[53317-08-1]  · C11H19B  · B-Allyl-9-borabicyclo[3.3.1]nonane  · (MW 162.07)

(allylating reagent for aldehydes, ketones, acid chlorides, amides, and acid anhydrides;2 reagent for diastereo- and enantioselective allylation of aldimines;6,8 diastereoselective allylation of acetals9)

Alternate Name: allyl-9-BBN.

Physical Data: bp 41.5-42 °C/0.05 mmHg.

Solubility: sol most organic solvents.

Preparative Methods: prepared from Allyl Bromide and B-methoxy-9-BBN according to a literature procedure.1 Crotyl-9-BBN and 2-methallyl-9-BBN are similarly prepared from the corresponding allylic bromides.

Handling, Storage, and Precautions: the neat liquid is highly flammable and must be stored in the absence of air and moisture. Allyl-9-BBN may be kept as a CH2Cl2 or THF solution under N2 atmosphere.2 Use in a fume hood.

Allylation of Carbonyl Compounds.2

The reaction of allyl-9-BBN with carbonyl compounds occurs cleanly with the allyl group undergoing transfer to the carbonyl carbon and the boron group to the carbonyl oxygen. Aldehydes and ketones react in a 1:1 stoichiometry, while acid chlorides, esters, and N,N-dimethylamides react with 2 equiv of the allylborane, and acid anhydrides utilize 4 equiv. Simple protonolysis of the boronic esters thus formed, either with water or Ethanolamine, affords excellent yields of the corresponding homoallylic alcohols.2 The allylation of chiral aldehydes with allyl-9-BBN normally produces low diastereoselectivity via nonchelated Cram transition states.3 The Cram/anti-Cram stereoselectivity in the allylation of (1)-(3)3 and (4)4 is 1:1-1.3:1, with only a slight preference for the syn diastereomers.

Allylation of Imines.

In contrast to the reactions with chiral aldehydes, the reactions with certain aldimines are highly or completely syn selective (eq 1).5,6 The aldimine having a chiral center at the a-position of the nitrogen substituent affords the Cram product (syn isomer) with high diastereoselectivity upon treatment with allyl-9-BBN (eq 2).5,6 The chiral a-imino ester, prepared from n-butyl glyoxylate and (S)-phenethylamine, reacts with allyl-9-BBN to give the allylation product with 92% de in 92% yield, which is hydrogenated to the butyl ester of L-norvaline.6,7

Aldimines derived from (2) gave the nonchelation product either exclusively or predominantly, regardless of the substituents at nitrogen (eq 3).8 On the other hand, the aldimine derived from (3) afforded the nonchelation adduct with good diastereoselectivity only when the (R)-phenethylamine group was used as an amine (eq 4).8

Allylation of Acetals.

Treatment of the chiral steroidal acetal (S,R,R) isomer, prepared from the steroidal aldehyde and (2R,4R)-(-)-pentanediol, with allyl-9-BBN in the presence of Titanium(IV) Chloride followed by the usual workup gave the (S,S) isomer exclusively (eq 5).9 On the other hand, the reaction of the (S,S,S) acetal isomer, prepared from the steroid aldehyde and (2S,4S)-(+)-pentanediol, with allyl-9-BBN produced an 88:12 mixture of the (S,S) and (S,R) isomers, suggesting that the 1,2-asymmetric induction is primarily controlled by the Cram rule rather than the chiral acetal template.

The TiCl4-mediated reaction of the chiral acetal (S,R,R) isomer, prepared from (3S)-3-t-butyldimethylsiloxybutanol and (2R,4R)-(-)-pentanediol, with allyl-9-BBN gave the corresponding homoallyl alcohol in 90% yield with predominant formation (90:10) of the chelation product,10 indicating that the 1,3-asymmetric induction is dictated by chelation rather than the acetal template.

1. Kramer, G. W.; Brown, H. C. JOM 1977, 132, 9.
2. Kramer, G. W.; Brown, H. C. JOC 1977, 42, 2292.
3. Yamamoto, Y.; Komatsu, T.; Maruyama, K. JOM 1985, 285, 31.
4. Vara Prasad, J. V. N.; Rich, D. H. TL 1990, 31, 1803.
5. Yamamoto, Y.; Komatsu, T.; Maruyama, K. JACS 1984, 106, 5031.
6. Yamamoto, Y.; Nishii, S.; Maruyama, K.; Komatsu, T.; Ito, W. JACS 1986, 108, 7778.
7. Yamamoto, Y.; Ito, W.; Maruyama, K. CC 1985, 1131.
8. Yamamoto, Y.; Komatsu, T.; Maruyama, K. CC 1985, 814.
9. Yamamoto, Y.; Nishii, S.; Yamada, J. JACS 1986, 108, 7116.
10. Yamamoto, Y.; Yamada, J. CC 1987, 1218.

Yoshinori Yamamoto

Tohoku University, Japan

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