Systems Involving Triple Bonds

Vinylacetylene,102 H C=CH—C=CH, and vinyl cyanide,103 have a planar bent structure with C=C—C bond angle 123° and C—C bond lengths 1.446 and 1.426 A, respectively, corresponding to 13 to 20 percent of double-bond character (Table 7-9 note that correction —0.06 A is made for adjacent double bond and triple bond). This agrees well with the value, 15 percent, found for butadiene. 

The single bond between a carbon-carbon triple bond and a carbon- 

The chemical properties of conjugated systems, including especially 

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Formation of rearranged products in the solvolysis of homopropargyl systems need not involve triple-bond participation and vinyl cations in all instances. Ward and Sherman investigated the formolysis of 4-phenyl-1-butyn-l-yl brosylate, 57 (80). At 80°C in the presence of one equivalent of pyridine, they observed formation of phenyl cyclopropyl ketone, 58, and... 

CS2CO3. Asymmetric hydrogenation of the reaction product in the RuCl2(PPh3)3-(/ )-BINAP system led to chiral bidentate phosphine ligand. Plausible reaction mechanism was proposed to involve triple bond activation by Cul as well as nucleophilic attack of the Cu phosphide species (Scheme 8.61) [144]. 

Compounds containing a double or triple bond, usually activated by additional unsaturation (carbonyl, cyano, nitro, phenyl, etc.) In the ap position, add to the I 4-positions of a conjugated (buta-1 3-diene) system with the formation of a ax-membered ring. The ethylenic or acetylenic compound is known as the dieTwphile and the second reactant as the diene the product is the adduct. The addition is generally termed the Diels-Alder reaction or the diene synthesis. The product in the case of an ethylenic dienophile is a cyctohexene and in that of an acetylenic dienophile is a cyctohexa-1 4-diene. The active unsaturated portion of the dienophile, or that of the diene, or those in both, may be involved in rings the adduct is then polycyclic. 

The suggested reaction mechanism involves a nucleophilic attack of the imine nitrogen at the activated triple bond, followed by a proton exchange, to give a benzimidazolinium system which, by intramolecular attack at the carbonyl group, leads to an epoxide that ring opens to the observed product. For the ethyl derivative (R = Et) a tub conformation could be established by X-ray crystallographic analysis.33... 

The selectivity for two-alkyne annulation can be increased by involving an intramolecular tethering of the carbene complex to both alkynes. This was accomplished by the synthesis of aryl-diynecarbene complexes 115 and 116 from the triynylcarbene complexes 113 and 114, respectively, and Danishefsky s diene in a Diels-Alder reaction [70a]. The diene adds chemoselectively to the triple bond next to the electrophilic carbene carbon. The thermally induced two-alkyne annulation of the complexes 115 and 116 was performed in benzene and yielded the steroid ring systems 117 and 118 (Scheme 51). This tandem Diels-Alder/two-alkyne annulation, which could also be applied in a one-pot procedure, offers new strategies for steroid synthesis in the class O—>ABCD. 

However, the observations of Ward and Sherman need not rule out triple-bond participation and vinyl cations in the systems studied by Hanack and co-workers (75-79). Presumably, the enol formate 61 itself arises via a transition state involving a rate-determining protonation and vinyl cation 62 (see previous section). A vinyl cation such as 62 with an adjacent phenyl group is considerably more stable and hence more accessible than a vinyl cation such as 63, stabilized only by a neighboring alkyl group. Hence, formation of enol formate 61 and its... 

Therefore, it appears that the overall agreement obtained for a variety of spectroseopie eonstants is comparable for the two methods while the present method allows us to use a more compact wavefunction. It should also be noted that a good Cl description of a triple bonded system involving a third period atom is much harder to achieve. It can be concluded that the shape of the theoretical potential energy curve reflects its experimental counterpart with acceptable accuracy in the interatomic region of interest. 

Thiazyl halide monomers undergo a variety of reactions that can be classified under the general headings (a) reactions involving the 7i-system of the N = S triple bond, (b) nucleophilic substitution, (c) halide abstraction, and (d) halide addition. The cycloaddition of NSF with hexafluoro-1,3-butadiene provides an example of a type (a) reaction. 

The first observations, done by Tsuji et al. in 1980 on substituted acetylenes, were reinvestigated by Brandsma in 1994 and concern the alkoxycarbonylation of these substrates, not to give an acrylate moiety, but instead acetylenic esters [111,112]. The catalytic system involves PdCl2/CuCl2/NaOAc. Copper chloride introduced in stoichiometric amounts is responsible for the CH activation, maintaining the triple bond. Such an activation recalls that observed on allenyl- and propynylhalides that classically gives the allenyl-esters [113]. 

For unsaturated lactones containing an endocyclic double bond also the two previously described mechanisms are presumably involved and the regio-selectivity of the cyclocarbonylation is governed by the presence of bulky substituents on the substrate. Inoue and his group have observed that the catalyst precursor needs to be the cationic complex [Pd(PhCN)2(dppb)]+ and not a neutral Pd(0) or Pd(II) complex [ 148,149]. It is suggested that the mechanism involves a cationic palladium-hydride that coordinates to the triple bond then a hydride transfer occurs through a czs-addition. Alper et al. have shown that addition of dihydrogen to the palladium(O) precursor Pd2(dba)3/dppb affords an active system, in our opinion a palladium-hydride species, that coordinates the alkyne [150]. 

Most metal mediated [2 + 2 + 2] cycloadditions involve two triple bonds which coordinate to a metal center to form a reactive metallocyclopentadiene species (vide infra). The corresponding reactions involving at least two double bonds and an intermediate met-allocyclopentane species are almost completely limited to norbornadiene systems. These reactions can be considered as homo Diels-Alder reactions. 

Some oxidations used the system TPAP/NMO/PMS/CH3CN (cf 2.4.2.2, Fig. 2.17) [155], cf mech. Ch. 1 [156], or TPAP/NMO/CH3CI3-CH3CN/PMS [66]. In one instance TBAP/NMO/PMS/CH Cl (rather than TPAP) was used for an oxidation in which a triple bond was affected by an oxidation involving 2-propar-gyloxy-cyclopentanone with a -CH CCH substituent [157]. 

Reactions of substituted a-ketoalkynes (RC=CCOR ) with 6-amino-l,3-dimethyluracil and a water-soluble nickel catalytic system [Ni(CN)2-CO-KCN-NaOH] afforded 2,4-dioxopyrido[2,3- pyrimidine derivatives 532 under very mild conditions (room temperature and atmospheric pressure). The mechanism involved a nucleophilic attack by Ni(0), formed in situ, onto the triple bond of the substrate. The reaction terminates within 30 min, giving 98% of 532, while in the absence of this catalytic system the reaction took a longer time (lOh) to reach a maximum yield of 30% <2001J(P1)2341>. A regioselective interaction of 6-aminouracil derivatives with GF3COCH2COR in boiling AcOH afforded the cyclized 5-trifluoromethylpyrido[2,3-, pyrimidines 533 <200381531 >. 

The general mechanism proposed for these additions starts with the interaction of the gold catalyst with the -system of the substrate (Scheme 8.1). This forms an intermediate where the double or triple bond is activated for nucleophilic attack. In most examples, addition is anti to gold delivering vinyl gold species. The final step of the reaction involves the release of the gold catalyst by protodemetallation and the addition product. 

Cycloaddition reactions represent a very versatile route to alicyclic compounds. The most important for six-membered rings is the Diels-Alder reaction, and its great utility lies in the fact that it is both regioselective and stereospecific. The reaction involves compounds containing a double or triple bond, usually activated by conjugation with additional multiply-bonded systems (carbonyl, cyano,... 

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