Reactivity photosubstitution reactions

Other factors, however, should also be effective. A specific influence of the solvent 101,116) added detergents 12 ) and remote electron donating substituents 119) has been observed. Steric hindrance, which certainly is of influence in the nucleophilic photosubstitution reactions of a-nitronaphthalenes, has been found to alter the reactivity of nitroanisoles... 

It is, thus, important that the ruthenium(II) complexes that are to be used as building blocks of the future machines contain sterically hindering chelates so as to force the coordination sphere of the metal to be distorted from the perfect octahedral geometry. We will discuss the photochemical reactivity of rotaxanes and catenanes of this family as well as non-interlocking systems like scorpionates since the lability of bulky monodentate ligands could also lead to useful photosubstitution reactions. 

A particularly promising feature of the Ru(terpy)(phen)(L)2+ series, in relation to future molecular machine and motors, is related to the pronounced effect of steric factors on the photochemical reactivity of the complexes [84]. When the bulkiness of the spectator phenanthroline moiety was increased, the steric congestion of the coordination sphere of the ruthenium complex also increased. This increased congestion was qualitatively correlated to the enhanced photoreactivities of these complexes (Fig. 14). More specifically, changing phen for dmp increased by one to two orders of magnitude the quantum yield of the photosubstitution reaction of L by pyridine with L = dimethylsulfide or 2,6-dimethoxybenzonitrile. 

Nucleophilic aromatic photosubstitution reactions have been divided into five mechanistic categories17 and each of these mechanistic types has its representatives in the class of aryl halides. Which reaction pathway is followed in any particular case depends on a number of factors such as the nature of the leaving group, the presence of electron-donating or electron-withdrawing substituents on the aromatic ring, the solvent, the multiplicity and the lifetime of the reactive excited state and the presence or absence of electron donors or acceptors in the reaction medium. This renders it rather difficult to make predictions about the mechanistic course that will be followed under a given set of circumstances. 

E. Havinga, Heterolytic photosubstitution reactions in aromatic compounds in Reactivity of the Photo-... 

Croup VI Carbonyl complexes In the case of substitution of neutral ligands by neutral ligands, pressure effects can be better correlated with the intrinsic volume changes associated with the mechanism. One such study dealt with the photosubstitution reactions of the hexacarbonyls M(CO)6 (M = Cr, Mo, W) to give M(CO)sL (Eq. 6.22) and M(CO)4L2, where L is a ligand such as pyridine [61]. For each M, Oco decreased with increasing pressure. Under the risky assumption that kn is independent of P, the pressure dependence of laser flash photolysis techniques have shown that CO loss to form the 5-coordinate intermediate M(CO)s occurs in less than 1 ps. For this reason, one cannot treat the ligand substitution pathway from the reactive ES in terms of mechanisms elucidated for bound excited states. Instead the positive... 

Examples of photosubstitution reactions of parent aromatic hydrocarbons are rare. Methylated anthracenes and dihydroanthracenes are, however, afforded by the irradiation of anthracene in the presence of methyl-lithium.98 The isomer distribution of methyl-anthracenes thus produced is very different from that observed from the thermal reaction, and the process appears to be only successful with higher methyl-lithium concentrations than were previously used.97 The reaction is also reported to occur with naphthalene and phenanthrene.98 The efficiency of light-induced substitution reactions of methoxyanthraquinones with nucleophiles has been shown to be very dependent upon the position of the methoxy-group and the particular nucleophile.98 In general, the 1-methoxy-derivative is more reactive than the 2-isomer, and whereas reaction was observed... 

Photoluminescence spectra, excited-state lifetimes, and quantum yields for photosubstitution reactions of [Rh(NH3)BX] + and [Rh(ND3)5X] - - ions (X=C1 or Br) have been determined. Individual rate constants for deactivation of the excited states by radiative, non-radiative, and reactive pathways were determined. For the deuterio-complexes, the rate of non-radiative deactivation is lower, and consequently quantum yields for photosubstitution and photoemission are larger. The rate constant for the excited-state substitution pathway is estimated to be 10 —10 times greater than for the thermal substitution process. ... 

The failure to observe photosubstitution in the presence of a sensitizer in which the latter is the principal absorber, the invariance of product quantum yield with wavelengths shorter than 350 nm (onset of n -> -n absorption), and the observation that chloride and bromide ions (known to catalyze S-+T intersystem crossing) strongly diminish the quantum yields of these reactions, strongly points to the lowest excited ir- n singlet state as the reactive species in these transformations. Excitation into the n->ir absorption band results in little product formation. A triplet state may, however, be involved in the photoamination of nitrobenzene.a41)... 

Fluorenone and sodium sorbate served to quench the unsensitized reaction, indicating a triplet excited state. Similarly, recent investigations of the photosubstitution of 3,5-dinitroanisole indicate a reactive triplet species(143) ... 

However gratifying this rationalization may seem to be in the case of azulene derivatives, it can of course not be used with naphthalene, biphenyl, etc. Here we have to confine ourselves to the statement that nucleophilic photosubstitution preferentially occurs at the positions that are known to be generally the most reactive in thermal reactions also. 

Both the excited states arising from the t2 J configuration12,13) and quartets7) from t2geg were invoked as possible reactive states. Definitive experimental evidence is now available which shows that the low lying doublet states are unreactive with respect to photosubstitution in Cr(CN)6.3-14) Reaction (4) occurs with Q.08 and 0.09 quantum efficiency upon 305 nm and 370 nm photolysis, respectively.15) The approximate energetic positions of the low lying LF states of Cr(CN) - are shown... 

Sterically hindered a-haloketones such as a-Cl- or a-Br-isobutyrophenone yield only a light-induced C-alkylation product with Me2C=N02 in DMSO when the phenyl ring carries ap-nitro or ap-cyano substituent (equation 53)247. This photosubstitution meets the usual criteria for an S l reaction mechanism. So does the photoreaction of the bridgehead chloride 63 with diphenylphosphide ions in liquid ammonia (equation 54)248. The reactivity of 63 is larger than that of similar non-keto bridgehead chlorides in S l reactions. 

Although the models have proved to be useful tools for rationalizing some aspects of the photosubstitutional behavior of simple transition metal complexes, they are not without deficiencies. For example, the predictions of reactivity made with the models are only qualitative. Thus a reaction that is predicted for a particular complex may not occur at all. Another important deficiency of the models was recently discussed by Ford (50). In the series of analogous rhodium(III) j omplexes, Rh(NH3)5X"" (X = NH3, H2O, 0H , Cl, Br, and I"), relative quantum yields of ligand substitution are strongly dependent on the rates of physical radiationless decay of the excited complexes to the ground state species. According to the Zink model, however, relative quantum yields within such a series should reflect "reactivities" of the excited state complexes (44,47). 

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