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Chapter 1
Silver Alkyls, Alkenyls, Aryls, and Alkynyls in Organic Synthesis
Rebecca H. Pouwer and Craig M. Williams
School of Chemistry and Molecular Biosciences, University of Queensland, Brisbane, Australia
1.1 Introduction
While the coordination and inorganic chemistry of silver compounds have been prolifically documented, the use of organosilver compounds to effect useful synthetic transformations is severely underrepresented in the synthetic organic chemistry literature. This has prompted us to present a review of literature reporting synthetically useful applications of organosilver compounds in the hope of inspiring further development in this field. The majority of the literature covered in this review concentrates on silver(I) organo-species as reagents, although on some occasions silver(II) and silver “ate” complexes will be discussed, in addition to organosilver intermediates. General reviews encompassing all classes of organosilver compounds have appeared previously.1–3
1.2 Csp3-Ag
1.2.1 Synthesis, Stability, and Reactivity of Alkylsilver Compounds
As a result of extremely low thermal stability, alkylsilver compounds have found only a narrow range of use in organic synthesis. Procedures for the synthesis of alkylsilver compounds as anything but fleeting proposed intermediates are limited to a handful. Semerano and Riccoboni first reported the synthesis of methyl-, ethyl-, and propylsilver in 1941 (Scheme 1.1). Reaction of silver nitrate and the corresponding tetralkyllead in alcohol at −80°C gave the compounds as brown precipitates that decomposed rapidly on warming to room temperature to give metallic silver and a mixture of hydrocarbons.4 This methodology has been utilized in a limited number of investigations into the mechanism of decomposition of alkylsilver compounds.5, 6 In these cases, the presence of the alkylsilver compound, and its subsequent decomposition, is inferred from the isolation of alkyl dimers.
Two plausible mechanistic pathways have been proposed for the thermal decomposition of alkylsilver compounds: either a radically-mediated cleavage of the carbon–silver bond or a process by which the breaking of the silver–carbon bond and formation of the carbon–carbon bond are concerted. Mechanistic studies by Whitesides and coworkers in which the product ratios obtained for the thermal process were compared to those for known radically-mediated reactions have suggested that a concerted process is more likely, although this has not proved to be general.7–9
The formation of methylsilver and dimethylargentate has been observed in the collision-induced dissociation MS3 spectrum of silver diacetate. Dimethylargentate is stable in the gas phase, and has been isolated for short periods (10 s) without significant decomposition.10
Alkylsilver compounds have been prepared by treatment of Grignard reagents with silver salts,11–19 and similarly undergo oxidative homocoupling to give alkyl dimers.11–13, 19, 20 Exploitation of this finding has resulted in the development of general methodology for silver-catalyzed alkyl–alkyl homocoupling of Grignard reagents (Table 1.1).21 The catalytic cycle of this reaction is proposed to proceed via the oxidation of metallic silver with 1,2-dibromoethane to generate silver bromide (Scheme 1.2).
Table 1.1 Silver-Catalyzed Dimerization of Alkylmagesium Halides.
Of particular note is the use of this reactivity to form small carbocycles. Whitesides and coworkers have shown that the treatment of primary bis(alkylmagnesium halides) with tributylphosphinesilver iodide produces carbocycles in a range of yields, with a strong dependence on ring size (Table 1.2). The best results were obtained for four-, five-, and six-membered rings. Although it was hoped that the aggregated nature of alkylsilver compounds would facilitate the formation of medium to large rings, compounds of this type were produced with only low yields.8
Table 1.2 Silver-Mediated Ring-Closing Reaction of Bis(Alkylmagnesium Halides)
It has also been shown that treatment of primary bis(alkylmagnesium halides) with silver trifluoromethanesulfonate effects ring closure under mild conditions for a range of substrates, thus highlighting the generality of this reaction for producing small carbocycles (Table 1.3).
Table 1.3 Silver-Mediated Ring-Closing Reaction of Bis(Alkylmagnesium Halides)
An equivalent reaction has been achieved via the treatment of hydroborated bisalkenes with alkaline silver nitrate solution (Table 1.4).22, 23 This method has been used to synthesize a number of small and medium-size carbocyclic rings in moderate to good yield. The selectivity for terminal cyclization observed for 1,6-heptadiene and 1,7-octadiene indicates that, in these cases, hydroboration of each of the alkenes occurs independently to yield acyclic boranes. It has, however, been found that both cyclic and acyclic boranes react under these conditions to yield the ring-closed products (Scheme 1.3).
Table 1.4 Silver-Mediated Ring Closing of Hydroborated Bisalkenes.
Intermolecular dimerization has also been effected by a comparable protocol.24–26 Treatment of triethylborane with silver nitrate and sodium hydroxide in water at 25°C led to the rapid evolution of ...
