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Pure Appl. Chem. 2017; 89(3): 281–286

Conference paper

Masaaki Yoshifuji* Sterically protected organophosphorus compounds of unusual structures

DOI 10.1515/pac-2016-1029

Abstract: The character of bis(2,4,6-tri-tert-butylphenyl)diphosphene is described experimentally and theo- retically. The diphosphene is stabilized by steric protection and the structure can be characterized by spec- troscopic as well as crystallographic analyses. Theoretical calculation on the diphosphene strongly suggests that the P=P bond is an isolated double bond and that the P–C bonds are single covalent bond. The reactivity has been investigated including photolysis, oxidation, sulfurization, selenation, transition-metal complex formation, and carbene addition. Plausible mechanistic scheme for the reaction of the diphosphene with 3,4,5,6-tetrachlorocyclohexa-3,5-diene-1,2-dione (or tetrachloro-o-benzoquinone) to a pentavalent spiro product is discussed based on the product analysis.

Keywords: diphosphene; ICPC-21; low coordination; steric protection.

Introduction

The first true “phosphobenzene,” bis(2,4,6-tri-tert-butylphenyl)diphosphene 1, was prepared as the phos- phorus analog of azobenzene in 1981, utilizing a sterically demanding 2,4,6-tri-tert-butylphenyl group (abbre- viated as Mes*) for protection of unstable P=P bonding system [1, 2]. The steric protection techniques have been widely used to isolate various kinds of unusual organophosphorus compounds with such uncommon structures as those carrying P=C bond and P≡C bond in low coordination states. Scheme 1 lists some of the examples with the Mes* group such as phosphaalkene 3 [3–5], 1,3-diphosphaallene 4 [6, 7], 1,4-diphosphabu- tatriene 5 [8, 9], 3,4-bis(phosphinidene)cyclobutene 6 [10, 11], and phosphaalkyne 2 [12–14]. Furthermore stable biradical species 7 [13, 14] could be isolated. Most of them are stable enough to analyze the structures by conventional spectroscopy as well as by X-ray crystallography. Beside the Mes* group, a number of substituents have been developed to isolate phosphorus-containing unusual bonding systems of interest [15, 16].

Results and discussion

Preparation of bis(2,4,6-tri-tert-butylphenyl)diphosphene

In our first paper on the isolation of diphosphene, we simply suggested a phosphinidene intermediate 9 for the preparation of diphosphene 1, as a dimer of 9 [1]. However, Protasiewicz investigated our established

Article note: A collection of invited papers based on presentations at the 21st International Conference on Phosphorous ­Chemistry (ICPC-21) held in Kazan, Russia, 5–10 June 2016.

*Corresponding author: Masaaki Yoshifuji, Department of Chemistry, Graduate School of Science, Tohoku University, 6-3 ­Aramaki Aza-Aoba, Aoba, Sendai, Miyagi, 980-0845, Japan; and Department of Chemistry, The University of Alabama, ­Tuscaloosa, Alabama, AL 35487-0336, USA, e-mail: [email protected]

© 2017 IUPAC & De Gruyter. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. For more information, please visit: http://creativecommons.org/licenses/by-nc-nd/4.0/ 282 M. Yoshifuji: Sterically protected organophosphorus compounds of unusual structures

2 4

3 567 1

Scheme 1: Some examples of unusual organophosphorus compounds with Mes*. reaction under strictly controlled conditions such as employing extremely dried THF with activated mag- nesium metal [17] to draw a conclusion that phosphinidene 9 does not seem to be incorporated under our regular experimental conditions, as an intramolecular C–H insertion occurs to afford exclusively 5,7-di-tert- butyl-3,3-dimethyl-2,3-dihydro-1H-phosphindole 8, instead of formation of 1 (“P” in Scheme 2). Taking this experimental result and the fact that phosphinidene 9 is known to give phosphindole 8 into account, under regular experimental conditions, most probably dimerization occurs to form a 1,2-dichloro-1,2-diphosphane at the earlier stage rather than generation of 9, leading to the dechlorination with magnesium to result in the formation of 1 (“Y” in Scheme 2).

Reactions of bis(2,4,6-tri-tert-butylphenyl)diphosphene

Selected reactivity of diphosphene 1 is summarized in Scheme 3 [16, 18–22], including isomerization, pho- tolysis [23, 24], oxidation [25], halogenation [26], reaction with nucleophiles [27], sulfurization [28, 29], selenation [30], carbene-addition [31–33], and coordination to the transition-metal complex formation [34–36]. In this article, only limited examples of photoisomerization [24] and reaction with a quinone [37] are discussed. First of all, photoreaction is to be discussed, which is wavelength and temperature dependent [24]. With a Pyrex filter, which cuts off higher energy (λ > 275 nm), E-diphosphene 1 shows photoisomerization at – 78 °C, indicating a photo-equilibrium with Z-diphosphene around the P=P bond [23, 24]. At room temperature, the Z-isomer goes back to E-diphosphene 1 probably because the ground state is high enough to easily exceed the energy barrier of the equilibrium due to the large steric repulsion within the Z-diphosphene. On the other hand, irradiation without a filter at – 78 °C is strong enough to cleave a double bond to give phosphinidene 9, which affords a phosphindole 8 through C–H insertion almost quantitatively. Formation of 9 on irradia- tion, however, does not seem simple, since unsymmetrical diphosphenes give symmetrical diphosphenes under certain photolysis conditions. E/Z-diphosphene photoisomerization on the carbonylmetal complexes (M=Cr, Mo, W) are studied and the theoretical calculations for the transition state are examined in terms of rotation or inversion mechanism [34]. Rotation energy (including P=P bond cleavage followed by rotation)

1

“ ”

9 “” Activated Mg extremely dried THF 8

Scheme 2: Is phosphinidene 9 a real reaction intermediate? See text for “P” [17] and “Y” [1]. M. Yoshifuji: Sterically protected organophosphorus compounds of unusual structures 283

R El R R PP PP RPCl2 Nu R [23] RP [24]

Cl2 [26] [27] Nu–/El+ hν R S R

Cl4 S PP PP Cl4 R [29] O [28] R S R R P O O PP [37] O R O R O [25] Se Se R PP [30] [31–33] :CCl 2 R RP + PP M Se R R M Se [34] R R R [35,36] PP – Cl PP P C P R 2 PP R C R R Cl2 M

Scheme 3: Selected reactions of diphosphene. is lower than inversion mechanism (on the C–P=P–C plane) in contrast to the nitrogen case, i.e. E/Z-diazene isomerization. Secondly, diphosphene 1 reacts with o-chloranil 10 (3,4,5,6-tetrachlorocyclohexa-3,5-diene-1,2-dione or tetrachloro-o-benzoquinone) at room temperature in toluene and the reaction products depend on the amount of the oxidant used (Scheme 4) [37]. With an excess amount of 10, the product is a pentava- lent spiro phosphorane 11 (δP 6.8) of TBP configuration and the structure is unambiguously confirmed by X-ray analysis. With an equimolar amount of 10, the product is a 1,3,2-diphospholane or 4,5,6,7-tet- rachloro-2-(2,4,6-tri-tert-butylphenyl)benzo[d][1,3,2]dioxaphosphole 12 (δP 218.1), indicating that 12 is the intermediate to 11 as a Ramirez reaction product [38]. By using an alternative tervalent 2-(2,4,6-tri-tert- butylphenyl)-4,5-benzo-1,3,2-diphospholane, the formation of the corresponding spiro phosphorane has been confirmed [37].

Theoretical calculation on bis(2,4,6-tri-tert-butylphenyl)diphosphene

Theoretical calculations on the P=P system with several simple model compounds such as unsubstituted 13 [2, 39] and diphenyldiphosphene 14 [40, 41] have been studied in various ab initio methods at a number of levels. These results rationalize their molecular geometries, physical properties, e.g. spectrum prediction and explanation for 31P NMR spectrum, UV/Vis spectrum, and cyclic voltammetry, and their chemical reactivity, e.g. mechanism for E/Z isomerization and coordination to transition metals.

12

1

11

Scheme 4: Reaction of diphosphene 1 with o-chloranil 10. 284 M. Yoshifuji: Sterically protected organophosphorus compounds of unusual structures

Bis(2,6-di-tert-butylphenyl)diphosphene 15 [42] has been prepared to examine its unimportant effect of the p-tert-butyl group on the stability of diphosphene 1 and the bonding scheme of the P=P in 15 has been theoretically discussed [43].

13 14 15

Ito (Tokyo Institute of Technology) has recently carried out AIM analysis (Atoms In Molecules [44]) for diphos- phene 1 with six tert-butyl groups on the M06-2X/6-31G* level [16] and found that the optimized structure is almost identical with that observed in the crystal at 300 K [1]. The calculated bond distance between two phosphorus atoms is 2.035 Å and is almost the same as that for the X-ray structure (2.034(2) Å). The torsion angle C2–P1–P49–C50 is calculated to be – 169.51°, whereas 172.2(1)° is observed in the crystal, indicating that the four atoms are almost planar but slightly distorted from the perfect planarity. The calculated bond angle P–P–C is 98.59° that is slightly narrower than the observed (102.8(1)°). The bond length between phosphorus and the aromatic ring is calculated to be 1.870 Å, whereas the observed is 1.862(2) Å. Time dependent DFT calculation indicates absorption bands at 481 (weak) and 330 (strong) nm, which correspond to HOMO (– 6.75 eV) → LUMO (– 1.28 eV) and HOMO– 1 (– 7.24 eV) → LUMO (– 1.28 eV) transition, respectively. The Laplacian map on the C2–P1–P49 plane is shown in Fig. 1 [16], indicating that the C–P–P–C atoms are covalently correlated. Furthermore, as for the P1–P49, both Laplacian of the charge density (∇2ρ value – 0.187) and the bond ellipticity (0.459) at BCP (bond critical point [44]) indicate strongly the double bond character, whereas the calculated ellipticity of C2–P1 bond is small (0.083) with ∇2ρ value – 0.238, suggesting a single bond character with a very small extent of conjugation between the aromatic ring and phosphorus atom [45]. One of the canonical structures with isolated P=P double bond is important as depicted on the left-hand side in Scheme 5 for 14 as a model of diaryldiphosphene.

Fig. 1: Laplacian map of diphosphene 1 on the C2–P1–P49 plane (blue: ∇2ρ > 0; red: ∇2ρ < 0; bond critical point: spot colored green; atom labels from the left: C2, P1, P49, and C50).

14

Scheme 5: Some resonance structures of 14. M. Yoshifuji: Sterically protected organophosphorus compounds of unusual structures 285

Conclusions

Sterically protected diphosphenes and the related species are of interest in terms both of physicochemical features and of chemical reactivity. Bis(2,4,6-tri-tert-butylphenyl)diphosphene is characterized experimen- tally and theoretically. Theoretical calculation strongly suggests that the P=P bond is an isolated double bond. Among various kinds of the reactivity, such reaction of diphosphene with o-chloranil as to give a pen- tavalent spiro product of TBP configuration via a 1,3,2-dioxaphospholane intermediate is described. By inte- grated theoretical consideration on these particular compounds might help in attaining a variety of goals in pure and applied chemistry.

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