Aryl transfer, from

The success of the asymmetric aryl transfer from boronic acids also relied on another important phenomenon. The presence of catalytic amounts (10 mol%) of DiMPEG (MW = 2000) increased the enantioselectivity of the process significantly [49]. For example, without DiMPEG the reaction between benzaldehyde 37b and 1-naphthylboronic acid 40c gave the corresponding diarylmethanol 27g with 31% ee in 56% yield, whereas in the presence of the polyether, 27g was obtained in 85% ee and 91% yield (Scheme 2.1.2.12) [49]. 

In this variant, the ipso substitution of 68 led to the cyclohexadienyl radical 69 which underwent C-P bond cleavage and expulsion of the tether to give biphenyl 70. A comparable mechanism is operative for the aryl transfer from silicon to carbon in which a comparable biphenyl 71 (c.f. 70) was synthesized from siloxane 72 (Scheme 26) [65, 133]. 

Scheme 13 Previously unobserved aryl transfer from palladium to a coordinated biaryl monophosphine ancillary ligand resulting in lower ring dearomatization...

Michael reaction. The title compound is a bidentate S,S-ligand for Rh. Complexes of the sort are used in mediating aryl transfer from ArB(OH)2 to 2-cycloalkenones and conjugated lactones under basic conditions. ... 

Conjugate addition. The parent chiral BINAMINE is an excellent ligand for CUCI2 to promote the conjugate addition of diorganozinc reagents. When complexed to carbene 3 palladium dicarboxylates exhibit catal3dic activity in the aryl transfer from ArB(OH)2 to... 

Aryl transfer from ArSi(OEt)3 to conjugated ketones, lactones and lactams is achieved with the aid of a palladium(ll) salt supported by 116. It is a variation of the reaction involving ArB(OH)2 with a similar system. The P,P -dioxide of the same ligand complements CuOTf to serve as catalyst for the addition of R2Zn to nitroalkenes. ... 

Rearrangement of oxonium ions. In the acid-catalysed cleavage of cumene hydroperoxide (to phenol and acetone), an important step is aryl transfer from carbon to oxygen in the intermediate oxonium ion ... 

Nishimura T, Katoh T, Hayashi T (2007) Rhodium-catalyzed aryl transfer from trisubstituted aryl methanols to a, P-unsaturated carbonyl compounds. Angew Chem Int Ed 46(26) 4937 939. doi 10.1002/anie.200700902... 

If an electron is transferred from a reducing agent to an arenediazonium ion, an aryldiazenyl radical (8.47) is formed. As discussed in this section, the latter dissociates rapidly into an aryl radical and N2 (Scheme 8-28). This type of dediazoniation was observed by Griess (1864 c), albeit not in our present formulation. He found that arenediazonium ions formed iodoarenes and N2 in the presence of iodide ions. More important for synthetic organic chemistry were some dediazonia-tions discovered in the late 19th and early 20th centuries, which are catalyzed by metals and metal ions, namely the Sandmeyer, Pschorr, Meerwein, and related syntheses (see Ch. 10). 

The formation of aryl radicals from benzenediazonium ions, initiated by electron transfer from a nitrite ion, has already been discussed in Section 8.6. It is an excellent example of a dediazoniation assisted by a donor species that is capable of forming a relatively stable species on release of an electron, in this case a nitrogen dioxide radical NO2 (Opgenorth and Rtichardt, 1974). 

The classical syntheses of phenanthrene and fluorenone fit well into the electron transfer scheme discussed in Section 8.6 and in this chapter. The aryl radical is formed by electron transfer from a Cu1 ion, iodide ion, pyridine, hypophosphorous acid, or by electrochemical transfer. The aryl radical attacks the neighboring phenyl ring, and the oxidized electron transfer reagent (e. g., Cu11) reduces the hexadienyl radical to the arenium ion, which is finally deprotonated by the solvent (Scheme 10-76). 

Laali and Lattimer (1989 see also Laali, 1990) observed arenediazonium ion/crown ether complexes in the gas phase by field desorption (FD) and by fast atom bombardment (FAB) mass spectrometry. The FAB-MS spectrum of benzenediazonium ion/18-crown-6 shows a 1 1 complex. In the FD spectrum, apart from the 1 1 complex, a one-cation/two-crown complex is also detected. Dicyclo-hexano-24-crown-6 appears to complex readily in the gas phase, whereas in solution this crown ether is rather poor for complexation (see earlier in this section) the presence of one or three methyl groups in the 2- or 2,4,6-positions respectively has little effect on the gas-phase complexation. With 4-nitrobenzenediazonium ion, 18-crown-6 even forms a 1 3 complex. The authors assume charge-transfer complexes such as 11.13 for all these species. There is also evidence for hydride ion transfer from the crown host within the 1 1 complex, and for either the arenediazonium ion or the aryl cation formed from it under the reaction conditions in the gas phase in tandem mass spectrometry (Laali, 1990). 

This cycle involves, first, a monoelectronic transfer from the nickel (0) complex to the aryl halide affording a Ni(I) complex and then an oxidative addition affording a 16 electron-nickel (II) which undergoes a nucleophilic substitution of Nu-, then a monoelectronic transfer occurs once again with a second aryl halide, and, last, a reductive elimination of the arylated nucleophile regenerates the active Ni(I) species. 

In this case the decisive redox process, which would be specifically inhibited by the hydroquinone, would be the formation of the copper(I) complex corresponding to a monoelectronic transfer from copper(O) to the aryl halide. 

The ratio ARH/ARj (monoalkylation/dialkylation) should depend principally on the electrophilic capability of RX. Thus it has been shown that in the case of t-butyl halides (due to the chemical and electrochemical stability of t-butyl free radical) the yield of mono alkylation is often good. Naturally, aryl sulphones may also be employed in the role of RX-type compounds. Indeed, the t-butylation of pyrene can be performed when reduced cathodically in the presence of CgHjSOjBu-t. Other alkylation reactions are also possible with sulphones possessing an ArS02 moiety bound to a tertiary carbon. In contrast, coupling reactions via redox catalysis do not occur in a good yield with primary and secondary sulphones. This is probably due to the disappearance of the mediator anion radical due to proton transfer from the acidic sulphone. 

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