Hydroalumination or hydrogallation
of a sterically encumbered hydrazone,
H10C5N–NC(C9H14) (NC5H10 = piperidine, C(C9H14) = 2-adamantdiyl), afforded hydrazides that, depending
on the steric shielding by the substituents at the metal atoms, had
different molecular structures. While both di(tert-butyl)metal derivatives (1a, 1b) are monomeric
in the solid state with highly strained MN2 heterocycles
(M = Al, Ga), the dimethylmetal compounds (1c, 1d) are dimeric with M2N2 heterocycles
and exocyclic N–N bonds. The latter compounds are highly dynamic
in solution. 1d crystallized as a mixture of cis- and trans-isomers as detected by crystal
structure determinations. These compounds react as active Lewis pairs
by their specific donor–acceptor functionality and are able
to activate C–H bonds of moderately acidic substrates. Reaction
of 1a (M = Al) with H–CC–C6H5 afforded by C–H bond activation and release
of H–CMe3 trialkynyl compound 4, in
which three alkynyl groups and a neutral hydrazine ligand are bound
to Al. 1b (M = Ga) gave only the known dimeric monoalkynyl
derivative [(Me3C)2Ga–CC–C6H5]2 (5b). The sterically
less shielded dimethyl compounds 1c and 1d similarly yielded trialkynylmetal compounds by methane and hydrogen
elimination. In this case a hydrazone ligand is coordinated to the
metal atoms. 1d reacted with pentafluorobenzene in an
unprecedented reaction to yield a diaryl-methylgallium compound with
the metal atom bound to two electron-withdrawing groups and a hydrazone
ligand completing the coordination sphere of Ga.
Abstract:Hydroalumination of H 10 C 5 N-N=C(Ad) with HAliBu 2 yielded the hydrazide H 10 C 5 N-N(AliBu 2 )Ad (4; Ad = adamantyl, NC 5 H 10 = piperidinyl). Compound 4 is monomeric and contains a highly strained AlN 2 heterocycle formed by a donor-acceptor interaction between the Al atom and the -N atom of the hydrazine. Two equivalents of the hydride afforded an adduct with an HAliBu 2 molecule coordinated to 4 by an Al-N and a 3c-2e Al-H-Al bond (5). tBuN=C=O did not insert into the relatively weak Al←N donor-acceptor bond of 4; instead, insertion
Treatment of the dimeric gallium hydrazide [Me2Ga‐N(2‐Ad)‐NC5H10]2 (5) with Me2GaH resulted in the formation of an adduct 6 by Ga–N bond cleavage and coordination of the metal hydride via a Ga–N and a 3c–2e Ga–H–Ga bond. This reaction reflects the typical behavior of frustrated Lewis pairs. Reactions with heterocumulenes R–N=C=X (R = Ph, CMe3, Dipp, X = O; R = Ph, X = S) or X=C=X (X = O, S) resulted in the formation of the cyclic Ga–N insertion products Me2Ga–N(R)C(O)N(2‐Ad)‐NC5H10 (7a–c), Me2GaS2C‐N(2‐Ad)‐NC5H10 (8) or Me2GaX2C‐N(2‐Ad)‐NC5H10 [X = O (9); S (10)] in moderate to good yields. Three different structural motifs were observed in the solid state: Five‐membered GaNCN2 heterocycles with exocyclic C=O bonds for compounds 7a–c, four‐membered GaSCN or GaSCS heterocycles for compounds 8 and 9 (chelating coordination of the Ga atoms by SCN and CS2 ligands) and an eight‐membered (GaOCO)2 heterocycle for the dimeric CO2 insertion product 10. Treatment of 5 with PhCN or Ph2CO resulted in a completely different reaction and afforded a dimeric Ga imide 11a or an alcoholate 11b. These reactions may start by retro‐hydrogallation with the formation of H10C5N–N=C(C9H14) and Me2GaH and proceed by addition of the metal hydride to the polar multiple bonds of the nitrile or ketone.
The series of differently substituted ketenimines 1 was hydroluminated using di-iso-butyl aluminum hydride. For the sterically congested ketenimine 1a, preferred hydroalumination of the C═N-bond was proven by X-ray crystallography (compound 5a). In situ treatment of the hydroaluminated ketenimines 5 with various heterocumulenes like carbodiimides, isocycanates, isothiocyanates and ketenimines as electrophiles and subsequent hydrolytic workup resulted in novel enamine derived amide species in case of N-attack (sterically less hindered ketenimines) under formation of a new C-N-bond or in 1,3-diimines by C-C-bond-formation in case of bulky substituents at the ketenimine-nitrogen atom. Furthermore, domino reactions with more than 1 equiv of the electrophile or by subsequent addition of two different electrophiles are possible and lead to polyfunctional amide derivatives of the biuret type which are otherwise not easily accessible.
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