The molecular structure of 3-tert-butyl-4,4-dimethyl-2-pentenal (3,3-di-tert-butylpropenal)

The highly strained conjugated aldehyde 3-tert-butyl-4,4-dimethyl-2-pentena l (3,3-di-tert-butylpropenal; abbreviated D33; 4) has been prepared, and it s molecular structure and conformation have been studied experimentally by the gas electron diffraction method and by theoretical ab initio (HF/ 6-31G *) calculations. The propenal skeleton assumes an anti conformation, and th e steric strain is primarily manifested in the following structural details : The C=C-C angle is substantially larger than normal [GED: 132.6(7)degrees ; HF/6-31G*: 132.5 degrees]; the (Z)-oriented tert-butyl group is twisted t o a nearly staggered position relative to the C=C double bond, forming a co gwheel system with the other tert-butyl group, which has the normal eclipse d conformation relative to C=C; the C-3-C-tBu (formally sp(2)-sp(3)) bonds are elongated compared to those in unstrained compounds, and are longer tha n the (formally sp(3)-sp(3)) C-CH3 bonds.