Benzyl carbonate

The unpaired electron in benzyl radical is shared by the benzylic carbon and by the nng carbons that are ortho and para to it as shown by the spin density surface in Figure 119 Delocalization of the unpaired electron from the benzylic carbon to the ortho and para positions can be explained on the basis of resonance contributions from the fol lowing structures... 

In orbital terms as represented m Figure 11 10 benzyl radical is stabilized by delo cahzation of electrons throughout the extended tt system formed by overlap of the p orbital of the benzylic carbon with the rr system of the ring... 

FIGURE 11 10 The lowest energy tt molecular orbital of benzyl radical shows the interaction of the 2p orbital of the benzylic carbon with the TT system of the aromatic ring... 

The positive charge m benzyl cation is shared by the carbons ortho and para to the benzylic carbon... 

FIGURE 1111 Cham propagation in polymerization of styrene The growing polymer chain has a free radical site at the benzylic carbon It adds to a molecule of styrene to extend the chain by one styrene unit The new polymer chain is also a benzylic radical it attacks another molecule of styrene and the process repeats over and over again... 

Section 11 16 Addition reactions to alkenylbenzenes occur at the double bond of the alkenyl substituent and the regioselectivity of electrophilic addition is governed by carbocation formation at the benzylic carbon See Table 11 2... 

Electrophilic addition (Section 11 16) An aryl group stabilizes a benzylic carbocation and con trols the regioselectivity of addition to a double bond involving the benzylic carbon Markovni kov s rule is obeyed... 

FIGURE 27 5 Tyrosine is the biosynthetic precursor to a number of neurotransmit ters Each transformation IS enzyme catalyzed Hydroxy lation of the aromatic ring of tyrosine converts it to 3 4 dihyd roxyphenylalanine (l dopa) decarboxylation of which gives dopamine Hy droxylation of the benzylic carbon of dopamine con verts It to norepinephrine (noradrenaline) and methy lation of the ammo group of norepinephrine yields epi nephrine (adrenaline)... 

Deprotection by this method rests on the ease with which benzyl esters are cleaved by nucleophilic attack at the benzylic carbon m the presence of strong acids Bromide ion IS the nucleophile... 

Benzylic carbon (Section 11 10) A carbon directly attached to a benzene nng A hydrogen attached to a benzylic carbon is a benzylic hydrogen A carbocation in which the benzylic carbon is positively charged is a benzylic carbocation A free radical in which the benzylic carbon bears the unpaired electron is a benzylic radical Benzyne (Section 23 8) The compound... 

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). 

A benzyl carbonate was prepared in 83% yield from the sodium alkoxide of glycerol and benzyl chloroformate (20°, 24 h). It is cleaved by hydrogenolysis (H2/ Pd-C, EtOH, 20°, 2 h, 2 atm, 76% yield) and electrolytic reduction (-2.7 V, R4N X, DMF, 70% yield). A benzyl carbonate was used to protect the hy-droxyl group in lactic acid during a peptide synthesis. 

Aiyl esters, prepared from the phenol and an acid chloride or anhydride in the presence of base, are readily cleaved by saponification. In general they are more readily cleaved than the related esters of alcohols, thus allowing selective removal of phenolic esters. 9-Fluorenecarboxylates and 9-xanthenecarboxylates are also cleaved by photolysis. To permit selective removal, a number of carbonate esters have been investigated aryl benzyl carbonates can be cleaved by hydrogenolysis aryl 2,2,2-trichloroethyl carbonates, by Zn/THF-H20. 

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. 

The efficiency of reduction of benzophenone derivatives is greatly diminished when an ortho alkyl substituent is present because a new photoreaction, intramolecular hydrogen-atom abstraction, then becomes the dominant process. The abstraction takes place from the benzylic position on the adjacent alkyl chain, giving an unstable enol that can revert to the original benzophenone without photoreduction. This process is known as photoenolization Photoenolization can be detected, even though no net transformation of the reactant occurs, by photolysis in deuterated hydroxylic solvents. The proton of the enolic hydroxyl is rapidly exchanged with solvent, so deuterium is introduced at the benzylic position. Deuterium is also introduced if the enol is protonated at the benzylic carbon by solvent ... 

Copyrolysis of 1,1-diehloroperfluoroindane and chlorodifluoromethane or tetrafluoroethylene gives 1-perfluoromethyleneindane as the major product and three minor products [3] (equation 2) Insertion of difluorocatbene into the benzylic carbon-chlorine bond and subsequent loss of a chlonne molecule is observed in the copyrolysis of chlorodifluoromethane and pentafluorobenzotnchlonde to give a-chloroperfluorostyrene as the major product. Aromatic carbon-chlorine bonds are unreactive to the difluorocarbene in this reaction [4] (equation 3). 

Benzylic carbon-hydrogen bonds in compounds such as methylpentafluoro-benzene, fluoromethylpentafluorobenzene, and difluoromethylpentafluoroben-zene are not capable of metalation by butyllithium Instead nucleophilic substitution of the para fluorines occurs m each example [55] (equation 13)... 

The aromaticity of the ring is retained when chlorine bonds to the benzylic carbon aromaticity is lost when chlorine bonds to one of the ring carbons. 

Aryl Benzyl Carbonate ArOCOOCH2C6Hg (Chart 4) Formation... 

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