ynaminoketones with 1,2-diaminobenzene form benzodiazepines with retention of the dialkylamino group. The reaction occurs as a nucleophilic addition in the absence of catalysts. With Q, /3-acetylenic ketones 1,2-diaminobenzene reacts in the same manner, but under proton-catalyzed conditions (72LA24). At the same time, ynamines and enynamines furnish with 1,2-diaminobenzene substituted benzimidazoles as aresultof double attack at the acetylene bond(83ZOR926 84ZOR1648). 

Reduction of enynones to dienones is structure sensitive and is often unsatisfactory if the acetylenic bond is attached directly to the carbonyl 30J 1,52). Selectivity is improved if the acetylenic bond is terminal 52,70,71). 

This procedure, which involves the addition of an anion derived from a nitrile to an unactivated acetylenic bond under rather mild conditions, is a convenient general method for the synthesis of a-vinyl-nitriles (see Table I). The reaction proceeds smoothly in either dimethyl sulfoxide or hexamethylphosphoric triamide (see p. 103 for warning) as solvent with a tetraalkylam monium salt as catalyst. The products thus prepared are obtained in yields higher2 than those obtained under conventional conditions, which generally require higher temperatures and elevated pressures.3-4... 

A plethora of reactions has been reported for the in situ generated (OC)5W=PPh, many of which have been the topic of previous reviews [2-4]. A typical account of the versatility of this reagent with olefinic and acetylenic bonds is visuaHzed in the wagon wheel . Only a brief account with selected ex-... 

An X-ray structure analysis of 74 (R=C4Hg) revealed that the unsaturated portion of the molecule was planar, with the angles between adjacent acetylenic bonds deviating by 13 -15° from 180°, the value for a strain-free molecule. Since the connection of the alkyne moieties to the aromatic rings was only shifted slightly (2-3°), distortion of the acetylene linkages appears as the major source of instability in these macrocycles. 

Structural elements such as allenic or acetylenic bonds, epoxydes, fnran-oxides, and C45 or C50 carotenoids are not found. 

With aluminium triacetylide, trimethylamine forms a complex which is extremely unstable. This is probably due to the presence of triple acetylene bonds. 

Fleming, I., and J. Harley-Mason A New Synthesis of Acetylenic Bonds... 

The reactivity of dichloro carbene towards acetylenic bonds was systematically investigated by Dehmlow19, 20 with respect to substitution of the acetylene, especially those containing additional C-C multiple bonds. It was shown that with aiyl alkyl acetylenes, e.g. 1-phenyl-butyne-l, often the normal cyclopropenone formation occurs only to a minor extent (to yield, e.g. 14), whilst the main reaction consists of an insertion of a second carbene moiety into the original acetylene-alkyl bond (giving, e.g. 15) ... 

Although acetylenic bonds are more reactive than C=C bonds, the reactions are often initiated by AIBN or UV radiation. Baldwin and Barden119 have used the latter method to treat a doubly labelled phenylacetylene with triphenyltin deuteride (Scheme 19). The addition of the triphenyltin deuteride was both regiospecific and gave a stereochemically pure product. A five-step synthesis (Scheme 20) converted this product into an optically pure trideuterophenylcyclopropane, which was used to study the thermal stereomutations that these compounds undergo. 

In the reaction of an unsymmetrical diyne with 9, the boryl group is introduced regioselectively to the terminal acetylenic bond (Equation (100)). 

An enyne undergoes the stannaborative cyclization with 9 in a completely regioselective fashion (Equation (101)). The boryl group is introduced exclusively to the acetylenic bond to give an isomerically pure product. These results obtained with the unsymmetrical substrates suggest that the more reactive unsaturated bond initially inserts into the Pd-B bond. 

Recently, we analyzed the role of electron repulsion relative to bond breaking and antiaromaticity effects on a quantitative basis using Natural Bond Orbital (NBO) analysis.24 Two other destabilizing factors were considered at the initial stage of the cyclization in addition to four-electron repulsion between the filled in-plane acetylenic re-orbitals - distortion/breaking of the acetylenic bonds as a result of their bending, and the fact that, at a distance of ca. 3 A, the in-plane re-orbitals become parallel and reach a geometry that resembles the antiaromatic TS of the symmetry forbidden [2S + 2S] cycloaddition (vide infra). 

An extensive computational analysis expanded the range of the c-d distances for reactive cyclic enediynes to 2.9-3.4 A.38 By comparing unsubstituted enediynes with dialkyl-substituted enediynes, it was found that the activation enthalpy is dependent on factors other than the c-d distance and that reactivity hinges on a subtle interplay of steric and electronic effects that accompany distortion caused by incorporation into a macrocycle. For example, since alkyl substituents stabilize acetylenic bonds to a greater extend than olefinic bonds,39 such substituents stabilize the starting material, thus increasing both the activation barrier and the reaction endothermicity. 

Where does the energy increase come from Can we offer an electronic description to the phenomenological term strain A priori, one can consider such factors as four-electron repulsion of filled in-plane orbitals and distortion/breaking of the acetylenic bonds as a result of their bending. 

Fig. 8 The changes in the NBO re-bond order of in-plane (re , circles) and out-of-plane (rec, diamonds) acetylenic bonds along the IRC pathway for the Bergman cyclization of (Z)-hex-3-ene-1,5-diyne.

Two basic structural modifications of acetylene bonding to all three... 

The present procedure9 represents another synthesis of cyelobutanone through the unique acetylenic bond participation in solvolysis. Cyclobutane derivatives prepared in this way include 2-methyl-, 2-ethyl-, 2-isopropyl-, and 2-trifluoromethylcyclobutanone from the corresponding acetylenic compounds.10... 

Fleming, P. R., and Hutchinson J. S. (1988), Representation of the Hamiltonian Matrix in Non-Local Coordinates for an Acetylene Bond-Mode Model, Comp. Phys. Comm. 51, 59. 

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