The origins of stereoselectivity in asymmetric reductions with boranes based on (+)-α-pinene - II. The geometries of competing transition-states and the nature of the reaction. A semiempirical study

Tetrahedron Asymmetry

Volumn 8, Issue 8, 1997, Pages 1287-1303

  Rogic, Milorad M ^a,d   Ramachandran, P Veeraraghavan ^b   Zinnen, Herman ^c   Brown, Leo D ^c   Zheng, Manli ^c  

^a Molec Modeling Research Institute   (United States)

^b PURDUE UNIVERSITY   (United States)

^c CAChe Scientific   (United States)

^d NONE   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

ALKANOL;

ARTICLE; CALCULATION; CHIRALITY; DRUG SYNTHESIS; ENTHALPY; PRIORITY JOURNAL; REACTION ANALYSIS; REDUCTION; STEREOCHEMISTRY;

EID: 0343035692     PISSN: 09574166     EISSN: None     Source Type: Journal    
DOI: 10.1016/S0957-4166(97)00106-7     Document Type: Article

References (60)

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42. 2b See also ref. 4f.
43. (b) The arguments why the reaction pathways based on the C and F conformations are not important were presented in reference 7.
44. (a) For the distinction between a conformation and a conformational isomer or conformer, see Eliel, E. L.; Wilen, S. H. Stereochemistry of Organic Compounds John Wiley and Sons, New York, 1994, p. 597.
45. (b) p. 1200.
46. Eliel, E. L. Stereochemistry of Carbon Compounds McGraw-Hill, New York, 1962, pp. 149-152.
47. 7 it is assumed that the relative calculated energies in the gas phase would reasonably approximate those in solution or under neat conditions.
48. (b) For a brief discussion how (in a relatively simple molecule) various factors contribute to torsional potential, see ref. 13a, p. 614 and references therein.
49. Li, Y.; Houk, K. N. J. Am. Chem. Soc. 1989, 111, 1236. In his ab initio calculations of transition-states for the reaction of formaldehyde with allylborane and allylboronic acid, Houk also identified an allylborane-aldehyde complex in which 'the boron is coordinated with an in-plane lone-pair on oxygen in both the complex and in the chair transition structure'. In this complex too the pyramidalization at the boron center was already in progress, while there was no appreciable change at the carbonyl carbon. See also ref. 9g.
50. From the electronic point of view the organoborane reductions could be considered (4+2) processes where the borane agent and the carbonyl compound appear (by analogy to other MPV type reductions from eq 1), as 'diene' and dienophile or as a nucleophile and electrophile. For example, according to AM1, the HOMO/LUMO energies of B-f-Bu-IpcBCl and acetophenone are -10.91/-4.07 eV and -11.28/-8.74 eV, respectively, indicating it is the borane's HOMO that interacts with the carbonyl's LUMO and, therefore, the initial charge-dipole interaction involves in-plane n electron-pair on oxygen rather than the Π-electron-pair.
51. (a) Zefirov, N. S.; Palyulin, V. A. Zh. Org. Chem. 1979, 15, 1098.
52. (b) Zefirov, N. S. Tetrahedron 1977, 33, 2719.
53. (a) Curtin, D. Y. Rec. Chem. Prog. 1954, 15, 111.
54. (b) For extensive discussion of the conformation/reactivity relationships, the Curtin-Hammett principle, and kinetic analyses, see Seeman, J. I. Chem. Rev. 1983, 83, 83.
55. (c) Winstein, S.; Holness, N. J. J. Am. Chem. Soc. 1955, 77, 5562.
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57. (e) Hutchins, R. O. J. Org. Chem. 1977, 42, 920.
58. For the discussion of the 'boundary conditions' where the Curtin-Hammett treatment and Winstein-Holness equation apply, see ref. 18b and ref. 13a, p. 649.
59. By 'simple nucleophiles' we mean here structures lacking reactive prostereogenic centers.
60. Wiegers, A.; Scharf, H. D. Tetrahedron: Asym. 1996, 7, 2303. These authors also concluded that the overall reduction is a two-step process and presented the evidence for the involvement of the carbonyl/borane complexes in the first step of the reaction.