Transformation Routes of P^V‐ and P^III‐N‐Substituted Acyclic Diaminocarbenes

Abstract

Herein we report on the synthesis of a set of transient N-(P III )-phosphanyl and N-(P V )-phosphoroselenoyl acyclic diaminocarbenes by deprotonation of the corresponding formamidinium salts. It is shown that N-(P V )-substituted acyclic diaminocarbenes undergo 1,2-phosphorus shift. Transformations of N-(P III )-substituted acyclic diaminocarbenes result in tetrahydroquinazolines as final products. The proposed mechanism includes the isomerization of the carbenes into transient [a] Deprotonation of tetrasubstituted formamidinium salts is the most common approach to acyclic carbenes. Formamidinium and amidinium salts can easily be prepared using a powerful electrophilic reagent developed by Alder et al. -compounds 2 (R = Me, iPr) named in the literature as Alder's dimer. [16,17] We extended this approach on a set of formamides. Compounds 2 were readily prepared in high yields by mixing formamides 1 with triflic acid anhydride in dichloromethane (Scheme 1). The Alder's dimers usually precipitated as analytically pure compounds, in some cases recrystallization from acetonitrile was required. For these species, characteristic low-field signals (≈ 9 ppm) of CH protons were found in 1 H NMR spectra. Scheme 1. Synthesis of Alder's dimers 2.N-Aryl phosphinoselenoic amides 3 reacted readily with Alder's dimers 2a-e in 1:1 molar ratio giving salts 4a-g in over 90 % yields. [18] Their 31 P NMR chemical shift values were in a range 136-143 ppm with the 1 J PSe coupling constants (814-834 Hz) typical for the P=Se double bond. In 1 H NMR spectra CH exhibited as a doublet at 8.45-8.68 ppm with a coupling constant ca. 5 Hz or as a broad singlet. 13 C NMR signals covered the area 152-156 ppm ( 2 J CP 4-11 Hz). These P V salts were readily reduced with hexamethylphosphorus triamide to the corresponding P III formamidinium salts 5a-g (Scheme 2). Scheme 2. Synthesis of formamidinium salts 4 and 5. Deprotonation of P V -and P III -N-Substituted Formamidinium SaltsDeprotonation of P V -N-substituted formamidinium salts 4 led to the corresponding carbenes 6 that quickly underwent 1,2phosphorus shift affording the corresponding phosphane selenides 7 in high yields.In our previous article [9] we have found that deprotonation at -50°C of the salt analogous to 4a, but bearing diisopropyl-Eur.