Neutral abstraction

Alkenes in (alkene)dicarbonyl(T -cyclopentadienyl)iron(l+) cations react with carbon nucleophiles to form new C —C bonds (M. Rosenblum, 1974 A.J. Pearson, 1987). Tricarbon-yi(ri -cycIohexadienyI)iron(l-h) cations, prepared from the T] -l,3-cyclohexadiene complexes by hydride abstraction with tritylium cations, react similarly to give 5-substituted 1,3-cyclo-hexadienes, and neutral tricarbonyl(n -l,3-cyciohexadiene)iron complexes can be coupled with olefins by hydrogen transfer at > 140°C. These reactions proceed regio- and stereospecifically in the successive cyanide addition and spirocyclization at an optically pure N-allyl-N-phenyl-1,3-cyclohexadiene-l-carboxamide iron complex (A.J. Pearson, 1989). 

Typical Cl processes in which neutral sample molecules (M) react with NH to give either (a) a protonated ion [M + HJ or (b) an adduct ion [M + NHJ+ the quasi-molecular ions are respectively 1 and 18 mass units greater than the true mass (M). In process (c), reagent ions (CjHf) abstract hydrogen, giving a quasi-molecular ion that is 1 mass unit less than M. 

R)-Pantothenic acid (1) contains two subunits, (R)-pantoic acid and P-alanine. The chemical abstract name is A/-(2,4-dihydroxy-3,3-dimethyl-l-oxobutyl)-P-alanine (11). Only (R)-pantothenic acid is biologically active. Pantothenic acid is unstable under alkaline or acidic conditions, but is stable under neutral conditions. Pantothenic acid is extremely hygroscopic, and there are stabiUty problems associated with the sodium salt of pantothenic acid. The major commercial source of this vitamin is thus the stable calcium salt (3) (calcium pantothenate). 

Common name [Chemical Abstracts name] CAS Registry Number Formula Neutralizing value Uses ... 

JA5190, 940M5132). Proton abstraction from 109 gives a neutral ti C) 2-thienyl complex, 110. Such a reaction becomes impossible in case of the 2,5-dimethylthiophene analog of 109. However, use of a strong base such as potassium hydroxide in methanol gives 111. An attempted transformation of 109 to 110 by protonation with triflic acid leads, however, to the thienylcarbene complex cation 112 where the aromaticity is disrupted. 

Another route to the amido complexes originates from [(>j-Tp )W(CO) (PhC=CMe)(OTf)l and benzylamine and yields [(i -Tp )W(CO)(PhC=CMe) (NHCH2Ph)] (96JA6916). The latter can be protonated with tetrafluoroboric acid to give the amine derivative [(> -Tp )W(CO)(PhC=CMe)(NH2CH2Ph)](Bp4), and this process can be reversed by -butyllithium. Hydride abstraction by silver tetrafiuoroborate, molecular iodine, or PhsCPEe leads to the cationic imine derivatives [(> -Tp )W(CO)(PhC=CMe)(HN=CHPh)]". -Butyllithium deproto-nates the product and gives the neutral azavinylidene species [(> -Tp )W(CO) (PhC=CMe)(N=CHPh)]. The latter with silver tetrafiuoroborate forms the cationic nitrile species [(j -Tp )W(CO)(PhC=CMe)(N=CPh)](Bp4). 

Aldol reactions, Like all carbonyl condensations, occur by nucleophilic addition of the enolate ion of the donor molecule to the carbonyl group of the acceptor molecule. The resultant tetrahedral intermediate is then protonated to give an alcohol product (Figure 23.2). The reverse process occurs in exactty the opposite manner base abstracts the -OH hydrogen from the aldol to yield a /3-keto alkoxide ion, which cleaves to give one molecule of enolate ion and one molecule of neutral carbonyl compound. 

As well as the neutral compounds, also cationic complexes are available from chlorosilyl compounds by chloride abstraction. One example 12 has been characterized by x-ray structure analysis (Fig. 2). 

Compounds containing the neutral (formally zwitterionic) group =N2 attached by one atom to carbon are named by adding the prefix diazo- to the name of the parent compound (Rule 931.4), e.g., diazomethane, ethyl diazoacetate. Diazo is a so-called characteristic group appearing only as a prefix in substitutive nomenclature. Chemical Abstracts and Beilstein indexing of diazo compounds is analogous to that mentioned above for diazonium ions and salts, but Diazo compounds is not... 

Typical chain transfer reactions involve the abstraction of an atom from a neutral saturated molecule, which may be solvent or a chain transfer agent added to the polymerisation mixture specifically to control the final size and distribution of molar masses in the polymer product. The chain transfer reaction may be represented as in Reaction 2.7. 

Although most acidity functions have been applied only to acidic solutions, some work has also been done with strongly basic solutions. The H function, which is used for highly acidic solutions when the base has a charge of — 1, can also be used for strongly basic solvents, in which case it measures the ability of these solvents to abstract a proton from a neutral acid BH. When a solvent becomes protonated, its conjugate acid is known as a lyonium ion. 

Mechanisms of aldehyde oxidation are not firmly established, but there seem to be at least two main types—a free-radical mechanism and an ionic one. In the free-radical process, the aldehydic hydrogen is abstracted to leave an acyl radical, which obtains OH from the oxidizing agent. In the ionic process, the first step is addition of a species OZ to the carbonyl bond to give 16 in alkaline solution and 17 in acid or neutral solution. The aldehydic hydrogen of 16 or 17 is then lost as a proton to a base, while Z leaves with its electron pair. 

However, just as real reactions do not include free electrons as reactants or products, they do not occur between or produce materials with (substantive) net charges. So whilst the equation above does represent the oxidation of bromide by acidified permanganate, it is an abstraction from a real chemical reaction where the bromide and the permanganate would be part of real substances that were (substantially) neutral. For example ... 

Hydrogen abstraction — The abstraction of a hydrogen atom H from a saturated carbon atom in a position allylic to the polyene chain can generate a resonance-stabilized neutral radical by homolytic cleavage of a C-H bond CAR = X - H. Then X - H -H R- X + RH. 

Dissociation of the gases SiH4 and H2 by electron impact will create reactive species (radicals) and/or neutrals (Si2H6 and even higher-order silanes [195-198]). Atomic hydrogen is an important particle because it is formed in nearly all electron impact collisions, and the H-abstraction reaction [199, 200] of (di)silane is an important process, as is seen from sensitivity study. Dissociation of SiHa can create different SiH (with x = 0, 1,2, 3) radicals. Only silylene (SiH2) and... 

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