Nicholas cation

Propargylic radicals are generated on reduction of metal-complex-stabilized Nicholas cations with Zn. This principle, which has been found very recently [21], can be used for ring-closure reactions via intramolecular radical recombination [Eq. (12)]. 

An alternative approach to the antitumor active enediyne systems (cf. eq 47) using Nicholas cations has been extensively studied and applied to the dynemycin core structure (eq 53). Another example of the use of such cations in forming strained rings is the preparation of a cobalt complex of a trithia-crown ether (eq 54), which has been shown to complex Cu and Ag ... 

This solid-phase technique has made possible efficient syntheses of bi- and polycyclic systems from precursors frequently generated via Nicholas cations, (e.g. eq 60) and has been used for intermolecular reactions with methy lenecyclopropane (eq 61) and methylenecyclobutane to give spirocyclic cyclopentenone derivatives. (1) is the majorproduct when the alkyne is terminal (R = H), but only (2) was isolated when EtC2Et or Me3 SiC2Me were used. ... 

Years earlier, Nicholas and Ladoulis had found another example of reactions catalyzed by Fe2(CO)9 127. They had shown that Fe2(CO)9 127 can be used as a catalyst for allylic alkylation of allylic acetates 129 by various malonate nucleophiles [109]. Although the regioselectivites were only moderately temperature-, solvent-, and substrate-dependent, further investigations concerned with the reaction mechanism and the catalytic species were undertaken [110]. Comparing stoichiometric reactions of cationic (ri -allyl)Fe(CO)4 and neutral (rj -crotyl ace-tate)Fe(CO)4 with different types of sodium malonates and the results of the Fe2(CO)9 127-catalyzed allylation they could show that these complexes are likely no reaction intermediates, because regioselectivites between stoichiometric and catalytic reactions differed. Examining the interaction of sodium dimethylmalonate 75 and Fe2(CO)9 127 they found some evidence for the involvement of a coordinated malonate species in the catalytic reactions. With an excess of malonate they... 

In acetonitrile (AN), the toluene cation-radical has high thermodynamic acidity, its pK is between -9 and -13 (Nicholas and Arnold 1982). In the same solvent (AN), neutral toluene has... 

Table 5.4 Adsorption of heavy metals on goethite as a function of pH. Data expressed as percent of initial amount of metalhc cation solution. Reprinted from Quirk JP, Posner AM (1975) Trace element adsorption by soil minerals. In Nicholas DJ, Egan AR (eds) Trace elements in soil plant animal system. Academic Press, New York pp 95-107. Copyright 1975 with permission of Elsevier...

Interestingly, seven-membered conjugated diyne complex 16 is produced by the acid-promoted Nicholas reaction of the dicobalt-coordinated bispropargylic complex (Equation (7)). The cyclization proceeds via an intramolecular coupling reaction between a propargyiic cation and an alkene produced after dehydration. The molecular structure of seven-membered diyne complex 16 is confirmed by X-ray analysis, although the decomplexation is not successfully carried out. 

Formation of radicals having a lower energy than that of the starting cation radicals is obviously favorable for their deprotonation. The cation radicals of toluene and other alkylbenzenes are illustrative examples. As shown (Sehested Holcman 1978), these cation radicals lose protons even in very acidic aqueous solutions. The deprotonation rate does not, in general, depend on the medium acidity. In acetonitrile (AN), the toluene cation radical has high thermodynamic acidity, its pKa is between 9 and -13 (Nicholas ... 

The idea of cation-77 interactions is receiving more and more acceptance. The interaction of an aromatic 77-syslcm with an alkali metal cation has been known for two decades (Sunner et al. 1981). Recently, it has been confirmed in several experimental (De Wall et al. 1999) and theoretical works (Kumpf Dougherty 1993 Dougherty 1996 Nicholas et al. 1999). 

Nicholas, J.B., Dixon, D.A., Hay, B.R 1999. Ab initio molecular orbital study of cationbinding between the alkali metal cations and benzene. J. Phys. Chem. 103 1394-1400. 

Later on, Schreiber used consecutively these two reactions in the key step for the synthesis of diterpene (-i-)-epoxydictimene (73), starting from natural (R)-pulegone [116,117]. This approach was built on their preliminary studies on Lewis acid mediated intermolecular Nicholas reactions [118]. They prepared functionalized enyne 70 bearing a mixed acetal. This compound was transformed into its dicobalt-hexacarbonyl complex and, in the presence of a carefully selected Lewis acid, it formed a stabilized carbocation by release of the more accessible ethyl moiety. This cation reacted intramolecularly with the allylsilane giving the central eight membered ring of the natural product... 

The chemistry and synthetic utility of cobalt complexed propargyl cations have been demonstrated by Nicholas in an impressive series of papers, and the area was reviewed in 1987.72 More recently, reviews of cluster-stabilized cations73 and propargylium complexes74 have appeared. Two general routes for the synthesis of dicobalt-propargylium complexes have been developed. The most commonly used method is the treatment of an alkynic ether or alkynic alcohol-hexacarbonyldicobalt complex with a Lewis or Bronsted acid [Eq. (7)]. 

The Nicholas reaction, that is, the addition reaction of nucleophiles to cobalt-complexed propargylic cations, formed in situ, has been extensively developed. " The... 

In a landmark paper, Breslow and coworkers described the determination of pA), values of weak hydrocarbon acids by use of thermochemical cycles involving electrochemical reduction data for triarylmethyl, cycloheptatrienyl, and triphenyl- and trialkylcyclopropenyl cations and radicals [9aj. Later, they derived pATa data from standard oxidation potentials and bond-dissociation energies [9b, c]. The methodology was further developed by Nicholas and Arnold [10a] for the determination of cation radical acidities, and later modified and extensively used by Bordwell and coworkers [10b, c] so that homolytic bond-dissociation energies and cation radical... 

Nicholas reaction Trapping of dicobalt hexacarbonyl-stabilized propargylic cations with various nucleophiles. 314... 

Green, J. R. Chemistry of propargyidicobalt cations recent developments in the Nicholas and related reactions. Curr. Org. Chem. 2001,5, 809-826. 

Padmanabhan, S., Nicholas, K. M. Carbon-13 NMR study of (propargyl)dicobalt hexacarbonyl cations a structurally unique class of metal-stabilized carbenium ions. J. Organomet. Chem. 1984, 268, C23-C27. 

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