Different reactivity of phosphorylallenes under the action of Brønsted or Lewis acids: a crucial role of involvement of the P=O group in intra- or intermolecular interactions at the formation of cationic intermediates.

Stanislav V Lozovskiy,Aleksander V Vasilyev,Alexander Yu Ivanov

doi:10.3762/bjoc.15.151

Abstract

3-Methylbuta-1,2-dien-1-ylphosphonic acid derivatives (phosphorylallenes) [X2(O=)P–CR=C=CMe2, X = Cl, OMe, NR2, or SAr] undergo intramolecular cyclization into the corresponding 1,2-oxaphospholium ions in the Brønsted superacid TfOH. These cations have been thoroughly studied by means of NMR spectroscopy. The hydrolysis of superacidic solutions of these species afforded cyclic phosphonic acids and other phosphorus-containing compounds. Contrary to Brønsted acids, 3-methylbuta-1,2-dien-1-ylphosphonic dichloride [Cl2(O=)P–HC=C=CMe2] reacted with the Lewis acid AlCl3 in an intermolecular way forming noncyclic intermediates, which were investigated by NMR spectroscopy and DFT calculations. Hydrolysis of these species resulted in the formation of phosphoryl-substituted allyl alcohols and 1,3-butadienes. A strong coordination of the oxygen of the P=O group with AlCl3 prevented the formation of cyclic 1,2-oxaphospholium ions and played a crucial role in the different reactivity of such phosphorylallenes under the action of Brønsted or Lewis acids. Apart from that, the reaction of dichlorophosphorylallenes with arenes and AlCl3 led to products of hydroarylation of the allene system, phosphoryl-substituted alkenes and/or indanes. This is the first example of a Lewis acid-promoted intermolecular hydroarylation of allenes bearing electron-withdrawing substituents. Plausible reaction mechanisms have been proposed on the basis of the investigated reactions, and NMR analysis and DFT studies of the intermediate cationic species.

Highlights

Electrophilic reactions of allenes have been intensely explored in organic synthesis [1,2,3]
We have recently reported on the generation, NMR characterization and reactions of oxaphospholium ions bearing phenyl or phenoxy substituents at the phosphorus atom of phosphorylallenes [13,14,15,16]
We explored allenes having different substituents at the phosphoryl group: chloro (1a–d), amino (1e–g), arylsulfanyl (1h,i), and methoxy (1j)

Summary

Introduction

Electrophilic reactions of allenes have been intensely explored in organic synthesis [1,2,3]. Reports on electrophilic activation of phosphorylallenes are numerous [4,5,6,7,8,9,10]. Miscellaneous electrophiles, such as sulfenyl, selenyl, and telluryl chlorides, were used in reactions with these allenes. Only a few studies have been focused on reactions of phosphorylallenes with Brønsted acids [11,12]. These reactions proceed through an intermediate formation of the corresponding 2,5dihydro-1,2-oxaphosphol-2-ium ions. The progenitor of the oxaphospholium ion family, 2,2-dichloro-5,5-dimethyl-1,2oxaphosphol-2-ium, was postulated for the first time in 1978 [12]

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Conclusion