Benzyl carbonate cleavage

Nitrogen benzylic carbon cleavage The deprotection conditions for the reductive amination products of phenylethylamine, also known as a methylbenzylamine, are Pd C (0.1 0.5mol% Pd), 8 12 bar (120 180psi) H2, MeOH, and 60 °C. Room temperature can be used, but then the Pd catalyst loading must be greatly increased 1.0 10.0 mol% Pd. 

Substitution Reactions on Side Chains. Because the benzyl carbon is the most reactive site on the propanoid side chain, many substitution reactions occur at this position. Typically, substitution reactions occur by attack of a nucleophilic reagent on a benzyl carbon present in the form of a carbonium ion or a methine group in a quinonemethide stmeture. In a reversal of the ether cleavage reactions described, benzyl alcohols and ethers may be transformed to alkyl or aryl ethers by acid-catalyzed etherifications or transetherifications with alcohol or phenol. The conversion of a benzyl alcohol or ether to a sulfonic acid group is among the most important side chain modification reactions because it is essential to the solubilization of lignin in the sulfite pulping process (17). 

H2/Pd-C, EtOH, 20°. < -Bromobenzyl carbonates have been developed for use in solid-phase peptide synthesis. An aryl o-bromobenzyl carbonate is stable to acidic cleavage (CF3CO2H) of a /-butyl carbamate a benzyl carbonate is cleaved. 

These results indicate that the aliphatic portion of the coal is very important in the initial phases of coal conversion. Weak linkages must be associated with the aliphatics in coal though they have not as yet been completely identified. Both of the above methods show an increase in the aromatic methyl content of SRCs at short times which indicates that cleavage at a benzylic carbon is important in dissolving the coal. 

The initial C-0 bond cleavage (by attack of a/the bromide ion at the benzylic carbon, followed by recyclization of the intermediate 301 by selective alkylation at sulfur) was suggested to be responsible for a stereospecific rearrangement of 2-substituted-1,3,2 -oxazaphospholidine-2-thiones 299 derived from (-) pseudoephedrine into... 

The addition of two ortho-methyl groups to cumyl derivatives also results in a decrease in the Hammett reaction constant from p+ = —4.3 for cleavage of X-[8]-(4-nitrobenzoate) to p+ = —3.0 for cleavage of X-[10]-(4-methoxy-benzoate).27 This provides additional support for the conclusion that rotation about the CAr—Ca bond, which will minimize destabilizing steric interactions between methyl groups in the transition state, also reduces stabilizing resonance electron donation from the ring substituent to the benzylic carbon (Scheme 9). 

Benzylsilanes are also oxidized under similar conditions with selective cleavage of the C Si bond (Scheme 4) [24,25], Nucleophiles are introduced onto the benzylic carbon exclusively. Nuclear substitution products are not formed. The high chemoselectivity observed for the reaction of p-(trimethylsilyl)benzyltrimethylsilane is interesting. Only the benzyl C-Si bond is cleaved without affecting the aromatic C-Si bond. 

Thermolytic Cleavage of Allvlic and Benzvlic Ethers and Polvethers. The thermal lability of the types of ethers which are of interest in the context of this study was discovered during a thorough study of the application of certain benzylic carbonates as labile protecting groups for alcohols and phenols [IS]. It was observed that, under acid catalysis, the bis-methylcarbonate derivative of l-(4-hydroxyphenyl)ethanol (4, in Equation 1) was transformed into the benzylic ether (5) which then underwent clean acid-catalyzed thermolysis to the corresponding styrene (6) in very high yield [14]. 

In this reaction, only the benzylic carbonate can react readily since cleavage of the benzylic carbon-oxygen bond leads to a stabilized carbocation while no such stabilization would exist for the hypothetical products which would be obtained by cleavage of the phenyl carbonate. The benzylic carbocation intermediate which is formed can either eliminate to the corresponding styrene 4 or recombine with the nucleophilic methanol which is formed by decarboxylation. It is the latter reaction which appears to prevail at room temperature as the ether 2 can be isolated in excellent yield. Subsequent heating in the presence of acid catalyst drives the reaction to the elimination product 6 and free methanol. 

Cleavage of a benzylic carbon-carbon bond is observed from aromatic radical-cations in cases where the resulting fragments are stabilised by substituents. Bond cleavage occurs, for example, with dialkylphenylcarbinols involving loss of the more stable alkyl radical and leaving a phenyl alkyl ketone [85, 86],... 

This electrochemical oxidation mediated by NHPI was applicable to benzylic carbons, allylic carbons, deprotection of acetals, oxidative cleavage of cyclic acetals and amide to afford benzoylated compounds, enones ", carbonyl compounds, -hydroxyethyl esters and imides, respectively (equations 31-35). 

In a number of nonenzymatic reactions catalyzed by pyridoxal, a metal ion complex is formed—a combination of a multivalent metal ion such as cupric oi aluminum ion with the Schiff base formed from the combination of an amino acid and pyridoxal (I). The electrostatic effect of the metal ion, as well as the electron sink of the pyridinium ion, facilitates the removal of an a -hydrogen atom to form the tautomeric Schiff base, II. Schiff base II is capable of a number of reactions characteristic of pyridoxal systems. Since the former asymmetric center of the amino acid has lost its asymmetry, donation of a proton to that center followed by hydrolytic cleavage of the system will result in racemic amino acid. On the other hand, donation of a proton to the benzylic carbon atom followed by hydrolytic cleavage of the system will result in a transamination reaction—that is, the amino acid will be converted to a keto acid and pyridoxal will be converted to pyridoxamine. Decarboxylation of the original amino acid can occur instead of the initial loss of a proton. In either case, a pair of electrons must be absorbed by the pyridoxal system, and in each case, the electrostatic effect of the metal ion facilitates this electron movement, as well as the subsequent hydrolytic cleavage (40, 43). 

Examples of the cleavage of support-bound carbonates are given in Table 3.36. Depending on the structure of the carbonate, acidolytic, base-induced, nucleophilic, or photolytic cleavage can be used to release the alcohol. Acidolysis of the benzylic C-O bond of resin-bound benzyl carbonates leads to the release of an unstable carbonic acid ester, which undergoes decarboxylation to yield the alcohol. 

The kinetic deuterium isotope effect indicates that C-H bond cleavage is paid of the rate-determining step and the negative p value shows that there is electron deficiency being produced on the benzylic carbon in the activated complex. Therefore... 

Eaborn and co-workers28 have reported product isotope ratios for the cleavage of benzyl-silicon, benzyl-tin, and aryl-tin bonds using an equimolar mixture of MeOH and MeOD as the solvent. A free carbanion would not be expected to discriminate between MeOH and MeOD in the fast step (24), and hence28 the product isotope ratio kH/kD should be unity. The values of kJkD in Table 11 indicate that the carbanions are not entirely free, but that some degree of electrophilic attack by the solvent at the benzyl carbon atom takes place, as in (V). 

The base peak in the mass spectrum of alkylbenzenes corresponds to carbon-carbon bond cleavage at the benzylic carbon. 

Styrene oxides can be reduced selectively to either 2-arylethanols or 1-arylethan-ols. Attack of hydride at the non-benzylic carbon atom can sometimes be achieved with LiAlH4, but most reducing agents, in particular under acidic reaction conditions, will lead to cleavage of the benzylic C-O bond (Scheme 4.62). 

Cleavage of (2S,3S)-3-phenylglycidol (1). This epoxide (1) is cleaved by a number of nucleophiles regioselectively at the benzylic carbon to give 1,2-diols. The cleavage usually occurs with almost complete inversion at the benzylic center (>99 1). Trimethylaluminum, however, reacts with retention (32 1, equation I). Triethylaluminum reacts with 1 to give the corresponding adducts in a 1 1 ratio. 

Further investigations on the field of oxidative bond cleavage even made single bonds accessible. Thus, biaryls 10 and 11 were similarly obtained by electrooxidation of 9,10-dihydrophenanthrene. Moreover, the cleaving reaction of benzylic carbons was also exploited in the synthesis p-tert-butylbenzaldehyde dimethyl acetale (3) starting from l,2-di-(p-ferf-butylphenyl)ethane (4, 1,2-DPTE) (Fig. 5.8) (Zollinger et al. 2004b). 

Electrolytic cleavage, 3 (review) of benzamides, to form amines, 356 of benzoate esters, to form alcohols, 102 of benzyl carbamates, to form amines, 336 of benzyl carbonates, to form alcohols, 109 of benzyl esters, to form carboxyl groups, 251... 

Compounds containing aromatic rings tend to fragment at the carbon (called a benzylic carbon) next to the aromatic ring. Such a cleavage forms a resonance-stabilized benzylic cation. 

---