[100701-75-5]  · C15H29BSi  · 9-[1-(Trimethylsilyl)-2(E)-butenyl]-9-borabicyclo[3.3.1]nonane  · (MW 248.34)

(anti-(Z)-selective allylation reagent for aldehydes and pyruvates;2 stereoselective synthesis of (Z)-2-butenylsilane1)

Alternate Name: 9-trimethylsilylcrotyl-9-borabicyclo[3.3.1]nonane.

Physical Data: bp 85-87 °C/0.03 mmHg.

Solubility: sol most organic solvents.

Preparative Methods: prepared as described by Yatagai et al.1

Handling, Storage, and Precautions: the neat liquid must be handled and stored in the absence of air. May be kept as CH2Cl2 or THF solutions under a N2 atmosphere. Use in a fume hood.

Anti-(Z)-Selective Crotylation of Aldehydes.

The reaction of a-trimethylsilyl-substituted crotyl-9-BBN with aldehydes in the presence of Pyridine affords anti-homoallyl alcohols having (Z) double bond geometry (eq 1).2 The use of 1 or 2 equiv of pyridine is essential to obtain high stereoselectivity. Without pyridine, a complex mixture of products, coupled both at the a- and g-position of the crotyl reagent, are obtained. Perhaps pyridine coordinates the boron atom to form the corresponding ate complex, which prevents allylic rearrangement of the crotylboron. The reaction with aldehydes proceeds through a six-membered chair transition state in which the trimethylsilyl group adopts an axial position, giving (Z) geometry of the double bond in the homoallyl alcohol products. The anti-(Z)-alkenylsilanes thus obtained are useful for the further elaboration of complex molecules. For example, epoxidation with m-Chloroperbenzoic Acid proceeds with high diastereoselectivity to give the corresponding epoxides in good yield (eq 2).

The reaction of methyl pyruvate with a-silyl-substituted crotyl-9-BBN gives the anti-(Z)-adduct as a single diastereoisomer in 60% yield along with the a-adduct (35%) (eq 3).3 n-Butyllithium is used instead of pyridine for converting the crotylboron reagent to the corresponding ate complex. The anti-(Z)-adduct can be converted into cis-crobarbatic acid methyl ester via the procedure4 of Magnus (eq 4). The reaction of simple crotyl-9-BBN with pyruvates produces the anti-isomers either predominantly or exclusively. The anti diastereoselectivity can be improved by increasing the steric bulk of the ester groups; anti/syn = 73:27 in the case of the methyl ester, whereas the ratio becomes 100:0 in the case of the 2,6-di-t-butyl-4-methylphenyl ester. However, as mentioned above, the a-silyl-substituted crotyl-9-BBN produces a 100:0 selectivity even in the case of the methyl ester.

a-Trimethylstannylallyl-9-BBN, prepared by a procedure similar to the synthesis of a-trimethylsilylallyl-9-BBN, also exhibits anti-(Z) diastereoselectivity in reactions with aldehydes and pyruvates, as observed in the case of the silyl reagent.

Synthesis of (Z)-2-Alkenylsilanes.

Protonolysis of 2-alkenyl-1-trimethylsilyl-9-BBN affords (Z)-2-alkenylsilanes with high stereoselectivity (eq 5).1 Similarly, protonolysis of 2-alkenyl-1-trimethylstannyl-9-BBN gives (Z)-2-alkenylstannanes.

Related Reagents.


1. Yatagai, H.; Yamamoto, Y.; Maruyama, K. JACS 1980, 102, 4548.
2. Yamamoto, Y.; Yatagai, H.; Maruyama, K. JACS 1981, 103, 3229.
3. (a) Yamamoto, Y.; Komatsu, T.; Maruyama, K. CC 1983, 191. (b) Yamamoto, Y.; Maruyama, K.; Komatsu, T.; Ito, W. JOC 1986, 51, 886.
4. Ehlinger, E.; Magnus, P. JACS 1980, 102, 5004.

Yoshinori Yamamoto

Tohoku University, Sendai, Japan

Copyright 1995-2000 by John Wiley & Sons, Ltd. All rights reserved.