One of the research interests of Prof. Zhenhua Gu @ University of Science and Technology of China is the divergent synthesis of atropisomers via efficient new-developed methodology. Stimulated by the advancement of pioneer work of Bringmann1 and Hiyashi2 in construction of axially chiral biaryls via a ring opening protocol of lactone and thioether, we have realized a series of transition metal catalyzed asymmetric ring-opening of cyclic diaryliodoniums with aniline (Chem, 2018, 4, 599), aliphatic amines (Adv. Synth. Catal., 2018, 360, 3877), potassium thioate (Org. Lett., 2019, 21, 3942), O-alkylhydroxylamines (Org. Lett., 2019, 21, 6374) and diarylphosphine oxide (ACS Catal., 2019, 9, 9852). Also, the same strategy has applied to the construction of biaryl atropisomers from 9H-Fluoren-9-ols successively (Chem, 2019, 5, 1834).
In these studies, an interesting phenomenon was found: twisted molecules are more likely to undergo ring-open reaction, even if their steric hindrance is larger than that of non-distorted molecules (Scheme 1a). On the basis of these findings, the authors were going to synthesize axially chiral silanol framework via a tandem C-Si bond cleavage and C-C bond forging of silafluorene.
The ring strain of distorted silafluorene 1d was around 12.58 kcal/mol by calculated hydrogenation energy of four different compounds 1a-1d. The silanes showed low reactivity in classic cross-couplings, i.e., Hiyama or Hiyama-Denmark Couplings3-4. Pleasingly, the Narasaka acylation5, which favored C-SiMe3 bond breaking over the C-SiMe2(OH), provided a potential solution for C-Si bond cleavage of silafluorene. However, challenges still remain: (a) only vinyl silanes underwent Narasaka acylation; (b) it is necessary to differentiate up to four C(aryl)-Si bonds in our systems. The releasing of non-cyclic aryl ring is the undesired pathway; (c) the classic Narasaka acylation is steric sensitive, and vinyltriphenylsilane displayed extremely low reactivity (Scheme 2). It was anticipated that the torsional strain of silafluorene would enable the first aryl Narasaka acylation reaction.
With [Rh(CO)2Cl]2 as the catalyst, primary investigations found that the TADDOL-based phosphoramidite gave satisfied stereo-induction and yield in the presence of Cs2CO3. Further investigation found that the amount of base has a significant effect on the yield and 30 mol% of base is optimal. A broad of substrates bearing different substituents were tested to be found that the methology has a good functional group toleration. The utility of the strategy was demonstrated that the product could be transferred to the hydroxysilanol, which displays a potential application in asymmetric catalysis.
In conclusion, Gu and coworkers reported a Rh-catalyzed asymmetric ring-opening acylation reaction for the synthesis of α-silyl biaryl atropisomers, a class of chiral bulky silanols. The torsional strain of five-membered silafluorenes enabled the success of selective cleavage of C(Ar)-Si bond, thus realized the first aryl-Narasaka acylation reaction. Additional notable merit of this work is the newly developed sulfonylpiperazine derived phosphoramidite ligands, which showed high stereo-induction in C(Ar)-Si cleavage/ring-opening reaction. More details of this work can be found here: “Catalytic Asymmetric C-Si Bond Activation via Torsional Strain-Promoted Rh-Catalyzed Aryl-Narasaka Acylation” in Nature Communications.
- Bringmann, G. & Menche, D. Stereoselective Total synthesis of axially chiral natural products via biaryl lactones. Chem. Res. 34, 615-624 (2001).
- Shimada, T., Cho, Y.-H. & Hayashi, T. Nickel-catalyzed asymmetric Grignard cross-coupling of dinaphthothiophene giving axially chiral 1, 1’-Binaphthyls. Am. Chem. Soc. 124, 13396-13397 (2002).
- Hatanaka, Y. & Hiyama, T. Cross-coupling of organosilanes with organic halides mediated by a palladium catalyst and tris(diethylamino)sulfonium difluorotrimethylsilicate, Org. Chem. 53, 918-920 (1988).
- Denmark, S. E., Wehrli, D. & Choi, J. Y. Convergence of mechanistic pathways in the palladium(0)-catalyzed cross-coupling of alkenylsilacyclobutanes and alkenylsilanols, Lett. 2, 2491-2494 (2002).
- Yamane, M., Uera, K. & Narasaka, K. Rhodium-catalyzed acylation of vinylsilanes with acid anhydrides: application to the transformation of α-acyloxy vinylsilanes to unsymmetrical 1,2-diketones, Chem. Lett. 33, 424-425 (2004).