A Powerful Palladium -Catalyzed Asymmetric Hydrosilylation
Through the development of asymmetric Pd-catalyzed hydrosilylation of maleimides, an unprecedented hydrosilylation –controlled construction of C-N axial chirality within ortho-substituted N-arylmaleimides was successfully established in this work.
The organosilicon-mediated organic synthesis (SiMOS) has attracted much attention over the recent decades. However, the use of organosilicon reagents with novel catalytic strategies remains to be explored. Our group has been focusing on developing silicon-mediated organic synthesis and the synthesis of chiral organosilicon compounds in the past decade.1 And in this field, the synthesis of organosilicon compounds via Si-C coupling is a basic process before next transformations for SiMOS. Very recently, we have developed a palladium-catalyzed Si-C cross-coupling transformation through the [4+2] annulation of cyclopropenes with benzosilacyclobutanes synergized with the formation of new C(sp2)-C(sp3) and Si-C(sp3) bonds.2 In this reaction, an array of newly elusive bicyclic skeleton with high strain, sila-bicyclo[4.1.0]heptanes, was formed in good yields and high enantioselectivites under mild conditions. Actually, the Si-C bond-forming reactions has been achieved many accomplishment, however, among various Si-C coupling reactions, the hydrosilylation of unsaturated carbon-carbon bonds with hydrosilanes is an industrially important transformations and yet remains highly challenging, especially for the stereoselective hydrosilylation.
In comparison to classic hydrosilylation of terminal alkenes, the enantioselective hydrosilylation of internal alkenes or its analogues are uncommon, and the synthetic capabilities for the catalytic asymmetric hydrosilylation of activated alkenes are also presently limited. In this context, our group has established an asymmetric Rh-catalyzed hydrosilylation of three-membered cyclopropenes with excellent enantioselectivities.3 And it is difficult to finish the medium cyclic alkenes with the aid of chiral Rh complex. In addition, the hydrosilylation of a,b-unsaturated carbonyl compounds may pose additional challenges because it can generate mixtures of reductive product, a- and b-adducts, and especially for a,b-unsaturated carbonyl compounds, silyl ketene acetals (O-silylation adduct with 1,4-addition), silyl ethers/amines (O-silylation adduct with 1,2-addition), and polymeric byproducts were also obtained depending on the catalyst systems. Based on our interests and experience in the palladium catalysis,4 we finally realized a highly enantioselective Si-C coupling by hydrosilylation of carbonyl-activated alkenes using a palladium catalyst with a chiral TADDOL-derived phosphoramidite ligand. And through the development of Pd-catalyzed hydrosilylation, we achieved remote hydrosilylation –controlled construction of C-N axial chirality within ortho-substituted N-arylmaleimides. More interestingly, this Si-C coupling hydrosilylation reaction provides a way to diversify synthetically useful intermediates and complex molecules benefited from the concept of silicon-mediated organic synthesis.
More details of this work can be found here: “Stereospecific Si-C Coupling and Remote Control of Axial Chirality by Enantioselective Palladium-Catalyzed Hydrosilylation of Maleimides” in Nature Communications (https://www.nature.com/articles/s41467-020-16716-5).
- Li, L., Wei, Y.-L., Xu, L.-W., Organosilicon-Mediated Organic Synthesis (SiMOS): A Personal Account, Synlett, 31, 21–34 (2020).
- Wang, X. B., Zheng, Z. J., Xie, J. L., Gu, X. W., Mu, Q. C., Yin, G. W., Ye, F., Xu, Z., Xu, L. W. Controllable Si-C Bond Activation Enables Stereocontrol in the Palladum-Catalyzed [4+2] Annulation of Cyclopropenes with Benzosilacyclobutanes, Angew. Chem. Int. Ed. 59, 790-797 (2020).
- Zhao, Z.-Y., Nie, Y.-X., Tang, R.-H., Yin, G.-W., Cao, J., Xu, Z., Cui, Y.-M., Zheng, Z.-J., Xu, L.-W. Enantioselective Rhodium-Catalyzed Desymmetric Hydrosilylation of Cyclopropenes, ACS Catal. 9, 9110−9116 (2019).
- Sun, Y. L., Wang, X. B., Sun, F. N., Chen, Q. Q., Cao, J., Xu, Z., Xu, L. W. Enantioselective Cross-Exchange between C-I and C-C s-Bonds. Angew. Chem. Int. Ed. 58, 6747-6751 (2019).