Synthesis of 5,6-dihydro-4 H-1,3-oxazines from neutral and cationic platinum(II) nitrile complexes. X-ray structure of trans-[Pt(CF 3){ [formula omitted]H 2CH 2}(PPh 3) 2]BF 4

Abstract

The 1,3-oxazine complexes cis- and trans-[PtCl 2{ N C(R)OCH 2CH 2C}H 2 2] ( cis: RCH 3 ( 1a), CH 2CH 3 ( 2a), (CH 3) 3C ( 3a), C 6H 5 ( 4a); trans:R CH 3 ( 1b), C 6H 5 ( 4b)) were obtained in 51-71% yield by reaction in THF at 0 °C of the corresponding nitrile complexes cis- and trans-[PtCl 2(NCR) 2] with 2 equiv. of −OCH 2CH 2CH 2Cl, generated by deprotonation of 3-chloro-1-propanol with n-BuLi. The cationic nitrile complexes trans-[Pt(CF 3)(NCR)(PPh 3) 2]BF 4 (RCH 3, C 6H 5) react with 1 equiv, of − OCH 2CH 2CH 2Cl to give a mixture of products, including the corresponding oxazine derivatives trans-[Pt(CF 3){ NC(R)OCH2CH2C CH 2}(PPh 3) 2]BF 4 ( 5 and 6), the chloro complex trans- [Pt(CF 3)Cl(PPh 3) 2] and free oxazine NC(R)OCH 2CH 2C H2. For short reaction times ( c. 5–15 min) the oxazine complexes 5 and 6 could be isolated in modest yield (37–49%) from the reaction mixtures and they could be separated from the corresponding chloro complex (yield 40%) by taking advantage of the higher solubility of the latter derivative in benzene. For longer reaction times (> 2 h), trans-[Pt(CF 3)Cl(PPh 3) 2] was the only isolated product. Complex 6 was crystallographically characterized and it was found to contain also crystals of trans- [PtCl{ NC(R)OCH 2CH 2C H 2}(PPh 3) 2]BF 4, which prevented a more detailed analysis of the bond lengths and angles within the metal coordination sphere. The 1,3-oxazine ring, which shows an overall planar arrangement, is characterized by high thermal values of the carbon atoms of the methylene groups indicative of disordering in this part of the molecule in agreement with fast dynamic ring processes suggested on the basis of 1H NMR spectra. It crystallizes in the trigonal space group P 3 , with a=22.590(4), b=15.970(3) Å, γ=120°, V=7058(1) Å 3 and Z=6. The structure was refined to R=0.059 for 3903 unique observed ( I⩾3σ( I)) reflections. A mechanism is proposed for the conversion of nitrile ligands to oxazines in Pt(II) complexes.