Studies On Some Electrophilic Addition And Cyclization Reactions Of Allenes

In recent years, electrophilic addition or cyclization reactions of functionalized allenes have been systematically explored in this group. Based on these work, I have conducted following studies:1. Compared with electrophilic addition reactions of functionalized allenes, such reactions of simple allenes are usually non-selective since there is no induction from functional groups or neighboring group participation effect. After screening of the reaction conditions, a regioselective bromohydroxylation of simple allenes affording2-bromoalk-2-enols in moderate to good yields was developed. Through this study it’s concluded that the regioselectivity may be determined by various factors including the steric and electronic effects of substitutents of the allene moiety. Substituents on allenes may be alkyl or aryl groups, allenes may be mono-substituted, di-substituted or tri-substituted. We also demonstrated the application of the products:the Suzuki reaction of2-bromoalken-3-ols with alkylboronic acids didn’t occur in the presence of PPh3. After screening, LB-Pho·HBF4developed in this group was identified for the Suzuki coupling reaction of2-bromoalken-3-ols with different alkylboronic acids affording the cross-coupling products in moderate to good yields. Different substitutions such as benzyl, phenyl, allyl, and alkyl can be tolerated at the1-and3-position of the enols. Furthermore, the reactions of both primary and secondary alkylboronic acids may also proceed smoothly.2. A method using quinidine and (R)-BINOL-derived phosphoric acid as a cocatalyst to catalyze the semipinacol rearrangment of2,3-allenols forming chiral3-enals (～70%ee) that contain a quaternary all-carbon stereocenter has been developed. After recrystalization or the recrystalization of hydrazone synthesized from3-enals and hydrazine, the products with high enatiomeric excess (98%ee) could be prepared. The nature of the catalyst and further optimization are still under investigation.3. We have developed a very mild and efficient method using easy available 2,3-allenols and N-non-protected untouched indoles followed by Au+/Ag+-catalyzed cyclization and aromatization to yield the final [4C+2C]-products, i.e., N-H carbazoles. The reaction enjoys the excellent regioselectivity of installing specific substituents to the different locations of the newly formed benzene ring in the carbazoles by utilizing the substituent-loading capability of2,3-allenols and the unprotected nature of the N-H unit for any further purpose-directed modification.4. Axailly chiral y-allenic acids could be obtained in a few steps from chiral propargyl alcohols. Five-membered lactone compounds could be formed by the electrophilic cyclization reaction of γ-allenic acids catalyzed by Au(IPr)OTs. In this process, the axially chirality of the γ-allenic acids was transferred to the chiral center almost completely due to the steric control of the gold catalyst. Some natural products containing five-membered ring lactone can be easily obtained in this way.