General strategy for inducing C–H bond fission (cycloiridation) in some aryl, heterocyclic, alkenyl or alkyl groups in azines derived from aldehydes or methyl ketones

Abstract

The phosphino hydrazone Z-PPh₂CH₂C(Bu^t)NNH₂I has been shown to be a convenient ‘reagent’ for converting aldehydes or ketones into azines which can then be cycloiridated rapidly with C–H bond fission to give iridium(III) hydrides. Condensation of I with a series of substituted benzaldehydes (RCHO) gave mixed azines of type Z,E-PPh₂CH₂C(Bu^t)N–NCHR [R = C₆H₂(OMe)₃-3,4,5 IIa, Ph IIb, C₆H₄OMe-4 IIc, C₆H₄NO₂-4 IId, C₆H₄Br-2 IIe, C₆H₄Cl-2 IIf, C₆H₄F-2 IIg, C₆H₄OH-2 IIh or C₆H₂(OH-2)(OMe)₂-4,6, IIi]. The Z,E configuration is necessary for the subsequent cyclometallation and when the azine IIa was treated with [IrCl(CO)₂(H₂NC₆H₄Me-p)] in benzene at 75 °C for 5 min it underwent an aryl C–H bond fission to give the cyclometallated chlorocarbonyliridium(III) hydride [[graphic omitted]H(OMe)₃-3,4,5]}]1a. The crystal structure of 1a showed that (i) oxidative addition of the aryl C–H bond to iridium was cis with the hydride ligand trans to chloride, and (II) the cyclometallated azine ligand was in the terdentate mer arrangement. Azines IIb–IIg reacted similarly. Treatment of IIh with [IrCl(CO)₂(H₂NC₆H₄Me-p)] in dichloromethane at 20 °C effected O–H bond fission to give the O-cyclometallated iridium(III) hydride [[graphic omitted])}], which isomerised in benzene at 75 °C to the C-metallated isomer. Similar treatment of IIi also gave an O-cyclometallated iridium(III) hydride. The phosphino hydrazone I condensed with ferrocenecarbaldehyde to give an azine, which with [IrCl(CO)₂(H₂NC₆H₄Me-p)] gave a cyclometallated iridium(III) hydride. Treatment of I with the 2-carbaldehydes of pyrrole, N-methylpyrrole or thiophene or with indole-3-carbaldehyde gave the corresponding azine phosphines with the required Z,E configuration. When the pyrrole azine was treated with [IrCl(CO)₂(H₂NC₆H₄Me-p)] the N-cyclometallated chlorocarbonyliridium(III) hydride [[graphic omitted])}] was formed via a N–H bond fission. The azine phosphines of N-methylpyrrole, thiophene and indole gave C-cyclometallated chlorocarbonyliridium(III) hydrides. Condensation of I with cinnamaldehyde or 3-methylpent-3-en-2-one gave azines which underwent oxidative addition of the olefinic C–H bonds readily to give corresponding C-cyclometallated iridium(III) hydrides. Phosphino hydrazone I also condensed with tert-butyl methyl ketone to give an azine which with [IrCl(CO)₂(H₂NC₆H₄Me-p)] gave the C-cyclometallated chlorocarbonyliridium(III) hydride, i.e. metallation in this case occurred on the single methyl group. Proton, ³¹P-{¹H} and some ¹³C-{¹H} NMR data are given.