Photochemistry of 2,3-dimethylcyclobutanone. Temperature and wavelength effects.

The ratio of products formed in the photochemistry of cis- and trans-2,3-dimethyl-cyclobutanone in alcohols is found to be wavelength and temperature dependent.     

Dirhenium paddlewheel compounds supported by N,N'-dialkylbenzamidinates: synthesis, structures, and photophysical properties

Dirhenium(III,III) compounds Re2(DMBA)4Cl2 (1, DMBA=N,N'-dimethylbenzamidinate) and Re2(DEBA)4Cl2 (2, DEBA=N,N'-diethylbenzamidinate) were synthesized via molten reactions between Re2(OAc)4Cl2 and the corresponding amidine. Re2(DMBA)4(NO3)2 (3) was obtained through reacting Re2(DMBA)4Cl2 with AgNO3. Single crystal X-ray diffraction studies revealed that the Re-Re distances in compounds 1-3 are 2.212(1), 2.217(1), and 2.173(1) A, respectively, which are consistent with the presence of a Re-Re quadruple bond. Voltammetric studies revealed that compound 2 exhibits two quasireversible couples, an oxidation and a reduction, and an irreversible reduction, while compound 1 displays irreversible couples at similar potentials. The three complexes exhibit 1deltadelta* absorption as a shoulder at approximately 440 nm (epsilon approximately 1500 M(-1) cm(-1)). Upon excitation of solid samples or CH2Cl2 solutions of 2 with visible light, emission is observed at 824 nm (77 K) and 833 nm (298 K), respectively. The luminescence is assigned as arising from the 3deltadelta* excited state.     

Stereocontrol within confined spaces: enantioselective photooxidation of enecarbamates inside zeolite supercages

Dye-exchanged Y zeolite is shown to be an effective medium to control the stereoselectivity in the photooxygenation of chiral oxazolidinone-functionalized Z/E-1 enecarbamates. An enantioselectivity (ee) as high as 80% was observed in the methyldesoxybenzoin (MDB) product, obtained in the methylene-blue-exchanged NaY zeolite at room temperature. The efficacy of the asymmetric induction in the MDB product depends on the Z/E geometry of the alkene, the Z-isomer being more effective than the corresponding E-isomer. The stereoselectivity is rationalized in terms of conformational effects through cationic interactions between the zeolite and the substrate.     

Reacting polymers with highly correlated initial conditions

We propose and theoretically study an experiment designed to measure short-time polymer reaction kinetics in melts or dilute solutions. The photolysis of groups centrally located along chain backbones, one group per chain, creates pairs of spatially highly correlated macroradicals. We calculate time-dependent rate coefficients κ(t) governing their first-order recombination kinetics, which are novel on account of the far-from-equilibrium initial conditions. In dilute solutions (good solvents) reaction kinetics are intrinsically weak, despite the highly reactive radical groups involved. This leads to a generalised mean-field kinetics in which the rate of radical density decay - ∼S(t), where S(t) ∼t ^{- (1 + g/3)} is the equilibrium return probability for 2 reactive groups, given initial contact. Here g≈ 0.27 is the correlation hole exponent for self-avoiding chain ends. For times beyond the longest coil relaxation time τ, - ∼S(t) remains true, but center of gravity coil diffusion takes over with rms displacement of reactive groups x(t) ∼t ^1/2 and S(t) ∼ 1/x ³(t). At the shortest times ( t 10^-6s), recombination is inhibited due to spin selection rules and we find ∼tS(t). In melts, kinetics are intrinsically diffusion-controlled, leading to entirely different rate laws. During the regime limited by spin selection rules, the density of radicals decays linearly, n(0) - n(t) ∼t. At longer times the standard result - ∼d ³(t)/d (for randomly distributed ends) is replaced by ∼d2x ³(t)/d ² for these correlated initial conditions. The long-time behavior, t > τ, has the same scaling form in time as for dilute solutions. Received 18 May 2000     

Aliphatic β‐Nitroalcohols for Therapeutic Corneoscleral Cross‐Linking: Chemical Stability Studies Using 1H‐NMR Spectroscopy

Recent studies suggest that aliphatic β‐nitro alcohols may represent a useful class of compounds for use as in vivo therapeutic corneoscleral cross‐linking agents with higher order nitroalcohols (HONAs) showing enhanced efficacy over the mono‐nitroalcohols. The current study was undertaken in order to evaluate the chemical stability of these compounds during storage conditions. Two mono‐nitroalcohols (2‐nitroethanol=2ne and 2‐nitro‐1‐propanol=2nprop) and two HONAs, a nitrodiol (2‐methyl‐2‐nitro‐1,3‐propanediol=MNPD), and a nitrotriol (2‐hydroxymethyl‐2‐nitro‐1,3‐propanediol=HNPD) were monitored for chemical stability by ¹H‐NMR for up to 7 months. Each compound was studied at two concentrations (1% and 10%) either in unbuffered H₂O or 0.2 m NaH₂PO₄/Na₂HPO₄ (pH=5), and at 0°C and room temperature (RT) for a total of eight conditions for each compound. The ¹H‐NMR spectra for the starting material were compared to subsequent spectra. Under all four of the conditions studied, both the nitrodiol (MNPD) and nitrotriol (HNPD) were stable for the duration of 7 months. 2nprop became unstable under all conditions at 3 months. 2ne was the most unstable of all the compounds tested. HONAs exhibit excellent chemical stability under long‐term storage conditions. In contrast, the nitromonols tested are significantly less stable. These findings are relevant to the translation of this technology into clinical use.     

Photocytotoxicity and photoinduced phosphine ligand exchange in a Ru(ii) polypyridyl complex

Two new tris-heteroleptic Ru(ii) complexes with triphenylphosphine (PPh₃) coordination, cis-[Ru(phen)₂(PPh₃)(CH₃CN)]²⁺ (1a, phen = 1,10-phenanthroline) and cis-[Ru(biq)(phen)(PPh₃)(CH₃CN)]²⁺ (2a, biq = 2,2′-biquinoline), were synthesized and characterized for photochemotherapeutic applications. Upon absorption of visible light, 1a exchanges a CH₃CN ligand for a solvent water molecule. Surprisingly, the steady-state irradiation of 2a followed by electronic absorption and NMR spectroscopies reveals the photosubstitution of the PPh₃ ligand. Phosphine photoinduced ligand exchange with visible light from a Ru(ii) polypyridyl complex has not previously been reported, and calculations reveal that it results from a trans-type influence in the excited state. Complexes 1a and 2a are not toxic against the triple negative breast cancer cell line MDA-MB-231 in the dark, but upon irradiation with blue light, the activity of both complexes increases by factors of >4.2 and 5.8, respectively. Experiments with PPh₃ alone show that the phototoxicity observed for 2a does not arise from the released phosphine ligand, indicating the role of the photochemically generated ruthenium aqua complex on the biological activity. These complexes represent a new design motif for the selective release of PPh₃ and CH₃CN for use in photochemotherapy.

New Ru(ii) complexes exhibit selective ligand dissociation driven by an excited state trans-type influence. The complexes are not toxic to triple-negative breast cancer cells in the dark, but induce cell death upon irradiation with visible light.