Captodative substitution

There have heen many studies on the polymerizability of a-substituted acrylic monomers.3jU35 33S It is established that the ceiling temperature for a-alkoxyacrylates decreases with the size of the alkoxy group. 35 However, it is of interest that polymerizations of a-(alkoxymethyl)acrylates (67)3 15 and a-(acyloxymethyl)acrylates (68)and captodative substituted monomers (69, 70) 39 appear to have much higher ceiling temperatures than the corresponding a-alkylacrylates methyl ethacrylate, MEA). For example, methyl a-... 

The concept of captodative substitution implies the simultaneous action of a captor (acceptor) and a donor substituent on a molecule. Furthermore, in the definition of Viehe et al. (1979), which was given for free radicals, both substituents are bonded to the same or to two vinylogous carbon atoms, i.e. 1,1- and 1,3-substitution, and so forth is considered. One might, however, also include 1,2-, 1,4-,. .. disubstitution, a situation which is more often referred to as push-pull substitution. Before discussing captodative substituent effects it might be helpful to analyse the terms capto and dative in more detail. 

Fig. 1 FMO diagram for the formation of a captodative-substituted radical c—C—d by successive interaction of (A) a carbon radical with a captor (c) and of (B) a captor-substituted carbon radical with a donor (d) substituent.

The foregoing discussion shows that the approach taken does not necessarily provide the organic chemist with an answer to the question of special effects on the radical centre in captodative-substituted radicals. Stabilization of the radical centre and stabilization of the complete radical structure must be considered separately. It is only the latter situation which can be dealt with by the approach of Leroy and coworkers. 

In order to find out whether captodative substitution of a methyl radical can lead to persistency, the rate of disappearance by bimolecular selfreaction was measured for typical sterically unhindered captodative radicals (Korth et al., 1983). The t-butoxy(cyano)methyl radical, t-butylthio(cyano)-methyl radical and methoxy(methoxycarbonyl)methyl radical have rate constants for bimolecular self-reactions between 1.0 x 10 and 1.5 X 10 1 mol s Mn the temperature range —60 to - -60°C. The dilTusion-controlled nature of these dimerizations is supported by the Arrhenius activation parameters. Thus, it has to be concluded that there is no kinetic stabilization for captodative-substituted methyl radicals. On the other hand, if captodative-substituted radicals are encountered which are kinetically stabilized (persistent) or which exist in equilibrium with their dimers, then other influences than the captodative substitution pattern alone must be added to account for this phenomenon. 

In addition to the stabilization by suitable substituents and the absence of other termination reactions than recombination, it is the strength of the bond formed in the dimerization which is a necessary cofactor for the observation of free radicals by esr spectroscopy. The stability of nitroxides [4] or hydrazyls [5] (Forrester et al., 1968) derives not only from their merostabilized or captodative character but also from a weak N-N bond in the dimer. The same should be the case for captodative-substituted aminyls... 

All these examples, and it would be possible to quote more, are a manifestation that captor and donor subtituents stabilize radicals. Judged by the temperature range where dissociation occurs it seems as if captodative substitution stabilizes better than dicaptor substitution (Stella et al., 1981). Mostly, however, these are qualitative or semiquantitative observations which do not allow one to evaluate the magnitude of stabilization in kcal mol". In particular, the question of a synergetic action of the captor and the donor substituent cannot be answered satisfactorily. In part, the observed effects might be related to steric interactions of the substituents. 

The discussion shows that it is difficult and sometimes ambiguous to interpret C—H BDEs in terms of radical stabilization only. Consequently, their usefulness in the context of captodative substitution appears to be questionable. 

Katritzky (Katritzky et al., 1986) has recently advanced the idea that captodative-substituted radicals should be stabilized significantly by polar solvents. This hypothesis, which is qualitatively derived from the polar resonance structures for these radicals, was supported by semiempirical molecular orbital calculations. An experimental test was carried out by Beckhaus and Riichardt (1987). For the dissociation of [24] and [25] into the radicals [21] and [28], they were unable to confirm Katritzky s hypothesis. The rate of thermolysis of [24] and [25] is not affected by a change in solvent polarity. If the stabilization were of the order of Katritzky s prediction, it should, however, have become evident in the rate measurements. The experiments thus suggest that the contribution of polar resonance structures to the ground state of the radicals is not appreciable. See, however, the results obtained by Koch (1986) on the dl meso isomerization of [47]. 

Similarly to the triphenylmethyl system, captodative-substituted 1,5-hexa-dienes, which can be cleaved thermally in solution into the corresponding substituted allyl radicals [15], dissociate more easily than dicaptor-substituted systems (Van Hoecke et al., 1986). Since ground-state and radical substituent effects cannot be separated cleanly, not only because of electronic but also because of steric effects, a conclusive answer cannot be provided. 

If the reduction of C—C BDEs by captodative substitution is interpreted with the appropriate caution, it can be stated that a conclusive answer as to the existence of a captodative effect in free radicals cannot be derived from these studies, If, furthermore, a consequent error-propagation analysis had been carried out, the outcome might have been that the error limits do not allow a definitive conclusion. However, the results convey a feeling that— regardless of the pros and cons for the different determination procedures— a possible captodative effect will not be great. 

The study of substituted allyl radicals (Sustmann and Brandes, 1976 Sustmann and Trill, 1974 Sustmann et al., 1972, 1977), where pronounced substituent effects were found as compared to the barrier in the parent system (Korth et al., 1981), initiated a study of the rotational barrier in a captodative-substituted allyl radical [32]/[33] (Korth et al., 1984). The concept behind these studies is derived from the stabilization of free radicals by delocalization of the unpaired spin (see, for instance, Walton, 1984). The... 

The rotational barrier about the C—O bond in the cyanomethoxymethyl radical, [35]/[36], constitutes a similar case, although the situation is somewhat more complicated (Beckwith and Brumby, 1987). As oxygen carries two lone pairs of electrons, the transition structure for rotation about the C—O bond can still be stabilized by conjugation. Compared to the methoxy-methyl radical, the barrier in the captodative-substituted radical is 1-2 kcal mol higher. 

The study of the rotational barriers in captodative-substituted radicals leads to the following conclusions the barriers are noticeably lower or higher than in cases of dicaptor or didonor substitution. This can be interpreted as the consequence of a captodative effect in these systems. However, the amount of special influence on the barrier height in energetic terms is small and may sometimes not exceed the numerical uncertainties. A derivation of absolute values for stabilization energies of captodative-substituted radicals by this procedure is not possible, since both ground and transition states are affected by substitution. The lowering of the barriers... 

Viehe s group has studied cis-trans isomerizations of captodative-substituted cyclopropanes in more detail (Table 14) (Merenyi et al., 1983 De Mesmaeker et al., 1982). The lowest activation energy is observed for X = OCH3 and the highest for X = COjCHj. Thus, a donor instead of a captor... 

Already, at an early stage of the studies on the captodative effect, Viehe s group (Lahousse et ai, 1984) measured relative rates for the addition of t-butoxyl radicals to 4,4 -disubstituted 1,1-diphenylethylenes and to substituted styrenes. This study did not reveal a special character of captodative-substituted olefins in such reactions. It might be that the stability of the radical to be formed does not influence the early transition state of the addition step. The rationalization of the kinetic studies mentioned above in terms of the FMO model indicates, indeed, an early transition state for these reactions, with the consequence that product properties should not influence the reactivity noticeably. 

In a systematic study of the addition of cyclohexyl radicals to a-substi-tuted methyl acrylates, Giese (1983) has shown that the captodative-substituted example fits the linear correlation line of log with o-values as perfectly as the other cases studied. Thus, no special character of the captodative-substituted olefin is displayed. More recently, arylthiyl radicals have been added to disubstituted olefins in order to uncover a captodative effect in the rate data (Ito et aL, 1988). Even though a-A, A -dimethyl-aminoacrylonitrile reacts fastest in these additions, this observation cannot per se be interpreted as the manifestation of a captodative effect. Owing to the lack of rate data for the corresponding dicaptor- and didonor-substituted olefins, it is not possible to postulate a special captodative effect. The result confirms only that the A, A -dimethylamino-group, as expected from its a, -value, enhances the addition rate. In the sequence a-alkoxy-, a-chloro-, a-acetoxy- and a-methyl-substituted acrylonitriles, it reacts fastest. 

The quantum chemical studies have not reached a unanimous conclusion. The more sophisticated procedures predict that in some captodative substituted systems an additive or a slightly more than additive substituent effect is possible. The calculations, particularly those of Leroy, have also contributed to the belief that the study of substituent effects requires the consideration of their influence in the ground and final states of the model system. 

Tanaka, H. Yoshida, S. Kinetic study of the radical homopolymerization of captodative substituted methyl a-(acyloxy)acrylates. Macromolecules 1995, 28, 8117-8121. 

Symmetric 3 m -excited 1,2-diarylethanediones (35) undergo highly regio- and stereoselective head to head additions to various captodative-substituted alkenes (2-aminopropenenitriles 36) forming oxetanes 37 in moderate to good yield (Sch. 9) [36]. 

Only few reports deal with a para photocycloaddition as the major reaction path. Recently, however, several cinnamide derivatives like 21 were efficiently transformed into the corresponding para adducts 22 (Sch. 6) [37]. Yields higher than 90% could be achieved. The para photocycloaddition is also observed with naphthalene derivatives like 1-acetylnaphthalene 23 and captodative enamino nitriles as 24 [38]. Other captodative substituted alkenes [39] as well as the fluorinated uracil derivative 26 [40] are transformed in the same way. Especially in the cases of 21 and 23, the... 

It is the purpose of this paper to review the implications of this unusual stabilization for the radical polymerization of captodatively substituted olefins. Also, the possibility of producing 1,4-tetramethylene diradicals from these olefins opens an important new area for the spontaneous polymerization of olefins. The latter will be discussed in the context of the Bond-Forming Initiation Theory [9-10],... 

All the above experiments were run using equimolar amounts of radicals and illustrate the extreme stabilization found in captodative radicals. This led Viehe and coworkers to conclude that the stabilized captodatively substituted radicals. .. do not undergo typical reactions such as polymerization or hydrogen abstraction but rather they trap another radical R or dimerize [2]. 

Table 15. Spontaneous polymerizations of captodative substituted acrylates CH2 = C(r/)COOR in bulk at 60 °C...

Another noticeable characteristic of captodative olefins is the influence of the reaction medium. The stabilizing effect of solvent on the persistency of a captodatively radical has been reported experimentally for the bond homolysis of bis(3,5,5-trimethyl-2-oxomorpholin-3-yl) [111], but was not found for the 2,3-diphenyl-2,3-dimethoxysuccinonitrile homolysis [112]. Theoretically the solvent-assisted stabilization las been predicted for the captodative substituted nitriles in solvent with large dielectric constants [113-114], Table 16 illustrates the solvent effect on the spontaneous thermal polymerizations [115]. The polymer yields are... 

Table 17. Copolymerization parameters for the copolymerization of captodative substituted acrylates CH2=C(rf)COOR (Mt) and styrene (M2) at 60 °C...

The Bond-Forming Initiation Theory gives a good interpretation of the observed spontaneous polymerizations of captodative monomers. The tetramethylene diradicals already implicated as initiators in the thermal (spontaneous) polymerizations of vinyl monomers can be particularly stabilized by captodative substituents. For comparison, and to initiate the polymerization of third monomers, captodative cyclobutanes and cyclopropanes are particularly appropriate precursors for generating tetra- and trimethylene diradicals. In particular the extensive work of Viehe [3,45,46] showed that thermolysis of captodative substituted cyclopropanes leads to trimethylene captodative diradicals at reasonable temperatures. Their initiating abilities for polymerization have not yet been determined. 

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