Proton transfer to olefins

There are a number of reactions which involve slow proton transfer to olefinic carbon as a first step followed by other fast steps which depend upon the olefin which is studied [42]. The mechanism (A—SE2) is shown in eqn. (21), viz. 

General acid catalysis is also clearly observable [47] in the hydration of cyanoketene dimethylacetal and other ketene acetals (24), viz. 

The evidence for the operation of the A—SE 2 mechanism in the hydration of olefins and other related reactions has been reviewed [42]. 

In a recent temperature-jump study [48] of the methanolysis of l,l-bis(p-dimethylaminophenyl)ethylene all rate coefficients shown in scheme (27) were measured and the mechanism was clearly identified. 

General acids and bases can take part in the first proton transfer. 

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The mechanism of hydrolysis of vinyl ethers (28) resembles the mechanism of hydration of olefins and has been studied extensively to obtain kinetic results for simple proton transfer to carbon [49]. As in many of the examples previously discussed in this section, the overall reaction is conveniently followed spectrophotometrically and the measured second-order rate coefficient refers to the rate coefficient for proton transfer to olefinic carbon (feH A ). 

In this section rate-equilibrium correlations for proton transfer to olefins and aromatic systems will be discussed. Although the kinetic behaviour varies from one unsaturated system to another some general features will become apparent. Most results for proton transfer involving unsaturated carbon have been obtained by studies of an overall reaction in which proton transfer to carbon is involved as a rate-determining step. The mechanisms of reactions of this type were discussed in Sects. 2.2.3 and 2.2.4. In these cases the rate coefficient for proton addition to form a carbonium ion is obtained. However, a few examples will be described where the equilibrium between an unsaturated system and a carbonium ion has been measured giving rate coefficients in both directions. 

The proposed reaction mechanism involves intermolecular nucleophilic addition of the amido ligand to the olefin to produce a zwitterionic intermediate, followed by proton transfer to form a new copper amido complex. Reaction with additional amine (presnmably via coordination to Cn) yields the hydroamination prodnct and regenerates the original copper catalyst (Scheme 2.15). In addition to the NHC complexes 94 and 95, copper amido complexes with the chelating diphosphine l,2-bis-(di-tert-bntylphosphino)-ethane also catalyse the reaction [81, 82]. 

The solvent isotope effect produces an A-ratio (HOH/DOD) of three with isotope-independent A// of 17-18 kJ/mol. This result is more difficult to interpret, because it is unknown how many isotopic sites in the enzyme or water structure contribute to the isotope effect of 2-3. If a single site should be the origin of the effect, then the site could reasonably be a solvent-derived protonic site of the enzyme involved in general-acid catalysis of the hydride transfer, most simply by protonic interaction with the carbonyl oxygen of cyclohexenone or possibly by proton transfer to an olefinic carbon of cyclohexenone. 

The general reaction mechanism of the Michael reaction is given below (Scheme 4). First, deprotonation of the Michael donor occurs to form a reactive nucleophile (A, C). This adds enantioselectively to the electron-deficient olefin under the action of the chiral catalyst. In the final step, proton transfer to the developed enolate (B, D) occurs from either a Michael donor or the conjugate acid of a catalyst or a base, affording the desired Michael adduct. It is noteworthy that the large difference of stability between the two enolate anions (A/B, C/D) is the driving force for the completion of the catalytic cycle. 

Proton transfer to paraffinic carbocations and self-alkylation of light olefins Q + iC4Hio + iC4Hg... 

The mechanism of this cyclization involves a conjugate addition of the enamine (100) to the nitroallyl ester (101) to give 102, which on elimination produced 103. The immonium salt 103 undergoes proton transfer to give enamino nitro olefin 104, which cyclizes to an enamine (107) via 105 and 106. Hydrolysis of 107 produces the ketone (108). Depending on the reaction conditions and the structure of the enamine and nitroolefin components employed, intermediates can be isolated (equation 19). 

A test of the validity of the hypothesis that the active site is a carbenium ion residue was carried out in the following manner (55) If the transformation of n-butenes into isobutylene via a monomolecular reaction is initiated by a simple proton transfer to the reactant, this transformation will involve an unstable primary carbenium ion, as shown previously. In contrast, for a larger olefin like pentene, the monomolecular reaction will occur via a secondary carbenium ion rather than a primary carbenium ion and take place more rapidly as follows ... 

First, the enzyme has at least two and probably three active-site basic groups involved in proton transfers to and from substrates, intermediates, and nascent products and all three bases are located on the si face of the substrate-PLP aldimine system as are the protons to be shuffled about, so all the proton transfers are likely to be economically suprafacial. Several pieces of stereochemical evidence suggest that the j5,y-olefinic PLP-p-quinoidal-a-anion (141) can rotate around its C(P)-C(ol) bond and also implicate that the cisoid isomer of this n complex and then the Z-isomer of the nascent aminocrotonate carry 80 % of the reaction flux. Furthermore, a 15% internal retention of the from the Pro-R methylene of ACPC (9) on B2H (85 % exchange with solvent, 15 % internal return) in the active site and the overall 22/78 H /H5 distribution at C(3) of the mono- and dideutero 2-ketobutyrate (138) products at C(3) are also noted. 

In the presence of alkenes, an excited ketone can become a sufficiently strongly oxidizing agent to abstract an electron from an alkene, leading to a radical-ion pair. A typical example of a ketone-olefin reaction which proceeds via PET and proton transfer to produce coupling products is shown in Scheme 5 [10]. 

Intramolecular addition reactions of arenes and aryl olefins with secondary and primary amines have proven to be of broader scope than the analogous reactions with tertiary amines. The intramolecular addition of nonconjugated o-allylanilines 51 to yield the 2-methylindolines 52 was reported by Koch-Pomeranz et al. in 1977. Intramolecular electron transfer from the singlet aniline to the ground state alkene followed by N-H proton transfer to the alkene terminal carbon was proposed to account for the regioselective formation of indolines. Proton transfer to the internal carbon would yield tetrahydroquinolines, which were... 

The alkenyl ion written in (14) has the ally lie structure, but alkenyl ions which are not allylic can also be formed in exothermic reactions. One can also write exothermic reactions for the formation of alkenyl ions which involve proton transfer to the olefin followed by loss of hydrogen molecule. All these reactions are strongly exothermic, and consequently extensive fragmentation of the (M H- 1)" and (M — I)" " ions is to be expected and is observed. 

Other pathways are available for hydroarylation of heterocycles. A mechanistic study by Bergman and Elhnan was conducted on intramolecular reactions of imidazole-type heterocycles. These mechanistic studies have shown that the C-H activation of the imidazole is followed by isomerization to generate an N-heterocyclic carbene ligand (Equation 18.60). Following this isomerization, the olefin appears to couple with the carbene by a [2+2] process to generate the carbon-carbon bond, followed by proton transfer to generate a rhodium hydride and reductive elimination to form the C-H bond. 

By trapping PX at liquid nitrogen temperature and transferring it to THF at —80° C, the nmr spectmm could be observed (9). It consists of two sharp peaks of equal area at chemical shifts of 5.10 and 6.49 ppm downfield from tetramethylsilane (TMS). The fact that any sharp peaks are observed at all attests to the absence of any significant concentration of unpaired electron spins, such as those that would be contributed by the biradical (11). Furthermore, the chemical shift of the ring protons, 6.49 ppm, is well upheld from the typical aromatic range and more characteristic of an oletinic proton. Thus the olefin stmcture (1) for PX is also supported by nmr. 

A low ion pair yield of products resulting from hydride transfer reactions is also noted when the additive molecules are unsaturated. Table I indicates, however, that hydride transfer reactions between alkyl ions and olefins do occur to some extent. The reduced yield can be accounted for by the occurrence of two additional reactions between alkyl ions and unsaturated hydrocarbon molecules—namely, proton transfer and condensation reactions, both of which will be discussed later. The total reaction rate of an ion with an olefin is much higher than reaction with a saturated molecule of comparable size. For example, the propyl ion reacts with cyclopentene and cyclohexene at rates which are, respectively, 3.05 and 3.07 times greater than the rate of hydride transfer with cyclobutane. This observation can probably be accounted for by a higher collision cross-section and /or a transmission coefficient for reaction which is close to unity. 

Several types of proton transfer reactions can be studied conveniently by a neutral product analysis. Until now, the most extensive investigations have been concerned with (1) proton transfer from H3+ and CH5 + to various hydrocarbon molecules, and (2) the transfer of a proton from carbonium ions to larger olefins or other organic compounds. 

A similar proton transfer from a growing chain end unit to give an olefinic linkage was observed in the cationic polymerization of 2-tert-butyl-7-oxabicycto[2.2.1 ]-heptane, although the proton liberated did not initiate the polymerization and hence this process was actually a termination34 . 

The organic substrates in Chart 8 can be divided into two main categories in which (i) the oxidation of olefins, sulfides, and selenides involves oxygen atom transfer to yield epoxides, sulfoxides, and selenoxides, respectively, whereas (ii) the oxidation of hydroquinones and quinone dioximes formally involves loss of two electrons and two protons to yield quinones and dinitrosobenzenes, respectively. In order to provide a unifying mechanistic theme for the seemingly disparate transformations in Chart 8, we note that nitrogen dioxide exists in equilibrium with its dimeric forms, namely, the predominant N—N bonded dimer 02N—N02 and the minor N—O bonded isomer ONO—N02 (equation 88). 

A close comparison between intramolecular proton transfer and intramolecular nucleophilic attack at carbon is provided by the varying amounts of olefins [46] which accompany the ring-closure reactions of 0--OC6H4O(CH2) 4Br [1] to the corresponding catechol polymethylene ethers [2] (Illuminati etal., 1975). 

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