Insertion Reactions into the Metal−Alkyl and Metal−Amido Bonds of 1,3-Di(silyl-η-amido)cyclopentadienyl Titanium and Zirconium Complexes

Formation of cyclic (boryl)iminomethane derivatives by the insertion of isocyanides into a boron-carbon bond

Isocyanides are highly valuable reagents in organic synthesis and have been widely used in multicomponent reactions. Although η²-imidoyl metal complexes, which are important intermediates in isocyanide chemistry, have been extensively explored, their boron species analogues have remained elusive. Hererin, we reported the synthesis of cyclic (boryl)iminomethanes via direct isocyanide insertion into the B-C bond of amino alkenyl boranes in a facile synthetic procedure. A family of well-defined cyclic (boryl)iminomethanes are characterized. Furthermore, the intrinsic ring strain of cyclic (boryl)iminomethanes primes them to further react with isocyanides and selectively afford double insertion products. Our results provide new insights into novel isocyanide chemistry involving boron species.

Insertion of Isonitriles into the M-C Bonds of Group 4 Dialkyl Complexes

The 2,3-dimethylbutadiene complexes of Group 4 metals with constrained geometry (cg) ligands have been prepared and found to adopt a supine orientation with σ²,π bonding. Treatment of cgTi(2,3-dimethylbutadiene) (1-Ti) with ^tBuNC leads to the formation of a titana-aziridine (3) with a coordinated cyclopentenimine that arises from the formal [4+1] addition of the diene to the isonitrile. In contrast, the reactions of cgZr(2,3-dimethylbutadiene) (1-Zr) or cgHf(2,3-dimethylbutadiene) (1-Hf) with 2 equiv of ^tBuNC or XyNC proceeded in a more sophisticated manner to yield unsymmetrical 2,5-diazametallacyclopentane derivatives (4, 6-Zr, and 6-Hf) or symmetrical 2,5-diazametallacyclopentene complexes (7-Zr and 7-Hf). The unsymmetrical products contain coordinated cyclopropanes; the strength of the interaction is measured by the reduction in the ¹ J_CC of the C-C bond that is coordinated. A detailed mechanistic analysis has been possible with the related cgM(Me)₂ (M = Ti and Hf) complexes. The first insertion is too fast to monitor, but allows complete conversion to an alkyl iminoacyl intermediate. The second isonitrile (RNC) may react with that intermediate by either of two different mechanisms, reductive elimination and coordination/insertion. In the first mechanism (Ti), rate-determining C-C coupling gives a titana-aziridine, followed by fast coordination of the isonitrile. In the second mechanism (Hf), coordination is the slow step; insertion to form a bis(iminoacyl) Hf complex is rapid.

Iminoacyl Alkyl Complexes of Zirconium Supported by a Ferrocene-Linked Diphosphinoamide Ligand Scaffold

Zirconium dialkyl complexes of the general formula fc(NPiPr2)2ZrR2 (where fc = 1,1′-ferrocenyl, R = CH3, CH2Ph, CH2tBu, tBu) have been synthesized and characterized via the addition of alkyl lithium or potassium benzyl derivatives to the dichloride complex fc(NPiPr2)2ZrCl2(THF). Addition of 2,6-dimethylphenylisocyanide to these alkyl derivatives generates the corresponding mono iminoacyl alkyl zirconium complexes. On thermolysis, the iminoacyl moiety containing a benzyl substituent undergoes rearrangement to yield a new complex that contains an alkene-amido fragment. Mechanistic studies point to a 1,2 hydrogen shift as the rate-determining step.

{2-[(η-Cyclo-penta-dien-yl)diphenyl-meth-yl]-1H-imidazolido-κN}bis-(N,N-diethyl-amido)-titanium(IV)

The chemically achiral title mol-ecule, [Ti(C(4)H(10)N)(2)(C(21)H(16)N(2))], crystallizes in the chiral space group P2(1). All three N atoms coordinating to the Ti(IV) atom adopt planar environments [sums of valence angles = 359.5 (6), 360.0 (7) and 360.0 (6)°], which is indicative of pπ-dπ donation from all of these N atoms to the metal and, thus, of the formal 18 e(-) nature of the complex. The overall coordination about the Ti(IV) atom is distorted tetra-hedral, assuming the cyclo-penta-dienyl ring occupies one coordination site. The Ti-N(imidazole) amide-type bond is longer by approximately 0.16 Å than the other two Ti-N(amide) bonds.

Efficient one-pot synthesis of N-containing heterocycles by multicomponent coupling of silicon-tethered diynes, nitriles, and isocyanides through intramolecular cyclization of iminoacyl-Zr intermediates

An efficient multicomponent synthesis of 5-azaindoles and dihydropyrrolo[3,2-c]azepines was achieved by zirconocene-mediated coupling of silicon-tethered diynes, nitriles, and isocyanides. The synthesis, structures, and intramolecular cyclization of mono- and bis(iminoacyl)--Zr intermediates were investigated to elucidate the reaction process. Upon hydrolysis, the isolated mono(iminoacyl)--Zr intermediates underwent intramolecular cyclization to afford tetrasubstituted 5-azaindoles, whereas intramolecular cyclization of bis(iminoacyl)--Zr intermediates led to the formation of dihydropyrrolo[3,2-c]azepines. The structure of a bis(iminoacyl)--Zr intermediate, formed through insertion of two molecules of CyNC into the Zr--C bond, and structures of two dihydropyrrolo[3,2-c]azepines were characterized by single-crystal X-ray structural analysis.

Synthesis and structural studies of amido, hydrazido and imido zirconium(IV) complexes incorporating a diamido/diamine cyclam-based ligand

The preparation, characterisation and structural analysis of a series of zirconium(IV) complexes that incorporate the diamido/diamine macrocyclic ligand Bn2Cyclam (Bn2Cyclam = 1,8-dibenzyl-1,4,8,11-tetraazacyclotetradecane) are described. The reaction of one or two equivalents of the appropriate LiNHR reagents with [(Bn2Cyclam)ZrCl2] give the corresponding amido-chloride [(Bn2Cyclam)ZrCl(NHR)] (R = tBu, (2,6-iPr)Ph) or bis(amido) [(Bn2Cyclam)Zr(NHR)2] (R = iPr, tBu, (2,6-Me)Ph) complexes, respectively. Treatment of [(Bn2Cyclam)ZrCl(NH(2,6-iPr)Ph)] with one equiv. of MgClMe gives the base-free, monomeric imido complex [(Bn2Cyclam)Zr(N(2,6-iPr)Ph)]. The reaction of the tBu analog with MgClMe generates a dimeric bridging imido species [{(Bn2Cyclam)Zr}2(mu-NR)2], which can also be obtained by thermal decomposition of [(Bn2Cyclam)Zr(NHtBu)2] in toluene. The bis(hydrazido) complex [(Bn2Cyclam)Zr(NHNPh2)2] was obtained by reaction of [(Bn2Cyclam)ZrCl2] and two equiv. of LiNHNPh2. A hydrazido-chloride compound [(Bn2Cyclam)ZrCl(N(Ph)NBu(Ph))] was generated in a one-pot reaction between [(Bn2Cyclam)ZrCl2], LiBu and azobenzene. DFT calculations on [(Bn2Cyclam)ZrXY] complexes indicate that the coordination geometry adopted by these species is dictated by the steric bulk of the ligands X and Y, varying between six-coordinate prismatic and four-coordinate tetrahedral.

Synthesis, Structural Characterization, Reactivity, and Thermal Stability of [η5:σ-Me2C(C5H4)(C2B10H10)]Ti(R)(NMe2)

Reaction of [eta:(5)sigma-Me2C(C5H4)(C2B10H10)]TiCl(NMe2) (1) with 1 equiv of PhCH2K, MeMgBr, or Me3SiCH2Li gave corresponding organotitanium alkyl complexes [eta:(5)sigma-Me2C(C5H4)(C2B10H10)]Ti(R)(NMe2) (R = CH2Ph (2), CH2SiMe3 (4), or Me (5)) in good yields. Treatment of 1 with 1 equiv of n-BuLi afforded the decomposition product {[eta:(5)sigma-Me2C(C5H4)(C2B10H10)]Ti}2(mu-NMe)(mu:sigma-CH2NMe) (3). Complex 5 slowly decomposed to generate a mixed-valence dinuclear species {[eta:(5)sigma-Me2C(C5H4)(C2B10H10)]Ti}2(mu-NMe2)(mu:sigma-CH2NMe) (6). Complex 1 reacted with 1 equiv of PhNCO or 2,6-Me2C6H3NC to afford the corresponding monoinsertion product [eta:(5)sigma-Me2C(C5H4)(C2B10H10)]Ti(Cl)[eta(2)-OC(NMe2)NPh] (7) or [eta:(5)sigma-Me2C(C5H4)(C2B10H10)]Ti(Cl)[eta(2)-C(NMe2)=N(2,6-Me2C6H3)] (8). Reaction of 4 or 5 with 1 equiv of R'NC gave the titanium eta(2)-iminoacyl complexes [eta:(5)sigma-Me2C(C5H4)(C2B10H10)]Ti(NMe2)[eta(2)-C(R)=N(R')] (R = CH2SiMe3, R' = 2,6-Me2C6H3 (9) or tBu (10); R = Me, R' = 2,6-Me2C6H3 (11) or tBu (12)). The results indicated that the unsaturated molecules inserted into the Ti-N bond only in the absence of the Ti-C(alkyl) bond and that the Ti-C(cage) bond remained intact. All complexes were fully characterized by various spectroscopic techniques and elemental analyses. Molecular structures of 2, 3, 6-8, and 10-12 were further confirmed by single-crystal X-ray analyses.