Protonation of alkyls

Table 7. H-N.m.r. parameters of and the protons of alkyl groups in 210a-d and 212b-c (t values)...

Protonation of alkyl- or aryl-substituted germanides in NH by NH,Br produces germanes in high yields ... 

When the substrate was more difficult to reduce the rate of protonation of alkyl cinnamates in the presence of phenol in DMF [209] increased. For aryl cinnamates the competing rate-determining dimerization of the radical anions is predominant [214]. In MeOH proton transfer occurs as a monomolecular reaction with the hydrogen-bonded radical anion complex (first-order rate constant kprot varies from 2.9 X 1Q2 s- to 3.5 X 10 s ) [215]. 

The protons of alkyl groups in the a- and y-positions relative to the pyridine nitrogen atom are, to a great extent, apt to be detached because of delocalization of a negative charge of the carbanion in the heterocycle. In the case of NlT salts and N-oxides, this tendency evidently increases. 

The aromatic protons of alkyl substituted benzene rings usually appear as a single broad peak near 7.1 ppm or a complex multiplet in the range from 6.9-7.5 ppm for highly branched chains such as the tert-butyl group. Aliphatic groups shield the ortho aromatic hydrogens by a factor of about 0.34 ppm, as evidenced by the aromatic resonance of mesitylene (1,3,5-trimethyl benzene) which appears at 6.69 ppm. 

These data on the hydrogen exchange in the alkyl groups of arenium ions and their precursors cover the examples when both substituents located at the ring sp -hybridized carbon are not hydrogen. A similar exchange scheme, however, is also possible for ions formed upon protonation of alkyl-substituted aromatic compounds. 

The temperature dependence of the hyperfine splitting width due to P-protons of alkyl radicals shows much of a mobile character in the case of the polyethylene chain in UPEC than in the case of bulk systems, as shown in Sect. 5 and vibration appears to occur around the chain axis since data analysis suggested vibrational motion of methylene groups. 

Fig. 8.1. Variation of line width and difference in hyperfine splittings due to two methylene protons of alkyl radicals with observation temperature 0> broad line NMR line width , ESR line width , difference in hyperfine splittings (Ref. )...

The proton of terminal acetylenes is acidic (pKa= 25), thus they can be deprotonated to give acetylide anions which can undergo substitution reactions with alkyl halides, carbonyls, epoxides, etc. to give other acetylenes. 

The thioboration of terminal alkynes with 9-(alkylthio)-9-borabicyclo[3.3.1]-nonanes (9-RS-9-BBN) proceeds regio- and stereoselectively by catalysis of Pd(Ph,P)4 to produce the 9-[(Z)-2-(alkylthio)-l-alkeny)]-9-BBN derivative 667 in high yields. The protonation of the product 667 with MeOH affords the Markownikov adduct 668 of thiol to 1-alkyne. One-pot synthesis of alkenyl sulfide derivatives 669 via the Pd-catalyzed thioboration-cross-coupling sequence is also possible. Another preparative method for alkenyl sulfides is the Pd-catalyzed cross-coupling of 9-alkyl-9-BBN with l-bromo-l-phe-nylthioethene or 2-bromo-l-phenylthio-l-alkene[534]. 

The protonation of the nitrogen atom of thiazole induces a large increase in reactivity of the 2-position (193, 194). This is in contrast to the pyridine series, where the reactivity of both positions adjacent to the nitrogen atom are enhanced (194). The phenylation of conjugate acid of 5-alkylthiazoles may then be considered as a preparative route to alkyl-thiazoles. The results (isomer percent and overall reactivity) are indicated in Tables III-31 (196) and 01-32 (196). 

In the case of alkyl radicals [e.g., methyl radical (197, 198) and cyclohexyl radical (198)], their nucleophilic behaviour enhances the reactivity of the 2-position. Here it is necessary to have full protonation of the nitrogen atom and to use specific solvents and radical sources. 

Dehydrohalogenation of alkyl halides (Sections 5 14-5 16) Strong bases cause a proton and a halide to be lost from adjacent carbons of an alkyl halide to yield an alkene Regioselectivity is in accord with the Zaitsev rule The order of halide reactivity is I > Br > Cl > F A concerted E2 reaction pathway is followed carbocations are not involved and rearrangements do not occur An anti coplanar arrangement of the proton being removed and the halide being lost characterizes the transition state... 

Section 5 15 Dehydrohalogenation of alkyl halides by alkoxide bases is not compli cated by rearrangements because carbocations are not intermediates The mechanism is E2 It is a concerted process m which the base abstracts a proton from the p carbon while the bond between the halogen and the a carbon undergoes heterolytic cleavage... 

Alkyl sulfonates are derivatives of sulfonic acids m which the proton of the hydroxyl group is replaced by an alkyl group They are prepared by treating an alcohol with the appropriate sulfonyl chloride usually m the presence of pyridine... 

Because electrophilic attack on benzene is simply another reaction available to a carbocation other carbocation precursors can be used m place of alkyl halides For exam pie alkenes which are converted to carbocations by protonation can be used to alkyl ate benzene... 

Intermediate formation of formyl chloride is not necessary since the actual alkylating agent, HCO", can be produced by protonation of carbon monoxide or its complexes. However, it is difficult to obtain an equimolar mixture of anhydrous hydrogen chloride and carbon monoxide. Suitable laboratory preparations involve the reaction of chlorosulfonic acid with formic acid or the reaction of ben2oyl chloride with formic acid ... 

Cocatalysts of two types occur (/) proton-donor substances, such as hydroxy compounds and proton acids, and (2) cation-forming substances (other than proton), including alkyl and acyl haUdes which form carbocations and other donor substances leading to oxonium, sulfonium, halonium, etc, complexes. 

Nuclear Magnetic Resonance. The nmr spectmm of aromatic amines shows resonance attributable to the N—H protons and the protons of any A/-alkyl substituents that are present. The N—H protons usually absorb in the 5 3.6—4.7 range. The position of the resonance peak varies with the concentration of the amine and the nature of the solvent employed. In aromatic amines, the resonance associated with N—CH protons occurs near 5 3.0, somewhat further downfield than those in the aliphatic amines. 

The initiating step in these reactions is the attachment of a group to the sulfoxide oxygen to produce an activated intermediate (5). Suitable groups are proton, acyl, alkyl, or almost any of the groups that also initiate the oxidations of alcohols with DMSO (40,48). In a reaction, eg, the one between DMSO and acetic anhydride, the second step is removal of a proton from an a-carbon to give an yUde (6). Release of an acetate ion generates the sulfur-stabilized carbonium ion (7), and the addition of acetate ion to the carbonium ion (7) results in the product (eq. 15) ... 

Other approaches to inhibiting intramolecular cycli2ations of erythromycin have also proven successhil. Erom a series of O-alkyl derivatives of erythromycin, clarithromycin (6-0-methylerythromycin) (37) was selected for clinical development (146,147). Another approach replaced the C-8 proton of erythromycin with duorine, which was accompHshed by both chemical and bioconversion methods to yield durithromycin (38) (148). 

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