Phenyllithium Explained

Phenyllithium is an organometallic agent with the empirical formula C₆H₅Li. It is most commonly used as a metalating agent in organic syntheses and a substitute for Grignard reagents for introducing phenyl groups in organic syntheses. Crystalline phenyllithium is colorless; however, solutions of phenyllithium are various shades of brown or red depending on the solvent used and the impurities present in the solute.^[1]

Preparation

Phenyllithium was first produced by the reaction of lithium metal with diphenylmercury:^[2]

(C₆Η₅)₂Ηg + 2Li → 2C₆Η₅Li + Ηg

Reaction of a phenyl halide with lithium metal produces phenyllithium:

X-Ph + 2Li → Ph-Li + LiX

Phenyllithium can also be synthesized with a metal-halogen exchange reaction:

n-BuLi + X-Ph → n-BuX + Ph-Li

The predominant method of producing phenyllithium today are the latter two syntheses.

Reactions

The primary use of PhLi is to facilitate formation of carbon-carbon bonds by nucleophilic addition and substitution reactions:

PhLi + R₂C=O → PhR₂COLi

2-Phenylpyridine is prepared by the reaction of phenyl lithium with pyridine, a process that entails an addition-elimination pathway:^[3]

C₆H₅Li + C₅H₅N → C₆H₅-C₅H₄N + LiH

Structure and properties

Phenyllithium is an organolithium compound that forms monoclinic crystals. Solid phenyl­lithium can be described as consisting of dimeric Li₂Ph₂ subunits. The Li atoms and the ipso carbons of the phenyl rings form a planar four-membered ring. The plane of the phenyl groups are perpendicular to the plane of this Li₂C₂ ring. Additional strong inter­molecular bonding occurs between these phenyllithium dimers and the π-electrons of the phenyl groups in the adjacent dimers, resulting in an infinite polymeric ladder structure.^[4]

In solution, it takes a variety of structures dependent on the organic solvent. In tetrahydrofuran, it equilibrates between monomer and dimer states. In ether, as it is commonly sold, phenyllithium exists as a tetramer. Four Li atoms and four ipso carbon centers occupy alter­nating vertices of a distorted cube. Phenyl groups are at the faces of the tetrahedron and bind to three of the nearest Li atoms.

The C–Li bond lengths are an average of 2.33 Å. An ether molecule binds to each of the Li sites through its oxygen atom. In the presence of LiBr, a byproduct of directly reacting lithium with a phenyl halide, the [(PhLi•Et₂O)₄] complex instead becomes [(PhLi•Et₂O)₃•LiBr]. The Li atom of LiBr occupies one of the lithium sites in the cubane-type cluster and Br atom sits in an adjacent carbon site.^[5]

Notes and References

1. Gilman, H. . Zoellner, E. A. . Selby, W. M. . An Improved Procedure for the Preparation of Organolithium Compounds . . 1932 . 54 . 5 . 1957–1962 . 10.1021/ja01344a033.
2. Encyclopedia: Green, D. P. . Zuev, D. . Encyclopedia of Reagents for Organic Synthesis . 2008 . Wiley and Sons . 10.1002/047084289X.rp076.pub2 . http://onlinelibrary.wiley.com/o/eros/articles/rp076/frame.html . 978-0471936237 . Phenyllithium .
3. Evans, J. C. W.; Allen, C. F. H. "2-Phenylpyridine" Organic Syntheses (1938), vol. 18, p. 70
4. Dinnebier . R. E. . Behrens . U. . Olbrich . F. . Lewis Base-Free Phenyllithium: Determination of the Solid-State Structure by Synchrotron Powder Diffraction . . 1998 . 120 . 7 . 1430–1433 . 10.1021/ja972816e.
5. Hope, H. . Power, P. P. . Isolation and Crystal Structures of the Halide-Free and Halide-Rich Phenyllithium Etherate Complexes [(PhLi•Et₂O)₄] and [(PhLi•Et₂O)₃•LiBr] . Journal of the American Chemical Society . 1983 . 105 . 16 . 5320–5324 . 10.1021/ja00354a022.