Science of Synthesis

Kantchev, E. A. B.; Organ, M. G., Science of Synthesis, (2009) 48, 83.

The reductive elimination[‌121‌] of two metal-bound cis-alkyl groups with concomitant lowering of the metal oxidation state by two is the last step of the alkyl–alkyl cross-coupling cycle (see Section ‌48.1.2.2‌). If such complexes are synthesized by alternative pathways (for instance, by treatment of metal halides with >2 equivalents of alkyl main-group organometallic reagents), alkyl–alkyl reductive elimination can also proceed. For alkyl groups possessing β-hydrogen atoms, β-hydride elimination followed by reductive alkyl–hydrogen elimination (see Section ‌48.1.1.2.2‌) is an important competing reaction. As β-hydride elimination is reversible, it can lead to isomerization of secondary or tertiary alkyl–metal complexes to the least sterically hindered, primary alkyl–metal complexes. For example, tert-butyldimethyl(triphenylphosphine)gold(III) isomerizes rapidly to the isobutyl analogue.[‌122‌] The stability of dialkyl transition-metal compounds varies from metal to metal, and then also with oxidation state and the presence of other ligands. Generally, alkyl–transition-metal complexes incorporating metal ions in higher oxidation states reductively eliminate more easily. Dialkyliron–bis(2,2′-bipyridyl) complexes provide a striking example of this. cis-Bis(2,2′-bipyridyl)diethyliron(II) undergoes thermal decomposition at 50°C to form ethane and ethene, but no butane. Electrochemical oxidation produces a cationic iron(III) complex that decomposes (half-life at 30°C: ∼30 min) with the formation of an ethyl radical. Further electrochemical oxidation yields a dicationic iron(IV) complex that undergoes smooth intramolecular alkyl–alkyl reductive elimination at room temperature resulting in the formation of butane (76%) and ∼2% ethene and ethane combined.[‌123‌] Similarly, the dipropyl analogue ‌58‌ yields 82% of hexane (‌Scheme 21‌). Trialkylgold(III)–triphenylphosphine complexes (e.g., ‌59‌, ‌Scheme 21‌) also undergo clean intramolecular alkyl–alkyl reductive elimination at 70–90°C with the formation of alkylgold(I)–triphenylphosphine complexes.[‌122‌,‌124‌] Another way to promote reductive elimination is the addition of π-acidic ligands, which have high affinity to the metal at the lower oxidation state attained after reductive elimination. The addition of chelating ligands has similar effect. Whereas trans-dimethylbis(triphenylphosphine)palladium(II) cannot be isolated at room temperature, trans-dimethylbis(triethylphosphine)palladium(II) is persistent for at least 3 hours.[‌109‌] The addition of [2-(methylsulfanyl)phenyl]diphenylphosphine (‌60‌) to dibromodimethylplatinum(IV) causes reductive elimination of ethane and the formation of platinum complex ‌61‌.[‌125‌] Similarly, the addition of π-acidic ligands such as maleic anhydride, tetracyanoethenes (‌Scheme 21‌), tetrafluoroethene, 2,2′-bipyridyl, or terpyridines (e.g., ‌31‌; see ‌Table 10‌, Section ‌48.1.2.2.2‌) to cis-dimethyl(N,N,N′,N′-tetramethylethylenediamine)nickel(II) (‌62‌) leads to rapid reductive elimination and ethane formation in the temperature range of −78 to −30°C.[‌25‌,‌126‌] These examples do not have practical utility for the synthesis of alkanes.

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References

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• 8.Meeeee-Meee, (8888) M 88-8, 8888.
• 8.Meeeee-Meee, (8888) M 88-8, 8888.