Steric Crowding and the Reactivity of Substituted tert-Butyl Perbenzoates

Abstract

SIDDICK ICLI, UNDIL A. KANDIL, C. THANKACHAN, and THOMAS T. TIDWELL. Can. J. Chem. 53,979 (1975).The relative rates of decomposition of peroxyesters in cumene at 100" and their activation parameters AH* (kcal/mol) and ASf (cal deg-I mol-') were tert-butyl perbenzoate (1) 1.0, 34.0, 9.3; tert-butyl pertoluate (7) 1.3, 32.2, 4.8; tert-butyl permesitoate (8) 11, 31.9, 8.4; and tert-butyl2,4,6-tri-tert-butylperbenzoate (9) 29, 32.9, 13.0. Each peroxyester yielded the corresponding acid as one of the products. The reactions are all interpreted as proceeding through a rate-determining scission of the 0-0 bond to yield a benzoyloxy radical, which then undergoes competitive decarboxylation and hydrogen abstraction from the solvent. The small rate acceleration due to apara substituent is interpreted as due to a polar factor and the larger rate enhancing effect of the ortho substituents is interpreted as due to destabilization of the ground state due to twisting of the carboxyl group out of conjugation with the ring. perbenzoate de tert-butyle (9) 29, 32.9, 13.0. Chaque peroxyester conduit entre autre ii l'acide correspondant. On interprete toutes les reactions en fonction d'une Ctape dkterminante de la reaction qui impliquerait la scission du lien 0-0 conduisant au radical benzoyloxy qui subirait ensuite une decarboxylation ou enleverait un hydrogene du solvant. On attribue la petite augmentation de vitesse due ii un substituant en para a des facteurs polaires; par ailleurs l'augmentation importante observee lorsqu'il y a des substituants en position ortho implique une dtstabilisation de 1'6tat fondamental due ii une rotation du groupement carboxyle hors du plan de conjugaison du cycle.[Traduit par le journal]

The influence of steric constraints on the conformational properties and on the 17O NMR shielding of ortho-substituted perbenzoates

“…~ The slope of eqn. (2) is smaller than that found4 for the dicoordinated oxygen of esters. This signal is assigned to the oxygen O(3) of peresters, which is less affected by ring twisting than the dicoordinated oxygen of esters.…”

“…Paramagnetic contributions aP(0) to the screening constant of oxygen nuclei of compounds 1-5 and of some reference peroxidic compounds followed by O ( 2 ) and O(3): for the latter nuclei the slope is close to 0.1. This behaviour appears qualitatively to confirm the experimental finding that the highest sensitivity of 7O chemical shifts to ring twisting is for O( 1) and O(2).…”

Thermolysis of organic peroxides in solution

“…As a result, the decomposition rates of peroxides 13c,d are increased dramatically (Table 16). 135 It is of note that the decomposition of peroxyacetate occurs several times more slowly than that of peroxybenzoates. The thermolysis of tert-alkyl peroxypivalates in the solvent cage 136 ± 138 affords equimolar amounts of tert-butyl and tertalkoxyl radicals as a result of concerted cleavage of two bonds.…”

Density Functional Studies on Secondary Amides: Role of Steric Factors in Cis/Trans Isomerization

“…With the A value > 4 [ 31 , 32 ] and υ ef value = 1.2 [ 33 , 34 ], the tert -butyl group can therefore be an ideal choice for “bulk”. In synthetic chemistry also, it is a common practice to use tert -butyl groups as bulky substitutions to analyze the effect of steric bulk [ 35 , 36 , 37 ].…”