Preparation and reaction of phosphorus peri-bridged naphthalenes and their adducts with Lewis acids

The reaction of dilithionaphthalene with R₂NPCl₂ (R = ⁱPr and Et) gave diisopropylamino-naphtho[1,8-bc]phosphete 3a and its diethyl analog 3b (phosphorus versions of single-atom peri-bridged naphthalene). The nature of the strained four-membered ring thus formed was examined by treating 3a with four types of electrophiles, BH₃, elemental sulfur, methyltriflate, and a metal fragment W(CO)₅, all of which gave the corresponding electrophile adducts with the strained ring retained. X-ray analysis was performed for 3b and all of the above adducts, showing that the trigonal plane of the amino group was commonly oriented almost perpendicular to the naphthalene plane, which was rationally understood on the basis of the favorable overlap between the lone pair orbital of the sp² nitrogen center and the σ¹ orbital(s) of the two P–C bonds. In addition, when the W(CO)₅ adduct, W(3a)(CO)₅, was treated with Pt(PPh₃)₄ and then with a CO gas, its four-membered ring was expanded to give a tungsten–platinum heterodinuclear complex having a five-membered platina–phospha heterocycle.