527 Heteroatom Chemistry Volume 8, Number 6, 1997 Steric Hindrance in the Synthesis and Properties of the Dimer of 1-(2,4,6-Tri-tert- butylphenyl)phosphole 1-Oxide Gyo ¨ rgy Keglevich* and La ´ szlo ´ To ˝ ke Department of Organic Chemical Technology, Technical Universityof Budapest, 1521 Budapest, Hungary Zsolt Bo ¨cskei and Veronika Harmat Laboratory of Theoretical Chemistry, Eo ¨ tvo ¨ s University of Sciences, 1518 Budapest, Hungary Received 3 April 1997 ABSTRACT Steric hindrance was observed during the oxidation of 2,4,6-tri-tert-butylphenylphosphole to the P oxide and in the dimerization of this latter species to the corre- sponding phosphanorbornene derivative. Single-crys- tal X-ray analysis of the dimer revealed considerable steric crowding around the P atoms. Deoxygenation of the dimer of the phosphole oxide by Cl 3 SiH–C 6 H 5 N could only be accomplished under forcing conditions at 110°C.  1997 John Wiley & Sons, Inc. Hetero- atom Chem 8: 527–531, 1997 INTRODUCTION Placing sterically demanding substituents on the P atom of different phosphorus compounds may have a serious impact on their geometry and properties. Recently, we have pointed out that the introduction of a 2,4,6-trialkylphenyl substituent on the P atom of phospholes results in some planarization of the Dedicated to Prof. William McEwen on the occasion of his sev- enty-fifth birthday. *To whom correspondence should be addressed.  1997 John Wiley & Sons, Inc. CCC 1042-7163/97/060527-05 phosphorus pyramid [1–4]. Among the trialkyl- phenyl groups, the tri-tert-butylphenyl substituent has had the most pronounced effect on the flattening [4]. In this article, we describe our experiences on the steric hindrance in the synthesis and properties of a phosphole oxide dimer with 2,4,6-tri-tert-butyl- phenyl substituents on the P atoms. RESULTS AND DISCUSSION Formation of the Dimer (3f) of the Tri-tert- butylphenylphosphole Oxide from Phosphole 1f Oxidation of the phospholes (1a–c) by peroxides is a well-known method for the preparation of the di- mers (3a–c) of phosphole oxides (2a–c), as the phos- phole oxides (2a–c) formed primarily are dimerized instantly in a regio- and stereospecific Diels-Alder cy- cloaddition reaction (Scheme 1) [5]. The phosphole oxide intermediates (2a–c) are usually of short lifetime and cannot be isolated from the mixtures. In several cases, the intermediates could, however, be observed for short periods of time. We could, for example, detect trialkylphenyl- phosphole oxides 2d and 2e and characterize them by 31 P NMR chemical shifts by carrying out the oxi- dations at 0°C [1,3]. The relative stabilities of intermediates 2d and 2e probably come from the steric hindrance in the