Substituted nitrogen-bridged diazocines

Novel nitrogen-bridged diazocines (triazocines) were synthesized that carry a formyl or an acetyl group at the CH2NR-bridge and bromo- or iodo-substituents at the distant phenyl ring. The photophysical properties were investigated in acetonitrile and water. As compared to previous approaches the yields of the intramolecular azo cyclizations were increased (from ≈40 to 60%) using an oxidative approach starting from the corresponding aniline precursors. The Z→E photoconversion yields in acetonitrile are 80–85% and the thermal half-lives of the metastable E configurations are 31–74 min. Particularly, the high photoconversion yields (≈70%) of the water-soluble diazocines are noteworthy, which makes them promising candidates for applications in photopharmacology. The halogen substituents allow further functionalization via cross-coupling reactions.


Introduction
Diazocines (bridged azobenzenes) are frequently used photoswitches with outstanding photophysical properties. Parent diazocine (CH 2 -CH 2 -bridged) exhibits well-separated n-π* transitions, which allow excellent photoconversion between the Z and E configurations ((Z→E) 385 nm = 92%, (E→Z) 525 nm > 99% in n-hexane) with light in the visible region [1]. Moreover, the Z-boat configuration is the thermodynamically stable isomer [2][3][4][5][6][7][8][9]. The latter property (i.e., the inverted stability compared to azobenzenes) makes them promising candidates for applications in photopharmacology. In the majority of azobenzenebased photopharmacophores, the bent Z configuration is biologically inactive [10][11][12]. Hence, (and in contrast to azobenzenes) the thermodynamically stable and biologically inactive Z-isomer can be administered and switched on with light at the site of interest with spatiotemporal resolution. Moreover, the photoconversion yield for the E→Z isomerization is quantitative (within the detection limit of UV and NMR spectroscopy). A high efficiency in switching the biological activity off is important to avoid side effects of residual concentrations of the active form [13].
(triazocines), we now explored this class of photoswitches and developed synthetic access to these photochromes ( Figure 1).

Synthesis
The first three stages of the synthesis of CH 2 -NR-bridged diazocines are analogous to the previously described synthesis of CH 2 -NH-bridged diazocine [15]. The single Boc-protected 1,2-phenylenediamine (2, Scheme 1) is reacted with halogensubstituted 2-nitrobenzyl bromides 3 [21] forming N-benzylanilines 4, which were protected with Fmoc chloride to accomplish an orthogonal protective group strategy. The removal of the Boc groups from compounds 5 with TFA gave the mixed aniline and nitro precursors 6.  UV-vis spectra of 3-bromo and 3-iodo, and unsubstituted CH 2 NAc-bridged (10a-c) and CH 2 NCHO-bridged (11a-c) diazocines. The spectra of Z-isomers are given in black, the photostationary states at 400 nm are represented as dashed red lines and the extrapolated spectra of the pure E-isomers are in blue.  In previous approaches, the nitro groups were reduced to hydroxylamines with zinc and oxidized to the corresponding nitroso compounds with iron(III) to perform an intramolecular Baeyer-Mills reaction [15,21]. We found that a complete reduction of the nitro group to aniline 7 and oxidation with mCPBA is increasing the yield of the intramolecular cyclization from 39% to 62% (over two steps) for the unsubstituted diazocine 8c as compared to the pathway via the hydroxylamine. The 3-bromo 8a and 3-iodo 8b compounds were obtained in 56% yield using the oxidative method of Trauner [22] with mCPBA. The Fmoc groups were removed with NEt 3 to yield the NH-diazocines 9. The acetylated diazocines 10a-c were synthesized using a mixed anhydride of acetic acid and T3P (propanephosphonic acid anhydride). The formylation of NH-diazocines 9a-c was accomplished with chloral [23] under non-acidic conditions.

Investigation of the photophysical properties
The UV-vis spectra of diazocines 10a-c, and 11a-c were recorded in acetonitrile at 25 °C. All compounds exhibit an n-π* transition at about 400 nm (Z→E conversion) and an n-π* transition at about 520 nm (E→Z conversion, Figure 2, Table 1).  Irradiation with 400 nm gives the metastable E-isomers of the acetylated and formylated derivatives 10 and 11 with good photoconversion yields (Γ) of 80-85% (Table 1)

in acetonitrile.
A complete E→Z conversion (>99%) can be achieved with light between 520 and 600 nm. The unsubstituted acetylated and formylated diazocines 10c and 11c exhibit similar conversion yields (88% and 85%) and halogenation as well does not have a significant influence. However, thermal half-lives (t 1/2 ) of the metastable E-isomers of the 3-bromo and 3-iodo N-acetyl diazocines 10a and 10b (≈30 min) are significantly smaller than the half-lives of the corresponding bromo and iodo N-formyl derivatives 11a and 11b (≈50 min). In general, halogenation decreases the half-lives compared to unsubstituted diazocines 10c and 11c. The activation barrier (E A ) of the E→Z isomerization (obtained by an Arrhenius plot) is higher in formylated compounds 11 compared to acetylated compounds 10 and is further increased by halogenation.
The unsubstituted N-formyl diazocine 11c and brominated NAc-diazocine 10a were also investigated in pure water since they are water-soluble (11c: ≈250 µM, 10a: ≈150 µM). The highest Z→E conversion yields are observed by irradiation with 400 nm in water and the back-isomerization E→Z can be accomplished by irradiation with light in the range of 525 and 600 nm ( Figure 3, Table 2).
The photoconversion yields (Z→E) of N-formyl diazocine 11c in water and bromo-NAc diazocine 10a are about 70%, which do not differ significantly from unsubstituted NAc diazocine 10c (72%) [15]. It is interesting to note that the half-lives and activation barriers (E→Z) are increasing (t 1/2 ≈ 2-2.5-fold) in water as compared to the less polar acetonitrile.

Conclusion
Five nitrogen-bridged diazocines (triazocines) were synthesized and characterized. Formyl (R = CHO) and acetyl groups (R = Ac) were introduced at the CH 2 NR bridge and the distant phenyl rings are Br and I substituted. In contrast to previous approaches, the azo cyclization (ring closure) was achieved via the oxidation of the bis-anilines 7 with mCPBA (≈60% yield). Among the nitrogen-bridged diazocines compounds 10a and 11c are water soluble and retained their high switching efficiency (≈70%) also in water. The half-lives of the metastable E-isomers are larger for the N-formyl diazocines 11a-c compared to the acetylated compounds 10a-c and generally, the half-lives are larger in water than in acetonitrile. Halogen atoms Br and I at the phenyl rings in 3-position as in 10a,b, and 11a,b are a good starting point for further functionalization [17,20,24]. We conclude that CH 2 NAc and CH 2 NCHO bridged diazocines (triazocines) are promising candidates for applications in biological environments and particularly as photoswitches in light-activatable drugs.

Supporting Information
Supporting Information File 1 Analytical equipment, experimental procedures, NMR and UV-vis spectra.