Protonation of olefins

Note Isoalkyl cations formed by fragmentation and protonation of olefins generally contain 4 to 10 carbon atoms. Some t-C.Hg+ s are formed In this manner, as are both DfH+ s and T s. This method Is thought to be the major method for production of the precursors for LE s and for DMH s (when sulfuric acid Is used as the catalyst). 

The expected change in Bronsted exponent with change in reactivity is illustrated by the results [49] shown in Table 9 for the hydrolysis of vinyl ethers (mono alkoxy-activated olefins) which occurs by initial slow protonation of olefinic carbon as in mechanism (28). The value of R which is the catalytic coefficient for an acid of pK 4.0 calculated from results for carboxylic acids with pK around 4.0 is taken as a measure of the reactivity of the system. The correlation of a with reactivity is scattered but the trend is in the expected direction. The results are quite similar to those shown for the ionization of ketones in Table 2. For the proton transfers shown in Table 9 the Bronsted exponent has not reached the limiting value of zero or unity even when reaction in one direction is very strongly thermodynamically favourable. The rate coefficient in the favourable direction is probably well below the diffusion limit, although this cannot be checked for the vinyl ethers. Non-limiting values for the Bronsted exponent have also been measured in the hydrolysis of other vinyl ethers [176]. 

Equilibrium protonation to give the carbonium ion can be observed in 80 % (v/v) methanol—water containing buffers. Proton removal from the carbonium ion (pX ca. 2.7 in this solvent) by acetate and chloroacetate is thermodynamically favourable but occurs with rate coefficients of 19.1 and 4.7 1 mole-1 sec-1, respectively, which are well below the values which would be found for normal proton transfer. Protonation of the olefin by hydrogen ion is thermodynamically favourable but occurs slowly with rate coefficient 23 1 mole-1 sec-1. These results clearly show that protonation of olefinic carbon belongs to the category of slow proton transfers. 

Despite the vast amount of work which has recently been done on the protonation of olefins in the context of the search and characterisation of carbenium ions, very little information is available on the kinetics of this proton transfer process in non-aqueous media. A study of this nature requires the use of an analytical stem capable of detecting quantitatively small concentrations of carbenium ions and possessing an adequate time response. Two t es of investigation have been carried out, namely those using polymerisable monomers and those using olefins which cannot be polymerised. We will not analyse here reactions in such basic sdvents as methanol, althou an excellent study has been conducted in this medium °° because they are outside our scope. 

There are no data available on the rate of formation of dialkyloxonium ions like protonated 1,3-dioxolane in Eq. (16). It is remarkable that the rate constants of formation of secondary onium ions from linear ethers, acetals, sulfides etc. are also utdcnovra. These should however be lower than the rate constants of proton transfer in water (an upper limit) being close to 10 mole 1 s but certainly higher than the rate constantsof protonation of olefins... 

Ferruginol (2) is a simple example. Disconnection of the central ring by riedel-Crafts reaction requires a carbonium ion which can be made by protonation of olefin <3). 

If the activity coefficient ratios of these different types of indicator vary in an analogous way with changes in medium acidity, the Hr and the equilibrium constant for protonation of olefins can be related to Hu (see 192-194). 

These relationships however, are not observed and protonation of olefins defines a new acidity function. Hr, viz-... 

Protonation of olefins to carbenium ions on acid sites,... 

The carbenium ions are initially formed from the supposed protonation of olefins, present as impurities or from thermal cracking, at the Bronsted acid sites or by hydride abstraction from a paraffin on a Lewis acid site. Once a carbenium ion is formed, cracking occurs by scission of the carbon chain at a bond located ft to the charged carbon atom to give an a-olefm and a smaller primary carbenium ion ... 

Pitcher, Buckingham, and Stone 285) have discussed the anomalous chemical shift of fluorine atoms bonded to the a-carbon atom of perfluoro-alkyl-transition metal derivatives in terms of mixing of nonbonding electrons of the halogen with orbitals of the metal. Bennett, Pratt, and Wilkinson (27) have discussed the shielding of protons of olefins in the complexes of these ligands with transition metals. 

Proton-catalyzed olefin cyclizations of open-chain educts may give tri- or tetracyclic products but low yields are typical (E.E. van Tamelen, 1968, 1977 see p. 91). More useful are cyclizations of monocyclic educts with appropriate side-chains. The chiral centre to which the chain is attached may direct the steric course of the cyclization, and several asymmetric centres may be formed stereoselectively since the cyclizations usually lead to traas-fused rings. 

The following acid-catalyzed cyclizations leading to steroid hormone precursors exemplify some important facts an acetylenic bond is less nucleophilic than an olelinic bond acetylenic bonds tend to form cyclopentane rather than cyclohexane derivatives, if there is a choice in proton-catalyzed olefin cyclizations the thermodynamically most stable Irons connection of cyclohexane rings is obtained selectively electroneutral nucleophilic agents such as ethylene carbonate can be used to terminate the cationic cyclization process forming stable enol derivatives which can be hydrolyzed to carbonyl compounds without this nucleophile and with trifluoroacetic acid the corresponding enol ester may be obtained (M.B. Gravestock, 1978, A,B P.E. Peterson, 1969). 

In the NMR spectrum of cis-l,2-bis[2-diethylamino-5-nitrothiazol-4-yl] ethylene (17) (1570), the nonequivalence of olefinic protons requires that the rotation of the NO2 group be hindered. 

Polar solvents shift the keto enol equilibrium toward the enol form (174b). Thus the NMR spectrum in DMSO of 2-phenyl-A-2-thiazoline-4-one is composed of three main signals +10.7 ppm (enolic proton). 7.7 ppm (aromatic protons), and 6.2 ppm (olefinic proton) associated with the enol form and a small signal associated with less than 10% of the keto form. In acetone, equal amounts of keto and enol forms were found (104). In general, a-methylene protons of keto forms appear at approximately 3.5 to 4.3 ppm as an AB spectra or a singlet (386, 419). A coupling constant, Jab - 15.5 Hz, has been reported for 2-[(S-carboxymethyl)thioimidyl]-A-2-thiazoline-4-one 175 (Scheme 92) (419). This high J b value could be of some help in the discussion on the structure of 178 (p. 423). 

The olefinic proton of the enol form emerges as a sharp singlet in the region 6.2 to 7.5 ppm (DMSO) (386). while the 5-methyl protons appear at approximately 2.2 ppm. 

Acid—Base Chemistry. Acetic acid dissociates in water, pK = 4.76 at 25°C. It is a mild acid which can be used for analysis of bases too weak to detect in water (26). It readily neutralizes the ordinary hydroxides of the alkaU metals and the alkaline earths to form the corresponding acetates. When the cmde material pyroligneous acid is neutralized with limestone or magnesia the commercial acetate of lime or acetate of magnesia is obtained (7). Acetic acid accepts protons only from the strongest acids such as nitric acid and sulfuric acid. Other acids exhibit very powerful, superacid properties in acetic acid solutions and are thus useful catalysts for esterifications of olefins and alcohols (27). Nitrations conducted in acetic acid solvent are effected because of the formation of the nitronium ion, NO Hexamethylenetetramine [100-97-0] may be nitrated in acetic acid solvent to yield the explosive cycl o trim ethyl en etrin itram in e [121 -82-4] also known as cyclonit or RDX. 

The initial step is the protonation of the aldehyde—e.g. formaldehyde—at the carbonyl oxygen. The hydroxycarbenium ion 6 is thus formed as reactive species, which reacts as electrophile with the carbon-carbon double bond of the olefinic substrate by formation of a carbenium ion species 7. A subsequent loss of a proton from 7 leads to formation of an allylic alcohol 4, while reaction with water, followed by loss of a proton, leads to formation of a 1,3-diol 3 " ... 

The polymerization reaction starts hy protonating the olefin and forming a carhocation. For example, protonating propene gives isopropyl car-hocation. The proton is provided hy the ionization of phosphoric acid ... 

Strong protonic acids can affect the polymerization of olefins (Chapter 3). Lewis acids, such as AICI3 or BF3, can also initiate polymerization. In this case, a trace amount of a proton donor (cocatalyst), such as water or methanol, is normally required. For example, water combined with BF3 forms a complex that provides the protons for the polymerization reaction. 

Micellar catalysis to enhance or diminish the rate of chemical reactions is well known [97]. Of somewhat greater interest is the influence of micelles on competing reactions, e.g., proton-catalyzed reactions. An example related to the effect of alkanesulfonates is the epoxidation of simple aliphatic olefins. The reaction of olefins and hydrogen peroxide catalyzed by strongly acidic Mo(VI)... 

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