Letter pubs.acs.org/OrgLett
Transition-Metal-Free Reductive Coupling of 1,3-Butadienes with Aldehydes Catalyzed by Dibutyliodotin Hydride Itaru Suzuki, Yuki Uji, Shujiro Kanaya, Ryosuke Ieki, Shinji Tsunoi, and Ikuya Shibata* Research Center for Environmental Preservation, Osaka University, 2-4 Yamadaoka, Suita, Osaka 565-0871, Japan S Supporting Information *
ABSTRACT: In this study, the Bu2SnIH-catalyzed direct coupling of 1,3dienes with aldehydes was developed. This reaction could be suitable for coupling without the use of transition-metal catalysts. Many types of aldehydes were applied to this reaction. The addition of MeOH promoted the catalytic cycle.
I
Table 1. Optimization of Coupling of 1,3-Butadiene 1 with Aldehyde 2 Catalyzed by Bu2SnIHa
n organic synthesis, allylation of aldehydes is one of the most important and reliable C−C bond formations for the production of homoallylic alcohols. Typically, isolated allylic metal reagents are prepared in advance from strong bases and allyl halides that are employed for allylation with many types of catalysts to either promote the reaction or induce enantioselectivity.1 Recently, reductive coupling between conjugated butadienes and aldehydes was developed by the Krische2 and Kimura3 groups. High efficiency and stereoselectivity were achieved using their methods, although precious transitionmetal catalysts were required. Our group has recently worked on the hydrostannylation of unsaturated bonds with dibutyltin halogen hydride (Bu2SnHalH)4 and reported a diene derivative, vinylcyclopropane, coupled with aldehydes catalyzed by dibutyliodotin hydride (Bu2SnIH).5 Herein, we report the effective catalytic activity of dibutyltin halogen hydride in the direct coupling of 1,3-dienes with aldehydes. To the best of our knowledge, this reaction is the first example of a coupling proceeding in the absence of transition metals. Initially, we investigated the optimized reaction conditions using 2,3-dimethyl-1,3-butadiene (1a) and 3-phenylpropanal (2a) at 25 °C for 24 h. According to the typical methods,6 Bu2SnIH was prepared in situ via redistribution between Bu2SnH2 and Bu2SnI2. The product 3aa was given in a high yield when the reaction conditions used in our previous reaction system were applied (Table 1, entry 1). The coupling was sluggish without the addition of MeOH (Table 1, entry 2). Other alcohols were ineffective in this reaction and afforded byproduct 4 (Table 1, entries 3 and 4) in greater amounts than the desired product 3aa. Byproduct 4 was formed by competitive reaction during the direct reduction of 2a. MeCN proved to be a better solvent than THF, toluene, or DMF (Table 1, entries 5−7). Hydrosilanes were also important in producing 3aa. Ph2SiH2 was the best hydride source (Table 1, entries 8−11). The radical scavenger TEMPO inhibited this reaction, which suggested the involvement of a radical pathway (Table 1, entry 12). With the optimized conditions in hand, various aldehydes were tested, and the results are shown in Table 2. The reactions were carried out on a 5 mmol scale. Aliphatic aldehydes were applicable to this coupling (Table 2, entries 1−6). The steric © 2017 American Chemical Society
yieldb (%) entry
Si-H/additive (mmol)
solvent
3a
4
1 2d 3 4 5 6 7 8 9 10 11 12
Ph2SiH2 (1.0)/MeOH (1.0) Ph2SiH2 (1.0) Ph2SiH2(1.0)/EtOH (1.0) Ph2SiH2 (1.0)/PrOH (1.0) Ph2SiH2 (1.0)/MeOH (1.0) Ph2SiH2 (1.0)/MeOH (1.0) Ph2SiH2(1.0)/MeOH(1.0) PhSiH3(1.0)/MeOH (1.0) Ph3SiH (1.0)/MeOH (1.0) MePhSiH2 (1.0)/MeOH (1.0) PMHS (1.0)/MeOH (1.0) Ph2SiH2(1.0)/MeOH (1.0)/ TEMPO (0.10)
MeCN MeCN MeCN MeCN THF toluene DMF MeCN MeCN MeCN MeCN MeCN
77 (67)c 33 43 50 36 45 14 50 0 32 5 3
7 67 44 43 30 30 5 43 13 12 0 0
a
All reactions were carried out under argon-flow conditions. bYields were determined by 1H NMR using an internal standard. cIsolated yield. dThe reaction was carried out for 48 h.
hindrance on the α position of carbonyls had little influence on the allylation that yielded products 3ab−ae (Table 2, entries 2−4). The product 3ac was isolated as a single diastereomer. Although its stereochemistry was not determined as yet, this could imply that the reaction proceeded via a Felkin−Anh model with prenyltin bulky nucleophiles.7 A cyclohexane ring was successfully introduced with this coupling, and afforded the Received: August 28, 2017 Published: September 11, 2017 5392
DOI: 10.1021/acs.orglett.7b02671 Org. Lett. 2017, 19, 5392−5394
Letter
Organic Letters Table 2. Various Types of Aldehydes 2 Applied to Coupling with 1,3-Butadiene 1a
Scheme 1. Plausible Reaction Mechanism
allylstannane 6, and the radical species is regenerated. After addition of allylstannane 6 to aldehyde 2,8 tin adduct 7 is protonated with MeOH. As a result, product 3 forms, accompanied by Bu2SnIOMe,9,10 and is transformed into Bu2SnIH. MeOH was a better additive than either EtOH or i-PrOH because the σ-bond metathesis between hydrosilanes and either Bu2SnIOEt or Bu2SnIOi-Pr would be disturbed by their bulkiness. This process was validated by the generation of Bu2SnIH from Bu2SnIOMe and Ph2SiH2 using 119Sn NMR (Figure S1). The prepared Bu2SnIOMe also was an effective catalyst for the coupling of 1a with 2a to afford the product 3aa in a yield that approximated the use of in situ generated Bu2SnIH from Bu2SnI2 and Bu2SnH2. Thus, the use of Bu2SnIOMe as a starting catalyst is a desirable alternative for the generation of Bu2SnIH because Bu2SnH2 is difficult to handle. Other dienes such as 1,3-butadiene (2b) and isoprene 2c reacted with aldehyde 2a to give the corresponding products in moderate yields (Scheme 2). An excess amount of 1,3-
a
All reactions were carried out under argon-flow conditions. bYields were determined by 1H NMR using an internal standard. Isolated yields are shown in parentheses. c3ac was isolated as a single diasteomer.
Scheme 2. Butadiene or Isoprene as Diene Sources for Coupling with 2a
product 3af. The couplings proceeded well when either electron-rich or -poor aromatic aldehydes were employed to afford the product 3ag−ak (Table 2, entry 7). Cinnamaldehyde (2l) coupled with the diene to give the product 3al with an intact E/Z ratio (Table 2, entry 8). Although the use of a ketone instead of an aldehyde would avoid the direct reduction of a carbonyl group, acetophenone was, unfortunately, not applicable to this reaction wherein the ketone was recovered quantitatively because of its low reactivity to allylation. A proposed catalytic cycle employing diene 1a is shown in Scheme 1. Initially, hydrostannylation to 1a occurs via a radicalchain mechanism mediated by in situ generated Bu2SnIH from Bu2SnI2 and Bu2SnH2. Bu2SnIH is homogeneously cleaved by V-70L to form a tin radical followed by addition to diene 1a. Hydrostannylation proceeds prior to the direct reduction of aldehyde 2. Allylic radical 5 is reduced by Bu2SnIH to give 5393
DOI: 10.1021/acs.orglett.7b02671 Org. Lett. 2017, 19, 5392−5394
Letter
Organic Letters
Yasuda, M.; Baba, A. Chem. Commun. 2007, 4913. (g) Hayashi, N.; Hirokawa, Y.; Shibata, I.; Yasuda, M.; Baba, A. J. Am. Chem. Soc. 2008, 130, 2912. (5) Ieki, R.; Kani, Y.; Tsunoi, S.; Shibata, I. Chem. - Eur. J. 2015, 21, 6295. (6) (a) Neumann, W. P.; Pedain, J. Tetrahedron Lett. 1964, 5, 2461. (b) Sawyer, A. K.; Brwon, Y. E.; Hanson, E. L. J. Organomet. Chem. 1965, 3, 464. (7) High diastereoselectivity via the Felkin−Anh model is due to the steric bulkiness of prenyltin nucleophile 6 in Scheme 1. See: Cherest, M.; Felkin, H.; Prudent, N. Tetrahedron Lett. 1968, 9, 2199. (8) Allylation of aldehydes with allylic stannanes has been studied. See: (a) Pereyre, M.; Quintard, P. J.; Rahm, A. In Tin in Organic Synthesis; Butterworth: London, 1987; pp 211−229. (b) Davies, A. G. In Organotin Chemistry; VCH:: Weinheim, 1997; pp 133−142. (c) Baba, A.; Shibata, I.; Yasuda, M. In Comprehensive Organometallic Chemistry III; Knochel, P., Ed.; Elsevier: Oxford, 2007; Vol. 9, pp 351− 358. (9) Bu2SnIOMe has already prepared and identified. See: Davies, A. G.; Harrison, P. G. J. Chem. Soc. C 1967, 298. (10) Yanagisawa’s group reported tin homoallylc alkoxides were protonated by MeOH and the generated tin methoxides were regenerated. See: (a) Yanagisawa, A.; Sekiguchi, T. Tetrahedron Lett. 2003, 44, 7163. (b) Yanagisawa, A.; Goudu, R.; Arai, T. Org. Lett. 2004, 6, 4281. (c) Yanagisawa, A.; Satou, T.; Izumiseki, A.; Tanaka, Y.; Miyagi, M.; Arai, T.; Yoshida, K. Chem. - Eur. J. 2009, 15, 11450.
butadiene was required to yield the product 3ba in a sufficient yield. Isoprene afforded two different products, 3ca and 3ca′, depending on the regioselectivity of the hydrostannylation to isoprene. A loss of diastereoselectivity for products 3ba and 3ca would imply that the hydrostannylation of the dienes in a radical manner was so rapid that allylic tins acted as mixtures of E- and Z- isomers. Unfortunately, terminally substituted dienes such as cyclohexadiene or piperylene were not reactive for the coupling as yet. In conclusion, we developed a transition metal-free reductive coupling of diene 1 with aldehyde 2. The reaction was catalyzed by Bu2SnIH and proceeded under mild conditions. The addition of MeOH was significant, and the product was smoothly obtained. Various aldehydes and dienes were applicable to the coupling.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b02671. Experimental procedures and spectral data (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Itaru Suzuki: 0000-0002-4817-9156 Ikuya Shibata: 0000-0002-9619-4019 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for financial support from The Naito Foundation. We also thank the Instrumental Analysis Center, Faculty of Engineering, Osaka University, for assistance with collecting the spectral data.
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REFERENCES
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DOI: 10.1021/acs.orglett.7b02671 Org. Lett. 2017, 19, 5392−5394
