Regioselective B-Cyclometalation of a Bulky o-Carboranyl Phosphine and the Unexpected Formation of a Dirhodium(II) Complex

Abstract

The bulky carboranyl monophosphine closo-1,2-B10H10C(H)C(PtBu2) (L) has been prepared in a one-pot procedure from o-carborane. The reaction of [PdCl2(NCPh)2] with L rapidly gave the binuclear B-cyclopalladate [Pd2(μ-Cl)2(κ2-L′)2] (L′ = L deprotonated at B3) as a mixture of two diastereoisomers, assigned structures 1a and 1a′. The Cl bridges of 1a/1a′ are cleaved by the addition of PEt3 to give the mononuclear [PdCl(κ2-L′)(PEt3)] (2) as a single isomer, with the P atoms mutually trans. The metalation occurs at boron positions 3 and 6 in the carborane cluster, and DFT calculations show that the 3,6-borometalate is lower in energy than the isomeric 4,5-borometalate and 2-carbometalate. Treatment of [Rh2Cl2(CO)4] with L led to the slow precipitation of the dirhodium(II) species [Rh2(μ-Cl)2(CO)2(κ2-L′)2] (3). The crystal structures of ligand L and complexes 1a, 2, and 3 have been determined.

Resisting B–H oxidative addition: The divergent reactivity of the o-carborane and carba-closo-dodecaborate ligand substituents

“…The presence of two carborane C-H signals suggests the formation of a diastereomeric mixture (6', 6''), resulting from competitive B-H oxidative addition at the B3 and B6 positions (Fig 2). Such competitive cyclometalations have been observed previously [14] with other metals, and occur preferentially at B3/B6 positions rather than at B4/B5, since the B3/B6 boron atoms are more activated (attached to 2-carbon atoms). All other ligand 1 H resonances from the two diastereomers 6' and 6'' are superimposed by coincidence.…”

Selective B(4)−H Activation of an o‐Carboranylthioamide Based on a Palladium Precursor

“…Each cage chelates aP dc enter through B(4)ÀHa nd the PdÀBb ond length is 2.0445(5) .A gostic B(5)ÀH(5)···Pd(1) intramolecular bonds are also present in the molecule. The B(5)ÀH(5)···Pd(1) angle is 130.280(5)8 and the PdÀPd distances are in the range 3.0873(10)-3.1773 (9) .T he distances between the Pd and S atoms are about 2.3 .T he PdÀBd istance of about 2.0 in 2 is similart ot hose in other borocyclopalladateds pecies. [14] The reactionp roceeded with palladation almoste xclusively at the B(4)ÀHv ertex.T he molecular structure of complex 2 is shown in Figure 1.…”

Palladium‐Catalyzed Oxidative Annulation of 1‐Hydroxy‐o‐Carborane with Internal Alkynes: Facile Synthesis of Carborane‐Fused Oxaboroles†

“…[ 13 ] Regioselective electrophilic attack at B(3)—H gives a palladacycle C . [ 14,15 ] Reductive elimination affords the final product 3 with the release of Pd(0). Pd(0) is then oxidized by PhI(OAc) 2 to regenerate the active Pd(II) catalyst.…”