First examples of methylene insertion into the phosphorus(III)±nitrogen bond Srinivasan Priya a, Maravanji S. Balakrishna a a,* , Joel T. Mague b Department of Chemistry, Indian Institute of Technology (Bombay), Mumbai 400 076, India b Department of Chemistry, Tulane University, New Orleans, LA 70118, USA Abstract The reaction of N -substituted phosphinous amides with paraformaldehyde leads to methylene insertion into the P±N bond, followed by the oxidation of phosphorus from P(III) to P(V) state. The product, Ph2 POCH2 NHPh was characterised by a singlecrystal X-ray diraction study. The reaction not only depends on the acidic proton on the nitrogen, but also on the oxidation state of phosphorus and it is considered to proceed through Staudinger±Wittig pathway. Keywords: Insertion; Phosphinoamine; Phosphine oxide; Paraformaldehyde 1. Introduction Insertion of CO, alkene, alkyne, CO2 , CS2 , etc., into the M±X bond (M metal or metalloid; X P, C, N, S, O, halide) are well documented in the literature [1±5]. The insertion of small molecules into C±H, N±H, O±H bonds with or without metal mediation is versatile in organic synthesis. Insertion of CO, alkene, etc., is an important step in many catalytic reactions, such as hydrogenation, hydroformylation [6]. Phosphines (R2 PH, RPH2 ) and secondary amines undergo nucleophilic addition reaction when treated with paraformaldehyde to give phosphinoalcohols (R2 PCH2 OH, RPCH2 OH2 ) and methylolamines, respectively. However, in the case of amines, the products undergo further condensation to give diamines. So far as the P±N bond is concerned, insertion into the P±N bond is scarcely seen in the literature [7±9]. As a part of our interest in designing and exploring multifunctional phosphorus-based ligands [10], we report here the reaction of N -diphenylphosphinoaniline with paraformaldehyde that leads to methylene insertion into the P±N bond followed by the oxidation of phosphorus from trivalent to pentavalent state instead of the anticipated amino alcohols of the type A (Scheme 1). 2. Experimental All manipulations were performed under rigorously anaerobic conditions using Schlenk techniques. The monophosphinoamine, PhN(H)PPh2 was prepared according to the literature procedure with slight modi®cation [11]. Solvents were dried and distilled under nitrogen atmosphere prior to use. Petroleum ether (60± 80°C), toluene, benzene and diethylether were distilled over Na±benzophenone and dichloromethane from calcium hydride. The 1 H and 31 P NMR spectra were recorded on a VXR 300S spectrometer operating at the appropriate frequencies using tetramethylsilane and 85% H3 PO4 as internal and external references, respec- Scheme 1. 438 tively. Positive shifts lie down®eld of the standard in all cases. Infrared spectra were recorded on a Nicolet Impact 400 FT IR instrument in KBr disk. Microanalyses were performed on a Carlo Erba model 1106 elemental analyser. 2.1. Synthesis of Ph2 P(O)CH2 N(H)Ph (2) Method (a): To a solution of N -diphenylphosphinoaniline (1) (0.5 g, 1.8 mmol) in toluene (10 ml), paraformaldehyde (0.057 g, 1.8 mmol) was added heated under re¯ux conditions for 4 h. The solution was then cooled to room temperature, concentrated to 4 ml and kept for 7 h to give crystalline product of 2 in 81% (0.450 g). Method (b): A mixture of N -diphenylphosphinoaniline (0.5 g, 1.8 mmol) and paraformaldehyde (0.057 g, 1.8 mmol) placed in a round-bottomed ¯ask ®tted with a condenser was heated with stirring in an oil bath maintained at 95±100°C for 2 h. The resultant clear melt was subjected to vacuum to remove any volatile material, then washed with hexane, ®ltered and dried under vacuum to obtain a white solid. The compound was crystallised from a mixture of CH2 Cl2 and n-hexane (2:1) at 0°C. Yield: 91% (0.51 g), m.p. 136±138°C. Anal.: Calc. for Ph2 POCH2 NHPh, C19 H18 NOP: C, 74.26; H, 5.90; N, 4.55%. Found: C, 74.44; H, 6.03; N, 3.95%. IR (KBr disk) cm 1 : 3255s, 3124m, 3052m, 2789m, 1598s, 1493s, 1433s, 1315s, 1210m, 1171s, 1124s, 867m, 755s, 696s, 578m, 532m. 1 H NMR (299.9 MHz, CDCl3 , 298 K): d 6.5±7.8 (m, phenyl, 15 H), 4.26 (br.s, NH , 1H), 3.94 (dd, CH2 , 2H, 2 JPH 8:8 Hz, 3 JHH 3:8 Hz). 31 Pf1 Hg NMR (121.427 MHz, CDCl3 , 298 K): d 30.2. HRMS: m=z 307:12. 3. Results and discussion The reaction of N -diphenylphosphinoaniline (1) with paraformaldehyde in toluene under re¯ux for 4 h afforded the methylene-inserted product, Ph2 POCH2 N(H)Ph (2), in good yield. The 31 P NMR spectrum of 2 shows a singlet at 30.2 ppm. The 1 H NMR spectrum shows a singlet at d 4:26 which is D2 O exchangeable and is assigned to NH, and a doublet of doublets at d 3.94 with a 2 JPH value of 8.8 Hz is assigned to the CH2 protons. The IR spectrum shows mNH at 3255 cm 1 , which is about 50 cm 1 lower in frequency when compared to the phosphinous amide mNH 3302 cm 1 , indicates the participation of the NH bond in either intra- or intermolecular interactions. The elemental analysis and HRMS could not explain the low-frequency shift of the NH bond and also the 31 P NMR data did not rule out the possibility of structure A as there is no signi®cant dierence in P(III) and P(V) chemical shifts. However, the single-crystal X-ray studies con®rmed that the product is not A and it is the methyleneinserted compound 2 (Fig. 1). The low-frequency shift of mNH when compared to phosphinous amide is due the intermolecular N±H . . . O@P hydrogen bonding between H(1N) and O of adjacent molecules [dN...O dH1N...O 2:022 A, N±H1N . . . O bond 2:8663 A, angle is 173(1)°] as the compound 2 exists as a dimer (Fig. 2). The anti conformation of the NH and the P@O of adjacent molecules facilitates this strong intermolec and P±Cav: ular hydrogen bonding. The P±O (1.481 A) (1.804 A) bond lengths agree well with the literature [14]. 2.2. Structure determination Crystals of the compound 2 obtained as described above were mounted on Pyrex ®laments with epoxy resin. General procedures for crystal alignment and collection of intensity data on the Enraf±Nonius CAD-4 diractometer have been published [12]. Details of the crystal and data collection for 2: C19 H18 NOP, 9.5474(5), b (A) M 307:31, monoclinic, a (A) 15.3265(11),3 c (A) 11.8320(10), b (deg) 107.452(6), 1651:72, T (K) 293(2). Space group Z 4, V A 3 P21 /n, D (calc) g=cm 1:236. l (Mo-Ka) 0.168 mm 1 , 3124 re¯ections collected, 2940 unique (R (int) 0.0348). The ®nal R1 was 0.0348 (all data) and wR2 0:0891 (all data). Periodic monitoring of check re¯ections showed stability of the intensity data. All calculations were performed with the S H E L X T L P L U S [13] program package. The data are deposited in the Cambridge Crystallographic Data Center and the CCDC reference number is 150093. Fig. 1. Perspective view of the compound (2). Selected bond lengths and bond angles (deg): P±O 1.4810 (14), P±C(1) 1.8006 (19), P± (A) C(7) 1.8033 (18), P±C(13) 1.8089 (19), N±C(13) 1.441 (2), N±C(14) 1.375 (2), N±H(1N) 0.85 (2); O±P±C(1) 112.94 (9), O±P±C(7) 111.27 (8), O±P±C(13) 114.56 (9), C(1)±P±C(7) 108.60 (9), C(1)±P±C(13) 107.14 (9), C(7)±P±C(13) 101.61 (8), P±C(13)±N 113.80 (13), C(13)±N± C(14) 120.87 (16), C(13)±N±H(1N) 116.5 (13), C(14)±N±H(1N) 118.1 (14). 439 Scheme 2. Fig. 2. Dimer of the compound (2) showing intermolecular N± H . . . O@P H-bonding. The phosphorus is in a slightly distorted tetrahedral environment. Mukaiyama and Yokota [15] and Hudson and coworkers [7] have reported the de-oxygenation reactions of isocyanates by the cyclic phosphoramidite (2-phenyl1,3,2-oxazaphospholidine) and acyclic phosphoramidite (diethyl-N -phenylphosphoramidite) to give the corresponding phosphine oxides and isonitrile which involves the preferential interaction of the oxygen and phosphorus atoms. Ivanov et al. [16] have obtained the same product (2) via the Arbuzov reaction of RR0 NCH2 OAc (R R0 Et; R H, R0 Ac, Bz, Ph) with PhR00 PNEt2 (R00 Ph, NEt2 ) as shown in Scheme 1. From the earlier reports [7,8], it was found that the hydrogen atom on nitrogen greatly accelerates the reaction. Also, no reaction occurred when the diphenylphosphinoamines containing P@O, P@S or P@Se bonds was treated with paraformaldehyde. This indicates that the reaction depends not only on the active hydrogen on nitrogen, but also on the oxidation state of phosphorus, i.e. the reaction occurs only if the phosphorus is in trivalent state. The mechanism of the reaction follows the Staudinger±Wittig pathway, which involves proton transfer from nitrogen to phosphorus giving the intermediate Ph2 P(O)H as shown in the proposed mechanism (Scheme 2). This may probably be due to the interaction of oxygen atom of the zwitterionic intermediate with the electrophilic phosphorus. 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