NHC aluminum chemistry on the rise.

This perspective highlights recent developments of the use of -heterocyclic carbenes (NHCs) and cyclic (alkyl)(amino)carbenes (cAACs) in alane and aluminum organyl chemistry. Especially in the last few years this flourishing research field led to some remarkable discoveries including various substitution patterns at the central aluminum atom, different oxidation states, neutral and charged compounds with varying coordination numbers and unique reactivities. Thereby NHCs play a vital role in the stabilization of these otherwise highly reactive compounds, which would not be realizable without the use of this intriguing class of ligands.

NHC-ligated indenyl- and fluorenyl-substituted Alanes and Gallanes: synthons towards indenyl- and fluorenyl-bridged (AlC)-heterocycles ( = 2,3).

Indenyl-(Ind) and fluorenyl-(Fl) substituted NHC-stabilized alanes and gallanes (NHC)·EHR 1-12 (NHC = IiPr, IiPr, IMe; E = Al, Ga; R = Ind, Fl) were prepared reaction of the corresponding NHC-iodoalanes and -gallanes with LiInd and LiFl, respectively. Analogously, the alane adducts with two Ind/Fl substituents (NHC)·AlHR13-18 (NHC = IiPr, IiPr, IMe; R = Ind, Fl) were obtained by using two equivalents of LiInd/LiFl. Elimination of indene and fluorene was induced thermally affording unusual dimeric and trimeric NHC-alane adducts {(NHC)·AlH}-μ-Fl 19-20 and {(NHC)·AlH-μ-R}21-23 (R = Ind, Fl; = 2, 3) with bridging indenyl and fluorenyl ligands.

How to Decarbonize N-Heterocyclic Carbenes (NHCs): The simple Alane Adducts (NHC) ⋅ AlR (R=H, Me, Et).

The reaction of the amine-stabilized alane (NMe) ⋅ AlH 1 with the backbone-saturated N-heterocyclic carbene (NHC) SIDipp (SIDipp=1,3-bis-{2,6-di-iso-propyl-phenyl}-imidazolidin-2-ylidene) at 0 °C yielded the NHC alane adduct (SIDipp) ⋅ AlH 2. Reaction at elevated temperatures or prolonged reaction at room temperature gave the product of a ring expansion reaction (RER) of the NHC, (NMe) ⋅ AlH(RER-SIDippH) 3 ⋅ (NMe). Subsequent reaction of the latter with sterically less hindered NHCs (IMe {=1,3,4,5-tetramethyl-imidazolin-2-ylidene}, IiPr {=1,3-di-iso-propyl-4,5-dimethyl-imidazolin-2-ylidene}, and IiPr {=1,3-di-iso-propyl-imidazolin-2-ylidene}) afforded the NHC-stabilized RER-products (NHC) ⋅ AlH(RER-SIDippH) 3 ⋅ (NHC) (NHC=IMe, IiPr, IiPr), while no reaction was observed with the sterically more demanding NHCs IDipp (=1,3-bis-{2,6-di-iso-propyl-phenyl}-imidazolin-2-ylidene), SIDipp and ItBu (=1,3-di-tert-butyl-imidazolin-2-ylidene).

Aluminum(III) Cations [(NHC) ⋅ AlMes ] : Synthesis, Characterization, and Application in FLP-Chemistry.

The three-coordinate aluminum cations ligated by N-heterocyclic carbenes (NHCs) [(NHC) ⋅ AlMes ] [B(C F ) ] (NHC=IMe 4, IiPr 5, IiPr 6, Mes=2,4,6-trimethylphenyl) were prepared via hydride abstraction of the alanes (NHC) ⋅ AlHMes (NHC=IMe 1, IiPr 2, IiPr 3) using [Ph C] [B(C F ) ] in toluene as hydride acceptor. If this reaction was performed in diethyl ether, the corresponding four-coordinate aluminum etherate cations [(NHC) ⋅ AlMes (OEt )] [B(C F ) ] 7-9 (NHC=IMe 7, IiPr 8, IiPr 9) were isolated. According to a theoretical and experimental assessment of the Lewis-acidity of the [(IMe ) ⋅ AlMes ] cation is the acidity larger than that of B(C F ) and of similar magnitude as reported for Al(C F ) .

NHC-Stabilized Dialanes(4) of Al Mes.

The synthesis and characterization of novel N-heterocyclic carbene (NHC) stabilized dialanes Al Mes as well as first investigations concerning the reactivity of these compounds are reported. The synthesis of these compounds proceeds via the mesityl-substituted alanes (NHC)⋅AlHMes (NHC=IMe {=1,3,4,5-tetramethyl-imidazolin-2-ylidene}, IiPr {=1,3-di-iso-propyl-4,5-dimethylimidazolin-2-ylidene}) and iodo-alanes (NHC)⋅AlIMes (NHC=IMe , IiPr ). Metallic reduction of (NHC)⋅AlIMes afforded the new NHC-stabilized dialanes (NHC) ⋅Al Mes (NHC=IMe , IiPr ).

Subvalent group 13 molecules by carbene-induced hydrogen abstraction.

The -Heterocyclic Carbene (NHC) alane and gallane adducts (NHC)·Cp*AlH (NHC = MeIm5, iPrIm6, DippIm 7) and (NHC)·Cp*GaH (NHC = MeIm8, iPrIm9, DippIm 10; RIm = 1,3-di-organyl-imidazolin-2-ylidene; Dipp = 2,6 diisopropylphenyl; MeIm = 1,3,4,5-tetra-methyl-imidazolin-2-ylidene; Cp* = CMe) were prepared either the reaction of (AlHCp*)1 with the NHC or by the treatment of (NHC)·GaHI (NHC = MeIm2, iPrIm3, DippIm 4) with KCp*. The reaction of (AlHCp*)1 with the backbone saturated NHC DippIm led to NHC ring expansion instead with the formation of (RER-DippImH)AlCp* 12. Heating solutions of the gallium compounds 8-10 triggered reductive elimination of Cp*H and afforded Cp*Ga16.

NHC induced radical formation homolytic cleavage of B-B bonds and its role in organic reactions.

New borylation methodologies have been reported recently, wherein diboron(4) compounds apparently participate in free radical couplings the homolytic cleavage of the B-B bond. We report herein that bis-NHC adducts of the type (NHC)·B(OR), which are thermally unstable and undergo intramolecular ring expansion reactions (RER), are sources of boryl radicals of the type NHC-BR˙, exemplified by MeIm·Bneop˙ 1a (MeIm = 1,3,4,5-tetramethyl-imidazolin-2-ylidene, neop = neopentylglycolato), which are formed by homolytic B-B bond cleavage. Attempts to apply the boryl moiety 1a in a metal-free borylation reaction by suppressing the RER failed.

N-Heterocyclic carbene and cyclic (alkyl)(amino)carbene adducts of gallium hydrides, gallium chlorides and gallium hydrochlorides.

The synthesis and characterization of N-heterocyclic carbene (NHC) and cyclic (alkyl)(amino)carbene (cAAC) gallane and chlorogallane adducts of the type (NHC)·GaH3 (NHC = Me2ImMe1, iPr2Im 2, iPr2ImMe3 and Dipp2ImH4; Me2ImMe = 1,3,4,5-tetra-methyl-imidazolin-2-ylidene; R2Im = 1,3-di-organyl-imidazolin-2-ylidene; Dipp = 2,6-diisopropylphenyl; Dipp2ImH = 1,3-bis(2,6-diisopropylphenyl)-imidazolidin-2-ylidene), (NHC)·GaH2Cl (NHC = iPr2ImMe9, Dipp2Im 10 and Dipp2ImH11; iPr2ImMe = 1,3 diisopropyl-4,5-dimethyl-imidazolin-2-ylidene), (NHC)·GaHCl2 (NHC = iPr2ImMe12, Dipp2Im 13 and Dipp2ImH14) and (cAACMe)·GaHCl215 is reported. Compounds 1-3 and 9-11 are unstable in solution as heating to the boiling temperature of toluene (110 °C) leads to decomposition into elemental gallium and the corresponding dihydroaminal NHC-H2. The reaction of the mono-NHC adducts with a second equivalent of NHC also afforded decomposition and formation of NHC-H2, whereas the reaction of the NHC-stabilized gallanes with one equivalent cAACMe leads to an insertion of the cAACMe carbene carbon atom into the Ga-H bond.

trans-Selective Insertional Dihydroboration of a cis-Diborene: Synthesis of Linear sp -sp -sp -Triboranes and Subsequent Cationization.

The reaction of aryl- and amino(dihydro)boranes with dibora[2]ferrocenophane 1 leads to the formation 1,3-trans-dihydrotriboranes by formal hydrogenation and insertion of a borylene unit into the B=B bond. The aryltriborane derivatives undergo reversible photoisomerization to the cis-1,2-μ-H-3-hydrotriboranes, while hydride abstraction affords cationic triboranes, which represent the first doubly base-stabilized B H analogues.

Spontaneous trans-Selective Transfer Hydrogenation of Apolar Boron-Boron Double Bonds.

The transfer hydrogenation of N-heterocyclic carbene (NHC)-supported diborenes with dimethylamine borane proceeds with high selectivity for the trans-1,2-dihydrodiboranes. DFT calculations, supported by kinetic studies and deuteration experiments, suggest a stepwise proton-first-hydride-second reaction mechanism via an intermediate μ-hydrodiboronium dimethylaminoborate ion pair.