Direct and sensitized valence photoisomerization of a substituted norbornadiene. Examination of the disparity between singlet- and triplet-state reactivities

Abstract

Direct photolysis of 1,2,3-trimethyl-5,6-dicyanonorbornadiene (4) at 366 nm induces valence isomerization to the corresponding quadricyclene compound, 5, with a quantum yield of 0.68 ±0.01. The same transformation occurs in the presence of a number of triplet photosensitizers. Thus Ru(bpy)32+ (bpy is 2,2'-bipyridine), whose emissive metal-to-ligand charge-transfer excited state is quenched by 4 with a rate constant of 2.0 ± 0.2 X 108 M"1 s'1, sensitizes the production of 5 at 546 nm with a limiting quantum yield of 0.06 ± 0.01. Limiting yields at 436 nm for the organic sensitizers biacetyl and 9-fluorenone are 0.11 ± 0.01 and 0.15 ± 0.01, respectively. The finding that direct photoisomerization occurs with much higher efficiency than the triplet-sensitized process suggests that the lowest excited singlet state of 4 is significantly more reactive than the triplet state. Interestingly, the opposite reactivity pattern obtains in the case of the unsubstituted norbornadiene molecule. Possible reasons for this substituent effect are discussed in terms of the excited-state potential energy surfaces that interconnect the norbornadiene and quadricyclene structures.

Visible Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis

“…In an early example, this pathway has been used to convert the substituted norbornadiene 371 to quadricyclene 372 (eq 14). 190 Triplet–triplet energy transfer from *Ru(bpy) 3 2+ to the norbornadiene 371 promotes this species to its triplet state, which then undergoes bond rearrangement to give 372 . Electron-transfer pathways are not possible in this transformation, as the oxidation ( E 1/2 +1/0 = +1.82 V vs SCE) and reduction potentials ( E 1/2 0/–1 = –1.39 V vs SCE) of 371 indicate that reductive or oxidative quenching of *Ru(bpy) 3 2+ would both be severely disfavored.…”

Photosensitized, energy transfer-mediated organometallic catalysis through electronically excited nickel(II)

“…Indeed, these studies stand as a direct analogy to the abundance of organic excited-state reactions that can be accessed via organocatalytic photosensitization (e.g., with benzophenone), as popularized in part by Schenck, Turro, and Hammond and colleagues in the 1950s and 1960s (18–20). Given all of the above, it is surprising to consider that photoinduced energy transfer from photocatalysts to transition metals as a means to access organometallic excited states has not previously been exploited as a general activation pathway for reaction invention—especially in light of the historical success of energy transfer mechanisms in organic photochemistry (21–23) (Fig. 1).…”

Visible Light Photocatalysis of [2+2] Styrene Cycloadditions by Energy Transfer

“…The first is the Ru(bpy) 3 2+ -mediated norbornadiene-to-quadricyclane valence isomerization studied by Kutal for solar energy storage applications. [8] The second is the photocatalytic dimerization of anthracene reported by Castellano to be sensitized by the related ruthenium photocatalyst Ru(dmb) 3 2+ (dmb = 4,4'-dimethyl-2,2'-bipyridine). [9] Thus, although the utility of UV-absorbing organic chromophores as triplet photosensitizers has been well established for decades, [10] synthetic applications of triplet sensitization with transition metal complexes that absorb in the visible range have not been extensively explored.We initiated our investigation by exploring the [2+2] photocycloaddition of styrene 3, a substrate whose oxidation potential (+ 1.42 V vs SCE) [11] has precluded its ability to participate in radical cation cycloadditions previously reported from our lab.…”