Nucleophilic trapping

The ferrocenyldiphynylpropargyl cation, 77, has an intrinsic delocalization nature exhibiting a valence tautomerization band at 856 nm, and its nucleophilic trapping reactions give rise to the formation of ferrocenyldiphyenylallenes (173). The bis(acetylide) mixed-valence complexes of ferrocene and the Ru complex moiety, 78, also behave as a fulvene-cumulene structure, 79, showing a u(M=C = C—C) band at 1985 cm-1 (174). Related alleylidene and cumulenylidene complexes of transition metals have been reviewed by Bruce (175). 

SCHEME 7.17 Electrostatic potential map of the protonated pyrido[l,2-<2]indole-based cyclopropyl quinone methide. The two possible nucleophile-trapping paths with the respective products are shown. 

Scheme 1.25. Cationic [4+3]-cycloaddition/nucleophilic trapping domino reaction in the synthesis of halocycloheptynes.

Nucleophilic Trapping of Radical Cations. To investigate some of the properties of Mh radical cations these intermediates have been generated in two one-electron oxidant systems. The first contains iodine as oxidant and pyridine as nucleophile and solvent (8-10), while the second contains Mn(0Ac) in acetic acid (10,11). Studies with a number of PAH indicate that the formation of pyridinium-PAH or acetoxy-PAH by one-electron oxidation with Mn(0Ac)3 or iodine, respectively, is related to the ionization potential (IP) of the PAH. For PAH with relatively high IP, such as phenanthrene, chrysene, 5-methyl chrysene and dibenz[a,h]anthracene, no reaction occurs with these two oxidant systems. Another important factor influencing the specific reactivity of PAH radical cations with nucleophiles is localization of the positive charge at one or a few carbon atoms in the radical cation. 

In addition to the fact that steric crowding can slow the reaction by hindering bromine approach to the double bond, it appears now that bulky substituents can modify the bromination mechanism by inhibiting nucleophilic solvent assistance to ionization of the CTC and/or nucleophilic trapping of the ionic intermediates. Assistance to the rate-limiting ionization step by... 

In conclusion, the reversibility of bromonium ion formation is at present inferred from particular experiments only nothing allows us to conclude that this mechanistic feature is general. However, when nucleophilic trapping... 

It is noteworthy that the nature of the ionic intermediate formed in bromine addition to olefins and the solvent properties also govern the competition between nucleophilic trapping and elimination. Thus 1,1-diphenylethylene, 11, gives the corresponding dibromide 13 (or solvent incorporated products, 14) and vinyl bromide, 12, in a ratio changing from 99 1 to 5 95 depending on solvent and on bromine concentration.(20) (see Table III results)... 

Analogously, bromine bridging is not the only factor affecting the elimination-nucleophilic trapping ratio. Pre-association with a nucleophilic solvent can explain the increased selectivity towards addition products with respect to elimination products, as observed in bromination of 1,1-diphenylethylene in methanol. 

Ion pair collapse in the acyloxy migration is so rapid as to preclude nucleophilic trapping of the contact ion pair even by intramolecular nucleophiles, which essentially precludes the use of acetates as leaving groups in tandem rearrangement reactions of the types discussed below [111, 127]. 

Crich and Gastaldi investigated the nucleophilic trapping of a dihydronaphthalene radical cation by octyl alcohol and noted that the stereoselectivity of the reaction, while not high, was a function of the substrate stereochemistry (Scheme 19) [134]. In terms of the general mechanism for fragmentation... 

Scheme 19 Nucleophilic trapping of a dihydronaphthalene radical cation by octyl alcohol...

The advantage of the nitro group as radical precursor is best seen in the context of intramolecular nucleophilic trapping of alkene radical cations by nitrogen nucleophiles, when no cyclization was observed prior to treatment... 

Scheme 30 Nucleophilic trapping followed by exo-digonal radical cyclization...

These results support the /3-elimination from 220 to give 221, towards which KOtBu acts as a base and a nucleophile. As in the case of 215, the addition occurs at the central allene carbon atom leading the allyl anion 222, which is protonated to yield 223. On the other hand, the deprotonation of the methylene group brings about 224, whose major amount is converted to naphthalene, but a small proportion, behaving as a nucleophile, traps 221, giving rise to the allylanion 225, which in turn reacts with 221 and, by a hydride transfer, furnishes 228 and the allyl anion 229. By protonation, the latter is converted into 226. By conducting this experiment in the presence of benzophenone, this mechanistic model was confirmed as the tertiary alcohols 227 and 230 were obtained in addition to naphthalene, 223 and 228. Apparently, the anions 224 and 229 were intercepted in part or totally, respectively, by benzophenone (Scheme 6.52) [137]. 

Many of the early reports of spin-trapping experiments were focused on mechanistic investigations, and some of these feature in the early reviews (see p. 4). Unfortunately, it is in this application that inferences drawn may be most suspect. For example, the inability of the method to differentiate between radical trapping on the one hand, and a combination of nucleophile trapping with one-electron oxidation on the other, is a serious shortcoming. An early example of this was the tentative conclusion that acetoxyl radicals were spin-trapped by PBN competitively with their decarboxylation in reactions of lead tetraacetate. In view of the rapidity of the decarboxylation reaction, trapping of acetate ion and subsequent oxidation seems a likely alternative. 

Aminyl spin adducts were also observed during Ag(0)-promoted dechlorination of N-chloroamines in the presence of MNP (Edwards et al., 1973), but the isolation of silver succinimide from the reaction of N-chlorosuccinimide again suggests that the possibility that these adducts arise by nucleophile trapping cannot be disregarded. 

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