Oxophilicity

Oxophilicity is the tendency of certain chemical compounds to form oxides by hydrolysis or abstraction of oxygen, often from organic compounds. The term is usually used to describe metal centers, commonly the early transition metals such as titanium, niobium, and tungsten. Oxophilic metals are classified as "hard" within the HSAB concept. Many main group compounds are also oxophilic, such as derivatives of aluminium, silicon, and phosphorus(V). The handling of oxophilic compounds often requires air-free techniques.

Typical reactivity

Complexes of oxophilic metals typically are prone to hydrolysis. For example, the high valent chlorides hydrolyze rapidly to give oxides:

TiCl₄ + 2 H₂O → TiO₂ + 4 HCl

These reactions proceed via oxychloride intermediates. For example WOCl₄ results from the partial hydrolysis of tungsten hexachloride. Hydroxide-containing intermediates are rarely observed for oxophilic metals. In contrast, the anhydrous halides of the later metals tend to hydrate, not hydrolyze, and they often form hydroxides.

Reduced complexes of oxophilic metals tend to generate oxides by reaction with oxygen. Typically the oxide-ligand is bridging, e.g.

2 (C₅H₅)₂TiCl + 1/2 O₂ → {(C₅H₅)₂TiCl}₂O

Only in rare cases do the products of oxygenation feature terminal oxo ligands.^[1]

Applications of oxophilicity in synthesis

Oxophilic reagents are often used to extract or exchange oxygen centers in organic substrates, especially carbonyls (esters, ketones, amides) and epoxides. The highly oxophilic reagent generated from tungsten hexachloride and butyl lithium is useful for the deoxygenation of epoxides.^[2] Such conversions are sometimes valuable in organic synthesis. In the McMurry reaction, ketones are converted into alkenes using oxophilic reagents:

2 R₂CO + "Ti" → R₂C=CR₂ + TiO₂

Similarly, Tebbe's reagent is used in olefination reactions:^[3]

Cp₂TiCH₂AlCl(CH₃)₂ + R₂C=O → "Cp₂TiOAlCl(CH₃)₂" + R₂C=CH₂

Oxophilic main group compounds are also well known and useful. The highly oxophilic reagent Si₂Cl₆ stereospecifically deoxygenates phosphine oxides.^[4] Phosphorus pentasulfide and the related Lawesson's reagent convert certain organic carbonyls to the corresponding sulfur derivatives:

P₄S₁₀ + n R₂C=O → P₄S_10-nO_n + n R₂C=S

References

1. ^ Nugent, W. A.; Mayer, J. M. "Metal-Ligand Multiple Bonds" J. Wiley: New York, 1988. ISBN 0471854409.
2. ^ M. A. Umbreit, K. B. Sharpless (1990), "Deoxygenation of Epoxides with Lower Valent Tungsten Halides: trans-Cyclododecene", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=CV7P0121 ; Coll. Vol. 7: 121 
3. ^ Pine, S. H.; Kim, G.; Lee, V. (1993), "Enol Ethers by Methylenation of Esters: 1-Phenoxy-1-phenylethene and 3,4-Dihydro-2-methylene-2H-1-benzopyran", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv8p0512 ; Coll. Vol. 8: 512 
4. ^ David P. Sebesta "Hexachlorodisilane" in Encyclopedia of Reagents for Organic Synthesis John Wiley, London, 2001. doi:10.1002/047084289X.rh007 Article Online Posting Date: April 15, 2001.