Intermediates spectroscopic observation

The mechanism of the Fischer cyclization outlined in equation 7.1 has been supported by spectroscopic observation of various intermediates[4] and by isolation of examples of intermediates in specialized structures[5]. In particular, it has been possible to isolate enehydrazines under neutral conditions and to demonstrate their conversion to indoles under the influence of acid cata-lysts[6]. 

Presence and nature of intermediates. Demonstration of an intermediate shows that a reaction is complex. An intermediate may be sufficiently stable to isolate, but more commonly it can only be detected by physical means (such as some form of spectroscopic observation) or by indirect chemical means ( trapping it in a side reaction). Despite the instability of some intermediates, these are ordinary chemical species whose properties can, in principle at least, be determined experimentally. 

Figure 18.5 Plausible sequence of steps responsible for rapid and selective reduction of O2 to H2O by mixed-valence CcO. The square frames signify the catalytic site (Fig. 18.4c) imidazole ligation of Cub is omitted for clarity in some or aU intermediates, Cub may additionally be ligated by an exogenous ligand, such as H2O (in Cu ) or OH (in Cu ) such ligation is not established, and hence is omitted in all but compound Pm and the putative hydroperoxo intermediate. The dashed frames signify the noncatalytic redox cofactors. Typically used phenomenological names of the spectroscopically observed intermediates (compounds A, E, H, etc.) are also indicated.

Abstract. We use intermediate resolution (II, 19 300) spectroscopic observations in the spectral region including the Li 6708 A line to study 341 stars in the star forming region (SFR) NGC 6530. Based on the optical color-magnitude diagrams (CMD), they are G, K and early M type pre-main sequence (PMS) cluster candidates. 72% of them are probable cluster members since are X-ray sources detected in a Chandra-ACIS observation ([2]). We use our spectroscopic measurements to confirm cluster membership by means of radial velocities and to investigate the Li abundance of cluster members. 

The donor-induced disproportionation in equation (91) leads to the EDA complex, i.e., [D, NO+]NO as the (first) directly observable intermediate. The critical role of the nitrosonium EDA complex in the electron-transfer activation in equation (92) is confirmed by the spectroscopic observation of the cation-radical intermediates (i.e., D+ ) as well as by an alternative (low-temperature) photochemical activation with deliberate irradiation of the charge-transfer band252 (equation 95). 

Clearly, mechanistic investigations can provide circumstantial evidence for the participation of particular intermediates in a reaction but, here, we are concerned with the definitive observation of these species. If the intermediates are relatively stable then direct spectroscopic observation of the species during a room-temperature reaction may be possible As a rather extreme example of this, the zero-valent manganese radicals, Mn(CO>3L2 (L phosphine) can be photochemically generated from Mh2(CO)gL2, and, in the absence of O2 or other radical scavengers, are stable in hydrocarbon solution for several weeks (2, 3) However, we are usually more anxious to probe reactions in which unstable intermediates are postulated. There are, broadly speaking, three approaches - continuous generation, instantaneous methods and matrix isolation. 

Substituted Phenyl 2,4,6-trinitrophenyl sulphides. By UV-VIS measurements of the reactions of 4 -substituted phenyl 2,4,6-trinitrophenyl sulphides with amines in DMSO, Crampton s group131 showed the presence of two well-separated processes which were interpreted by Scheme 7129. In each case a rapid reversible equilibrium was established leading to the 3-adduct (10). They also observed a second, much slower process resulting in formation of the N-substituted picramide derivatives, 13. The final spectra were identical to those of the independently prepared products, 13. Chamberlain and Crampton133 showed that the reaction products are in rapid equilibrium with anions derived from them by amine addition at the 3-position and/or loss of a side-chain proton, but they did not find evidence for the accumulation of spectroscopically observable concentrations of intermediates such as 12. 

The difference between this catalytic system and Wilkinson s catalyst lies in the sequence of the oxidative addition and the alkene complexation. As mentioned above, for the cationic catalysts the intermediate alkene (enamide) complex has been spectroscopically observed. Subsequently oxidative addition of H2 and insertion of the alkene occurs, followed by reductive elimination of the hydrogenation product. 

Metal-substituted hemoglobin hybrids, [MP, Fe " (H20)P] are particularly attractive for the study of long-range electron transfer within protein complexes. Both photoinitiated and thermally activated electron transfer can be studied by flash excitation of Zn- or Mg-substituted complexes. Direct spectroscopic observation of the charge-separated intermediate, [(MP), Fe " P], unambiguously demonstrates photoinitiated ET, and the time course of this ET process indicates the presence of thermal ET. Replacement of the coordinated H2O in the protein containing the ferric heme with anionic ligands (CN , F , Nj ) dramatically lowers the photoinitiated rate constant, k(, but has a relatively minor effect on the thermal rate, kg. 

The last years have seen a rapid development of the direct spectroscopic observation of stable carbonium ion intermediates in strongly acidic solutions. Thus many alkylcarbonium ions, previously suggested... 

For the rational design of transition metal catalyzed reactions, as well as for fine-tuning, it is vital to know about the catalytic mechanism in as much detail as possible. Apart from kinetic measurements, the only way to learn about mechanistic details is direct spectroscopic observation of reactive intermediates. In this chapter, we have demonstrated that NMR spectroscopy is an invaluable tool in this respect. In combination with other physicochemical effects (such as parahydrogen induced nuclear polarization) even reactive intermediates, which are present at only very low concentrations, can be observed and fully characterized. Therefore, it might be worthwhile not only to apply standard experiments, but to go and exploit some of the more exotic techniques that are now available and ready to use. The successful story of homogeneous hydrogenation with rhodium catalysts demonstrates impressively that this really might be worth the effort. 

S. Hirota, T. Iwamoto, S. Kishishita, T. Okajima, O. Yamauchi, K. Tanizawa, Spectroscopic observation of intermediates formed during the oxidative half-reaction of copper/topa quinone-containing phenylethylamine oxidase, Biochemistry 40 (2001) 15789-15796. 

Determination of the mode of bonding of the allylic intermediate formed would provide direct evidence that an M-O-C species produces acrolein. This is difficult because formation of the allylic intermediate is rate-determining and its subsequent reaction is fast. Thus, the surface concentration of the intermediate is small and not amenable to standard spectroscopic observation. 

Scheme 13 Pathway for the oxidation of 3,5-di-tert-butylphenolate to the corresponding catechol and quinone products using dicopper species 28, via the spectroscopically observed bis(/x-oxo)phenolate intermediate 29 [190]...

Because reactive intermediates (RIs) undergo facile intra- and/or intermolecular reactions, they are, by definition, short lived. As described in the other chapters in this volume, the transient namre of RIs provides a challenge to their direct spectroscopic observation. When the spectra of a putative RI can be obtained, it may not be certain that the observed spectra actually belong to the RI. 

Most presolar silicon carbide and oxide grains and a significant fraction of presolar silicate grains found in meteorites come from low- to intermediate-mass stars in the asymptotic giant branch (AGB) phase (see Chapter 3). Evidence for this conclusion derives from two sources (1) spectroscopic observations of the envelopes of these stars and (2) comparison... 

Proof that the intermediate cyclopropanones are involved in these reactions was shown by spectroscopic observation, isolation and trapping with various dipolarophiles.1 3 Yields of the double ring expansion products are quite good ranging up to 90% for the parent cyclobutanone,1 but the separation of the regioisomeric mixture can be tedious. 

The allyl cation has never been characterized as a persistent species in solution. If prepared, it would be the smallest carbenium ion universally accepted to have been formed in condensed media (this title has for many years been held by the isopropyl cation). No allyl cation derivatives, e.g., 10, were observed in a 1990 report of a CAVERN study of butadiene on HY and HZSM-5 (104). The same year, Hutchings reported a flow reactor study of allyl alcohol on HZSM-5 (105) and Gorte (106) reported a TPD study of allyl alcohol on the same zeolite. Hutchings and co-workers found that allyl alcohol had two reaction paths, one in which propanal was formed and another that formed hydrocarbons. The latter route was proposed to proceed through an allyl cation intermediate, but no claim for its persistence or spectroscopic observation was made or implied by Hutchings. Gorte... 

Transient absorption spectra from valerophenone in Na-ZSM-5 and Cs-ZSM-5 could be assigned to triplet-triplet transitions no spectra which can be attributed to the hydroxy-1,4-biradicals were detected. Furthermore, the decay of the transient signals could not be fit to either a single or double exponential expression, and samples prepared under apparently identical conditions exhibited half-lives that varied by a factor of 2 [292], All of these spectroscopic observations suggest that the valerophenone molecules reside in a distribution of sites within a zeolite and migration among them under the experimental conditions is slow. Thus, the Norrish II photoproduct ratios must be interpreted in terms of not only the relative populations of alkanophenones at each site type, but also the quantum efficiencies of each and the conformational preferences of the intermediate BR in each environment. 

Reactions of the HNiL3CN complex with 1,3-cyclopentadiene, 1,3-cyclo-hexadiene, and 1,3-cyclooctadiene gave intermediates with decreasing stabilities in that order the 1,3-cyclooctadiene intermediate was not spectroscopically observable. The cyclohexadiene adduct was shown to be the cyclohexadienyl complex 12 by its proton spectra, with resonances of H , Hb, and —(CH2)3— at 14.53, 6.06, and 8.47, respectively these values are close to the chemical shifts found earlier (51) for 13 14.52,5.86, and 8.48. The reaction of DNi[P(OMe)3]X with cyclopentadiene gives 13-d, with addition of D and Ni to the same side of the ring (52). Backvall and Andell (55) have shown, using Ni[P(OPh)3]4 and deuterium cyanide (DCN), that addition of D and CN to cyclohexadiene is stereospecifically cis, as expected for jt-allyl intermediate 12. 

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