Acetylenic units

This excess hydrogen is normally carried forward to be compressed into the synthesis loop, from which it is ultimately purged as fuel. Addition of by-product CO2 where available may be advantageous in that it serves to adjust the reformed gas to a more stoichiometric composition gas for methanol production, which results in a decrease in natural gas consumption (8). Carbon-rich off-gases from other sources, such as acetylene units, can also be used to provide supplemental synthesis gas. Alternatively, the hydrogen-rich purge gas can be an attractive feedstock for ammonia production (9). 

Protection of an acetylenic hydrogen is often necessary because of its acidity. The bulk of a silane can protect an acetylene against catalytic hydrogenation because of rate differences between an olefin (primary or secondary) vs. the more hindered protected alkyne. Trialkylsilylacetylenes are often used as a convenient method for introducing an acetylenic unit because they tend to be easily handled liquids or solids, as opposed to gaseous acetylene. 

The penultimate example of macrocycles based on phenyl and acetylenic units has been the very recent report by Tobe [801 and Rubin [81] of cyclophane 134. Both groups generated 134 in the mass spectrometer by laser desorption of hexa-protected polyynes 135 (robust) and 136 (unstable), respectively (Scheme 31). 

Fig. 2. A poly(phenylacetylene) dendrimer with an energy gradient across the conjugation length of the acetylene units from the periphery to the focal point...

The scope of the SH reaction encompasses sp-sp2 (alkenyl, aryl (29),143 heteroaryl) and sp-sp couplings (the modified Cadiot-Chodkiewicz reaction).142 Iodides are most frequently used as electrophilic coupling partners, though the use of bromides, triflates (30),144 or even some reactive chlorides (31)145 is also possible. Due to the low steric bulk of the acetylenic unit, as well as its exceptional ability in the transduction of electronic effects, the SH reaction is well suited for construction of new (e.g., star-like) molecular architectures through polysubstitution (32).146... 

No defined complexes could be isolated from reactions of complex 1 with acetone Me2C=0. Complexes 2a and 2b react with acetone to give the zirconafuranone 2c, which is an interesting zirconocene precursor in view of its extremely good solubility in hydrocarbon solvents and because of its ability to dissociate into the alkyne complex [2f], It is also possible to cleanly substitute the bis(trimethylsilyl)acetylene unit so as to obtain the complex 47, or, alternatively, to substitute the acetone with formation of the zirconafuranone 95 (Fig. 10.14) [2f],... 

As may be anticipated from the chemistry of the related iron complex, which has been extensively reviewed (108), the interaction of alkynes with both Ru3(CO)i2 and Os3(CO)j2 leads to a range of complexes involving addition of one or more acetylenic units to the cluster. 

Reaction of acetylenic complexes with triosmium dodecacarbonyl leads to a variety of products involving one, two, or three acetylenic units. As with ruthenium, for the monosubstituted alkynes, hydrogen transfer can occur to the metal cluster. Thus, Os3(CO)12 and phenyl-acetylene (L) yield, in refluxing benzene, the derivatives Os3(CO)10L, Os3(CO)10L2, Os3(CO)9L, and HOs3(CO)9(L-H). The general chemistry is summarized in Scheme 2 (131). 

In 1999, Sekiguchi et al. prepared the first silyl acetylene dendrimers 255 and 256 with up to 22 Si atoms and 21 acetylene units (Schemes 35 and 36).358 The preparation of 255 and 256 has been achieved by the reaction of the triple Grignard reagent 257 with the trisila diyne 258 and the heptasila hexayne 259, respectively. 

Photoelectron and electron transmission spectroscopy indicate that there is appreciable interaction between the acetylene units of [129] (Houk et al., 1985). Both homoconjugation and hyperconjugation are proposed. Dewar and Holloway (1984) suggest that the through-bond interactions dominate. Similar thermochemical studies to those performed with the triquinacene series were carried out on [129] and some acyclic homoconjugated acetylenes (Scott et al., 1988). From these data it was concluded that decamethyl[5]pericyclyne should be classed as a homoaromatic molecule. As already discussed for the triquinacene series, the species used as non-homoaromatic models (and the calculated compensations for strain energies) may be inappropriate and thus this conclusion should be treated with some caution. Using our probes for homoaromaticity we were not able to obtain any evidence in support of the homoaromaticity of [129] (Williams and Kurtz, unpublished results). 

Kukula H,Veit S, Godt A (1999) Eur J Org Chem 277 Synthesis of monodisperse oligo-PEs using orthogonal protecting groups with different polarity for terminal acetylene units... 

The starting Fischer-type carbene complexes 1 were obtained by Michael addition of dimethylamine to the carbon-carbon triple bond of the corresponding ethoxy-(phenylethynyl)carbenes. In this regard, de Meijere and co-workers observed that the reactions of several primary and secondary amines with this sort of carbenes, in particular chromium derivatives 3 containing bulky substituents at the terminal carbon of the acetylenic unit, result in formation of the aminoallenylidene derivatives 5 as by-products of the expected Michael adducts 4 (Scheme 2) [20-24]. 

Ethynylated dihydrofullerenes serve as precursors for buckydumbbells (Scheme 3.3). Coupling of the desilylated compound 10 with CuCl leads to the dimer 11 [24]. Reaction of Cjq with the acetylide Li-C=C-Li leads to the dimer with a bridge consisting of one acetylene unit only. Electronic interaction between the fullerene-units in these two buckydumbbells is negligible. Further examples of CgQ-acetylene-hybrids synthesized by using cyclopropanation reactions are shown in Chapter 4. 

Pinho e Melo et al. (89) employed an intramolecular miinchnone cycloaddition to constmct several l/7-pyrrolo[l,2-c]thiazole derivatives from N-acylthiazolidines and acetic anhydride. Martinelli and co-workers (90,91) employed an intramolecular miinchnone cycloaddition to craft a series of 4-keto, 5,6,7-tetrahydroindoles (168-171) in two steps. The requisite acetylenic precursors were prepared from glutaric anhydride (or 3-methylglutaric anhydride). The overall sequence is illustrated for the synthesis of 168. An electrophilic acetylenic unit appears to be necessary for successful intramolecular 1,3-dipolar cycloaddition. 

Amino acids in general Crown ether and open chain polymer of transtetra-hydro furan, 2,5-bis acetylene unit SLM Highly selective separation and enantiomeric resolution [98,99]... 

Acetylene unites with bromine, forming acetylenic dibro mides—... 

When disubstituted acetylenes are employed, the >/2-acetylene complex, [Co(i/5-C5H5)(PPh3)(RC=CR)], can be isolated. In such cases, one can construct a cobaltacyclopentadiene with two different acetylene units by addition of other acetylenes, or one can synthesize cobaltacyclopentene complexes6 by addition of olefins. 

The authors also considered the relative influence of para substitution in the phenylethynyl compared to simply phenyl (i.e., compared to the analogous styrenes). They found that over the four substituents noted above, the stabilization energy from Eq. (6.14) varied by 5.2 kJ mol for phenylethynyl and 7.0 kJ mol for phenyl. Thus, insertion of the acetylene unit between the radical center and the aromatic ring is predicted to decrease the influence of the aryl substituent by only about 25 percent. 

SAMPLE SOLUTION (a) An examination of the structural formula of 1-heptyne reveals it to have a pentyl group attached to an acetylene unit. Alkylation of acetylene, by way of its anion, with a pentyl halide is a suitable synthetic route to 1-heptyne. 

When judged in its entirety, the information currently available on the hetero[13]annulenes allows one to draw the following conclusions (i) the presence of a highly mobile lone pair such as those associated with N", NH, and S may induce the system to undergo the type of skeletal flattening that is necessary for w delocalization only when two or three trans double bonds are present, e.g., 77, 78, 87b and 80, 81, respectively but not when one such function is available, cf. 88b and (ii) the skeletal restriction imposed by the presence by two properly located acetylenic units imparts sufficient rigidity (AS0 advantage) to the system to allow... 

While the carbonyl group is a very common starting point for bond disconnections in retrosynthetic analysis, an olefinic or acetylenic unit is also a useful reference point in many instances. This is because a terminal acetylene can be used as an... 

Structural features of 1 are an oxazinone moiety and a cyclopropyl acetylene unit connected to a chiral quaternary carbon center. 

Compound similar to 132 but lacking the bond linking the two acetylenic units. 

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