Organometallic intermediate

Reactions via organometallic intermediates achieve position selectivity on the basis of prior substitution, for example through halogen-metal e.xchange... 

There are several mechanisms by which net nucleophilic aromatic substitution can occur. In this section we will discuss the addition-elimination mechanism and the elimination-addition mechanism. Substitutions via organometallic intermediates and via aryl diazo-nium ions will be considered in Chapter 11 of Part B. 

The preparation of aromatic fluorine compounds may be accomplished by direct fluorination or by fluonnation of organometallic intermediates. Tlte ipso fluorination of an aryl organometallic derivative with a positive fluorine reagent allows control over the regioselectivity of the fluorination and offers advantages in the preparation of F-labeled materials [II, 50],... 

Provided electron transfer between the electrode and solute species is not interrupted by the coating, even electroinactive films can offer interesting applications. Thus, a chiral environment in the surface layer may impose stereoselectivity in the follow-up reactions of organic or organometallic intermediates. Furthermore, polymer layers may be used to obtain diffusional permeation selectivity for certain substrates, or as a preconcentration medium for analyzing low concentration species. 

It is not entirely clear whether these reactions involve a redox reaction at sulfur or if they proceed by organometallic intermediates. 

A striking feature of the reactions of indium and allylic halides is that they can be carried out in aqueous solution.192 The aldehyde traps the organometallic intermediate as it is formed. 

Clearly RR is attractive as a technique for detecting intermediates. It has, for example, been very successful with intermediates in organic and bioinorganic reactions (55). Unfortunately the high laser intensities that are needed in RR may be incompatible with the photosensitivity of most organometallic intermediates. For example, in an attempt to detect Cr(CO)5 in solution by RR, the only detectable transient signal was emission from excited Cr atoms (55). 

It will be clear that in these examples the short-lived transients have been detected by their V(C-O) vibrations. Church (51) has managed to observe V(N-N) of Cr(CO)5(N2) and Weitz has observed V(N-O) of transients produced in the reaction of Fe(CO) with NO (52 ). However, at this stage it looks as though detection of organometallic intermediates via, for example, V(C-H) or V(C=C) absorptions, will require extensive improvements in sensitivity. 

Direct electrophilic silylation of thiadiazole 321 with bromotrimethylsilane (TMSBr) under basic conditions provides easy access to C-silyl thiadiazole 322, which can serve as a synthetic equivalent of an organometallic intermediate or a silyl-protected azole <06S 1279>. 

Organometallic Intermediates by CO Loss. There are many studies where the primary photoproduct generated by CO loss has been characterized in a matrix. One example will illustrate. 

Whatever the best explanation may be, an indication that allylic alkali metal compounds or allylic carbanions do in fact form the less stable of the two possible acids on neutralization is found in the results of the reduction of aromatic compounds by dissolving metals.376The detection of a paramagnetic intermediate in a similar system and polaro-graphic evidence indicate a one electron transfer in the rate and potential determining step.878 376 The mechanism therefore involves ions (or organometallic intermediates) like the following ... 

Another synthetic route which gives a good yield of the labelled tin hydride involves the hydrolysis of an organometallic intermediate such as a trialkylstannyllithium46 with deuterated or tritiated water. The trialkylstannyllithium can be prepared by treating the trialkyltin chloride with lithium metal in THF46. This process is shown in equation 42. 

Structural studies on the nature of the organometallic intermediates following chelation-assisted CH additions of pincer iridium complexes have been carried out. The product was found to have an unexpected /ram-disposition of the hydride with respect to the metallated aromatic group. This is not the expected direct outcome of a chelation-assisted reaction since coordination of oxygen to iridium prior to C-H activation would be expected to afford the m-isorner (Equation (97)). 

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