[77882-24-7]  · C30H51B  · Dilongifolylborane  · (MW 422.55)

(reagent for asymmetric hydroboration of prochiral alkenes1,2)

Alternate Name: Lgf2BH.

Physical Data: mp 160-161 °C (sealed evacuated capillary).

Solubility: sparingly sol common organic solvents, i.e. THF, CCl4, CH2Cl2, CHCl3. The suspended material is active.

Form Supplied in: white solid, mp 161-163 °C.

Preparative Method: although now commercially available, the reagent is readily prepared by hydroboration of (+)-longifolene with Borane-Dimethyl Sulfide (BH3.SMe2) in a 2:1 ratio (eq 1).

Handling, Storage, and Precautions: guidelines for handling air- and moisture-sensitive reagents should be followed.1c The pure reagent is described as an irritant.

Asymmetric Hydroboration.2

Dilongifolylborane (Lgf2BH) is a chiral dialkylborane intermediate in steric requirement between the two widely investigated chiral organoboranes derived from a-pinene: Monoisopinocampheylborane (IpcBH2)3 and Diisopinocampheylborane (Ipc2BH).4

Due to the hindered exo face environment of the longifolene double bond, the addition of borane proceeds exclusively from the endo face. Even in the presence of excess longifolene, hydroboration stops cleanly at the dialkylborane stage.

The reported levels of asymmetric induction achieved with this reagent in the hydroboration-oxidation of representative alkenes are in the range of 59-78% ee for cis and 45-75% ee for trisubstituted alkenes. The highest levels of asymmetric induction have been recorded for cis-2-butene (eq 2) and 2-methyl-2-pentene (eq 3).2

Trisubstituted alkenes such as 2,4,4-trimethyl-2-pentene and 1-phenylcyclopentene, which apparently exceed the steric requirements of Lgf2BH, fail to react after 4 days at 35 °C.

Although the ready availability of only the (+)-longifolene enantiomer potentially limits the use of Lgf2BH as a reagent for natural product synthesis, this form proved appropriate for the preparation of an optically enriched intermediate in a synthesis of the naturally occurring enantiomer of verrucarinic acid (eq 4).5

(+)-Longifolene of >99% ee can be liberated from Lgf2BH which has been diastereomerically enriched by recrystallization from THF at 0 °C (eq 5).6

1. (a) Brown, H. C.; Jadhav, P. K.; Mandal, A. K. T 1981, 37, 3547. (b) Brown, H. C.; Jadhav, P. K. In Asymmetric Synthesis; Morrison, J. D., Ed.; Academic: New York, 1983; Vol. 2, Chapter 1. (c) Brown, H. C.; Kramer, G. W.; Levy, A. B.; Midland, M. M. In Organic Synthesis via Boranes; Wiley: New York, 1975. (d) Smith, K.; Pelter, A. COS 1991, 8, Chapter 3.10.
2. Jadhav, P. K.; Brown, H. C. JOC 1981, 46, 2988.
3. Brown, H. C.; Yoon, N. M. JACS 1977, 99, 5514.
4. Brown, H. C.; Zweifel, G. JACS 1961, 83, 486.
5. Herold, P.; Mohr, P.; Tamm, C. HCA 1983, 66, 744.
6. Jadhav, P. K.; Vara Prasad, J. V. N.; Brown, H. C. JOC 1985, 50, 3203.

Robert P. Short

Polaroid Corporation, Cambridge, MA, USA

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