Intramolecular connection

Intramolecular heteroatom coordination may also influence the stabilities or structures of catenated tellurium compounds. For example, a rare example of a tritelluride, bis[2-(2-pyridyl)phenyl]tritelluride, is stabilized by a Te N contact of 2.55 The ditelluride (2-MeOCelFtCOTe) has an unusual planar structure. Although the C=0 Te interaction is longer (3.11 A) than the Me 0 contact (2.76 A), ab initio molecular orbital calculations indicate that the planarity results predominantly from the former intramolecular connection. 

Reference to the infinite structures as networks would seem inconsistent with our assumption, introduced as an approximation, that no intramolecular reactions occur. A randomly branched structure devoid of intramolecular linkages could hardly be called a network the latter term conveys the notion of circuitous interconnections within the structure. Actually, as will appear later, the assumption referred to need only be applied to the finite molecular species its extension to the infinite structure is superfluous. Certainly it will contain an abundance of intramolecular connections, which, in fact, is an essential feature of the gel structure... 

A significant part of the examples of transition metal catalyzed formation of five membered heterocycles utilizes a carbon-heteroatom bond forming reaction as the concluding step. The palladium or copper promoted addition of amines or alcohols onto unsaturated bonds (acetylene, olefin, allene or allyl moieties) is a prime example. This chapter summarises all those catalytic transformations, where the five membered ring is formed in the intramolecular connection of a carbon atom and a heteroatom, except for annulation reactions, involving the formation of a carbon-heteroatom bond, which are discussed in Chapter 3.4. 

The third term - X RT n cfrf) is the entropic correction for the total of mn - m- n+ 1 intramolecular connections in the [MmLn] complex. Terms 2 and 3 correspond to Ercolani s model. 

Do not add hydrogen atoms yet. SYBYL works with a force field. In a force field, atom characteristics and intramolecular connections are defined by atom types. 

The challenge is to derive intramolecular connectivity from chemical composition. Once this has been achieved all further molecular properties can, in principle, be deduced. However, the first step is clearly impossible As the number of atoms per molecule increases, so does the number of possible isomers, and there are no criteria for ranking the isomers. The only way to predict connectivity is from a historical record, such as a synthetic pathway, which, in turn, depends upon reaction conditions and the nature of primary reactants. 

The dimerization also brings the eight residues attached to the urea groups into a unique mutual position. This means that the residues are preorganized for controlled reactions between them, for instance, for their intramolecular connection. 

To obtain selectively a single, well-defined product, it is obviously not sufficient to arrange the reacting groups in an appropriate mutual position. A perfect preorganization also demands the separation of those functional groups which should not be involved in the reaction. For an intramolecular connection between reactive functions attached to the urea residues, this preorganization is possible in heterodimers with a non-reactive tetra-urea calix[4]arene. 

The intramolecular connection of two functional groups attached to the wide rim of calix[4]arene can be achieved under high-dilution conditions [50], If the number of functional groups increases, a selective connection becomes more and more difficult, as illustrated in Figure 5.7. For methyl ethers, the situation is even more complicated by the presence of several conformations. 

Figure 5.7 Possible side reactions and complications for the controlled intramolecular connection of functional groups R attached to the wide rim of a calix[4]arene.

Equations have been developed relating a to the extent of reaction, P. In general, the observed results of e slightly higher than those calculated indicating that some of the reactive sites are lost in intramolecular connections. Flory cites the case of pentaerythritol (/ = 4) and adipic acid. For this case, a = P2 and = 1/3 = 0.577. The experimental result was Pc = 0.63. 

Even if we consider that the chain of atoms connecting the two binding site of the ligand is long and flexible enough to produce no additional strain during the intramolecular connection... 

FIGURE 84 Intermolecular and intramolecular connection processes operating during complexation. 

When both/j- and cf are constant for all inter- and intramolecular connections in [R L ] ", the self-assembly is said to be statistical or noncooperative (Ercolani, 2003). The initial application of Eq. (99) for rationalizing the formation of Lehn s famous helicate [Cu3(Ll)2] " (Figure 3), indeed evidenced the occurrence of a noncooperative process (Ercolani, 2003), despite previous claims for positive cooperativity based on the erroneous neglect of the separation of intra- and intermolecular... 

Some examples for the dimerization of allyl compounds are also known. An intramolecular connection was reported by Bogdanovic [64]. He... 

Cavitands are more-or-less rigid, bowl shaped molecules with an enforced cavity. Many examples have been prepared from resorcarenes by intramolecular connection of the hydroxyl functions of adjacent resorcinol units [19]. Asymmetric cavitands [20] have been obtained using a similar principle as described above by introducing just two different bridges (A, B) as shown in 15. 

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