Regio- and chemoselective oxidation of the bis(bidentate) phosphine cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutaneviacobalt(II) mediated dioxygen activation

Abstract

The bis(bidentate) phosphine cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane (dppcb) has been regioselectively oxidized leading to novel, hemilabile ligands. [Co₂Cl₄(dppcb)] (1a) is transformed viacobalt(II) mediated dioxygen activation into [Co₂Cl₄(2,3-trans-dppcbO₂)] (2a) in excellent yield, where 2,3-trans-dppcbO₂ is cis,trans,cis-2,3-bis(diphenylphosphinoyl)-1,4-bis(diphenylphosphino)cyclobutane. By contrast, the in situ presence of dioxygen during the synthesis of Co₂Br₄(dppcb)] (1b) produces both [Co₂Br₄(2,3-trans-dppcbO₂)] (2b) and [Co₂Br₄(1,3-trans-dppcbO₂)] (3), where 1,3-trans-dppcbO₂ is cis,trans,cis-1,3-bis(diphenylphosphinoyl)-2,4-bis(diphenylphosphino)cyclobutane. The new compounds 2a, 2b and 3 have been obtained as pure, crystalline solids and all three X-ray structure analyses have been performed showing folded cyclobutane rings. Interestingly, the corresponding reaction using [Co₂I₄(dppcb)] (1c) proceeds chemoselectively. Thus, [Co₂I₄(dppcbO₃)] (4), where dppcbO₃ is cis,trans,cis-1,2,3-tris(diphenylphosphinoyl)-4-diphenylphosphinocyclobutane, is formed in excellent yield and also fully characterized by an X-ray structure analysis showing two different conformations of 4. However, [Co₂(NO₃)₄(dppcb)] (1d) shows no dioxygen activation at all. Therefore, in order to reveal the mechanism of this oxidation [Co₂I₄(DMF)₂(dppcb)] (5) has been prepared and its X-ray structure is presented. The synthesis of [Co₂I₄(PMe₂Ph)₂(dppcb)] (6) proves that this is a common reaction pathway. Furthermore, because the product distribution of the oxidation strongly depends on the kind of halides present, the whole series Co₂X₄(dppcbO₄)] (X = Cl, 7a; Br, 7b; I, 7c) has been prepared, where dppcbO₄ is cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphinoyl)cyclobutane, and all three X-ray structures are given, also showing folded cyclobutane rings. It seems likely that coordination of dppcb to cobalt(II) is essential to form the regio- and chemoselectively oxygenated molecules.