[7772-99-8] · Cl2Sn · Tin(II) Chloride · (MW 189.61)
(reducing agent for many functional groups; carbonyl allylation; Lewis acid catalyst in C-C bond-forming reactions; catalyst with AgClO4 for the synthesis of a-glycosides; synthesis of alkenes, dienes, cis-vinyloxiranes, and allylic selenides; deoxygenation of 1,4-endoperoxides; protection of carboxylic acids as 1,3-dithianes; selective p-methoxybenzyl ether cleavage reagent; additive in hydroformylation and carbonylation reactions1)
Alternate Name: stannous chloride.
Physical Data: mp 246.8 °C; bp 623 °C; d 3.95 g cm-3.
Solubility: insol xylenes; sol water, ethanol, acetone, diethyl ether, methyl acrylate, methyl ethyl ketone, isobutyl alcohol.
Form Supplied in: white crystalline solid; widely available.
Preparative Methods: analytical reagent grade tin(II) chloride is prepared by adding the dihydrate to a vigorously stirred solution of acetic anhydride (120 g salt per 100 g anhydride). After ca. one hour, the anhydrous SnCl2 is filtered on a dry sintered glass or Buchner funnel, washed free from acetic acid with dry Et2O, and dried in vacuo. It is best stored in a sealed container.2
SnCl2 is an effective reducing agent under acidic conditions.3 SnCl2 selectively reduces aromatic nitro compounds under nonacidic and nonaqueous conditions. Nearly quantitative yields of arylamines are obtained by using SnCl2.2H2O in ethanol or ethyl acetate, or anhydrous SnCl2 in alcohol at 70 °C. Under these conditions, other reducible or acid sensitive groups such as ketones, esters, nitriles, halogen, oximes, and alkenes are unaffected (eq 1).4
SnCl2 is used in the reduction of nitriles to aldehydes.5 For example, treatment of the nitrile (1) with hydrochloric acid generates the intermediate (2). This is reduced with anhydrous SnCl2 to (3), which precipitates as a complex with SnCl4 and is then hydrolyzed to the aldehyde (4) (eq 2).2 This protocol, known as the Stephen reaction, is most successful with aromatic nitriles, but it has been reported for aliphatic nitriles with up to six carbons. An alternative procedure in which (2) is obtained from the N-phenylamide, ArCONHPh, by treatment with Phosphorus(V) Chloride is known as the Sonn-Müller method.6 Carboxylic acids can be prepared by reduction of cyanosilyl ethers (5) with SnCl2 in AcOH and HCl (eq 3).7
Organic azides are reduced by SnCl2 in methanol to the corresponding amines in 85-98% yield.8 For less reactive azides, it is necessary to initiate the reaction by adding catalytic amounts of Aluminum Chloride
a-Halo ketones are hydrodehalogenated to the corresponding ketones in good yield using SnCl2 in combination with a variety of additives, e.g. sodium halides (chloride, bromide, iodide) and pseudohalides (thiocyanate, cyanate, chloride),9 sulfur compounds (NaSH, Na2S, NaHSO3, Na2SO3), and aromatic compounds (benzene, pyridine, aniline) (eq 4).10 SnCl2 is more effective than other metal halides, such as SnCl4, FeCl2, AlCl3, and CrCl3.
Treatment of an a-bromo carbonyl compound with SnCl2 and Diethylaluminum Chloride effects dehydrohalogenation with formation of an aluminum enolate, which can react with an aldehyde or ketone to give the b-hydroxy carbonyl compound in 61-95% yield, in a modification of the Reformatsky reaction (eq 5).11 Catalytic quantities of Tetrakis(triphenylphosphine)palladium(0) promote the reduction and improve the yield of the desired aldol adducts.
The reagent prepared from Sodium Cyanoborohydride and SnCl2 in a 2:1 ratio will reduce tertiary, allyl, and benzyl halides in good yields and is thus comparable to NaBH3CN-ZnCl2. However, other functional groups such as aldehydes, ketones, and acid chlorides are reduced to alcohols. Esters and amides are unaffected.12
Asymmetric reduction of prochiral ketones,13 a-, b-, and g-keto esters,14 and prochiral hydroxy ketones15 can be achieved with a reagent prepared from SnCl2 and Diisobutylaluminum Hydride, in the presence of a chiral diamine derived from (S)-proline (eq 6).
Aldehydes are efficiently converted to b-diketo esters in 50-90% yield by addition of Ethyl Diazoacetate in the presence of SnCl2 (eq 7). Although the reaction can be effected by a variety of Lewis acids, SnCl2, BF3, and GeCl2 are the most effective.16
Treatment of a number of trisubstituted alkenes with Ozone followed by the addition of SnCl2 and ethyl diazoacetate affords the b-diketo esters in 47-90% yield (eq 8). For monosubstituted alkenes to be successful, they must first be treated with ozone in the presence of methanol to generate methoxy hydroperoxides, which can subsequently be reduced to the b-diketo esters in 47-74% yield.17 1,3-Diketones can be prepared in 42-90% yield by SnCl2-catalyzed reaction of a-diazo ketones with aldehydes (eq 9).18
High regio- and diastereoselectivity can be achieved in the reaction of cinnamyl chloride with aldehydes using SnCl2 and Aluminum.19 The reaction proceeds through zerovalent tin and affords the anti-adducts with high selectivity (eq 10). Under the same conditions, enals provide 1,2-adducts.
SnCl2 acts as a reducing agent in the allylation of carbonyl compounds using allylic alcohols and Pd0; this reaction proceeds through a p-allylpalladium complex (eq 11). The order of reactivity of allylating agents is allylic carbonate > allylic alcohol > allylic acetate, and that of carbonyl compounds is aldehyde > ketone. High regioselectivity is obtained in polar solvents such as DMF, DMI (1,3-dimethyl-2-imidazolidone), and DMSO, where the carbonyl compound attacks the more substituted allylic position of the p-allylpalladium complex.
Diastereocontrol in the Pd-catalyzed carbonyl allylation of allylic alcohols with SnCl2 is dependent on the nature of the solvent. Use of DMSO at 25 °C affords the syn-adduct, while anti selection is found in THF, DMF, and DMI. The addition of water in any solvent accelerates the rate of the reaction and enhances both regioselectivity and diastereoselectivity (eq 12). For reactions in DMSO-H2O, the ratio of syn:anti products can be controlled by the amount of water added to the reaction.20 SnF2 and Sn(OAc)2 cannot replace SnCl2 in this allylation reaction.
This reaction has been extended to the synthesis of a-methylene-g-hydroxy lactones by allylation of carbonyl compounds with ethyl a-(hydroxymethyl)acrylate in DMI/H2O (eq 13).21 When the hydroxymethyl group bears an alkyl substituent, the product is obtained with high syn selectivity (eq 14).
Acetals, aldehydes, orthoesters, and a,b-unsaturated ketones are sufficiently activated by a combination of SnCl2 and Chlorotrimethylsilane22 or SnCl2 and trityl chloride23 to react with silyl enol ethers to give the corresponding addition products in 70-97% yield (eq 15). These reagents are also effective in facilitating the reaction of activated alkenes, such as 3,4-dihydro-2H-pyran, vinyl ethers, and styrene, with acetals to afford the corresponding adducts in 55-85% yield under extremely mild conditions.24 SnCl2 facilitates the workup in the Titanium(IV) Chloride-catalyzed reaction of aldehydes with silyl enol ethers by inhibiting b-elimination and formation of polymers (eq 16).25
a-Glycosides are formed predominantly by reaction of an alcohol with 2,3,4,6-tetra-O-benzyl-b-glucosyl fluoride in the presence of Tin(II) Chloride-Silver(I) Perchlorate (eq 17).
vic-Dinitro compounds can be denitrated to yield alkenes using either anhydrous SnCl2 or the dihydrate. This method fails for aliphatic vic-dinitro compounds.27 Alkenes can also be obtained in good yield from the vic-dibromo compounds using SnCl2 and DIBAL (eq 18).16
Reaction of a variety of aldehydes with 1-bromo-3-iodopropene in the presence of 2 equiv of SnCl2 yields the corresponding (E)-1,3-dienes in 40-70% yield (eq 19).28 When an enal is employed a terminal conjugated triene is obtained. Treatment of 1-chloro-3-iodopropene with 1 equiv of SnCl2 in DMF, and reaction with an aldehyde followed by NaOMe yieldes cis-vinyloxiranes in 51-53% yield (eq 20).29
Propargylic iodides form an adduct with SnCl2, which can react with aldehydes to form a mixture of a-hydroxy allenes and b-hydroxy alkynes, in which the former usually predominates. DMI is the preferred solvent (eq 21).30
1,4-Endoperoxides are converted to the corresponding 1,3-dienes in 15-70% yield using SnCl2 (eq 22).
Addition of Trimethylsilylmethyllithium to a-phenylseleno aldehydes generates 2-hydroxy-3-trimethylsilylpropyl selenides which rearrange in the presence of SnCl2 to allylic selenides (eq 23).
Reactions of carboxylic acids with 1,3,2-dithiaborinane-dimethyl sulfide in the presence of SnCl2 in THF affords the corresponding 1,3-dithianes in high yields. This method is also sufficient to distinguish between carboxylic acids and isolated double bonds (eq 24).
p-Methoxybenzyl (PMB) ethers can be cleaved selectively in the presence of benzyl ethers by employing TMSCl/anisole and a catalytic amount of SnCl2 (eq 25).
Cerium(III) Chloride-Tin(II) Chloride;
Margaret M. Faul
Eli Lilly & Company, Indianapolis, IN, USA.