Synthesis of Aromatic Sulfones from SO 2 and Organosilanes, Under Metal-free Conditions

: The conversion of SO 2 to arylsulfones has been achieved for the first time under metal-free conditions, by reacting SO 2 with (hetero)arylsilanes and alkylhalides, in the presence of a fluoride source. The mechanism of this transformation has been elucidated based on DFT calculations, which highlight the influence of SO 2 in promoting C–Si bond cleavage. The

Scheme 1. Some representative sulfone-containing drugs and trends in state of the art sulfone synthesis from SO2.
Organosilanes are very mild nucleophiles and a fluoride source or a base is needed to activate the low polar C-Si bond and facilitate transmetallation to a metal catalyst. [9] Interestingly, we have recently reported on the fluoride-mediated carboxylation of heteroarylsilanes with CO2, where CO2 acts both as a reagent and as a catalyst. [10] Reasoning that SO2 is a stronger Lewis acid than CO2, we have sought to promote the sulfonylation of 2-(trimethylsilyl)pyridine (1) with SO2 adduct DABSO. Addition of DABSO to a CH2Cl2 solution of 1 and methyliodide, in the presence of tetrabutylammonium triphenyldifluorosilicate (TBAT) as a fluoride source, resulted in the formation of the expected methylsulfone 1a in >90% yield within 17 h at 25 °C (Eq. 1). DABCO, Ph3SiF, tetrabutylammonium iodide and Me3SiF were formed as byproducts.
To the best of our knowledge, the conversion of 1 to 2a represents the first example of the direct sulfonylation of an organosilane reagent under metal-free conditions. Importantly, 1 selectively reacts with SO2 and no trace of pyridine or 2-methylpyridine was detected, that would result from quenching of the pyridine anion with protons or MeI, It is notable that, in the absence of SO2, no conversion of 1 was observed after 17 h at 25 °C, thereby suggesting that SO2 facilitates the C-Si bond scission. [11]  Reaction conditions: Ar-Si(Me)3 (0.5 mmol), TBAT (0.5 mmol), RX (0.5 mmol), DABSO (0.25 mmol), CH2Cl2 (1 mL). a) NMR yield.
As pointed out by Willis et al., DABSO is a practical surrogate for SO2, as it is bench-stable and avoids the manipulation of a toxic and corrosive gas. [12] Nonetheless, 2a could also be successfully obtained in 71% yield by replacing DABSO with SO2 gas, highlighting that DABCO does not play an essential role in this transformation. Replacing MeI with other organo-halides, a large variety of 2-pyridylsulfone derivatives were successfully prepared (Eq. 2, Table 1). For example, sulfones 2a-2e were formed in excellent 71-96% yields using primary alkyl halides such as iodomethane, iodoethane, 1-iodohexane, 1bromohexane, allyl-iodide and -bromide and benzylbromide respectively (Table 1, entries 1-5). Although TBAT can be easily recycled by addition of TBAF to fluorotriphenylsilane, [13] the atom efficiency of the present transformation can be improved by utilizing CsF as a fluoride source, in a polar solvent such as CH3CN. Under these conditions 2e is isolated in 77% yield (see SI). This reaction is tolerant towards several functional groups such as iodides (2g), esters (2h), ketones (2i) and nitriles (2j) ( Table 1, entries 7-10). Introducing, an electron donating (3-CH3, 5-CH3 or 6-CH3) (EDGs) or withdrawing (4-CF3) (EWGs) group on the pyridine ring of 1 did not influence significantly the reactivity of the organosilane reagent and sulfones 4a-4d were formed in 64-90% yield, with 4b (5-CH3) being the most reactive (Scheme 2).
[a] NMR yield determined with mesitylene as internal standard.
Disappointingly, trimethylphenylsilane (5) exhibits no reactivity in the presence of DABSO, EtI and CsF, thereby showing that the pyridine ring has a positive influence on the reaction of 1 with SO2. In order to promote the metal-free conversion of arylsilanes to aromatic sulfones, we sought to increase the Lewis acidity of silicon to facilitate the activation of the arylsilane by CsF. The sulfonation of triethoxyphenylsilane (6a) with DABSO, EtI and CsF indeed enables the formation of phenylethylsulfone 7a after 3 h at 80 °C in a low 5 % yield. This poor conversion could be attributed to the competitive formation of the anion FSO2‾. [14] To shift this equilibrium towards the release of the active fluoride and SO2 sources, the conversion of a variety of aryltriethoxylsilanes possessing EDGs and EWGs groups (6a-6e) was attempted, with 6 equiv. CsF, 3 equiv. DABSO and 4 equiv. EtI, at 120 °C (Scheme 3). The corresponding arylethylsulfones 7a-7e were successfully isolated in 33-97% with the electron deficient C6F5Si(OEt)3 (6e) derivative being the most reactive. The formation of 2a-2j, 4a-4d and 7a-7e represents the first examples of a metal-free synthesis of sulfones from (hetero)arylsilanes. From a mechanistic viewpoint, experiments showed that, in the presence of a fluoride source, the C-Si bond scission of arylsilanes does not take place in the absence of SO2. Second, a marked difference in reactivity was observed between pyridyl-and phenyl-silane derivatives, the latter being less reactive. To address these questions, DFT calculations were performed for the sulfonylation of 2-(trimethylsilyl)pyridine (1) and trimethylphenylsilane (5) (5). b) Proposed mechanism for the metal-free sulfonylation of silanes 1 and 5. c) Representation of the HOMO in TS4 with X = CH (5) showing a strong electron delocalization over the SO2-fragment.
The simplest pathway would rely on a fluoride transfer from the fluoride source to the organosilane and subsequent C-Si bond cleavage of the hypervalent intermediate (8) to release the free pyridyl or phenyl anion (Scheme 4a). Nevertheless, this sequence can be discarded as it involves the unstabilized carbanions (ΔG>28.7 kcal.mol -1 ) and transition states lying at least 35.6 kcal.mol -1 higher than the starting materials. As SO2 has a positive influence on the early stages of the reaction, its role in the formation of 8 and the activation of the organosilane was investigated computationally (Scheme 4b). SO2 is a potent electrophile and it readily abstracts a fluoride anion from Me3SiF2‾ to yield the stable FSO2‾ anion, as observed experimentally (ΔG=-12.7 kcal.mol -1 ; ΔG ‡ (TS3)=6.3 kcal.mol -1 ). The stable FSO2‾ can then act as a fluoride transfer agent and a transition state (TS4) was located that connects phenylsilane 5 to the sulfinate anion 9, in the presence of a second molecule of SO2. TS4 only lies 20.4 kcal.mol -1 above the starting materials and its low energy is attributed to the strong electrophilic character of SO2, able to stabilize the charge build-up on the aryl ring upon cleavage of the C-Si bond. Indeed, the highest occupied molecular orbital (HOMO) of TS4 shows a large delocalization of the carbon lone pair onto the SO2 fragment, with a minimum structural perturbation of SO2 (OSO=114.4° vs. 117.6° in free SO2, mean S-O bond lengths of 1.47 Å vs. 1.45 Å in free SO2) (Scheme 4c). The so-formed arylsulfinate anion 9 (-28.0 kcal.mol -1 ) could then undergo S-alkylation via transition state TS5 (-11.5 kcal.mol -1 ) to afford the final sulfone product (-47.8 kcal.mol -1 ). This pathway helps rationalize the positive effect of SO2 in the activation of the organosilane reagent. Nonetheless, it fails to account for the difference in reactivity between 1 and 5, as similar energy barriers are computed for the two substrates (20.0 vs 20.4 kcal.mol -1 ). Reasoning that the nitrogen atom of the pyridyl ring could also interact with SO2, the formation of adduct 10 was computed. Interestingly, sulfonylation of 1 leading to 10 is barrier-less and only slightly endergonic (ΔG=12.2 kcal.mol -1 ). The sulfonylation of the C-Si bond, in the presence of FSO2‾, then proceeds via TS6 to yield sulfinate 11. Decoordination of SO2 from 11 is again barrier-free and it releases the expected sulfinate anion 9. Importantly, TS6 displays similar features to TS4 and the extra stabilization provided by the N-SO2 interaction translates into a 3.3 kcal.mol -1 stabilization of TS6 compared to TS4. Overall, these mechanistic trends unveil a multiple catalytic influence of SO2 in the sulfonylation of 1 and 5. First, SO2 behaves as an efficient fluoride carrier via the reversible formation of FSO2‾. Second, it facilitates the C-Si bond cleavage of 1 by reversible coordination to the N atom of the pyridine ring. This kinetic behavior is attributed to the electronic and geometric structure of SO2. With a bent structure and a low lying π* vacant orbital perpendicular to the (O,S,O) plane and polarized towards the sulfur atom, SO2 is poised towards nucleophilic attack. In fact, we computed that the elongation of the S-O bonds does not exceed 9.8% and the bending of the O-S-O angle remains small (9.7%) upon formation of FSO2‾, 9, 10 and 11. These findings suggest that the sulfonylation of other mild nucleophiles could be facilitated by SO2, such as the sulfonylation of C-H, N-H or N-Si bonds, under metal-free conditions.
In conclusion, we have shown that sulfones can be accessed under metal-free conditions for the first time from both heteroaryl and arylsilanes, in the presence of SO2 or a surrogate, the activation of the organosilane being promoted by a fluoride source. Mechanistic investigations show that SO2 is an excellent electrophile, able to synchronize a fluoride mediated C-Si bond cleavage with C-S bond formation.

Experimental Section
Detailed descriptions of experimental and computational methods are given in the Supporting Information.