Protons production

In the UV spectrum of the protonation products there is a hypsochromic shift of the absorption maximum of enyne system compared to the bases (74DIS) this agrees with the data of the protonation of simple enamines and dienic amines (69MI1). 

Notice in the list of Lewis bases just given that some compounds, such as carboxylic acids, esters, and amides, have more than one atom ivith a lone pair of electrons and can therefore react at more than one site. Acetic acid, for example, can be protonated either on the doubly bonded oxygen atom or on the singly bonded oxygen atom. Reaction normally occurs only once in such instances, and the more stable of the two possible protonation products is formed. For acetic add, protonation by reaction with sulfuric acid occurs on... 

On reaction with acid, 4-pvrone is protonated on the carbonyl-group oxygen to give a stable cationic product. Using resonance structures and the Hiickel 4n 4- 2 rule, explain why the protonated product is so stable. 

Even with a limited amount of the alkylating agent, the equilibria between protonated product and the neutral starting amine are sufficiently fast that a mixture of products may be obtained. For this reason, when monoalkylation of an amine is desired, the reaction is usually best carried out by reductive amination, a reaction that is discussed in Chapter 5. If complete alkylation to the quaternary salt is desired, use of excess alkylating agent and a base to neutralize the liberated acid normally results in complete reaction. 

Selective reduction of the cyclopropenone carbonyl group to a CH2 group has been described for diphenyl cyclopropenone utilizing its protonation product 294 or the diphenyl chloro cyclopropenium cation 292, which yielded 1,2-diphenyl-A1,2-cyclopropene (293) on treatment with trimethylamine borane210 ... 

X = 0, CH2, CHCOOH, C(COOH)2, NH, NCH3 N(CH2CH=CH2), N(CHs)2 Cl Bobrowski and Das published a series of papers on the transients in the pulse radiolysis of retinyl polyenes31-37, due to their importance in a variety of biomolecular processes. They studied32 the kinetics and mechanisms of protonation reaction. The protons were released by pulse radiolysis, on a nanosecond time scale, of 2-propanol air-saturated solutions containing, in addition to the retinyl polyenes, also 0.5 M acetone and 0.2 M CCI4. Within less than 300 ns, the electron beam pulse results in formation of HC1. The protonated products of retinyl polyenes were found to absorb optically with Xmax at the range of 475-585 nm and were measured by this absorption. They found that the protonation rate constants of polyene s Schiff bases depend on the polyene chain... 

FIGURE 2.38. Yields of electron transfer + protonation product vs. H-atom transfer product (Scheme 2.22) in constant-potential exhaustive electrolytes as a function of the competition parameter, a ECE electron transfer b DISP electron transfer. 

Figure 4.J. Proton production upon hydrolysis and hydrogenation of lrans [RhCl(CO)(TPPMS)2] at various constant pH in the 4 < pH < 10 range. [Rh] = 2.4 x 10" M, [TPPMS] = 7.2x10 M, 1 bar Ar or H, T = 35 "C, Reprinted with permission from Chem. Eur. J. 2001, 7, 193. Copyright (2001) Wiley-VCH VerlagGmbH.

Protonation to the conjugate acid (iminium cation) increases the potential of the itnine to act as an electrophile (compare carbonyl see Section 7.1), and this is followed by nucleophilic attack of water. The protonated product is in equilibrium with the other mono-protonated species in which the nitrogen carries the charge. We shall meet this mechanistic feature from time to time, and it is usually represented in a mechanism simply by putting H+, +H+ over the equilibrium arrows. Do not interpret this as an internal transfer of a proton such transfer would not be possible, and it is necessary to have solvent to supply and remove protons. 

The pattern in Figure 10.10 contains four energy minima occupied by the initial hydride, the dihydrogen-bonded complex, the intimate ion pair, and the solvent-separated ion pair as a final product of the reaction. For simplicity, the homo-conjugated species [RO- -H- OR] , assisting proton transfer and stabilizing the protonated products, can be ruled out. 

It is worth noting that use of i-PrMgCl in ether (not in THF) and toluene as solvent for the reaction is essential for the reaction to proceed. Otherwise, the protonated product of magnesium carbenoid 78, obtained as by-product, was very difficult to separate from the desired compound (79). Interestingly, the 1,3-C,H insertion reaction is highly regioselective. 

Table 2. Competition between C-protonation (formation of nitro compounds 45) and O-proton-ation (formation of the Nef-product 47) in the protonation of nitronate ions 44 (s. Scheme 8). The percentage of C-protonated product 45 for different buffers (pyridine/p-toluenesulfonic acid) and unbuffered p-toluenesulfonic acid (48) is listed...

In contrast to X -phosphorins, X -phosphorins can be protonated. The basicity is very much influenced by the nature of the substituents R and R at the phosphorus. 1.1-Dialkyl or 1.1-diaryl-X -phosphorins are even protonated by aqueous HCl the salts are deprotonated by aqueous NaOH. Strong acids in organic solvents, e. g. trifluoroacetic acid in hexane or benzene, (see p. 106), are required to proto-nate 1.1-dialkoxy-X -phosphorins. Addition of tert-butoxide deprotonates the salt. By studying the NMR spectra of 1. l-dimethoxy-2.4.6-tris-pentadeuterophenyl-X -phosphorin 185 in benzene solutions containing H and D-trifluoroacetic acid Stade could show that two different protonation products are formed in a ratio of 3 1. One product is the result of C—2 protonation 186 the other of C-4 protonation 187 (Fig. 38). Similar results were observed in the case of 1.1-bis-dimethylamino-2.4.6-triphenyl-X -phosphorin... 

Reductive decyanations of 2-cyanotetrahydropyran derivatives with sodium in ammonia yield predominantly axially protonated products. The observations are consistent with the reductive decyanation proceeding via the pyramidal, axial radical which accepts a second electron to give a configurationally stable carbanion, which in turn abstracts a proton from ammonia with retention of configuration (Rych-novsky, S. D. Powers, J. P. LePage, T. J., J. Am. Chem. Soc., 1992, 114, 8375-8384). Provide an explanation for the axial preference of the intermediate free radical on the basis of orbital interactions. Hint The title of the paper by Rychnovsky et al. is Conformation and Reactivity of Anomeric Radicals. ... 

It should be mentioned that with superacidic electrophilic oxygenation of methane either to methanol (with protonated hydrogen peroxide) or to formaldehyde (with protonated ozone), the products formed are indeed the protonated products (CH3OH2 and CH2=OH+, respectively), which are protected from further electrophilic oxygenation, which happens only too readily in conventional oxidations. 

The protonation of [Ru3(CO)10(NO)] shows a dependence on the acid.131 Whilst CF S03H leads to the O-protonated product, CF3C02H gives the species HRu3(CO)10(NO) in which H+ addition has occurred at the metal unit. The two isomeric forms do not interconvert in inert solvents such as hexane, ether and CH2C12, However, addition of a catalytic amount of [CF3C02]- causes the rapid O—H to M—H rearrangement. 

A beautiful extension of this reaction has recently been communicated by Nozaki, Oshima, and Utimo-to.184 These workers simply admixed f-butyl iodide (3 equiv.), benzaldehyde (1 equiv.), methyl vinyl ketone (1 equiv.) and triethylborane (1 equiv.) in benzene (Scheme 60). After 5 min at 25 C, the reaction was subjected to standard extractive work-up and the crude product was purified by chromatography to give (54) in 63% yield. If methanol is substituted for benzaldehyde, the protonated product (55) is isolated in 79% yield. Although enones are equivalents of synthon (56), such a direct coupling of radical and ionic reactions had not been achieved previously. 

The most important feature of organocobalt cyclizations is that a variety of functionalized products can be obtained, depending on the nature of the substrate and the reaction conditions. The most common transformation has been formation of an alkene by cobalt hydride elimination. Alkenes are often formed in situ during the photolysis, and with activated alkene acceptors the formation of these products by cobalt hydride elimination is very facile. Scheme 31 provides a representative example from the work of Baldwin and Li.143 The alkene that is formed by cobalt hydride elimination maintains the correct oxidation state in the product (54) for formation of the pyrimidone ring of acromelic acid. Under acidic conditions, protonation of the cyclic organocobalt compound may compete 144 however, if protonated products are desired, the cyclization can probably be conducted by the reductive method with only catalytic quantities of cobalt (see Section 4.2.2.2.2). 

Even though radical intermediates are involved in many reductive metallations, cyclizations of alkyl radicals by reductive trapping methods are relatively uncommon. When protonated products are formed, the overall transformations are similar to a tin hydride reductions however, reductive methods may facilitate purification of the cyclic product because tin by-products are absent Two examples of simple reductive cyclizations are shown in Scheme 35. The first example illustrates that Sheffold s procedure, which uses catalytic quantities of vitamin B12 with a chemical or electrochemical coreductant, is attrac-... 

Protonation of the side chain in arginine occurs on the double-bonded nitrogen atom. Draw three resonance structures of the protonated product. 

A novel system for the enantioconvergent decarboxylative protonation of racemic /3-kclo esters has been developed.48 The reaction tolerates a variety of substitution and functionality and delivers products of high enantiopurity in excellent yield. The enan-tioinduction in the observed protonated products is consistent with the intermediacy of an enolate that is intimately associated with a chiral Pd complex. 

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