"Golden" Cascade Cyclization to Benzo[c]-Phenanthridines

Abstract

Herein, we describe a gold-catalyzed cascade cyclization of Boc-protected benzylamines bearing two tethered alkyne moieties in a domino reaction initiated by a 6-endo-dig cyclization. The reaction was screened intensively, and the scope was explored, resulting in nine new Boc-protected dihydrobenzo[c]phenanthridines with yields of up to 98 %; even a π-extension and two bidirectional approaches were successful. Furthermore, thermal cleavage of the Boc group and subsequent oxidation gave substituted benzo[c]phenanthridines in up to quantitative yields. Two bidirectional approaches under the optimized conditions were successful, and the resulting π-extended molecules were tested as organic semiconductors in organic thin-film transistors.

Keywords: benzo[c]phenanthridines; benzylamines; cascade reactions; homogeneous gold catalysis.

Scheme 1

Comparable previous cascade cyclizations of anilines and our benzylamine approach.

Scheme 2

First evaluations for the synthesis of isoquinoline 2 and benzo[c]phenanthridine 4.

Scheme 3

Gold‐catalyzed cascade cyclization of Boc‐protected diyne 5 a.

Scheme 4

Screened reaction of diyne 5 a to give 6 a via intermediate 7 a.

Figure 1

Chemical structure of some of the ligands used for the screening.

Scheme 5

Proposed mechanism.

Scheme 6

Conditions for the thermal Boc deprotection and subsequent oxidation to benzo[c]phenanthridine 13 a.

Scheme 7

Conditions for the scope of the reaction.^[a]For 5 c additional 2.5 mol% catalyst were used.

Figure 2

Solid state molecular structures of 6 c (left) and 13 a (right).

Figure 3

Solid state molecular structures of compounds generated in a bidirectional manner. Top left: 6 k; top right: side view of 6 k (the Ph‐substituents are omitted for clarity); bottom left: 13 k; bottom right: 13 l.