Intermolecular dimeric complexes

Fig. 3.7. Structures of Kaifer s intermolecular dimeric complexes formed by viologen-substituted -CDs [29].

Fig. 3.8. Structures of Kaneda s intermolecular dimeric complexes and tetramer formed by azobenzene-derivative-substituted permethylated a-CDs [31].

Of course, the strength of the intermolecular three-electron bond depicted is lower than that of the intramolecular three-electron bond (see Section 3.2.3). Interestingly, no dimer complex... 

Pyrroles having electron-withdrawing substituents in the /3-position are usually less soluble in nonpolar solvents than are the corresponding a-substituted compounds. As the importance of the zwitterionic canonical form to the resonance hybrid is less for the /3-isomers than for the a-substituted compounds, the lower solubility of the -substituted pyrroles most probably results from a difference in the intermolecular association of the two compounds. Pyrroles that have carbonyl substituents in the a-position readily form dimeric complexes via NH-0=C hydrogen bonds (25) in the liquid phase and in concentrated solution. The dimeric species may also exist in the solid phase and only in very dilute solutions (< ca. 10-4 M) do the com-... 

Of course, the strength of the intermolecular three-electron bond depicted here is lower than that of the intramolecular three-electron bond (see section 3.2.3). Interestingly, no dimer complex (t-Bu2S. . St-Bu2)+ was formed. The strength of the S S bond in this dimer is smaller than the strength of the S. . O bond in the (t-Bu2S OH2)+ complex (As-... 

An important physical property of 66 and its simple 1-alkyl homologs is the capacity to form dimeric complexes in solution as the temperature is lowered. No covalent bond is formed the complexes are believed to be 7t-complexes, and their formation is accompanied spectroscopically by the appearance of characteristic intermolecular charge-transfer transitions.245... 

In contrast to intermolecular reactions intramolecular substitutions may offer higher diastereoselectivities but again, complete lack of stereoselectivity is often encountered. Allylsi-lanes can serve as precursors for the stoichiometric preparation of 7c-allylpalladium(II) complexes12. If a stereogenic center is present in the molecule, diastereomeric complexes are formed, whose ratio can be determined by H NMR. Ionization of the dimeric complex 3 with triphenylphosphane gives a zwitterion which cyclizes in a 3-exo-trig mode to the vinyl cyclopropane derivative 4 with unknown diastereomeric ratio. For the cyclization step, dimethylsul-foxide and acetonitrile arc effective solvents, whereas with tetrahydrofuran and methylene chloride no reaction is observed. 

Kaifer et al. [29] reported electrochemically controlled intermolecular dimeric com-plexation of viologen-modified fi-CDs in aqueous solutions (Fig. 3.7). Harada et al. [30] have found that the H NMR signal of the phenyl part of the 6-hydroxycinnna-moyl a-CD (6-HyCiO-a-CD) gave slight peak-shifts as the concentration increased. They found that 6-HyCiO-a-CD formed a weak intermolecular complex in aqueous solutions (Fig. 3.6 (b)). Kaneda et al. [32, 33] previously reported the formation of a cyclic dimer formed by azobenzene-derivative-substituted permethylated a-CDs in equilibrium (Fig. 3.8). Liu et al. [34, 35] reported on the dimerization... 

The octakis(crown-ether) (78) does not present the same changes in absorption spectrum as described above with the tetrakis(crown-ether) (72) in the presence of metal ions (Li, Na, K ) there was no observable change in the spectrum. It was concluded that K" " forms an intramolecular sandwich-type complex and not an intermolecular dimer as does (72) <89CB291>. 

With propargylic alcohols, in general, allenylidenes are formed. However, in the case of HC=CCMe2(OH), an intermolecular dimerization of the allenylidene ligands takes place to yield a vinylidene arbene complex. In the presence of base, this dimeric complex transforms into the corresponding alkynyl-carbene ruthenium complex. 

The first hydroaminations by this mechanism were reported by Bergman with zircono-cene complexes and by Livinghouse with monocyclopentadienyl titanium and zirconium complexes. Bergman reported the intermolecular addition of a hindered aniline to an alkyne. The hindrance of the aniline was important to prevent formation of stable dimeric complexes containing bridging imido groups. Livinghouse reported intramolecular reactions that occurred at lower temperatures over shorter times. The intramolecularity of this process allows the [2+2] cycloaddition of the imido complex with the alkyne to be faster than the dimerization. 

Muller then intensively studied rhodium dimer complexes as catalysts for the intermolecular amination of alkanes using iminoiodinanes, typically PhI=NTs and PhI=NNs [28-30]. Good yields were obtained for amination of benzyUc, aDylic and tertiary C—H bonds, which are the most reactive C— H bonds towards metal nitrenes. For instance the amination of indane provided the corresponding benzylic amine in... 

One feature of these dimeric complexes is the long intermolecular Pt—Pt distances between dimers in the pyrimidine series the dimers are essentially unique. The association of two head-to-head dimers appears... 

In the late 1990s de Mendoza et al. designed hybrids between calix[4]arene compounds and two cytosine-like amino-pyrimidinone derivatives (27 and 28) which are attached to the scaffold at the 1,3-positions. The resulting hybrids adopt a 1,3-alternate conformation and form non-covalent dimers with two sets of four intermolecular DDAA hydrogen bonds (Fig. 24.13). The dimeric complexes... 

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