Tin(II) Bromide

SnBr2

[10031-24-0]  · Br2Sn  · Tin(II) Bromide  · (MW 278.51)

(mild Lewis acid used in halogenative allylation and Friedel-Crafts allylation; synthesis of a-nucleosides; a-methylation of ketones; reduction of acetals and aromatic nitro compounds; reductive dimerization of imines)

Alternate Name: stannous bromide.

Physical Data: mp 215.5 °C; bp 620 °C; d 5.1 g cm-3.

Solubility: sol water (85.2 g/100 mL at 0 °C, 222.5 g/100 mL at 100 °C), alcohol, ether, acetone.

Form Supplied in: pale yellow rhombic crystals; commercially available.

Handling, Storage, and Precautions: corrosive and moisture sensitive; should be stored in a tightly closed container protected from light.

Allylations.

Oriyama1 has shown that aromatic acetals can be converted in 63-87% yield to a-allylbenzyl bromides utilizing Allyltrimethylsilane and a catalytic amount of SnBr2 in combination with excess Acetyl Bromide (eq 1). This reaction works equally well with SnCl2 and Acetyl Chloride to yield the corresponding a-allylbenzyl chlorides. The Friedel-Crafts allylation of anisole with allylic bromides catalyzed by SnBr2 has also been reported (eq 2).2

a-Methylene Ketones.

Silyl enol ethers react with Bromomethyl Methyl Ether in the presence of a catalytic amount of SnBr2 to yield a-bromomethyl ketones. The a-bromomethyl ketones can be isolated or converted in a one-pot procedure to a-methylene ketones by treatment with 1,8-Diazabicyclo[5.4.0]undec-7-ene (eq 3).3 Other tin halides such as Tin(II) Fluoride and Tin(II) Chloride can be used successfully in the reaction. This one-pot procedure is a convenient method for the introduction of a-methylene groups to ketones. Other methods such as the introduction of dialkylaminomethyl4 or arylthiomethyl5 groups a to a ketone, followed by activation and elimination, are two-step procedures. This methodology has been used in the synthesis of the antitumor antibiotic sarkomycin (1) (eq 4).

Reductive Dimerization of Imines.

The reductive dimerization of N-alkyl imines to vicinal diamines can be effected in 62-90% yields with Aluminum and a catalytic amount of a SnII or PbII salt in the presence of Trifluoroacetic Acid or Aluminum Bromide (eq 5).6 The PbBr2/Al bimetal redox system is the best combination although SnBr2/Al is also effective. The reaction works best on both substituted and unsubstituted aromatic imines; however, aliphatic and aromatic ketimines will undergo reductive dimerization under refluxing conditions. This method produces slightly better yields than two other recently developed methods for the preparation of vicinal diamines, namely the niobium(IV) promoted dimerization of N-trimethylsilylimines7 and the reductive dimerization of N-alkylbenzylimines with titanium(0).8

Nucleosides from Acyclic Epoxy Aldehydes.

Acyclic epoxy aldehydes undergo an addition-cyclization process with the tin(II) salts of nucleoside bases in the presence of SnBr2 to yield nucleosides having predominately the a configuration (eq 6).9 This methodology can be used to prepare stereoselectively 2-deoxyribonucleosides that are difficult to prepare by other means.

Reductions.

Aromatic acetals can be reductively halogenated in high yield to give benzylic bromides1 by employing a catalytic amount of SnBr2 in the presence of Triethylsilane and excess acetyl bromide. The intermediate bromides can be further reduced with Lithium Aluminum Hydride or Tri-n-butylstannane to the corresponding hydrocarbon (eq 7).10 This methodology has been extended to the reduction of aliphatic and aromatic ketone acetals; however, the product may be contaminated with small amounts of alkene as a byproduct (eq 8). This three-step conversion of ketones and aldehydes to the corresponding alkanes represents a mild and convenient alternative to the Wolff-Kishner and Clemmensen reductions.

It has been reported that m-bromobenzaldehyde can be prepared by reduction of m-nitrobenzaldehyde with SnBr2/HCl, followed by diazotization of the resulting aminoaldehyde with Sodium Nitrite, and treatment of the diazonium ion solution with Copper(I) Bromide/HBr (eq 9).11 The use of SnBr2 is important in the reduction step because substitution of SnCl2 leads to a product mixture containing up to 20% of m-chlorobenzaldehyde.


1. Oriyama, T.; Iwanami, K.; Tsukamoto, K.; Ichimura, Y.; Koga, G. BCJ 1991, 64, 1410.
2. Yamaguchi, J.; Takagi, Y.; Nakayama, A.; Fujiwara, T.; Takeda, T. CL 1991, 133.
3. Hayashi, M.; Mukaiyama, T. CL 1987, 1283.
4. (a) For a review see: Tramontini, M. S 1973, 703. (b) Schreiber, J.; Maag, H.; Hashimoto, N.; Eschenmoser, A. AG(E) 1971, 10, 330.
5. Fleming, I.; Goldhill, J.; Paterson, I. TL 1979, 3205, 3209.
6. Tanaka, H.; Dhimane, H.; Fujita, H.; Ikemoto, Y.; Torii, S. TL 1988, 29, 3811.
7. Roskamp, E.; Pedersen, S. JACS 1987, 109, 3152.
8. Betschart, C.; Seebach, D. HCA 1987, 70, 2215.
9. Miwa, T.; Narasaka, K.; Mukaiyama, T. CL 1984, 1093.
10. Oriyama, T.; Ichimura, Y.; Koga, G. BCJ 1991, 64, 2581.
11. Buck, J.; Ide, W. OSC 1943, 2, 130.

Perry C. Heath

Eli Lilly and Company, Indianapolis, IN, USA



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