Reactive E=C [p‐p]π Systems, 45[]. 1‐Aza‐3,4‐diphospholenes by [3 + 2] Cycloaddition of 2‐(Diisopropylamino)phosphaethyne to Diazo Derivatives R¹R²C=N₂

Abstract

The reaction of the (diisopropy1amino)phosphaethyne (1) with diazo compounds of the type R1R2C=N2 (2a-2c) unexpectedly leads in high yields (SO-SO%) to the l-aza-3,4-diphospholene derivatives 3a-3c, a new class of heterocycles.NMR investigations of the analogous reaction of 1 with diazocyclopentadiene 2d show that the multi-step formation of 3a-3c proceeds via the l-aza-3,4-diphospholes 6a-6d as intermediates.Phosphaalkynes RC=P exhibit a pronounced reactivity towards 1,3-dipoles and, therefore, undergo [3 + 21 cycloaddition with diazo compounds R1(H)C=N2 or R1R2C=N2 yielding regiospecifically 1 H-l,2,4-or 3 H-l,2,4-diazaphospholes[2]. Scheme 1In order to examine the cycloaddition potential of aminosubstituted phosphaalkynes R2N-C=P['s3I in comparison with the alkyl compounds RC=P we studied reactions of iPr2N-C=P (1) with diazomethane derivatives. With the reactants R'(H)C=N2 the 1,3-dipolar cycloaddition of 1 proceeds as in the case of the alkyl-substituted phosphaalkynes [eq. (la)] affording 1 H-l,2,4-dia~aphospholes[~], however, with loss of regioselectivity. Hence formation of the regioisomeric 1 H-l,2,3-diazaphospholes as side products is observed. Using R1R2C=N2 (2a-2c) as 1,3-dipolar reagents, we found a surprising substituent effect for 1 leading to the novel phosphaheterocycles 3a-3c according to Scheme 2 and not to the expected 3H-or 4H-1,2,4-diazaphospholes [eq. (1 b)].The reactions of 1 with 2a-2c take place at 25 "C in toluene solution with evolution of N2 and elimination of iPr2N-C=N which is collected in a cold trap (-196°C) [ and detected by GC-MS spectroscopy. During this process the color of the reaction mixture changes from red to brown. 31P-NMR control measurements indicate complete consumption of 1 together with an almost quantitative generation of 3a-3c after 3 to 5 hours. While 3a and 3c are stable at room temperature, even in organic solvents, the mixture of the diastereomers 3b undergoes a slow decomposition in toluene solution giving rise to the formation of a series of unidentified phosphorus-containing compounds. The molecular structures of 3a-3c were deduced from spectroscopic investigations and conclusively ascertained by an X-ray diffraction study of single crystals of 3c. The mass spectra (70 eV) of 3a and 3c show the molecular peaks M+(3a) = 520 and M+(3c) = 516 with relative intensities of 52 and 22%, respectively. The 31P resonances of 3a-3c result from an AB spin system and appear in the high-field region (lip = -109.6 to -128.6) with lJ(PA,PB) couplings of 190-203 Hz. Chemical shifts and coupling constants are characteristic of diph~sphjranes [~], especially for spirocyclic derivativesL61. Due to the chirality of the P atoms in 3a-3c

A Convenient Synthesis of 1,2,4‐ and 1,3,4‐Azadiphospholes

“…The annulated analogue ADP2 was formed by the reaction of 2‐phosphanylaniline with P(NMe 2 ) 3 with elimination of three equivalents of HNMe 2 . The spiro compound ADP3 is the cycloaddition product of ( i Pr) 2 N‐C≡P and 5‐diazocyclopenta‐1,3‐diene . Compounds ADP4 and ADP5 were isolated from a mixture obtained following the reaction of 1‐piperidino‐cyanide with a tungsten‐coordinated azaphosphirene and an excess of t Bu‐C≡P .…”

“…[33] The spiro compound ADP3 is the cycloaddition product of (iPr) 2 N-CPa nd 5-diazocyclopenta-1,3-diene. [34] Compounds ADP4 and ADP5 were isolated from am ixture obtainedf ollowing the reaction of 1-piperidino-cyanide with at ungsten-coordinated azaphosphirene anda ne xcess of tBu-CP. [35] Cycloaddition reactions of tBu-CPw ith metal-imido complexes (of vanadium, titanium, or zirconium)y ieldedt he 1,2,4-azadiphospholes ADP6 as majori somers.…”

Synthesis and Structure of K+[iPrNCP]-, a 1-Aza-3λ3-phospha-3-allenide

“…13 2-(2,2,6,6-Tetramethylpiperidino)-1λ 3 -phosphaalkyne (A) 15 and the thermally unstable 2-(diisopropylamino)-1λ 3 -phosphaalkyne (B) 16 have both been structurally characterized. No reactions, however, have been reported for the former, whereas several products of the latter are known: Grobe et al treated i Pr 2 NsCtP with diazo compounds, 14,17 and Hahn and co-workers treated it with Arduengo carbenes 13 to give five-membered heterocycles. Furthermore, Grobe et al reacted i Pr 2 NsCtP with F 3 CsPdCF 2 to furnish a 1,2dihydro-1λ 3 ,3λ 3 -diphosphete via [2+2] cycloaddition; they also studied the methylation of the λ 3 -phosphaalkyne as well as reactions with chalcogens and with several metal complexes.…”