Four-electron reduction of CO2: from formaldehyde and acetal synthesis to complex transformations

Abstract

The expansive and dynamic field of the CO₂ Reduction Reaction (CO₂RR) seeks to harness CO₂ as a sustainable carbon source or energy carrier. While significant progress has been made in two, six, and eight-electron reductions of CO₂, the four-electron reduction remains understudied. This review fills this gap, comprehensively exploring CO₂ reduction into formaldehyde (HCHO) or acetal-type compounds (EOCH₂OE, with E = [Si], [B], [Zr], [U], [Y], [Nb], [Ta] or –R) using various CO₂RR systems. These encompass (photo)electro-, bio-, and thermal reduction processes with diverse reductants. Formaldehyde, a versatile C₁ product, is challenging to synthesize and isolate from the CO₂RR. The review also discusses acetal compounds, emphasizing their significance as pathways to formaldehyde with distinct reactivity. Providing an overview of the state of four-electron CO₂ reduction, this review highlights achievements, challenges, and the potential of the produced compounds – formaldehyde and acetals – as sustainable sources for valuable product synthesis, including chiral compounds.