[2 + 3] Cycloaddition of Nitrones to Platinum-Bound Organonitriles:  Effect of Metal Oxidation State and of Nitrile Substituent

Abstract

The ligated benzonitriles in the platinum(II) complex [PtCl2(PhCN)2] undergo metal-mediated [2 + 3] cycloaddition with nitrones -ON+(R3)=C(R1)(R2) [R1/R2/R3 = H/Ph/Me, H/p-MeC6H4/Me, H/Ph/CH2Ph] to give delta 4-1,2,4-oxadiazoline complexes, [PtCl2(N=C(Ph)O-N(R3)-C(R1)(R2))2] (2a, 4a, 6a), as a 1:1 mixture of two diastereoisomers, in 60-75% yields, while [PtCl2(MeCN)2] is inactive toward the addition. However, a strong activation of acetonitrile was reached by application of the platinum(IV) complex [PtCl4(MeCN)2] and both [PtCl4(RCN)2] (R = Me, Ph) react smoothly with various nitrones to give [PtCl4(N=C(R)O-N(R3)-C(R1)(R2))2] (1b-6b). The latter were reduced to the corresponding platinum(II) complexes [PtCl2(N=C(R)O-N(R3)-C(R1)(R2))2] (1a-6a) by treatment with PhCH2NHOH, while the reverse reaction, i.e. conversion of 1a-6a to 1b-6b, was achieved by chlorination with Cl2. The diastereoisomers of [PtCl2(N=C(R)O-N(R3)-C(R1)(R2))2] (1a-6a) exhibit different kinetic labilities, and liberation of the delta 4-1,2,4-oxadiazolines by substitution with 1,2-bis(diphenylphosphino)ethane (dppe) in CDCl3 proceeds at different reaction rates to give free N=C(R)O-N(R3)-C(R1)(R2) and [PtCl2(dppe)] in almost quantitative NMR yield. All prepared compounds were characterized by elemental analyses, FAB mass spectrometry, and IR and 1H, 13C(1H), and 195Pt (metal complexes) NMR spectroscopies; X-ray structure determination of the first (delta 4-1,2,4-oxadiazoline)Pt(II) complexes was performed for (S,S)/(R,R)-rac-[PtCl2(N=C(Me)O-N(Me)-C(H)Ph)2] (1a) (a = 9.3562(4), b = 9.8046(3), c = 13.1146(5) A; alpha = 76.155(2), beta = 83.421(2), gamma = 73.285(2) degrees; V = 1117.39(7) A3; triclinic, P1, Z = 2), (R,S)-meso-[PtCl2(N=C(Ph)O-N(Me)-C(H)Ph)2] (2a) (a = 8.9689(9), b = 9.1365(5), c = 10.1846(10) A; alpha = 64.328(6), beta = 72.532(4), gamma = 67.744(6) degrees; V = 686.82(11) A3; triclinic, P1, Z = 1), (S,S)/(R,R)-rac-[PtCl2(N=C(Me)O-N(Me)-C(H)(p-C6H4Me))2] (3a) (a = 11.6378(2), b = 19.0767(7), c = 11.5782(4) A; beta = 111.062(2) degrees; V = 2398.76(13) A3; monoclinic, P2(1)/c, Z = 4), and (S,S)/(R,R)-rac-[PtCl2(N=C(Me)O-N(CH2Ph)-C(H)Ph2] (5a) (a = 10.664(2), b = 10.879(2), c = 14.388(3) A; alpha = 73.11(3), beta = 78.30(3), gamma = 88.88(3) degrees; V = 1562.6(6) A3; triclinic, P1, Z = 2).

Oxadiazoles

“…If the Lewis acid is coordinated to the nitrile and strong enough, the process requires a stoichiometric amount of Lewis acid and forms a stable Lewis acid-product complex. 13.3.3 Structural Aspects 13.3.3.1 X-Ray Diffraction X-Ray data of many 1,2.4-oxadiazoles confirms that the ring is planar [149][150][151][152][153][154].…”

Metal‐Mediated [2+3] Cycloaddition of Nitrones to Palladium‐Bound Isonitriles

“…In the course of our studies on metal-mediated nucleophilic additions and cycloadditions to isonitriles RNC, [9] and upon further extension of our project on cycloadditions to ligated nitriles RCN, [10,11] we have succeeded for the first time in performing the [2+3] cycloaddition of an allyl-anion-type dipole, that is, a nitrone, to coordinated isonitriles, isolating and characterizing novel carbene complexes derived from the cycloaddition of both acyclic and cyclic nitrones. Details of the results obtained are elaborated in this article.…”

A Route to 1,2,4-Oxadiazoles and Their Complexes via Platinum-Mediated 1,3-Dipolar Cycloaddition of Nitrile Oxides to Organonitriles

“…22,23 Two relatively stable aryl nitrile oxides, i.e., 2,4,6-Me 3 C 6 H 2 CNO and 2,4,6-(MeO) 3 C 6 H 2 CNO, 27 have been chosen as dipoles. The reaction between [PtCl 4 (EtCN) 2 ], exhibiting moderate solubility in CH 2 Cl 2 , and the nitrile oxides in a molar ratio of 1:4 proceeds smoothly in a suspension at room temperature and is complete after 1 day (Scheme 1), and the final products were isolated in good yields.…”

“…20 A few years ago, within the framework of our continuous project on reactions of metal-activated nitriles (this topic has been reviewed by two of us 20 ), it was found that the platinum(IV) center in [PtCl 4 (RCN) 2 ] complexes provides sufficiently strong activation of the nitriles to assist the facile [2 + 3] cycloaddition between RCN ligands and various nitrones (R 1 )(R 2 )Cd(R 3 )-N + O -to achieve the first examples of ∆ 4 -1,2,4-oxadiazoline complexes [PtCl 4 (∆ 4 -1,2,4-oxadiazoline) 2 ]. 22,23 In this work, we endeavored to extend the [2 + 3] cycloaddition of complexed RCN species from nitrones to nitrile oxides. The main goals of this work are the following ones: (i) to determine whether the results disclosing the enhanced reactivity of Pt(IV)-bound nitriles are specific for dipoles of allyl anion type, 24 e.g., nitrones, or the reactions can be spread out to dipoles of the propargyl/allenyl anion type, 24 e.g., nitrile oxides; (ii) to develop a general route to 1,2,4-oxadiazoles which is based on the [2 + 3] cycloaddition between the metal-activated nitriles and nitrile oxides and to perform the synthesis under mild conditions, thus preventing the nitrile oxides from dimerizing.…”