Communication pubs.acs.org/Organometallics
Mo(CO)6-Catalyzed Reduction of N,N-Dimethylthioformamide by Silanes, Germanes, and Stannanes (R3EH) To Produce Trimethylamine and Group 14 Thioethers (R3E−S−ER3) Renzo Arias-Ugarte, Hemant K. Sharma, Alejandro J. Metta-Magaña, and Keith H. Pannell* Department of Chemistry, The University of Texas at El Paso, El Paso, Texas 79968, United States S Supporting Information *
ABSTRACT: Treatment of N,N-dimethylthioformamide (DMTF) with a variety of silanes, germanes, and stannanes (R 3EH; E = Si, Ge, Sn) in the presence of Mo(CO)6 leads to facile reduction to trimethylamine and the corresponding group 14 thioethers, (R 3E)2S. In the reactions we are able to observe the intermediacy of the catalytically active Mo(CO) 5(SCHNMe2) and completely characterize it, including single-crystal X-ray structural analysis. In the case of R 3SnH, the reduction occurs without the need for a metal catalyst, presumably via a free radical process.
T
here has been a considerable research effort on the use of organosilanes to effect the facile reduction of amides to amines first reported by the Voronkov group in 1985 (eq 1). 1
Table 1. Synthesis of Disila-, Digerma-, and Distannathianes, R3E−S−ER3
(1)
To date, a wide range of transition-metal catalysts facilitate this reaction.2−6 We recently reported that the catalytically useful transition-metal alkyl complex (η5-C5H5)Fe(CO)2CH3, (1a)7 was also efficient in the photolytic reduction of N,N-dimethylformamide (DMF) by silanes.8 Subsequently we noted that the Mo complexes (η5-C5H5)Mo(CO)2(L)CH3 (L = CO (2a), PPh3 (2b)), Mo(CO)6 (3a), and Mo(CO)5NMe3 (3b) also catalyzed this reduction, both photochemically and thermally, and in addition observed that germanes and stannanes could also accomplish the reduction.9 We have suggested that initial activation of the amide via coordination to the metal center was important, followed by hydrosilylation to form R3SiOCH2NMe2, an isolable intermediate in the case of using 3a as catalyst. This intermediate is then further reduced by the silane, a thermal reaction that can proceed without a catalyst in the presence of DMF. We have extended our studies to the reduction of N,Ndimethylthioformamide (DMTF) by group 14 hydrides and now report the successful outcome of these attempts. Thus, using a range of group 14 hydrides R3EH (E = Si, R3 = Me2Ph, MePh2, Et3; E = Ge, R3 = Et3; Bu3, Ph3; E = Sn, R3 = Bu3, Ph3) and using the molybdenum catalyst 3a, we obtain good to excellent yields of the appropriate group 14 sulfides (eq 2 and Table 1), together with Me3N.10 This is a noteworthy result, since in general whereas hydrosilylations of CO bonds are very well-established,11 those of CS linkages are rare,12 and thus this simple capacity to reduce the CS by group 14 hydrides may have a wide applicability. © 2011 American Chemical Society
R3
E
cat.
Δ
time
yield, %a,b
PhMe2 PhMe2 Ph2Me Et3 Et3 Bu3 Ph3 Bu3 Ph3 PhMe2 Et3 Bu3 Ph3 PhMe2 Et3
Si Si Si Si Ge Ge Ge Sn Sn Si Ge Sn Sn Si Ge
Mo(CO)6 Mo(CO)6 Mo(CO)6 Mo(CO)6 Mo(CO)6 Mo(CO)6 Mo(CO)6 Mo(CO)6 Mo(CO)6
Δ hv Δ Δ Δ Δ Δ Δ Δ Δ Δ Δ Δ Δ Δ
18 h 2 days 3 days 2 days 2 days 2 days 2 days 3h 3h 2 days 2 days 3h 3h 4 days 4 days
87 (100) TR 63 (100) 62 (100) 63 (100) 67 (100) 53 (90) 80 (100) 60 (100) NR NR (100) (100) NR NR
c c
a
Isolated yields. NR = no reaction; TR = traces. bYields based upon H NMR given in parentheses. cBenzoyl peroxide or AIBN used as radical initiator.
1
(2)
In a typical reaction 1.00 mmol of PhMe2SiH and 3 mmol of HC(S)NMe2 together with 5% equivalent of Mo(CO)6 were dissolved in 2 mL of C6D6 and placed in a Pyrex NMR tube. The solution was degassed via two freeze−thaw cycles and the tube sealed under vacuum and placed in a oil bath at 120 °C. The reaction was monitored by 13C and 29Si NMR spectroscopy, Received: August 11, 2011 Published: November 28, 2011 6506
dx.doi.org/10.1021/om200753t | Organometallics 2011, 30, 6506−6509
Organometallics
Communication
Figure 1. NMR monitoring of the reaction between PhMe2SiH (*) (29Si NMR: −17.3 ppm) and HC(S)NMe2 (◇) catalyzed by Mo(CO)6 (3a), showing the formation of Mo(CO)5(SCHNMe2) (3c) and disappearance of PhMe2SiH to form PhMe2SiSSiMe2Ph (△) (29Si NMR: 7.1 ppm) and Me3N: (A) 29Si spectra; (B) 13C spectra.
and such a procees is presented in Figure 1. The spectra illustrate the clean tranformation, and for example in the 29Si spectral sequence (Figure 1A) the removal of the resonance at −17.3 ppm (PhMe2SiH) occurs concomitantly with the sole appearance of the resonance at 7.1 ppm due to the product (PhMe2Si)2S. Likewise, the 13C spectra (Figure 1B) show the disappearance of the resonances for the starting silane at −3.5 ppm (PhMe2SiH) and 128.1, 129.5, 134.2, and 137.1 ppm and their transformation to the appropriate product (PhMe2Si)2S resonances at 2.6, 128.1, 129.8, 133.8, and 138.6 ppm, respectively. The growth of the resonance at 47.1 ppm due to Me3N is also apparent. The disilyl thioethers were purified by column chromatography and their spectroscopic data compared to those of known samples reported in the literature.10 In the case of the silane reductions of DMF catalyzed by 3a we were able to observe, isolate, and characterize the initial hydrosilylated products R 3SiOCH2NMe2, which are key intermediates in this process.9 In the reductions of N,Ndimethylthioformamide noted in eq 2, also demonstrated in Figure 1, we have not observed such intermediates for any of group 14 hydrides used. However, in this chemistry, by monitoring the reactions by 13C and 1H NMR spectroscopy, we were able to detect both the presence of Mo(CO)5(S CHNMe2) (3c; 13C NMR 212.6 (CO), 205.2 (CO), 189.3 (CS), 46.0 (Me), 38.0 ppm (Me)) and, as expected from our results with the reduction of DMF by silanes, trace amounts of Mo(CO)5(NMe3) (3b; 13C NMR 58.0 ppm, NMe3). In separate experiments we have shown that 3b reacts rapidly at room temperature with HC(S)NMe2 to form 3c. We have independently synthesized 3c, using a literature procedure by Ishaq et al.,13 and confirmed that the spectral data noted in the
catalytic process were indeed those of the proposed intermediate. Complex 3c is also an efficient catalyst for the reduction process as is 3b with 80−90% recovered yields of the disilyl thioethers.10 Scheme 1. Proposed Mechanism for the Reduction of Thioformamide by Silanes (LM-CO = Mo(CO)6)
We propose an overall mechanism (Scheme 1) similar to that for the reduction of DMF,8 without the intermediacy of free R3EOCH2NMe2. We suggest that the initially formed hydrosilylated (CO)5MoS(R3Si)CH2NMe2 can be further reduced by R3SiH in situ. One might reasonably expect the Mo−S bond to be strong, and indeed, despite our inability to note such intermediates, Mo(CO)5SR2 (R = Me, Ph) complexes have been reported.14 Certainly the reduction in the base character of the sulfur atom in the Si−S−R system will be less dramatic than the reduction in basicity of the corresponding siloxane. A search of the Cambridge Crystallographic Data Base on the 6507
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structures with Si−S−C (and Si−O−C) linkages indicates that the mean Si−S−C bond angle is ∼106°, whereas that of the Si−O−C bond angle is ∼130°.10 Such data imply that the delocalization of sulfur lone pairs is less pronounced in Si−S−C systems than the related delocalization of the oxygen lone pairs.15 Regardless of the cause, the greater availability of the electron pair on the S atom resulting in stronger bonding to the metal center, will create an electron deficiency at the C atom, thereby facilitating its further reduction by R3EH. The thermal reaction between Bu3SnH and DMTF (Table 1) suggests that a possible radical process may be important, since no catalyst is required. Certainly it has been established that stannyl radicals react with thiocarbonyl derivatives: e.g., methyl xanthates to produce stannyl thioethers such as R3SnSMe.16 However, despite the potential for stannyl radical chemistry providing a distinct mechanistic route, our attempts to induce the silane (germane) reductions in a radical manner by introduction of either benzoyl peroxide or AIBN have been unsuccessful (Table 1) and in such reactions no new chemistry was noted. We have suggested that it is the initial coordination of the amide (thioamide) to the metal center, (CO) 5Mo−E CHNMe2 (E = O, S), that activates the amide toward hydrosilylation. In the case of the DMF reduction we were unable to observe this (known) species during the chemical reaction; however, in the present case 3c is observable and we have demonstrated its role in the catalytic process. Furthermore, in a separate reaction we have treated a C 6D6 solution of 3c with an excess of PhMe2SiH at 120 °C (the reaction conditions used for the catalysis noted in eq 2) and obtained an 85% yield of PhMe2Si−S−SiPhMe2 (based upon the amount of 3c used), along with trace amounts of PhMe2SiNMe2 and Mo(CO)6. No new intermediate species were observed in this chemistry. The molecular structure of 3c is presented in Figure 2, along with some pertinent bond angles and lengths. The geometry at Mo is octahedral, as expected, and the SCCN atoms of the
thioformamide are planar. The C−S bond length of 166.7(8) pm is unchanged from that of the free ligand at 166.6(1) pm, from a structure determined at 90 K.17 This result is similar to those noted for other thioformamide metal complexes: for example, [M(L)6][ClO4]2 (M = Fe, 166(3)−169(3) pm;18 M = Ni, 166.5(3)−166.7(3) pm) and related complexes.19 However, this result is in contrast with other data involving related thioamide metal complexes, where either significant CS bond length reductions are noted for η1 complexes20 or significant elongations are seen in the case of η2-(SN) complexes.21 The Mo−S bond length of 258.4(3) pm is somewhat shorter than some other thione−Mo interactions, e.g. Mo(CO)5(1,3,4, 5-tetramethylimidazoline-2-thione) (4),22 where the Mo−S bond length is ∼261 pm. However, despite this difference for both 3c and 4 the CO group trans to the S coordination exhibits a distinctly reduced Mo−C bond length (1.97(1) Å vs ∼2.05(1) Å (cis)) and elongated C−O bond (1.15(1) Å vs ∼1.12(1) Å (cis)) as a response to the stronger donation of electrons to the Mo center from the S atom, as noted for other thioamide Cr and W carbonyl complexes.23 In summary, we have developed a new catalytic route for the synthesis of bis(group 14) thioethers (R3E)2S in high yields by the unprecedented reduction of thioamides by group 14 hydrides R3EH, along with formation of trimethylamine. It is noteworthy that the new chemistry is a desulfurization process that could exhibit further utility, and our studies are ongoing.
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ASSOCIATED CONTENT * Supporting Information Text giving synthetic details and full spectroscopic data for products, graphical displays of the comparison of literature Si− O−Si, Si−O−C and Si−S−Si, Si−O−C bond angles of R3SiESiR (E = O, S), and a CIF file giving crystallographic data for 3c. This material is available free of charge via the Internet at http://pubs.acs.org. S
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ACKNOWLEDGMENTS This work was supported by the Welch Foundation, Houston, TX (Grant AH-546).
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REFERENCES
(1) Kopylova, L. I.; Ivanova, N. D.; Voronkov, M. G. Zh. Obshch. Khim. 1985, 55, 1649−51. (2) Zhou, S.; Junge, K.; Adis, D.; Das, S.; Beller, M. Angew. Chem., Int. Ed. 2009, 48, 9507−9510. (3) (a) Motoyama, Y.; Mitsui, K.; Ishida, T.; Nagashima, H. J. Am. Chem. Soc. 2005, 127, 13150−13151. (b) Hanada, S.; Motoyama, Y.; Nagashima, H. Tetrahedron Lett. 2006, 47, 6173−6177. (4) (a) Matsubara, K.; Iura, T.; Maki, T.; Nagashima, H. J. Org. Chem. 2002, 67, 4985−4988. (b) Hanada, S.; Ishida, T.; Motoyama, Y.; Nagashima, H. J. Org. Chem. 2002, 67, 4985−4988. (5) (a) Sunada, Y.; Kawakami, H.; Imaoka, T.; Motoyama, Y.; Nagashima, H. Angew. Chem., Int. Ed. 2009, 48, 9511−9514. (b) Motoyama, Y.; Aoki, M.; Takaoka, N.; Aoto, R. Chem. Commun. 2009, 1574−1576. (6) Sakai, N.; Fujii, K.; Konokahara, T. Tetrahedron Lett 2008, 49, 6873−75. (7) Sharma, H. K.; Pannell, K. H. Organometallics 2010, 29, 4741− 4745. (8) Sharma, H. K.; Pannell, K. H. Angew. Chem., Int. Ed. 2009, 48, 7052−7054. This report has been confirmed by others: Itazaki, M.; Ueda, K.; Nakazawa, H. Angew. Chem., Int. Ed. 2009, 48, 6938. (9) Arias-Ugarte, R.; Sharma, H. K.; Cervantes, E.; Pannell, K. H. Submitted for publication in J. Am. Chem. Soc.
Figure 2. Molecular structure of the complex Mo(CO) 5(thioformamide) (3c), with the ellipsoids drawn at the 40% probability level. Selected distances (Å): S1−Mo1 = 2.584(3), C6−S1 = 1.667(8), C6−N1 = 1.287(11), N1−C8 = 1.453(12), N1−C7 = 1.466(12), Mo1-Cn = 1.968(12), 2.058(9), 2.044(8), 2.039(9), 2.052(9) for n = 1−5, respectively. Selected angles (deg): C1−Mo1−S1 = 177.0(3), Mo1−S1−C6 = 111.5(3), S1−C6−N1 = 126.6(7), C7−N1−C6 = 121.2(8), C8−N1−C6 = 122.1(7), Mo1−S1−C6−N1 = 178.9(4), S1−C6−N1−C7 = 179.5(8), S1−C6−N1−C8 = −1.3(4). 6508
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(10) For experimental details and literature references to the various R3E−S−ER3 species, see the Supporting Information. (11) Marciniec, B. Comprehensive Handbook on Hydrosilylation; Pergamon Press: Oxford, U.K., 1990; Chapter 3. (12) Harrison, D. J.; McDonald, R.; Rosenberg, L. Organometallics 2005, 24, 1398−1400. (13) Baghlaf, A. O.; Al-Shaikh, A. H.; Ishaq, M. J. Chem. Soc. Pakistan 1987, 9, 555−558. (14) Herberhold, H.; Suss, G. J. Chem. Res., Synop. 1977, 246. (15) Alternatively, as suggested by a reviewer, the repulsion of Si−O bonding pairs is greater due to polarization of the bonding pairs toward the more electronegative O atom, causing the greater SiOSi angle. Of course, second-order Jahn−Teller effects should also be considered. (16) Barton, D. H. R.; Jang, D. O.; Jaszberenyi, J. C. Tetrahedron Lett. 1990, 31, 3991−4. (17) Borrmann, H.; Persson, I.; Sandström, M.; Stalhandske, C. M. V. Perkin 2 2000, 393−402. (18) Baumgartner, O. Acta Crystallogr. 1986, C42, 1723. (19) Kristiansson, O.; Persson, I.; Bobicz, D.; Xu, D. Inorg. Chim. Acta 2003, 344, 15−27. (20) Spofford, W. A. III; Boldrini, P.; Amma, E. L. Inorg. Chim. Acta 1971, 5, 70−74. (21) (a) Reisner, M. G.; Bernal, I.; Brunner, H.; Wachter, J. J. Organomet. Chem. 1977, 137, 329−47. (b) Luo, X.-L.; Kubas, G. J.; Burns, C. J.; Butcher, R. J. Organometallics 1995, 14, 3370−3376. (22) Saito, K.; Kawano, Y.; Shima, M. Eur. J. Inorg. Chem. 2007, 3195−3200. (23) (a) Szesni, N.; Drexler, M.; Weibert, B.; Fischer, H. Z. Naturforsch. B: Chem. Sci. 2007, 62, 346−356. (b) Fischer, H.; Treier, K.; Hofmann, J. J. Organomet. Chem. 1990, 384, 305−314.
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