Highly Selective Nickel-Catalyzed Isomerization-Hydroboration of Alkenes Affords Terminal Functionalization at Remote C-H Position.

We report herein the synthesis and characterization of nickel complexes supported by tridentate and bidentate phosphino(silyl) ancillary ligands, along with the successful application of these complexes as precatalysts for the hydroboration of terminal and internal alkenes using pinacolborane (HBPin). These reactions proceeded with low nickel loadings of 2.5-5 mol % in the absence of co-solvent, and in some cases at room temperature. Isomerization to afford exclusively the terminal hydroboration product was obtained across a range of internal alkenes, including tri- and tetra-substituted examples. This reactivity is unprecedented for nickel and offers a powerful means of achieving functionalization at a C-H position remote from the C=C double bond. Nickel-catalyzed deuteroboration experiments using DBPin support a mechanism involving 1,2-insertion of the alkene and subsequent chain-walking, which results in isotopic scrambling.

Synthetic investigations of low-coordinate (N-phosphino-amidinate) nickel chemistry: agostic alkyl complexes and benzene insertion into Ni-H.

Treatment of (PN)NiX (X = NHdipp or OtBu; PN = N-phosphinoamidinate ligand) with Me2PhSiH in benzene solvent afforded the crystallographically characterized, antifacial-coordinated, dinuclear species 1, the formation of which corresponds to the hitherto unknown net Ni-H addition of two equivalents of the putative (PN)NiH intermediate across C[double bond, length as m-dash]C units within a single benzene molecule. Computational analysis supports the view of 1 as being comprised of two cationic (PN)NiII fragments ligated by a substituted butadiene dianion µ2-η3:η3-C6H82- bridging group. Also described is the formation and characterization of three-coordinate (PN)Ni(alkyl) complexes stabilized by ß-agostic (alkyl = Et, 2; n-Bu, 3; n-hexyl, 4) or γ-agostic (alkyl = neopentyl, 5) interactions, and our efforts to employ 2 and 3 as synthons for the generation of (PN)NiHvia ß-hydride elimination. Notably, compound 5 represents both the first crystallographically characterized three-coordinate Ni-alkyl complex featuring a heterobidentate ligation, and the first neutral γ-agostic NiII-alkyl complex.

A comparative analysis of hydrosilative amide reduction catalyzed by first-row transition metal (Mn, Fe, Co, and Ni) N-phosphinoamidinate complexes.

A comparative study of the performance of (PN)M(N(SiMe3)2) (M = Mn, Fe, Co, and Ni) pre-catalysts supported by N-phosphinoamidinate ligation, as well as M(N(SiMe3)2)n (M = Li, Na, K, Mn, Fe, and Co) pre-catalysts, in the hydrosilative reduction of selected tertiary amide test substrates using PhSiH3 is reported. Encouraged by the performance observed herein for (PN)Ni(N(SiMe3)2) in the reduction of both N,N-dibenzylbenzamide and N,N-diisopropylbenzamide, further competitive testing involving the known complex (PN)Ni(NHdipp) (dipp = 2,6-diisopropylphenyl), as well as the new and crystallographically characterized mononuclear complexes (PN)Ni(OR) (R = 2,6-dimethylphenyl or tBu), revealed (PN)Ni(OtBu) to be particularly effective in such reduction chemistry, including transformations involving the secondary amides N-benzylbenzamide and caprolactam.

A Manganese Pre-Catalyst: Mild Reduction of Amides, Ketones, Aldehydes, and Esters.

A new (N-phosphinoamidinate)manganese complex is shown to be a useful pre-catalyst for the hydrosilative reduction of carbonyl compounds, and in most cases at room temperature. The Mn-catalyzed reduction of tertiary amides to tertiary amines, with a useful scope, is demonstrated for the first time by use of this catalyst, and is competitive with the most effective transition-metal catalysts known for such transformations. Ketones, aldehydes, and esters were also successfully reduced under mild conditions by using this new Mn catalyst.

Dehydrogenative B-H/C(sp3 )-H Benzylic Borylation within the Coordination Sphere of Platinum(II).

The first examples of stoichiometric dehydrogenative B-H/C(sp3 )-H benzylic borylation reactions, which are of relevance to catalytic methylarene (di)borylation, are reported. These unusual transformations involving a (κ2 -P,N)Pt(η3 -benzyl) complex, and either pinacolborane or catecholborane, proceed cleanly at room temperature. Density functional calculations suggest that borylation occurs via successive σ-bond metathesis steps, whereby a PtII -H intermediate engages in C(sp3 )-H bond activation-induced dehydrogenation.

Synthesis and characterization of five-coordinate, 16-electron RuII complexes supported by tridentate bis(phosphino)silyl ligation.

The synthesis of 16-electron complexes of the type (Cy-PSiP)RuX(L) (Cy-PSiP = κ3-(2-Cy2PC6H4)2SiMe) is reported. Treatment of (Cy-PSiP)H with one equiv. RuCl2(PPh3)3 in the presence of Et3N afforded a mixture of (Cy-PSiP)RuCl(PPh3) (1) and [(Cy-PSiP)RuCl]2 (2), which exists in a temperature-dependent equilibrium process involving reversible dissociation of PPh3 from 1 to form 2. While treatment of this equilibrium mixture with PMe3 afforded (Cy-PSiP)RuCl(PMe3) (3) cleanly, this product was more straightforwardly prepared by the addition of PMe3 to 2 (97% yield). Attempts to form an alkyl complex of the type (Cy-PSiP)RuMe(PMe3) via treatment of 3 with MeMgBr led to the dehydrogenation and cyclometalation of a cyclohexyl phosphino group to give the isolable 18-electron complex [MeSi(C6H4PCy2)(C6H4PCy(η3-C6H8))]RuPMe3 (4, 78% yield). The hydrido complex (Cy-PSiP)RuH(PMe3) (5) was identified as a minor by-product in this dehydrogenative process. Whereas simple 16-electron alkyl complexes of the type (Cy-PSiP)RuR(PMe3) remained elusive, the isolable allyl complex (Cy-PSiP)Ru(η3-C3H5) (9) was successfully prepared from 2 and (C3H5)MgCl (79% yield). Treatment of 3 with Me3SiN3 or NaN3 did not generate a five-coordinate azido species. However, the putative dinuclear complex [(Cy-PSiP)RuN3]2 (10) was observed to react with one equiv. of PMe3 to afford the isolable five-coordinate azido complex (Cy-PSiP)Ru(N3)(PMe3) (11; 94% yield). Crystallographic data for 3, 4, 5, 9 and 11 are presented.

Synthesis and Reactivity of a Neutral, Three-Coordinate Platinum(II) Complex Featuring Terminal Amido Ligation.

A crystallographically characterized three-coordinate, formally 14 electron Pt(II) complex 1 featuring terminal amido ligation is reported. Computational analysis revealed relatively weak π donation from the amide lone pair to platinum and supports a 14-electron assignment for 1. Stoichiometric reactivity studies confirmed the viability of net O-H and C-H addition across, as well as isonitrile insertion into, the terminal platinum-amido linkage of 1.

(N-phosphinoamidinate)cobalt-catalyzed hydroboration: alkene isomerization affords terminal selectivity.

Herein we establish the utility of a three-coordinate (N-phosphinoamidinate)cobalt(amido) pre-catalyst that is capable of effecting challenging alkene isomerization/hydroboration processes at room temperature, leading to the selective terminal addition of the boron group.

'Hemilabile' silyl pincer ligation: platinum group PSiN complexes and triple C-H activation to form a (PSiC)Ru carbene complex.

The first example of PSiN mixed-donor silyl pincer ligation is described. Studies involving platinum group metal complexes of [(2-(t)Bu(2)PC(6)H(4))(2-Me(2)NC(6)H(4))SiMe](-) ((t)Bu-PSiN-Me) confirmed that the ligand amino donor is labile. Within the coordination sphere of Ru, (t)Bu-PSiN-Me is transformed into a PSiC ligand via multiple C-H bond activation events.

Four-coordinate, 14-electron Ru(II) complexes: unusual trigonal pyramidal geometry enforced by bis(phosphino)silyl ligation.

Unprecedented diamagnetic, four-coordinate, formally 14-electron (Cy-PSiP)RuX (Cy-PSiP = [κ(3)-(2-R(2)PC(6)H(4))(2)SiMe](-); X = amido, alkoxo) complexes that do not require agostic stabilization and that adopt a highly unusual trigonal pyramidal coordination geometry are reported. The tertiary silane [(2-Cy(2)PC(6)H(4))(2)SiMe]H ((Cy-PSiP)H) reacted with 0.5 [(p-cymene)RuCl(2)](2) in the presence of Et(3)N and PCy(3) to afford [(Cy-PSiP)RuCl](2) (1) in 74% yield. Treatment of 1 with KO(t)Bu led to the formation of (Cy-PSiP)RuO(t)Bu (2, 97% yield), which was crystallographically characterized and shown to adopt a trigonal pyramidal coordination geometry in the solid state. Treatment of 1 with NaN(SiMe(3))(2) led to the formation of (Cy-PSiP)RuN(SiMe(3))(2) (3, 70% yield), which was also found to adopt a trigonal pyramidal coordination geometry in the solid state. The related anilido complexes (Cy-PSiP)RuNH(2,6-R(2)C(6)H(3)) (4, R = H; 5, R = Me) were also prepared in >90% yields by treating 1 with LiNH(2,6-R(2)C(6)H(3)) (R = H, Me) reagents. The solid state structure of 5 indicates a monomeric trigonal pyramidal complex that features a C-H agostic interaction. Complexes 2 and 3 were found to react readily with 1 equiv of H(2)O to form the dimeric hydroxo-bridged complex [(Cy-PSiP)RuOH](2) (6, 94% yield), which was crystallographically characterized. Complexes 2 and 3 also reacted with 1 equiv of PhOH to form the new 18-electron η(5)-oxocyclohexadienyl complex (Cy-PSiP)Ru(η(5)-C(6)H(5)O) (7, 84% yield). Both amido and alkoxo (Cy-PSiP)RuX complexes reacted with H(3)B·NHRR' reagents to form bis(σ-B-H) complexes of the type (Cy-PSiP)RuH(η(2):η(2)-H(2)BNRR') (8, R = R' = H; 9, R = R' = Me; 10, R = H, R' = (t)Bu), which illustrates that such four-coordinate (Cy-PSiP)RuX (X = amido, alkoxo) complexes are able to undergo multiple E-H (E = main group element) bond activation steps. Computational methods were used to investigate structurally related PCP, PPP, PNP, and PSiP four-coordinate Ru complexes and confirmed the key role of the strongly σ-donating silyl group of the PSiP ligand set in enforcing the unusual trigonal pyramidal coordination geometry featured in complexes 2-5, thus substantiating a new strategy for the synthesis of low-coordinate Ru species. The mechanism of the activation of ammonia-borane by such low-coordinate (R-PSiP)RuX (X = amido, alkoxo) species was also studied computationally and was determined to proceed most likely in a stepwise fashion via intramolecular deprotonation of ammonia and subsequent borane B-H bond oxidative addition steps.

Rhodium and iridium amido complexes supported by silyl pincer ligation: ammonia N-H bond activation by a [PSiP]Ir complex.

Ir silyl pincer complexes insert into the N-H bonds of anilines and ammonia under mild conditions to form isolable [Cy-PSiP]Ir(H)(NHR) complexes that are resistant to N-H bond reductive elimination at room temperature, even in the presence of arenes, alkenes, and phosphines.

Room temperature benzene C-H activation by a new [PSiP]Ir pincer complex.

The synthesis and reactivity of coordinatively unsaturated Rh and Ir complexes supported by the new bis(phosphino)silyl pincer ligand [kappa(3)-(2-Cy(2)PC(6)H(4))(2)SiMe](-) ([Cy-PSiP](-)) are reported, including the first example of facile, room temperature intermolecular arene C-H bond activation mediated by a silyl pincer complex.