Cleavage of Carbon-Carbon Bonds in Aldehydes and Ketones

The disconnection of carbon-carbon bonds has a relevant role in organic chemistry in the same way as the formation of these bonds and is probably even more challenging. An interesting and sometimes overlooked transformation involves the hydroxide-mediated cleavage of carbon-carbon bonds in aldehydes and ketones which has been known for more than a century. The generated fragments are the carboxylate and various neutral residues, such as ketones, nitroalkanes, sulphonyl alkanes, trihaloalkanes (haloform reaction)1 and other moieties. The neutral residues are all very weak acids with pKa values between 10 and 40. We have discovered by serendipity that toluene residues with a pK_a of about 41 can also be cleaved from ketones with hydroxide in generally good yields. Herein, we present studies of the cleavage of different substituted benzylic ketones and aldehydes promoted by hydroxide sources in various solvent systems with the aim to investigate the scope of the reaction and clarify the mechanism. Kinetic data resulting from Hammett correlation plots were investigated and compared with theoretical values from density functional theory (DFT) calculations. DFT calculations were also conducted to determine the relative free energies of possible intermediates and transition states. 
Dehydrogenative decarbonylation of alcohols is an attractive reaction based on two individual processes: the acceptorless dehydrogenation of an alcohol and the decarbonylation of the resulting aldehyde. In this transformation, valuable products are formed, such as the unfunctionalized organic residue and two gases, hydrogen and carbon monoxide, respectively. The gaseous mixture is also known as synthesis gas (SynGas) and has many applications ranging from energy production to chemical manufacture. Homogeneous catalysis has previously been investigated to mediate this process with the aid of metal species based on rhodium and iridium complexes. However, both metals showed limitations in the scope and affordability. In this work, a cheaper alternative is presented, based on the system Ru(COD)Cl₂ and the phosphine P(o-tolyl)₃ for the dehydrogenative decarbonylation of alcohols. The reaction was applied to both benzylic and long chain linear aliphatic alcohols. The intermediate aldehyde can be observed during the transformation, which is therefore believed to proceed through two separate catalytic cycles involving first dehydrogenation of the alcohol, followed by decarbonylation of the resulting aldehyde.