<table><caption>**Table 10** Best catalytic performances of the different reduction systems reported for the generation of acetal compounds and further use of the acetal as C₁ and/or Cₙ sources in subsequent transformations</caption><thead><tr><th>Reduction system</th><th>Hydrogenation</th><th>Hydrosilylation</th><th>Hydroboration</th><th>Hydrozirconation</th></tr></thead><tbody><tr><td>TON</td><td>3874</td><td>3400</td><td>690</td><td>—</td></tr><tr><td>TOF (h<sup>-1</sup>)</td><td>194</td><td>10.4</td><td>516</td><td>—</td></tr><tr><td>Yield (%)</td><td>—</td><td>99</td><td>97</td><td>—</td></tr><tr><td>Scale</td><td>2.88 mmol</td><td>4.70 mmol</td><td>0.55 mmol</td><td>—</td></tr><tr><td>C<sub>1</sub> source</td><td>×</td><td>CH<sub>2</sub>-N, -O, -S, -C</td><td>CH<sub>2</sub>-N, -O, -P, -C</td><td>CH<sub>2</sub>-C</td></tr><tr><td>C<sub>n</sub> source</td><td>×</td><td>×</td><td>C<sub>2</sub>, C<sub>3</sub>, C<sub>4</sub></td><td>×</td></tr><tr><td>Asymmetric C center</td><td>×</td><td>×</td><td>Diastereo- and enantio-selective transformations</td><td>×</td></tr></tbody></table>

In hydrogenation reactions, the highest reported yield of formaldehyde (HCHO) was 0.037 M g$_{cat}$$^{-1}$ h$^{-1}$ achieved with a Ru-based heterogeneous catalyst in an aqueous medium.¹⁴⁸ Another notable instance involved electroreduction, where a film of electropolymerized chromium vinylterpyridine complex exhibited 90% faradaic efficiency (FE) and a turnover number (TON) of 10⁴.¹⁷⁰ In photoelectroreduction, optimal performances were observed at 16 500 µmol h$^{-1}$ g$_{cat}$$^{-1}$ under 7 bars of CO$_{2}$ and 80 °C with a TiO$_{2}$ catalyst, achieving 70% selectivity.¹⁸⁹ Remarkably, in all these systems, the generated formaldehyde was utilized only once as a substrate in a cascade reaction to form a C-N bond, leaving significant untapped potential in this area for future exploration.¹⁹⁴ Further investigations are thus imperative, both for fundamental understanding and catalyst development, and it is crucial to gather solid evidence regarding the origin of the generated formaldehyde in each case. In addition to the challenge of generating formaldehyde, detecting and isolating it proves to be a formidable task due to its reactive nature.

On the other hand, the reduction of CO$_{2}$ into acetal products has seen successful developments in thermic processes, specifically in (i) the hydrogenation of CO$_{2}$ in the presence of alcohol and (ii) hydroelementation reactions involving hydrosilanes, hydroboranes, and early transition metal hydride species. In contrast to the synthesis of formaldehyde, numerous acetal compounds were selectively and efficiently generated, and eventually isolated. Notably, the formation of acetal from the CO$_{2}$RR currently stands as the sole pathway to formaldehyde, enabling its consistent use not only as a C$_{1}$ source but also as a C$_{n}$ source. Bis(silyl)acetal (BSA) and bis(boryl)acetal (BBA) were indeed shown to release formaldehyde, which was then employed in subsequent C$_{1}$ and C$_{n}$ transformations. It was also demonstrated that these acetal could serve as a formaldehyde surrogate without releasing formaldehyde. The primary drawback of the acetal approach is its lack of atom efficiency. While the regeneration of hydroborane or hydrosilane in a sustainable reduction process would be an ideal solution, the formation of high value-added compounds *via*

complex transformations may compensate for this issue to some extent. Table 10 compiles the highest reported performances (TON, TOF, yield, selectivity) in each of the reductive processes leading to acetal compounds. While the chemistry of formaldehyde is well-established and developed, the acetal compounds obtained from the four-electron reduction of CO$_{2}$ are, for the most part, novel molecules, and their reactivities are still under investigation. Pioneering studies on their reactivity have led to the utilization of several BSA, BBA, and **BZrA 1** as a C$_{1}$ source in multicomponent reactions. This facilitated the incorporation of one carbon atom from CO$_{2}$ into very diverse molecules in the form of methylene. Table 10 also summarizes the types of C-E bonds (E = N, O, S, P) formed from BSA, BBA, and **BZrA 1**. Moreover, two BBA compounds (**BBA 2** and **4**) were employed as a C$_{n}$ source, generating C$_{2}$, C$_{3}$, and C$_{4}$ carbohydrates. In the case of the latter two, asymmetric carbon centres were even generated in a diastereo- and enantioselective manner, respectively. Formaldehyde was directly observed only once in 4% yield with hydrosilanes and only once among a mixture of other products with hydroboranes. The intermediary of acetal (**BSA 3**, **BBA 2** and **4**, **BZrA 1**) was essential for the selective generation of formaldehyde, as it was quantitatively released either spontaneously, from hydrolysis, or from CsF activation depending on the acetal properties.

To conclude, the 4e$^{-}$ reduction of CO$_{2}$ is a two-face journey. On one hand, the generation of acetal compounds witnessed important advances both to control this challenging reduction stage and to use it in subsequent complex transformations but at the expense of low atom-efficiency. On the other hand, the generation of formaldehyde from hydrogenation or photo(electro) reduction is more sustainable, but still in its infancy compared to other C$_{1}$ reduction compounds such as CO or methanol.

We believe that intertwined processes from the different fields involved in the CO$_{2}$RR (single vs. cascade processes, coupling between electro- and organo-catalysis) may be instrumental in addressing the challenging and sustainability issues associated with the 4e$^{-}$ reduction of CO$_{2}$.

<figure><img src="image_102.png" /></figure>
## Data availability

Correspondence and requests should be addressed to julien.bonin@sorbonne-universite.fr, sebastien.bontemps@lcc-toulouse.fr or marc.robert@sorbonne-universite.fr.

## Author contributions

S. B. conceptualized the review. S. B, S. D., J. B. and M. R. wrote and edited the review.

## Conflicts of interest

The authors declare no competing financial interest.

## Acknowledgements

Partial financial support to M. R. from the Institut Universitaire de France (IUF) is warmly thanked. S. D. thanks Région Midi-Pyrénées and Université Fédérale de Toulouse for a doctoral fellowship. S. B. thanks the CNRS for financial and technical support.

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