Lewis macrocycle complex

Base hydrolysis of O-coordinated JV,JV-dimethylformamide in the complex [Co([15]aneN3)DMF]3+ (40)187 is accelerated some 327-fold compared with the free ligand. The effect is considerably less marked in the macrocyclic complex when comparisons are made with the [Co(NH3)5(DMF)]2+ complex (rate acceleration ca. 104). The Lewis acidity of Co111 is presumably reduced by the stronger cr-donors of the macrocyclic ligand. 

Many examples are known in which multiple components are brought together about a metal ion to form macrocyclic complexes. Typical examples include the formation of meso-tetraphenylporphyrin (6.21) from benzaldehyde and pyrrole (Fig. 6-20, [4+4]), or phthalocyanine (6.6) from phthalonitrile (Fig. 6-21). The formation of the tetraphenyl-porphyrin is catalysed by a range of Lewis acids, and the facile preparation from aldehydes and pyrroles has obvious implications for the bioevolution of porphyrin pigments. Virtually any benzene derivative with ortho carbon-bearing substituents can be converted to a phthalocyanine complex on heating with a metal or metal salt in the presence of ammonia or some other nitrogen source. 

Ligands for the lanthanides must strongly chelate lanthanide ions but not inactivate them as catalysts. Coordination sites must be available for catalysis and the metal ion should retain a high degree of Lewis acidity. An overall positive charge on the complex may aid in catalysis, as discussed in subsequent paragraphs. Macrocyclic complexes of the lanthanides abound (24). Few lanthanide macrocyclic complexes, however, are inert to metal ion release in water. For example, perhaps the most well-known class of lanthanide macrocyclic compounds are the crown ethers. Crown ether complexes are synthesized under anhydrous conditions and are known to hydrolyze in water. 

Wuest. J.D. Zacharie, B. Multidentate Lewis acids. Complex of macrocyclic tetradentate organomercuric perfluoroglutarate. J. Am. Chem. Soc. 1987, 109, 4714-4715. 

Nadeau, F. Simard. M. Wuest, J.D. Multidentate Lewis acids. Complex of a macrocyclic host with a bidentate guest. Organometallics 1990. 9. 1311-1314. 

The interlocking of secondary amines with polyether macrocycles enables amines to activate H2 in the presence of B(CgF5)3. DF calculations show that the amine-macrocycle complex both preorganizes a frustrated Lewis pair minimum for H2 activation and stabilizes the product through strong hydrogen bonds. ... 

Transition metals have been used to complex Lewis-basic centers in metathesis substrates and to arrange the reacting olefins in such a way that cycliza-tion is facilitated. Olefin metathesis of 122, for example, proceeds with good yield to the bispyridine macrocycle 123 (Eq. 17) [115]. 

Hawthorne and co-workers have also produced a series of macrocyclic Lewis acid hosts called mercuracarborands (156, 157, and 158) (Fig. 84) with structures incorporating electron-withdrawing icosahedral carboranes and electrophilic mercury centers. They were synthesized by a kinetic halide ion template effect that afforded tetrameric cycles or cyclic trimers in the presence or absence of halide ion templates, respectively.163 These complexes, which can bind a variety of electron-rich guests, are ideal for catalytic and ion-sensing applications, as well as for the assembly of supramolecular architectures. 

Finally, the presence of the substituents on the nitrogen donors also influences the reactivity of the corresponding complexes significantly. Thus, utilization of the permethylated derivative in place of H2L23 drastically alters the Lewis-acidity of the metal ions, that is the coligands in the [Ni2(L19)(L )]+ compounds become more polarized than in the [Ni2(L23)(L )]+ complexes. In the latter complexes, the Lewis-acidity of the divalent nickel(II) ions is in large part consumed for the polarization of the six secondary NH functions. This explains why only the complexes of the permethylated macrocycle are able to activate and transform small molecules such as H20 or C02 (210,239,241). 

Also in macrocyclizations the conformation of the starting diene has an influence on the RCM/ADMET ratio. Substituents on the diene which form hydrogen bonds between each other or with the intermediate carbene complex, or external Lewis acids can have an important effect on the course of the reaction. Such effects, often subtle, might explain the variable results sometimes obtained in macrocyclizations [842], but make synthesis-planning and the formulation of general guidelines difficult. 

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