Elimination modes

The presence of /3-unsaturation normally leads to the formation of the corresponding conjugated system. The usual order of reactivity of a /3 proton is thus benzylic, allylic > CH3 > CH2 > CH. However, this preference can be offset by additional substitution at the more activated position competition between the /3-elimination modes leads to a mixture of regio-isomers as shown in Scheme 20. 

Since the expulsion of the leaving group is largely dependent on the mesomeric effect of the nucleophilic moiety incorporated in the molecule, one might expect that the use of thio nucleophiles should shift the mechanism to the addition-elimination mode. Indeed, when the reaction of 113 with thiophenoxide was performed in dimethoxyethane, the addition-elimination product 117 was exclusively obtained (equation 161). However, in methanol the ionic bicyclobutane mechanism was found to be operative in spite of the weaker mesomeric effect of the thio group (equation 162). ... 

Elimination modes = transfer to third party + collection + discarding in MSW. 

There are three major elimination modes for equipment and batteries (Figure 12) ... 

Elimination modes = Transfer to third party + Collection + Discarding in MSW... 

According to the type of equipment, one or the other of the elimination modes is favoured. For dust busters, discarding represents the majority of the declared cases. For cordless power tools, transfer is the preferred mode of elimination (resale, exchange... 

In all cases, when combining collection and transfer data, they represent the large majority of elimination modes of equipment and batteries (more than two thirds of eliminated equipment and batteries versus less than one third for the MSW discarding mode). 

Figure 12. Elimination Modes for Equipment and Replacement Batteries...

This survey has confirmed the correspondence (consistency) between this method of second hand acquisition and the elimination mode by transfer. 

Figure 3.7. Large volume injection of 100 (il of a mixture of n-alkanes of wide volatility using the PTV injector in the cold split solvent elimination mode. The vaporizing chamber was thermostated at 0°C, split flow 250 ml/min with the split vent closed after 2.5 min. For splitless transfer the vaporizing chamber was heated at 4°C/s to 325°C with the puige flow started after 1.5 min. (From ref. [68] Wiley-VCH).

The outlook is much better with the Peterson reaction here, at least trisubstituted double bonds with E- as well as with Z-configuration will result if the syn-elimination mode 147 or the corresponding anti version 148 is employed. 

The backward interval elimination mode acts similar to the forward one, but from an opposite starting point. In this case the global model (7 intervals) is used as a basis and then intervals are eliminated, one at a time. As for forward... 

Like homopropargylic alcohols, homoallylic alcohols undergo with dimethyltitanium dichloride carbotitanation followed by elimination of titanium hydride. As a consequence of the yw-addition/syw-elimination mode, the configiuation of internal double bonds (as in 21 or 22) is inverted whereas terminal substrates (20) minimize steric repulsion and afford tra 5-3-penten-l-ols (Scheme 1-17). The optimum medium is a mixture of pentanes and the slowly evaporating dimethyl ether. Obviously the titanium species has to be coordinatively unsaturated to accomplish the critical carbometalation step. ° ... 

All kinds of bases, the counter-ion free amines and alcoholates or metal amides as well, favor in polar, i.e., ionizing, media the anti- over the 5y -periplanar elimination mode. Exceptions are rare. In less polar solvents, the metal seeks the proximity to the... 

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