Light‐Controlled Orthogonal Covalent Bond Formation at Two Different Wavelengths

Abstract

We report light-induced reactions in at wo-chromophore system capable of sequence-independent l-orthogonal reactivity relying solely on the choice of wavelength and solvent. In as olution of water and acetonitrile,L ED irradiation at l max = 285 nm leads to full conversion of 2,5-diphenyltetrazoles with N-ethylmaleimide to the pyrazoline ligation products.S imultaneously present o-methylbenzaldehyde thioethers are retained. Conversely,L ED irradiation at l max = 382 nm is used to induce ligation of the o-methylbenzaldehydes in acetonitrile with N-ethylmaleimide via o-quinodimethanes, while 2,5-diphenyltetrazoles also present are retained. This unprecedented photochemical selectivity is achieved through control of the number and wavelength of incident photons as well as favorable optical properties and quantum yields of the reactants in their environment. Photoinducedligationreactionsinaone-potsystemthatcanbe controlled by the choice of irradiation wavelength without the need for additives or catalysts are highly desirable tools in the fields of organic, biological, and macromolecular chemistry as well as materials science.Until recently,this orthogonality,a lso described as l-orthogonality, [1] wavelength selectivity, [2] or chromatic orthogonality, [3] has-with the exception of photocleavage or photodeprotection systems, [4] the use of additional protecting groups, [5] added catalysts, [6] release from plasmon resonant liposomes, [7] and photoisomerization systems (photo switches) [3,8] -been achieved in only asequencedependent fashion. [1,2, 9] Clearly,t he design of ac atalyst-free, sequence-independent orthogonal ligation system is af ormidable challenge. [10] Generally,s equence-dependent l-orthogonality is possible,i fo ne photoactivatable chromophore absorbs light in ar egion of the ultraviolet-visible spectrum where the other chromophore does not absorb (long-wavelength irradiation induces the first reaction). Thei nverted order (short-wavelength irradiation induces the first reaction) of photoreactions usually results in undesired nonselective activation of both chromophores.I n2 000, Bochet and coworkers introduced ap air of protecting groups that can be cleaved with different wavelengths in ao ne-pot system with acceptable selectivity: [4a,b] One of the photoactivatable substances was transformed with 254 nm light to 92 %c onversion, while the other photocleavable compound was retained to adegree of 83 %. In the wavelength-inverted approach, the latter substance can be transformed by 420 nm light to 93 % conversion, while 92 %o fthe first compound is retained. While this selectivity for ap hotodeprotection is notable,itis far from being quantitative and addresses bond cleavage rather than formation. Examples that show control over ligation reactions with different colors of light without the limitation of sequence dependence are scarce:I tw as shown indirectly by combining av isible-light-triggered "off switch" of athermal ligation reaction with aUV-light-induced ligation reaction. [11] Orthogonal la...

Green light triggered [2+2] cycloaddition of halochromic styrylquinoxaline—controlling photoreactivity by pH

“…ight-gated reactions are important tools in soft matter materials design and the biological sciences due to their unique spatial and temporal control over the chemical processes [1][2][3][4][5] . Critically, by switching the wavelength of irradiation in photo-regulated processes, it is possible to trigger specific reactions in a sequence as well as establish true wavelength orthogonality [6][7][8][9][10][11][12][13][14] . This distinct feature has become a key concept in the design of photo-responsive systems 15 .…”

Predicting wavelength-dependent photochemical reactivity and selectivity

“…The absorbance of A, NEM, and AP was reported previously 21,26 , but is shown in Fig. 2b to allow comparison with wavelength-dependent reaction quantum yields, obtained in this study from experiments that were carried out in triplicate, refer to Supplementary Information, section 2.2.1.…”

“…A significant limitation of most examples is the competition between two reaction pathways, if one irradiation step activates both chromophores [40][41][42] . Selectivity can be achieved through control of the photo-kinetics by tuning both the degree of absorbance and the ratio of the relevant reaction quantum yields 26,[43][44][45] . We have previously reported that a combination of A with a diaryltetrazole can be addressed with a very high degree of selectivity at two different wavelengths, employing LEDs centered at 285 and 380 nm 26 .…”

Sunscreen-Assisted Selective Photochemical Transformations

“…Notable works in this field were reported by Bochet et al [20,21], for example by harnessing kinetic isotope effects to achieve chromatic orthogonality between two types of 2-nitrobenzyl based photolabile protecting groups (PPGs) [22] and even demonstrating the chromatic orthogonal total synthesis of a natural product [23]. Additional significant advancements in the field were reported by Barner-Kowollik et al, who developed a series of chromatic orthogonal photocyclization reactions with applications in polymer chemistry and material science [24][25][26][27][28]. In addition, several chromatic selective systems where two chromophores may be reacted sequentially have been developed with great efficiency [29][30][31][32].…”