Substituent effects small rings

Substituents on both sides of the double bond are considered separately. Additional vinyl carbons are treated as if they were alkyl carbons. The method is applicable to alicyclic alkenes in small rings carbons are counted twice, i.e., from both sides of the double bond where applicable. The constant in the equation is the chemical shift for ethylene. The effect of other substituent groups is tabulated below. 

Diels-Alder reactions, 4, 842 flash vapour phase pyrolysis, 4, 846 reactions with 6-dimethylaminofuKenov, 4, 844 reactions with JV,n-diphenylnitrone, 4, 841 reactions with mesitonitrile oxide, 4, 841 structure, 4, 715, 725 synthesis, 4, 725, 767-769, 930 theoretical methods, 4, 3 tricarbonyl iron complexes, 4, 847 dipole moments, 4, 716 n-directing effect, 4, 44 2,5-disubstituted synthesis, 4, 116-117 from l,3-dithiolylium-4-olates, 6, 826 electrocyclization, 4, 748-750 electron bombardment, 4, 739 electronic deformation, 4, 722-723 electronic structure, 4, 715 electrophilic substitution, 4, 43, 44, 717-719, 751 directing effects, 4, 752-753 fluorescence spectra, 4, 735-736 fluorinated derivatives, 4, 679 H NMR, 4, 731 Friedel-Crafts acylation, 4, 777 with fused six-membered heterocyclic rings, 4, 973-1036 fused small rings structure, 4, 720-721 gas phase UV spectrum, 4, 734 H NMR, 4, 7, 728-731, 939 solvent effects, 4, 730 substituent constants, 4, 731 halo... 

The very small p- and m-values observed for the fast bromination of a-methoxystyrenes deserve comment since they are the smallest found for this electrophilic addition. The rates, almost but not quite diffusion-controlled, are amongst the highest. The sensitivity to polar effects of ring substituents is very attenuated but still significant that to resonance is nil. These unusually low p-values for a reaction leading to a benzylic carbocation are accompanied by a very small sensitivity to the solvent. All these data support a very early transition state for this olefin series. Accordingly, for the still more reactive acetophenone enols, the bromination of which is diffusion-controlled, the usual sensitivity to substituents is annulled. 

In conclusion, therefore, a judicious use of CV methodology may lead to absolute thermodynamic data that are accurate to ca. 15 kJ mol-1. Relative values (i.e., differences between bond dissociation enthalpies in similar compounds) can be more reliable, but the approximations described suggest that some caution be exercised when using the results to draw conclusions that rely on small differences between bond dissociation enthalpies. This is the case, for example, for ring substituent effects on the O-H bond dissociation enthalpies in substituted phenols [346,349],... 

It is instructive to consider why the presence of relatively bulky N-substituents results in the formation of small rings. Of the reactions of phosphorus trichloride with amines, that with t-butylamine has been widely studied (Scheme 6) Intermediates (17) and (18) have been isolated, but not (79), which evidently undergoes a facile cyclisation step to give (20). Two effects can be identified, which are expected to favour the formation of small rings. Firstly, an n.m.r. study of (18)... 

A second effect, widely believed to favour small ring formation concerns the steric interactions between the P- and N-skeletal substituents (Scheme 8). In this case steric interference between the N-substituents is minimised in four membered rings (25), but greater in larger rings or polymers 24). This effect could equally well apply to phosphorus substituents. 

Further work by Anson s group sought to find the effects that would cause the four-electron reaction to occur as the primary process. Studies with ruthenated complexes [[98], and references therein], (23), demonstrated that 7T back-bonding interactions are more important than intramolecular electron transfer in causing cobalt porphyrins to promote the four-electron process over the two-electron reaction. Ruthenated complexes result in the formation of water as the product of the primary catalytic process. Attempts to simulate this behavior without the use of transition-metal substituents (e.g. ruthenated moieties) to enhance the transfer of electron density from the meso position to the porphyrin ring [99] met with limited success. Also, the use of jO-hydroxy substituents produced small positive shifts in the potential at which catalysis occurs. 

The H-NMR methyl substituent effect is very different on the various ring protons of TP from this Makisumi et al. (64CPB204) concluded the ring system possesses only a small aromaticity. The simple HMO method cannot be used for calculations of tautomerism of isomeric oxotriazolopyr-... 

As seen in the preceding sections, 2-chloro and 2-alkoxy substituents are very labile, due to the effect of the adjacent ether oxygen atom and the small ring. 2-Oxetanol has no significant existence, the tautomeric 3-hydroxypropanaI being much more stable. 

---