Cyclic peroxides. 44. A convenient and efficient preparation of aromatic .alpha.-hydroperoxy acids via oxygenation of .alpha.-lithio enolates, prepared by direct .alpha.-lithiation of arylacetic acids

Abstract

Direct a-lithiation of aromatic acetic acids by n-butyllithium in THF at -40 O C affords essentially quantitatively lithium a-lithiocarboxylates. The method can be employed as a convenient titration of alkyllithiums. a-Deuteration and bistrimethylsilylation of the a-lithiocarboxylates takes place essentially quantitatively. These are reliable electrophiles for the quantitative determination of the a-lithiocarboxylates. Direct oxygenation with molecular oxygen at room temperature affords good yields of the respectiye a-hydroxy acids, while reaction with molecular oxygen at dry ice temperature by inverse addition is an excellent method for the preparation of a-hydroperoxy acids derived from arylacetic acids. a-Hydroperoxy acids 1, which on cyclodehydration serve as precursors to a-peroxylactones 2 (eq 1),3 are extremelybase-and acid-sensitive compounds in view of the facile Grob-type fragmentations4 depicted in the respective transition states 3a and 3b. Since these decarboxylations efficiently destroy the a-hydroperoxy acids 1, it was essential to circumvent this problem by working under neutral conditions. We took advantage of the oxygenophilic propensity of the trimethylsilyl group and prepared a number of a-hydroperoxy acids 1 by singlet oxygenation of ketene bis(trimethylsily1) acetals 4 and subsequent desilylation of the peroxy ester 5 with methanol (eq 2). Analogous silatropic shifts have been ob-0 0 1 served in the singlet oxygenation of trimethylsilyl enol ether^.^ This novel a-hydroperoxylation lacks unfortunately generality since the classical prototropics shifts (ene reaction) compete with the silatropic shift when R is a nontertiary alkyl group6 in the ketene acetal 4. When R is an aromatic group, the classical [2 + 41 cycloaddition of the styryl unit with singlet oxygen competes.7Recently, we and others8 have shown that a-lithio enolates can be directly oxygenated to afford a-hydroperoxy acids 1 after acidification (eq 3). Unfortunately, this most direct a-0 0 6 1 oxygenation is limited to nonaromatic substrates since an attempt to a-oxygenate a-lithio-a-phenylacetate gave only decomposition and reduction products.8b To suppress the Grob fragmentation process, since benzaldehyde was the major product, we did not use hexamethylphosphoramide (HMPA) and pumped off the diisopropylamine prior to aoxygenation. While this process gave significantly improved results for the lower a-alkyl-and a,a-dialkylacetic acids, only poor yields of impure a-hydroperoxy acids could be realized for phenylacetic acid.9 It was clear to us that even traces of amines exerted detrimental effects in the preparation of ahydroperoxy acids derived from arylacetic acids via a-lithiation with lithium diisopropylamide (LDA). Although aliphatic carboxylates form ketones on treatment with alkyllithiums,1° Ivanov and colleagues demonstrated" that arylacetates can be a-lithiated with alkyllithiums directly; however, generally the yields were poor (19-35%). It was not clear whether the metalation process worked poorly or whether the titrati...

Oxidative addition of halosilanes to zero-valent platinum complexes

“…90-120 °C. (10) The coupling constant of 'H-1,sPt is 25.2 Hz which is similar to the value reported for rrans-Me3SiPtCl(PEtj)j (24.6 Hz).11 (11) Glockling, F.; Hooton, K. A. J. Chem. Soc.…”

Revision of Singlet Quantum Yields in the Catalyzed Decomposition of Cyclic Peroxides

“…The following cyclic peroxides were prepared according to adapted literature procedures: diphenoyl peroxide ( 1 ), 3,3-dimethyl-1,2-dioxetanone ( 2 ), spiro-adamantyl-1,2-dioxetanone ( 3 ) and spiro-cyclopentyl-1,2-dioxetanone ( 4 ). − All cyclic peroxides, including the hitherto unknown α-peroxylactone derivative 4 , were characterized by low-temperature 1 H and 13 C NMR spectroscopy. In the case of the known compounds 1 – 3 , these were fully characterized for the first time only recently. − Due to the difficulty of preparing such unstable compounds, some critical details on their storage, purification, and characterization are given elsewhere…”

Synthesis of unstable cyclic peroxides for chemiluminescence studies

“…No evidences were found for the presence of starting materials, diphenic anhydride or benzocoumarin, a possible product formed in the thermolysis of 3 (Scheme 5). 27 1,2-Dioxetanones (4-6) 1,2-Dioxetanones 4-6 were prepared according to a general literature procedure, [63][64][65][66][67] introducing specific modifications for each of the derivatives (see the Experimental section for details). The methodology consists in cyclization of an a-hydroperoxyacid derivative (7)(8)(9) with N,N'-dicyclohexylcarbodiimide (DCC), in which 7-9 are obtained by a-lithiation of the corresponding carboxylic acids, followed by low-temperature autoxidation (Scheme 6).…”