Alkynes symmetric

The alkynyltitanium complexes TiCI((C=CR) (R typically = alkyl groups) are prepared in situ by reacting 1-alkynes with TiCU in the presence of Et3N. The synthetic potential of these compounds has been illustrated in reactions with a number of electrophiles. In the presence of terminal alkynes, symmetrical 1,3-diynes are formed. The reaction with chlorodiphenylphosphine produces the corresponding alkynyl phosphine, while treatment with triethylphosphite affords tris(phenylethynyl) phosphine by substitution of the three alkoxo groups. The reaction with trimethyl orthoformate gives acetals.6-9... 

The addition of phthalimidylnitrene (374) to simple alkynes affords 1-azirines in yields of 1-15% (Scheme 10). In this reaction, which is of no real preparative value, the symmetrical 2-azirines (375) were suggested as the most plausible intermediates and unequivocal proof of the existence of such species was demonstrated from a series of 1,2,3-triazole pyrolysis reactions <71CC1518). Extrusion of nitrogen from the regioisomeric 4,5-disubstituted 1,2,3-triazoles (376) during flash vacuum pyrolysis furnished identical product mixtures which included both regioisomeric 1-azirines (377). 

The Glaser reaction is an oxidative coupling of terminal alkynes 1 to yield a symmetrical Z -acetylene 2 the coupling step is catalyzed by a copper salt. Closely related is the Eglinton reaction, which differs from the Glaser reaction mainly by the use of stoichiometric amounts of copper salt as oxidizing agent. 

The two reactions described above can be applied for the synthesis of symmetrical -acetylenes only. Unsymmetrical bis-acetylenes can be prepared by using the Cadiot-Chodkiew icz reaction For that method a terminal alkyne 1 is reacted with a bromoalkyne 8 in the presence of a copper catalyst, to yield an unsymmetrical coupling product 9 ... 

Alkynes show a C=C stretching absorption at 2100 to 2260 cm i, an absorption that is much more intense for terminal alkynes than for internal alkynes. In fact symmetrically substituted triple bonds like that in 3-hexyne show no absorption at all, lor reasons we won t go into. Terminal alkynes such as 1-hexyne also have a characteristic =C-H stretch at 3300 cm-1 (Figure 12.14c). This band is diagnostic for terminal alkynes because it is fairly intense and quite sharp. 

In 1986 Yamashida et al. found that the reaction of the (morpholino)phenyl-carbene complex 46 with symmetric alkynes 47 gave the morpholinylindene derivatives 48 and 49, as well as the indanones 50 derived from the latter by hydrolysis, in excellent yields (Scheme 9) [54]. This contrasts with the behavior of the corresponding (methoxy)phenylcarbene complex, which solely undergoes the Dotz reaction [55]. This transformation of the amino-substituted complex 46 apparently does not involve a CO insertion, which is an important feature of the Dotz benzannelation. 

Terminal alkynes can be coupled by heating with stoichiometric amounts of cupric salts in pyridine or a similar base. This reaction, which produces symmetrical diynes... 

In 2002, Braga el al. employed a chiral C2-symmetric oxazolidine disulfide as a ligand for the enantioselective synthesis of propargylic alcohols by direct addition of alkynes to aldehydes (Scheme 3.64). Good yields but moderate enantioselectivities (<58% ee) were obtained for the enantioselective alkyny-lation of aldehydes in the presence of ZnEt2. 

One drawback to this alkyne annulation chemistry is that it requires either symmetrical alkynes or unsymmetrical alkynes in which the two substitutents on the internal alkyne are sterically quite different or else one obtains mixtures of regioisomers. One way to overcome this problem is to prepare the corresponding arylalkyne through catalytic Pd/Cu chemistry and then effect electrophilic cyclization using organic halides and a Pd catalyst (Scheme 8).9... 

A combination of a metathesis and a Diels-Alder reaction was published by North and coworkers [263]. However, this is not a true domino reaction, as the dienophile (e. g., maleic anhydride) was added after the in situ formation of the his-butadiene 6/3-89 from the fois-alkyne 6/3-88 and ethylene. The final product is the fois-cycloadduct 6/3-90, which was obtained in 34% yield. Using styrene as an un-symmetrical alkene instead of ethylene, the mono-cycloadduct 6/3-91 was formed as a mixture of double-bond isomers, in 38% yield (Scheme 6/3.26). 

Organic compounds such as terminal alkynes can undergo direct mercuration using various mercury salts. For instance, alkyne 61 has been shown to react with Hg(OAc)2 to form the symmetrical bis-alkyl-mercury complex 62 (Equation (21)).73... 

The Boger group synthesis commences with a Stille coupling of a stannyl acetylene with two equivalents of a highly oxygenated bromobenzene to yield a symmetrical diarylalkyne (82). The subsequent reaction of this alkyne (82) with 3,6-dicabomethoxytetrazine under Diels-Alder/retrograde Diels-Alder... 

The symmetrical dienyne 58a was converted to a fused bicyclo [4.3.0] ring in 95% yield [17] (Eq. 27). With substrate 58c containing an unsymmetrical diene tether, two different products, 59c and 59c, were obtained in a ratio of 1 to 1 (Eq. 28). The reaction course in the formation of the different bicyclic rings is shown in Scheme 8. This dienyne metathesis is also catalyzed by tungsten or molybdenum complex 62 or 63 (Fig. 1), and a dienyne bearing terminal alkyne 58b could be cyclized to give 59b in 97% yield. 

Intermolecular-enyne metathesis, if it is possible, is very unique because the double bond of the alkene is cleaved and each alkylidene part is then introduced onto each alkyne carbon, respectively, as shown in Scheme 9. If metathesis is carried out between alkene and alkyne, many olefins, dienes and polymers would be produced, because intermolecular enyne metathesis includes alkene metathesis, alkyne metathesis and enyne metathesis. The reaction course for intermolecular enyne metathesis between a symmetrical alkyne and an unsym-metrical alkene is shown in Scheme 9. The reaction course is very complicated, and it seems impossible to develop this reaction in synthetic organic chemistry. 

Reaction of the unsymmetrical substituted acetylene Me3SiC=CPy with 1 yields, after substitution of the alkyne, the kinetically favored unsymmetrical substituted complex 55a as well as the symmetrically substituted, thermodynamically more stable product 55b [38]. 

Grigg employed a tandem Pd-catalyzed cyclization to synthesize 3,3-biindole 204 from symmetrical alkyne 203 [198], The presumed intermediate exocyclic alkene is not detected. 

Under basic conditions, obviously only one isomerization step takes place and thus a terminal alkyne will deliver 1,2-dienes selectively. With internal alkynes, on the other hand, selectivity can only be achieved when the alkyne is either symmetrical as in 14 [34] (Scheme 1.6) or has a tertiary center on one side as in 16 [35, 36] (Scheme 1.7). So, unlike potassium 3-aminopropylamide in 1,3-diaminopropane, where the Jt-bonds can migrate over a long distance by a sequence of deprotonations and reprotonations, here the stoichiometric deprotonation delivers one specific anion which is then reprotonated (in 16 after transmetalation). 

Vollhardt and colleagues have explored the reactions between diynes and enami-nes338-341. The reactions between symmetrically substituted alkynes and alkyne tethered uracil derivatives proceeded in moderate yields, producing adducts with predominant anti configurations342-343. On the other hand, the reactions between diynes and uracil derivatives produced predominantly syn isomers. 

Alkynes react with haloethenes [38] to yield but-l-en-3-ynes (55-80%), when the reaction is catalysed by Cu(I) and Pd(0) in the presence of a quaternary ammonium salt. The formation of pent-l-en-4-ynes, obtained from the Cu(I)-catalysed reaction of equimolar amounts of alk-l-ynes and allyl halides, has greater applicability and versatility when conducted in the presence of a phase-transfer catalyst [39, 40] although, under strongly basic conditions, 5-arylpent-l-en-4-ynes isomerize. Symmetrical 1,3-diynes are produced by the catalysed dimerization of terminal alkynes in the presence of Pd(0) and a catalytic amount of allyl bromide [41]. No reaction occurs in the absence of the allyl bromide, and an increased amount of the bromide also significantly reduces the yield of the diyne with concomitant formation of an endiyene. The reaction probably involves the initial allylation of the ethnyl carbanion and subsequent displacement of the allyl group by a second ethynyl carbanion on the Pd(0) complex. 

This information confirmed that a symmetrical intermediate could not lie along the reaction coordinate, although the alkyne complex 254 was still theoretically a viable intermediate. However, the stereochemical study... 

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