[7783-47-3] · F2Sn · Tin(II) Fluoride · (MW 156.71)
(Knoevenagel2 and aldol-type3 catalyst; oxidative addition to carbon-halogen bonds;4,5 stereocontrolled [3 + n] (n = 3, 4, or 5) annulations;6,8,9 conversion of allylic phosphonates to allyltin intermediates7)
Alternate Name: stannous fluoride.
Physical Data: mp 213 °C, 219.5 °C; bp 850 °C; d 4.570 g cm-3.
Solubility: v sol water (>30% w/w); sol aq potassium hydroxide, fluoride solutions, mineral acids. Readily dissolves in donor solvents such as acetone, ethyl acetate, or pyridine, forming presumed pyramidal (&PPsi;-tetrahedral) complexes.
Form Supplied in: white, monoclinic crystals; widely available.
Handling, Storage, and Precautions: slowly hydrolyzes in water to SnO. Converts to SnOF2 upon exposure to air. Stable in aq mineral acids. Aq solutions are readily oxidized (catalyzed by light). Fluoride solutions react with SnF2, providing SnF3- (pK = 10) which absorbs oxygen but does not oxidize. Containers of SnF2 should be stored under a dry, inert atmosphere. LD50 128.4 mg/kg (mouse), 188.2 mg/kg (rat).
Among 14 metallic fluorides, Lelean2 has shown that tin(II) fluoride is a superior catalyst in the Knoevenagel condensation of a tetrahydropyrimidine with 2-thiophenecarbaldehyde in methylcyclohexane at reflux (eq 1). Complete conversion (based upon the amount of water formed) is observed with only 9.3 mol % of catalyst.
Tin(II) fluoride undergoes oxidative insertion with a,a-dibromoacetophenone derivatives (eq 2) (R1 = Me, Et), affording the tin enolate which undergoes addition to aliphatic and aromatic aldehydes (R2 = PhCH2CH2, Ph, n-C8H17, p-ClC6H4) at -45 °C. Treatment of the aldol intermediate with Cesium Fluoride (or Triethylamine) promotes closure to the a,b-epoxy ketone in 52-81% yields.3
Mukaiyama4 has shown that Allyl Iodide reacts with SnF2 to give an allyltin species (eq 3) which adds to aldehydes (R1 = PhCH2CH2, PhCH=CH, Ph) to give an intermediate which, in turn, can be trapped with acid chlorides (R2 = Me, PhCH2CH2), affording esters of homoallylic alcohols.
This methodology has been applied to a four-pot synthesis of 2-deoxy-D-ribose (eq 4).4 Allylation/trapping of 1,2-O-isopropylidene-D-glyceraldehyde with phenoxyacetyl chloride gives a 78% yield of the erythro-homoallylic alcohol (admixed with 19% of the threo-isomer). Cleavage of the phenoxyacetyl group with Ammonia affords the acetonide in 71% yield. Hydrolysis then gives the triol in 95% yield, and this affords 2-deoxy-D-ribose in 75% yield after ozonolysis with a reductive workup.
In a similar manner, reaction of Carbon Tetrabromide with SnF2 in DMSO at 25 °C gives an intermediate which adds to aldehydes (R = Ph, 1-furyl, PhCH2CH2, n-C8H17, PhCH=CH, PhCO, PhCH(Me)), affording the corresponding tribromoethanols in 46-80% yields (eq 5).
Tribromoethanols are useful precursors of a-hydroxy carboxylic acids and have been employed in the synthesis of 2,3-diacetyl-D-erythronolactone5 from 1,2-O-isopropylidene-D-glyceraldehyde (eq 6).
Molander reports the stereocontrolled synthesis of cyclopentanoids6 (eq 7) using a 1,2-dicarbonyl component and a 1,3-dianion synthon derived from the treatment of 3-halo-2-[(trimethylsilyl)methyl]propenes (X = I, Br) with SnF2 in THF at 25 °C. Several 1,2-diketones (R, R1 = Me, Et, Ph, n-Pr, Cl(CH2)4, -(CH2)4-; 0% for R = R1 = Ph) afford the cis-diols.
SnF2 appears unique for this annulation. After generation of the allylstannane, the Sn4+ serves as a Lewis acid catalyst and provides internal chelation. The fluoride counterion is an ideal nucleophile for the activation of the allylsilane. The high stereoselectivity (25:1 to 75:1) is attributed to chelation (eq 8).6
Reaction of an asymmetrically substituted 1,2-dione with the allylstannane as described above gives predominantly the diastereomer predicted by the Felkin-Ahn model (eq 9).6
Cyclohexanediols are prepared in approximately 20-60% yields (diastereomeric ratios from 4:1 to 50:1) by a [3 + 3] annulation using a,b-epoxy aldehydes (R = alkyl, H) as the dielectrophilic partner (eq 10).8 The high degree of stereoselectivity and the lack of appreciable [3 + 2] annulation is attributed to internal chelation and an internal trans-diaxial epoxide opening (eq 11).8
Annulations with 1,4-dicarbonyl or 1,5-dicarbonyl substrates (R = aryl, aliphatic) give seven- and eight-membered ethers in high yields (eqs 12 and 13).9 The ethers are obtained since the initial adduct may form an internal acetal (eq 14).9
Allylic phosphonates (R = H or Me) react with Et2AlSnClF3 (formed in situ with SnF2 and Diethylaluminum Chloride with a catalytic amount of Tetrakis(triphenylphosphine)palladium(0)), giving an allyltin reagent which adds to benzaldehyde or hexanal in high yields (eq 15).7 The syn/anti selectivities of the homoallylic alcohols range from 25/75 to 55/45.
Leland O. Weigel
Eli Lilly and Company, Indianapolis, IN, USA