Tri-n-butyl(1-methoxymethoxy-2-butenyl)stannane

[84510-37-2]  · C18H38O2Sn  · Tri-n-butyl(1-methoxymethoxy-2-butenyl)stannane  · (MW 405.27)

(allylation of aldehydes1)

Alternate Names: a-methoxymethoxycrotylstannane; tributyl(1-methoxymethoxy-2-butenyl)tin.

Physical Data: colorless, odorless oil.

Solubility: sol CH2Cl2, THF, ether; insol H2O.

Form Supplied in: not commercially available.

Preparative Methods: racemic forms can be prepared from Crotonaldehyde via addition of Tri-n-butylstannyllithium, followed by protection of the resulting hydroxystannane with Chloromethyl Methyl Ether (MOM-Cl) and Diisopropylethylamine (eq 1).1 Tributyl(1-benzyloxymethoxy-2-butenyl)tin can be similarly prepared.

Optically enriched tributyl(1-methoxymethoxy-2-butenyl)tin can be prepared by asymmetric reduction of the corresponding acyltin, which is obtained by oxidation of the racemic hydroxystannane with 1,1Ž-(Azodicarbonyl)dipiperidine (ADD) or DIAD (diisopropyl azodicarboxylate), (eq 2).1,2 The reduction of acylstannanes by (R)-BINAL-H produces hydroxystannanes in the (S)-configuration in greater than 95% ee. The results of asymmetric induction for acylstannanes are different from those of normal ketones, i.e. the reduction of normal prochiral ketones by (R)-BINAL-H usually produces (R)-alcohols. This fact was explained by Marshall with the assumption of a pentacoordinated tin complex (1).1 a-Ethoxy analogs can be prepared from diethoxymethyltributyltin via treatment with acetyl chloride, followed by a vinyl Grignard reagent (eq 3).3

Handling, Storage, and Precautions: stable over a period of several months if stored in a refrigerator. However, decomposition occurs slowly if exposed to air at rt. It is recommended that tributyl(1-methoxymethoxy-2-butenyl)tins are freshly prepared before use.

Allylation of Aldehydes.

In the presence of Boron Trifluoride Etherate, tributyl(1-methoxymethoxy-2-butenyl)tin undergoes facile addition to aldehydes via an SEŽ mechanism (eqs 4 and 5).1 The major product is the syn-(E)-isomer for aliphatic aldehydes. Comparable results can be achieved using tributyl(1-benzyloxymethoxy-2-heptenyl)tin (eq 6).

In contrast, additions to aromatic aldehydes give products with almost exclusively syn-(Z) stereochemistry (eq 7).4 The reactions occur through an SEŽ mechanism with respect to the a-alkoxyallylstannanes. The aldehydes, when complexed to BF3.Et2O, are sufficiently electrophilic to attack the double bond of the allyltributyltin.

An important feature of the Lewis acid-catalyzed reaction is that the C-C bond formation occurs stereospecifically anti to the Bu3Sn group of the allylic stannanes. This is consistent with steric and stereoelectronic principles.5 Possible transition states producing the different results for aliphatic and aromatic aldehydes have been suggested (eqs 8 and 9).4 Of the two favorable staggered transition states, aliphatic aldehydes slightly prefer the antiperiplanar arrangement in order to avoid gauche interactions between the methyl group and the R1 group. In contrast, aromatic aldehydes prefer the synclinal arrangement due to their strong anti complexation with the Lewis acid.6 In the antiperiplanar transition state, a syn-pentane interaction exists between the methyl group and the BF3. While the loose complexation of aliphatic aldehydes with BF3 tolerates this unfavorable interaction, the tight complexation of aromatic aldehydes with BF3 prefers the synclinal arrangement. The synclinal arrangement allows the inside alkoxy alignment of the methoxymethoxy group. This conformation has been suggested to be favored in electrophilic additions to the allylic system.7

In the absence of a Lewis acid, high temperature is required to complete the reaction of an aldehyde with tributyl(1-methoxymethoxy-2-butenyl)tin.8 Based on the stereochemistry of the product, a cyclic transition state has been suggested for the thermal reactions (eq 10).

Intramolecular Allylations via a-Alkoxyallyltributyltins.

Because of the relative stability of a-alkoxyallyltributyltins towards the aldehyde function at room temperature, the intramolecular version of the allylation reactions has been applied to form macrocycles (eq 11).2,9 These experiments are carried out under highly dilute conditions (ca. 0.001 M). A solution of the substrate (an a-alkoxyallyltributyltin aldehyde) is added dropwise to a solution of BF3.Et2O in CH2Cl2 over several hours. Usually, the reaction is completed when the addition is finished. Consistent high yields are obtained with the substrate shown in eq 11. Several members of the cembranolide family of natural products have been synthesized using this strategy.2

Isomerization of Tributyl(1-methoxymethoxy-2-butenyl)tins.11

Treatment of (S)-tributyl(1-methoxymethoxy-2-butenyl)tin with BF3.Et2O at -78 °C leads to a facile 1,3-stereospecific isomerization to produce (S)-(g-methoxymethoxyallyl)tin (eq 12). Mechanistic studies of the rearrangement have shown that the 1,3-tin migration is intermolecular.10,11 The product of the 1,3-isomerization is a useful reagent for the enantioselective synthesis of carbohydrates.12

Double Diastereofacial Differentiation.

Although the p-facial selectivity of the allylation by tributyl(1-methoxymethoxy-2-butenyl)tin is only moderate on aliphatic aldehydes (eqs 4 and 5), much greater selectivity can be achieved by using a-chiral aldehydes. Excellent p-facial selectivity is obtained when matched pairs of a-chiral aldehydes and (1-methoxymethoxy-2-butenyl)stannanes are allowed to react (eq 13).13 On the other hand, poor selectivity is observed when mismatched pairs are the reacting partners (eq 14).

Kinetic Resolution.

When a racemic mixture of tributyl(methoxymethoxy-2-butenyl)tin is allowed to react with an optically pure a-chiral aldehyde, a single adduct can be isolated along with the recovery of enantiomerically enriched (g-methoxymethoxyallyl)tin (eq 15).13 The (S)-enantiomer of the allylstannane reacts much faster than the (R)-enantiomer with the a-chiral aldehyde to produce the adduct. The (R)-enantiomer rearranges to give the (R)-(g-methoxymethoxyallyl)tin (eq 16).

Summary.

The allylation of aldehydes using tributyl(1-methoxymethoxy-2-butenyl)tin leads to adducts with two new stereocenters. These reactions proceed under mild conditions and in high yields. The stereochemistry of the products can be predicted by controlling the reaction conditions and substrate structures. BF3-catalyzed additions are highly syn selective. In contrast to many other allylic metallic reagents, (1-alkoxyallyl)stannanes are stable and isolable intermediates, making the intramolecular version of the allylation reaction practical.


1. Marshall, J. A.; Gung, W. Y. T 1989, 45, 1043.
2. Marshall, J. A.; Gung, B. W. Isr. J. Chem. 1991, 31, 199.
3. Quintard, J. P.; Dumartin, G.; Elissondo, B.; Rahm, A.; Pereyre, M. T 1989, 45, 1017.
4. (a) Gung, B. W.; Smith, D. T.; Wolf, M. A. T 1992, 48, 5455. (b) Gung, B. W.; Peat, A. J.; Snook, B. M.; Smith, D. T. TL 1991, 32, 453.
5. McGarvey, G. J.; Williams, J. M. JACS 1985, 107, 1435.
6. Gung, B. W.; Wolf, M. A. JOC 1992, 57, 1370.
7. Houk, K. N.; Moses, S. R.; Wu, Y. D.; Rodan, N. G.; Jager, V.; Schohe, R.; Fronczek, F. R. JACS 1984, 106, 3880.
8. Pratt, A. J.; Thomas, E. J. CC 1982, 1115; Jephcote, V. J.; Pratt, A. J.; Thomas, E. J. CC 1984, 800.
9. Marshall, J. A.; Gung, W. Y. TL 1988, 29, 1657.
10. Marshall, J. A.; Gung, B. W. TL 1989, 30, 7349.
11. Marshall, J. A.; Welmaker, G. S.; Gung, B. W. JACS 1991, 113, 647.
12. Marshall, J. A.; Luke, G. P. JOC 1991, 56, 483.
13. Marshall, J. A.; Yashunsky, D. V. JOC 1991, 56, 5493.

Benjamin W. Gung

Miami University, Oxford, OH, USA



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