Reactivity of 3,3,3-Trifluoropropyne at Rhodium Complexes: Development of Hydroboration Reactions

Competing C-H and C-F bond activation reactions of a fluorinated olefin at Rh: a fluorido vinylidene complex as an intermediate in an unprecedented dehydrofluorination step

The hydrofluoroolefin Z-1,3,3,3-tetrafluoropropene has been activated via an initial C-F bond activation and subsequent C-H bond activation using [Rh(H)(PEt3)3] (1) or via C-H bond activation at [Rh(CH3)(PEt3)3] (8). In both cases the formation of [Rh{(E)-CF[double bond, length as m-dash]CHCF3}(PEt3)3] (3) was observed. Importantly, the C-F activation product [Rh{(E)-CH[double bond, length as m-dash]CHCF3}(PEt3)3] (2) reacts in the presence of Z-1,3,3,3-tetrafluoropropene into 3. The latter converted into [Rh(C[triple bond, length as m-dash]CCF3)(PEt3)3] (6) by an unprecedented dehydrofluorination reaction, presumably via a vinylidene complex as intermediate. When the carbonyl complex [Rh(C[triple bond, length as m-dash]CCF3)(CO)(PEt3)3] (12) was treated with an excess of NEt3·3HF or HBF4 at low temperature, the formation of the phosphonioalkenyl compounds [Rh{(Z)-C(PEt3)[double bond, length as m-dash]CHCF3}(CO)(PEt3)2]X (X = F(HF) x , BF4) (13) was observed. The formation of 13 can be explained by an attack of PEt3 at the electrophilic α-carbon atom of an intermediate vinylidene complex. The employment of PiPr3 derivatives as model compounds allowed for the isolation of the unique fluorido vinylidene complex trans-[Rh(F)([double bond, length as m-dash]C[double bond, length as m-dash]CHCF3)(PiPr3)2] (16), which in the presence of PEt3 transforms into [Rh(C[triple bond, length as m-dash]CCF3)(PEt3)3] (6).

The transition metal-catalysed hydroboration reaction

This review covers the development of the transition metal-catalysed hydroboration reaction, from its beginnings in the 1980s to more recent developments including earth-abundant catalysts and an ever-expanding array of substrates.

Versatile Reaction Pathways of 1,1,3,3,3-Pentafluoropropene at Rh(I) Complexes [Rh(E)(PEt3 )3 ] (E=H, GePh3 , Si(OEt)3 , F, Cl): C-F versus C-H Bond Activation Steps

The reaction of the rhodium(I) complexes [Rh(E)(PEt₃)₃] (E=GePh₃ (1), H (6), F (7)) with 1,1,3,3,3‐pentafluoropropene afforded the defluorinative germylation products Z/E‐2‐(triphenylgermyl)‐1,3,3,3‐tetrafluoropropene and the fluorido complex [Rh(F)(CF₃CHCF₂)(PEt₃)₂] (2) together with the fluorophosphorane E‐(CF₃)CH=CF(PFEt₃). For [Rh(Si(OEt)₃)(PEt₃)₃] (4) the coordination of the fluoroolefin was found to give [Rh{Si(OEt)₃}(CF₃CHCF₂)(PEt₃)₂] (5). Two equivalents of complex 2 reacted further by C−F bond oxidative addition to yield [Rh(CF=CHCF₃)(PEt₃)₂(μ‐F)₃Rh(CF₃CHCF₂)(PEt₃)] (9). The role of the fluorido ligand on the reactivity of complex 2 was assessed by comparison with the analogous chlorido complex. The use of complexes 1, 4 and 6 as catalysts for the derivatization of 1,1,3,3,3‐pentafluoropropene provided products, which were generated by hydrodefluorination, hydrometallation and germylation reactions.

A SF5 Derivative of Triphenylphosphine as an Electron-Poor Ligand Precursor for Rh and Ir Complexes

The synthesis of the triarylphosphine, P(p-C₆H₄SF₅)₃ containing a SF₅ group, has been achieved. The experimental and theoretical studies showed that P(p-C₆H₄SF₅)₃ is a weaker σ-donor when compared with other substituted triarylphosphines, which is consistent with the electron-withdrawing effect of the SF₅ moiety. The studies also revealed a moderate air stability of the phosphine. The σ-donor capabilities of P(p-C₆H₄SF₅)₃ were estimated from the phosphorus-selenium coupling constant in SeP(p-C₆H₄SF₅)₃ and by DFT calculations. The behavior of P(p-C₆H₄SF₅)₃ as ligand has been investigated by the synthesis of the iridium and rhodium complexes [MCl(COD){P(p-C₆H₄SF₅)₃}], [MCl(CO)₂{P(p-C₆H₄SF₅)₃}₂] (M = Ir, Rh), or [Rh(µ-Cl)(COE){P(p-C₆H₄SF₅)₃}]₂, and the molecular structures of [IrCl(COD){P(p-C₆H₄SF₅)₃}] and [Rh(µ-Cl)(COE){P(p-C₆H₄SF₅)₃}]₂ were determined by single X-ray diffraction. The structures revealed a slightly larger cone angle for P(p-C₆H₄SF₅)₃ when compared to other para-substituted triarylphosphines.

Rhodium(I) Complexes as Useful Tools for the Activation of Fluoroolefins

In this account we describe studies on the reactivity of rhodium(I) complexes of the type [Rh(E)(PEt3)3], where E represents hydrido, fluorido, germyl, boryl or silyl ligands, towards fluorinated olefins. The results are compared with those reported by other research groups on fluoroolefins, as well as with the chemistry of compounds [Rh(E)(PEt3)3] towards fluoroaromatics in terms of selectivity and mechanisms.

1 Introduction

2 Reactivity Towards Fluoroolefins

2.1 Reactivity of Hexafluoropropene

2.2 Reactivity of (E)-1,2,3,3,3-Pentafluoropropene

2.3 Reactivity of 2,3,3,3-Tetrafluoropropene and (E)-1,3,3,3-Tetrafluoropropene

2.4 Reactivity of 3,3,3-Trifluoropropene

2.5 Reactivity of Pentafluorostyrene

3 Conclusion and Perspective

C-H and C-F bond activation reactions of pentafluorostyrene at rhodium complexes

The rhodium(i) complexes [Rh(Bpin)(PEt₃)₃] (1), [Rh(H)(PEt₃)₃] (5) and [Rh(Me)(PEt₃)₃] (14) were employed in reactions with pentafluorostyrene affording coordination of the olefin and C-F or C-H bond activation. Control of the reaction conditions allowed for selective activation reactions at different positions at the fluorinated aromatic ring. The rhodacycle trans-[Rh(F)(CH₂CH₂(2-C₆F₄))(PEt₃)₂] (7) was identified as an intermediate for an activation at the 2-position. Reactivity studies of the latter with CO led to the generation of trans-[Rh(F)(CH₂CH₂C₆F₄)(CO)(PEt₃)₂] (10). Stoichiometric and catalytic hydroboration reactions were achieved using complexes 1 or 5 as catalysts.