Reduction and Cycloaddition of Heteroalkenes at Ga(I) Bisamide Center

: The reactivity of the complex [(dpp-bian)GaNa(DME) 2 ] ( 1 ) (dpp-bian = 1,2-bis[(2,6-di-isopropylphenyl)imino]acenaphthene) towards isocyanates, benzophenone, diphenylketene, and 1,2-dibenzylidenehydrazine has been studied. Treatment of 1 with isocyanates led to derivatives of imido-formamide [(dpp-bian)Ga{C(=NPh) 2 } 2 –NPh][Na(DME) 3 ] ( 2 ), biuret [(dpp-bian)Ga(NCy) 2 (CO) 2 NCy] [Na(DME)] ( 3 ), or carbamic acids [(dpp-bian)GaN(Cy)C(O)O] 2 [Na(THF)(Et 2 O)] ( 4 ), [(dpp-bian)GaC (=NCy)N(Cy)C(O)O][Na(Py) 3 ] ( 5 ). Treatment of 1 with 2 equiv. of Ph 2 CO resulted in gallium pinacolate [(dpp-bian)GaO(CPh 2 ) 2 O][Na(Py) 2 ] ( 9 ), while the reaction of 1 with 2 equiv. Ph 2 CCO gave divinyl ether derivative [(dpp-bian)Ga{C(=CPh 2 )O} 2 ][Na(DME) 3 ] ( 10 ). Complex 1 treated with 2 equiv. 1,2-dibenzylidenehydrazine underwent [1+2+2] cycloaddition to give C–C coupling product [(dpp-bian)Ga{N(NCHPh)} 2 (CHPh) 2 ][Na(DME) 3 ] ( 11 ). When complex 1 was sequentially treated with 1 equiv. of 1,2-dibenzylidenehydrazine and 1 equiv. of pyridine or pyridine-d5; it gave [1+2+2] cycloaddition product [(dpp-bian)GaN(NCHPh)C(Ph)CN][Na(DME) 3 ] ( 12 ). Compounds 2 – 12 were characterized by NMR and IR spectroscopy, and their molecular structures were established by single-crystal X-ray diffraction analysis.


Introduction
The discovery of stable carbenes at the end of the 20th century marked a new era in chemistry.These compounds underwent rapid examination and garnered significant favor as ligands for transition metal atoms.Notably, palladium and ruthenium complexes based on N-heterocyclic carbenes have proven effective as catalysts for cross-coupling and olefin metathesis reactions [1,2].Carbenes themselves are highly reactive and showcase unique properties [3][4][5], including cycloadditions [6].
Attempts have been made to apply these concepts, but as of now, the achievements in this area are quite limited, in contrast to applied low-valent surfaces and solid-state chemistry [79,80].Metallylenes were employed for selective coupling of carbonyl compounds, isocyanates, nitriles, imines, and azides with pyridine [81] or benzophenone [66].They promoted a facile synthesis of phosphine PH 3 from white phosphorus and ammonia [82].Alumene's adducts of olefins can undergo a reversible insertion of CO [83].Benzene was converted into acyclic 1,3,5-triene products at a low-valent aluminum center [84].Recently, we reported a direct transformation of RN=C=NR to guanidinates and iminoguanidinates [85,86], activation and modification of CO 2 into carboxylic acid derivatives [87] mediated by gallylene complex [(dpp-bian)GaNa(DME) 2 ] (1).
Here, we report on novel transformations of RN=C=O, Ph 2 C=O, Ph 2 C=C=O, N 2 O, COS and PhHC=N-N=CHPh at the gallium center of complex 1 and draw certain generalizations regarding the patterns that govern them.

Materials and Methods
General procedure for synthesis of 2-12.All the manipulations with air-and moisturesensitive compounds were carried out in a vacuum or under argon using the standard Schlenk technique or under an argon atmosphere in a drybox.A solution of 1 equiv. of [(dpp-bian)GaNa(DME) 2 ] [88] was prepared in situ by stirring 0.5 equiv.[(dpp-bian)Ga] 2 with 1 equiv. of sodium metal in a relevant ether solvent until the metal completely dissolved.The solution became yellow-green.Then, 1 (or 2) equiv. of heteroalkene was added to the solution.Within a few minutes, the solution's color became green/green-blue.Further crystallization afforded green/green-blue diamagnetic crystals of compounds 2-12.

Results and Discussion
Metallylene's [(dpp-bian)GaNa(DME) 2 ] (1) reactivity against isocyanates was tested in three conditions: (1) 1 with 1 equiv. of PhNCO in DME solution, (2) 1 with 1 equiv. of CyNCO in DME solution, and (3) 1 with 2 equiv. of CyNCO in THF solution at 25 • C. A rapid solution color change from yellow-green to green-blue was observed.However, despite similar reaction conditions, the isolated products were strikingly different in each case (Scheme 3).The imidoformamide 2 was isolated from the first reaction, biuret (3) and carbamic acid (4) derivatives from the second reaction, and carbamic acid (5) from the third reaction, respectively.The products appeared to be especially moisture-and air-sensitive crystals of various green-blue tones indicative of dpp-bian 2− [31] that were isolated in moderate 11-18% yields.Compounds 3 and 4 were isolated from the same reaction mixture simultaneously.
isolated in moderate 11-18% yields.Compounds 3 and 4 were isolated from the same reaction mixture simultaneously.Complex 6 was also synthesized by the direct reaction of gallylene 1 with 1 equiv.N2O (Scheme 4) as green crystals in 66% yield.Spectral characteristics of 6 from both experiments coincided.For the sake of 6 synthesis, compound 1 was also treated with 1 equiv.carbonyl sulfide.That, however, resulted in sulfide 7 as green crystals in 35% yield.(6), on top of crystals of complex 2. The relatively low isolated yields of 2-5 may be attributed to the existence of few energetically close intermediates

INT[GaO], INT[GaNR], INT[GaRNCO]
along with the presence of competitive reaction pathways that, in fact, resulted in a mixture of similar products.Complex 6 was also synthesized by the direct reaction of gallylene 1 with 1 equiv.N 2 O (Scheme 4) as green crystals in 66% yield.Spectral characteristics of 6 from both experiments coincided.For the sake of 6 synthesis, compound 1 was also treated with 1 equiv.carbonyl sulfide.That, however, resulted in sulfide 7 as green crystals in 35% yield.Scheme 4. Reaction of gallylene 1 with 1 equiv. of N2O and 1 equiv. of COS.
Compounds 2-6 are diamagnetic and produce resolved NMR spectra.Complexes of dpp-bian that have symmetry elements produce a characteristic pattern of the ligand NMR [87].Likewise, the NMR spectra of a solution of 2 contain a simplified set of signals due to two symmetry planes.Eight methyl groups and four methine groups of iPr are  Compounds 2-6 are diamagnetic and produce resolved NMR spectra.Complexes of dpp-bian that have symmetry elements produce a characteristic pattern of the ligand NMR [87].Likewise, the NMR spectra of a solution of 2 contain a simplified set of signals due to two symmetry planes.Eight methyl groups and four methine groups of iPr are presented by two doublets (δ 0.75, 0.89 ppm, 12H each) and one septet (3.74 ppm, 4H), respectively.Protons of the phenyl group produce signals at 7.56 (2H), 7.25 (2H), 7.01 (4H), 6.78 (4H), 7.06 (1H), and 6.57 (2H) ppm.The GaC(NPh)N carbon atom produces a signal at 152.8 ppm that is similar to 158.1 ppm of azacyclobutane C(=NPh)C(Me2)N(Ph)C(=NPh) [89].Compounds 3 and 4 were characterized only by XRD analysis because of very close solubilities.The NMR spectrum of complex 5 demonstrates a simplified signal set due to a symmetry plane.Protons of the methyl and methine groups of iPr give four doublets (6H each) and two septets (2H each).Protons of the naphthalene part appear as two doublets (2H each) at 6.29 and 7.12 ppm and one doublet of doublets (2Н) at 6.86 ppm.Protons of methylene groups cyclohexyl substitutes give six multiplets at 2.  Complex 8 is diamagnetic but shows no resolved NMR spectra at room temperature, probably due to molecular motion in the cyclohexyl substituent.The spectrum resolution improved at −20 • C (Figure S19).Protons from the isopropyl group produced signals at 1.20, 1.03, 0.74, and 0.71 ppm (12H each) and 3.84 and 3.48 ppm (4H each).Protons of cyclohexyl substitutions appeared at 3.07 (1H), 2.20 (4H), 1.99 (4H), and 1.12 (2H) ppm.Atom H(5) of the imino group presented as a doublet at 5.79 ppm.The IR spectrum of product 8 contains absorption bands corresponding to stretching vibrations of N-H (3364 cm −1 ) bonds and a carboxyl group (1591 and 1438 cm −1 ).
Even though the isocyanate oligomerization phenomenon under basic or reductive conditions is known [90,91], and we reported a few isocyanate transformations on the gallium center [27], most of the structures and transformations reported herein are novel.For example, complex 2 is the first metallocycloiminoacylamidine of the main group metal.Three metallocycloiminoacylamidines of the transition metals were characterized, though.They were isolated by the insertion of 2,6-dimethylphenyl isocyanide to the M-N bond of platinu-mazacyclopropane [(RCN) 2 PtC(=NR)NR)] (R = 2,6-dimethylphenyl) [92] and transformation of isonitrile in the coordination sphere of iron in the complex [Fe(dppe)(CNR) 4 ](ClO 4 ) 2 ] (R = p-Tol; dppe = 1,2-bis(diphenylphosphino)ethane)) by its treatment with excess KOH [93].Similarly, complex 3 is the second main group metal biuret to be deposited in the Cambridge Structural Database (CSD) [27].Isocyanate oligomerizations were reported to give six-member metallacycles on nickel [94], palladium [95], and chromium [96] centers.To the best of our knowledge, the generation of compounds with five-membered metallacycle MC(=NR)C(=O)O, like in complex 5, is also unprecedented.
To better understand the reactivity of 1 against C=O-containing compounds, it was treated with 2 equiv. of benzophenone or 2 equiv. of diphenylketene.Pinacolate coupling was reported in the reaction of [NacNacAl] and benzophenone [98].Magnesium complex [(dpp-bian)Mg(THF) 3 ] also reacts with benzophenone to create binuclear magnesium pinacolate [{(dpp-bian)Mg(THF)} 2 {µ-O 2 C 2 Ph 4 }], which can dissociate in toluene solution into two biradical species [99].Analogously to these cases, generation of pinacolate 9 can be viewed through ketyl radical formation.Meanwhile, C-O coupling instead of C-C in the reaction of 1 with diphenylketene may account for the [1+2+2] cycloaddition mechanism.Besides transition metals [100] and phosphorus [97], ketene cyclization product 10 is the first to be reported across the main group metals.
A similar mechanism ambiguity between reduction and [1+2+2] cycloaddition appeared in the reaction of complex 1 with 2 equiv.A similar mechanism ambiguity between reduction and [1+2+2] cycloaddition appeared in the reaction of complex 1 with 2 equiv. of 1,2-dibenzylidenehydrazine.It resulted in complex [(dpp-bian)Ga{N(NCHPh)}2(CHPh)2][Na(DME)3] (11) in the form of turquoise crystals at 39% yield.During the reaction, two benzaldazine molecules combined in the gallium atom coordination sphere.One C-C bond and two Ga-N bonds formed to create a five-membered galladiazametallacycle (Scheme 7).Compound 11 is diamagnetic and exhibits resolved NMR spectra.In the THF-d8 solution, isopropyl substituents of the ligand appeared as a set of four doublets 6H and two septets 2H, indicative of a symmetry plane.The methine protons of benzaldazine are represented by two singlets at 7.37 and 5.08 ppm (2H each).The latter signal corresponds to the hydrogen atoms at the carbon atoms that formed a new bond.It shifted into the higher field compared to the starting 1,2-dibenzylidenehydrazine (cf.δ (NCHPh2)2 = 8.65 ppm).The naphthalene protons were not equivalent, which allowed us to conclude that the symmetry plane coincides with the naphthalene plane.The addition reaction was also nicely illustrated by 13  Curiously, when the DME solution of 1 was treated with 1 equiv. of 1,2dibenzylidenehydrazine and then the products were dissolved in pyridine, a pyridinebenzaldazine coupling product [(dpp-bian)GaN(NCHPh)C(Ph)CN][Na(DME)3] (12) was isolated in 40% yield as green crystals (Scheme 7).Using pyridine-d5 as a solvent, an isostructural complex 12-d 5 was isolated.
In the THF-d8 solution, the molecule of 12 was asymmetric, which was seen from eight doublets and four septets arising from iPr-substituents of dpp-bian (Figure 1).The methine protons of the azine fragment produced two distinct signals (1H each) at 7.73 and 4.58 ppm.The last corresponded to the hydrogen atom at the sp 3 -hybrydized carbon atom, forming a new bond.Five signals of protons in the pyridine fragment (6.62, 5.44, 4.29, 4.23, 4.00 ppm) were significantly shifted in comparison to free pyridine (8.61, 7.66, 7.28 ppm), which confirmed its dearomatization.The assignment of these signals became clear when 1 H NMR spectra of 12 and 12-d 5 were compared (Figure 1).The 13 C NMR spectrum also confirmed the structure of the resulting compound.The signal of the sp 3  Compound 11 is diamagnetic and exhibits resolved NMR spectra.In the THF-d 8 solution, isopropyl substituents of the ligand appeared as a set of four doublets 6H and two septets 2H, indicative of a symmetry plane.The methine protons of benzaldazine are represented by two singlets at 7.37 and 5.08 ppm (2H each).The latter signal corresponds to the hydrogen atoms at the carbon atoms that formed a new bond.It shifted into the higher field compared to the starting 1,2-dibenzylidenehydrazine (cf.δ (NCHPh 2 ) 2 = 8.65 ppm).The naphthalene protons were not equivalent, which allowed us to conclude that the symmetry plane coincides with the naphthalene plane.The addition reaction was also nicely illustrated by 13 C NMR signals of 131.2 ppm (H-C=N) and 68.8 ppm (H-C-N) shifted in comparison to the starting material (cf.δ (-N=CHPh) 2 = 161.8ppm).The presence of the symmetry plane confirmed the formation of one optical isomer-the meso-form.
In the THF-d 8 solution, the molecule of 12 was asymmetric, which was seen from eight doublets and four septets arising from iPr-substituents of dpp-bian (Figure 1).The methine protons of the azine fragment produced two distinct signals (1H each) at 7. 143.7, 127.5, 104.6, 92.5, and 62.5 ppm.(cf.C5H5N 149.9, 135.9, and 123.75 ppm in pyridine).Since only one diastereomer was identified in both reactions of 1 with benzaldazine, it may be believed to proceed through the cycloaddition mechanism.

Molecular Structures of Compounds 2-12
According to XRD analysis of 2-12, interatomic bond distances within fivemembered metallacycles correspond to single C-N and double C=C bonds and agree well with the geometry of complex 1 [103].This anticipates that dpp-bian preserved the dianion state during the reactions, while electron transfer occurred exclusively from the metal atom.

Molecular Structures of Compounds 2-12
According to XRD analysis of 2-12, interatomic bond distances within five-membered metallacycles correspond to single C-N and double C=C bonds and agree well with the geometry of complex 1 [103].This anticipates that dpp-bian preserved the dianion state during the reactions, while electron transfer occurred exclusively from the metal atom.
The XRD analysis of 2 (Figure 2 In the crystal, molecules of 3 presented by species dimerized through sodium cations and placed on the crystallographic center of symmetry (Figure 3  In the crystal, molecules of 3 presented by species dimerized through sodium cations and placed on the crystallographic center of symmetry (Figure 3).Distances O(1)-C( 43  In the crystal, molecules of 3 presented by species dimerized through sodium cations and placed on the crystallographic center of symmetry (Figure 3).Distances O(1)-C( 43 3) Å) are shorter and correspond to double bonds.The structure of the organic fragment in 5 is parallel to the product of the insertion of carbodiimide into a carbamate, propane-2-yl (diphenylcarbamimidoyl)(phenyl)carbamate PhNHC(=NPh)NPhC(=O)OiPr, that was isolated from the reaction of titanium isopropoxide with phenylisocyanate [106].

Scheme 3 .
Scheme 3. Reaction of gallylene 1 with CyNCO and PhNCO to yield complexes 2-5.Counter ions and acenaphthene parts are omitted for clarity.

Scheme 3 .
Scheme 3. Reaction of gallylene 1 with CyNCO and PhNCO to yield complexes 2-5.Counter ions and acenaphthene parts are omitted for clarity.Formation of 2-5 can be rationalized through the mechanism suggested earlier for the reaction of 1 with O=C=O [87].It anticipates the formation of intermediate adduct INT[GaRNCO] (Scheme 3).The reaction is followed by the [3+2] or [3+2+2] cycloaddition of a second and a third molecule of isocyanate with concomitant extrusion of isonitrile or CO.Moreover, the intermediate INT[GaRNCO] is supposed to have two competitive decomposition pathways to the oxide INT[GaO] and the imide INT[GaNR], which are unstable towards dimerization [87].According to the proposed mechanism, the generation of species 2 should have led to a noticeable amount of INT[GaO], which was well supported by an experiment.A deeper inspection of the reaction mixture of 1 with PhNCO allowed the isolation of green crystals of an INT[GaO] dimer, the gallium oxide [(dppbian)GaO] 2 [Na(DME) 2 ] 2(6), on top of crystals of complex 2. The relatively low isolated yields of 2-5 may be attributed to the existence of few energetically close intermediatesINT[GaO], INT[GaNR], INT[GaRNCO]along with the presence of competitive reaction pathways that, in fact, resulted in a mixture of similar products.Complex 6 was also synthesized by the direct reaction of gallylene 1 with 1 equiv.N 2 O (Scheme 4) as green crystals in 66% yield.Spectral characteristics of 6 from both experiments coincided.For the sake of 6 synthesis, compound 1 was also treated with 1 equiv.carbonyl sulfide.That, however, resulted in sulfide 7 as green crystals in 35% yield.
79 (2Н), 1.89 (2Н), 1.70 (4Н), 1.57 (4Н), 1.37 (4Н), and 1.18 (4Н) ppm.Protons of the methine groups produce two broad singlets (1Н each) at 4.82 and 3.54 ppm.Compound 6 is poorly soluble in ether solvents, aggravating its NMR identification.The 1 H NMR spectra of 6 exhibited two doublets (24H each) at 0.96 and 0.86 ppm and one septet (8H) at 3.77 ppm for the protons isopropyl group indicative of three symmetry planes.The reactions from Scheme 3 seem to be unselective; NMR-scale experiments resulted in untrackable mixtures of products.The IR spectra of the compounds 2-6 support the proposed structures.Compound 2 features a strong C=N absorption band (1640 cm −1 ).The mixture of 3 and 4 exhibits absorption at 1628 cm −1 and 1602 cm −1 , corresponding to C=O and C=N vibrations.The IR spectrum of compound 5 consists of the absorption of 1714 cm −1 and 1661 cm −1 , which are characteristics of double C=O and C=N bonds, respectively.No -N=C=O absorption bands were registered in the 2000-2300 cm −1 region.In a separate experiment, adventitious contact of a reaction mixture of 1 with 1 equiv.CyNCO in THF with air led to the isolation of green crystals of compound [(dppbian)GaOC(NHCy)O(O)][Na(DME)3] (8) in 9% yield (Scheme 5).
C NMR signals of 131.2 ppm (H-C=N) and 68.8 ppm (H-C-N) shifted in comparison to the starting material (cf.δ (-N=CHPh)2 = 161.8ppm).The presence of the symmetry plane confirmed the formation of one optical isomer-the mesoform.
73 and  4.58  ppm.The last corresponded to the hydrogen atom at the sp3 -hybrydized carbon atom, forming a new bond.Five signals of protons in the pyridine fragment(6.62,5.44, 4.29, 4.23,  4.00 ppm) were significantly shifted in comparison to free pyridine (8.61, 7.66, 7.28 ppm), which confirmed its dearomatization.The assignment of these signals became clear when 1 H NMR spectra of 12 and 12-d5 were compared (Figure1).The 13 C NMR spectrum also confirmed the structure of the resulting compound.The signal of the sp3 -hybridized carbon atom H-C-N of the azine fragment shifted upfield (73.3 ppm), and the sp 2 -hybridized carbon atom H-C=N produced a signal at 131.5 ppm (cf.δ (-N=CHPh) 2 161.8 ppm).The carbon atoms of the pyridine fragment of 12 produced 13 C NMR signals at 143.7, 127.5, 104.6, 92.5, and 62.5 ppm.(cf.C 5 H 5 N 149.9, 135.9, and 123.75 ppm in pyridine).Since only one diastereomer was identified in both reactions of 1 with benzaldazine, it may be believed to proceed through the cycloaddition mechanism.Similar reductions of benzaldazine were reported by [(C 5 Me 5 ) 2 Sm(THF) 2 ] [101] and [Cp 2 Ti(η 2 -Me 3 SiC 2 SiMe 3 )] [102].The authors suggested a one-electron reduction of the imino group, which led to C-C coupling binuclei complexes.Processes of chemoselective coupling of carbonyl compounds with pyridine were previously reported for [NacNacAl:] (NacNac = [ArNC-(Me)CHC(Me)NAr] − , Ar = 2,6-iPr 2 C 6 H 3 ) [81].However, the coupling of two azine molecules and one azine molecule with a pyridine molecule in the coordination sphere of one metal atom was observed for the first time.