Contacted Ion-Pair Lithium Alkylamidoaluminates: Intramolecular

Oct 9, 2009 - E-mail: [email protected] (R.E.M.) and [email protected] .... Elaine Crosbie , Alan R. Kennedy , and Stuart D. Robertson...
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Organometallics 2009, 28, 6462–6468 DOI: 10.1021/om900736a

Contacted Ion-Pair Lithium Alkylamidoaluminates: Intramolecular Alumination (Al-H Exchange) Traps for TMEDA and PMDETA ,‡  Ben Conway,† Joaquı´ n Garcı´ a-Alvarez,* Eva Hevia,† Alan R. Kennedy,† ,† Robert E. Mulvey,* and Stuart D. Robertson† †

WestCHEM, Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow G1 1XL, U.K., and ‡Departamento de Quı´mica Org anica e Inorg anica, Instituto Universitario de Quı´mica Organomet alica “Enrique Moles” (Unidad Asociada al CSIC), Facultad de Quı´mica, Universidad de Oviedo, E-33071 Oviedo, Spain Received August 24, 2009

Exploring the reactivity of mixed-metal synergic bases, it is found that the lithium TMP aluminate Bu3Al(TMP)Li functions as a dual TMP/alkyl base, exhibiting 2-fold AMMAl (alkali-metal-mediated alumination) toward TMEDA to yield the heterobimetallic bis-(deprotonated TMEDA) derivative [Li{Me2NCH2CH2N(Me)CH2}2Al(iBu)2] (2). In contrast, the amide enriched aluminate iBu2Al(TMP)2Li acts as only a single-fold amido base toward TMEDA or PMDETA to afford the aminedeprotonated derivatives [Li{Me2NCH2CH2N(Me)CH2}(TMP)Al(iBu)2] (4) and [Li{Me2NCH2CH2N(Me)CH2CH2N(Me)CH2}(TMP)Al(iBu)2] (5), respectively. On their own, the aluminum compounds iBu3Al or iBu2Al(TMP) are not sufficiently strong bases to metalate TMEDA or PMDETA, so in 2, 4, and 5, the R-deprotonations of TMEDA and PMDETA are synergic in origin, as the intramolecular communication between Li and Al appears to activate the TMP and iBu bases. This special behavior can be attributed to intramolecular proximity effects between the active base component (TMP or iBu) and the ligating TMEDA or PMDETA molecule. X-ray crystallography studies reveal 2 is a contacted ion-pair ate containing two R-aluminated TMEDA ligands, which chelate the lithium cation which is linked to the distorted tetrahedral Al center by two N bridges from the metalated junction of the TMEDA molecules. In contrast, 4 and 5 have a mixed NCH2-TMP bridging ligand set, completed by two terminal iBu ligands on Al and a chelating metalated TMEDA or PMDETA ligand attached to Li, respectively. In addition, the 1H, 7Li, and 13C{1H} spectra of 2 (recorded in C6D6 solutions), 4, and 5 (recorded in cyclohexane solutions-d12) are disclosed. i

Introduction First reported in 2004 by Uchiyama et al., lithium TMPaluminate “iBu3Al(TMP)Li” (where TMP is the sterically demanding amide 2,2,6,6-tetramethylpiperidide) is an excellent base reagent in THF, realizing highly chemo- and regioselective deprotonative aluminations of functionalized aromatic and heteroaromatic compounds as well as of functionalized allylic compounds.1 Theoretical and experimental studies have revealed that this TMP-aluminate acts as an amido base, exhibiting only single-step directed *To whom correspondence should be addressed. E-mail: r.e.mulvey@ strath.ac.uk (R.E.M.) and [email protected] (J.G.-A.). (1) (a) Uchiyama, M.; Naka, H.; Matsumoto, Y.; Ohwada, T. J. Am. Chem. Soc. 2004, 126, 10526. (b) Naka, H.; Uchiyama, M.; Matsumoto, Y.; Wheatley, A. E. H.; McPartlin, M.; Morey, J. V.; Kondo, Y. J. Am. Chem. Soc. 2007, 129, 1921. (2) Naka, H.; Morey, J. V.; Haywood, J.; Eisler, D. J.; McPartlin, M.; Garcia, F.; Kudo, H.; Kondo, Y.; Uchiyama, M.; Wheatley, A. E. H. J. Am. Chem. Soc. 2008, 130, 16193. (3) This is in contrast to most TMP-zincate bases which exhibit twostep DoM, see: Nobuto, D.; Uchiyama, M. J. Org. Chem. 2008, 73, 1117, although the second step can be inhibited by coordination saturation at the zinc center, see: Clegg, W.; Conway, B.; Graham, D. V.; Hevia, E.; Kennedy, A. R.; Mulvey, R. E.; Russo, L.; Wright, D. S. Chem.;Eur. J. 2009, 15, 7074. pubs.acs.org/Organometallics

Published on Web 10/09/2009

ortho-metalation (DoM) reactivity.2-4 Furthermore, on the basis of our synthetic, reactivity, NMR spectroscopic, and X-ray crystallographic studies, a reaction scheme involving previously unconsidered solvent-separated ion-pair species and a dismutation process was postulated.5 In addition, we were able to report5 the complex [(THF)3 3 Li{O(dC)N(iPr)2(C6H4)}Al(iBu)3] as the first tangible metallo intermediate of a direct alumination reaction (performed in THF solutions) of N,N-diisopropylbenzamide, or indeed of any functionalized aromatic compound, to be structurally defined and fully characterized.6 We also described that carrying out the same reaction in a nonpolar solvent (hexane) (4) For an authoritative review on “complex-induced proximity effect (CIPE)” in DoM see: Whisler, M. C.; MacNeil, S.; Snieckus, V.; Beak, P. Angew Chem., Int. Ed. 2004, 43, 2206. For reviews in DoM of aromatic systems through alkali-metal-mediated alumination (and metalation in general), see: (a) Mulvey, R. E.; Mongin, F.; Uchiyama, M.; Kondo, Y. Angew. Chem., Int. Ed. 2007, 46, 3802. (b) Mulvey, R. E. Acc. Chem. Res. 2009, 42, 743. (5) Conway, B.; Hevia, E.; Garcia-Alvarez, J.; Graham, D. V.; Kennedy, A. R.; Mulvey, R. E. Chem. Commun. 2007, 5241. (6) Crystal structures of three different Lewis base-stabilized derivatives [L 3 Li(μ-TMP)(μ-iBu)(iBu)2] have been previously described by us in: Garcia-Alvarez, J.; Hevia, E.; Kennedy, A. R.; Klett, J.; Mulvey, R. E. Chem. Commun. 2007, 2402. r 2009 American Chemical Society

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Figure 1. Structure of [{PhC(dO)N(iPr)2} 3 Li{2-[1-C(dO)N(iPr)2]C6H4}{Me2NCH2CH2N(Me)CH2}Al(iBu)2] (1).

instead of THF, the aforementioned LiTMP-aluminate can function as a dual TMP/alkyl base in the synthesis of the complex [{PhC(dO)N(iPr)2} 3 Li{2-[1-C(dO)N(iPr)2]C6H4}{Me2NCH2CH2N(Me)CH2}Al(iBu)2] (1) (see Figure 1) that combines ortho-alumination of N,N-diisopropylbenzamide with methyl-alumination of TMEDA (N,N,N0 ,N0 -tetramethylethylenediamine, Me2NCH2CH2NMe2).7 As previously noted by Clayden,8 “direct deprotonation R to nitrogen is usually impossible to achieve (except for superbasic reagents) unless the lone pair of the nitrogen atom is involved in conjugation with a carbonyl group or delocalized around an aromatic ring.” This general process, direct Rmetalation of tertiary amines, is a desirable synthetic route to polar heteroatom-containing organometallics but is hampered due, as previously mentioned, to the destabilization of the developing carbanions by the repulsion with the lone pair electron density of the adjacent nitrogen atom.9 Usually, multistep procedures such as BF3-activation of amines,10 transmetalation (in particular, tin/lithium exchange),11 or C-S/C-Te bond breakages12 are necessary to access these useful synthetic reagents, but generally these provide only moderate yields. Only a few examples exist for which direct deprotonation of otherwise nonfunctionalized amines has been demonstrated. Although TMEDA’s primary role in organometallic chemistry13 is as a bidentate N donor ligand, it does, on prolonged exposure to certain organoalkali reagents, undergo different direct R-metalation reactions, (7) Garcia-Alvarez, J.; Graham, D. V.; Kennedy, A. R.; Mulvey, R. E.; Weatherstone, S. Chem. Commun. 2006, 3208. (8) Clayden, J. In Organolithiums: Selectivity for Synthesis, Tetrahedron Organic Chemistry Series, 2002, 23, p 14. (9) (a) Boche, G.; Lohrenz, J. C. W.; Opel, A.; Sapse, A.-M.; Schleyer, P. v. R. in Lithium Chemistry; John Wiley & Sons: New York, 1995, p. 195. (b) Bordwell, N. R.; Vanler, R.; Zhang, X. J. Am. Chem. Soc. 1991, 113, 9856. (c) Schlosser, M., Ed. Organometallics in Synthesis;A Manual 2nd ed.; Wiley: New York, 2002. (10) Kessar, S. V.; Singh, P. Chem. Rev. 1997, 97, 721. (11) (a) Seyferth, D.; Weiner, M. A. J. Org. Chem. 1959, 24, 1395. (b) Peterson, D. J. J. Organomet. Chem. 1970, 21, 63. (c) Peterson, D. J. J. Am. Chem. Soc. 1971, 93, 4027. (d) Tian, X.; Woski, M.; Lustig, C.; Pape, T.; Fr€ ohlich, R.; Le Van, D.; Bergander, K.; Mitzel, N. W. Organometallics 2005, 24, 82. (e) Tian, X.; Fr€ohlich, R.; Pape, T.; Mitzel, N. W. Organometallics 2005, 24, 5294. (f) Tian, X.; Fr€ohlich, R.; Mitzel, N. W. Dalton Trans. 2005, 380. (12) (a) Gawley, R. E.; Zhang, Q. J. Org. Chem. 1995, 60, 5763. (b) Quintard, J.-P.; Elissondo, B.; Jousseaume, B. Synthesis 1984, 495. (c) Trost, B. M., Fleming, I., Eds. in Comprehensive Organic Synthesis; Pergamon Press: New York, 1991. (d) Strohmann, C.; Abele, B. C. Angew. Chem., Int. Ed. Engl. 1996, 35, 2378. (13) For an enlightening discussion on the role of TMEDA in group 1 chemistry, see: (a) Collum, D. B. Acc. Chem. Rev. 1992, 25, 448. See also: (b) Langer, A. W. Polyamine-Chelate Alkali Metal Compounds; Advances in Chemistry Series; American Chemical Society: Washington, DC, 1974; p 130. (14) (a) K€ ohler, F. H.; Hertkorn, N.; Bl€ umel, J. Chem. Ber. 1987, 120, 2081. (b) Harder, S.; Lutz, M. Organometallics 1994, 13, 5173. (c) Hildebrand, A.; Lonnecke, P.; Silaghi-Dumitrescu, L.; Silaghi-Dumitrescu, I.; Hey-Hawkins, E. Dalton Trans. 2006, 967. (d) Gessner, V. H.; Strohmann, C. J. Am. Chem. Soc. 2008, 130, 14412.

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the regioselectivity of which depends on the identity of the deprotonating agent. Thus, by means of tBuLi14a or n BuLi,14b,c lithiation of TMEDA in the terminal methyl group is achieved, while its methylene metalation is observed when the superbase LICKOR is used,14a followed by elimination of potassium dimethylamide. Only a select few methods have been proven to be effective in direct deprotonation of other nonfunctionalized amines. To date, these include deprotonation of: (i) monodentate amines as N-methylpiperidine (metalated at a CH3 position by the Lochmann-Schlosser base),15 (ii) bidentate amines as 1,3-dimethyl-1,3-diazacyclohexane (deprotonated at the disfavored position between the two nitrogen atoms with t BuLi)16a,b and N,N,N0 ,N0 -tetramethylcyclohexane-1,2-diamine ((R,R)-TMCDA, lithiated at a terminal CH3 group by t BuLi),17 (iii) tridentate amines as N,N0 ,N00 -trimethyl-1,4,7triazacyclononane18 (lithiated at a CH3 position by nBuLi), MeN[(CH2)2NMe2]2 (N,N,N0 ,N00 ,N00 -pentamethyldiethylenetriamine, PMDETA, lithiated at a terminal or central CH3 group depending on the concentration of the alkyl lithium reagent),19 and 1,3,5-trimethyl-1,3,5-triazacyclohexane (TMTAC, deprotonated at a methylenic (CH2) group between the two nitrogen atoms with tBuLi),16a,b and (iv) tetradentate amines as bis(3-methyl-1,3-diazacyclohex-1yl)methane and its five-ring analogue (regioselectively deprotonated at both endocyclic NCH2N units by tBuLi).16c In this work, we present the regioselective direct lithiummediated methyl-alumination of TMEDA by two different LiTMP-aluminates. Thus, iBu3Al(TMP)Li is capable of metalating two molecules of TMEDA in a one-pot reaction, establishing, as previously described in a communication by us,7 that this TMP-aluminate can function as a dual TMP/ alkyl base, while moving to the dialkyl-diamido aluminate i Bu2Al(TMP)2Li only one molecule of TMEDA could be metalated. Significantly, this dialkyl-diamido aluminate i Bu2Al(TMP)2Li can also undergo lithium-mediated methyl-alumination of a terminal CH3-group of PMDETA.

Results and Discussion As we have previously described, mixing LiTMP, iBu3Al, TMEDA, and N,N-diisopropylbenzamide in bulk hydrocarbon solvent generates the heterobimetallic trianionic complex [{PhC(dO)N(iPr)2} 3 Li{2-[1-C(dO)N(iPr)2]C6H4}{Me2NCH2CH2N(Me)CH2}Al(iBu)2] (1) (see Figure 1). Attempting to shed more light on this unexpected R-alumination of TMEDA, the same reaction methodology was employed but avoiding the addition of the aromatic amide. (15) (a) Schlosser, M.; Hartmann, J. Angew. Chem. 1973, 85, 544. Angew. Chem., Int. Ed. 1973, 12, 508. (b) Bauer, W.; Lochmann, L. J. Am. Chem. Soc. 1992, 114, 7482. (16) (a) Bojer, D.; Kamps, I.; Tian, X.; Hepp, A.; Pape, T.; Fr€ ohlich, R.; Mitzel, N. W. Angew. Chem. 2007, 119, 4254. Angew. Chem., Int. Ed. 2007, 46, 4176. (b) See also: K€ohn, R. D.; Seifert, G.; Kociok-K€ohn, G. Chem. Ber. 1996, 129, 1327. (c) Kamps, I.; Mix, A.; Berger, R. J. F.; Neumann, B.; Stammler, H.-G.; Mitzel, N. W. Chem. Commun. 2009, DOI: 10.1039/b909561f. (17) (a) Strohmann, C.; Gessner, V. H. Angew. Chem. 2007, 119, 8429 (b) Angew. Chem., Int. Ed. 2007, 46, 8281. (18) Arnold, J.; Knapp, V.; Schmidt, J. A. R.; Shafir, A. J. Chem. Soc., Dalton Trans. 2002, 3273. (19) (a) Schakel, M.; Aarnts, M. P.; Klumpp, G. W. Recl. Trav. Chim. Pays-Bas 1990, 109, 305. (b) Klumpp, G. W.; Luitjes, H.; Schakel, M.; de Kanter, E. J. J.; Schmitz, R. F.; van Eikema Hommes, N. J. R. C. Angew. Chem. 1992, 104, 624. Angew. Chem., Int. Ed. Engl. 1992, 31, 633. (c) Luitjes, H.; Schakel; Aarnts, M. P.; Schmitz, R. F.; de Kanter, E. J. J.; Klumpp, G. W. Tetrahedron 1997, 53, 9977. (d) Gessner, V. H.; Strohmann, C. Angew. Chem. 2007, 119, 4650; Angew. Chem., Int. Ed. 2007, 46, 4566.

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Figure 2. Molecular structure of [Li{Me2NCH2CH2N(Me)CH2}2Al(iBu)2] (2) with selective atom labeling. Hydrogen atoms have been omitted for clarity. Selected bond lengths (A˚) and angles (deg): Al(1)-C(1) 2.032(2), Al(1)-C(7) 2.029(2), Al(1)-C(13) 2.017(2), Al(1)-C(17) 2.013(2), Li(1)-N(1) 2.060(3), Li(1)-N(2) 2.198(3), Li(1)-N(3) 2.045(3), Li(1)-N(4) 2.182(4); C(1)-Al(1)-C(7) 103.83(9), C(1)-Al(1)-C(13) 109.89(9), C(1)-Al(1)-C(17) 117.29(10), C(7)-Al(1)-C(13) 113.91(10), C(7)-Al(1)-C(17) 101.78(9), C(13)-Al(1)-C(17) 109.92(10), N(1)-Li(1)-N(2) 87.85(13), N(1)-Li(1)-N(3) 113.48(16), N(1)-Li(1)-N(4) 127.99(17), N(2)-Li(1)-N(3) 128.38(17), N(2)-Li(1)-N(4) 116.61(16), N(3)-Li(1)-N(4) 87.35(13). Scheme 1. Lithium-Mediated Alumination of Two Molecules of TMEDA

n

BuLi was converted to LiTMP by initial addition of the amine TMP(H) followed by TMEDA (or vice versa), and the reaction (see Scheme 1) was completed by addition of the trialkylaluminium (iBu3Al) reagent. Surprisingly, the crystalline product obtained [Li{Me2NCH2CH2N(Me)CH2}2Al(iBu)2] (2) contains two molecules of methyl-aluminated TMEDA, which bridge both metals in a contacted ionpair type molecular structure (Figure 2). Moreover, as 2 was still obtained even when a deficiency of TMEDA was employed (in the 1:1 stoichiometric reaction), it can be inferred that the putative intermediate [Li{Me2NCH2CH2N(Me)CH2}(iBu)Al(iBu)2] or [Li{Me2NCH2CH2N(Me)CH2}(TMP)Al(iBu)2] (products of amido and alkyl basicity, respectively) is more reactive than the initial lithium TMPaluminate “iBu3Al(TMP)Li”. Thus, the reaction was therefore repeated in line with the stoichiometry found in 2 by adding an extra molar equivalent of TMEDA and this caused an increase in the yield of 2 (from 15 to 48%, see Scheme 1). This result establishes that TMP-aluminates can function as dual alkyl/amido bases7 even with relatively nonacidic (N)C-H bonds as ultimately one methyl group from each of two molecules of TMEDA are deprotonated in this one-pot reaction. It is important to note that on its own i Bu3Al is not a strong enough base to metalate TMEDA, so both deprotonations of this reaction are synergic in origin, as the contacted Li appears to activate the Al-attached TMP and one of the iBu groups of the base. The molecular structure of 2 (see Figure 2), belonging to the space group

Conway et al.

P21/c, can be considered a contacted ion pair, with both N atoms of the metalated junction of TMEDA chiral as each is attached to four different substituents, although overall the crystals are racemic. The anionic moiety comprises a distorted tetrahedral Al center (range of C-Al-C bond angles 101.78(9)-117.29(10)°) made up of four C atoms, from two terminal iBu ligands (with the shortest Al-C bonds (Al(1)-C(13) 2.017(2) A˚, Al(1)-C(17) 2.013(2) A˚]) and two R-deprotonated TMEDA molecules (with the longest Al-C bonds (Al(1)-C(1) 2.032(2) A˚, Al(1)-C(7) 2.029(2) A˚), in the same range as the Al-C bond length found in the metalated TMEDA units of 1 (2.0327(17) A˚)).7 Contact to the cationic moiety is through the two N atoms from the metalated junction of the TMEDA molecules, which bind to Li to complete a distorted tetrahedral coordination sphere of N atoms (range of N-Li-N bond angles 87.35(13)-128.38(17)°), the remaining two of which belong to the unmetalated end of TMEDA. The Li-N bonds involving the metalated part of TMEDA (Li(1)-N(1) 2.060(3)A˚, Li(1)-N(3) 2.045(3) A˚)) are significantly shorter than the Li-N bonds in the unmetalated part (Li(1)-N(2) 2.198(3) A˚, Li(1)-N(4) 2.182(4) A˚). Overall, the Al and Li centers are held together in an irregularly shaped 12-membered LiNCCNCAlCNCCN tricyclic ring, containing a smaller six-membered AlNCLiNC ring, which, surprisingly, does not have the expected chair conformation but displays a boat-shape conformation. This probably occurs because this six-membered ring is also attached to another two five-atom rings (LiNCCN) to build together the larger dodecatricyclic ring. 1 H, 13C, and 7Li NMR spectra recorded in benzene-d6 solution were obtained for 2 with only one set of resonances, supporting the diastereoselective formation of this compound (note that two stereogenic centers are generated, namely both N atoms of the metalated junction of TMEDA). All the anticipated different types of hydrogen atoms in 2 are accounted for and well-resolved in its 1H NMR spectrum, the most significant features being the two doublet signals (AB spin system, JHA, HB = 12.9 Hz) at 1.15 and 2.02 ppm for both protons of the metalated NCH2 unit of TMEDA. The Me2N and Me0 N resonances appear at 1.63 and 2.27 ppm, respectively, while all four H atoms of the two unique CH2 groups are inequivalent, having separate resonances at 1.53, 1.80, 2.21, and 2.53 ppm, consistent with the rigid, tricyclic conformation of the structure. Similarly, the 13C NMR spectrum of 2 is easily fully assignable (see Experimental Section for details). The reaction filtrate that remained after isolating the crystalline product 2 was also probed by NMR techniques. The 1H NMR spectrum indicated that the oily filtrate appeared to contain the aforementioned signals for 2 and free TMP(H) (consistent with the amido basicity of 2) but accompanied with a complicated mixture of products in minor yield. As we have previously demonstrated, deprotonations of both TMEDA molecules in 2 are synergic in origin, as the contacted Li appears to activate the Al-attached TMP and i Bu bases. To probe whether the remaining terminal two iBu ligands in 2 were able to bring about a metalation of a third (or even fourth) TMEDA molecule, we decided to repeat the synthetic process for 2 (see Scheme 1) but adding an extra excess (3 or 4 mol equiv) of TMEDA. However, this made no difference as we always recover from these reactions [Li{Me2NCH2CH2N(Me)CH2}2Al(iBu)2] (2) as a crystalline solid in similar yields to those found when the reaction is carried out with 2 equiv of TMEDA. Furthermore, the

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Scheme 2. Lithium-Mediated Alumination of One Molecule of TMEDA by a Dialkyldiamidoaluminate Base

1 H NMR analysis of the filtrate revealed the presence of nonmetalated TMEDA and free TMP(H), showing that the two remaining iBu groups in 2 fail to metalate any additional molecule of TMEDA or even the more acidic amine TMP(H). This lack of basicity could be related to the fact that in compound 2, lithium is coordinatively saturated and therefore it cannot bind to another molecule of TMEDA, which we believe should be the first step to induce an intramolecular R-deprotonation (vide infra, Scheme 3). This was supported by our findings when we investigated the deprotonation of TMEDA using an alternative solvent-separated ion-pair lithium aluminate. Thus, we prepared iBu3Al(TMP)Li in THF solution following literature procedures,1 which was proposed to contain solvent-separated ion pairs of general formula [{Li(THF)x}þ{Al(TMP)n(iBu)4-n}-]5 then added an equimolar amount of TMEDA to the reaction mixture. However, this procedure gave only a white oily product. Its 1H NMR spectrum indicates that the oil contains unmetalated TMEDA. Thus, this observation seems to rule out the use of separated ion pairs of general formula [{Li(THF)x}þ{Al(TMP)n(iBu)4-n}-] to achieve metalation of TMEDA, as the aforementioned close proximity between the substrate TMEDA and the Al(TMP)n(iBu)4-n fragment, thought necessary to promote the intramolecular deprotonation, is no longer present. The success in the metalation of TMEDA observed in 2 by using iBu3Al(TMP)Li prompted us to investigate its amide enriched congener, that is, iBu2Al(TMP)2Li (previously proposed as a component in the dismutation process of iBu3Al(TMP)Li in pure THF),5 obtained by mixing LiTMP with i Bu2Al(TMP) (3). Organoaluminum amides (R2AlNR0 2)20 can be easily prepared from the metathesis reaction of the corresponding lithium amide and the dialkylaluminum halide. For iBu2Al(TMP) 3, even though it was previously found to be highly effective for the Fischer indole synthesis21a and asymmetrization of meso-cyclic ketones,21b to the best of our knowledge, no NMR spectroscopic data for it have been reported. We report such data here. In particular, the 1H NMR spectrum is very informative, showing the presence of two well-defined sets of signals, one for the iBu groups (0.17 (4H, d, CH2-Al iBu), 1.01 (12H, d, CH3-iBu), and 1.94 (2H, sept, CH-iBu) ppm) and the other for the amide TMP (1.29 (12H, s, CH3 of TMP), 1.31 (4H, m, β-TMP), 1.72 (2H, m, γ-TMP) ppm), in a 2:1 ratio. By simply mixing together a hexane solution of freshly prepared LiTMP with the diisobutylaluminium amide iBu2Al(TMP) before a molar equivalent of the Lewis base (TMEDA) was introduced, we obtained the heterobimetallic-

(20) (a) Mole, T.; Jeffrey, E. A. In Organoaluminium Compounds; Elsevier: Amsterdam, 1972. (b) Ooi, T.; Maruoka, K. Sci. Synth.s 2004, 7, 225. (21) (a) Naruse, Y.; Yamamoto, H. Tetrahedron 1988, 44, 6021. (b) Maruoka, K.; Oishi, M.; Yamamoto, H. J. Org. Chem. 1993, 58, 7638.

Figure 3. Molecular structure of [Li{Me2NCH2CH2N(Me)CH2}(TMP)Al(iBu)2] (4) with selective atom labeling. Hydrogen atoms have been omitted for clarity. Selected bond lengths (A˚) and angles (deg): Al(1)-N(3) 1.9869(13), Al(1)-C(12) 2.0658(17), Al(1)-C(30) 2.0340(16), Al(1)-C(40) 2.0319(17), Li(1)-N(1) 2.000(3), Li(1)-N(2) 2.086(3), Li(1)-N(3) 2.035(3); N(3)-Al(1)-C(12) 99.57(6), N(3)-Al(1)-C(30) 121.47(7), N(3)-Al(1)-C(40) 115.57(6), C(12)-Al(1)-C(30) 106.57(7), C(12)-Al(1)-C(40) 110.56(7), C(30)-Al(1)-C(40) 102.71(7), N(1)-Li(1)-N(2) 88.58(12), N(1)-Li(1)-N(3) 114.87(14), N(2)-Li(1)-N(3) 151.46(15).

heterotrianionic complex [Li{Me2NCH2CH2N(Me)CH2}(TMP)Al(iBu)2] (4) in an isolated crystalline yield of 55%. 1 H, 13C, and 7Li NMR spectra recorded in cyclohexane-d12 solution were obtained for 4. The 1H NMR spectrum clearly shows the expected pattern for methyl-aluminated TMEDA, mimicking that previously described for 2 (NCH2Al 1.39 and 1.58 (1H each, AB spin system, d, JHA, HB = 14.0 Hz) ppm; NCH2 1.96 (2H, m), 2.73 and 2.94 (1H each, m) ppm; N(CH3)2 (2.24, 2.73, and 2.74 (3H each, s) ppm). This 1H NMR spectrum also reveals the presence of two iBu groups and one TMP ligand (see Supporting Information). The reaction filtrate that remained after isolating the crystalline product 4 was also probed by NMR techniques. The 1H NMR spectrum indicated that this reaction is almost quantitative, as the oily filtrate appears to contain only the aforementioned signals for 4 and free TMP(H). Thus, toward TMEDA, the lithium dialkyldiamidoaluminate i Bu2Al(TMP)2Li acts as an amido base, exhibiting only single-step AMMAl (alkali-metal-mediated alumination) reactivity in contrast to the dual amido/alkyl basicity of the trialkylamidoaluminate iBu3Al(TMP)Li. It should be noted that, as aforementioned for 2 with iBu3Al, the organoaluminium amide iBu2Al(TMP) (3) is not a strong enough base to metalate TMEDA, so the deprotonation observed in this reaction is also synergic in origin, as the contacted Li appears to activate one of the Al-attached TMP ligands. The molecular structure of 4 (Figure 3) features a five-atom, fourelement LiNCAlN ring, with a mixed NCH2-TMP bridging ligand set, and is completed by two terminal iBu ligands on Al and a chelating metalated TMEDA (N,N-attached) to Li. Overall, the Al and Li centers are held together in an irregularly shaped octa-LiNCCNCAlN bycyclic ring, with a Li-N hinge. The Al center displays a distorted tetrahedral geometry (subtending bond angles from 99.57(6)° {N(3)-Al(1)-C(12)} to 121.47(6)° {N(3)-Al(1)-C(30)}) made up of three C atoms, from two terminal iBu ligands (with the shortest Al-C bonds {Al(1)-C(40) 2.0319(17) A˚, Al(1)-C(30) 2.0340(16) A˚}) and one deprotonated TMEDA molecule (with the longest Al-C bonds {Al(1)-C(12) 2.0658(17) A˚}), as previously reported for 2, with the fourth

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Conway et al.

Scheme 3. Proposed Stepwise Reaction Pathway for the Formation of 2

atom the N of the amido TMP (Al(1)-N(3) 1.9869(13) A˚), with a bond length in the same range as the Al-N distances found in other structures of general formula (donor 3 Li(TMP)Al(iBu)3).1b,6 The wide Al(1)-C(12)-N(1) bond angle at 116.86(10)° is marginally less than those in 2 (Al(1)C(1)-N(1), 118.28(13)°; Al(1)-C(7)-N(3), 120.06(13)°) and these fall in a similar range to those reported in the closely related terminally dialuminated TMEDA complexes R2AlCH2N(Me)CH2CH2N(Me)CH2AlR2 (R = Me, 113.9(1)°; R = tBu, 116.3(2)/116.2(2)°), which in contrast to 2 and 4 were prepared by a conventional metathesis approach from the corresponding dilithium complex.11e Note that an even wider Al-C(H)2-N bond angle (121.5(2)°) is found in (tBu2AlCH2NiPr2 3 LiCl)2.11f Contact from aluminum to the lithium atom in 4 is through one amido (TMP) nitrogen atom (Li(1)-N(3) 2.035(3) A˚) and one nitrogen of the metalated junction of TMEDA (Li(1)-N(1) 2.000(3) A˚). The remaining nitrogen atom of TMEDA also binds to Li, displaying the longest Li-N bond distance (Li(1)-N(2) 2.086(3) A˚) to complete a distorted trigonal coordination sphere of N atoms (subtending bond angles from 88.58(12)° {N(1)-Li(1)-N(2)} to 151.46(15)° {N(2)-Li(1)-N(3)}). After the successful isolation of the monometalated TMEDA compound 4, which established that the lithium dialkyldiamidoaluminate iBu2Al(TMP)2Li acts as only a single-step amido base, we investigated whether the remaining amido bridging ligand or the terminal iBu groups were able to bring about a metalation of a second TMEDA molecule as seen in compound 2. Thus, we repeated the aforementioned synthesis of 4 but adding an extra molar equivalent of TMEDA. However, this led only to recovery of [Li{Me2NCH2CH2N(Me)CH2}(TMP)Al(iBu)2] (4) as a crystalline solid with the excess of unmetalated TMEDA and free TMP(H) observed in the filtrate, even when more drastic reaction conditions were employed.22 This observation apparently rules out 4 as an intermediate in the formation of 2 and suggests that the presence of an additional bridging TMP ligand in 4 inhibits the metalation of a second molar equivalent of TMEDA. A reaction sequence can be proposed (see Scheme 3) to rationalize the formation of 2. While the structure of the starting complex is as yet not known, the sodium congener has been found to adopt this structure and it is likely given (22) Long reaction times at room temperature (48 h) or refluxing conditions (hexane or toluene) only yield 4 as the major product.

the smaller size of lithium, that in the lithium case TMEDA will be in closer proximity to the TMP bridge to facilitate a subsequent intramolecular Me deprotonation. In the first step, TMEDA is intramolecularly R-aluminated by activated TMP, with concomitant elimination of TMPH to yield the intermediate A (not isolated or detected), which is more reactive than the initial lithium TMP-aluminate “iBu3Al(TMP)Li” as 2 was still obtained even when a deficiency of TMEDA was employed. This putative intermediate can react now in two different ways: (i) With an additional equivalent of TMEDA (pathway a), where the chelating diamine could coordinate to lithium, because the Li 3 3 3 iBu contact in A can be expected to be relatively weak,23 allowing the formation of intermediate24 B (not isolated or detected). In a final step, one of the neighboring iBu groups attached to Al intramoleculary deprotonates one NMe2 arm of the TMEDA, to close a six-member {LiNCAlCN} ring, affording compound 2 and releasing iBuH. (ii) Alternatively, A could react with the concomitant TMP(H) formed during the metalation of the first equivalent of TMEDA (pathway b), affording compound 4. In this compound, the TMP ligand forms a strong bridge between Li and Al, which is retained in solution as evidenced by the inequivalence of the four methyl groups of the TMP ligand in the 1H and 13C{1H} spectra (see Experimental Section). As above-mentioned, our reactivity studies show that 4 (prepared by reacting iBu2Al(TMP)2Li with 1 equiv of TMEDA) fails to metalate TMEDA. This can now be rationalized in terms of the coordination sphere of Li, which forms three strong Li-N bonds (Li-N bond distances: 2.000(3), 2.086(3), and 2. 0340(16) A˚), making the alkali-metal coordinatively and electronically saturated and leaving no place for the coordination of an additional molecule of TMEDA. Because this precoordination seems to be crucial for the deprotonation to take place, the formation of 2 must occur through the alternative reaction pathway a. The stronger Lewis basicity of TMEDA versus that of

(23) NMR studies on the related [(THF) 3 Li(TMP)Al(iBu3)] showed that in solution the three isobutyl groups appear equivalent, suggesting that these Li 3 3 3 C bonds are forming/breaking fast in the NMR time scale, see ref 1b. (24) A similar open contacted ion-pair motif with a tetracoordinated lithium bridged to aluminium by an ortho-metalated benzamide in compound [(THF)3 3 Li{O(dC)N(iPr)2(C6H4)}Al(iBu)3] has been previously reported by us in ref 5.

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Scheme 4. Lithium-Mediated Alumination of One Molecule of PMDETA by a Dialkyldiamidoaluminate Base

Figure 4. Structure of [Li{Me2NCH2CH2N(Me)CH2CH2N(Me)CH2}(TMP)Al(iBu)2] (5).

bulky TMP(H) and also its bidentate nature may be important factors in favoring this reaction pathway. After the success in the R-metalation of TMEDA by means of lithium(TMP)aluminates, we decided to extend this chemistry to the tridentate ligand PMDETA, which is commonly used as a coordinating additive to break up organolithium aggregates and thus to aid solubility and reaction kinetics.25 As previously mentioned, PMDETA is lithiated at a terminal or central CH3 group depending on the concentration of the alkyllithium compound.19,26 Thus, attempting to achieve Ralumination of PMDETA, the same reaction methodology used in the synthesis of 2 was employed but substituting TMEDA by PMDETA. However, this procedure gave only a white oily product. In this case, a 1H NMR spectrum indicates that the oil appeared to contain metalated PMDETA but accompanied with a complicated mixture of products. As no useful structural information could be gleaned from this oil, we decided to utilize the lithium dialkyldiamidoaluminate iBu2Al(TMP)2Li in anticipation of a directed Ralumination of PMDETA (see Scheme 4). This was indeed realized as the solid product of the reaction was the heterobimetallic-heterotrianionic complex [Li{Me2NCH2CH2N(Me)CH2CH2N(Me)CH2}(TMP)Al(iBu)2] (5) obtained in a crystalline yield of 47%. 1 H, 13C, and 7Li NMR spectra recorded in cyclohexaned12 solution were obtained for 5 with only one set of resonances, supporting the diastereoselective formation of this compound (note that two stereogenic centers are generated, namely the two N atoms of the metalated junction of PMDETA). The 1H NMR spectrum clearly shows the expected pattern for methyl-aluminated PMDETA, similar to that for TMEDA previously described for 2 and 4 (NCH2Al 1.07 and 1.82 (d, AB spin system, JHA, HB = 13.4 Hz, ppm; NCH2 1.98, 2.12, and 2.95 (1H, m), 2.42 (3H, m), and 2.73 (2H, m) ppm; N(CH3)2 2.22 (6H, s), 2.34, and 2.37 (3H each, s) ppm). This 1H NMR spectrum also shows the presence of two iBu groups and one TMP ligand in 5 (see Supporting Information). Thus, again, the lithium dialkyldiamidoaluminate iBu2Al(TMP)2Li acts as only a single-step amido base toward PMDETA. The reaction filtrate that remained after isolating the crystalline product 5 was also checked by NMR spectroscopy. A 1H NMR spectrum indicates that this reaction is almost quantitative as the oily (25) See for example: (a) Sch€ umann, U.; Kopf, J.; Weiss, E. Angew. Chem. 1985, 97, 222. Angew. Chem., Int. Ed. Engl. 1985, 24, 215. (b) Engelhardt, L. M.; Leung, W.-P.; Raston, C. L.; Salem, G.; Twiss, P.; White, A. W. J. Chem. Soc., Dalton Trans. 1988, 2403. (c) Karsch, H. H.; Zellner, K.; Mikulcik, P.; Lachmann, J.; M€uller, G. Organometallics 1990, 9, 190. (d) Lappert, M. F.; Engelhardt, L. M.; Raston, C. L.; White, A. H. J. Chem. Soc., Chem. Commun. 1982, 1323. (26) Polar metalation of PMDETA has been described in: (a) Ellerman, J.; Sch€ utz, M.; Heinemann, F. W.; Moll, M.; Bauer, W. Chem. Ber. 1997, 130, 141. (b) Izod, K.; Steward, J. C.; Clegg, W.; Harrington, R. H. Dalton Trans. 2007, 257.

filtrate appears to contain only the aforementioned resonances for 5 and free TMP(H). Again, we decided to check if the remaining amido bridging ligand or the terminal iBu groups were able to bring about a second metalation of another methyl group of PMDETA. Therefore, we repeated the aforementioned synthesis of 5 but employed more drastic reaction conditions (refluxing the mixture in toluene solution for 2 h). However, even by using 2 equiv of PMDETA, we always recovered only [Li{Me2NCH2CH2N(Me)CH2CH2N(Me)CH2}(TMP)Al(iBu)2] (5) as a crystalline material. It should be mentioned that, as aforementioned for 4, the organoaluminium amide iBu2Al(TMP) (3) is not a powerful enough base to metalate PMDETA, so the deprotonation observed in this reaction is also synergic in origin, as the contacted Li appears to activate one of the Alattached TMP ligands. A low resolution crystal structure of 5 confirmed its previously discussed components and their connectivity, featuring an undeca LiNCCNCCNCAlN tricyclic ring, containing two five-atom, three-element LiNCCN rings and a five-atom, four-element LiNCAlN ring, with a mixed NCH2-TMP bridging ligand set completed by two terminal iBu ligands on Al and a chelating metalated PMDETA (N,N,N-attached) to Li, in agreement with findings from the aforementioned NMR spectroscopic studies (Figure 4). In conclusion, this work establishes that lithium TMP aluminates can function as dual TMP/alkyl (for iBu3Al(TMP)Li) or as mono amido bases (for iBu2Al(TMP)2Li) exhibiting two-fold or single-fold AMMAl (alkali-metalmediated alumination), respectively, toward TMEDA or PMDETA. On their own, iBu3Al or iBu2Al(TMP) are not sufficiently strong bases to metalate TMEDA or PMDETA, so in 2, 4, and 5, the R-deprotonations of TMEDA and PMDETA are synergic based, as the intramolecular communication between Li and Al appears to activate the TMP and iBu bases. Finally, it further establishes that by using AMMAl the normal patterns of reactivity can be reversed with a TMEDA nonacidic C-H bond breaking in preference to an acidic TMP(H) N-H bond. This special behavior can be attributed to intramolecular proximity effects between the active base component (TMP or iBu) and the ligating TMEDA molecule. It remains to be established whether these bimetallic alumination traps can be utilized to deprotonate other donor ligand molecules usually resistant to conventional organometallic bases.

Experimental Section General Procedures. All reactions were carried out under a protective argon atmosphere using standard Schlenk techniques. Hexane was dried by heating to reflux over sodium benzophenone ketyl and distilled under nitrogen prior to use. n BuLi, iBu3Al, and iBu2AlCl were purchased from Aldrich Chemicals and used as received. NMR spectra were recorded

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on a Bruker DPX 400 MHz spectrometer, operating at 400.15 MHz for 1H, 150.32 MHz for 7Li, and 100.62 MHz for 13C. X-ray Crystallography. Crystal data for 2: C20H48AlLiN4, M = 378.54, monoclinic, P21/c, a = 9.2581(3), b= 15.4470(5), c= 18.2432(5) A˚, β = 99.457(2)°, V = 2573.50(14) A˚3, Z = 4, T = 223 K. Data were collected on a Nonius Kappa CCD diffractometer with Mo KR radiation (λ = 0.71073 A˚), 31196 reflections collected, 5063 were unique, Rint 0.053. Final refinement27 to convergence was on F2, R = 0.0501 (F, 3351 obsd data only) and Rw = 0.1291, F2 all data), GOF = 1.013, 293 refined parameters, residual electron density max and min 0.294 and -0.224 e A˚-3. Crystal data for 4: C23H51AlLiN3, M = 403.59, monoclinic, P21/n, a = 10.3165(4), b= 15.3891(6), c= 17.0248(7) A˚, β = 105.505(4)°, V = 2604.52(18) A˚3, Z = 4, T = 123 K. Data were collected on an Oxford Diffraction Gemini S diffractometer with Mo KR radiation (λ = 0.71073 A˚), 23211 reflections collected, 5742 were unique, Rint 0.0441, R = 0.0457 (F, 4043 obsd data only), and Rw = 0.1136, F2 all data), GOF = 0.999, 287 refined parameters, residual electron density max and min 0.337 and -0.242 e A˚-3. Synthesis of [Li{Me2NCH2CH2N(Me)CH2}2Al(iBu)2] (2). In a Schlenk tube, 4 mmol of TMEDA (0.60 mL) were added to a hexane solution of LiTMP (prepared freshly from a mixture of n BuLi (2 mmol, 1.25 mL of a 1.6 M solution in hexane) and TMPH (2 mmol, 0.34 mL)) to give a yellow solution. After the solution had been stirred for 30 min, iBu3Al (2 mmol, 2 mL of a 1 M solution in hexane) was introduced and the mixture changed from a yellow to a slightly cloudy yellow solution. This mixture was heated gently to form a yellow solution, which was allowed to cool to ambient temperature. Freezer cooling of this solution at -27 °C afforded colorless crystals of 2 (0.37 g, 48%). Note that adding only 1 mol equiv of TMEDA also produced 2, but in a smaller yield. 1H NMR (400.13 MHz, benzene-d6, 300 K): 0.43 (4H, m, CH2-Al iBu), 1.15 and 2.02 (2H each, d, AB spin system, JHA, HB = 12.9 Hz), Al-CH2N of TMEDA), 1.46 and 1.48 (6H each, d, 3JHH = 6.8 Hz, CH3-iBu), 1.53, 1.80, 2.21, and 2.53 (2H each, m, 4 CH2-TMEDA), 1.63 (12H, broad s, 4 CH3-TMEDA), 2.27 (6H, s, 2 Al-CH2N(CH3) of TMEDA), 2.45 (2H, sept, 3JHH = 6.8 Hz, CH-iBu). 13C{H} NMR (100.63 MHz, benzene-d6, 300 K): 29.87 (CH of iBu), 30.36 and 30.58 (CH3 of iBu), 31.10 (Al-CH2N TMEDA), 46.87 (Al-CH2N(CH3) TMEDA), 49.92 (CH3 TMEDA), 58.87 and 61.96 (CH2-TMEDA). Signal for AlCH2 of iBu was not observed. 7Li NMR (155.50 MHz, benzened6, 300 K, reference LiCl in D2O at 0.00 ppm): -0.13. Synthesis of iBu2Al(TMP) (3). Ten mL of hexane were added to an oven-dried Schlenk tube. Next, 1.25 mL (2 mmol) of 1.6 M solution of nBuLi were added, followed by 0.34 mL (2 mmol) of TMP(H) at room temperature. The reaction mixture was left to stir for 30 min and then 0.38 mL (2 mmol) of iBu2AlCl were then injected into the Schlenk tube, producing a white suspension almost immediately. The reaction mixture was left to stir for 45 min and was then filtered through celite and glasswool, which was then washed with a further 10 mL of hexane. The solvent was then removed in vacuo to yield a clear oil (quantitative yield by NMR). 1H NMR (400.13 MHz, cyclohexane-d12, 30 0K): 0.17 (4H, d, 3JHH = 6.8 Hz, CH2-Al iBu), 1.01 (12H, d, 3JHH = 6.8 Hz, CH3-iBu), 1.29 (12H, s, CH3 of TMP), 1.31 (4H, m, β-TMP), 1.72 (2H, m, γ-TMP), 1.94 (1H, sept, 3JHH = 6.8 Hz, CH-iBu). 13C{H} NMR (100.63 MHz, cyclohexane-d12, 300 K): 18.85 (γ-TMP), 25.81 (CH2 of iBu), 27.68 (CH3 of iBu), 28.44 (CH of iBu), 32.45 (CH3-TMP), 39.08 (β-TMP), 50.87 (R-TMP). (27) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112.

Conway et al. Synthesis of [Li{Me2NCH2CH2N(Me)CH2}(TMP)Al(iBu)2] (4). iBu2Al(TMP) was synthesized as above and added via canula to a freshly prepared solution of LiTMP in 10 mL of hexane (from a mixture of nBuLi (2 mmol, 1.25 mL of a 1.6 M solution in hexane) and TMPH (2 mmol, 0.34 mL) to give an homogeneous solution. Finally, 0.30 mL (2 mmol) of TMEDA were injected and the reaction mixture left to stir for 30 min before the Schlenk tube was placed in the freezer overnight (-28 °C). A crop (0.44 g, 55%) of colorless crystals formed in solution that were suitable for X-ray crystallographic analysis. 1 H NMR (400.13 MHz, cyclohexane-d12, 300 K): 0.17 (4H, m, CH2-Al iBu), 0.76 and 0.89 (1H each, m, β-TMP), 0.90, 0.94, 0.99, and 1.00 (3H each, d, 3JHH = 6.4 Hz, CH3-iBu), 1.31 (6H, s, 2 CH3 of TMP), 1.34 and 1.44 (3H each, s, CH3 of TMP), 1.39 and 1.58 (1H each, d, AB spin system, JHA, HB = 14.0 Hz, Al-CH2N of TMEDA), 1.69 (2H, m, β-TMP), 1.80 (1H, sept, 3 JHH = 6.4 Hz, CH-iBu), 1.96 (5H, m, 2H CH2-TMEDA, 2H γTMP, and 1H CH-iBu), 2.24, 2.73, and 2.74 (3H each, s, CH3TMEDA), 2.73 and 2.94 (1H each, m, CH2-TMEDA). 13C{H} NMR (100.63 MHz, cyclohexane-d12, 300 K): 18.10 (γ-TMP), 26.08 (CH of iBu), 27.06, 27.16, 27.45, and 27.67 (CH3 of iBu), 28.68, 28.80, and 29.59 (CH3-TMP), 35.39 and 36.34 (β-TMP), 45.34, 46.43, and 48.07 (CH3 TMEDA), 47.39 (Al-CH2N TMEDA), 51.36 and 52.30 (R-TMP), 56.36 and 59.9 (CH2TMEDA). Signal for Al-CH2 of iBu was not observed. 7Li NMR (155.50 MHz, cyclohexane-d12, 300 K, reference LiCl in D2O at 0.00 ppm): 1.20. Synthesis of [Li{Me2NCH2CH2N(Me)CH2CH2N(Me)CH2}(TMP)Al(iBu)2] (5). Complex 5, isolated as colorless crystals (0.43 g, 47%), was prepared as previously described for 4 but using the N-donor ligand PMDETA (2 mmol, 0.42 mL) instead of TMEDA. 1H NMR (400.13 MHz, cyclohexane-d12, 300 K): 0.23 (4H, m, CH2-Al iBu), 0.91 and 0.95 (3H each, d, 3JHH = 6.4 Hz, CH3-iBu), 0.99 (6H, d, 3JHH = 6.4 Hz, CH3-iBu), 1.07 and 1.82 (1H each, d, AB spin system, JHA, HB = 13.4 Hz, Al-CH2N of PMDETA), 1.18 and 1.71 (2H each, m, β-TMP), 1.34 and 1.37 (6H each, s, CH3 of TMP), 1.71 (m, 2H, γ-TMP), 1.87 (1H, m, CH-iBu), 1.98 (2H, m, 1H of CH2-PMDETA and 1H of CH-iBu), 2.12 and 2.95 (1H each, m, CH2-PMDETA), 2.27 (6H, CH3-PMDETA), 2.34 and 2.37 (3H each, CH3-PMDETA), 2.42 (3H, m, CH2-PMDETA), 2.73 (2H, m, CH2-PMDETA). 13C{H} NMR (100.63 MHz, cyclohexane-d12, 300 K): 18.32 (γ-TMP), 26.08 (CH of iBu), 27.14, 27.22, 27.53, and 27.77 (CH3 of iBu), 29.19 and 29.76 (CH3-TMP), 32.58 (b, CH2 of i Bu), 44.35 (2C, β-TMP), 46.38, 48.64, and 51.96 (CH3 PMDETA), 47.28 (Al-CH2N PMDETA), 51.96 (2C, R-TMP), 54.10, 57.26, 59.00, and 61.15 (CH2-PMDETA). 7Li NMR (155.50 MHz, cyclohexane-d12, 300 K, reference LiCl in D2O at 0.00 ppm): 0.43.

Acknowledgment. We thank the EPSRC and the Royal Society (International Travel Grant to E.H. and J.G.-A. and a Wolfson merit award to R.E.M.) for sponsoring this research. In addition, E.H. thanks the Royal Society for a University Research Fellowship and J.G-A. thanks the Ministerio de Ciencia y Educacion (MEC) of Spain and the European Social Fund for the award of a “Juan de la Cierva” contract. Supporting Information Available: NMR spectra for 2, 3, 4, and 5 and CIF file giving crystallographic data for compounds 2 and 4. This material is available free of charge via the Internet at http://pubs.acs.org.