-hydrazination

Oct 20, 2010 - Jürgen Koller and Robert G. Bergman*. Department of .... Peter D. Schweizer , Hubert Wadepohl , and Lutz H. Gade. Organometallics 2014...
0 downloads 0 Views 1MB Size
Download PDF
5946

Organometallics 2010, 29, 5946–5952 DOI: 10.1021/om100735q

Highly Efficient Aluminum-Catalyzed Hydro-amination/-hydrazination of Carbodiimides J€ urgen Koller and Robert G. Bergman* Department of Chemistry, University of California, Berkeley, California 94720, United States Received July 26, 2010

The catalytic activity of commercially available [Al(NMe2)3]2 (1) and a dimethyl aluminum guanidinate complex toward the hydro-amination/-hydrazination of carbodiimides was studied. The guanidinate-supported complex 2 was prepared via salt metathesis reactions of AlMe2Cl and an in situ generated lithium guanidinate reagent. X-ray crystallographic studies revealed the influence of the guanidinate ligand on the Al metal center. Hydroamination reactions were successfully carried out at room temperature with 2 as the catalyst, while 1 proved to be ineffective under these conditions. On the contrary, both 1 and 2 were active toward the hydro-hydrazination of carbodiimides, which were run at elevated temperatures (120 °C). Consequently, the reaction temperature had a significant influence on the choice of the catalyst since the catalytically active species can be generated from various precatalysts under different conditions. The formation of guanidines and aminoguanidines showed a high functional group tolerance and typically proceeded with excellent yields at low catalyst loadings. X-ray crystallographic studies of compound 4a revealed interesting structural features of the previously unknown aminoguanidine products. The independently isolated Al aminoguanidinate complex 5 showed catalytic activity toward hydro-hydrazination chemistry and provided valuable evidence in support of the proposed reaction mechanism.

Introduction The catalytic addition of amine N-H bonds across carbodiimides, also known as guanylation, is a highly efficient and atom economical route toward a series of substituted guanidines. Besides their importance in biological and pharmaceutical chemistry,1,2 guanidines enjoy extensive presence in the literature as ancillary ligands for a wide variety of transition, f-block, and main group metals due to their electronic similarities to the widely used cyclopentadienyl (Cp) ligand.3-10 Similarly, functionalized guanidines not only are important building blocks for various natural products but have also been shown to exhibit interesting *To whom correspondence should be addressed. Tel: þ1-510-6422156. Fax: þ1-510-642-7714. E-mail: [email protected]. (1) Ekelund, S.; Nygren, P.; Larsson, R. Biochem. Pharmacol. 2001, 61, 1183–1193. (2) Katritzky, A. R.; Rogovoy, B. V. Arkivoc 2005, 49–87. (3) Barker, J.; Kilner, M. Coord. Chem. Rev. 1994, 133, 219–300. (4) Bailey, P. J.; Pace, S. Coord. Chem. Rev. 2001, 214, 91–141. (5) Brussee, E. A. C.; Meetsma, A.; Hessen, B.; Teuben, J. H. Organometallics 1998, 17, 4090–4095. (6) Coles, M. P.; Jordan, R. F. J. Am. Chem. Soc. 1997, 119, 8125– 8126. (7) Dagorne, S.; Guzei, T. A.; Coles, M. P.; Jordan, R. F. J. Am. Chem. Soc. 2000, 122, 274–289. (8) Cotton, F. A.; Ren, T. J. Am. Chem. Soc. 1992, 114, 2237–2242. (9) Berry, J. F.; Cotton, F. A.; Ibragimov, S. A.; Murillo, C. A.; Wang, X. Inorg. Chem. 2005, 44, 6129–6137. (10) Kondo, H.; Yamaguchi, Y.; Nagashima, H. J. Am. Chem. Soc. 2001, 123, 500–501. (11) Anselmi, C.; Ettorre, A.; Andreassi, M.; Centini, M.; Neri, P.; Di Stefano, A. Biochem. Pharmacol. 2002, 63, 437–453. (12) Byk, G.; Soto, J.; Mattler, C.; Frederic, M.; Scherman, D. Biotechnol. Bioeng. 1998, 61, 81–87. pubs.acs.org/Organometallics

Published on Web 10/20/2010

biological properties.11-13 Aminoguanidines in particular display dopamine β-oxidase inhibition and antihypertensive properties.14 In recent years, the guanylation chemistry involving arylamines has enjoyed increasing attention sparked by the discovery of highly active transition metal and lanthanide catalysts.15-21 Additionally, group 1 and 2 metal complexes have recently been shown to exhibit high activity toward this organic transformation, thus expanding the list of active guanylation catalysts to main group metals.22-24 Although (13) Wermann, K.; Walther, M.; Anders, E. Arkivoc 2002, 24–33. (14) Augstein, J.; Green, S. M.; Monro, A. M.; Wrigley, T. I.; Katritzky, A. R.; Tiddy, G. J. T. J. Med. Chem. 1967, 10, 391–400. (15) Zhu, X.; Du, Z.; Xu, F.; Shen, Q. J. Org. Chem. 2009, 74, 6347– 6349. (16) Du, Z.; Li, W.; Zhu, X.; Xu, F.; Shen, Q. J. Org. Chem. 2008, 73, 8966–8972. (17) Zhang, W.-X.; Nishiura, M.; Hou, Z. Chem.-Eur. J. 2007, 13, 4037–4051. (18) Montilla, F.; Pastor, A.; Galindo, A. J. Organomet. Chem. 2004, 689, 993–996. (19) Li, Q.; Wang, S.; Zhou, S.; Yang, G.; Zhu, X.; Liu, Y. J. Org. Chem. 2007, 72, 6763–6767. (20) Ong, T.-G.; Yap, G. P. A.; Richeson, D. S. J. Am. Chem. Soc. 2003, 125, 8100–8101. (21) Wu, Y.; Wang, S.; Zhang, L.; Yang, G.; Zhu, X.; Liu, C.; Yin, C.; Rong, J. Inorg. Chim. Acta 2009, 362, 2814–2819. (22) Alonso-Moreno, C.; Carrillo-Hermosilla, F.; Garces, A.; Otero, A.; L opez-Solera, I.; Rodrı´ guez, A. M.; Anti~ nolo, A. Organometallics 2010, 29, 2789–2795. (23) Lachs, J. R.; Barrett, A. G. M.; Crimmin, M. R.; Kociok-Kohn, G.; Hill, M. S.; Mahon, M. F.; Procopiou, P. A. Eur. J. Inorg. Chem. 2008, 4173–4179. (24) Ong, T.-G.; O’Brien, J. S.; Korobkov, I.; Richeson, D. S. Organometallics 2006, 25, 4728–4730. r 2010 American Chemical Society

Article

Organometallics, Vol. 29, No. 22, 2010

5947

Scheme 1. Synthesis of Compound 2

the insertion of carbodiimides into Al-C/N bonds of Al alkyl/amide species such as [Al(NMe2)3]2 (1) is known,25 examples of catalytic guanylation of amines using organometallic catalysts involving such species have remained undocumented until recently.26 Despite the similarities of primary arylamines and 1,1-disubstituted hydrazines, to the best of our knowledge, metal-catalyzed hydroamination of carbodiimides with hydrazines to form aminoguanidines has not been reported. Herein we report the highly efficient Al-catalyzed formation of substituted guanidines and aminoguanidines via hydro-amination/-hydrazination of various carbodiimides with arylamines and 1,1-disubstituted hydrazines.

Table 1. Selected Bond Distances (A˚) and Angles (deg) for Compound 2 Al1-N1 Al1-N2 C1-N1 C1-N2 C1-N3 Al1-C32 Al1-C33

1.9287(15) 1.9239(16) 1.356(2) 1.354(2) 1.371(2) 1.9688(19) 1.9726(19)

N1-Al1-N2 N1-C1-N2 N1-C1-N3 N2-C1-N3 C32-Al1-C33

69.06(6) 107.39(15) 125.75(15) 126.87(16) 114.96(9)

Results and Discussion Synthesis of 2. The guanidinate ligand precursor for compound 2 was prepared by treating N,N0 -bis(2,6-diisopropylphenyl)carbodiimide with LiNiPr2 at ambient temperature to generate the corresponding lithium guanidinate reagent as described in the literature.27-29 The intermediate Li salt was not isolated but rather treated in situ with AlMe2Cl to form 2 via salt metathesis, accompanied by elimination of LiCl upon warming the solution to room temperature and stirring overnight (Scheme 1). Analytically pure 2 was isolated as colorless crystals via filtration after recrystallization of the crude product from pentane at -35 °C. The 1H and 13C NMR spectra show upfield shifted resonances at -0.16 and -6.89 ppm, respectively, which correspond well to magnetically equivalent Al-bound methyl moieties. A single resonance in the 13C NMR spectrum at 165.91 ppm is diagnostic for the central C atom of the guanidinate ligand and further corroborates the structural assignment.16,30 The diastereotopic nature of the isopropyl substituents on the flanking aryl groups is evidenced by wellseparated resonances (two doublets at 1.29 and 1.31 ppm) in the 1 H NMR spectrum. Solid-State Structure of Compound 2. To confirm the monomeric nature of 2, single crystals were grown from a pentane solution at -35 °C and analyzed by X-ray crystallography. Crystal data and structure refinement for 2 can be found in Table S1 (Supporting Information). Selected bond lengths and angles can be found in Table 1. Unsurprisingly, (25) Chang, C. C.; Hsiung, C. S.; Su, H. L.; Srinivas, B.; Chiang, M. Y.; Lee, G. H.; Wang, Y. Organometallics 1998, 17, 1595–1601. (26) Rowley, C. N.; Ong, T.-G.; Priem, J.; Woo, T. K.; Richeson, D. S. Inorg. Chem. 2008, 47, 9660–9668. (27) Wilder, C. B.; Reitfort, L. L.; Abboud, K. A.; McElwee-White, L. Inorg. Chem. 2006, 45, 263–268. (28) Duncan, A. P.; Mullins, S. M.; Arnold, J.; Bergman, R. G. Organometallics 2001, 20, 1808–1819. (29) Wood; Yap, G. P. A.; Richeson, D. S. Inorg. Chem. 1999, 38, 5788–5794. (30) Zhou, S.; Wang, S.; Yang, G.; Li, Q.; Zhang, L.; Yao, Z.; Zhou, Z.; Song, H.-b. Organometallics 2007, 26, 3755–3761.

Figure 1. Thermal ellipsoids diagram of the molecular structure of 2. Thermal ellipsoids are drawn at 50% probability. H atoms are omitted for clarity.

the structural features are similar to other Al complexes supported by guanidinate ligands.31 Compound 2 adopts a distorted tetrahedral coordination environment as shown in the ORTEP representation of 2 (Figure 1). The Al1-N1 and Al1-N2 bonds (1.9287(15) and 1.9239(16) A˚, respectively) of the guanidinate ligand are almost identical, suggesting a high degree of electron delocalization within the guanidinate backbone, an observation that is further substantiated by virtually identical C1-N1 and C1-N2 bond distances of 1.356(2) and 1.354(2) A˚, respectively. Further electron delocalization involving the N3-based lone pair is restricted by a typical twist in the guanidinate backbone (28.4°) due to the sterically demanding isopropyl substituents on N3, as evidenced by the significantly (31) Koller, J.; Bergman, R. G. Organometallics 2010, 29, 3350–3356.

5948

Organometallics, Vol. 29, No. 22, 2010

longer C1-N3 bond (1.371(2) A˚).32 The Al-C bond distances of 1.9688(19) and 1.9726(19) A˚ for Al1-C32 and Al1-C33, respectively, reflect the symmetrical coordination of the guanidinate ligand to the Al metal center. The small bite angle of 69.06(6)° is characteristic for guanidinate ligands on relatively small metals such as Al or Li and is significantly larger compared to the corresponding angles in transition metal guanidinate complexes.33,34 The notable deviation of the C32-Al1-C33 bond angle (114.96(9)°) from the ideal 109.5° for tetrahedral coordination has been documented for similar compounds and is a further reflection of the ample space generated by the small guanidinate bite angle. The flanking aryl groups adopt a perpendicular orientation with respect to the metal-ligand plane, which is most likely due to the sterically demanding 2,6-substitution pattern. In recent reports of successful guanylation of carbodiimides with aniline derivatives mitigated by mixed Al alkyl/halide catalyst precursors, Al was established as a potent metal center to catalyze these types of organic transformations.35 The catalytic cycle was proposed to proceed via a highly active threecoordinate Al amide species, a finding that tempted us to investigate the catalytic activity of commercially available [Al(NMe2)3]2 (1) toward the hydroamination of various carbodiimides. NMR-scale reactions (in C6D6) carried out at room temperature revealed 1 to be inactive toward catalytic conversion of N,N0 -diisopropylcarbodiimide and p-toluidine to the corresponding guanidine. Raising the temperature above 75 °C, however, resulted in quantitative conversion to 1,3-diisopropyl2-p-tolylguanidine within a few minutes of heating time. This finding is consistent with literature reports of the remarkable thermal stability of Al amide/alkoxide dimers, preventing dissociation of the dimer into reactive three-coordinate species (eq 1).36,37

Koller and Bergman Table 2. Catalytic Formation of Guanidines 3a-f

R

R0

product

catalyst

time (min)

yield (%)a

Pr Pr Cy i Pr Cy i Pr Cy

Me Me Me H H F F

3a 3a 3b 3c 3d 3e 3f

1 2 2 2 2 2 2

60 30 30 30 30 30 30

0 84 97 90 97 92 99

entry 1 2 3 4 5 6 7 a

i i

Isolated yields.

Figure 2. Proposed catalytic cycle for the formation of guanidines.

The use of a monomeric Al source such as compound 2 is a convenient way to avoid the need for elevated temperatures. Upon stirring N,N0 -dialkylcarbodiimides with various aniline derivatives in the presence of 2 (1 mol %), formation of the corresponding guanidines was observed within minutes at room temperature (Table 2, entries 2-7). Substitution of the aniline derivative in the para position with either electron-donating or -withdrawing groups was tolerated by the catalyst, and the reactions proceeded under equally mild conditions. Analogous to the AlMexCl3-x-catalyzed systems,35 the reaction is proposed to proceed via amine exchange to form a catalytically active three-coordinate Al species (structure I, Figure 2), which can subsequently insert carbodiimides to form Al guanidinate species similar to the precatalyst itself (structure II, Figure 2). The catalytically active species can then be regenerated by another amine (32) Barrett, A. G. M.; Crimmin, M. R.; Hill, M. S.; Hitchcock, P. B.; Procopiou, P. A. Dalton Trans. 2008, 4474–4481. (33) Aeilts, S. L.; Coles, M. P.; Swenson, D. G.; Jordan, R. F.; Young, V. G. Organometallics 1998, 17, 3265–3270. (34) Rische, D.; Baunemann, A.; Winter, M.; Fischer, R. A. Inorg. Chem. 2006, 45, 269–277. (35) Zhang, W.-X.; Li, D.; Wang, Z.; Xi, Z. Organometallics 2009, 28, 882–887. (36) Chisholm, M. H.; DiStasi, V. F.; Streib, W. E. Polyhedron 1990, 9, 253–255. (37) Brazeau, A. L.; DiLabio, G. A.; Kreisel, K. A.; Monillas, W.; Yap, G. P. A.; Barry, S. T. Dalton Trans. 2007, 3297–3304.

exchange reaction with free aniline. The absence of diagnostic methane signals in the 1H NMR spectra (at catalyst loadings up to 20 mol %) suggests that the methyl moieties on the metal center act as spectator ligands under the reaction conditions and are not involved in the catalytic activity. Although counterintuitive at first, this finding is supported by reports of the remarkable stability of Al-Me bonds toward protonolysis by anilines.38,39 The similarities of primary arylamines and 1,1-disubstituted hydrazines tempted us to investigate the corresponding reactivity of hydrazines with carbodiimides to form aminoguanidines. Initial NMR-scale reactions showed that the reaction of 1,1-dimethylhydrazine and N,N0 -diisopropylcarbodiimide in the presence of 2 (1 mol %) required temperatures above 100 °C to proceed at a reasonable rate. On the basis of the observations with the aniline derivatives, we attempted the same reaction in the presence of 1 (1 mol %) as the catalyst since the dissociation of 1 into its monomeric form [Al(NMe2)3] should be significant under these conditions. Unsurprisingly, conversion of the substrates to the corresponding aminoguanidine product was observed within a few hours at 120 °C. Control experiments revealed that catalysis is not influenced by the presence of a noncoordinating base such as 1,2,2,6,6-pentamethylpiperidine, thus ruling out the possibility of acid catalysis (Table 3, entry 1). Additionally, no product formation was observed upon (38) Waggoner, K. M.; Power, P. P. J. Am. Chem. Soc. 1991, 113, 3385–3393. (39) Timoshkin, A. Y. Coord. Chem. Rev. 2005, 249, 2094–2131.

Article

Organometallics, Vol. 29, No. 22, 2010

5949

Table 3. Catalytic Formation of Aminoguanidines 4a-g

R

NR0 2

product

yield (%)a

Pr Pr i Pr Cy Cy 2,6-iPr-C6H3 2,6-iPr-C6H3

NMe2 pip NPh2 NMe2 pip NMe2 pip

4a 4b 4c 4d 4e 4f 4g

88 (86)b (89)c 93 90 99 98 84 95

entry 1 2 3 4 5 6 7

i i

a Isolated yields. b Reaction run in the presence of 2 mol % of 1,2,2,6,6-pentamethylpiperidine. c Reaction run with 1 mol % of 5 as the catalyst.

heating the substrates at 135 °C for 4 h in the absence of any catalyst. Encouraged by the NMR-scale experiments, we attempted the synthesis of various aminoguanidines on a preparative scale. The reactions proceeded with generally excellent yields (Table 3, entries 1-7), and the products were isolated on a gram scale as colorless crystalline solids. Both alkyl- and aryl-substituted carbodiimides were tolerated by the catalyst, and even the sterically demanding 2,6-diisopropylphenyl substituents did not have a significant impact on product formation. Furthermore, various substituents (alkyl, cyclic alkyl, aryl) on the hydrazine moiety were incorporated successfully into the final products without any significant change in the overall excellent reaction yields. To establish the structural conformation of the aminoguanidine products, single crystals of compound 4a were grown from a concentrated pentane solution at -20 °C and analyzed by X-ray crystallography. Crystal data and structure refinement for 4a can be found in Table S6 (Supporting Information). Compound 4a shows interesting structural features, as shown in the ORTEP representation of 4a (Figure 3). Selected bond lengths and angles can be found in Table 4. The trigonal coordination around C1 (sum of bond angles 359.9°) is consistent with sp2 hybridization. The short C1-N3 bond distance of 1.3098(14) A˚ is indicative of a C-N double bond and is remarkably similar to structures obtained from the guanidine counterparts.30 Similar C1-N1 and C1-N2 bond lengths (1.3807(14) and 1.3616(14) A˚, respectively) further corroborate this finding. The unusually long N3-N4 bond (1.4640(12) A˚) is indicative of a N-N single bond with very little electron delocalization between the N atoms, a phenomenon that is typically observed in hydrazines.40,41 Average bond angles of 109.0° around N4 are in good agreement with a tetrahedral arrangement around N4, further limiting the ability of electron delocalization through population of a formally sp3-hybridized orbital. Moreover, this lone pair on N4 is further immobilized by a hydrogen-bonding interaction with H1 (bond length of 2.071(14) A˚). Additionally, a weak hydrogen bond between H2 and N3 of a neighboring molecule (bond length 2.522(15) A˚) is present in the solid-state structure. (40) Schlegel, H. B.; Skancke, A. J. Am. Chem. Soc. 1993, 115, 7465– 7471. (41) Koller, J.; Ajmera, H. M.; Abboud, K. A.; Anderson, T. J.; McElwee-White, L. Inorg. Chem. 2008, 47, 4457–4462.

Figure 3. Thermal ellipsoids diagram of the molecular structure of 4a. Thermal ellipsoids are drawn at 50% probability. H atoms (except H1 and H2) are omitted for clarity. Table 4. Selected Bond Distances (A˚) and Angles (deg) for Compound 4a C1-N1 C1-N2 C1-N3 N3-N4 N1-H1 N2-H2

1.3807(14) 1.3616(14) 1.3098(14) 1.4640(12) 0.877(15) 0.826(14)

N1-C1-N2 N1-C1-N3 N2-C1-N3 C1-N3-N4 N3-N4-C8 N3-N4-C9

116.80(10) 123.47(10) 119.62(10) 110.22(8) 108.32(9) 107.69(9)

The aforementioned hydrogen-bonding interaction between H1 and N4 of compound 4a seems to be sustained in solution (on the NMR time scale at room temperature), as evidenced by chemically inequivalent isopropyl groups on N1 and N2 as well as a significantly downfield shifted resonance corresponding to H1 (5.84 ppm) compared to the resonance assigned to H2 (2.78 ppm). Remarkably similar 1H NMR patterns can be observed for all other aminoguanidine products, indicating that this bonding scheme is common for this class of molecules. This H-bonding interaction is also manifested in the infrared spectra of compounds 4a-g, where relatively sharp peaks corresponding to the H-bonded N-H functionalities (shifted to lower wavenumbers compared to typical secondary amine stretches) are apparent. These experimental findings are in accordance with a recent computational study by Bharatam et al., where this structural arrangement has been predicted to be the most stable conformation for aminoguanidines.42 In analogy with the guanidine chemistry, the catalytic cycle is proposed to proceed via a formal amine exchange reaction resulting in the generation of the catalytically active Al species (structure I, Figure 4). The κ2-N,N0 (“side-on”) coordination ability of the hydrazide ligand likely stabilizes the proposed intermediate and thus lowers its reactivity toward carbodiimide insertion. Similar coordination complexes involving hydrazide ligands coordinated to Al have been reported in the literature and may explain the need for substantially higher temperatures to achieve catalytic turnover (compared to the guanylation reactions).43 Subsequent activation of the carbodiimide by the Al hydrazide intermediate (structure II, Figure 3) followed by protonation with (42) Bharatam, P. V.; Iqbal, P.; Malde, A.; Tiwari, R. J. Phys. Chem. A 2004, 108, 10509–10517. (43) Uhl, W.; Molter, J.; Koch, R. Eur. J. Inorg. Chem. 2000, 2255– 2262.

5950

Organometallics, Vol. 29, No. 22, 2010

Koller and Bergman

The commercially available [Al(NMe2)3]2 (1) dimer is active in transformations that occur at temperatures above 75 °C, whereas monomeric species such as the guanidinate-supported dimethyl Al complex 2 show increased catalytic activity at low reaction temperatures. The presented synthetic protocols allow convenient access to various guanidine and previously unknown aminoguanidine species at low catalyst loadings with the added benefit of facile product isolation procedures. X-ray crystallographic analysis of [(iPr)NHC(NNMe2)NH(iPr)] (4a) provides valuable insight into the interesting bonding behavior of aminoguanidines and confirms computational predictions involving such species. The independently prepared Al aminoguanidinate complex 5 proved to be catalytically active and thus corroborates the proposed reaction mechanism.

Experimental Section

Figure 4. Proposed catalytic cycle for the formation of aminoguanidines. Scheme 2. Synthesis of Compound 5

free hydrazine results in formation of the aminoguanidine product while simultaneously regenerating the active species. To provide further evidence for the proposed reaction mechanism, we attempted the independent synthesis of an Al aminoguanidinate complex akin to structure II (Figure 4). In fact, a simple amine exchange reaction of 1 with two equivalents of aminoguanidine 4a at 100 °C provided access to compound 5 (Scheme 2), which was isolated as a pale yellow oil. The 1H NMR spectrum of 5 shows resonances that are remarkably similar to those of 4a in addition to resonances corresponding to magnetically equivalent dimethylamide functionalities on the Al metal center. As expected, the hydrogen-bonded N-H moiety of 4a is activated preferentially during the amine exchange reaction, as evidenced by the disappearance of the N-H resonance of the free ligand at 5.84 ppm. The emergence of the remaining N-H proton as a doublet, in combination with the apparent doublet of septets assigned to the methine proton of an isopropyl substituent, corroborates the structural assignment. Subjecting compound 5 to catalytic conditions revealed its activity toward hydro-hydrazination chemistry with isolated reaction yields similar to those of reactions catalyzed by 1 (Table 3, entry 1). These findings are consistent with the proposed reaction mechanism and indicate that Al aminoguanidinate complexes such as 5 may in fact be intermediates during the conversion of 1,1-dialkylhydrazines and carbodiimides to aminoguanidines.

Conclusions We have established aluminum as a highly active metal toward the hydro-amination/-hydrazination of carbodiimides.

General Procedures. Unless otherwise noted, all reactions and manipulations were performed in an inert atmosphere (N2) glovebox or using standard Schlenk and high-vacuum line techniques. Glassware was dried overnight at 150 °C before use. 1 H and 13C NMR spectra were recorded at room temperature (except where noted) on Bruker AVB 400 MHz, AVQ 400 MHz, or DRX 500 MHz spectrometers using residual protons of deuterated solvents for reference. Elemental analyses were performed at the University of California, Berkeley, Microanalytical Facility, on a Perkin-Elmer 2400 Series II CHNO/S analyzer. IR spectra were measured neat on a Nicolet iS10 FT-IR spectrometer with a diamond attenuated total reflective (ATR) accessory. Peak intensities are reported as broad (b), weak (w), medium (m), or strong (s). Only peaks in the functional group region (4000-1300 cm-1) are reported. Highresolution mass spectral data were obtained at the QB3/Chemistry Mass Spectrometry Facility operated by the College of Chemistry, University of California, Berkeley. Electrospray injection (ESI) mass spectra were recorded on a LTQ Orbitrap (Thermo) instrument. Low-resolution mass spectral data were obtained on a 6890N gas chromatograph system coupled to a 5973N mass selective detector (Agilent Technologies). Sealed NMR tubes were prepared by attaching the NMR tube directly to a Kontes highvacuum stopcock via a Cajon Ultra-Torr reducing union followed by flame-sealing on a vacuum line. Materials. Unless otherwise noted, reagents were purchased from commercial suppliers and used without further purification. Pentane, hexane, diethyl ether, and toluene were purified by passing the degassed solvents through a column of activated alumina (type A2, size 12-32, Purifry Co.) under nitrogen pressure followed by additional sparging with N2 prior to use. Deuterated solvents (Cambridge Isotope Laboratories) and 1,1-diphenylhydrazine were degassed by three freeze-pump-thaw cycles and stored over activated 3 A˚ molecular sieves. [Al(NMe2)3]2 was purchased from Aldrich and sublimed prior to use. Aniline, 4-fluoroaniline, 1,1-dimethylhydrazine, N-aminopiperidine, and N,N0 -diisopropylcarbodiimide were distilled from CaH2 and stored over activated 3 A˚ molecular sieves. Crystallographic Analysis. X-ray structural determinations were performed at CHEXRAY, University of California, Berkeley. Single crystals of 2 and 4a were coated in Paratone-N oil, mounted on a Kaptan loop, transferred to a Bruker APEX-I CCD area detector instrument, centered in the beam, and cooled by a nitrogen flow low-temperature apparatus that was previously calibrated by a thermocouple placed at the same position as the crystal. Preliminary orientation matrices and cell constants were determined by collection of 60 10 s frames, followed by spot integration and least-squares refinement. An arbitrary hemisphere of data was collected, and the raw data were integrated using SAINT. Cell dimensions reported were calculated from all reflections with I > 10σ. The data were corrected for Lorentz and polarization effects, but no correction for crystal decay was

Article applied. Data were analyzed for agreement and possible absorption using XPREP. An empirical absorption correction based on S3 comparison of redundant and equivalent reflections was applied using SADABS.44 The structures were solved using SHELXS and refined on all data by full-matrix least-squares with SHELXL-97.45 Thermal parameters for all non-hydrogen atoms were refined anisotropically. Hydrogen atoms were calculated in ideal positions and were riding on their respective carbon atoms. Nitrogen-bound hydrogen atoms were located on the electron density map and fully refined. For compound 2, a total of 358 parameters were refined in the final cycle of refinement using 4564 reflections with I > 2σ(I) to yield R1 and wR2 of 4.65% and 11.29%, respectively. For compound 4a, a total of 136 parameters were refined in the final cycle of refinement using 2042 reflections with I > 2σ(I) to yield R1 and wR2 of 3.99% and 10.02%, respectively. Refinement was done using F2. ORTEP diagrams were created using the ORTEP-3 software package and rendered using Pov-Ray 3.6. [(2,6-iPr2C6H3)NC(N(iPr)2)N(2,6-iPr2C6H3)]AlMe2 (2). To a stirred solution of LiN(iPr)2 (241 mg, 2.25 mmol) in Et2O (10 mL) was added a solution of N,N0 -bis(2,6-diisopropylphenyl)carbodiimide (816 mg, 2.25 mmol) in Et2O (10 mL). The resulting mixture was stirred at ambient temperature for 60 min. The pale yellow solution was subsequently cooled to 0 °C, and AlClMe2 (2.50 mL, 0.9 M solution in heptane, 2.25 mmol) was added dropwise. The resulting mixture was allowed to warm to room temperature and stirred for another 18 h. All volatiles were removed in vacuo, and the product was extracted with pentane (2  5 mL). The combined extracts were filtered and cooled to -35 °C to yield pure 2 as colorless crystals, which were isolated by filtration. Yield: 518 mg (40%, 1.00 mmol). 1H NMR (benzened6, 500 MHz): δ 7.09 (bs, 6H, Ph), 3.98 (sept, 2H, NCH(CH3)2), 3.73 (sept, 4H, CH(CH3)2), 1.31 (d, 12H, J = 6.9 Hz, CH(CH3)2), 1.29 (d, 12H, J = 6.7 Hz, CH(CH3)2), 0.80 (d, 12H, J = 7.0 Hz, NCH(CH3)2), -0.16 (s, 6H, AlCH3). 13C NMR (benzene-d6, 125 MHz): δ 165.91 (s, N3C), 145.65 (s, Ph), 139.73 (s, Ph), 126.14 (s, Ph), 124.55 (s, Ph), 50.19 (s, NCH(CH3)2), 28.70 (s, CH(CH3)2), 27.62 (s, CH(CH3)2), 24.13 (s, CH(CH3)2), 23.97 (s, NCH(CH3)2), -6.89 (s, AlCH3). Anal. Calcd for C33H54N3Al: C, 76.25; H, 10.47; N, 8.08. Found: C, 75.88; H, 10.52; N, 8.15. General Procedure for Formation of Guanidines. A 20 mL vial equipped with a Teflon stir bar was charged with the respective carbodiimide (1 mmol), aniline (1 mmol), and 4 mL of hexane. A solution of 2 (0.01 mmol) in hexane (1 mL) was added via pipet, and the solution was stirred at ambient temperature for 30 min, during which time the product precipitated as a white solid. The vial was removed from the glovebox, and the product was collected via filtration, washed with 5 mL of cold (-20 °C) pentane, and subsequently dried in vacuo. Spectroscopic data are in good agreement with literature data for known compounds 3a-f.16,19,46 1,3-Diisopropyl-2-p-tolylguanidine (3a): 19 colorless solid. Yield: 196 mg (84%, 0.840 mmol). 1H NMR (benzene-d6, 400 MHz): δ 7.08 (s, 4H, Ph), 3.67 (bs, 2H, NCH(CH3)2), 3.45 (bs, 2H, NH), 2.18 (s, 3H, CH3), 0.91 (d, 12H, J = 5.8 Hz, NCH(CH3)2). 13C NMR (benzene-d6, 101 MHz): δ 150.09 (s, N3C), 149.38 (s, Ph), 130.68 (s, Ph), 130.53 (s, Ph), 123.95 (s, Ph), 43.59 (s, NCH(CH3)2), 23.62 (s, NCH(CH3)2), 21.23 (s, CH3). MS (EI, þve): m/z (%) 233 (64) [M]þ. 1,3-Dicyclohexyl-2-p-tolylguanidine (3b): 19 colorless solid. Yield: 303 mg (97%, 0.967 mmol). 1H NMR (benzene-d6, 500 MHz): δ 7.14-7.09 (m, 4H, Ph), 3.63 (bs, 2H, NH), 3.50 (bs, 2H, NCH(CH3)2), 2.19 (s, 3H, CH3), 1.94 (m, 4H, Cy), 1.50 (m, 4H, Cy), 1.38 (m, 2H, Cy), 1.14-1.10 (m, 4H, Cy), 0.97-0.85 (m, 6H, Cy). 13 C NMR (benzene-d6, 125 MHz): δ 149.95 (s, N3C), 149.50 (44) SADABS; Bruker-AXS: Madison, WI, 2001. (45) SHELXTL6; Bruker-AXS: Madison, WI, 2000. (46) Shen, H.; Chan, H.-S.; Xie, Z. Organometallics 2006, 25, 5515– 5517.

Organometallics, Vol. 29, No. 22, 2010

5951

(s, Ph), 130.68 (s, Ph), 130.53 (s, Ph), 124.06 (s, Ph), 50.77 (s, NCH), 34.35 (s, Cy), 26.30 (s, Cy), 25.63 (s, Cy), 21.24 (s, CH3). MS (EI, þve): m/z (%) 313 (29) [M]þ. 1,3-Diisopropyl-2-phenylguanidine (3c): 46 colorless solid. Yield: 197 mg (90%, 0.898 mmol). 1H NMR (benzene-d6, 400 MHz): δ 7.29-7.25 (m, 2H, Ph), 7.14 (m, 2H, Ph), 6.94-6.91 (m, 1H, Ph), 3.64 (bs, 2H, NCH(CH3)2), 3.41 (bs, 2H, NH), 0.89 (d, 12H, J = 6.0 Hz, NCH(CH3)2). 13C NMR (benzene-d6, 101 MHz): δ 152.04 (s, N3C), 149.88 (s, Ph), 130.04 (s, Ph), 124.10 (s, Ph), 121.76 (s, Ph), 43.59 (s, NCH(CH3)2), 23.57 (s, NCH(CH3)2). MS (EI, þve): m/z (%) 219 (47) [M]þ. 1,3-Dicyclohexyl-2-phenylguanidine (3d): 46 colorless solid. Yield: 290 mg (97%, 0.968 mmol). 1H NMR (benzene-d6, 400 MHz): δ 7.30-7.27 (m, 2H, Ph), 7.19 (m, 2H, Ph), 6.96-6.92 (m, 1H, Ph), 3.61 (bd, 2H, J = 6.0 Hz, NH), 3.47 (bs, 2H, NCH(CH3)2), 1.93-1.91 (m, 4H, Cy), 1.51-1.47 (m, 4H, Cy), 1.39-1.37 (m, 2H, Cy), 1.17-1.08 (m, 4H, Cy), 0.97-0.80 (m, 6H, Cy). 13C NMR (benzene-d6, 101 MHz): δ 152.17 (s, N3C), 149.79 (s, Ph), 130.04 (s, Ph), 124.22 (s, Ph), 121.75 (s, Ph), 50.77 (s, NCH), 34.29 (s, Cy), 26.26 (s, Cy), 25.61 (s, Cy). MS (EI, þve): m/z (%) 299 (35) [M]þ. 1,3-Diisopropyl-2-(4-fluorophenyl)guanidine (3e): 16 colorless solid. Yield: 219 mg (92%, 0.923 mmol). 1H NMR (benzene-d6, 400 MHz): δ 6.89 (d, 4H, J = 6.8 Hz, Ph), 3.60 (bs, 2H, NCH(CH3)2), 3.33 (bd, 2H, J = 6.8 Hz, NH), 0.88 (d, 12H, J = 5.9 Hz, NCH(CH3)2). 13C NMR (benzene-d6, 101 MHz): δ 160.10 (s, N3C), 157.73 (s, Ph), 150.23 (s, Ph), 147.94 (d, J = 2.2 Hz, Ph), 124.88 (d, J = 7.6 Hz, Ph), 116.49 (d, J = 21.8 Hz, Ph), 43.55 (s, NCH(CH3)2), 23.55 (s, NCH(CH3)2). MS (EI, þve): m/z (%) = 237 (100) [M]þ. 1,3-Dicyclohexyl-2-(4-fluorophenyl)guanidine (3f): 16 colorless solid. Yield: 315 mg (99%, 0.992 mmol). 1H NMR (benzene-d6, 400 MHz): δ 6.96-6.89 (m, 4H, Ph), 3.52 (bd, 2H, J = 6.7 Hz, NH), 3.45 (bs, 2H, NCH(CH3)2), 1.90 (m, 4H, Cy), 1.51-1.47 (m, 4H, Cy), 1.39-1.36 (m, 2H, Cy), 1.16-1.07 (m, 4H, Cy), 0.97-0.82 (m, 6H, Cy). 13C NMR (benzene-d6, 101 MHz): δ 160.12 (s, N3C), 150.11 (s, Ph), 148.06 (d, J = 3.0 Hz, Ph), 124.98 (d, J = 7.4 Hz, Ph), 116.50 (d, J = 21.8 Hz, Ph), 50.72 (s, NCH), 34.28 (s, Cy), 26.24 (s, Cy), 25.57 (s, Cy). MS (EI, þve): m/z (%) 317 (43) [M]þ. General Procedure for Formation of Aminoguanidines. A Schlenk bomb was charged with the respective carbodiimide (5 mmol), 1, 1-disubstituted hydrazine (5 mmol), 1 (0.05 mmol), and 5 mL of toluene. The clear solution was heated at 120 °C for 4 h followed by solvent removal in vacuo after cooling to ambient temperature. The resulting white solid was extracted with 2  10 mL of pentane, and the solution was subsequently filtered to remove all insolubles. The pure product was obtained as a colorless solid via crystallization from pentane at -20 °C. [(iPr)NHC(NNMe2)NH(iPr)] (4a): colorless solid. Yield: 0.82 g (88%, 4.39 mmol). 1H NMR (benzene-d6, 400 MHz): δ 5.84 (d, 1H, J = 9.2 Hz, NH), 4.18 (m, 1H, NCH(CH3)2), 2.99 (dsept, 1H, J = 10.1, 6.3 Hz, NCH(CH3)2), 2.78 (d, 1H, J = 5.7 Hz, NH), 2.51 (s, 6H, NN(CH3)2), 1.08 (d, 6H, J = 6.4 Hz, NCH(CH3)2), 0.90 (d, 6H, J = 6.3 Hz, NCH(CH3)2). 13C NMR (benzene-d6, 101 MHz): δ 157.01 (s, N3C), 48.84 (s, NN(CH3)2), 44.01 (s, NCH(CH3)2), 42.50 (s, NCH(CH3)2), 24.16 (s, NCH(CH3)2), 23.72 (s, NCH(CH3)2). IR (neat): ν~ 3310 (b), 3255 (w), 2970 (m), 2939 (w), 2853 (b), 1598, (s), 1530 (s), 1466 (w), 1443 (w), 1378 (m), 1362 (s), 1332 (w). HRMS (ESI): calcd for [C9H23N4]þ 187.1917, found 187.1915. [(iPr)NHC(Npip)NH(iPr)] (4b): colorless solid. Yield: 1.06 g (93%, 4.67 mmol). 1H NMR (benzene-d6, 400 MHz): δ 5.92 (d, 1H, J = 9.4 Hz, NH), 4.20 (m, 1H, NCH(CH3)2), 3.01 (m, 1H, NCH(CH3)2), 2.98 (m, 2H, NCH2), 2.82 (d, 1H, J = 6.1 Hz, NH), 2.67 (m, 2H, NCH2), 1.61-1.58 (m, 4H, CH2), 1.51 (m, 1H, CH2), 1.16 (m, 1H, CH2), 1.09 (d, 6H, J = 6.4 Hz, NCH(CH3)2), 0.94 (d, 6H, J = 6.3 Hz, NCH(CH3)2). 13C NMR (benzene-d6, 101 MHz): δ 156.88 (s, N3C), 58.04 (s, NCH2), 44.13 (s, NCH(CH3)2), 42.55 (s, NCH(CH3)2), 26.93 (s, CH2),

5952

Organometallics, Vol. 29, No. 22, 2010

24.81 (s, CH2), 24.28 (s, NCH(CH3)2), 23.72 (s, NCH(CH3)2). IR (neat): ν~ 3310 (b), 2961 (m), 2933 (m), 2825 (w), 1596 (s), 1540 (s), 1469 (w), 1441 (w), 1378 (m), 1360 (m). HRMS (ESI): calcd for [C12H27N4]þ 227.2230, found 227.2231. [(iPr)NHC(NNPh2)NH(iPr)] (4c): colorless solid. Yield: 1.38 g (90%, 4.45 mmol). 1H NMR (benzene-d6, 400 MHz): δ 7.44 (d, 4H, J = 8.2 Hz, Ph), 7.19 (d, 4H, J = 8.3 Hz, Ph), 6.88 (t, 2H, J = 7.2 Hz, Ph), 5.19 (d, 1H, J = 9.2 Hz, NH), 4.27 (m, 1H, NCH(CH3)2), 2.95 (d, 1H, J = 7.1 Hz, NH), 2.80 (m, 1H, NCH(CH3)2), 1.10 (d, 6H, J = 6.4 Hz, NCH(CH3)2), 0.68 (d, 6H, J = 6.3 Hz, NCH(CH3)2). 13C NMR (benzene-d6, 101 MHz): δ 159.63 (s, N3C), 150.63 (s, Ph), 149.93 (s, Ph), 144.01 (s, Ph), 129.87 (s, Ph), 129.50 (s, Ph), 129.39 (s, Ph), 122.41 (s, Ph), 122.17 (s, Ph), 121.35 (s, Ph), 120.09 (s, Ph), 118.52 (s, Ph), 44.14 (s, NCH(CH3)2), 43.05 (s, NCH(CH3)2), 23.92 (s, NCH(CH3)2), 23.84 (s, NCH(CH3)2). IR (neat): ν~ 3401 (w), 3368 (w), 2978 (w), 2965 (m), 2978 (b), 1587 (s), 1516 (m), 1485 (s), 1381 (w), 1303 (w). HRMS (ESI): calcd for [C19H27N4]þ 311.2230, found 311.2229. [(Cy)NHC(NNMe2)NH(Cy)] (4d): colorless solid. Yield: 1.32 g (99%, 4.94 mmol). 1H NMR (benzene-d6, 400 MHz): δ 6.05 (d, 1H, J = 9.7 Hz, NH), 3.90 (m, 1H, NCH(CH3)2), 2.99 (d, 1H, J = 7.1 Hz, NH), 2.86 (m, 1H, NCH(CH3)2), 2.53 (s, 6H, NN(CH3)2), 2.15-2.11 (m, 2H, Cy), 1.84-1.81 (m, 2H, Cy), 1.60-1.53 (m, 4H, Cy), 1.45-1.19 (m, 6H, Cy), 1.12-0.94 (m, 8H, Cy). 13C NMR (benzene-d6, 101 MHz): δ 156.93 (s, N3C), 51.31 (s, NCH), 49.50 (s, NCH), 48.90 (s, NN(CH3)2), 34.87 (s, Cy), 34.35 (s, Cy), 26.74 (s, Cy), 26.30 (s, Cy), 25.83 (s, Cy), 25.49 (s, Cy). IR (neat): ν~ 3329 (w), 3277 (w), 2928 (s), 2850 (m), 1596 (s), 1526 (s), 1447 (w), 1387 (w), 1346 (w). HRMS (ESI): calcd for [C15H31N4]þ 267.2543, found 267.2544. [(Cy)NHC(Npip)NH(Cy)] (4e): colorless solid. Yield: 1.50 g (98%, 4.91 mmol). 1H NMR (benzene-d6, 400 MHz): δ 6.18 (d, 1H, J = 9.7 Hz, NH), 3.93 (m, 1H, NCH), 3.01 (d, 1H, J = 7.2 Hz, NH), 2.90 (m, 3H, NCH and CH2), 2.63 (m, 2H, CH2), 2.14 (m, 2H, CH2), 1.82 (m, 2H, CH2), 1.62-1.36 (m, 11H, CH2), 1.30-0.98 (m, 11H, CH2). 13C NMR (benzene-d6, 101 MHz): δ 156.87 (s, N3C), 58.08 (s, NCH2), 51.19 (s, NCH), 49.52 (s, NCH), 34.83 (s, CH2), 34.42 (s, CH2), 27.06 (s, CH2), 26.76 (s, CH2), 26.34 (s, CH2), 25.82 (s, CH2), 25.30 (s, CH2), 24.84 (s, CH2). IR (neat): ν~ 3334 (b), 3276 (w), 2931 (s), 2851 (m), 2802 (w), 1595 (s), 1526 (s), 1446 (w), 1415 (w), 1388 (w), 1360 (w), 1346 (w). HRMS (ESI): calcd for [C18H35N4]þ 307.2856, found 307.2856. [(2,6-iPr2C6H3)NHC(NNMe2)NH(2,6-iPr2C6H3)] (4f): colorless solid. Yield: 1.78 g (84%, 4.21 mmol). 1H NMR (benzene-d6, 400 MHz): δ 7.29-7.20 (m, 5H, Ph), 7.13-7.10 (m, 1H, Ph), 6.94 (s, 1H, NH), 4.30 (s, 1H, NH), 3.66 (sept, 2H, CH(CH3)2), 3.40 (sept, 2H, CH(CH3)2), 1.90 (s, 6H, NN(CH3)2), 1.36 (bd, 18H, J = 6.9 Hz, CH(CH3)2), 1.17 (d, 6H, J = 6.9 Hz, CH(CH3)2). 13C NMR (benzene-d6, 101 MHz): δ 147.61 (s, N3C), 146.76 (s, Ph), 144.03 (s, Ph), 141.06 (s, Ph), 134.40 (s, Ph), 123.53 (s, Ph), 123.37

Koller and Bergman (s, Ph), 122.92 (s, Ph), 47.76 (s, NN(CH3)2), 29.04 (s, CH(CH3)2), 28.13 (s, CH(CH3)2), 24.47 (s, CH(CH3)2), 24.20 (s, CH(CH3)2). IR (neat): ν~ 3367 (b), 3308 (w), 2957 (m), 2929 (w), 2866 (w), 1649 (s), 1586 (w), 1491 (s), 1435 (m), 1381 (w), 1360 (w), 1329 (w). HRMS (ESI): calcd for [C27H43N4]þ 423.3482, found 423.3474. [(2,6-iPr2C6H3)NC(Npip)N(2,6-iPr2C6H3)] (4g). Due to solubility issues, compound 4g was extracted with diethyl ether (2  10 mL). The pure product was crystallized from diethyl ether as a colorless solid. Yield: 2.20 g (95%, 4.76 mmol). 1H NMR (benzene-d6, 400 MHz): δ 7.29-7.21 (m, 5H, Ph), 7.14-7.12 (m, 1H, Ph), 7.09 (m, 1H, NH), 4.47 (s, 1H, NH), 3.70 (sept, 2H, CH(CH3)2), 3.44 (sept, 2H, CH(CH3)2), 2.85 (m, 2H, NCH2), 1.48 (m, 2H, CH2), 1.39 (d, 12H, J = 7.0 Hz, CH(CH3)2), 1.36 (m, 6H, CH2), 1.18 (d, 12H, J = 6.9 Hz, CH(CH3)2). 13C NMR (benzene-d6, 101 MHz): δ 147.49 (s, N3C), 146.82 (s, Ph), 144.11 (s, Ph), 141.15 (s, Ph), 134.64 (s, Ph), 123.55 (s, Ph), 123.38 (s, Ph), 122.91 (s, Ph), 57.64 (s, NCH2), 29.14 (s, CH(CH3)2), 28.19 (s, CH(CH3)2), 26.28 (s, CH2), 24.54 (s, CH(CH3)2), 24.22 (s, CH(CH3)2), 23.02 (s, CH2). IR (neat): ν~ 3390 (b), 2955 (b), 2863 (w), 1644 (s), 1587 (m), 1490 (s), 1464 (m), 1437 (m), 1380 (w), 1358 (w), 1330 (w). HRMS (ESI): calcd for [C30H47N4]þ 463.3795, found 463.3782. [(iPr)NC(NNMe2)NH(iPr)]Al(NMe2)2 (5). A Schlenk bomb was charged with a solution of 1 (500 mg, 1.57 mmol) and 4a (585 mg, 3.14 mmol) in 5 mL of toluene. The clear solution was heated at 100 °C for 60 min. After cooling to ambient temperature, 2 mL of pentane was added, and all insolubles were removed by filtration. Removal of all volatiles yielded analytically pure 5 as a pale yellow oil. Yield: 820 mg (87%, 2.73 mmol). 1 H NMR (benzene-d6, 400 MHz): δ 4.09 (dsept, 1H, NCH(CH3)2), 3.10 (bd, 1H, J = 7.4 Hz, NH), 3.04 (sept, 1H, NCH(CH3)2), 2.78 (s, 12H, AlN(CH3)2), 2.32 (s, 6H, NN(CH3)2), 1.17 (d, 6H, J = 6.3 Hz, NCH(CH3)2), 1.04 (d, 6H, J = 6.4 Hz, NCH(CH3)2). 13C NMR (benzene-d6, 101 MHz): δ 161.56 (s, N3C), 49.22 (s, NN(CH3)2), 44.17 (s, NCH(CH3)2), 42.52 (s, NCH(CH3)2), 41.69 (s, AlN(CH3)2), 23.94 (s, NCH(CH3)2), 23.36 (s, NCH(CH3)2). Anal. Calcd for C13H33N6Al: C, 51.97; H, 11.07; N, 27.97. Found: C, 51.67; H, 11.44; N, 27.64.

Acknowledgment. This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, and the Division of Chemical Sciences, Geosciences, and Biosciences of the U.S. Department of Energy at LBNL under Contract DE-AC02-05CH11231. Supporting Information Available: X-ray data are available as crystallographic information files (CIF) for compounds 2 and 4a. 1H and 13C NMR data of all compounds as pdf files. This material is available free of charge via the Internet at http:// pubs.acs.org.