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

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