Samarium triflate-catalyzed dimerization of vinylarenes

We report the preparation of substituted indanes and their dimeric isomers via the samarium triflate-mediated [Sm(OTf) 3 , 10 mol%] self-dimerization of vinylarenes in MeNO 2 at 25 o C for 10 h. The diverse products were obtained in moderate to high yields. The synthesis involves a (3+2) annulation via the formation of carbon-carbon bonds. Plausible mechanisms are proposed and discussed. The investigation of various rare metal triflates catalyst loadings, reaction conditions, and substrate scope led to an operationally easy one-pot Friedel-Crafts reaction protocol.


Introduction
The catalytic self-dimerization reaction of vinyl arenes (e.g.α-methylstyrenes) to derive functionalized indanes is one of the most straightforward and useful transformations used to construct carbon-carbon (C-C) bonds.After pioneer work by Bergmann and co-workers, 1 various promoter-mediated synthetic routes have been documented via the intermolecular hydroarylation of vinyl arenes followed by spontaneous intramolecular ring-closure of the resulting dimeric isomers (Scheme 1).9][20][21] On the basis of observations, attempts to develop new and efficient catalyst systems for the self-dimerization of vinyl arenes are still in demand.0] To the best of our knowledge, no examples have been reported for Ln(OTf)3-mediated self-dimerizations of this type.Scheme 1. Self-dimerization of vinyl arenes.
Further variations in the reaction parameters, such as catalyst loading, the solvent system, temperature and reaction time, were carried out as follows.In entry 2, decreasing the catalytic equivalent of Sm(OTf)3 (10 → 5 mol%) diminished the yield of 4a (66 → 39%).Entry 3 showed that excess amounts (20 mol%) of Sm(OTf)3 did not increase the catalytic ability to provide a better yield, and a similar yield (64%) was observed.After elevating the temperature (25 → 75 o C), 4a was isolated in only a 41% yield (entry 4).Under a refluxing MeNO2 (101 o C) condition, 4a was isolated in a low yield (35%), and a very complex mixture was detected, as shown in entry 5.With longer reaction times (5 → 10, 20 h), 4a was formed in higher yield (83%, 80%) (entries 6-7).Furthermore, controlling the reaction conditions for the combination of Sm(OTf)3 (10 mol%), 25 o C and 10 h, different solvents were examined.After changing the reaction solvent from MeNO2 to PhMe and CH2Cl2, 4a was produced in lower yields (28%, 60%) than MeNO2 (entries 8 & 9).In particular, entry 10 showed that the use of DMF did not give the desired product 4a.On the basis of TLC monitoring, only the starting material 3a was detected.However, treatment of 3a with diluted MeNO2 afforded 4a in a similar yield (80%) (entry 11).This meant that the reaction concentration was not a main factor affecting the 4a yield.
On the basis of the highest observed yield (entry 6, 83%), the combination of 10 mol% Sm(OTf)3/MeNO2 (5 mL)/25 o C/10 h was selected as the optimal reaction conditions for the formation of dimer 4a.On the other hand, Sm(OTf)3 has been reported as a catalyst for different reaction types, including Ferrier rearrangement, 31 Friedel-Crafts alkylation, 32 aza-Diels−Alder cycloaddition, 33 conjugated addition 34 and cross-coupling. 357][38][39][40] With the optimal reaction conditions in hand (Table 1, Entry 6), we then explored the scope of the conversion with other substrates (Table 2).
α-Phenylstyrene (3q) was also studied (Table 2).On the basis of the styrene skeleton, changing the α-methyl group to α-phenyl was tested.Under the above-mentioned conditions, treatment of 3q with Sm(OTf)3 afforded 4q in a 56% yield along with a 25% yield of unknown and unanalyzed products mixture (Scheme 2).

Scheme 2. Synthesis of indane 4q.
Based on experimental results, a possible reaction mechanism with both electron-withdrawing 4trifluoromethylphenyl group and electron-donating 3,4-dimethoxyphenyl groups is shown in Scheme 3. How were two dimers 4b-1 and 4h produced?The event is initiated to form A by complexation of an olefinic moiety of 3 with Sm(OTf)3.After releasing a triflate anion, B, with a methylene samarium arm, is generated.Then, participation of another 3 converts the resulting B into C having a tertiary carbocation.On the basis of the structure on C, path a (green) shows that the 4-CF3 group decreases the electron density of Ar such that the triflate anion could trap the proton to stabilize carbocation.Following in-situ formed triflic acid-promoted protodemetalation of D, the removal of Sm(OTf)3 afforded 4b-1.For path b (red), owing to Ar = 3,4-(MeO)2, the electron-rich arene could attack the carbocation to give E via the five-membered ring closure procedure.Following the triflate anion-mediated dehydrogenative aromatization of E, and then, triflic acid-promoted protodemetalation of F, tetramethoxyindane 4h is obtained along with the recovery of Sm(OTf)3.From the plausible mechanisms, we understand that electron-density on arene is a key factor in affecting the reaction pathway and product distribution.Scheme 3. Plausible mechanism.

AUTHOR(S)
In particular, when Ar with a 2,4-dimethoxy group was treated with the above reaction conditions, only 4k was generated in a 76% yield.The spiro structure of bis-indane 4k was determined by single-crystal X-ray crystallography. 41We postulated that three equivalents of 3k were involved to generate 4k (Scheme 4).According to a series of reaction steps in Scheme 3, path b, II is produced from I with an oxygen-chelated samarium complex conformation.By removal of 1,3-dimethoxybenzene, III should be formed.This is a very important step for the formation of a bis-indane skeleton because the formed 3-carbon fragment could construct a spiro ring.Furthermore, intermolecular Friedel-Crafts type coupling of the corresponding III with another I produces IV.Next, electron-rich arene attacks the carbocation to give V via intramolecular benzannulation.Following the above-mentioned steps (triflate anion-mediated dehydrogenative aromatization of V followed by triflic acid-promoted protodemetalation of VI), tetramethoxy spiro-indane 4k is obtained along with the recovery of Sm(OTf)3.Compared with the formation of oxygenated indanes 4h-k, only 4k was produced as a spiro system under similar reaction conditions.The detailed reasons are still unclear, however, we believe that the 2-MeO group of 3k plays a role in triggering the removal of 1,3dimethoxybenzene more easily than other oxygenated vinylarenes 3h-j during the conversion process from II to III.Scheme 4. Plausible Mechanism of 4k.

Conclusions
We have developed a mild synthesis of substituted indanes and dimeric isomers in moderate yields via a 10 mol% Sm(OTf)3-mediated self-dimerization reaction of substituted vinylarenes in MeNO2 at 25 o C for 10 h.The control of reaction parameters such as the lanthanide triflates catalyst loading, the reaction temperature, the solvent and the time, had to be finely tuned to explore optimal conditions.Furthermore, the proposed mechanisms for the formation of 4b-1, 4h and 4k are discussed.Further investigation regarding synthetic applications of lanthanide triflates will be conducted and published in due course.