Four-electron reduction of CO 2 : from formaldehyde and acetal synthesis to complex transformations.

The expansive and dynamic field of the CO 2 Reduction Reaction (CO 2 RR) seeks to harness CO 2 as a sustainable carbon source or energy carrier. While significant progress has been made in two, six, and eight-electron reductions of CO 2 , the four-electron reduction remains understudied. This review fills this gap, comprehensively exploring CO 2 reduction into formaldehyde (HCHO) or acetal-type compounds (EOCH 2 OE, with E = [Si], [B], [Zr], [U], [Y], [Nb], [Ta] or -R) using various CO 2 RR systems. These encompass (photo)electro-, bio-, and thermal reduction processes with diverse reductants. Formaldehyde, a versatile C 1 product, is challenging to synthesize and isolate from the CO 2 RR. 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 CO 2 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.