· (MW 331.11)
(allylating reagent for many compounds, including alkyl halides, carbonyl compounds, imines, acetals, thioacetals, and sulfoximides)
Alternate Name: allyltributyltin.
Physical Data: bp 88-92 °C/0.2 mmHg; fp > 110 °C; d 1.068 g cm-3.
Solubility: sol dichloromethane, diethyl ether, THF, toluene, benzene.
Form Supplied in: colorless liquid; widely available.
Handling, Storage, and Precautions: all organotin compounds are highly toxic.
Allylstannanes are widely used as allyl anion equivalents.1 They are less reactive than the corresponding magnesium or lithium reagents and, hence, can be classified as
storable organometallic reagents.13b This reduced activity increases the ease of handling in the laboratory; however, higher reaction temperatures or activation with Lewis acids are necessary. The relative reactivities of allyltriphenylsilane, -germane, and -stannane with diaryl carbenium ions are 1, 5.6, and 1600, respectively.2 Allyltributylstannane is more reactive than Allyltriphenylstannane by three orders of magnitude. Allyl- and crotyltrialkyltin reagents undergo transmetalation reactions with strong Lewis acids through an SE2´ pathway.3 Competing transmetalation processes can affect the mechanistic pathway
and product distribution.4 Radical processes can also be exploited in allylation reactions employing stannanes.5 Because of the high toxicity of organotin reagents, allyltributyltin is more widely used than the more volatile allyltrimethyltin.
Additions to Aldehydes.
Allyltrialkyltin reagents, such as allyltributyltin (1), react with carbonyl compounds1 to form homoallylic alcohols under photolytic,6 thermal,7 high pressure,8 or, more commonly, Lewis acidic conditions.9 The order of reactivity is aldehydes > methyl ketones > internal ketones. A number of stereochemical issues are important when substituted allylic stannanes are utilized (see Crotyltributylstannane for details).
Selective conversion of protected a-hydroxy aldehydes (2) to monoprotected derivatives of syn- or anti-1,2-diols by reaction with allyltrialkylstannanes is realized with judicious choice of Lewis acid and protecting group.10 Magnesium Bromide, Titanium(IV) Chloride, and Zinc Iodide favor syn products (3), especially with the benzyloxy derivative (2a), while use of Boron Trifluoride Etherate favors the anti products (4), particularly with the t-butyldimethylsilyl ether (2b) (eq 1).
The Tin(IV) Chloride promoted allylation of a-methylthio aldehydes with allyltriphenyltin is highly selective for anti products, while the selectivity from BF3.OEt2 mediation is variable.11 A very mild promoter system, 5 M Lithium Perchlorate in diethyl ether, was used to allylate dialdose derivatives with high selectivity.12 Acetals, ethers, and silyl ethers survive the allylation of aldehydes with this promoter.
b-Alkoxy aldehydes, with alkyl groups at C-2, readily form stable chelates with TiCl4, SnCl4, and MgBr2 and consquently show high levels of anti selectivity in allylations with allyltributyltin.13 High levels of diastereofacial selectivity in the Lewis acid mediated additions of allylstannanes to b-alkoxy aldehydes with substituents at C-3 are achieved when (a) the protecting group permits effective bidentate chelation between the aldehyde carbonyl and the ether oxygen and (b) the protecting group provides enough steric bulk to force C-3 substituents into an axial position in the six-membered chelate formed with the Lewis acid. TiCl4 shows the highest anti selectivity when the protecting group is benzyl (R = n-hexyl; 96:1) and poor
selectivity for methyl protection (R = n-hexyl; 3.8:1) (eq 2). SnCl4 provides poor selectivities for all C-3 alkyl substituted b-alkoxy aldehydes. These results are consistent with predictions based upon ground state solution structures which show that the preferred conformation for TiCl4 and MgBr2 chelates has the alkyl group in a pseudoaxial position when the protecting group is ethyl or benzyl. Chelation is not involved in the reactions of a-or b-siloxy aldehydes.14 In TiCl4 promoted allyltriphenylstannane additions to b-alkoxyaldehydes with a methyl group at C-3, benzyl protection provides superior anti selectivity (29:1) to (methylthio)methyl (2:1) and (benzyloxy)methyl (9:1) groups.15
Allylation of Ketones.
While irradiation of mixtures of aromatic ketones and allyltrialkylstannanes usually affords coupling products which are allylated at the carbonyl carbon,6a,b selective allylation at the a-carbon of aromatic a,b-epoxy ketones is observed (eq 3).16 Yields of the a-allyl-b-hydroxy aryl ketones are highest when the para substituent is an electron-withdrawing group (CN) and lowest when it is an electron donor (MeO). Quinones undergo 1,4-monoallylation with allyltributyltin in the presence of BF3 etherate (eq 4). However, 4-substituted 1,2-naphthoquinones and sterically hindered 3,5-di-t-butyl-o-quinones undergo 1,2-addition (eq 5).17
Allyltributylstannane in the presence of BF3 etherate is a more efficient a-allylating reagent for quinones than the allylsilane-TiCl4 reagent system. Eleutherin and isoleutherin were synthesized, in part, by this method.18
Unsymmetrical aryl alkyl a-diketones are regioselectively allylated by allylstannanes at the benzylic carbon under photolytic conditions and allylated at the acyl carbon in the presence of BF3 etherate (eq 6).19 Stannylated cyclopentanes are formed from the reaction of allyltributylstannane with aluminum trichloride-activated a,-b-unsatured acyliron complexes.20 Stereochemistry about the alkene is preserved in this reaction.
Allylation of Organohalides.
Alkyl halides21 and selenides22 are allylated by allylstannes under thermal (with Azobisisobutyronitrile), photochemical (with a tungsten lamp), or palladium-catalyzed conditions in high yield (eqs 7 and 8). Palladium catalyzes many reactions of allyltin reagents with various electrophiles, including allyl halides, aryl iodides and bromides, activated aryl chlorides, acid chlorides, vinyl halides, vinyl triflates, a-halo ketones and esters, and a-halo lactones.23
Aliphatic, aromatic, and heterocyclic acid chlorides react with allyltrialkylstannanes to give ketones in high yield (eq 9). Functional groups such as nitro, nitrile, haloaryl, methoxy, ester, and aldehyde are tolerated. An alternative palladium-catalyzed ketone synthesis involves the coupling of primary, secondary, or tertiary halides with carbon monoxide and allyltin (eq 10). Allyltributyltin adds to a-alkoxy-b-siloxy acylsilanes, with high syn selectivity (syn:anti = 91:9) in the presence of Zinc Chloride.24 A monoprotected syn-1,2,3 triol results from protiodesilylation. The palladium-catalyzed reaction of a-halo ketones with acetonyl- and allylstannanes produces oxiranes, oxetanes,
and tetrahydrofurans in good yield.25 Most allylic acetates do not react, although cinnamyl acetate and allyl acetate are exceptions.
The reaction between phenyl triflates (a vinyl triflate) and allylic stannanes is useful for the synthesis of substituted aromatic compounds (eq 11).26 The reaction works well with most highly substituted phenols except for hexasubstituted ones.27 The reaction has been extended to the less expensive aryl fluorosulfonates28 and aryl arenesulfonates.29 These reactions proceed in good yield unless the aryl ring contains electron-donating substituents.
Allylation of Acetals.
In the presence of a Lewis acid, 1,3-dioxolanes can be allylated with allyltributylstannanes or allylsilanes.30 The Lewis acid promoted cleavage of chiral acetals with allylstannes affords chiral ethers with reported diastereoselectivites of > 500:1 (eq 12).31 Allylation of monothioacetals and dithioacetals occurs in a highly syn selective fashion to form homoallyl sulfides in good yield, particularly with GaCl3 as the Lewis acid promoter.32
Allylation of Imines.
Aldimines are converted to homoallylamines by allyltributyltin with Lewis acid promotion in moderate to high yield.33 Likewise, b-methyl homoallylamines (predominantly syn) result from the reaction of Crotyltributylstannane with the TiCl4 chelate of aldimines. In the TiCl4-mediated allylstanne addition to (5), the Cram product is favored (92:8) (eq 13).34
Allyltributyltin is also useful for a-allylations of N-acyl heterocycles, including pyridinium salts.35 Isoquinolines (or dihydroisoquinolines) can be simultaneously acylated and allylated by the addition of a,b,g,d-unsaturated acyl chloride and allyltributyltin. The resulting adduct undergoes a Diels-Alder cyclization yielding an isoquinoline alkaloid precursor (eq 14).36 Acylation and allylation of imidazoles is a particularly useful route to highly substituted 2-allylimidazolines (eq 15).37
Acylimininium ions, formed by the reaction of a-alkoxy carbamates with Lewis acids, undergo allyl transfer from allylstannanes or silanes (eq 16).38
Allylation of Sulfoximidoyl Chlorides.
A variety of S-allylsulfoximines can be synthesized in high yield by the allylation of sulfoximidoyl chlorides (eq 17).39 Thiocarbonates are also allylated under photolytic conditions.40
In addition to ionic pathways, radical processes can also be employed in allylations using stannanes.5,21 The 1,2-asymmetric induction in radical allylations of a-alkoxycarbonyl radicals has been investigated. The observed selectivities, ranging from 1:1 to 99:1, are consistent with transition-state models which incorporate favorable stereoelectronic effects and the minimization of A1,2, A1,3, and torsional strain.41 The camphorsultam derivative (6) undergoes thermal allylation (10% AIBN, 80 °C, benzene) with stannanes to give the allylated products in excellent yield with diastereoselectivities of 12:1 (eq 18).42 Allyl transfer to quinones and a,b-epoxy ketones by single electron transfer pathways has also been investigated.43
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- 4. (a) Keck, G. E.; Castellino, S.; Andrus, M. B. Selectivities in Lewis Acid Promoted Reactions, Schinzer, D., Ed.; Kluwer: Dordrecht, 1989; pp 73-105. (b) Keck, G. E.; Andrus, M. B.; Castellino, S. JACS 1989, 111, 8136. (c) Denmark, S. E.; Wilson, T.; Wilson, T. M. JACS 1988, 110, 984. (d) Boaretto, A.; Marton, D.; Tagliavini, G.; Ganis, P. JOM 1987, 321, 199. (e) Yamamoto, Y.; Maeda, N.; Maruyama, K. CC 1983, 742. (f) Quintard, J. P.; Elissondo, B.; Pereyre, M. JOC 1983, 48, 1559.
- 5. (a) Pereyre, M.; Quintard, J. P.; Rahm, A. Tin in Organic Synthesis; Butterworths: London, 1987. (b) Yamamoto, Y. T, 1989, 45, 909. (c) Ref. 1f.
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- 14. Keck, G. E.; Castellino, S. TL 1987, 28, 281.
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- 16. Hasegawa, E.; Ishiyama, K.; Horaguchi, T.; Shimizu, T. TL 1991, 32, 2029.
- 17. (a) Maruyama, K.; Takuwa, A.; Naruta, Y.; Satao, K.; Soga, O. CL 1981, 47. (b) Maruyama, K.; Naruta, Y. CL 1978, 431. (c) Naruta, Y.; Maruyama, K. CL 1979, 885. (d) Naruta, Y.; Maruyama, K. CL 1979, 881.
- 18. Naruta, Y.; Uno, H.; Maruyama, K. CC 1981, 1277.
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- 23. For a review see: Stille, J. K. AG(E) 1986, 25, 508.
- 24. Cirillo, P. F.; Panek, J. S. JOC 1990, 55, 6071.
- 25.Pri-Bar, I.; Pearlman, P. S.; Stille, J. K. JOC 1983, 48, 4629.
- 26. (a) Stille, J. K. PAC 1985, 57, 1771. (b) Scott, W. J.; McMurray, J. E. ACR 1988, 21, 47. (c) Ref. 24.
- 27. Saá, J. M.; Martorell, G.; García-Raso, A. JOC 1992, 57, 678.
- 28. Roth, G. P.; Fuller, C. E. JOC 1991, 56, 3493.
- 29. Badone, D.; Cecchi, R.; Guzzi, U. JOC 1992, 57, 6321.
- 30. (a) Yamamoto, Y.; Abe, H.; Nishii, S.; Yamada, J. JCS(P1) 1991, 3253. (b) Denmark, S. E.; Almstead, N. G. JOC, 1991, 56, 6458.
- 31. (a) Sammakia, T.; Smith, R. S. JACS 1992, 114, 10998. (b) Denmark, S. E.; Almstead, N. G. JOC, 1991, 56, 6485.
- 32. (a) Sato, T.; Otera, J.; Nozaki, H. JOC 1990, 55, 6116. (b) Saigo, K.; Hashimoto, Y.; Kihara, N. CL, 1990, 1097.
- 33. Keck, G. E.; Enholm, E. J. JOC 1985, 50, 146.
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- 35. (a) Yamaguchi, R.; Moriyasu, M.; Yoshioka, M.; Kawanisi, M. JOC 1985, 50, 287. (b) Yamaguchi, R.; Moriyasu, M.; Yoshioka, M.; Kawanisi, M. JOC 1988, 53, 3507.
- 36. Yamaguchi, R.; Otsuji, A.; Utimoto, K. JACS 1988, 110, 2186.
- 37. Itoh, T.; Hasegawa, H.; Nagata, K.; Okada, M.; Ohsawa, A. TL 1992, 33, 5399.
- 38. (a) Yamamoto, T.; Schmid, M. CC 1989, 1310. (b) Yamamoto, Y. Sato, H.; Yamada, J. SL 1991, 339. (c) Wanner, K. T.; Wadenstorfer, E. Kärtner, A. SL 1991, 797.
- 39. Harmata, M.; Claassen, R. J. II TL 1991, 32, 6497.
- 40. Kelly, M. J.; Roberts, S. M. JCS(P1), 1991, 787.
- 41. (a) Hart, D. J.; Krishnamurthy, R. JOC 1992, 57, 4457. (b) Hart, D. J.; Krishnamurthy, R. SL, 1991, 412.
- 42. Curran, D. P.; Shen, W.; Zhang, J.; Heffner, T. A. JACS 1990, 112, 6738.
- 43. (a) Maruyama, K.; Imahori, H. BCJ 1989, 62, 816. (b) Hasegawa, E.; Ishiyama, K.; Horaguchi, T.; Shimizu, T. TL 1991, 32, 2029.
Rhône-Poulenc, Research Triangle Park, NC, USA
David E. Volk
North Dakota State University, Fargo, ND, USA
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