On the Electronic Character of Localized Singlet 2,2-Dimethoxycyclopentane-1,3-diyl Diradicals:  Substituent Effects on the Lifetime

Abstract

Photodenitrogenation of the diazenes 4 affords exclusively the housanes 5 through intramolecular cyclization of the spectrally detected and characterized singlet diradicals 3. The lifetime of singlet diradical 3, determined by transient absorption measurements, depends on the Y and Z substituents at the para position of the phenyl ring and has the following order: Y, Z = OMe, OMe > OMe, CN > CN, CN > OMe, H > Cl, Cl approximately CN, H approximately Me, Me > H, H. This unprecedented substituent effect reveals stabilization of the singlet 2,2-dimethoxycyclopentane-1,3-diyl diradicals 3 through radical, zwitterionic, pi-bonding, and hyperconjugative structures.

Stabile Heterocyclopentan‐1,3‐diyle

“…CO ist ein besonders vielversprechendes Molekül für die Aktivierung, da sowohl 1,1-als auch 1,2-Verbrückung von Interesse sind, denn es würde entweder ein [1. [4,5,10,[13][14][15][16] oder auch Cyclopentene nach Gruppenumlagerung [11,17] -isoliert werden. Kleine Moleküle der 15.…”

Is π-Single Bonding (C–π–C) Possible? A Challenge in Organic Chemistry

“…To understand the substituent effect on the lifetime of the singlet diradicaloid, the Mülliken bond order (BO) was computed for the model CP4 (Figure 8e). 22 The π-single bonding character (BO = 0.58) for the parent CP4a (Z = H) was computed to be higher than that for CP4b (Z = OH, BO = 0.35) and CP4c (Z = CN, BO = 0.40). Therefore, the singlet diradicaloid CP4 possesses a mixed character of ZI, CπC, and CP (Figure 8d).…”

“…900 ns, Entry 1), 18 and CP3b (OR = OR¤ = OCH 3 ,¸2 93 = ca. 300 ns, Entry 2) 22 at 293 K. The substituent effect on the lifetime can be rationalized by the repulsive interaction of the phenyl ring with the alkoxy group at the transition state during the disrotatory ring-closing reaction (Figure 8).…”

“…28 To experimentally obtain more detail concerning the electronic character of the singlet diradicaloids, the para-substituent group (Y) effect of the aryl group on the lifetime was investigated (Entries 2, 7, and 8, Table 2). 22 The singlet diradicaloid CP3g (OR = OR¤ = OCH 3 , Y = CN,¸2 93 = 625 ns) was found to be a longer-lived species than the parent CP3b (OR = OR¤ = OCH 3 , Y = H,¸2 93 = 320 ns) in benzene. The substituent effect can be rationalized by the higher benzyl-radical stabilization ability of the p-cyanophenyl group than that of the parent phenyl group, because the radical stabilization ability (·•) of the p-cyano group (·• = 0.47) 27 is known to be higher than that of the phenyl group (·• = 0.00) in the cumyl type of benzyl radical R (Figure 8c).…”

“…In fact, the bond order between C1 and C3 in CP1d was calculated to be 0.58. 22 The maximum π-bond energy between C1C3 is estimated to be ca. 12 kcal mol ¹1 for CP1e, because the singlettriplet energy gap is found to be ¹12 kcal mol…”