Reversible Coupling of Germylone with Isocyanates.

The germylone dimNHCGe (5, dimNHC=diimino N-heterocyclic carbene) undergoes a [2+2] cycloaddition with isocyanates RNCO (R=4-tolyl or 3,5-xylyl) to furnish novel alkyl carboxamido germylenes 7 (R=4-tolyl) and 8 (R=3,5-xylyl), featuring a C-C bond between the former carbene carbon and the isocyanate moiety. Heating a mixture of 8 with 4-tolyl isocyanate to 100 °C results in isocyanate metathesis, demonstrating reversible C-C bond formation on the reduced germanium compound. DFT calculations suggest that this process occurs via the reductive dissociation of isocyanate from 8 that regenerates the parent Ge(0) compound 5.

Small Molecule Activation on a Base-stabilized Phosphinidene.

Reduction of (NP)PCl2 (NP=phosphinoamidinate [PhC(NAr)(=NPPri 2 )]- ) with KC8 affords the phosphinoamidinato-supported phosphinidene (NP)P (9). Reaction of 9 with a N-heterocyclic carbene (MeC(NMe))2 C: results in the NHC-adduct NHC→P-P(Pri 2 )=NC(Ph)=NAr featuring an iminophosphinyl group. Reactions of 9 with HBpin and H3 SiPh led to the metathesis products (NP)Bpin and (NP)SiH2 Ph, respectively, whereas with HPPh2 a base-stabilized phosphido-phosphinidene, the product of N-P and H-P bond metathesis, was obtained. Reaction of 9 with tetrachlorobenzaquinone results in oxidation of P(I) to P(III), accompanied by oxidation of the amidophosphine ligand into P(V). Addition of benzaldehyde to 9 results in a phospha-Wittig reaction affording a product of P=P and C=O bond metathesis. Related reaction with phenylisocyanate results in a product of N-P(=O)Pri 2 addition to the C=N bond of an intermediate iminophosphaalkene to produce a phosphinidene intramolecularly stabilized by a diaminocarbene.

Base-Stabilized Phosphinidene Oxide, Imide and Sulfide.

Oxidation of a base-stabilized phosphinidene (κ2 -NNP)P (12, NNP=phosphinoamidinate) with N2 O afforded a labile phosphinidene oxide (κ2 -NNP)P=O (16) which was characterized by NMR spectroscopy. Further oxidation of 16 by N2 O or reaction of 12 with two equivalents of pyridine oxide afforded the isolable dioxide (κ2 -NNP)PO2 which was characterized by NMR and SC XRD. Trapping of 16 with tolyl isocyanate resulted in P=O/N=C metathesis, eventually affording a urea-ligated phosphine (κ1 -NNP)P(NTol)2 C=O (17) The mechanism of this reaction was elucidated by DFT calculations. Reactions of phosphinidene 12 with azides generated transient imines (NNP)P=NR, which in the case of R=Tol underwent cycloaddition with tolyl Isocyanate to afford the urea product 17, and in the case of R=SiMe3 reacts with N3 SiMe3 via the addition of N-Si across the P=N bond affording, after the extrusion of dinitrogen, a P,N-heterocyclic compound. Both products of the reactions with azides have been fully characterized, both in solution and the solid-state. Finally, reaction of phosphinidene 12 with one equivalent of sulfur resulted in the isolation of the base-stabilized phosphinidene sulfide (κ2 -NNP)P=S that has also been fully characterized.

A Guanidine-Supported π-Complex of Germanium Amenable to Intramolecular C-C Cleavage in Arene and Ge Atom Transfer.

The germylone dimNHCGe (dimNHC=diimino N-heterocyclic carbene) reacts with azides N3 R (R=SiMe3 or p-tolyl) to furnish the first examples of germanium π-complexes, i. e. guanidine-ligated compounds (dimNHI-SiMe3 )Ge (NHI=N-heterocyclic imine, R=SiMe3 ) and (dimNHI-Tol)Ge (R=p-tolyl). DFT calculations suggest that these species are formed by a Staudinger type replacement of dinitrogen in the azide by a nucleophilic germylone, leading to a transient carbene adduct of iminogermylidene. Heating a solution of compound (dimNHI-SiMe3 )Ge to 70 °C results in extrusion of the iminogermylidene that further aggregates to produce the known [Me3 SiNGe]4 tetramer, whereas the imidazolylidene fragment transforms into an unusual heptatriene species that can be considered as a product of carbene insertion into the C-C bond of a pendant Ar substituent at the imidazolylidene nitrogen of the dimNHC. Reaction of (dimNHI-SiMe3 )Ge with tetrachloro-o-benzoquinone results in the net transfer of a germanium atom and formation of the free diimino-guanidine ligand. This ligand also forms when (dimNHI-SiMe3 )Ge is treated with azide N3 (p-Tol), with the germanium product being [(p-Tol)NGe]n.

The Insertion of EII and EIV Chlorides (E=Si, Ge) into the Si-Si Bond of Disilylene.

Reactions of silicon and germanium dichlorides L⋅ECl2 (E=Si, L=IPr; E=Ge, L=dioxane) with the phosphinoamidinato-supported disilylene ({κ2 (N,P)-NNP}Si)2 resulted in formal tetrylene insertions into the Si-Si bond. In the case of the reaction with silylene, two products were isolated. The first product ({κ2 (N,P)-NNP}Si)2 SiCl2 , is the formal product of direct SiCl2 insertion into the Si-Si bond of ({κ2 (N,P)-NNP}Si)2 and thus features two separated silylamido silylene centers. Over time, migration of the SiCl2 group to a lateral position afforded the second product, the disilylene {κ2 (Si,P)-SiCl2 NNP}Si-Si{κ2 (N,P)-NNP}. In contrast, insertion of GeCl2 resulted only in the isolation of the germanium analogue of {κ2 (Si,P)-SiCl2 NNP}Si-Si{κ2 (N,P)-NNP}, containing a Ge atom in the central position namely, compound {κ2 (Si,P)-SiCl2 NNP}Ge-Si{κ2 (N,P)-NNP}, which is a rare example of a silylene-germylene. Finally, reaction of disilylene ({κ2 (N,P)-NNP}Si)2 with SiCl4 and SiHCl3 led to the formation of the new bis(silyl)silylene, ({NNP}SiCl2 )2 Si:. All four new products from these insertion reactions have been characterized by multinuclear NMR and single-crystal X-ray diffraction studies.

An Isolable Gallium-Substituted Nitrilimine and its Reactivity with B-H, Si-H and B-B Bonds.

Reaction of the Ga(I) compound NacNacGa (9) with the diazo compound N2 CHSiMe3 affords the nitrilimine compound NacNacGa(N-NCSiMe3 )(CH2 SiMe3 ) (10). Carrying out this reaction in the presence of pyridine does not lead to C-H activation on the transient alkylidene NacNacGa=CHSiMe3 but generates a metallated diazo species NacNacGa(NHN=CHSiMe3 )(CN2 SiMe3 ) (13) that further rearranges into the isonitrile compound NacNacGa(NHN=CHSiMe3 )(N(NC)SiMe3 ) (15). Reactions of 10 with the silane H3 SiPh and the borane HBcat furnished products of 1,3 addition to the nitrilimine moiety NacNacGa{N(ERn )NCSiMe3 }(CH2 SiMe3 ), whereas reaction with the diborane B2 cat2 gave the product of formal nitrene insertion into the B-B bond. DFT calculations suggest that the interaction of 9 with N2 CHSiMe3 proceeds through intermediate formation of an alkylidene compound that undergoes CH activation with a second molecule of N2 CHSiMe3 . Insertion into the B-B bond likely proceeds through an initial 1,3-addition of the diborane, followed by boryl migration to the former nitrene center.

Selective Cross-Coupling of Unsaturated Substrates on AlI.

The AlI compound NacNacAl (1, NacNac = [ArNC(Me)CHC(Me)NAr]- , Ar = 2,6-iPr2 C6 H3 ) serves as a template for the chemoselective coupling between carbonyls (benzophenone, fenchone, isophorone, p-tolyl benzoate, N,N-dimethylbenzamide, (1-phenylethylidene)aniline) and pyridine. With the CH-acidic ketone (1R)-(+) camphor, the reaction affords a hydrido alkoxide compound of Al, formed as the result of enolization, whereas an enolizable imine, (1-phenylethylidene)aniline, and the bulky ketone isophorone, still chemoselectively couple with pyridine. In contrast, reaction with the ester p-tolyl benzoate results in cleavage of the ester bond together with replacement of the alkoxy group by a hydrogen atom of the pyridine moiety. This study demonstrates that for carbonyl substrates featuring phenyl substituents, the reaction proceeds via intermediate formation of η2 (C,X)-coordinated (X = O, N) carbonyl adducts, whereas the reaction of 1 with (R)-(-)-fenchone in the absence of pyridine leads to CH activation in the pendant isopropyl group of the Ar substituent of the NacNac ligand.

Reactivity of a Phosphinoamidinate-Stabilized Disilylene toward H-X Bonds.

An efficient method for the preparation of a phosphinoamidinate-supported disilylene was developed, and its reactivity toward H-E bonds (E = elements from Groups 13-15) was studied. With HBpin, transfer of the ligand from silicon to boron was observed to afford (NP)Bpin. Reaction with a silane (H3SiPh) took place only at elevated temperatures, at which point oxidative addition of the N-P bond of the NP-ligand to one of the silicon atoms of the disilylene occurred prior to Si-H addition involving the remaining silylene center. In contrast, reaction of the disilylene with phosphine, HPPh2 furnished the phosphidosilylene (NP)SiPPh2 (16) together with a highly transient species that, on the basis of a trapping experiment with H3SiPh, is proposed to be the hydridosilylene (NP)SiH, 17. Interestingly, 16 reacts with HPPh2 to give the diphosphine (PPh2)2, most likely via a direct σ-bond metathesis process. The aforementioned products have been characterized by multinuclear NMR and single-crystal X-ray diffraction studies.

Ge(0) Compound Stabilized by a Diimino-Carbene Ligand: Synthesis and Ambiphilic Reactivity.

The germylone dimNHCGe (5, dimNHC = diimino N-heterocyclic carbene) was successfully prepared via the reduction of the germanium cation [dimNHCGeCl]+ with KC8. The molecular structure of 5 was unambiguously established by both NMR spectroscopy and single-crystal X-ray diffraction. The reactivity of 5 was investigated, revealing that it undergoes oxidative addition of HCl, CH3I, and PhI, accompanied by an unusual migration of the H, Me, and Ph groups from germanium to the carbene ligand. Related chemistry was also observed with C5F5N, which results in the migration of the fluorinated pyridine moiety to the carbene ligand. Compound 5 also undergoes cycloaddition with tetrachloro-o-benzoquinone to afford a Ge(IV) adduct.

Adduct of NacNacAl with Benzophenone and Its Coupling Chemistry.

Reaction of NacNacAl (NacNac=[DippNC(Me)CHC(Me)NDipp]- ) with one equivalent of benzophenone affords a ketylate species NacNacAl(η2 (C,O)-OCPh2 ) that undergoes easy cyclization reactions with unsaturated substrates. The scope of substrates included benzophenone, aldimine (PhNC(Ph)H), quinoline, phenyl nitrile, trimethylsilyl azide, and a saturated cyclic thiourea. The latter substrate reacted by an unusual C-N cleavage that left the C=S functionality intact. The new products were characterized by NMR spectroscopy and X-ray diffraction analysis.

Oxidative Addition and Reductive Elimination at Main-Group Element Centers.

Oxidative addition and reductive elimination are key steps in a wide variety of catalytic reactions mediated by transition-metal complexes. Historically, this reactivity has been considered to be the exclusive domain of d-block elements. However, this paradigm has changed in recent years with the demonstration of transition-metal-like reactivity by main-group compounds. This Review highlights the substantial progress achieved in the past decade for the activation of robust single bonds by main-group compounds and the more recently realized activation of multiple bonds by these elements. We also discuss the significant discovery of reversible activation of single bonds and distinct examples of reductive elimination at main-group element centers. The review consists of three major parts, starting with oxidative addition of single bonds, proceeding to cleavage of multiple bonds, and culminated by the discussion of reversible bond activation and reductive elimination. Within each subsection, the discussion is arranged according to the type of bond being cleaved or formed and considers elements from the left to the right of each period and down each group of the periodic table. The majority of results discussed in this Review come from the past decade; however, earlier reports are also included to ensure completeness.

Shedding Light on the Diverse Reactivity of NacNacAl with N-Heterocycles.

The aluminum(I) compound NacNacAl (NacNac=[ArNC(Me)CHC(Me)NAr]- , Ar=2,6-iPr2 C6 H3 , 1) shows diverse and substrate-controlled reactivity in reactions with N-heterocycles. 4-Dimethylaminopyridine (DMAP), a basic substrate in which the 4-position is blocked, induces rearrangement of NacNacAl by shifting a hydrogen atom from the methyl group of the NacNac backbone to the aluminum center. In contrast, C-H activation of the methyl group of 4-picoline takes place to produce a species with a reactive terminal methylene. Reaction of 1 with 3,5-lutidine results in the first example of an uncatalyzed, room-temperature cleavage of an sp2 C-H bond (in the 4-position) by an AlI species. Another reactivity mode was observed for quinoline, which undergoes 2,2'-coupling. Finally, the reaction of 1 with phthalazine produces the product of N-N bond cleavage.

Ligand Effect in Alkali-Metal-Catalyzed Transfer Hydrogenation of Ketones.

This work unveils the reactivity patterns, as well as ligand and additive effect on alkali-metal-base-catalyzed transfer hydrogenation of ketones. Crucially to this reactivity is the presence of a Lewis acid (alkali cation), as opposed to a simple base effect. With aryl ketones, the observed reactivity order is Na+ >Li+ >K+ , whereas for aliphatic substrates it follows the expected Lewis acidity, Li+ >Na+ >K+ . Importantly, the reactivity pattern can be drastically changed by adding ligands and additives. Kinetic, labelling, and competition experiments as well as DFT calculations suggested that the reaction proceeds through a concerted direct hydride-transfer mechanism, originally suggested by Woodward. The lithium cation was found to be intrinsically more active than heavier congeners, but in the case of aryl ketones a decrease in reaction rate was observed at ≈40 % conversion with lithium cations. Noncovalent-interaction analysis revealed that this deceleration effect originated from specific noncovalent interactions between the aryl moiety of 1-phenylethanol and the carbonyl group of acetophenone, which stabilize the product in the coordination sphere of lithium and thus poison the catalyst. The ligand/additive effect is a complicated phenomenon that includes a combination of several factors, such as the decrease of activation energy by ligation (confirmed by distortion/interaction calculations of N,N,N',N'-tetramethylethylenediamine, TMEDA) and the change in relative stabilization of reagents and substrates in the solution and the coordination sphere of the metal. Finally, we observed that lithium-base-catalyzed transfer hydrogenation can be further facilitated by the addition of an inexpensive and benign reagent, LiCl, which likely operates by re-initiating the reaction on a new lithium center.

Interaction of Multiple Bonds with NacNacGa: Oxidative Cleavage vs Coupling and Cyclization.

The reactivity of the gallium(I) compound NacNacGa (4) to a variety of unsaturated compounds has been studied. Whereas 4 proved surprisingly unreactive toward olefins, ketones, guanidines, and thioureas, it reacts with isocyanates and carbodiimides to furnish the products of coupling of two heterocumulenes. With isothiocyanate, the C═S cleavage occurs, leading to the dimer (NacNacGa)2(µ-S)(µ-CNPh) and the cyclization product NacNacGa(S2C═NPh). These compounds were characterized by multinuclear NMR spectroscopy and X-ray crystal structure analyses. Oxidative addition of S═PPh3 occurs at elevated temperatures and results in the known dimer [NacNacGa(µ-S)]2.

Sequential Oxidation and C-H Bond Activation at a Gallium(I) Center.

In situ oxidation of the GaI compound NacNacGa by either N2 O or pyridine oxide results in the generation of a labile monomeric oxide, NacNacGa(O), which can easily cleave the C-H bonds of aliphatic and aromatic substrates featuring good donor sites. The products of this reaction are gallium organyl hydroxides. DFT calculations show that these reactions start with the formation of NacNac-Ga(O)(L) adducts, the oxo ligand of which can easily abstract protons from nearby C-H bonds, even for sp2 -hybridized carbon centers. Aliphatic amines do not enter this reaction for kinetic reasons, presumably because of the unfavorable sterics.

Oxidative Cleavage of the C═N Bond on Al(I).

Reaction of the cyclic guanidine TolN═SIMe with the aluminum(I) compound NacNacAl (1) results in the unprecedented cleavage of the C-N multiple bond to give, after rearrangement, the carbene-ligated Al(III) amide, NacNac'Al(NHTol)(SIMe) (6). DFT calculations revealed that these reactions proceed via a bimolecular mechanism in which either the basic Al(I) center or the transient Al═NTol species deprotonates the methyl group of the NacNac ligand.

Unusual Reactions of NacNacAl with Urea and Phosphine Oxides.

The reaction of cyclic urea 1,3-dimethyl-2-imidazolidinone with the aluminum(I) compound NacNacAl (1) gives an unexpected adduct of urea with the isomerized aluminum(III) hydride NacNac'AlH(O═SIMe) (3). A related reaction of 1 with phosphine oxides results in cleavage of the P═O bond and formation of hydroxyl derivatives NacNac'Al(OH)(O═PR3) [R = Ph (5) and Et (6)]. Density functional theory calculations revealed that these reactions proceed via a bimolecular mechanism in which either the basic aluminum(I) center or the transient Al═O species deprotonate the methyl group of the NacNac ligand.

Oxidative Addition of Disulfides, Alkyl Sulfides, and Diphosphides to an Aluminum(I) Center.

The aluminum(I) compound NacNacAl (1) reacts with diphenyl disulfide and diethyl sulfide to form the respective four-coordinate bis(phenyl sulfide) complex NacNacAl(SPh)2 (2) and alkyl thiolate aluminum complex NacNacAlEt(SEt) (3). As well, reaction of 1 with tetraphenyl diphosphine furnishes the bis(diphenyl phosphido) complex NacNacAl(PPh2)2 (4). Production of 3 and 4 are the first examples of C(sp(3))-S and R2P-PR2 activation by a main-group element complex. All three complexes were characterized by multinuclear NMR spectroscopy and X-ray crystal structure analysis. Furthermore, a variable-temperature NMR spectroscopic study was undertaken on 4 to study its dynamic behavior in solution.

Oxidative Cleavage of C=S and P=S Bonds at an AlI Center: Preparation of Terminally Bound Aluminum Sulfides.

The treatment of cyclic thioureas with the aluminum(I) compound NacNacAl (1; NacNac=[ArNC(Me)CHC(Me)NAr]- , Ar=2,6-Pri2 C6 H3 ) resulted in oxidative cleavage of the C=S bond and the formation of 3 and 5, the first monomeric aluminum complexes with an Al=S double bond stabilized by N-heterocyclic carbenes. Compound 1 also reacted with triphenylphosphine sulfide in a similar manner, which resulted in cleavage of the P=S bond and production of the adduct [NacNacAl=S(S=PPh3 )] (8). The Al=S double bond in 3 can react with phenyl isothiocyanate to furnish the cycloaddition product 9 and zwitterion 10 as a result of coupling between the liberated carbene and PhN=C=S. All novel complexes were characterized by multinuclear NMR spectroscopy, and the structures of 5, 9, and 10 were confirmed by X-ray diffraction analysis. The nature of the Al=S bond in 5 was also probed by DFT calculations.

Oxidative addition of σ bonds to an Al(I) center.

The Al(I) compound NacNacAl (1, NacNac = [ArNC(Me)CHC(Me)NAr](-) and Ar = 2,6-Pr(i)2C6H3) reacts with H-X (X = H, Si, B, Al, C, N, P, O) σ bonds of H2, silanes, borane (HBpin, pin = pinacolate), allane (NacNacAlH2), phosphine (HPPh2), amines, alcohol (Pr(i)OH), and Cp*H (Cp* = pentamethylcyclopentadiene) to give a series of hydride derivatives of the four-coordinate aluminum NacNacAlH(X), which are characterized herein by spectroscopic methods (NMR and IR) and X-ray diffraction. This method allows for the syntheses of the first boryl hydride of aluminum and novel silyl hydride and phosphido hydride derivatives. In the case of the addition of NacNacAlH2, the reaction is reversible, proving the possibility of reductive elimination from the species NacNacAlH(X).