This manuscript describes a new method for the synthesis of cyclopentenes in which the alkene forms part of an alpha, beta-unsaturated aryl ketone motif. The reaction is that of an di-ortho-substituted aryl methyl ketone with a 1-aryl or 1-alkyl-cyclopropyl methanol promoted by t-BuOK (1 equiv.) and a Manganese complex (3 mol%) in toluene at 140 C for 72 h. 28 examples are presented (Tables 2-3, 40-88% yields) along with some synthetic transformations of one of the products (Scheme 2), some experiments to probe the mechanism (Scheme 3) and some DFT studies (Figures 1-3).

The manuscript is logically constructed and easy to follow, and well referenced. From a synthetic perspective, the scope of the disclosed reaction is limited by the requirement for the aryl ketone component to be di-ortho-substituted, likely in order to slow down competing aldol-type reaction. The cyclopropylmethanol can comprise both aryl and alkyl substituents. Although cyclopentene motifs are often found in naturally occurring substances of e.g., medicinal interest the aforementioned scope limitations preclude direct application to many of these. Given also the high temperature and long duration of the reaction, I would not consider that the transformation merits publication in Chem Sci. on its synthetic merits alone. However, the authors draw attention to their hypothesis that this reaction proceeds via an unusual mechanism catalysed by an earth abundant metal (Mn). To me, the proposed mechanism raises several questions: The role of the Mn complex in effecting dehydrogenation of the cyclopropylmethanol makes sense via the process explored in Figure 1, liberating H2 gas – presumably this step has been verified under the reaction conditions and in the absence of the aryl methyl ketone? The subsequent condensation of with the aryl methyl ketone is simply an aldol condensation which Scheme 3, part (b) entry 2 (also 7.2 in SI) shows requires no Mn to proceed and can be promoted by the t-BuOK. This same experiment demonstrates that the subsequent ring expansion does not require Mn either. So what is the key evidence that the Mn plays a role in these steps? It seems TEMPO inhibits not only the Mn containing reaction (Scheme 3, part (b) entry 4) but also the non-Mn-containing congener (7.4.2 in SI), so this does not to my mind provide evidence of radical involvement. I see no control reactions using the cyclopropyl ketone proposed intermediate which is presumably easy to make and use. Additionally, I find the suggestion that tert-butoxide deprotonates toluene being a viable step in the proposed mechanism somewhat surprising – what is driving this usually thermodynamically uphill step? (Figure 2). Apologies if I have missed something fundamental here but it would be good to see the =DFY computed profiles for the non-Mn catalysed steps corresponding to Figures 2 and 3 for comparison with the proposed pathway.
In summary, I would like to be convinced that the reservations expressed above about the authors’ mechanistic hypothesis are unfounded before recommending for publication in Chem. Sci.