Photoreduction of aliphatic and aromatic thioketals. new acces to the reduction of carbonyl group by desulfurization process

GABRIELA OKSDATH-MANSILLA; JUAN E. ARGÜELLO; MARC ROBERT; ALICIA B. PEÑÉÑORY

Conferencia; 12a Conferencia Latinoamericana de Físico-Química Orgánica; 2013

Carbonyl reduction is very well described and useful reaction in organic chemistry. A carbonyl group can be reduced to the alcohol or the oxygen atom can be removed altogether, a process called deoxygenation. Carbonyl reduction to methylene group appears as an interesting alternative following a Friedel-Crafts acylation or an α-substitution of a ketone anion enolate. To achieved this chemical transformation the Clemmensen reduction is often employed (Hg(Zn)/HCl). However, the substrate must be stable in the strongly acidic conditions, acid sensitive substrates should be reacted in the Wolff-Kishner reduction, which utilizes strongly basic conditions (NH2-NH2/KOH); a further, milder method is the Mozingo reduction. In this reaction a thioketal is first produced by reaction of the ketone with an appropriate thiol. The product is then hydrogenolyzed to the alkane, using Raney nickel. Following this last concept, we are interested in developing a novel photoinduced reductive process for aromatic and aliphatic ketones transformation to methylene by desulfurization of the corresponding dithioketals. This protocol is and environmentally friendly alternative that avoids the use of transition metals and toxic/explosive reagents such us mercury, hidrazine and hydrogen. To prove the concept, the reduction of a family of 1,3-dithianes and 1,3-dithiolanes in the presence of t-BuOK/1,4-cyclohexadiene under photostimulation will be presented. The synthetic outcome as well as a detailed mechanistic exploration will be discussed based on quantitative photochemical measurements together with a careful determination of the electrochemical properties of the thioketals.