Science of Synthesis

Gandon, V.; Malacria, M., Science of Synthesis, (2008) 44, 214.

The reaction mixture composed of dichlorodiisopropoxytitanium(IV) [or trichloro(isopropoxy)titanium(IV)], tris(dimethylamino)(methylene)phosphorane [(Me2N)3P=CH2], and sodium hexamethyldisilazanide acts as both a phosphorus ylide and a titanium carbene.[‌139‌–‌142‌] It is thus able to promote the double alkenation of carbonyl compounds to give allenes ‌104‌. The best results for such condensation reaction are obtained with aromatic aldehydes. In its intramolecular version, this reaction allows the formation of macrocyclic allenes from two linked aldehydes (‌Scheme 61‌). Formally, both a phosphine oxide and a titanium oxide are eliminated during the reaction. Thus, this method unites both the Wittig reaction and the alkenyltitanocene route to allenes discussed in Section ‌44.2.2.3.2‌. Examples of homocoupling of aldehydes are provided in ‌Scheme 62‌.

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