Reactivity of tetrahydrochromeno[2,3‐b]indoles: chromic indicators of cyanide

Abstract

The synthesis and reactivity of a tetrahydrochromeno [2,3-b]indoles are reported. Evidence for reversible ring-opening is based on H/D exchange and trapping experiments. These compounds readily undergo reaction with tetra-nbutylammonium cyanide. The cyanide reaction is 10-100× faster when the solution is irradiated with 350 nm light. Reaction with trimethylsilyl cyanide occurs only with UV irradiation demonstrating photoreactivity. The rate of tetrahydrochromeno[2,3-b]indole ring-opening is greater for (i) Me substitution at the hemiaminal carbon (compared to Ph), and (ii) substitution of fluorine at the 9-position of the indole. Under acidic conditions, the ring-opened indolium ion is observed.

Transient spectroscopic characterization of the ring‐opening reaction of tetrahydrochromeno[2,3‐dimethyl]indole

“…Steady‐state absorption measurements were performed using an Ocean Optics component system (Ocean Optics, Inc., 830 Douglas Ave, Dunedin, FL 34698, USA) with a HR2000+ detector, qpod TM sample holder, DH 2,000 source and Spectra Suite software for data collection. The synthesis of 5a,6,10b‐trimethyl‐2‐nitro‐5a,6,10b,11‐tetrahydrochromeno[2,3‐b]indole ( CR1 ) has been previously described …”

“…Molecular orbitals were computed at the same level of theory, including integral equation formalism polarizable continuum model solvation model. The starting geometry for CR1 was taken from the published X‐ray structure . The geometry of CR1-ROP was determined by a series of optimizations as the length of the [C–O] was increased in 0.1 Å increments.…”

“…We have previously described the synthesis of a system analogous to OX, tetrahydrochromeno[2,3‐b]indole (CR) (Scheme ) . The impetus for the present work was to further study the details of the ring‐opening process in this class of chromophores that involve C–O bond cleavage at hemiaminal carbon.…”

“…The potential for applications of light‐induced molecular switching has been demonstrated in a wide range of examples including photoresponsive materials, optical switches, memory storage, and colormetric analytics . In particular, the use of photochromic compounds in photonic devices has garnered much attention, as each isomer of the photochromic compound can represent a “0” or “1” in the binary number system .…”

“…Like OX, the CR system does not require cis–trans isomerization as part of its photocycle. We have reported the photoactivity of CR using steady‐state spectroscopic techniques, wherein the ring‐opened photoisomer reacted with trimethylsilyl cyanide (trapping was selective for the putative intermediate and did not occur with CR) . Our results suggest that CR is a viable chromic system with similar reactivity to OX.…”