Alkynes substituent control, regiochemistry

Steric differences have been shown to affect the regio-chemistry of cyclization, as the CO inserts in the a-position of the larger alkynyl substituent. However, theoretical studies on dicobalt hexacarbonyl alkyne complexes have suggested that electronic differences in alkyne substituents may also exert control over the regiochemistry of cyclization. Calculations have also provided information on geometries and configurational stabilities of these complexes. 

The regiochemistry can be controlled by the nature of the substituents. With a tri-methylsilyl-substituted acetylene, the trimethylsilyl groups are placed in a positions of zirconacyclopentadienes with excellent selectivity (Eq. 2.8) [20]. With a phenyl-substituted alkyne, regioselective reactions are usually observed, although in some cases a mixture of two isomers may be formed. 

---