Platinum Assisted Tandem P-C Bond Cleavage and P-N Bond Formation in Amide Functionalized Bisphosphine o-Ph2PC6H4C(O)N(H)C6H4PPh2-o: Synthesis, Mechanistic, and Catalytic Studies

The reactions of amide functionalized bisphosphine o-Ph₂PC₆H₄C(O)N(H)C₆H₄PPh₂-o (1) with platinum salts are described. Treatment of 1 with [Pt(COD)Cl₂] yielded a chelate complex, [PtCl₂{o-Ph₂PC₆H₄C(O)N(H)C₆H₄PPh₂-o}κ²-P,P] (2), which on subsequent treatment with LiHMDS formed a novel 1,2-azaphospholene-phosphine complex [Pt(C₆H₅)Cl{o-C₆H₄{C(O)N(o-PPh₂(C₆H₄))P(Ph)}}κ²-P,P] (3) involving a tandem P-C bond cleavage and P-N bond formation. The same complex 3 on passing dry HCl gas afforded the dichloro complex [PtCl₂{o-C₆H₄{C(O)N(o-PPh₂(C₆H₄))P(Ph)}}κ²-P,P] (5). Complex 2 upon refluxing in toluene or treatment of 1 with [Pt(COD)Cl₂] in the presence of a base at room temperature resulted in the pincer complex [PtCl{o-Ph₂PC₆H₄C(O)N(C₆H₄PPh₂-o)}κ³-P,N,P] (4). Reaction of 1 with [Pt(COD)ClMe] at room temperature also afforded the pincer complex [PtMe{o-Ph₂PC₆H₄C(O)N(C₆H₄PPh₂-o)}κ³-P,N,P] (6). Mechanistic studies on 1,2-azaphospholene formation showed the reductive elimination of LiCl to form a phosphonium salt that readily adds one of the P-C bonds oxidatively to the in situ generated Pt⁰ species to form a chelate complex 3. The analogous palladium complex [PdCl₂{o-C₆H₄{C(O)N(o-PPh₂(C₆H₄))P(Ph)}}κ²-P,P] (7) showed excellent catalytic activity toward N-alkylation of amines with alcohols with a very low catalyst loading (0.05 mol %), and the methodology is very efficient toward the gram-scale synthesis of many N-alkylated amines.

Contrasting reactivity of (boryl)(aryl)lithium-amide with electrophiles: N- vs. p-aryl-C-nucleophilic substitution

Herein we report two different reactivity modes of lithium(aryl)(boryl)amide, 4, when it is reacted with chlorosilanes such as SiCl4 and MeSiHCl2, and chlorophosphine, Ph2PCl. Thus, the reaction of lithium(aryl)(boryl)amide, 4, with MeSiHCl2 leads exclusively to an N-substitution product, 6. On the other hand, the reaction of 4 with SiCl4 and Ph2PCl proceeds completely differently affording exclusively p-aryl-C-substitution products, 5 and 7, respectively.

FLP reactivity of [Ph3C]+ and (o-tolyl)3P and the capture of a Staudinger reaction intermediate

The frustrated Lewis pair (FLP) derived from the trityl cation and (o-tolyl)₃P effects the activation of 1,4-cyclohexadiene and 1-bromo-4-ethynylbenzene and heterolytically cleaves the S-S bond of diphenyl disulfide. The FLP also captures pentafluorophenyl azide as the Staudinger reaction intermediate, a species that reacts with Ph₃SiH to give the silyl analog.

Synthesis and complexation of superbulky imidazolium-2-dithiocarboxylate ligands

The superbulky N-heterocyclic carbenes (NHCs) 2,6-bis(diphenylmethyl)-4-methylimidazol-2-ylidene (IDip*Me) and its 4-methoxy analogue (IDip*OMe) reacted instantaneously with carbon disulfide to afford the corresponding imidazolium-2-dithiocarboxylate zwitterions in high yields. These new dithiolate ligands were fully characterized and their coordination chemistry toward common Re(i) and Ru(ii) metal sources was thoroughly investigated. Neutral [ReBr(CO)₃(S₂C·NHC)] chelates featured three facially-arranged carbonyl groups on a distorted octahedron, whereas cationic [RuCl(p-cymene)(S₂C·NHC)]PF₆ complexes displayed a piano-stool geometry. The molecular structures of the six new compounds revealed that the NHC·CS₂ inner salts were highly flexible. Indeed, the torsion angle between their anionic and cationic moieties varied between ca. 63° in the free ligands and 3° in the ruthenium complexes. Concomitantly, the S-C-S bite angle underwent a contraction from 131° to 110-113° upon chelation. Computation of the %V_Bur parameter showed that the dithiocarboxylate unit of the NHC·CS₂ betaines chiefly determined the steric requirements of the imidazolium moieties, irrespective of the metal center involved in the complexation. The replacement of the p-methyl substituents of IDip*Me with p-methoxy groups in IDip*OMe did not significantly affect the ligand bulkiness. The more electron-donating methoxy group led, however, to small changes in various IR wavenumbers used to probe the electron donor properties of the carbene moiety.

Ring-Shaped Phosphinoamido-Magnesium-Hydride Complexes: Syntheses, Structures, Reactivity, and Catalysis

A series of magnesium(II) complexes bearing the sterically demanding phosphinoamide ligand, L(-) =Ph2 PNDip(-) , Dip=2,6-diisopropylphenyl, including heteroleptic magnesium alkyl and hydride complexes are described. The ligand geometry enforces various novel ring and cluster geometries for the heteroleptic compounds. We have studied the stoichiometric reactivity of [(LMgH)4 ] towards unsaturated substrates, and investigated catalytic hydroborations and hydrosilylations of ketones and pyridines. We found that hydroborations of two ketones with pinacolborane using various Mg precatalysts is very rapid at room temperature with very low catalyst loadings, and ketone hydrosilylation using phenylsilane is rapid at 70 °C. Our studies point to an insertion/σ-bond metathesis catalytic cycle of an in situ formed "MgH2 " active species.