Unsymmetrical acetylenes

Addition of zirconocene to unsymmetric acetylenes is often regiospe-cific. Thus the reaction of 1-trimethylsilyl-l-propyne (50) with zirconocene dichloride and magnesium amalgam gives the zirconocycle 51, where the large trimethylsilyl groups are adjacent to zirconium.21 Iodination of 51 affords diiodide 52, which has been converted to the corresponding di-stibaferrocene 55 and dibismaferrocene 56. 

W(CCMe3)(0CMe3)3 reacts rapidly with unsymmetric acetylenes to give the initial metathesis products, RC=CCMe3 and/or R C CCMe3, and the symmetric acetylenes catalytically. The most impressive is the metathesis of 3-heptyne where the value for k (M-1 sec-1) 1s between 1 and 10. Therefore, in neat 3-heptyne ( 1 M) at 25° the number of turnovers 1s of the order of several per second. If we assume that W(VI) or Mo(VI) alkylidyne sites or complexes are responsible for the relatively slow metathesis in the known heterogeneous (30) and homogeneous (31) systems, then 1t becomes clear that the concentration of active species on the surface or 1n solution must be extremely small. 

As well as undergoing carbonyl addition, titanacyclopentadiene intermediates generated from two unsymmetrical acetylenes have been shown to react with ethynyl para-tolyl sul-fone to afford an aryltitanium compound of the structure shown in Scheme 9.6 [34], The reaction may proceed according to path a or path b, as shown in Scheme 9.7. In path a, the first step should be regioselective [4+2] cycloaddition of the titanacyclopentadiene with the sulfonylacetylene to afford the bicyclic titanacycle, at least in an equilibrium concen-... 

Another problem with this method of synthesis is that unsymmetrical acetylenes can, and usually do, give two isomeric triazoles (Scheme 2). The sterically less hindered isomer is by no means always the major product In addition to phenylacetylene, for example, the 5-phenyl-triazole often predominates in the product mixture. 

Few unsymmetrical acetylenes give exclusively one isomer. Ynamines are exceptional in that they are reported to give solely the isomer with... 

In 05JOC5545, unsymmetrical acetylene derivatives 58 and 59 were described. 

Compounds 332a, b possess an azomethine imine structure and should undergo cycloaddition reactions.4208 Although dimerization of 332b has not been observed,335 dipolarophiles yield cycloadducts 444 and Michael adducts 445.420a The structure of the product obtained from methyl propiolate, an unsymmetrical acetylene, has been studied by NMR and shown to be 446.420b... 

The A pyrroline-A-oxides (SO) undergo violently exothermic reactions with DM AD in the absence of solvent.95 In ether at room temperature, unstable products (51) that rearrange exothermically to the pyrrolines (52) are formed this ring fission recalls that of isoxazolium salts.96 With unsymmetrical acetylenes, two modes of cycloaddition are possible. Propiolic acid adds to 50 (R = H), giving an unstable solid... 

The very unsymmetrical acetylene 48, with a ketone on one side of the alkyne and a cA-alkene on the other, hydrates completely regioselectively9 to the diketone 49. 

An unsymmetrical acetylenic intermediate is formed from 3-bromo-2-cyclooctenone (177) which gave on reaction with base in methanol-d a substituted methoxyether (179) with over 90% vinylic deuterium (Eaton and Stubbs, 1967). The intermediate was shown to be 2-cyclo-... 

A detailed investigation of the reaction of protoporphyrin 80b with TONE in chloroform demonstrated the kinetically controlled formation of [2 + 2]-adducts (83), which either rearranged to [4 + 2]-adducts (84) or lost TCNE in a cycloreversion. A dipolar intermediate was assumed for these reactions (Scheme 4). Depending on reaction conditions, [2 + 2]- (85), [4 + 2]- (82, 86c,d), and mixed-type (86a,c) adducts were obtained (80JOC5196). Regio-and stereospecificity of the Diels-Alder reaction of 80b with unsymmetric acetylenes has been studied (86JOC1094). 

It was also discovered that the exo-isomer was formed exclusively in the reaction (except in cases involving norbornadiene where the formed exoisomer underwent irreversible isomerisation to the entfo-isomer, catalysed by dicobalt octacarbonyl) and noted when the products are derived from unsymmetrical acetylenes (MeC=CH or PhC=CH) only a single position isomer is isolated... it is readily shown that the single substituent always occurs on the carbon atom adjacent to the carbonyl group , 3... 

In the addition to unsymmetrical acetylenes the orientation is controlled by electronic effects. Thus, while in the reaction of phenyl azide with phenylacetylene the two isomeric diphenyltriazoles 252 and 253 were obtained in almost equal amounts , methyl propiolate yielded mainly l-phenyl-4-carbomethoxy-1,2,3-triazole (254) and only minor amounts of the isomeric 255 °. Dimethylaminoacety-lene yielded l-phenyl-5-dimethylamino-1,2,3-triazole (256) as the sole product... 

Ti( 72-propene)(OPr1)2 reacts with the two unsymmetrical acetylenes But02C-G=C-C6H13 and HC=C-G6H13 to give a dialkoxo-titanacyclopentadiene derivative (Scheme 99) which upon treatment with ethynyl tolyl sulfone produces a new single aryl-titanium compound.194... 

H n.m.r. spectra of complexes of unsymmetrical acetylenes and phosphines show coupling to two nonequivalent phosphorus atoms showing that the square planar environment is maintained in solution 3>. However rotation of both ethylene and acetylene in zerovalent complexes has been predicted 115T... 

Aryl dialdehydes were reacted with symmetrical dialkylacetylenes in the presence of a niobium(lll) catalyst to afford 2, dialkyl-1-naphthols in high yield. With unsymmetrical acetylenes, the formation of two products is highly dependent upon the structure of the alkyl groups (ref.77). 

Unsymmetrical acetylenes (81 R = CsFs or i-CgF ) are formed in high yields by pyrolysis (720 or 680 °C, respectively) of the corresponding bispentafluoro-phenylpyridazines (79), prepared as shown (Scheme 20). Tetrakispentafluoro-... 

The insertion of disubstituted unsymmetrical acetylenes R C CR into the Zr—H bond of [Zr(Cl)(H)( j-C 5115)2] gives mixtures of the two isomeric cw-vinylic products (1) and (2) in which the direction of the cis addition is dictated by the bulk of the substituents R and R . The initial mixture is catalytically converted after a few hours by excess hydride into one with higher regioselectivity, possibly via an intermediate dimetallated alkyl, [(r -C5H5)2Zr(Cl)CHRiCHR2(Cl)Zr( /-C5H5)2]. 

Electrophilic Additions to Acetylenes.— Addition of aluminium-hydrogen across unsymmetrical acetylenes (165) can be directed to yield cis or trans addition products. cw-Addition is promoted by the presence of one equivalent of tertiary amine. The aluminium-carbon bond in the products is... 

HgRUg (Ug-COMe) (CO)g or HRu ( i-COMe) (CO) g with acetylenes results in coupling to yield complexes such as (178) with unsymmetrical acetylenes, regioisomers are obtained. Hydrogenation of (178)... 

Acetylene itself can be used in cross-coupling using the method described. The use of water, however, enables one to add calcium carbide to the reaction mixture to generate C2H2 in situ, thus allowing the hazards of handling gaseous acetylene to be avoided. This approach opens the route to a one-pot procedure for the assembly of disubstituted unsymmetrical acetylenes by... 

With unsymmetric acetylenes the cycloaddition proceeds with complete regioselectivity. 

---