Our studies commenced with 1-phenyl-1,3-butadiene (1a) and 4-methylbenzenesulfinic acid (2a). After considerable screening, the allylic sulfone (3aa) could be obtained in 95% yield with excellent regioselectivity using dichloromethane (DCM) as solvent and boron trifluoride ether (BF3·OEt2) as catalyst at 60°C for 6 hours (Table S1).
Encouraged by the aforementioned results, we explored the scope and generality of the present process (Fig. 2). First, the limitation of the reaction was assessed with various 1,3-dienes. In the beginning, the substituent effect at the C4 position of the phenyl group of 1a was examined. The desired products 3ba and 3ca were obtained in 81% and 35% respectively when the C4 position of the phenyl ring was substituted with electron-donating methyl- and methoxyl- groups. Halogen groups (-F, -Cl, -Br) at the C4 position gave the corresponding allylic sulfones (3da-3fa) in 48–75% yields. Products with substitutions at the C3 and C2 position of phenyl group were also generated in the yields of 20%-50% (3ga-3ja). When the phenyl group of 1 was changed with 2-naphthyl, the desired product 3ka was prepared in 69% yield. It’s always challenging to control the selectivity of the hydro-functionalization reactions when multi-substituted 1,3-diene was used. In this case, the reaction performed with excellent regioselectivity for multi-substituted substrates affording 3la in the yield of 98%. Next, we probed the scope of the reaction with respect to the sulfinic acid. Phenyl sulfinic acid and 4-tert-Bu-phenyl sulfinic acid offering 3ab and 3ac in 68% yield. Electron-absorbing groups, such as halogen (-Cl, -Br), trifluoromethoxy, nitro on the phenyl ring were also well-tolerated, giving the desired allylic sulfones (3ad-3ag) in 62%-80% yield. Product with multiple halogen group 3ah was also obtained in 43% yield. Substrates with methyl group at the C3 and C2 position of the phenyl group of 2 offered 3ai and 3aj in 70% and 52% yields respectively. When we replaced the phenyl group with 2-naphthyl and 2-thienyl groups, 3ak and 3al were synthesized in 70% and 58% yield respectively. Except for these aromatic sulfinic acids, aliphatic sulfinic acid 2m can also be efficient sulfonylation reagents, giving 3am in 55% yield.
To demonstrate the utility of this method, the reaction was carried out at 10 mmol scale, and 3aa could be prepared in 65% yield at gram scale. The generated allyl sulfone products can be rapidly deprotonation in the presence of sodium hydroxide to obtain tertiary alkyl sulfones, which could be further transformed to various complex molecules through nickel-catalyzed Suzuki-Miyaura cross-coupling (Ariki et al. 2018). 4a-4c were prepared in high yields of 89–95% at room temperature from 3aa and halides (Fig. 3). Epoxy group is well founded in natural products, pharmaceuticals and reaction intermediates (Meninno et al. 2017; Petsi et al. 2021; Sartori et al. 2021; Vilotijevic and Jamison 2009). Herein, the epoxidation of 3aa was achieved with m-CPBA to offer β-epoxy-sulfone 4d in 50% yield.
In the end, a plausible mechanism for this reaction could be proposed (Kancharla et al. 2019; Kumar et al. 2019; La and Kim 2018b; La and Kim 2018a; Markos et al. 2021; Wang et al. 2019), as shown in Fig. 4. The BF3·OEt2-catalyzed hydrosulfonylation of 1-phenyl-1,3-butadiene 1a with 4-methylbenzenesulfinic acid 2a is assumed to proceed through three steps: 1) coordination of the 2a to BF3·OEt2 generates the Lewis adduct A; 2) proton transfer from the 2a O-H group to the 1,3-diene forms an intermediate B; 3) electrophilic attack of the allyl carbocation to the benzenesulfinic acid anion give the intermediate C (C-S formation), then through the dissociation of BF3 provides the desired hydrosulfonylation product 3aa.