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Organometallics 1999, 18, 2782-2785
A Novel Copper(II)/Tin(II) Reagent for Regio- and Chemoselective Carbonyl Propargylation Abhijit Kundu,† Sripadi Prabhakar,‡ M. Vairamani,‡ and Sujit Roy*,† Metallo-organic Laboratory, Inorganic Division, and Mass Spectrometry Center, Indian Institute of Chemical Technology, Hyderabad 500007, India Received January 20, 1999
A convenient and synthetically attractive protocol for the regioselective synthesis of homopropargyl alcohols from aldehydes and 2-propynyl halides having terminal alkynes under the aegis of catalytic cupric bromide and stannous chloride in THF is described. Insitu probing by 1H NMR and MIKE-CID spectrum provides evidence for the copper(I)catalyzed formation of allenyltrihalostannane. Introduction By virtue of their synthetic usefulness and mechanistic intrigue, organic reactivities of propargyl and allenyl organometallics have received considerable attention in the past two decades.1 These reagents, like their allylic counterpart, can be generated by Grignard, as well as Barbier, methodologies.2 The metallotropic rearrangement3 between propargyl and allenyl organometallics often result in poor regioselection in the end organic product, for example in the case of reaction with carbonyl compounds and other electrophiles (Scheme 1). Hence, a pertinent synthetic challenge is to tune the regioselectivity toward either acetylenic or allenic species. In this context the synthesis of homopropargyl alcohols by the umpolung approach from propargylpalladium described by Tamaru et al. and the synthesis of homoallenyl alcohols from transient propargylic stannanes reported by Marshall et al. are noteworthy (Scheme 2).4 Our recent success in a Cu(II)/Sn(II)-promoted carbonyl allylation reaction5 prompted us to explore the further utility of this reagent for carbonyl propargylation. To our delight, we could harness 100% regioselective syntheses of homopropargyl alcohols from 2-pro-
Scheme 1
Scheme 2
†
Metallo-organic Laboratory, Inorganic Division. Mass Spectrometry Center. (1) For review see: (a) Yamamoto, H. In Comprehensive Organic Synthesis; Trost, B. M., Ed.; Pergamon: Oxford, U.K., 1991; Vol. 2, pp 81-98. (b) Panek, J. S. In Comprehensive Organic Synthesis; Schreiber, S. L., Ed.; Pergamon: Oxford, U.K., 1991; Vol. 1, p 595 and references therein. (2) (a) Cabezas, J. A.; Alvarez, L. X. Tetrahedron Lett. 1998, 39, 3935 and references therein. (b) Tanaka, H.; Hamatani, T.; Yamashita, S.; Torii, S. Chem. Lett. 1986, 1461. (c) Dabdoub, J. M.; Totta, J. C. G. Synlett 1996, 526. (d) Larock, R. C.; Chow, M.-S. Tetrahedron Lett. 1984, 25, 2727. (e) Issac, M. B.; Chan, T.-H. J. Chem. Soc., Chem. Commun. 1995, 1003. (3) (a) Doherty, S.; Corrigan, J. F.; Carty, A. J.; Sappa, E. Adv. Organomet. Chem. 1995, 37, 39. (b) Wojicicki, A. New. J. Chem. 1994, 18, 61. (c) Tsuji, J.; Mandai, T. Angew. Chem., Int. Ed. Engl. 1995, 34, 2589. (d) Ogoshi, S.; Fukunishi, Y.; Tsutsumi, K.; Kurosawa, H. J. Chem. Soc., Chem. Commun. 1995, 2485. (e) Hoffmann, R. W.; Lanz, J.; Metternich, R.; Tarava, G.; Hoppe, D. Angew. Chem., Int. Ed. Engl. 1987, 26, 1145. (4) (a) Tamaru, Y.; Goto, S.; Tanaka, A.; Shimizu, M.; Kimura, M. Angew. Chem., Int. Ed. Engl. 1996, 35, 878. (b) Marshall, J. A.; Yu, R. H.; Perkins, J. F. J. Org. Chem. 1995, 60, 5550. (5) Kundu, A.; Prabhakar, S.; Vairamani, M.; Roy, S. Organometallics 1997, 16, 4796. ‡
Scheme 3
pynyl halides and various aldehydes (Scheme 3). Much to our surprise, we noted that the reaction is unusually chemoselective for a halide bearing a terminal alkyne in preference to an internal alkyne. Results and Discussion The reaction of stannous chloride dihydrate with 2-propynyl bromide 1 in the presence of catalytic cupric bromide in refluxing THF after 4 h gave rise to the
10.1021/om9900317 CCC: $18.00 © 1999 American Chemical Society Publication on Web 06/22/1999
A Cu(II)/Sn(II) Reagent for Carbonyl Propargylation
Organometallics, Vol. 18, No. 15, 1999 2783
Table 1. Propargylationa of Aldehydes RCHO with 2-Propynyl Bromides HCtCC(R1R2)Br R1
R2
1 2 3 4
H
H
H
Me
5 6 7 8
Me
Me
entry
9 10 11 12 c
R
time (h)
yieldb (%)
4-chlorophenyl cinnamyl 1-naphthyl
4 3 2 2
55 48 85c 90d
4-chlorophenyl benzyl
4 8 6 5
90 75 78 70
24f 6 16f 5
53 72 48 55
ferrocenyl -CH2(CH2)3CH2- e Me
Et
piperonyl 4-chlorophenyl 2-furyl ferrocenyl
a Unless otherwise stated all reactions were carried out in refluxing THF. b Isolated yields after chromatography based on aldehydes. Syn:anti ) 30:70. d Syn:anti ) 45:55. e Chloride instead of bromide was used. f Reaction at room temperature.
Scheme 4
desired homopropargyl alcohol 2 in 55% isolated yield. The reaction proceeds well with cuprous bromide also.6 THF was found to be a better solvent in comparison to THF-H2O, DCM, and DCM-H2O. On the other hand, reaction without cupric bromide showed
