Abstract:The photo-NOCAS reaction that combines methanol, sewing as the nucleophile, and the radical cation of 4-methyl-1,3-pentadiene (14+'), substituting on the 1,4-dicyanobenzene radical anion (I-'), yields (E)-I-(4-cyanopheny1)-4-methoxy-4-methyl-2-pentene (15) as the major product. This regioisomer arises from bonding of methanol to C-4, the more heavily alkylsubstituted carbon of the diene, giving the less alkyl-substituted allylic radical. All previous examples of the photo-NOCAS reaction have yielded major adduct(s) having regiochemistry consistent with the anti-Markovnikov rule; the more heavily substituted (more stable?) P-alkoxyalkyl radical was the predominant intermediate. Empirically derived heats of formation and high-level ab initio molecular orbital calculations (MP216-3 1G*//HF/6-3 lG*) provide convincing evidence that of the two alternative allylic radicals, generated upon addition of methanol to 14+', that which has the more alkyl substituted allylic radical moiety is, in fact, not the more stable. Of course, the total structure of the intermediate must be considered; the stabilizing effect of alkyl substitution on the carbon next to the oxygen of the ether moiety cannot be ignored. Ab initio molecular orbital calculations (MP216-3 1 G*//HFl6-3 lG*) are reported for the radical cations of 2-methylpropene (2+'), 2-methyl-2-butene (6+'), 2-methyl-l,3-butadiene (9"). 4-methyl-l,3-pentadiene (14+'), and 2,4-dimethyl-l,3-pentadiene (18"). Calculations were also carried out on possible intermediates (bridged radical cations, distonic radical cations, and P-alkoxyalkyl radicals) involved upon reaction of these radical cations with methanol. Results of these calculations provide a basis for explaininglpredicting the regiochemistry of the photo-NOCAS reaction involving methanol as the nucleophile: the major adduct(s) result(s) from attachment of methanol to that end of the alkene or diene which gives rise to the more stable intermediate radical. The more stable radical is not necessarily the more heavily alkyl substituted.Key words: photoinduced electron transfer, radicals, radical cations, ab initio molecular orbital calculations.RCsumC : La rCaction de photo-NOCAS permet de combiner le mCthanol, qui agit comme nucltophile, et le cation radical du 4-mtthylpenta-l,3-dikne (14+'), qui se substitue sur l'anion radical du 1,4-dicyanobenzkne (I-'), et fournit du (E)-1-(4-cyanophCnyl)-4-mCthoxy-4-m&hylpent-2-2 (15) comme produit principal. Le rCgioisomkre rCsulte d'une liaison du mCthanol au C-4, le carbone du dikne le plus substituC par des groupes alkyles, qui conduit au radical allylique le moins substitut par des groupes alkyles. Tous les exemples de rCactions photo-NOCAS rapportCs antkrieurement ont toujours fourni des produits majoritaires dont la rtgiochimie Ctait en accord avec la rkgle anti-Markovnikov correspondant i un intermkdiaire predominant comportant le radical P-alkoxyalkyle le plus substituC (le plus stable?). Les chaleurs de formation que l'on peut obtenir empiriquement de m&me que des c...
The preparation and evaluation of a new class of photoresponsive polymers are described on the basis of a process called quantum amplified isomerization (QAI). The QAI process utilizes photoinitiated, cation radical isomerization chemistry in a polymeric medium. Two classes of materials are described: one where the QAI reactant is molecularly doped in the polymer matrix and another where the reactant is part of a functionalized polymer. Quantum yield experiments demonstrate that the isomerization reaction can proceed by a chain process with modest efficiencies. Photochemical conversion experiments show that high extents of conversion of the QAI reactants are possible. The rate and extent of conversion are strongly correlated to the glass transition temperature of the polymer. For molecularly doped polymers, hypotheses to explain the high conversions based on diffusion or phase separation of the reactants were tested and excluded. Models are discussed to rationalize experimental factors that affect the quantum yields and the photochemical conversions.
The scope of the photochemical nucleophile-olefin combination, aromatic substitution (photo-NOCAS) reaction has been extended to include conjugated dienes: 1,3-butadiene (9), 2-methyl-l,3-butadiene (lo), 2,3-dimethyl-l,3-butadiene (1 l), and 2,5-dimethyl-2,4-hexadiene (12). Acetonitrile-methanol solutions of the dienes 9, 10, and 11, and 1,4-dicyanobenzene (I), with and without codonor (biphenyl (5)), were irradiated with a medium-pressure mercury vapour lamp through Pyrex. Both 1,2-and 1,4-addition products were formed in approximately equal amounts (combined yields of photo-NOCAS products, 50-65%). In marked contrast, when an acetonitrile-methanol solution of 2,5-dimethyl-2,4-hexadiene (12), 1, and 5 was irradiated, only the 1,4-addition product, trans-2-(4-cyanophenyl)-5-methoxy-2,5-dimethyl-3-hexene (22, 82%), was obtained. This photolysate also contained a small amount of another 1,4-addition product, that which had incorporated cyanide ion instead of methanol, tratzs-2-(4-cyanophenyl)-2,2,5-trimethyl-3-hexenenitrile (23,2%). Irradiation of an acetonitrile solution (no methanol) of 12,1, and 5 gave 23 in good yield (68%). An excellent yield (80%) of 23 was obtained upon irradiation of an acetonitrile solution of 1, 12,5, potassium cyanide, and 18-crown-6. Addition of 2,4,6-trimethylpyridine (collidine, 25), a mild, non-nucleophilic base, to the reaction mixture diverts the reaction involving 12 from photo-NOCAS products to 1: 1 substitution products; 3-(4-cyanopheny1)-2,5-dimethyl-I ,4-hexadiene (26), trans-5-(4-cyanopheny1)-2,5-dimethyl-1,3-hexadiene (27), (Z)-1 -(4-cyanopheny1)-2,5-dimethyl-2,4-hexadiene (28), and (E)-l-(4-cyanophenyl-2,5-dimethyl-2,4-hexadiene (29) were formed. The mechanisms of these reactions are discussed and an explanation for the observed regio-and stereoselectivity is given. KIMBERLY A. MCMANUS et DONALD R. ARNOLD. Can. J. Chem. 72,2291Chem. 72, (1994. On a Clargi le domaine d'application de la combinaison photochimique nuclCophile-olkfine, la rCaction de substitution aromatique photo-NOCAS, de f a~o n ? i inclure les diknes conjugues suivants : buta-1,3-dikne (9). 2-mtthylbuta-l,3-dikne (lo), 2,3-dimkthylbuta-l,3-dikne (11) et 2,5-dimithylbuta-l,3-dikne (12). On a soumis des solutions acktonitrile-methanol des diknes 9, 10 et 11 avec le 1,4-dicyanobenzhne (I), avec ou sans codonneur (biphenyle 5 ) , avec une lampe i vapeur dc mercure i pression moyenne h travers du Pyrex. Les produits d'additions 1,2 et 1,4 se forment en quantitCs approximativement Cgales (rendements combinks des produits de photo-NOCAS, 50-65%). Par ailleurs, lorsqu'on irradie une solution acttonitrile-mbthanol du 2,5-dimCthylbuta-l,3-dikne (12) avec les composCs 1 et 5, il ne se forme que du produit d'addition 1,4, le trar~s-2-(4-cyanophCny1)-5-methoxy-2,5-dimkthylhex-3-kne (22,82%). Ce photolysat contenait aussi une faible quantitC d'un autre produit d'addition 1,4, celui qui incorpore un ion cyanure 2 la place du methanol, le trans-2-(4-cyanophCnyl)-2,2,5-trimCthylhex-3-knenitrile (23,2%).Une irradiation d'un...
Photophysical parameters relevant to photodynamic therapy have been studied for a novel lipophilic opp-dibenzoporphyrin (DBP), 2,12-diethyl-3,13-dimethyldibenzo[g,q]porphyrin, in N,N-dimethylformamide and TX-100 micelles. The structure of DBP is intermediate between a porphyrin and a phthalocyanine and is associated with higher molar extinction coefficients in the red Q-bands than those found in the haematoporphyrin derivative currently used for photodynamic therapy. The ultrafast measurements on DBP in DMF revealed a fast (ps) lifetime for the second excited singlet state. Observed lifetimes of the first excited singlet state were found to be similar in DMF and TX-100 (12.2 and 14.7 ns), but excited triplet lifetimes were different in the two solvents (0.46 and 2.86 micros). The fluorescent quantum yields of DBP in DMF and TX-100 were twice that of free-base tetraphenylporphyrin and the singlet oxygen quantum yield in DMF and TX-100 was high (0.56-0.65). The combination of stable chemical structure, stronger red-absorption, high singlet oxygen quantum yields, and high fluorescent quantum yields suggests that DBP is an potential chromophore for applications in photodynamic therapy.
The scope of the photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction has been extended to include cyanide anion as the nucleophile. Highest yields of adducts were obtained when the alkene or diene has an oxidation potential less than ca. 1.5 V (SCE). No adducts were obtained from 2-methylpropene (9), oxidation potential 2.6 V. Oxidation of cyanide anion, by the radical cation of the alkene or diene, can compete with the combination. With the alkenes, 2,3-dimethyl-2-butene (2) and 2-methyl-2-butene (10), both nitriles and isonitriles were obtained; isonitriles were not detected from the reactions involving the dienes, 2-methyl-1,3-butadiene (11), 2,3-dimethyl-1,3-butadiene (12), 4-methyl-1,3-pentadiene (13), 2,4-dimethyl-1,3-pentadiene (14), and 2,5-dimethyl-2,4-hexadiene (6). The specificity, nitrile versus isonitrile, is explained in terms of the Hard-Soft-Acid-Base (HSAB) principle. The photo-NOCAS reaction also occurs with the allene, 2,4-dimethyl-2,3-pentadiene (15), cyanide combining at the central carbon. Factors influencing the regiochemistry of the combination step, Markovnikov versus anti-Markovnikov, have been defined. Cyanide anion adds preferentially to the less alkyl-substituted, less sterically hindered, end of an unsymmetric alkene or conjugated diene radical cation, forming the more heavily alkyl-substituted radical intermediate. High-level abinitio molecular orbital calculations (MP2/6-31G*//HF/6-31G*) have been used to determine the effect of alkyl substitution on the stability of the intermediates, β-cyano and β-isocyano alkyl radicals, and products, alkyl cyanides and isocyanides. The more heavily alkyl-substituted radical is not necessarily the more stable. The product ratio (Markovnikov versus anti-Markovnikov) must be kinetically controlled. Keywords: photochemistry, radical ions, electron transfer, nitriles, isonitriles.
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