Insertion reactions of nitriles in cationic alkylbis(cyclopentadienyl)titanium complexes:  the facile synthesis of azaalkenylidene titanium complexes and the crystal and molecular structure of [(indenyl)2Ti(NCMePh)(NCPh)]BPh4

Bochmann, Manfred; Wilson, L. M.; Hursthouse, Michael B.; Motevalli, Majid

doi:10.1021/om00095a020

Cited by 100 publications

(35 citation statements)

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“…Based on these spectral data, we postulate that the imine nitrogen is covalently bound to titanium as shown by 35 in Scheme 4. [25] The nature of the titanium center is more difficult to specify, as methanol may be exchanging with the isopropoxy ligands on the metal, and the imine-titanium complex may be dinuclear. [26] In order to further corroborate structure 35, we attempted to prepare it from an alternate route (Scheme 5).…”

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confidence: 99%

Direct Titanium‐Mediated Conversion of Ketones into Enamides with Ammonia and Acetic Anhydride

et al. 2012

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N-Acyl enamides are useful compounds in organic synthesis. In the realm of catalytic asymmetric hydrogenation, they are among the most exhaustively studied class of substrates, and provide access to valuable chiral amine building blocks. [1] These substrates have also demonstrated broad utility in catalytic asymmetric CÀC bond forming processes such as aza-ene, [2] Michael, [3] Friedel-Crafts, [4] cycloaddition, [5] and arylation [6] reactions. Despite the extensive applications of Nacyl enamides, their preparation remains challenging. The direct condensation of acetamide with ketones, while attractive in its simplicity, proceeds either in low yields or not at all for the majority of ketone substrates.[7] The Pd-catalyzed cross-coupling of vinyl electrophiles with amides [8] and the Heck reaction of N-vinylacetamide with aryl halides [9] often require additional steps for preparation of coupling precursors and employ a costly transition metal catalyst.[10] The addition of alkyl magnesium or alkyl lithium reagents to nitriles followed by trapping with Ac 2 O or AcCl has limited functional-group tolerance [11] and requires low reaction temperatures.[12] By far the most commonly employed procedure is the two-step conversion of ketones through ketoximes (Scheme 1). [13] This reaction was first described in 1975 by Barton and coworkers. After a first step of oxime formation, the ketoxime was then treated with Ac 2 O and either pyridine at reflux, Cr(OAc) 2 , or Ti(OAc) 3 for reductive acylation.[13] Subsequently, numerous alternative reducing agents were developed.[14] The most commonly employed reductant is Fe powder, which was first demonstrated by Barton and Zard in 1985 [14a] and subsequently developed by Burk and Zhang in 1998. [14b,c] From a large scale perspective, the use of highenergy hydroxylamine, generating a high-energy oxime intermediate, and reducing the oxime at high temperatures present safety concerns. In addition, the workup of the Fe process is often tedious, requiring filtration of large amounts of inorganic salts. While several alternatives to Fe metal have emerged recently, these still rely on the same overall two-step process through a ketoxime.[14d-g] Our own requirements for large-scale synthesis of N-acetyl enamides for asymmetric hydrogenation prompted us to develop a more direct and process-friendly alternative in which hydroxylamine is replaced with ammonia. Herein we describe a direct, redoxfree synthesis of enamides from ketones, ammonia, and Ac 2 O mediated by Ti(OiPr) 4 . In addition, we introduce the use of edte (N,N,N',N'-tetrakis(2-hydroxyethyl)ethylenediamine) to effect water solubilization of the Ti and allow a simple extractive workup.Our strategy for enamide synthesis was based on condensation of a ketone with ammonia to give an N-unsubstituted imine or enamine, followed by N-acetylation on addition of Ac 2 O. The imine formation presented a challenge due to the volatility of ammonia, which excluded the common method for imine formation by azeotropic distillation for remo...

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confidence: 99%

Direct Titanium‐Mediated Conversion of Ketones into Enamides with Ammonia and Acetic Anhydride

et al. 2012

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“…The formation of trimethylsilyl isocyanide complexes upon treatment with Me 3 SiCN, which has been reported for rhenium(I), is thought to be driven thermodynamically by the stronger net π-acceptor/σ-donor properties of isocyanides as compared to nitriles. 29 The silyl isonitrile complexes are so stable that their formation acts as thermodynamic sinks in the cleavage of the C−C bond of RCN in combination with a silyl ligand on the metal, as reported for rhodium 31 Although N and C atoms can be hardly distinguished by Xray diffraction, the isonitrile formulation gives a physically more realistic pattern for the thermal parameters of the atoms involved than the alternative Fe−NCSiMe 3 description (see Supporting Information). 34 The Fe−C and C−N distances in 18 (Table 5) 862(4) and 1.157(6) Å).…”

Section: Scheme 5 Asymmetric Strecker Reactionmentioning

confidence: 86%

“…Calculations suggest that the isonitrile form (Me 3 SiNC) is 3−5 kcal/mol less stable than Me 3 SiCN with an activation barrier of 28−30 kcal mol −1 for the noncatalyzed interconversion. 28 However, the thermodynamic preference for the cyano form can be reversed by coordination to a transition metal, as documented for titanium 29 and rhenium 30 complexes. The formation of trimethylsilyl isocyanide complexes upon treatment with Me 3 SiCN, which has been reported for rhenium(I), is thought to be driven thermodynamically by the stronger net π-acceptor/σ-donor properties of isocyanides as compared to nitriles.…”

Section: Scheme 5 Asymmetric Strecker Reactionmentioning

confidence: 97%

Chiral Iron(II) NPPN Complexes: Synthesis and Application in the Asymmetric Strecker Reaction of Azomethine Imines

2015

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The open-chain NPPN ligand (1S,1′S)-1,1′-((ethane-1,2-diylbis(phenylphosphanediyl))bis(2,1-phenylene))bis(N-cyclohexylmethanimine) (1) was prepared by condensation of cyclohexylamine with enantiomerically pure (1S,1′S)-2,2′-(ethane-1,2-diylbis(phenylphosphanediyl))dibenzaldehyde ((S,S)-6). Ligand 1 coordinates to [Fe(OH 2 ) 6 ](BF 4 ) 2 or [Fe(MeCN) 6 ](SbF 6 ) 2 in acetonitrile to give the dicationic complex [Fe(MeCN) 2 (1)](X) 2 (2) (X = BF 4 − or SbF 6 − ). The corresponding carbonyl (3), bromocarbonyl (4), and bis(tert-butylisonitrile) (5) derivatives were prepared and fully characterized. Complex 2 reacts with Me 3 SiCN to give the corresponding trimethylsilyl isocyanide derivative 18 featuring a Fe−CNSiMe 3 linkage. The X-ray structures of 2, 3, 5, and 18 show that ligand 1 assumes the Λ-cis-α geometry, which allows comparing the trans influence of these ligands. Complexes 2, 3, 5, and 18 were applied in the asymmetric addition of trimethylsilyl cyanide to azomethine imines (Strecker reaction), whose enantioselectivity reached 22% ee. The low enantioselectivity can be explained on the basis of the Me 3 SiCN/ Me 3 SiNC isomerization and of the reaction product partially displacing the NPPN ligand from iron. ■ INTRODUCTIONBeing able to replace precious metals with iron provides several advantages: iron is cheap, nontoxic, and environmentally benign. Catalysts based on iron are employed for a broad range of transformations, 1 and nature uses it for some of the most challenging reactions, such as the direct C−H oxidation of methane. 2 There are, however, serious obstacles that have to be overcome in order to successfully use ironand base metals, in generalin catalysis. In particular, 3d metals give weaker metal−ligand bonds, which often results in high-spin electron configurations. The resulting paramagnetic species not only elude characterization by traditional NMR experiments but also are less stable than their low-spin counterparts because of the partial occupation of σ*-antibonding orbitals (e g orbitals in the case of octahedral complexes).A common approach to tackle these problems is to exploit the chelate effect by using pincer 3 or tetradentate 4 ligands. Strong-field ligands such as CO, isonitriles, or hydrides are also often applied to enforce the low-spin electron configuration. Furthermore, the macrocyclic effect 5 uses ligand preorganization and rigidity to provide a pocket of appropriate size for the metal and to increase the kinetic inertness of the ligand framework, which can be exploited to build robust systems. Accordingly, iron−porphyrin systems are abundant in nature as the active site of enzymes and are widely used. 2 We recently capitalized on the macrocyclic effect with iron complexes of tetradentate N 2 P 2 -macrocycles, 6 whereas Gao has successfully applied N 4 P 2 macrocycles in asymmetric hydrogenation of ketones with iron. 7 Notably, the diphosphine (S,S)-6 that we developed to prepare C 2 -symmetric macrocycles is a new, P-stereogenic synthon that gives access also to diastereo...

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“…Treatment of 7c with either trityltetraphenylborate [20] or N,N-dimethylanilinium tetraphenylborate [21] (3c) is given in Table 6.…”

Section: ( Me)]mentioning

confidence: 99%

Comparison of Characteristic Structural Features among the Triade of Tris(cyclopentadienyl)(Group-4 Metal) Complex Cations: a Combined Theoretical and Experimental Study

Jacobsen¹,

Berke²,

Brackemeyer³

et al. 1998

HCA

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A density functional theory computational chemistry study has revealed a fundamental structural difference between [Ti(Cp)3]+ and its congeners [Zr(Cp)3]+ and [Hf(Cp)3]+/(Cp=cyclopentadienyl). Whereas the latter two are found to contain three uniformely η5‐coordinated Cp ligands (3η5‐structural type), [Ti(Cp)3]+ is shown to prefer a 2η5η2 structure. [Ti(Cp)3]+[B(C6F5)3(Me)]− (10⋅[B(C6F5)3(Me)]−) was experimentally generated by treatment of [Ti(Cp)3(Me)] (7a) with B(C6F5)3 (Scheme 3). Low‐temperature 1H‐NMR spectroscopy in CDFCl2 (143 K, 600 MHz; Fig. 8) showed a splitting of the Cp resonance into five lines in a 2 : 5 : 2 : 5 : 1 ratio which would be in accord with the theoretically predicted 2η5η2‐type structure of [Ti(Cp)3]+. The precursor [Ti(Cp)3(Me)] (7a) exhibits two 1H‐NMR Cp resonances in a 10 : 5 ratio in CD2Cl2 at 223 K. Treatment of [HfCl(Cp)2(Me)] (6c) with sodium cyclopentadienide gave [Hf(Cp)3(Me)] (7c) (Scheme 1). Its reaction with B(C6F5)3 furnished the salt [Hf(Cp)3]+[B(C6F5)3(Me)]− (8⋅[B(C6F5)3(Me)]−), which reacted with tert‐butyl isocyanide to give the cationic complex [Hf(Cp)3(C=N−CMe3)]+ (9a; with counterion [B(C6F5)3(Me)]− (Scheme 2). Complex cation 9a was characterized by X‐ray diffraction (Fig. 7). Its Hf(Cp3) moiety is of the 3η5‐type. The structure is distorted trigonal‐pyramidal with an average D−Hf−D angle of 118.8° and an average D−Hf−C(1) angle of 96.5° (D denotes the centroids of the Cp rings; Table 6). Cation 9a is a typical d0‐isocyanide complex exhibiting structural parameters of the C≡N−CMe3 group (d(C(1)−N(2))=1.146 (5) Å; IR: v˜(C≡N) 2211 cm−1) very similar to free uncomplexed isonitrile. Analogous treatment of 8 with carbon monoxide yielded the carbonyl (d0‐group‐4‐metal) complex [Hf(Cp)3(CO)]+ (9b; with counterion [B(C6F5)3(Me)]−) (Scheme 2) that was also characterized by X‐ray crystal‐structure analysis (Fig. 6). Complex 9b is also of the 3η5‐structural type, similar to the peviously described cationic complex [Zr(Cp)3(CO)]+, and exhibits properties of the CO ligand (d(C−O)=1.11 (2) Å; IR: v˜(C≡O) 2137 cm−1) very similar to the free carbon monoxide molecule.

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Insertion reactions of nitriles in cationic alkylbis(cyclopentadienyl)titanium complexes: the facile synthesis of azaalkenylidene titanium complexes and the crystal and molecular structure of [(indenyl)2Ti(NCMePh)(NCPh)]BPh4

Cited by 100 publications

References 1 publication

Direct Titanium‐Mediated Conversion of Ketones into Enamides with Ammonia and Acetic Anhydride

Direct Titanium‐Mediated Conversion of Ketones into Enamides with Ammonia and Acetic Anhydride

Chiral Iron(II) NPPN Complexes: Synthesis and Application in the Asymmetric Strecker Reaction of Azomethine Imines

Comparison of Characteristic Structural Features among the Triade of Tris(cyclopentadienyl)(Group-4 Metal) Complex Cations: a Combined Theoretical and Experimental Study

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