Using tBu Mo IV (OTf)2 as a catalyst, full conversion was observed at a catalyst loading of 2 mol% within less than one hour at ambient temperature and the expected coupling products were isolated as mixtures of orthoand para regio-isomers in good yields (92% for the coupling of 1 and 2, Scheme 3a). The catalytic activity of tBu Mo IV (OTf)2 was further tested in a 3,3-cyclocoupling reaction of allylic alcohol 3 with p-cresol. This reaction -of interest to generate chroman moieties found notably in several pharmaceutical drugs -may be described as a domino reaction, where an allylic substitution is followed by a Lewis acidcatalyzed cyclization step. This reaction was found challenging in the original report using the in-situ generated catalyst obtained with R Mo IV Cl2/AgSbF6 (1:2 ratio) mixtures, with a reported yield of only 27%. 14 1 H NMR analysis of the reaction using 2 mol% of tBu Mo IV (OTf)2 as a catalyst revealed the formation of the initial C-C coupling product 4 and its full conversion to the cyclized product 5 over prolonged reaction times (Scheme 3b and Fig. ). Chroman 5 could be isolated in 62% yield, a significant improvement with respect to the previously reported yield obtained using in-situ generated catalysts (vide supra). This reaction was previously reported to be catalyzed by Lewis- or Brønsted 29 acids under comparably harsh conditions, typically requiring elevated temperature (up to 150 °C), high catalyst loadings (5-15 mol%) or a large excess of p-cresol (up to 30 equiv.). To the best of our knowledge, the conditions for the preparation of chroman 5 described here (2 mol% catalyst loading, p-cresol (1.3 equiv.), 25 °C, no additives) are among the mildest reported to date. Intrigued by the change of color of the reaction mixture from red to green under catalytic conditions, we attempted identifying the new species generated in-situ and responsible of this color change. During the catalytic substitution of allylic alcohols, water (H2O) is expected as the main by-product. Here, we were able to show that [Mo V (O)( tBu diket)2(OTf)] ( tBu Mo V (O)OTf) is formed upon hydrolysis of tBu Mo IV (OTf)2 (Scheme 4). To probe this assumption, we independently investigated the reactivity of tBu Mo IV (OTf)2 with stoichiometric amounts of H2O. When a slight excess of H2O (5 equiv.) was added to a THF solution of tBu Mo IV (OTf)2, a broad signal at 2.6 ppm appeared in the 1 H NMR spectrum. Cooling this reaction mixture in acetonitrile at -35 °C afforded bright green crystals of tBu Mo V (O)OTf (Fig. ). Interestingly, while the NMR and UV-Vis spectra of tBu Mo IV (OTf)2 do not change upon addition of (excess) 2, addition of stoichiometric amounts of allylic alcohol 1 to tBu Mo IV (OTf)2 led to the fast appearance of the same broad signal at 𝛿 = 2.6 ppm in the 1 H NMR spectrum of the mixture, pointing towards the in-situ generation of tBu Mo V (O)OTf (Fig. ). Relatedly, 1 H NMR and UV-Vis spectra recorded in-situ in the conditions used for the catalytic allylic substitution reaction of 1 with 2 (1 mol% tBu Mo IV (OTf)2) confirm the fast formation of tBu Mo V (O)OTf under catalytic conditions (Fig. and). Further, the spectroscopic data highlights that the conversion of the substrates is still observed when tBu Mo V (O)OTf is the main Mo species, suggesting that it may also be a catalytically active species. To test this hypothesis, tBu Mo V (O)OTf (1 mol%) was used as a catalyst for the allylic substitution of 1 with 2. Full conversion was observed after 4 h at room temperature. However, following this reaction in-situ by 1 H NMR spectroscopy revealed a much lower product formation rate, as if using tBu Mo IV (OTf)2 as the catalyst (Fig. ). A long induction period of ca. 40 min was observed, suggesting that tBu Mo V (O)OTf is not the actual catalytically active species, but a resting state of the catalyst. In addition, time-dependent UV-Vis spectroscopic studies carried out under catalytic conditions (Fig. ) did not exhibit significant changes over the course of the reaction, ruling out the insitu regeneration of the original Mo(IV) catalyst, and suggesting the formation of a new catalytically competent species. Assuming that the generation of the Mo(V) resting state is mainly driven by the accumulation of H2O released during the catalytic run, we attempted to optimize reaction conditions to prevent its formation. The presence of 4 Å molecular sieves (MS) as an additive to standard catalytic conditions using the Mo(IV) complex, led to much faster catalysis (98% conversion after 7 minutes, Fig. ). This suggests that the H2O by-product can be efficiently displaced by the MS, to significantly increase the lifetime of the Mo(IV) catalyst and disfavor the formation of Mo(V) oxo resting states. Based on these findings, we propose that under standard catalytic conditions, two separated catalytic cycles involving either the Mo(IV)-or the Mo(V) complex as active species are operative (Scheme S2), even though for catalyst loadings > 1 mol%, the latter can be neglected due to its comparably slow product formation rate. tBu Mo IV (OTf)2 is also an active catalyst promoting C-N bond formation reactions, enabling the formation of allylic azides (Scheme 3c and Fig. ). Interestingly, the catalytic reaction mixture did not display the characteristic change of color to green observed in C-C coupling reaction, but instead turned dark brown, suggesting the formation of at least one additional new species. We explored the reactivity of the second reactant, TMSN3 (TMS = SiMe3) with tBu Mo IV (OTf)2. At the difference of 2, which did not display any significant reactivity with tBu Mo IV (OTf)2, a strong change of color to dark brown was observed immediately after addition of stoichiometric amounts of TMSN3 (1.2 equiv.) to tBu Mo IV (OTf)2. Single crystal XRD measurements pointed to the formation of a new Mo(VI) species, namely [Mo VI (N)( tBu diket)2(OTf)] ( tBu Mo VI (N)OTf, Fig. ). The fact, that under catalytic conditions, TMSN3 is present in large excess with respect to tBu Mo IV (OTf)2, suggests that the abovedescribed reaction is likely to be present in significant amounts in catalytic conditions. Similarly to what observed with tBu Mo V (O)OTf, tBu Mo VI (N)OTf can catalyze allylic substitution reactions but is much more sluggish (ca. 25% conversion after 12 h) than the parent tBu Mo IV (OTf)2 complex. The in-depth investigation of the coordination chemistry properties of a series of β-diketonate Mo(IV) complexes enabled to identify that the strongly Lewis acidic complexes R Mo IV (OTf)2 are highly active catalysts for the substitution of allylic alcohols.