Transient species

Transient Species.— Treatment of carbonyl compounds with hydrogen sulphide under pressure in the presence of molybdenum sulphide leads to reduction, and involves the formation of thioketonic intermediates. Aliphatic thioketones and a )8-unsaturated thioketone (27) are proposed as intermediates in the reaction of Grignard reagents with various dithioesters. A mechanistic 

Metal Complexes.—Thiobenzophenones give complexes with metals and ( r/Ao-metallated complexes with metal carbonyls. The latter complexes give or/Ao-metallation on cleavage with mercuric acetate. Dithio-a-diketone complexes have been studied. A paper on the synthesis and physical properties of dithiosquarate complexes has appeared.  

Metal chelates of enamino-thioketones have been described. The synthesis and the chemical and physical properties of metal chelates of monothio- and dithio-analogues of jS-diketones are the subject of numerous papers. The 

Reactions.—The chemical behaviour of thioketones has mainly been investigated in four directions reaction with organometallic compounds, reaction with nucleophiles, thermal cycloadditions, and photochemical reactions. 

Nakamura, M. Uohama, S. Oka, T. Yamabe, and S. Nagata, Bull. Chem. Soc. Japan, 1975, 

Transient Species.—Because of their instability, no attempt at identifying the intermediacy of thio- or seleno-aldehydes has yet been successful. However, product analyses and mechanistic considerations suggest that thio- or selenoaldehydes are formed as intermediates in certain reactions. For example, thioformaldehyde has been proposed as a transient species in the reaction of nitroalkanes with sodium methanethiolate, as shown in reactions (1) and (2). The 

The reaction of an aldehyde with sodium hydrogen selenide and an amine hydrochloride generates a selenoaldehyde, which is reduced in situ to a symmetric diselenide by sodium borohydride.  

Transient Species.—Pioneering studies by Johnson and co-workers have shown that monomeric thioformaldehyde is generated when methane reacts with small sulphur-containing molecules, such as carbon disulphide, carbonyl sulphide, or hydrogen sulphide, at low pressure in a radio-frequency discharge tube. On the basis of these results, Johns and Olson prepared thioformaldehyde, under slightly different conditions, from dimethyl disulphide in the presence of sulphur hexafluoride. The reaction path suggested was  

Accordingly, the formation of hydrogen fluoride could be verified spectroscopically. The thioformaldehyde generated in this way was unam-bigously identified by means of its microwave and i.r. spectra, and its half-life was determined to be a few minutes. 

The formation of l,4-dianthronylidenebut-2-ene (13) by the photolysis of diazoanthrone in thiophen has been explained by a mechanism involving the thioaldehyde (11a) as an intermediate. It was considered likely that the initially formed carbene (9) reacts with the solvent to give (10), which then undergoes valence isomerization to the thioaldehyde (11a). The two 

Cauquis, B. Divisia, and G. Reverdy, Bull. Soc. chim. France, 1971, 3027. 

Metal Complexes.— Two interesting thioaldehydes, unknown as free ligands, have been characterized as their nickel(ii) complexes. Thus (14) was obtained as a black partially crystalline mixture by treatment of 

Transient Species.— The formation of seven different products by the photolysis reaction of S-(cis-prop-l-enyl)-L-cysteine was explained by assuming different reaction courses of a common precursor, the initially formed radical (15). The transient existence of (15) was supported by e.s.r. 

Reactions.—The conversion of thials of the structure (10) into l,6,6a-S -tri-thiapentalenes by means of phosphorus pentasulphide is well known. However, Davy and Vialle have now reported that thioacetamide may also effect this conversion. Reaction of the thials (11) with ethylenediamine or trimethylenediamine yielded tetra-aza macrocyclic compounds.  

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Lindinger W and Smith D 1983 Influenoe of translational and internal energy on ion-neutral reaotions Reactions of Small Transient Species ed A Fonti]n and M A A Clyne (New York Aoademio)... 

Vibrational spectroscopy has been, and will continue to be, one of the most important teclmiques in physical chemistry. In fact, the vibrational absorption of a single acetylene molecule on a Cu(lOO) surface was recently reported [ ]. Its endurance is due to the fact that it provides detailed infonnation on structure, dynamics and enviromnent. It is employed in a wide variety of circumstances, from routine analytical applications, to identifying novel (often transient) species, to providing some of the most important data for advancing the understanding of intramolecular and intemiolecular interactions. 

Kincaid J R 1995 Structure and dynamics of transient species using time-resolved resonance Raman-spectroscopy Biochemical Spectroscopy Methods Enzymol. vol 246, ed K Sauer (San Diego, CA Academic) pp 460-501... 

Until this time alkyl cations were considered only transient species. Their existence had been indirectly inferred from kinetic and stereochemical studies, but no reliable spectroscopic or other physical measurements of simple alkyl cations in solution or in the solid state were obtained. 

Transient species, existing for periods of time of the order of a microsecond (lO s) or a nanosecond (10 s), may be produced by photolysis using far-ultraviolet radiation. Electronic spectroscopy is one of the most sensitive methods for detecting such species, whether they are produced in the solid, liquid or gas phase, but a special technique, that of flash photolysis devised by Norrish and Porter in 1949, is necessary. 

Both the binary and complex fluorides of aluminum have played a significant role in the aluminum industry. Aluminum trifluoride [7784-18-17, A1F., and its trihydrate [15098-87-0] 3 3H2O, have thus far remained to be the only binary fluorides of industrial interest. The nonahydrate [15098-89-2] 3 9H2O, and the monohydrate [12252-28-7, 15621 -55-3], AIF 20, are of only academic curiosity. The monofluoride [13595-82-9], AIF, and the difluoride [13569-23-8], AIF2, have been observed as transient species at high temperatures. 

Modem real time instmmental methods permit analyses of unstable transient species and the free-radical intermediates as well. These methods have gready expanded the scope and power of VPO studies, but important basic questions remain unresolved. Another complication is the role of surface. Peroxide decompositions and radical termination reactions can occur on a surface so that, depending on circumstances, surfaces can have either an inhibiting or accelerating effect. Each surface has varying amounts of adventitious contaminants and also accumulates deposits during reaction. Thus no two surfaces are exactly alike and each changes with time. 

The chemistry of ketenes is dominated by their high reactivity most of them are not stable under normal conditions, many exist only as transient Species. Nucleophilic attack at the j -carbon, [2 + 2] cycloadditions, and ketene iasertion iato single bonds are the most important and widely used reactions of such compounds. 

G. L. Hug, Optical Spectra ofNonmetallic Inorganic Transient Species in Aqueous Solution, NSRDS-NBS 69, National Bureau of Standards, Washington,... 

Representative pentacarbonyls are restricted to the iron, mthenium, and osmium group. AH three pentacarbonyls possess trigonal bipyramidal stmctures as shown in stmcture (2). The pentacarbonyls of mthenium and osmium are thermally unstable. Osmium pentacarbonyl [16406-49-8] rapidly polymerizes at room temperature to form polynuclear species. The transient species Cr(CO) (1 )> Mo(CO) (1 )> W(CO) (19) have been investigated. 

Dicarbocyanine and trie arbo cyanine laser dyes such as stmcture (1) (n = 2 and n = 3, X = oxygen) and stmcture (34) (n = 3) are photoexcited in ethanol solution to produce relatively long-Hved photoisomers (lO " -10 s), and the absorption spectra are shifted to longer wavelength by several tens of nanometers (41,42). In polar media like ethanol, the excited state relaxation times for trie arbo cyanine (34) (n = 3) are independent of the anion, but in less polar solvent (dichloroethane) significant dependence on the anion occurs (43). The carbocyanine from stmcture (34) (n = 1) exists as a tight ion pair with borate anions, represented RB(CgH5 )g, in benzene solution photoexcitation of this dye—anion pair yields a new, transient species, presumably due to intra-ion pair electron transfer from the borate to yield the neutral dye radical (ie, the reduced state of the dye) (44). 

All other spectroscopic methods are applicable, in principle, to the detection of reaction intermediates so long as the method provides sufficient structural information to assist in the identification of the transient species. In the use of all methods, including those discussed above, it must be remembered that simple detection of a species does not prove that it is an intermediate. It also must be shown that the species is converted to product. In favorable cases, this may be done by isolation or trapping experiments. More often, it may be necessary to determine the kinetic behavior of the appearance and disappearance of the intermediate and demonstrate that this behavior is consistent with the species being an intermediate. 

Ehminations of HX to give double bonds offer considerable scope for selectivity and choice of reaction conditions. The dehydration of alcohols is the most common example of this class and may be achieved directly or through intermediate derivatives. In most cases, such derivatives are transient species formed in situ, but sometimes e.g. sulfonates, certain other esters and halides) they are isolated and characterized. Eliminations from jS-substituted ketones are very facile. The dehydration of jS-hydroxy ketones has been covered in section V. 

Intermediate (Section 3.9) Transient species formed during a chemical reaction. Typically, an intermediate is not stable under the conditions of its formation and proceeds further to form the product. Unlike a transition state, which corresponds to a maximum along a potential energy surface, an intermediate lies at a potential energy minimum. 

The stability of this derivative is attributed to the strong mesomeric effect of the NMca group, which changes the nature of the N=S bond. The selenium analogues RNSe have only been detected as transient species. 

Isoindole (1) and its analogs, isobenzofuran (2) and isothianaphthene (3), have attracted considerable theoretical and synthetic interest. -Of the jjarent heterocycles, only isothianaphthene (3) has proved sufficiently stable for isolation, - although both (1) and (2) have been detected as transient species, and it seems likely that a more complete characterization will be forthcoming with improved experimental techniques. This review is concerned with the chemistry of... 

An interesting result has been observed when 4-formylantipyrine 89 was converted into the corresponding pyridinium salt 90 and reacted with alkyl 3-aminobut-2-enoates. Tire expected 1,4-dihydropyridines 91 are transient species in these syntheses and readily lose the 4-substituent (antipyrine, 93) so that dialkyl 2,6-dimethylpyridine-3,5-dicarboxylates 92 are obtained (85-95%) (94H815). Protonation of the pyrazole ring by the evolved hydrochloric acid accounts for this particular behavior (Scheme 29). 

Most radicals are transient species. They (e.%. 1-10) decay by self-reaction with rates at or close to the diffusion-controlled limit (Section 1.4). This situation also pertains in conventional radical polymerization. Certain radicals, however, have thermodynamic stability, kinetic stability (persistence) or both that is conferred by appropriate substitution. Some well-known examples of stable radicals are diphenylpicrylhydrazyl (DPPH), nitroxides such as 2,2,6,6-tetramethylpiperidin-A -oxyl (TEMPO), triphenylniethyl radical (13) and galvinoxyl (14). Some examples of carbon-centered radicals which are persistent but which do not have intrinsic thermodynamic stability are shown in Section 1.4.3.2. These radicals (DPPH, TEMPO, 13, 14) are comparatively stable in isolation as solids or in solution and either do not react or react very slowly with compounds usually thought of as substrates for radical reactions. They may, nonetheless, react with less stable radicals at close to diffusion controlled rates. In polymer synthesis these species find use as inhibitors (to stabilize monomers against polymerization or to quench radical reactions - Section 5,3.1) and as reversible termination agents (in living radical polymerization - Section 9.3). 

The flash photolysis of s-trinitrobenzene (TNB) aerated solns in alcohols generated a transient species with absorption maxima at 430 and 51 Onm (Ref 27). The yield of the transient was a function of oxygen concn, and its rate of formation was viscosity dependent. In deaerated solns, instead of the transient, a brown permanent product, identified as a charge transfer... 

Capellos and Suryanarayanan (Ref 28) described a ruby laser nanosecond flash photolysis system to study the chemical reactivity of electrically excited state of aromatic nitrocompds. The system was capable of recording absorption spectra of transient species with half-lives in the range of 20 nanoseconds (20 x lO sec) to 1 millisecond (1 O 3sec). Kinetic data pertaining to the lifetime of electronically excited states could be recorded by following the transient absorption as a function of time. Preliminary data on the spectroscopic and kinetic behavior of 1,4-dinitronaphthalene triplet excited state were obtained with this equipment... 

Nanosecond flash photolysis of 1,4-dinitro-naphthalene in aerated and deaerated solvents showed a transient species with absorption maximum at 545nm. The maximum of the transient absorption was independent of solvent polarity and its lifetime seemed to be a function of the hydrogen donor efficiency of the solvent. The transient absorption was attributed to the lowest excited triplet state of 1,4-dinitronaphthalene. Based on spectroscopic and kinetic evidence, the triplet state of 1,4-dinitronaphthalene behaved as an n - Tt state in nonpolar solvents,... 

Cooper and coworkers30 measured also the absorption spectrum of transient species produced in the radiolysis of pure liquid DMSO-d6 and found the same absorption of the first two bands, however, the intensity of the absorption is about 30% larger in the case of the deuterated compound for both of the absorption bands. The intensity of the absorption is given by Ge, but as the same change was found for both bands it seems most reasonable that the 30% difference arises from a change in G rather than in e. This is similar to water, where the fraction of ions which become free ions is substantially larger for the deuterated compound32. 

Sulfonyl radicals are highly reactive transient species despite their short lifetime, they figure in a wide variety of organosulfur reactions, some of which are of great synthetic... 

Formed by either process, carbocations are most often short-lived transient species and react further without being isolated. The intrinsic barriers to formation and reaction of carbocations has been studied. ... 

The focus is on the primary formation of bonds, not on subsequent reactions of the products to form other bonds. These latter reactions are covered at the places where the formation of those bonds is described. Reactions in which atoms merely change their oxidation states are not included, nor are reactions in which the same pairs of elements come together again in the product (for example, in metatheses or redistributions). Physical and spectroscopic properties or structural details of the products are not covered by the reaction volumes which are concerned with synthetic utility based on yield, economy of ingredients, purity of product, specificity, etc. The preparation of short-lived transient species is not described. 

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