Fluoroalkyl-metal compounds

The proton magnetic resonance spectrum is a powerful tool for determination of the nature of a hydrocarbon group in organometallic compounds 65). Its application for this purpose is very similar to its use in organic chemistry. The use of F spectra has been described in the review of fluoroalkyl metal compounds by Treichel and Stone (7). Other nuclei such as P, Mn, and Co have given useful information on molecular configuration and electron distribution in organometallic compounds but they provide little help in the characterization of the C—M bond. 

Fluoroalfyl Derivatives Table IV). The F magnetic resonance spectra available for fluoroalkyl metal compounds thus far are mainly for derivatives of transition metals, only a few spectra having yet been reported for the many known derivatives of the main group elements. 

Scheme 2.109 Principal methods for generation of fluorinated carbanions and perfluoroalkyl metal compounds (Rp = (per)fluoroalkyl, Arp = (per)fluoroaryl, M = metal, X = halogen, B = base, S = solvent). The particularly unstable trifluoromethyl anion can be stabilized by formation of an adduct with a suitable solvent, for example DMF, which itself acts as a CFj"" source in a haloform-like reaction (bottom).

Organo-metallic or -metalloid reagents provide efficient routes to fluoroalkylated carbenes difluoromethylfluorocarbene (6.55A) is readily generated by a general route which involves the pyrolysis of fluoroaUcylsilicon compounds [84] (Figure 6.55). 

The reaction is reversible. If a fluorinated olefin is used, it would be expected that the resulting fluoroalkyl compound would be readily obtained, since the greater strength of the metal-fluorocarbon bond as compared to the metal-hydrocarbon bond favors the insertion reaction 267). The reaction of hydroplatinum(II) complexes with fluoroolefins has been extensively studied. A surprising product from a number of these reactions is a fluorovinyl compound rather than the anticipated fluoroalkyl complex. In Fig. 4 are shown the reactions of PtHCl(PEt3)2 with a series of fluoroolefins to give PtCl(CF=CF2)(PEt3)2 (XXXIV)... 

So far as possible, the same order has been used for the fluoroalkyl derivatives in each group of transition metals a-bonded alkyl, alkenyl, and alkynyl derivatives, 7r-bonded derivatives, and then derivatives bonded via N, P, O, or S in that order. In some cases, notably olefin complexes, there is ambiguity about the bonding and this should be borne in mind when searching for particular compounds. Fluoroaryl and heteroaryl derivatives form a more homogeneous group of compounds, and their parameters are considered together across the transition series. 

The interpretation of the experimental a constants for the fluoroalkyl and nitroxide radicals is complicated because the stereochemistry at the radical center is not well defined. This complication does not exist for radicals derived from aromatic compounds. Consequently, several research groups have investigated aromatic radicals to characterize the factors governing spin delocalization to /3-fluorine atoms. One aspect of this work concerns the epr spectra of nitrobenzene anion radicals. Another concerns the contact chemical shifts of paramagnetic transition metal complexes. The latter approach was initiated by Eaton, Josey, and Sheppard who examined stable bis(phenylaminotroponiminato)-nickel(II) complexes (55a). More recently, we have examined the contact chemical shifts in the nmr spectra of nickel acetylacetonate complexes of aniline derivatives (556). 

Electron-withdrawing substituents, especially fluoroalkyl groups, stabilize transition metal derivatives. Compounds such as octafiuorotetramethyl-eneiron tetracarbonyl display remarkable thermal and oxidative stability. There has been speculation that these compounds are stabilized by negative hyperconjugation or no bond resonance. Spectroscopic evidence (105, 105a) has been cited in favor of resonance structures such as... 

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