SYNTHESIS OF BENZO[b]CARBAZOLS BY TANDEM Au(I)-CATALYZED CYCLIZATION/MIGRATION/CYCLIZATION

The initial results on a novel procedure for the synthesis of benzo[b]carbazols via gold(I)-catalyzed tandem cyclization/Migration/cyclization is described. The procedure allowed the access to a highly functionalized benzo[b]carbazols in a one-pot process starting from tertiary anilines. The mechanism of this reaction by using gold(I) catalysis, interestingly proceeded via 5-endo-dig cyclization with successively migration of benzyl substituent on 2 and 3 position of indole, leading to the formation of benzyl substituted indole derivatives. We are proposing a cyclodimerization and aromatization via 6-exo-dig with the help of gold(I)-carbene.

Preparation of Starting material tertiary aniline required four step- Step 1. Methylation -( 2-iodo aniline, Methyl-iodide, NaH, 0 ⁰C, 30 min. then 23 ⁰C. 12 hr). Step 2. Bis-alkylation-(Methylated compound 1, Benzyl bromide, K₂CO_3, Reflux, 24 hr). Step 3. Sonogashira Cross-coupling reaction (Bis-alkylated compound 2, Tri-methyl-silyl-acetylene, Pd(PPh₃)₂Cl_2, CuI, Et₃N, 23 ⁰C, 2 hr). Step 4. Deprotection of TMS (Compound 3, K₂CO₃, MeOH, 23 ⁰C, 4 hr and finally, we get Tertiary-aniline 4, as a starting material. Initial optimization with Tertiary Aniline 4, 5mol% of (Acetonitrile)[(2-biphenyl)di-tert-butylphosphine]gold(I) hexafluoroantimonate as Gold(I)-catalyst, Dry-DCM or Dry-DCE at 23 ⁰C. But unfortunately, we do not get result. Then we rise the temp of reaction in Dry-DCE upto 60 ⁰C with 10mol% of gold(I) catalyst and then we get substituted indole as a cyclized product with 95% yield which is confirmed by H¹, C¹³ and NOE spectrum.

Overall, we get first intermolecular cyclized product of benzyl substituted indole with excellent yield.