Intermediates in reactions

Entry 4 shows that reaction of a secondary 2-octyl system with the moderately good nucleophile acetate ion occurs wifii complete inversion. The results cited in entry 5 serve to illustrate the importance of solvation of ion-pair intermediates in reactions of secondary substrates. The data show fiiat partial racemization occurs in aqueous dioxane but that an added nucleophile (azide ion) results in complete inversion, both in the product resulting from reaction with azide ion and in the alcohol resulting from reaction with water. The alcohol of retained configuration is attributed to an intermediate oxonium ion resulting from reaction of the ion pair with the dioxane solvent. This would react until water to give product of retained configuratioiL When azide ion is present, dioxane does not efiTectively conqiete for tiie ion-p intermediate, and all of the alcohol arises from tiie inversion mechanism. ... 

Common variants include reduction of a metal oxide or halide in the presence of N2 and the formation of a metal amide as an intermediate in reactions in liquid NH3 ... 

Ni- and Pt-Cycles as intermediates in reactions of Ni(0)- and Pt(0)-complexes with benzyne or related small-ring alkynes 97CB1029. 

This review describes the current status of silenes (silaethylenes, silaethenes), molecules which contain a silicon-carbon double bond. The heart of the material is derived from a computer-based search of the literature which we believe reports all silenes that have been described to date, either as isolated species, chemically trapped species, proposed intermediates (in reactions where some experimental evidence has been provided), or as the result of molecular orbital calculations. Ionized species... 

The preparation of imines, enamines, nitroalkenes and N-sulfonylimines proceeds via the azeotropic removal of water from the intermediate in reactions that are normally catalyzed by p-toluenesulfonic acid, titanium(IV) chloride, or montmorillonite K 10 clay. A Dean-Stark apparatus is traditionally used which requires a large excess of aromatic hydrocarbons such as benzene or toluene for azeotropic water elimination. 

The similar rate laws (Eqs. (15) and (18)), pH profiles, and the values of the observed second-order rate constants ( and k s) suggest a common reactive intermediate in reactions of Eqs. (14) and (17) (oxidized Fe-TAML in Scheme 6). Taking all three steps... 

Nitroxyl radicals are formed as intermediates in reactions of polymer stabilization by steri-cally hindered amines as light stabilizers (HALS) [30,34,39,59]. The very important peculiarity of nitroxyl radicals as antioxidants of polymer degradation is their ability to participate in cyclic mechanisms of chain termination. This mechanism involves alternation of reactions involving alkyl and peroxyl radicals with regeneration of nitroxyl radical [60 64],... 

ALKOXY-5(4H)-OXAZOLONES AS INTERMEDIATES IN REACTIONS OF A/-ALKOXYCARBONYLAMINO ACIDS... 

FIGURE 1.18 2-AI koxy-5(4/7)-oxazol ones as intermediates in reactions of IV-alkoxycarbo-nylamino acids.22 After removal of the symmetrical anhydride from a reaction mixture containing Boc-valine and ethyl-(3-dimethylaminopropyl)-carbodiimide hydrochloride, the filtrate contained a novel activated form of Boc-valine (20% yield) that was established to be the 2-alkoxy-5(4H)-oxazolone. Slow addition of Boc-valine to ethyl-(3-dimethylamino-propyl)-carbodiimide hydrochloride in dilute solution gave a 55% yield. Petrol = petroleum ether, bp 40-60°. 

Carbon atoms in organic molecules are most often neutral. Positively charged carbocations have attracted the interest of synthetic organic chemists, because of their use as intermediates in reactions leading to formation of carbon-carbon bonds. Our work on carbocations has focused on defining the stability of these species as intermediates of solvolysis reactions, through the determination of rate and equilibrium constants for these stepwise reactions (Scheme 1). This has led to the development of experimental methods to characterize these parameters for carbocations that are sufficiently stable to form in aqueous solution. 

A. Polarization and Electrophilic Svhstitidion From the various accounts that have been given of the role of 77-complexes and <7-complexes as possible intermediates in reaction mechanisms, that described by Olah e al.(1961)is selected for special attention, since it... 

If aspartic acid-52 acts as a nucleophile in lysozyme reactions a glycosyl enzyme intermediate will be formed [60]. There is no evidence, kinetic or otherwise, for substituted enzyme intermediates, but rapid breakdown might preclude attainment of detectable concentrations. Formation of a substituted enzyme could explain the observed retention of configuration at the anomeric carbon in transglycosidation reactions, provided backside attack in a subsequent reaction is chemically reasonable. It has therefore been important to attempt to understand the chemistry of acylal hydrolysis so as to assess the properties that would be expected of an acylal intermediate in reactions catalysed by the enzyme. 

MeCo-COCl reacts with NaMnfCOlj in THF at 0 C to give Me CO CO Mn(CO)5 (33% yield). The product is stable, but could not be isolated from the carbonylation of MeCO MnfCO) at 80 C and high pressure of carbon monoxide, suggesting that such a compound is not an intermediate in reactions of MeCO MnlCOlj. ... 

Because many of them are nearly inert, ionic liquids have been used to stabilize highly polar or ionic transition states. Ionic liquids provide favorable media for the formation and stabilization of intermediates in reactions that proceed through charged intermediates. An example is the Baylis-Hillman reaction catalyzed by 1,4-diazabicyclo (222). octane (DABCO) (Scheme 8) (162). 

---