Catalytic C–N bond formation in guanylation reaction by N-heterocyclic carbene supported magnesium(II) and zinc(II) amide complexes

Zinc hydride catalyzed hydroboration of esters

The conjugated bis-guanidinate (CBG)-supported zinc hydride {LZnH}2; L = {(ArHN)(ArN)-CN-C(NAr)(NHAr); Ar = 2,6-Et2-C6H3} (I) is utilized as a catalyst for the hydroboration of esters with pinacolborane (HBpin) under mild reaction conditions. Various aryl and alkyl substrates containing electron-donating, withdrawing, and cyclic groups of esters are effectively converted into alkoxy boronate esters as products upon hydroboration. Furthermore, stoichiometric experiments have been performed to understand the plausible reaction mechanism for the hydroboration of esters. Additionally, complex (I) was used for the hydroboration of carbonate, carboxylic acid, and anhydride substrates to showcase the broad substrate scope.

Reactivity of the carbodiphosphorane, (Ph3P)2C, towards main group metal alkyl compounds: coordination and cyclometalation

The carbodiphosphorane, (Ph3P)2C, reacts with Me3Al and Me3Ga to afford the adducts, [(Ph3P)2C]AlMe3 and [(Ph3P)2C]GaMe3, which have been structurally characterized by X-ray diffraction. (Ph3P)2C also reacts with Me2Zn and Me2Cd to generate an adduct but the formation is reversible on the NMR time scale. At elevated temperatures, however, elimination of methane and cyclometalation occurs to afford [κ2-Ph3PC{PPh2(C6H4)}]ZnMe and [κ2-Ph3PC{PPh2(C6H4)}]CdMe. Analogous cyclometalated products, [κ2-Ph3P{CPPh2(C6H4)}]ZnN(SiMe3)2 and [κ2-Ph3P{CPPh2(C6H4)}]CdN(SiMe3)2, are also obtained upon reaction of (Ph3P)2C with Zn[N(SiMe3)2]2 and Cd[N(SiMe3)2]2. The magnesium compounds, Me2Mg and {Mg[N(SiMe3)2]2}2, likewise react with (Ph3P)2C to afford cyclometalated derivatives, namely [κ2-Ph3PC{PPh2(C6H4)}]MgN(SiMe3)2 and {[κ2-Ph3PC{PPh2(C6H4)}]MgMe}2. While this reactivity is similar to the zinc system, the magnesium methyl complex is a dimer with bridging methyl groups, whereas the zinc complex is a monomer. The greater tendency of the methyl groups to bridge magnesium centers rather than zinc centers is supported by density functional theory calculations.

NHCs in Main Group Chemistry

Since the discovery of the first stable N-heterocyclic carbene (NHC) in the beginning of the 1990s, these divalent carbon species have become a common and available class of compounds, which have found numerous applications in academic and industrial research. Their important role as two-electron donor ligands, especially in transition metal chemistry and catalysis, is difficult to overestimate. In the past decade, there has been tremendous research attention given to the chemistry of low-coordinate main group element compounds. Significant progress has been achieved in stabilization and isolation of such species as Lewis acid/base adducts with highly tunable NHC ligands. This has allowed investigation of numerous novel types of compounds with unique electronic structures and opened new opportunities in the rational design of novel organic catalysts and materials. This Review gives a general overview of this research, basic synthetic approaches, key features of NHC-main group element adducts, and might be useful for the broad research community.

Organomagnesium amide catalyzed cross-dehydrocoupling of organosilanes with amines

Heteroleptic organomagnesium complex pre-catalyzed cross-dehydrocoupling of silanes with amines.

Solvent dependence of the solid-state structures of salicylaldiminate magnesium amide complexes

There are challenges in using magnesium coordination complexes as reagents owing to their tendency to adopt varying aggregation states in solution and thus impacting the reactivity of the complexes. Many magnesium complexes are prone to ligand redistributionviaSchlenk equilibrium due to the ionic character within the metal–ligand interactions. The role of the supporting ligand is often crucial for providing stability to the heteroleptic complex. Strategies to minimize ligand redistribution in alkaline earth metal complexes could include using a supporting ligand with tunable sterics and electronics to influence the degree of association to the metal atom. Magnesium bis(hexamethyldisilazide) was reacted with salicylaldimines [1L=N-(2,6-diisopropylphenyl)salicylaldimine and2L= 3,5-di-tert-butyl-N-(2,6-diisopropylphenyl)salicylaldimine] in either nondonor (toluene) or donor solvents [tetrahydrofuran (THF) or pyridine]. The structures of the magnesium complexes were studied in the solid stateviaX-ray diffraction. In the nondonor solvent,i.e.toluene, the heteroleptic complex bis{μ-2-[(2,6-diisopropylphenyl)iminomethyl]phenolato}-κ3N,O:O;κ3O:N,O-bis[(hexamethyldisilazido-κN)magnesium(II)], [Mg2(C19H22NO)2(C6H18NSi2)2] or [1LMgN(SiMe3)2]2, (1), was favored, while in the donor solvent,i.e.pyridine (pyr), the formation of the homoleptic complex {2,4-di-tert-butyl-6-[(2,6-diisopropylphenyl)iminomethyl]phenolato-κ2N,O}bis(pyridine-κN)magnesium(II) toluene monosolvate, [Mg(C27H38NO)2(C5H5N)2]·C5H5N or [{2L2Mg2(pyr)2}·pyr], (2), predominated. Heteroleptic complex (1) was crystallized from toluene, while homoleptic complexes (2) and the previously reported [1L2Mg·THF] [Quinqueet al.(2011).Eur. J. Inorg. Chem.pp. 3321–3326] were crystallized from pyridine and THF, respectively. These studies support solvent-dependent ligand redistribution in solution.In-situ1H NMR experiments were carried out to further probe the solution behavior of these systems.

Alkaline earths as main group reagents in molecular catalysis

The past decade has witnessed some remarkable advances in our appreciation of the structural and reaction chemistry of the heavier alkaline earth (Ae = Mg, Ca, Sr, Ba) elements. Derived from complexes of these metals in their immutable +2 oxidation state, a broad and widely applicable catalytic chemistry has also emerged, driven by considerations of cost and inherent low toxicity. The considerable adjustments incurred to ionic radius and resultant cation charge density also provide reactivity with significant mechanistic and kinetic variability as group 2 is descended. In an attempt to place these advances in the broader context of contemporary main group element chemistry, this review focusses on the developing state of the art in both multiple bond heterofunctionalisation and cross coupling catalysis. We review specific advances in alkene and alkyne hydroamination and hydrophosphination catalysis and related extensions of this reactivity that allow the synthesis of a wide variety of acyclic and heterocyclic small molecules. The use of heavier alkaline earth hydride derivatives as pre-catalysts and intermediates in multiple bond hydrogenation, hydrosilylation and hydroboration is also described along with the emergence of these and related reagents in a variety of dehydrocoupling processes that allow that facile catalytic construction of Si-C, Si-N and B-N bonds.

Mixed guanidinato-amido Ge(iv) and Sn(iv) complexes with GeE (E = S, Se) double bond and SnS4, Sn2Se2 rings

The oxidative addition of chalcogens (S and Se) to the guanidinate supported Ge(ii) and Sn(ii) amide complexes.

Two alternative approaches to access mixed hydride-amido zinc complexes: synthetic, structural and solution implications

In the presence of a bulky N-heterocyclic carbene, Zn(HMDS)₂can be converted in to either a mononuclear amido/hydride complex or a tetranuclear hydride rich cluster depending on the hydride source and reaction conditions employed.

Synthesis and characterization of bis(amidate) rare-earth metal amides and their application in catalytic addition of amines to carbodiimides

Five bis(amidate) rare-earth metal amides were successfully employed in guanidination, and the Nd-based catalyst showed the highest reactivity.

Catalyst free C–N bond formation by the reaction of amines with diimides: bulky guanidines

Catalyst free direct addition of cyclic secondary amines to various N,N′-bisaryl substituted carbodiimides led to the formation of bulky guanidines. Furthermore, two equivalents of N,N′-bisaryl substituted carbodiimides upon treatment with piperazine led to the formation of bis guanidines.