Ring Species

Ring opening polymerization of strained cyclic olefins, discussed previously, might exemplify such a system. The metalla-carbene, Met=CHR, is the active, and presumably labile form, while the ring species,... 

Fig. 10. Schematic drawing of the Gas ring species K2(GaC6H3-2,6-Mes2)3 [63]. H atoms are not shown...

The initial results on the [NaA ] clusters have been extended to the heterocyclic ring species [XAls] (X = Si, Ge, Sn, Pb) bearing a single charge and as a result are not prone to dissociation of an electron like the [Al4] or [Ga4H2] anions [69]. A planar four-membered ring structure is in all cases the most stable structural isomer with Al—Al distances in the range 2.57-2.61 A. 

Emphasis in the early years of macrocyclic chemistry was given to the synthesis of a large number of N4-donor cyclic systems but synthetic procedures were subsequently developed for obtaining larger-ring species incorporating more than four donors [such as (38) and (39)]. 

The results are not particularly consistent here either. This last reaction, exothermic by ca —206 kJmol-1, is not consistent with either result in equation 43. We strongly doubt the principal source of error is the difference of choice of phases employed57. The difference of the hexahydrotriazine enthalpies of formation, —18 — (—132) = 114 kJmol-1, is comparable to three times the difference of liquid pyrrolidine and piperidine, 132 kJ mol-1. (The difference for other pairs of corresponding 5- and 6-membered ring species includes 166 kJ mol-1 for cyclopentane and cyclohexane, and 162 for perhydroindene and decalin.) The origin of the ca 20 and 50 kJmol-1 discrepancies remains enigmatic. 

Ethylbenzene Isomerization Isomerization of EB requires both metal and acid function. Hydrogenation results in an intermediate naphthene. The acid function is required to isomerize the naphthene to a methyl-ethyl-substituted five-mem-bered ring species that can further convert to a dimethyl-substituted six-membered ring naphthene. This can be dehydrogenated by the metal function to a xylene isomer, OX in the example shown in Figure 14.9. 

Anions (9), (10a) and (11) derived from cycloctatetraene have been studied using the selected ion flow tube technique and MO calculations, as a continuation of previous work on the structures and energetics of eight-membered-ring species, CgH and... 

Interestingly, two of the other species in Table 3 are nitrolates, i.e. ethers of a-nitrooximes, an otherwise thermochemically unprecedented class of compounds. We already have briefly discussed one, 3-nitroisoxazoline, and the second is 1-nitroacetaldehyde 0-(l,l-dinitroethyl)oxime (ONo-ld-dinitroethyl acetonitronate), MeC (NOala—O—N=C(N02)Me. The latter acyclic species is a derivative of 1,1-dinitroethanol—we know of the enthalpy of formation of no other a-nitroalcohol or derivative. Nonetheless, we may ask if the two calorimetric data are internally consistent. Consider the condensed phase reaction 47, which involves formal cleavage of the O — bond in the nitroisoxazoline by the C—H bond of the dinitromethane. It is assumed that the isoxazoline has the same strain energy as the archetypal 5-atom ring species cyclopentane and cyclopentene, ca 30 kJ mol . ... 

Unlike aromatic borirene, only thiirene of the antiaromatic three-membered ring species (100), (123), and (127) (see Table XII) could be experimentally detected by means of matrix isolation at low temperature [81JA(103)486]. The IR spectrum of thiirene is consistent with the data of ab initio calculation [80JA2507 also see 83JA( 105)396]. 

The nitrogen-containing porphyrin ring species can be found in most asphaltic crude oils. The porphyrin ring usually holds a metal ion such as nickel or vanadium. These metals are known as powerful poisons for refining catalysts. 

Over the years it became apparent, especially from matrix-isolation studies, that the seven-membered ring species was better formulated as cycloheptatetraene (72), a cyclic allene, rather than as the carbene (71). The question of the intermediacy of 73 remains unresolved Theory supports Baron s early suggestion, but the molecule itself has never been detected in a simple system. 

Reficha also measured infrared spectra of C6H6 and of QD6 adsorbed on Ni/Al203 (252). Apart from absorption bands characteristic of physically adsorbed QH6 and C6D6, a low-wavenumber vCD band at 2050 cm"1 was observed in the latter case and also assigned to a cr-bonded Q-ring species such as Haaland had considered. 

The photolysis of 1,2,4-oxadiazoles in the presence of sulfur nucleophiles has been shown to afford 1,2,4-thiadiazoles. N—S bond formation between the ring species and the sulfur nucleophile is thought to account for the observed products.96 A review has appeared which includes an account of the rearrangement of 1,2,3-thiadiazoles to other heterocycles such as 1,2,3-triazoles and 1,2,3,4-thiatriazoles.97... 

For example, consider hydrogenation enthalpies. Besides the multi-ring species just discussed, our analysis also includes the variously phenylated and tolylated ethylenes presented in the beginning... 

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