Arylation cationic route

The diazonium salt 42 is stable at 0-5 °C but decomposes to N2 and an unstable aryl cation 43 on warming to room temperature. The empty orbital of 43 is in an sp2 orbital in the plane of the aromatic ring, quite unlike the normal p orbital for cations like 20. Reaction occurs with any available nucleophile, even water, and this is a route to phenols 45. 

Figure 3.3 General regiochemical outcome in Mizoroki-Heck arylation proceeding via the neutral or cationic route.

The general mechanism of the rearrangement of aryl and diaryl-thiazoles seems to exclude the zwitterion route. Instead it takes place through bending of thiazoles bonds (98.213). Moreover, tricyclic sul-fonium cation intermediates, after irradiation of deuterated phenyl-thiazoles, have been suggested by several workers (98). 

Alkyl and aryl derivatives of poly(dichlorophosphazene) are not efficiently synthesized by nucleophilic reaction of LXXXIV with metal alkyls or aryls. The halogen substitution reaction occurs but is accompanied by polymer chain cleavage. Use of poly(difluorophosphazene) or introduction of aryl and alkyl groups at the monomer stage offer some improvement, but neither method is fully satisfactory. The best route to alkyl and aryl derivatives is polymerization of A-(trimethylsilyl)-/).P-dialkyl-.P-halophosphoranimines at moderate temperatures (25-60°C) in the presence of a Lewis acid [Allcock et al., 1996, 2000, 2001a,b Neilson and Wisian-Neilson, 1988]. The reaction proceeds as a cationic chain polymerization ... 

G. Angelini, M. Speranza, A.P. Wolf, C.Y. Shiue, Nucleophilic aromatic-substitution of activated cationic groups by F-18-labeled fluoride—A useful route to no-carrier-added (nca) F-18-labeled aryl fluorides, J. Fluor. Chem. 27 (1985) 177-191. 

A number of publications have appeared on the synthesis of cationic cobalt(I) complexes by known routes. The cations are of the well-established type [Co(CNR)5]+ (R = aryl) (119-122). Reactions of aromatic isocyanides with Co2(CO)g in refluxing toluene have given the fully substituted cobalt(O) dimer Co CNR) (R = xylyl, C6H2Me3-2,4,6, C6H2Br-4-Me2-2,6) (25,123). 

Using cationic tricarbonyl(q5-cyclohexadienyl)iron complexes as starting materials, different synthetic routes to a large number of carbazole alkaloids have been developed [51, 58, 67]. The first step is an electrophilic substitution of a substituted arylamine using the cyclohexadienyliron complex and provides the corresponding 5-aryl-substituted cyclohexadiene-iron complexes (Scheme 1.29). 

PET cyclizations of (3,y-unsaturated oximes (27) have been established by Armesto and Horspool [38]. This 9,10-dicyanoanthracene (DCA) sensitized reaction provides an efficient route for the synthesis of dihydroisoxazole derivatives (28) in reasonable to good yields (Sch. 17). Key step in the mechanistic scenario is an electron transfer from the oxime group to DCA in its excited singlet state. The radical cationic intermediate thus generated undergoes subsequent exo-cyclization with the olefinic moiety. Further, proton and electron transfer steps complete the reaction. A major limitation of this process is the restriction to molecules incorporating two aryl groups [38a]. 

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