Crotyltributylstannane1

(E)

[35998-93-7]  · C16H34Sn  · Crotyltributylstannane  · (MW 345.13) (Z)

[35998-94-8]

(allylating reagent for many compounds, including alkyl halides, carbonyl compounds, imines, acetals, thioacetals, and sulfoximides)

Alternate Name: 2-butenyltributyltin.

Physical Data: bp 100-110 °C/1 mmHg;2f 1H NMR (E) isomer d 5.95-5.10 (m, 2 H), 2.00-0.70 (m, 32 H); 13C NMR (E) isomer d 130.5, 120.3, 29.3, 27.5, 27.4, 17.9, 13.8, 9.2;2f 1H NMR (Z) isomer d 6.84-5.96 (m, 2 H), 3.30-1.43 (m, 32 H); 13C NMR (Z) isomer d 129.6, 118.2, 29.3, 27.5, 27.4, 13.8, 12.5, 9.4.2f

Solubility: sol methylene chloride, diethyl ether, tetrahydrofuran, toluene, and benzene.

Form Supplied in: not commercially available.

Preparative Methods: several methods have been described.2

Purification: by distillation.

Handling, Storage, and Precautions: all organotin compounds are highly toxic. This reagent therefore must be used in a well-ventilated fume hood. Contact with the eyes, skin, and respiratory system should be avoided.

Introduction.

Allylstannanes are widely used storable allyl anion equivalents.1 Since the chemistry associated with allystannanes and crotylstannanes is, in most cases, very similar, this section will focus on the reactions of crotyltrialkylstannane and crotyl derivatives (methallyltributylstannane, b-methylcrotylstannane, pent-3-en-2-yltrialkylstannanes, and cinnamylstannane. A more comprehensive review of allylation reactions employing allylic stannanes is presented in the article on Allyltributylstannane. Related allylic stannane chemistry is also found under articles on a-alkoxyallylstannanes and Allyltriphenylstannane derivatives.

Additions to Aldehydes and Ketones.

The addition of allylic reagents to carbonyl compounds has become a very attractive synthetic strategy in carbon-carbon bond formations. g-Substituted allylmetal reagents are especially attractive in natural product synthesis since additions can provide two new adjacent stereocenters. Thus methods to control the stereochemical outcome of both diastereofacial and bond construction selectivity have received a lot of attention.

Crotyltrialkyltin reagents can undergo transmetalation reactions in the presence of Lewis acids.3 Competing transmetalation process can affect the mechanistic pathway and product distribution (eq 1).4 1,3-Isomerization results in linear (a) adducts, while branched (g) adducts occur through the Lewis acid-carbonyl complex or the transmetalated intermediate, a crotyltin halide. The course of the reaction depends upon the Lewis acid, temperature, substrate, stoichiometry, and order of addition.

In thermal reactions, the stereochemical outcome of crotyltin additions to aldehydes is dependent upon the geometry of the but-2-enyl unit. Thus the (E) isomer produces anti alcohols while the (Z) isomer produces the syn alcohols.5 The allylic coupling to aldehydes also occurs at rt under high pressure.6

The Lewis acid-mediated reactions of crotylstannanes and -silanes produces syn homoallylic alcohols predominantly, regardless of the geometry of the but-2-enyl unit (eq 2). The selectivity was rationalized with an extended acyclic transition state geometry. This is in contrast to other crotyl organometallic reagents in which the crotyl geometry is linked to the observed stereoselectivity.7

(E)-Crotyltributylstannane has been shown to react faster with aldehyde-Lewis acid complexes than the corresponding (Z) isomer.8 Thus even higher levels of syn selectivity (25:1; 94% yield) can be achieved when 2 equiv of crotylstannane, which consists of a mixture of the (E) and (Z) isomers, is employed in the Boron Trifluoride Etherate-mediated addition to aldehydes (eq 3). A decrease in selectivity to 9:1 is observed when only 1 equiv of stannane is used. Titanium(IV) Chloride-mediated reactions show syn or anti selectivity depending upon the order of addition. For example normal addition (crotyltin added last) shows high syn selectivity (93:7) and only minor amounts of the linear alcohols (<3%). Under reverse addition (stannane addition to TiCl4, followed by aldehyde addition) anti selectivity is observed, again with only minor amounts of the linear alcohols (<5%). The use of chiral Lewis acid catalyst, as opposed to Lewis acid promoters, has also been successfully employed in the stereoselective allylations of aldehydes.9

The effect of addition modes upon product distributions for (E)- and (Z)-crotyltributyltins with various Lewis acids and aldehydes has been investigated.4d,10 The linear (a) homoallylic alcohol adduct with (Z) double bond geometry can be obtained selectively when (Z)-crotyltributylstannane is combined with an aldehyde followed by the addition of n-Butyltrichlorostannane (eq 4).11 The reaction is believed to proceed through transmetallation to form the methallylbutyldichlorotin. The selectivity for the linear homoallylic products is reduced or lost when the order of addition of the aldehyde and BuSnCl3 is reversed or when an (E/Z) mixture of crotylin is used. Similarly, the transmetalation of (E)-g-(phenyl)allyltributyltin with BuSnCl3 in the presence of an aldehyde produces the corresponding linear homoallylic alcohol adduct with (Z) double bond geometry.12

The addition of crotyltributylstannane to aldehydes in the presence of Cobalt(II) Chloride also affords the linear homoallylic alcohols exclusively (yields: 54-74%) (eq 5).13 The regiochemistry is not affected by the steric hinderance of the aldehyde or the order of addition.

In the Lewis acid-mediated additions of crotyltributylstannane to a-alkoxyaldehydes, MgBr2 and TiCl4 provide chelation control diastereofacial selectivity (&egt;99:1) (eq 6). The diastereofacial selectivity for all crotylin additions is higher than the corresponding allyl additions. Both diastereofacial and bond construction selectivity increase with the size of R1 (cyclohexyl > n-butyl > Me). Within the chelation control manifold, syn selectivity for bond construction is &egt;90:10; MgBr2 is the Lewis acid of choice.14

The Lewis acid and hydroxy-protecting group also strongly influence stereoselectivity in the crotylstannane additions to 2-methyl-3-hydroxypropanal derivatives (b-alkoxyaldehydes). The promoter BF3.OEt2 shows high syn diastereofacial selectivity (Cram) with the characteristic syn bond construction selectivity. The syn,syn isomer is formed in 92% yield with high diastereofacial selectivity (18:1) and essentially complete bond construction selectivity when the t-butyldimethylsilyl protecting group is present. Chelation control diastereofacial selectivity (anti) is high for all Lewis acids capable of chelation; however, bond construction selectivity is low. MgBr2 provides the anti,syn isomer with 91:9 anti facial selectivity and 89:11 syn bond construction selectivity.8 Using chelation control conditions, the protected b-alkoxyaldehyde (1) was selectively allylated with crotyltriphenylstannane (as a 1:1 cis/trans mixture) to provide the desired stereoisomer in 60% yield and a 20% yield of a regiomeric crotyl addition product (eq 7).15

High syn diastereofacial control and anti bond construction selectivity is observed, under chelation control conditions, for the additions of b-methylcrotylstannanes to 2-(benzyloxy)-propanal (eq 8).16

The Lewis acid-mediated addition of crotyltributylstannane to pyruvates slightly favors the anti products when the ester group is small; however, syn products dominate as the size of the ester group is increased (eq 9).17 The BF3 etherate-mediated addition to glyoxylate esters is also influenced by the size of the ester group, providing a syn:anti ratio of 90:10 for the isopropyl ester and decreasing to 75:25 for the methyl ester.18 Crotyltin addition to meso-dimethylglutaric hemialdehyde, also mediated by BF3 etherate, shows chelation control syn selectivity. The selectivity is based upon the conformationally rigid substrate, which prefers a chelate-like conformation.19

Iminines and Iminium Ions.

Crotyltributylstannane additions to aliphatic and furyl imines (R1 = i-Pr, Cy, furyl; R2 = Bn) provide addition products with very high syn selectivity (syn:anti > 20:1) (eq 10).20 This high selectivity requires extended Lewis acid-imine complexation at -78 °C (2.5 h) prior to crotylstannane addition, otherwise a modest 4:1 syn selectivity is attained. Crotyltributylstannane additions to aromatic imines (R1 = Ph; R2 = Ph, p-tolyl) occur with much lower diastereoselectivity (syn:anti > 75:25).21

Methallyl- and crotyltin reagents add to pyridine activated by Methyl Chloroformate (i.e. prior iminium salt formation) with poor regioselectivity (a:g = 48:52 to 36:64) (eq 11).22 Allylstannanes add in a highly a-selective fashion (86-99%) while prenyltributylstannane additions occur exclusively at the g-carbon of pyridine. Crotyltin reagents add to larger aromatic N-heterocycles with excellent regioselectivity (>95%) (eq 12).

Conjugate Addition.

Conjugate additions of crotyltributylstannanes to Michael acceptors occur with poor to modest anti selectivity in variable yields (eq 13).23 Tin(IV) Chloride promotion provides the highest selectivity for the crotyltributylstannane addition to an enoate (90:10; 52%). Other Lewis acids provide slightly lower selectivities (>80:20) in higher yields (>94%). Conjugate additions to nitroalkenes occur with very low selectivity (anti:syn = 45:55 to 70:30). Yields are higher for the phenyl-substituted nitroalkene (53-92%) than for the methyl derivative (29-42%). The trialkylsilyl triflate-promoted conjugate addition of allylstannes to a,b-enones affords the corresponding b-alkylated silyl enol ethers in high yields (85-95%).24

Reactions with Orthoamides.

The BF3.OEt2 promoted addition of crotyltributylstannane to orthoamides occurs with complete stereoselectivity (eq 14).25 Interestingly, the TiCl4-promoted reaction provides the other diastereomer exclusively.

Intramolecular Cyclizations.

Intramolecular condensation reactions between allylstannanes and a carbonyl group joined by a one-carbon tether afford silyloxyvinylcyclopropanes.26 A variety of 1-butenyl- and 5-hexenyltributylstannanes undergo intramolecular cyclization in the presence of N-Phenylselenophthalimide and SnCl4.27

Radical Reactions.28

The radical allyl transfer reactions from allyl- or methallyltributylstannane to a variety of substrates under various initiation conditions have been demonstrated to be synthetically useful.29 For example, thiophenyl glycosides are converted into C-gylcosides by reaction with methallyltributylstannane. The stereoselectivity is dependent on the reaction conditions. Using photochemical initiation, the reaction of (2) with methallylbutylstannane gives an 87% yield of the C-glycoside with a 92:8 ratio of a:b anomers (eq 15). In contrast, the reaction in the presence of catalytic (0.2 equiv) tributylstannyl triflate provides a 95% yield preferentially of the b-isomer (99:1).30

Crotylstannanes are less reactive than allyltributylstannane towards carbon-centered radicals. In contrast to allylstannanes, which readily react by radical pathways with alkyl halides, the use of crotylstannane affords only reduction products under a variety of initiation processes.31 However, the successful allyl transfer reaction of crotylstannane has been reported for a bromine-substituted glycine (eq 16).32 Thus a mixture of (E) and (Z) isomers (ca. 1:1) of crotyltributyltin reacted with (3) to produce a 57% yield of the allylated product as a 1:1 mixture of diastereomers. The reduction product and 5-methylallylglycine were not observed in the reaction mixture. Radical SH2 substitution reaction using hetero-centered radicals (RS&bdot;, RSO2&bdot;, and PhSe&bdot;) and crotylstannane has also been demonstrated.33

Irradiation of benzil in the presence of (E)- and (Z)-crotyl, -pent-2-enyl-, or -hex-2-enyltributylstannane affords predominantly the linear homoallylic alcohols in good yields with retention of the double bond configuration.34

Related Reagents.

Allyltributylstannane; Allyltrimethylsilane; Allyltriphenylstannane; trans-Cinnamyltributylstannane; Crotyltrimethylsilane; Tri-n-butyl(1-methoxymethoxy-2-butenyl)stannane; Trimethyl(3-methyl-2-butenyl)stannane.


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Stephen Castellino

Rhône-Poulenc Ag. Co., Research Triangle Park, NC, USA

David E. Volk

North Dakota State University, Fargo, ND, USA



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