Intramolecular solvation

The lack of solvent separated pairs raises the question whether some alternative mode of solvation should be considered. The ester group of the penultimate unit of the polymer, or the one preceeding it, could act as a solvating agent. The idea of intramolecular solvation was proposed by several workers in the field 37) and it is supported by the results of nmr studies of polymethyl methacrylate formed under various experimental conditions 38). Hypothetical structures such as those depicted below were proposed 39 h... 

The proposed intramolecular solvation is not the only feature differentiating between the polystyryl and polymethyl methacrylate salts. The former are classified as the salts of carbanions, whereas the latter are ambident salts having the character of allylic enolates with the cation interacting with the partially negatively charged carbon and oxygen atoms. The degree to which the one or the other interaction is favored is affected by the size of the cation. 

In dimethoxyethane DME, a more powerful solvating agent than THF, solvation by solvent molecules competes with the intramolecular solvation, increasing the reactivity of ion-pairs. Indeed, the propagation constants of Na+ and Cs+ salts of polymethyl methacrylate are higher in that solvent than in THF, although again both salts are nearly equally reactive 39) as shown in Fig. 5. 

The behaviour of the barium salt of poly-4-vinyl pyridine is similar to that of barium poly-2-vinyl pyridine85). However, its ionic dissociation is substantially higher than that of the latter salt because the nitrogen in para position cannot contribute to intramolecular solvation of the cation that binds it to the chain and prevents its dissociation. 

A special case of the internal stabilization of a cationic chain end is the intramolecular solvation of the cationic centre. This can proceed with the assistance of suitable substituents at the polymeric backbone which possess donor ability (for instance methoxy groups 109)). This stabilization can lead to an increase in molecular weight and to a decrease in non-uniformity of the products. The two effects named above were obtained during the transition from vinyl ethers U0) to the cis-l,2-dimethoxy ethylene (DME)1U). An intramolecular stabilization is discussed for the case of vinyl ether polymerization by assuming a six-membered cyclic oxonium ion 2) as well as for the case of cationic polymerization of oxygen heterocycles112). Contrary to normal vinyl ethers, DME can form 5- and 7-membe red cyclic intermediates beside 6-membered ringsIl2). 

Optimization of the valence and dihedral angles yields planar cyclic structures for the 3- to 5-ring intermediates in contrast to a chair conformation for that of the 6-ring. In the cases of n = 4, 5, 6 the oxygen atom is placed almost in the plane of the three C-atoms directly bonded to it. Therefore, an intramolecular solvation of the cationic chain end by methoxy groups which are bonded to the polymer backbone is preferred in the gas phase. The calculations show that for a non-polar solvent such as CH2C12 a decrease in stability of the cyclic intermediates exists. But this decrease does not result in a total break of the intramolecular solvation (Table 13). An equilibrium between open chain and cyclic intermediates must only be taken into account in more polar solvents, due to the competition of intra- and intermolecular solvation. 

A kinetic study of the previously reported substitution of aromatic nitro groups by tervalent phosphorus has established an aromatic 5n2 mechanism. Similarities in values of activation energies, and in relative reactivities of phosphite and phosphonite esters, between this displacement and the Arbusov reaction suggest a related mechanism (31), while the lack of reactivity of p-dinitrobenzene is attributed to the need for intramolecular solvation (32). The exclusive formation of ethyl nitrite, rather than other isomers, is confirmed from the decomposition of triethoxy-(ethyl)phosphonium fluoroborate (33) in the presence of silver nitrite. A mechanism involving quinquevalent phosphorus (34) still seems applicable, particularly in view of the recent mechanistic work on the Arbusov reaction. ... 

The higher reactivity of 2-vinylpyridine relative to styrene has been attributed to a combination of intramolecular solvation and triple-ion formation [Sigwalt, 1975 Soum et al., 1977]. 

This indicates that a close contact of the carbanion with the counterion favours isotactic placements as well as short sequence length (corresponding to persistence ratios below 1). In the system Cs/THF the marked non-Bernoullian behaviour can be described by Markovian statistics rather than the Coleman-Fox model, i.e. the penultimate monomer unit influences the stereochemistry of the monomer addition (29). This effect can be interpreted by decreasing external solvation (III,IV) and increasing intramolecular solvation (I,II). 

These elimination processes were described in terms of an intimate ion-pair mechanism involving a neighboring COOCH3 group participation and an intramolecular solvation of the chloride ion (X = Cl), which may proceed via two directions giving noncyclic and cyclic products (equation 74). 

The reported analyses of the products suggested that the COOCH2CH3 substituent provided anchimeric assistance for the reaction paths 2 and 3. The mechanism was explained in terms of an intramolecular solvation of the bromide ion through an intimate ion-pair intermediate which decomposes in two different directions (equation 74, where X = Br and CH3CH2 replaces CH3). The formation of bromobutyric acid and ethylene (path 3) indicated a normal six-membered transition state for their formation as found in primary ethyl esters pyrolyses in the gas phase. Bromobutyric acid, which is known to be unstable at room temperature, rapidly produced butyrolactone. The consecutive reaction of path 4, under the experimental conditions, was ascribed to a similar mechanism where an intimate ion-pair intermediate is formed through COOH participation (equation 76). 

There is no doubt that the CH3S group is providing anchimeric assistance through the favorable cyclic five-membered transition state. In this respect, the reaction mechanism via an ion-pair intermediate suggested intramolecular solvation of the leaving chloride ion to form only tetrahydrothiophene and CH3C1 (equation 89). 

The solvation of ion pairs may also arise from intramolecular interaction. For example, the high reactivity of living poly(2-vinylpyridine) is probably caused by the intramolecular solvation of the Na+ ion by the adjacent pyridine rings (16, 23, 32). Interesting example of such a solvation has been discovered by Smets and van Beylen (30), who studied anionic polymerization of p- and o-methoxystyrene. The ion pair of the latter living polymer, but not of the former, showed exceptional reactivity, and the model reveals that only the o-methoxy group can participate in the intramolecular solvation. 

In Sections 5, 6 and 7, three different approaches to the problem of silylium ions in solution are described. First, the typical gas phase versus solution phase ab initio (DFT) description of silyl compounds and silylium ions is given (Section 5). In Section 6, the NMR/ab initio/IGLO and NMR/DFT/IGLO methods are used to investigate solvation of silylium ions in different solvents. This work demonstrates how complex the solvation process of a silylium ion can be and, therefore, there is a need to generate silylium ions under well-defined situations in solution which simplify investigations. Out of this necessity, the idea of intramolecular solvation of silylium ions was born, which is discussed in Section 7. 

The incoming monomer unit would then be forced, either because of steric interactions, or by the interaction of its carboxyl group with lithium at the chain-end, to add in a specific manner to re-form the same loose ring structure present initially. One variant of this mechanism [192] involves a covalently bonded six membered ring formed by enolization of the active chain end followed by alkoxide ion attack on the penultimate carboxyl group. In polar solvents, or in the presence of moderate amounts of them, competition for solvation of the counter-ion would be produced and the intramolecular solvation producing the stereospecificity would be reduced in effectiveness as the ether concentration is increased. Replacing the lithium counter-ion with sodium or other alkali metal would be... 

This result argued for the alternate view of the EEC Cp mechanism sequence, involving initial face-selective attack on the chiral radical-cation intermediate by methoxyl radical (path A), followed by acetal cyclization and proton loss. A rationale for the increased chemical yields observed for the systems studied in Eqs. (44) and (45) was not addressed in this work, although intramolecular solvation and stabilization of the intermediate radical cation by the appended hydroxyether side chain was suggested as one of the possible factors involved in the observed stereodifferentiation [101,102]. 

Intramolecular solvated tetramers are observed for 3-lithio-l-methoxybutane (49) and frrom 1-di-methylamino-3-lithiopropane (50) in the solid state. Hiese tetramers are shown in generalized form as (51) and (52), respectively. Note the significant difference between the aggregates (51) and (52). Variable temperature Li NMR as well as NMR suggest that although the major form of 1-dimethylami-no-3-lithiopropane (50) is the diastereomer (51), this structure is presumed to be in equilibrium with (52)... 

Since HLiCO and LiCOH are intramolecular solvated organolithium species - in contrast to LiH - it is a question of interest, whether oligomerization is still a strongly exothermic process. Therefore again ab initio calculations on HF/6-3 IG level of theory are used to explore geometries and reaction enthalpies of dimers and tetramers, when the lithium to carbon monoxide ratio is 1 1. 

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