Edward T. Hessell

Organophosphorus and organosilicon derivatives of sterically hindered phenols

Abstract A general methodology for the preparation of sterically hindered hydroxyaryl derivatives of both phosphorus and silicon is described. The reaction of 3,5-di-t-butyl-4-trimethylsilyloxyphenyllithium ( 4 ), which was prepared by the metalation of 1-bromo-3,5-di-t-butyl-4-trimethylsilyloxybenzene with n-butyllithium at −78°C, with chlorodiphenylphosphine gave the protected phenolic derivative 3,5-di-t-butyl-4-trimethylsilyloxyphenyl(diphenyl)phosphine ( 5 ). Similarly, the reaction of 4 with dichlorophenylphosphine and phosphorus trichloride afforded the bis- and tris-adducts, bis(3,5-di-t-butyl-4-trimethylsilyloxyphenyl)phenylphosphine ( 7a ) and tris(3,5-di-t-butyl-4-trimethylsilyloxyphenyl)phosphine ( 7b ), respectively. Aqueous hydrolysis of 5 and 7a , 7b gave the corresponding 3,5-di-t-butyl-4-hydroxyphenylphosphines 1b and 8a , 8b , respectively. The reaction of 4 with the dimethyl-, diphenyl-, and methyl-substituted dichlorosilanes 9a – c gave the corresponding bis(3,5-di-t-butyl-4-trimethylsilyloxyphenyl)silanes 10a – 10c . Removal of the trimethylsilyl protecting group with N,N,N,N -tetra-n-butylammonium fluoride trihydrate (TBAF, trihydrate) gave the corresponding bis(3,5-di-t-butyl-4-hydroxyphenyl)silanes 11a – 11c , respectively. In the case of 10c , besides the expected derivative 11c , the fluoro-substituted silane bis(3,5-di-t-butyl-4-hydroxyphenyl)methylfluorosilane ( 12 ) was isolated. A plausible mechanism for the formation of 12 involving a pentacoordinate siliconate anion intermediate or transition state 13 was suggested. The reaction of 4 with silicon(IV) chloride followed by aqueous hydrolysis gave the trisubstitution product tris(3,5-di-t-butyl-4-trimethylsilyloxyphenyl)silanol ( 14 ). Deprotection of 14 with TBAF · trihydrate gave the trisadduct tris(3,5-di-t-butyl-4-hydroxyphenyl)silanol ( 15 ).

Nucleophilic subsitution at silicon: Synthesis of sterically-hindered bis(2,6-di-t-butylaryloxy)silanes

Abstract The reaction of the monomethylsilane (8a) with two equivalents of the 4-(carboalkoxy)-2,6-di-t-butyl-substituted phenol (7b) in toulene using triethylamine as an acid acceptor gave the bis(aryloxy) adduct (9a). The analogus reaction of the dimethylsilane (8b) with sodium 2,6-di-t-butyl-4-(methoxycarboxyl)-phenolate (7a) gave only the monosubstitution product (10a). The reaction of the corresponding phenolate (7e) with 8b gave a mixture of 7a, 10a, and bis-adduct (9b), whereas, in the presence of 15-crown-5, the bis-adduct 9b was obtained. The bis-adducts 9c–e were prepared in an analogous manner. The reaction of n-hexyl 3,5-di-t-butyl-4-hydoxylbenzoate (7h) with the diphenylsilane (8c) gave only the monosubstitution product 12, while forcing conditions gave, unexpectedly, the methyl ether 13. The reaction of 4-(carboalkoxyethyl)-2,6-di-t-butylphenol (16a) with 8a gave the bis adduct. The reaction of 16a with 8b in THF, without a crown ether, gave a low yield of the monosubstitution product. The bis-adducts 17b–c were obtained by the reaction of 8b with the corresponding phenolates (16a–b) in tetraglyme. Compound 17b was also obtained by the reaction of 8b with 16a in THF with a crown ether. These results are discussed in terms of charge dispersal in the phenolate ion and the corresponding effect upon both ion-pairing and aggregation in solution.

MECHANISM OF THE REACTION OF SOME STABLE HEMIMERCAPTALS WITH Secondary-AMINES: EVIDENCE AGAINST THE INTERMEDIACY OF A METHYLENESULFONIUM ION

Abstract The reaction of the arenethiols 4a-b with formaldehyde and the sec-amines 5a-c gave the aminomethyl aryl sulfides 6a-d. The reaction of the hemimercaptals 3a-b with 5a in methanol gave 6a-b in high yield. In acetonitrile reaction media, 6b was obtained by the reaction of 3b with 5a which suggested that 7b was not an intermediate in the formation of 6b in methanolic media. The absence of 7b in methanolic media suggests that the methylenesulfonium ion 8b is not an reaction intermediate. The formation of 7b was observed in the reaction of 3b with methanol when catalyzed by the Lewis acid tetrafluoroboric acid diethyl ether complex. The experimental observations are best explained by a mechanism whereby 3a-b are in rapid equilibrium with 4a-b under the basic reaction conditions. Rapid reaction of the liberated formaldehyde with 5a leads to the normal Mannich reaction pathway. Consistent with this mechanism, the reaction of a mixture of 3a-b and 12 with 5a gave both 6a-b and 13.

STERICALLY HINDERED 4-AMINO-3,5-DIALKYLBENZENETHIOL DERIVATIVES : SYNTHESIS AND CHARACTERIZATION

Abstract The synthesis and characterization of a sterically hindered aminobenzenethiol 1 (4-NH2-3-1Bu-5-MeC6H2SH) is described. The addition of 1 to electron-deficient olefins as well as the Mannich-type reaction of 1 with formaldehyde and a sec-amine was investigated. Phosphorothiolate derivatives were prepared by the reaction of the corresponding sulfenyl chloride 4-NH2-3-1Bu-5-MeC6H2SCl with tricoordinate P(III) esters.