Crimmin, MR

Gates, SJ

Nako, AE

Schädel, N

White, AJ

English

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Journal Name Cite this: DOI: 10.1039/c0xx00000x www.rsc.org/xxxxxx Dynamic Article Links ► ARTICLE TYPE  This journal is © The Royal Society of Chemistry [year] [journal], [year], [vol], 00–00  |  1 Yttrium-Catalysed Dehydrocoupling of Alanes with Amines Adi E. Nako, Sarah J. Gates, Nicole Schädel, Andrew J. P. White and Mark R. Crimmin* Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x We report [Y{N(SiMe3)2}3] as a precatalyst for the 5 dehydrocoupling of sterically demanding amines with  β-diketiminate stabilised aluminium dihydrides. While simple anilines readily undergo Al–H/N–H dehydrocoupling under thermal conditions, catalytic methods are required to achieve reasonable rates of reaction for ortho-substituted anilines or 10 hindered aliphatic amines. The drive to control the rate of hydrogen release from amineborane has given rise to a number of mechanistically diverse catalyst systems capable of the dehydrocoupling of protic and hydridic substrates.1 While new catalysts for amine-borane 15 (or phosphine-borane) dehydrogenation continue to attract attention,2 in the past few years related methods for heteroatom–heteroatom bond formation by dehydrocoupling have emerged.3 For example, the catalytic reaction of amines with silanes provides an atom-efficient route to form nitrogen–silicon bonds 20 and incorporate silyl-protecting groups into organic molecules.4 Although a number of precatalysts have been developed that are capable of catalysing the release of multiple equivalents of H2 from H3N⋅BH3,1 the formation of unsaturated products or intermediates, such as silylimines [R2Si=NR], from the 25 dehydrocoupling of R2SiH2 and H2NR are rare.5     Fig. 1. The dehydrogenation of amineboranes and the dehydrocoupling of 30 amines with silanes. Amine-adducts of AlH3 have been proposed as hydrogen storage materials, and aluminium amides themselves may act as single source precursors for the chemical vapour deposition of aluminium nitride.6-7 Although the dehydrocoupling (or 35 protonolysis) of aluminium hydrides with amines is a common method to make Al–N bonds, these reactions can suffer from slow reaction rates and often salt-metathesis methods are preferred.8 For substrates containing more than one Al–H bond achieving a selective reaction is also a potential problem. 40 Catalytic approaches to the dehydrocoupling of amines and alanes offer an opportunity to address the issue of selectivity while vastly increasing reaction rates, to the best of our knowledge catalysts for this reaction remain unreported. Herein we report our initial findings on the thermal and catalytic 45 dehydrocoupling of amines with sterically demanding alanes. Thermal Dehydrocoupling of Anilines with Alanes: The reaction of 4-fluoroaniline with 1a-b in toluene gave the corresponding mono-substitution products in good yield within 2h at 25 oC. Variation of the substrate to aniline or 2,4,6-50 trifluoroaniline had little effect on the efficiency of the reaction with 1a. The corresponding aluminium amides 2a-d were isolated in 39-95% yield following preparative scale experiments. The variation in the isolated yields is more likely a consequence of the conditions of crystallisation rather than the efficiency of thermal 55 dehydrocoupling. Table 1. The thermal dehydrocoupling of sterically demanding alanes with anilines.  The reaction scope was extended to include to the 5-coordinate 60 aluminium dihydride, 1d (Scheme 1).9 Reaction with 2 equiv. of 4-fluoroaniline proceeds rapidly at 25 oC to yield 3. The reaction is not selective and attempts to modify the conditions to isolate the hydrido/amide intermediate failed. Although we have previously demonstrated that 1d remains 5-coordinate in 65 solution,9 in the current case, we propose that the dissociation of the pendant amine becomes favourable following the first Al–N bond formation. As a result the rate of the second dehydrocoupling becomes comparable with the first and the reaction is no longer selective. 70 Journal Name Cite this: DOI: 10.1039/c0xx00000x www.rsc.org/xxxxxx Dynamic Article Links ► ARTICLE TYPE  This journal is © The Royal Society of Chemistry [year] [journal], [year], [vol], 00–00  |  2  Figure 2. The crystal structure of (a) 2a, (b) 4b (disordered toluene molecule omitted for clarity), and (c) 3. Selected bond angles (o) and bond lengths (Å): 2a, Al–N1 1.8893(11), Al–N13 1.81.32(16), N1–Al–N1’ 96.52; 4b, Al–N1 1.8951(16), Al–N3 1.9021(15), Al–N30 1.8182(16), N1–Al–N3 95.55(7);  3, Al–N1 1.9084(17), Al–N3 1.905(2), Al–N31 1.7998(19), Al–N41 1.805(19), N1–Al–N3 94.17(8). 5  Scheme 1. The thermal dehydrocoupling of a 5-coordinate alane with p-fluoroaniline. In order to test this hypothesis, 1H–15N HMBC experiments were conducted on C6D6 solutions of 1d, 3 and a reaction mixture 10 containing the hydrido/amide intermediate 3’ at 298K. 15N chemical shifts of δ = 28.1, 29.8 and 22.4 ppm were recorded for the NMe2 group of 1d, 3’ and 3 respectively. While these data are consistent with amine dissociation in 3, the difference between the chemical shifts of the β-diketiminate nitrogens Δδ = 67.8, 15 59.2 and 4.2 ppm of 1d, 3’ and 3 respectively, provides direct evidence for a dramatic change in the electronic structure of the ligand. Hence, 1d and 3’ possess a trigonal bipyramidal structure with the equatorial and axial positions being magnetically and chemically distinct while 3 possess a four-coordinate geometry. 20 In combination with an X-ray crystallography study (Figure 2) these data show that the pendant amine of 3 not coordinated to Al either in solution or the solid-state. While 3’ appears to remain 5-coordinate in solution at 298 K, it can be expected that amine dissociation will more facile than that of 1d and open up the 25 coordination at Al, favouring a second dehydrocoupling. Catalytic Dehydrocoupling: Attempts to extend the scope of the thermal reaction to hindered aliphatic amines (pKa ∼ 40 – 45) or ortho-substituted anilines (pKa ∼ 30) failed to proceed selectively at useful reaction rates. For example, the reaction of 30 an excess of tert-butylamine with 1a in C6D6 (0.1-0.2 M in 1a) only reached completion after 12 d at 80 oC.     Table 2. [Y{N(SiMe3)2}3] catalysed dehydrocoupling of sterically demanding alanes with ortho-substituted anilines and 35 amines.   In contrast, at 25 oC with addition of 5 mol % [Y{N(SiMe3)2}3] vigorous gas evolution was observed. Monitoring the latter 40  This journal is © The Royal Society of Chemistry [year] Journal Name, [year], [vol], 00–00  |  3 reaction by 1H NMR spectroscopy revealed not only the formation of H2 (δ = 4.49 ppm) but complete and selective formation of 4d within 30 minutes at 25 oC. The catalyst loading could be lowered to 0.5 mol % without a detrimental effect on the yield or reaction time. These data allow a coarse estimation of a 5 minimum catalyst TOF of 500 h-1 for this reaction; a value that compares favourably to that of >1000 h-1 at 50 oC for boratabenzene yttrium alkyl catalysts employed for the dehydrogenation of dimethylamine-borane.10c Scale up of this reaction in toluene gave the product in a 64% isolated yield. 10   The catalytic dehydrocoupling of sterically demanding alanes can be generalised to a number of ortho-substituted anilines (Table 2, 4a-c, 4g) primary amines (Table 2, 4d-e) and a secondary amine (Table 2, 4f).¶  It is noteworthy that rare-earth bis(trimethylsilyl)amides have proven efficient catalysts for 15 amine-silane dehydrocoupling, amine-borane dehydrogenation, and the dehydrocoupling of triphenylphosphonium methylide with phenylsilane.4,10,11 Furthermore, aluminium hydrides themselves have recently emerged as efficient catalysts for amine-borane dehydrocoupling.12 20  In all cases, thermal control reactions show either trace or no conversion under the same conditions and require both elevated temperatures and extended reaction times to reach similar yields as the catalytic experiments (Table 2). For example, complex 4g has been previously reported by H. Roesky and co-workers and 25 may be synthesized by the thermolysis of 1b in neat 2,6-di-iso-propylaniline at 150 oC for 1h. Conducting this reaction in a flame-sealed NMR tube in toluene-d8 gave only trace conversion after 1d at 150 oC.13 With 5 mol% [Y{N(SiMe3)2}3] the reaction proceeds at 25 oC giving 4g in 87% isolated yield, albeit with a 30 longer reaction time of 15 h (Table 2, 4g). Under these catalytic conditions, hindered 2o amines such as di-iso-propylamine and hexamethyldisilazane gave no reaction with 1a despite prolonged heating at 80oC.   The reaction products have been characterised by multinuclear 35 NMR and infrared spectroscopy and CHN analysis. Complexes 2a-d and 4a-h display Al–H stretches in the region of 1788-1875 cm-1, while in some cases N–H stretches were observed between 3344-3449 cm-1 but these bands were often weak and broad. Multinuclear NMR data supports the assignment and while the 40 Al–H resonances are observed as extremely broad peaks in the region of δ = 3.0–5.0 ppm, the aliphatic N–H protons of 4d and 4e demonstrate a 3JH–H coupling to the Al–H and resonate at δ = 0.56 (d, J = 3.6 Hz) and 0.36 (d, J = 3.2 Hz) ppm respectively. Complexes 2a, 3 and 4b have been characterised by single crystal 45 X-ray diffraction and while the metrical parameters do not warrant discussion it is noteworthy that the pendant amine of 3 is not coordinated to aluminium in the solid state.   In summary, we have reported [Y{N(SiMe3)2}3] as a precatalyst for the dehydrocoupling of amines with sterically 50 demanding alanes. Catalysis proceeds rapidly at 25 oC and is significantly more efficient than thermal methods. While these findings may open up new possibilities for the application of aluminium amides as materials precursors, as ligands for transition metals, or as intermediates in catalysis, if a further 55 catalytic dehydrogenation of the aluminium hydrido/amide complexes reported herein could be achieved it may generate a reactive 3-coordinate aluminium imido, BDIAl=NR.8b This latter moiety has been proposed as an intermediate in intramolecular C–H activation and [2+2] cycloaddition reactions.14-15 60 Notes and references a Department of Chemistry, Imperial College London, South Kensington, London SW7 2AZ, U.K. Tel: +44(0)2075942846; E-mail: m.crimmin@imperial.ac.uk † Electronic Supplementary Information (ESI) available: Including full 65 experimental procedures, X-ray crystallography data and details of calculations. See DOI: 10.1039/b000000x/  ¶ General experimental procedure for catalytic dehydrocoupling: In a glovebox, the alane (0.33 mmol) and [Y{N(SiMe3)2}3] (0.016 mmol) were weighed out into a 20 mL glass scintillation vial. Dry toluene or 70 diethyl ether (2 mL) was added followed by addition of the amine (0.33 mmol). Vigorous effervescence was observed and the vial was sealed. The reaction mixture was left for 0.5 – 15 h at 25 oC before the mixture was passed through glass fibre filter paper and the solvent removed under reduced pressure. The crude reaction mixture was then either 75 recrystallized or washed with cold hexane to give the pure product (48 – 89 % yield).   1 (a) T. J. Clark, K. Lee and I. Manners, Chem. Eur. J. 2006, 12, 8634. (b) F.  Gauvin, J. F. Harrod and H. G. Woo, Ad. 80 Organomet. Chem. 1998, 42, 363. 2 (a) M. C. Denney, V. Pons, T. J. Hebden, D. M. Heinekey, K. I. Goldberg, J. Am. Chem. Soc. 2006, 128, 12048. (b) C. A. Jaska, I. Manners, J. Am. Chem. Soc. 2004, 126, 9776. 3 (a) E. M. Leitao, T. Jurca, I. Manners, Nature Chem. 2013, 5, 85 817. (b) R. J. Less, R. L. Melen, D. S. Wright, RSC Advances 2012, 2, 2191. 4 (a) W. Xie, H. Hu, C. Cui, Angew. Chem., Int. Ed. 2012, 51, 11141; (b) M. S. Hill, D. J. Liptrot, D. J. MacDougall, M. F. Mahon, T. P. Robinson, Chem. Sci. 2013, 4, 4212; (c) F. Buch, 90 S. Harder, Organometallics 2007, 26, 5132; (d) J. F. Dunne, S. R. Neal, J. Engelkemier, A. Ellern, A. Sadow, J. Am. Chem. Soc. 2011, 133, 16782. 5 Y. Chen, H. Song, C. Cui, Angew. Chem., Int. Ed. 2010, 49, 8958. 95 6 (a) D. J. Grant and D. A. Dixon, J. Phys. Chem. A, 2005, 109, 10138; (b) B. M. Wong, D. Lacina, I. M. B. Nielsen, J. Graetz and M. D. Allendorf, J. Phys. Chem. C, 2011, 115, 7778. 7 (a) F. C. Sauls and L. V. Interrante, Coord. Chem. Rev., 1993, 128, 193; (b) Y. Saito, Y. Sugahara and K. Kuroda, J. Am. 100 Ceram. Soc, 2000, 83, 2436; (c) J. Khanderi, D. Rische, H. W. Becker and R. A. Fischer, J. Mater. Chem, 2004, 14, 3210; (d) U. Werner, Structure and Bonding, 2003, 105, 41. 8 (a) A. Y. Timoshkin, Coordination Chemistry Reviews, 2005, 249, 2094-2131; (b) J. Koller, R. G. Bergman, 105 Organometallics 2010, 29, 3350. (c) A. Heine and D. Stalke, Angew. Chem., Int. Ed. 1992, 31, 854. (d) M. L. Montero, H. Wessel, H. W. Roesky, M. Teichert, I. Usón, Angew. Chem., Int. Ed. 1997, 36, 629. 9 A. E. Nako, S. J. Gates, A. J. P. White, M. R. Crimmin, 110 Dalton Trans. 2013, 42, 15199. 10 For amine-borane dehydrogenation see (a) M. S. Hill, G. Kociok-Köhn and T. P. Robinson, Chem. Commun., 2010, 46, 7587; (b) P. Cui, T. P. Spaniol, L. Maron and J. Okuda, Chem. Eur. J., 2013, 19, 13437; (c) E. Lu, Y. Yuan, Y. Chen and W. 115 Xia, ACS Catal., 2013, 3, 521.  11 A. E. Nako, A. J. P. White, M. R. Crimmin, Chem. Sci. 2013, 4, 691. 12 (a) H. J. Cowley, M. S. Holt, R. L. Melen, J. M. Rawson and D. S. Wright, Chem. Commun. 2011, 47, 2682. (b) M. M. 120 Hansmann, R. L. Melen and D. S. Wright, Chem. Sci. 2011, 2, 1554. (c) R. J. Less, H. R. Simmonds, D. S. Wright, Dalton Trans. 2014, 43, 5785.  This journal is © The Royal Society of Chemistry [year] Journal Name, [year], [vol], 00–00  |  4 13 H. Zhu, Z. Yang, J. Magull, H. W. Roesky, H.-G. Schmidt and M. Noltemeyer, Organometallics 2005, 24, 6420. 14 For putatative three-coordinate LnAl=NAr see: (a) N. J. Hardman,  C. Cui, H. W. Roesky, W. H. Fink and P. P. Power, Angew. Chem. Int. Ed. 2001, 40, 2172; (b) H. Zhu, J. Chai, V. 5 Chandrasekhar, H. W. Roesky, J. Magull, D. Vidovic, H.-G. Schmidt, M. Noltemeyer, P. P. Power and W. A. Merrill, J. Am. Chem. Soc. 2004, 126, 9472; (c) H. Zhu, J. Chai, V. Jancik, H. W. Roesky, W. A. Merrill and P. P. Power, J. Am. Chem. Soc. 2005, 127, 10170.  10 15 For an isolated four-coordinate LnAl=NAr see: J. Li, X. Li, W. Huang, H. Hu, J. Zhang and C. Cui, Chem. Eur. J. 2012, 18, 15263.   15 

Yttrium-catalysed dehydrocoupling of alanes with amines.

Adi E. Nako

Sarah J. Gates

Nicole Schädel

Andrew J. P. White

Mark R. Crimmin

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Chemical Communications

Yttrium-catalysed dehydrocoupling of alanes with amines

http://spiral.imperial.ac.uk/bitstream/10044/1/15255/2/Chemical%20Communications_67_2014.pdf

Yttrium-catalysed dehydrocoupling of alanes with amines.

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