Reactive intermediates solution-phase studies

The model process Eq. (15) has been studied by means of the MINDO/3 method to clarify the energetic conditions during the formation of cyclic reactive intermediates in cationic propagation of alkoxy-substituted monomers. The enthalpies of formation in the gas phase AH°g of both the alternative structures e and /were supplemented by the solvation energies Eso]v for transition into solvent CH2C12 with the assistance of the continuum model of Huron and Claverie which leads to heats of formation in solution AH° s. Table 13 contains the calculated results. 

A point of interest at this stop in our tour is that fragmentation of organometallic ions in ESI-MS often proceeds via ligand dissociation (e.g., phosphane loss) to generate coordinatively unsaturated organometallic ions [1-9]. One of the strengths of this technique is that such unsaturated ions are typically proposed as reactive intermediates in catalytic reactions carried out in solution (vide infra), allowing ESI-tandem-MS systems to study directly the gas-phase reactivity of such species. 

Most of the reactions of triplet carbenes discussed in this chapter will deal with reactions in solution, but some reactions in the gas phase will also be included. Triplet carbenes may be expected to show a radical-like behaviour, since their reactions usually involve only one of their two electrons. In this, triplet carbenes differ from singlet carbenes, which resemble both carbenium ions (electron sextet) and carbanions (free electron pair). Radical like behaviour may, also be expected in the first excited singlet state Sr e.g. the state in CH2) since here, too, two unpaired electrons are present in the reactive intermediate. These Sj-carbenes are magnetically inert, i.e., should not show ESR activity. Since in a number of studies ESR spectra could be taken of the triplet carbene, the reactions most probably involved the Ti-carbene state. However, this question should be studied in more detail. 

It is true that in some cases, the spectroscopic data on a reactive intermediate are so persuasive that the connection between structure and spectroscopic features is firm. However, in general this will not be the case, and additional spectroscopic or preparative criteria will have to be provided. So we are faced with the question How can we connect the information obtained, for example, from observations in matrices or in solution-phase fast kinetic studies, to molecular structure How do we know that the results of these experiments, using what we hopefully call direct methods, really pertain to the species we are trying to characterize I attempt to deal with this issue in what follows. Since the methods used vary from one class of non-Kekule species to another, specific classes are individually discussed, and special techniques are introduced as needed. Electron spin resonance spectroscopy has played such a pervasive role that it will be useful to give first a brief outline of that method. 

One of the most commonly apphed types of spectroscopy in the picosecond realm is pump-probe electronic absorption spectroscopy. The absorption spectra of reactive intermediates are usually just as featureless as those of the other two time domains described in this volume. It is simply the inherent nature of these spectra in condensed phases, most typically in solution. Spectroscopic studies in solution most closely mimic reaction conditions that reactive intermediates may find themselves involved in when they are formed and consumed during the course of an organic chemical reaction. 

The development of mass spectrometric ionization methods at atmospheric pressures (API), such as the atmospheric pressure chemical ionization (APCI)99 and the electrospray ionization mass spectrometry (ESI-MS)100 has made it possible to study liquid-phase solutions by mass spectrometry. Electrospray ionization mass spectrometry coupled to a micro-reactor was used to investigate radical cation chain reaction is solution101. The tris (p-bromophenyl)aminium hexachloro antimonate mediated [2 + 2] cycloaddition of trans-anethole to give l,2-bis(4-methoxyphenyl)-3,4-dimethylcyclobutane was investigated and the transient intermediates 9 + and 10 + were detected and characterized directly in the reacting solution. However, steady state conditions are necessary for the detection of reactive intermediates and therefore it is crucial that the reaction must not be complete at the moment of electrospray ionization to be able to detect the intermediates. 

Although considerable difficulties have been encountered in generating silylenium ions in solution, either as stable species or as reactive intermediates, such has not been the case in the gas phase. Numerous studies of silyl cations in the gas phase have been reported17"24, both by traditional mass spectrometric methods and by ion cyclotron resonance. 

In the context of this work we found it interesting to make the transition from ion spectroscopy in the gas phase to ion photochemistry in liquid solution. While the basic processes are still the same the different environment drastically modifies the results known from gas phase studies. The mobility of ions and electrons in solution is greatly different, the energy of the spectroscopie states is changed, and reactive interaction with the solvent or other added species becomes possible. A very first step in this direction of preparative ionic photochemistry was reported before from our laboratory treating the benzene molecule as an example. More results have now been obtained relating to the mechanism of ion fonnation and its yield, the nature of the intermediates, and the conditions favouring product yield. As a first case the conversion of benzene to phenol and biphenyl in aqueous solution was studied followed by the study of reactions of benzene derivatives in water and other solvents /3/. 

All these cases suggest the necessity to perform a rigorous analysis to probe unambiguously that the species detected by ESI are the ones prevailing in solution, and more importantly to confirm that they are indeed reactive intermediates on the reaction path. An outstanding methodology is to isolate in the gas phase the species assumed to participate in the reaction mechanism and perform ion/molecule reactions with the substrate of the reaction solution. This methodology is a very powerful way to reject side-products and to assure the reliability of the analysis. Another important method is to study well-known reactions and compare the data obtained by ESI-MS with other spectroscopic techniques. 

Of course, aU these studies were conducted in the gas-phase, and so solution-phase reactivity may be different. The ability to detect and manipulate species that are unstable or undetectable in solution is incredibly powerful, and has been used to investigate mechanistic aspects of metathesis chemistry. These techniques show further potential for the investigation of structure/ac-tivity relationships in metathesis and other catalytic reactions, as they can isolate and study key intermediates such as the alkylidene complexes 142. 

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