Open-chain analogues

Conjugated dienes sucb as butadiene and its open-chain analogues can act as 17 ligands the complexes are u.sunlly prepared from melal carbonyl complexes by direct replacement of 2CO by the diene. Isomerization or rearrangement of the diene may occur a.s indicated schematically below ... 

Compound [5a] is the open-chain analogue of [5b], but, if [5a] is diprotonated, the two positive charges can avoid each other. Therefore log Ki and log K2 values only differ by 0.8 units (see Table 2). For statistical reasons, the pK i and pi a2 of a diacid with identical acidic functions differ by 0.6 pATa units. Thus in [5a] there is hardly any influence of the first on the second protonation. 

A comparison between concave pyridines [13] monomacrocychc compounds [14] and [15] and open-chain analogues [16] shows a macrocyclic effect. The open-chain analogues [16] possess very small basicities [(log K)re = —1.3 to —1.6]. In the monomacrocychc systems, those which contain ether oxygens, [14] and [15b], had larger (log A)rei values. 

Table 18 p/fa values of macrocycle [36] containing two intra-annular carboxylic groups and an open-chain analogue [37] in DMSO/H2O (2 1 v/v). 

Ni(cyclam)l2+ gives smaller coordination equilibrium constants than the open chain analogue, [Ni(2,3,2-tet)] +. A comparative study of donor solvent coordination to Ni11 complexes of 2,3,2-tet, cyclam, the smaller [12]aneN4, and [14]aneS4 has been performed.1594... 

A careful examination of the results given in Tables 7 and 8 reveal that with the exception of 2,3-dimethyl-2-butene (in the case of 47) only cyclic guest molecules are taken up into the lattices of the host compounds 46-48, but not the respective open-chain analogues. Saturated 2,3-dimethylbutane, as a compound for comparison, is not accommodated either, either by 46 or by 47. Moreover, only cycles with distinct ring sizes (five- to eight-membered rings) are effective, indicating the presence... 

The second mechanism, due to the permutational properties of the electronic wave function is referred to as the permutational mechanism. It was introduced in Section I for the H4 system, and above for pericyclic reactions and is closely related to the aromaticity of the reaction. Following Evans principle, an aromatic transition state is defined in analogy with the hybrid of the two Kekule structures of benzene. A cyclic transition state in pericyclic reactions is defined as aromatic or antiaromatic according to whether it is more stable or less stable than the open chain analogue, respectively. In [32], it was assumed that the in-phase combination in Eq. (14) lies always the on the ground state potential. As discussed above, it can be shown that the ground state of aromatic systems is always represented by the in-phase combination of Eq. (14), and antiaromatic ones—by the out-of-phase combination. 

Thermodynamic aspects of the interaction of metal ions with macrocyclic ligands have been well studied. In many instances such studies have involved a comparison of the behaviour of cyclic ligand systems with that of their open-chain analogues. In this manner, information concerning the thermodynamic consequences arising from the cyclic nature of the macrocyclic ligand has been obtained. Frequently these studies have been restricted to stability constant (log K) measurements and, for such studies, a variety of techniques has been employed (Izatt etal., 1985). 

Table 6.1 summarizes the thermodynamic parameters relating to the macrocyclic effect for the high-spin Ni(n) complexes of four tetraaza-macrocyclic ligands and their open-chain analogues (the open-chain derivative which yields the most stable nickel complex was used in each case) (Micheloni, Paoletti Sabatini, 1983). Clearly, the enthalpy and entropy terms make substantially different contributions to complex stability along the series. Thus, the small macrocyclic effect which occurs for the first complex results from a favourable entropy term which overrides an unfavourable enthalpy term. Similar trends are apparent for the next two systems but, for these, entropy terms are larger and a more pronounced macrocyclic effect is evident. For the fourth (cyclam) system, the considerable macrocyclic effect is a reflection of both a favourable entropy term and a favourable enthalpy term. 

While the rates of alkaline and pH-independent hydrolysis of [93] and [94] are similar, hydronium ion catalysis is 2500 times less favourable in the case of the cyclic acylal. The large rate difference in comparison with the open chain analogue may reflect the fact that, if a carbonium ion intermediate with a carboxyl group held rigidly adjacent to the carbonium carbon atom were formed in an A-1 reaction, rapid reclosure of the ring would result from recapture of the carbonium carbon by the carboxyl group [equation (55)]. A greatly diminished rate of hydrolysis would result. Perhaps there is a... 

Table 3.3. THE ACTIVITY OF OPEN CHAIN ANALOGUES OF GUANETHIDINE r r N-CH2-CH2-NH-C( NH)NH2...

Maleuric acid (A -carbamoylmaleamic acid, XVIII), when injected into mice bearing Ehrlich ascites tumours, can produce cytoplasmic abnormalities in all phases of mitosis. This acid also inhibits the incorporation of tritiated thymidine into DNA, and prevents the progression of premitotic cells into mitosis [217]. This substance, which is an open-chain analogue of orotic acid, may possibly be an antimetabolic of this pyrimidine or related compounds. 

A variety of concave pyridines 3 (Table 1) and open-chain analogues have been tested in the addition of ethanol to diphenylketene (59a). Pseudo-first-order rate constants in dichloromethane have been determined photometrically at 25 °C by recording the disappearance of the ketene absorption [47]. In comparison to the uncatalyzed addition of ethanol to the ketene 59a, accelerations of 3 to 25(X) were found under the reaction conditions chosen. Two factors determine the effectiveness of a catalyst basicity and sterical shielding. Using a Bronsted plot, these two influences could be separated from one another. Figure 4 shows a Bronsted plot for some selected concave pyridines 3 and pyridine itself (50). 

Kapadia, G. J., Y. N. Shuykla, and S. P. Basak. 1978. Melovinone, an open chain analogue of melochinone from Melochia tomentosa. Phytochemistry 17 1444-1445. 

Various open-chain analogues [74] of 1-deoxynojirimycin, such as (32), were synthesized as potential glucosidase inhibitors and, surprisingly, exhibited good... 

The basicities of saturated heterocycles are similar to those of analogous open chain systems, with the exception of three-membered heterocycles, in which the basicity is markedly reduced. Table 1 gives pvalues for the equilibria between free and monoprotonated heterocycles. As the ring size increases, the protonated species become more stable and the pKa values approach those of the open chain analogues. Increasing basicity (thiirane < oxirane < aziridine) prevails in gas phase proton affinities (Table 2) (80JA5151). 

Heterocycle Proton affinity (kJ mol ) Open chain analogue Proton affinity (kJ mol )... 

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