Arylethynyl Derivatives of the Dihydroazulene/Vinylheptafulvene Photo/Thermoswitch: Tuning the Switching Event

Abstract

A selection of dihydroazulene (DHA) photoswitches incorporating an arylethynyl-substituent in the seven-membered ring was prepared by palladium-catalyzed Sonogashira cross-coupling reactions employing a suitable bromo-functionalized DHA. Shielding of the alkyne bridge and separating the aryl and DHA units, by sterically demanding groups, was required to obtain stable compounds. The DHAs underwent a light-induced ring-opening to vinylheptafulvenes (VHFs) which were thermally converted to a mixture of two DHA regioisomers, one of which was the original one. The influence of the aryl groups on the DHA and VHF absorptions and on their interconversion was investigated in detail. The rates of the switching events were finely tuned by the donor or acceptor strength of the aryl group. The thermal ring closure was found to proceed most readily in the presence of an electron-donating group on the seven-membered ring. The rate constant was found to follow a Hammett linear free energy correlation, which signals that stabilization of a positive charge in the seven-membered ring plays a crucial role in the ring-closure reaction. In view of these findings, it was possible to control the switching event by protonation/deprotonation of an anilino-substituted DHA. Also, the light-induced ring opening reaction was strongly controlled by acid/base. In addition to the mesomeric effects exerted by an arylethynyl group, the inductive effects exerted by different groups on the thermal ring closure were elucidated. Although the alkyne bridge transmits the electronic character of the aryl group, the ring-closure is retarded for all the ethynylated compounds relative to the parent unsubstituted compound. Along with our synthesis of suitable arylalkynes, we discovered an interesting byproduct in a Sonogashira cross-coupling reaction involving a nitrophenyl group, namely a diaryl azoxy compound. Its structure was confirmed by X-ray crystallographic analysis.

Gaining Control: Direct Suzuki Arylation of Dihydroazulenes and Tuning of Photo‐ and Thermochromism

“…[5, 6,8] Instead the Suzuki cross-coupling was carried out using fluoride to activate the aryl boronic acid. [9] The choice of fluoride source proved to be important; the use of Bu 4 NF was accompanied by substantial azulene formation.…”

“…[6] It was found that the electronic properties of the substituent on the phenyl ring (R = NH 2 , H, or NO 2 ) was transmitted through the alkyne bridge. Property tuning is a desirable feature of photochromic systems, and by moving the substituent closer to the seven-membered ring, the influence of the substituent should be more pronounced.…”

Synthetic protocols for the key functionalizations of the photochromic dihydroazulene scaffold

“…Thermal ring-closure hereof gave a mixture of the original 7-bromo DHA 17 and the 6-bromo DHA 22 (Scheme 9). 33 The two DHA regioisomers can be easily distinguished by 1 H-NMR spectroscopy, and a COSY spectrum supported the assignment of the new DHA isomer as being the C-6 substituted. Regioselective methods for obtaining DHA regioisomers with functional groups at the remaining positions in the seven-membered ring are still unknown.…”

“…[31][32][33][34] It consists of a sequential bromination/elimination protocol, which furnished selectively the 7-bromo DHA 17 starting from DHA 1 (Scheme 7). Addition of up to three molar equivalents of elemental bromine was possible in the first step (Scheme 8).…”

“…The straightforward access to the 7-bromo DHA 17 has opened the way via metal-catalyzed cross-couplings to a wide variety of 7-substituted DHA derivatives. 26,32,33 Thus, while the 3-bromo-derivative 15 described above was unreactive to such coupling conditions, the 7-bromosubstituted DHA is readily functionalized by aryl and alkynyl groups. Another important step on DHA functionalization was taken while investigating light/heat cycles on 17.…”