Enhancing the reactivity of 1,2-diphospholes in cycloaddition reactions

Two different approaches have been employed to enhance the reactivity of 1-alkyl-1,2-diphospholes – the introduction of electron-withdrawing groups either at the phosphorus atoms or in the para-position of the arene ring. The alkylation of sodium 1,2-diphospha-3,4,5-triphenylcyclopentadienide with alkyl halides Hal-CH2-R (R = CN, COOEt, OMe, CH2OEt) results in corresponding 1-alkyl-3,4,5-triphenyl-1,2-diphospholes (alkyl = CH2CN (1a), CH2COOEt (1b), CH2OMe (1c), and (CH2)2OEt (1d)), which spontaneously undergo the intermolecular [4 + 2] cycloaddition reactions at room temperature to form the mixture of the cycloadducts, 2a–c, respectively. However the alkylation of sodium 1,2-diphospha-3,4,5-tri(p-fluorophenyl)cyclopentadienide with ethyl iodide leads to stable 1-ethyl-3,4,5-tris(p-fluorophenyl)-1,2-diphosphole (1e), which forms the [4 + 2] cycloadduct 2,3,4,4a,5,6-hexa(p-fluorophenyl)-1-ethyl-1,7,7a-triphospha-4,7-(ethylphosphinidene)indene (2e) only upon heating up to 60 °C. With further heating to 120 °C with N-phenylmaleimide, the cycloadducts 2a–c and 2e undergo the retro-Diels–Alder reaction and form only one product of the [4 + 2] cycloaddition reaction 3a–с, 3e with good yields up to 65%.


Introduction
Phospholes are weakly aromatic heterocycles and demonstrate rather different properties from those of their S, N and O counterparts [1,2]. Due to low their aromaticity, phospholes are of significant interest for the preparation of highly effective catalysts, materials for light-emitting diodes and nonlinear optics [3,4]. In contrast to furans, thiophenes and pyrroles, phospholes display cycloaddition and complexation reactions and can be used as starting materials for caged phosphines, phosphinidenes, etc. [2]. At the same time, the presence of electron-withdrawing substituents (cyano-, alkoxy-, or halo-) at the phosphorus atom reduces the aromaticity of the monophosphole ring and facilitates cycloaddition reactions resulting in novel 7-phosphanorbornenes [5,6], which was verified by theoretical calculations and experimental work [7,8]. At the same time both the presence of the P=C bond in phospholes as well as the transient 2H-phospholes [3] increase the cycloaddition reactivity. It was previously demonstrated that 1-alkyl-1,2-diphospholes combine the properties of both 1H-phospholes (with thermal stability up to 190 °C) and 2H-phospholes (exhibiting high reactivity in the cycloaddition reaction at 25 °C) [9][10][11]. In the present work, attempts to increase the reactivity of the dienic system of 1,2-diphospholes using two different approaches are described: (a) by the introduction of electron-withdrawing groups (EWGs) at the phosphorus atom or (b) the introduction of EWGs to the carbon atoms of aryl substituents. This work will provide access to new polycyclic, organophosphorus compounds having significant potential as weak, bulky ligands in homogeneous catalysis [12][13][14].
Remarkably, the coupling constants 1 J PР increase with the decrease of the electron-withdrawing properties of the substitutents in the diphosphole ring (CN > COOEt > OMe > CH 2 OEt) in the series 1a-1d. A large phosphorus-phosphorus coupling constant, 1 J PР , usually indicates significant σ-π delocalization of the lone pair of the tricoordinated phosphorus atom into the diphosphole ring system. Thus, the 1 J PP coupling constant for a non-aromatic 1,2-diphosphacyclopentene is observed at around 220 Hz [16], although both phosphorus atoms of the highly aromatic 1-(2,4,6-tri-tert-butylphenyl)-1H-1,2-diphosphole are coupled with a larger phosphorus-phosphorus constant ( 1 J PР = 528.2 Hz) [17]. The same phenomena was noted for the 1,2,4triphosphole with the planar tricoordinated phosphorus [18]. Thus, the increase of 1 J PР in a sequence from 1a to 1d could imply the increasing delocalization of the RP-fragment within the diphosphole system that reflects the stability and the reactivity of 1,2-diphosphole.
Indeed, the compounds 1a,b are stable only at temperatures below +5 °C, while 1c is stable at room temperature for a few hours. 1,2-Diphosphole 1d is more stable and no cycloaddition was observed upon heating in toluene. Upon standing, the diphospholes 1a-c undergo spontaneous [4 + 2] cycloaddition reactions leading to a mixture of cycloadducts (Scheme 2). The 31 P NMR spectra of the reaction mixtures showed many multi-plets at 80 and −40 ppm with a coupling constant 1 J PP ca. 200-210 Hz characteristic for the products of [4 + 2] cycloaddition reaction -1,7-diphosphanorbornadienes [9,11].
The molecular structure of 2e (Figure 1) was verified by X-ray crystallography. The crystal structure analysis of 2e showed that only the endo isomer was formed with the alkyl group in antiorientation with respect to the double bond of the ring.
In the case of 2a-c, it would be assumed that similar [4 + 2] cycloadducts are formed according to the range of signals in the 31 P NMR spectra. However, in this case, several stereoisomers are formed due to the high reactivity of the 1,2-diphospholes containing EWGs on the phosphorus atom. It should be noted that this is the first example of [4 + 2] cycloaddition between two diphosphole molecules where 1,2-diphosphole acts as a diene and a dienophile in one reaction. Therefore, these isomeric cycloadducts, 2a-с, can be a source of reactive 1,2-diphospholes containing EWGs 1a-с in the retro-Diels-Alder reaction [19].
Indeed, upon further heating up to 120 °C, the mixture of the cycloadducts 2a-c as well the cycloadduct 2e underwent the retro-Diels-Alder reaction to form monomeric 1,2-diphospholes, 1a-c, 1e, which were trapped by N-phenylmaleimide to form the compounds 3a-c, 3e with yields of 55-65% (Scheme 3).  [20], we can conclude that only an anti-endo-isomer was formed in each case.

Supporting Information
Supporting Information File 1 Experimental procedures and characterization data.