3.6.1 Introduction

Transition metals can be defined as an element whose atoms have an incomplete d -shell or which can give rise to cations with an incomplete shell. Due to incompletely filled d -orbitals it can exhibit a large number of oxidation states. The lower oxidation states can act as reducing agent while the higher oxidation states act as oxidizing agents. Some transition metals exhibit a pair of oxidation states in which the process of redox reaction is very facile. Pd(0)/Pd(II), Cu(I)/Cu(II) and Ni(II)/Ni(IV) are examples of such oxidation states. As such, these oxidation states may support catalytic cycles.

Transition metals can also bind to an organic ligand thus forming a metal-carbon bond. The metal-carbon bond may have σ-character or π-character or both. In case of a bond formation between a d -orbital of the metal with a s -containing hybrid orbital of carbon it gives rise to a σ-organometallic compound. Similarly, if the bond is formed between a d-orbital of a metal with π-orbitals of ligand then it gives rise to π-organometallic compound. In the latter case, bonding is provided by both donation from a filled π-orbital of the ligand to the empty orbital of metal and by the interaction of an unfilled π*-orbital of the ligand with the filled metal d -orbital. Gilman's reagent is an example of the former type of organometallic compounds while Zeise complex is an example of the latter variety.

3.6.1 Titanium Catalysts

Titanium is a 3d series transition element belonging to Group IV. Thus, it has a low valence shell electron density compared to other transition metals belonging to other group. Since the valence shell electrons are strongly attracted by the nucleus, titanium has a high ionization potential. However, once ionized, Ti can attain up to +IV oxidation state. The cationic species are strong Lewis acids and, therefore, they have found application in a wide variety of Lewis acid catalyzed reactions.

3.6.1.1 Lewis Acid Catalysis

The Lewis acidity of TiCl₄ is applied in case of aldol reaction, Micheal addition, Claisen condensation and reductive hydrolysis among many other reactions. Some examples of these are exemplified in Scheme 1.

Scheme 1