19-e intermediates

A femtosecond photolysis study of Re2(CO)io in CCI4 and hexane has been interpreted in terms of the solvent acting as a viscous medium for Re(CO)5 radicals. Those Re(CO)5 radicals that do diffuse out of the solvent cage may react with the chlorinated solvent to form Re(CO)5Cl. The absence of IR bands for intermediates other than the Re(CO)s radical rule out the possibihty of a 19-e intermediate, and support a mechanism in which involves only C-Cl bond activation. ... 

The role of 19 e intermediates has been a source of controversy, in particular many mechanisms can be explained without their proposal, and few 19 e complexes have been crystallographically characterized. Ultrafast photochemical studies of phosphite substitution in W(CO)3Cp allowed the direct detection of infrared bands assigned to W(CO)3(PR3)Cp in Equation 10.44 with decay times on the order of 280 ns63... 

The 17 e- mononuclear metal radicals once formed have been shown to be dramatically more reactive toward ligand substitution reactions than 18 e-mononuclear complexes or their dinuclear precursors [42], Indeed the formation of mononuclear radical intermediates can serve as catalysts for the substitution chemistry of the dinuclear parents owing to the recombination of the radicals to give the substituted dimers [43]. After considerable mechanistic investigation, it was concluded that the high radical reactivity can be attributed to facile associative reactions of the 17 e species with the nucleophiles to form 19 e intermediates (eq. 8) [39a,44]. 

The 19 e intermediates not only can undergo facile ligand dissociation to give a substituted 17 e" radical, but are also powerful reductants and can transfer an electron to other species in solution (eq.9) [6,45]. 

The sum of all the cathodic partial reactions is included in e.g., oxygen reduction according to Eq. (2-17) and hydrogen evolution according to Eq. (2-19). The intermediate formation of anode metal ions of anomalous valence is also possible ... 

The alternative situation where k- zero order in [X], and the clear disparitybetween stoichiometric coefficients and reaction orders demands the existence of an intermediate. The situation occurs in acid- and base-catalysed reactions of ketones with reactive electrophiles (e.g. X = CI2, Br2 or I2), which are usually zero order in the electrophilic reagent, unless concentrations of the electrophiles are extremely low [19]. The intermediate reacting with the electrophile may be the enol tautomer of the ketone or its enolate anion, formed catalytically from the ketone. 

E. Kuhle, E. Klauke, Fluorinated isocyanates and their derivatives as intermediates for biologically active compounds, Angew. Chem., Int. Ed. Engl. 16 (1977) 735. 

Interaction of A-B with the 17 e radical MLn leads to formation of the 19 e complex MLn(A-B). According to the simple molecular orbital theory, such 19 e complexes are stabilized by the formation of a half bond between the metal and A-B.1 Much of the reactivity of 17 e radicals keys off these elusive 19 e adducts that can form in rapid associative reactions. The 19 e adduct MLn(A-B) can eject a ligand to form a substituted radical complex via the lower reaction in Scheme 10.1. If the A-B bond is particularly weak, homolysis to A-.V1L, and a free "B radical can occur. The "B radical generated as such may then combine with MLn(A-B) to form B-.V1L, or enter different reaction channels. If the A-B bond is strong, a second mole of radical may be required to break the A-B bond. In that case, an intermediate adduct LnM(A-B)MLn forms and then fragments to A-.V1L, and B-.V1L,. These are the same products obtained in homolytic (single metal) cleavage, but the important difference is that free "B radicals are not formed in the reaction. 

In addition to these atom transfer reactions, electron transfer reactions can occur. Reduction of 17 e MLn to 18 e M L anions is a common reaction. Oxidation of 19 e MLn(A-B) adducts to 18 e ML (A-B)+ cations is also frequently observed. Thus, an additional reaction pathway of the intermediate L M (A-B)ML is disproportionation to an ionic compound L M (A-B)]+ M L . The detailed mechanisms of these reactions are more complex than what is shown in Scheme 10.1. Many of the reactions are reversible and, in addition, they can couple to other reactions. 

Propene passed ca. 7.5 hrs. into a stirred soln. of bromodicyanomethane in methylene chloride with initial removal of oxygen and subsequent irradiation with a high-pressure Hg-lamp crude 3-bromo-l,l-dicyanobutane (Y 90%) boiled 3 hrs. with HCl (d. 1.19), the intermediate crude 4-hydroxy-2-carboxypentanoic acid lactone (5.5 from 9.2 g.) heated 3 hrs. at 145 and distilled 4-hydroxypentanoic acid lactone (Y 78% based on 3-bromo-l,l-dicyanobutane). F. e., also stereospecific ring closure, and from 1,1-dicyanocyclopropanes, s. P. Boldt, W. Thieleche, and J. Etzemuller, B. 102, 4157 (1969). 

Figure 4.19. Reversible intermediate section trajectories of sharp auto-extractive distillation ofideal ternary mixture ( fi > K2 > K3) (a) D < E, and (b) D > E. Component 1, overhead prodnct component 3, entrainer x and y, composition in arbitrary cross-section x j), composition of psendoprodnct.

The theory of sublimation, t.e. the direct conversion from the vapour to the sohd state without the intermediate formation of the liquid state, has been discussed in Section 1,19. The number of compounds which can be purified by sublimation under normal pressure is comparatively small (these include naphthalene, anthracene, benzoic acid, hexachloroethane, camphor, and the quinones). The process does, in general, yield products of high purity, but considerable loss of product may occur. 

Special reactions of hydrazides and azides are illustrated by the conversion of the hydrazide (205) into the azide (206) by nitrous acid (60JOC1950) and thence into the urethane (207) by ethanol (64FES(19)105Q) the conversion of the same azide (206) into the N-alkylamide (208) by ethylamine the formation of the hydrazone (209) from acetaldehyde and the hydrazide (205) and the IV-acylation of the hydrazide (205) to give, for example, the formylhydrazide (210) (65FES(20)259). It is evident that there is an isocyanate intermediate between (206) and (207) such compounds have been isolated sometimes, e.g. (211). Several of the above reactions are involved in some Curtius degradations. 

JOC1537). The mechanisms of these transformations may involve homolytic or heterolytic C —S bond fission. A sulfur-walk mechanism has been proposed to account for isomerization or automerization of Dewar thiophenes and their 5-oxides e.g. 31 in Scheme 17) (76JA4325). Calculations show that a symmetrical pyramidal intermediate with the sulfur atom centered over the plane of the four carbon atoms is unlikely <79JOU140l). Reactions which may be mechanistically similar to that shown in Scheme 18 are the thermal isomerization of thiirane (32 Scheme 19) (70CB949) and the rearrangement of (6) to a benzothio-phene (80JOC4366). 

Functionalized rubbers. Butyl rubber (isobutylene with about 2% iso-prene) has been functionalized through the residual double bonds via the bro-mobutyl intermediate to produce a material with 2% conjugated diene (see Fig. 19). This resin shows high reactivity towards e-beam or UV (free radical or cationic [53]). The bromo butyl intermediate has also been used to attach acrylate or photoinitiator groups to the butyl backbone [54]. 

Reactions that fit this model are called ping-pong or double-displacement reactions. Two distinctive features of this mechanism are the obligatory formation of a modified enzyme intermediate, E, and the pattern of parallel lines obtained in double-reciprocal plots (Figure 14.19). 

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