Exchange reactions, photochemical

Dyotropic rearrangements are uncatalyzed concerted dihydrogen exchange reactions, another class of orbital symmetry controlled processes, which involve the simultaneous migration of two cr-bonds. These conversions can be both thermal and photochemical. They can be subdivided into two types (1) reactions in which two migrating cr-bonds interchange their positions (equation 78), and (2) reactions without such positional interchange (equation 79)91,92. 

Borazine + D - Nadler and Porter reported on the photochemical exchange reaction of borazine with D2 at 184.9 nm. Products observed include B-mono-, B-di,... 

F . 9. Data ftom a mass spectrometric study of the photochemical exchange reaction of (-BHNH-)3 with D2. Initial conditions are indicated... 

The photolysis reaction of borazine with a second reagent can be classified on the basis of two types of bimolecular processes. In process A, borazine is the photochemical recent, and the products are formed by bimolecular exchange reactions between an excited borazine molecule and the second recent The preparation of a large number of B-substituted borazines is linked to this type of process (Table 3). In process B, the photochemical recent is a species other than borazine, as for example in the reaction of ammonia with borazine. 

Quantum yield data for photochemically initiated ligand exchange reactions of a number of chromium(III) complexes having ligand fields of effective Oh symmetry are given in Table III. These data show that... 

Much less is known about photoinitiated ligand exchange reactions of coordination complexes of cobalt(III) and other de ions than is the case for chromium(III). With the exception of the cobalt(III) ammines, however, available data suggest that photochemical ligand exchange reactions of cobalt(III) and d6 systems involve the thermally equilibrated 17 g and/or 37, states of the complex as intermediates. The reasoning is completely analogous to that described earlier with respect to chromium(III) photochemistry. 

Electrophilic deuterium exchange reaction is also photosensitized. Thermal and photochemical processes show a different orientation of deuterium exchange. 

Intra- and intermolecular hydrogen transfer processes are important in a wide variety of chemical processes, ranging from free radical reactions (which make up the foundation of radiation chemistry) and tautomeriza-tion in the ground and excited states (a fundamental photochemical process) to bulk and surface diffusion (critical for heterogeneous catalytic processes). The exchange reaction H2 + H has always been the preeminent model for testing basic concepts of chemical dynamics theory because it is amenable to carrying out exact three-dimensional fully quantum mechanical calculations. This reaction is now studied in low-temperature solids as well. 

Allylation of carbonyl and imino groups is one of the most convenient methods for the introduction of allylic functions.107-110 Allylic tin compounds have high interaction between C=C and C-Sn bonds which makes them more reactive than the corresponding silicon derivatives.111,112 In spite of their high reactivity, tin compounds are stable enough to be isolated and to react at ambient temperature under aerobic conditions. These factors allow them to be applicable to various types of reactions, for example, thermal,113 high-pressure,116 transition metal-catalyzed,117,118 radical,119,120 photochemical,121,122 tin-lithium exchange reactions,108,113 and so on. A broad... 

CO substitution in heteronuclear cobalt carbonyl complexes has also been studied. Several phosphines were used in exchange reactions with CO in the complex CpMo(CO)3-Co(CO)4. Both a CO dissociative pathway, leading to substitution, and a radical chain pathway, initiated by associative attack of the phosphine and subsequent disproportionation (see Disproportionation), were found depending on reaction conditions (thermal or photochemical treatment) and the basicity and sterics of the phosphine ligands. As seen in substitution reactions with Co2(CO)g, strongly basic phosphines add to the complex via disproportionation, while less basic phosphines add via substitution. However, the metal centers help determine where the phosphine adds because the rates of substitution on each metal center differ depending on which pathway, dissociative or radical chain, is operative. ... 

Recently, it was shown that the exchange reaction can be photochemically see Photochemistry) induced by irradiating at A, > 280 nm the reaction mixture of an alkyl iodide and 0.5 eqniv of R2Zn (R = Et or i-Pr) (eqnationd). ... 

Photoexcitation of complex ions need not necessarily lead to electron transfer even as a secondary stage. In a recent study of the photochemistry of various cobaltic and chromic complex ions by Adamson and Sporer (I) it was shown that oxidation-reduction, apart from aquation, racemization, or exchange reactions, occurred not only by photoexcitation in the electron transfer band region but also in ligand field bands where electron transfer is out of the question. Very similar observations had been made by Copestake and Uri (7) in a detailed study of the photochemical and thermal decomposition of the trioxalatocobaltate(III) complex. In this case electron transfer was not postulated as the primary reaction. The suggested scheme was... 

Numerous applications of [Cr(NH3)6]3+ and its derivatives to mechanistic studies of conventional or photo-assisted ligand-exchange reactions have been extensively reviewed.7,270-276 Among other common Cr111 complexes, [Cr(NH3)6]3+ has been used for the studies of reactivities of muonium and positronium atoms in aqueous solutions.313-315 Several computational methods, including DFT calculations,316 a combination of molecular mechanics and angular overlap model calculations, or vibrational analysis have been used for the prediction and interpretation of electronic spectra and photochemical properties of [Cr(NH3)6]3+. 

The apparent enthalpy of the exchange reaction, from values measured between 30 and 60 C, is -416 J mol (from ionization chamber measurements) and -423 J mol" (from Gelger-MUller counter measurements), compared to the calculated value of -414 J mol" [1963]. The kinetics of both the photochemical [1964] and the thermal [1965] reactions have also been measured. 

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