Elimination/addition reactions features

This special feature arises from the combination of the transition metal behavior such as the coordination of a carbon-carbon multiple bond, oxidative addition, reductive elimination, P-hydride elimination, addition reactions and the behavior of classical c-carbanion towards electrophiles. 

Having established structural and electronic analogies between metal oxides and alkoxides of molybdenum and tungsten, the key remaining feature to be examined is the reactivity patterns of the metal-alkoxides. Metal-metal bonds provide both a source and a returning place for electrons in oxidative-addition and reductive elimination reactions. Stepwise transformations of M-M bond order, from 3 to 4 (37,38), 3 to 2 and 1 (39) have now been documented. The alkoxides M2(0R)6 (MiM) are coordinatively unsaturated, as is evident from their facile reversible reactions with donor ligands, eq. 1, and are readily oxidized in addition reactions of the type shown in equations 2 (39) and 3 (39). 

Geometric features of transition states for addition reactions of olefins (Chapter 3), bimolecular substitutions (Si f3 reactions p. 36), bimolecular eliminations (E2 reactions p. loi) and many other reactions have been defined from studies of a variety of steroid systems (e.g. Fig. la). Reactions proceeding through carboniumions (e.g. Fig.Tb seep. 228) are sensitive to... 

Table IV compares the reactivity ratios of a soft (PhS-) to a hard (MeO-) nucleophile in vinylic substitution. PhS is always more reactive, and ratios lower than unity, as for 4, X = Br (4), are certainly due to elimination-addition with MeO . The ratios change by >2000-fold and are sensitive to the geometry of the substrate. An important feature is that for (3-halo-p-nitrostyrenes the ratio decreases strongly with the increased hardness of the (3-halogen (38). The lowest ratios are for the (3-fluoro derivative, whereas the differences between the chloro and bromo compounds are not so large. This behavior is similar to that in SNAr reactions. This behavior can be rationalized by symbiotic effects, which favor the soft-soft PhS--Br interaction and the hard-hard MeO-F interaction. A reactivity-selectivity relationship for vinyl bromides of different electrophilicities does not exist.

A characteristic and advantageous feature of the oxidative addition reactions of transition metal complexes compared with those of nontransition metal compounds is their reversibility. Grignard reactions cannot be made reversible, but the reverse reaction of oxidative addition is possible with transition metals in many cases. This reverse reaction is called reductive elimination. The ability of the transition metals to donate as well as to accept elec-... 

One of the key features in Scheme 27 is the intramolecular cycloaddition of an o-quinodimethane intermediate, and this reaction has featured in several other new approaches to steroidal and related polycyclic carbon skeletons (Scheme 28). The last example in Scheme 28 also illustrates a new route to the cyclization precursor (119) via a nucleophilic induced elimination-addition of an a-halogeno-... 

Some of the reactions in this chapter operate by still other mechanisms, among them an addition-elimination mechanism (see 13-15). A new mechanism has been reported in aromatic chemistry, a reductively activated polar nucleophilic aromatic substitution. The reaction of phenoxide with p-dinitrobenzene in DMF shows radical features that cannot be attributed to a radical anion, and it is not Srn2. The new designation was proposed to account for these results. 

---