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. Author manuscript; available in PMC: 2010 Feb 1.
Published in final edited form as: Acta Crystallogr C. 2004 Feb 10;60(Pt 3):o173. doi: 10.1107/S0108270104000666

A 1,3,2-oxazaborolidine dimer derived from (S)-α,α-diphenylprolinol

Margarita Ortiz-Marciales a,‡, Melvin De Jesús a, Eduvigis Gonzáles a, Raphael G Raptis b, Peter Baran b,*
PMCID: PMC2814345  NIHMSID: NIHMS160417  PMID: 15004373

The reaction of (S)-α,α-diphenylprolinol with an excess of borane–tetrahydrofuran complex yields a stable crystalline material with the composition C34H38B2N2O2, which features a borane adduct of a spirocyclic structure with two oxazaborolidine rings joined by a central tetrahedral B atom. This dimeric oxazaborolidine complex, 3,3,3′,3′-tetraphenyl-1,1′-spirobi(3a,4,5,6-tetrahydro-3H-pyrrolo[1,2-c][1,3,2]oxaza-borole)–7-borane, is the dominant product under various reaction conditions; its crystal structure is consistent with 11B, 1H and 13C NMR and IR analyses.

Comment

Oxazaborolidines derived from chiral β-amino alcohols are regarded as important catalysts for the enantioselective reduction of C==O and C==N bonds (Corey & Helal, 1998; Deloux & Srebnik, 1993; Singh, 1992; Puigjaner et al., 1999), and for other enantioselective organic transformations (Wallbaum & Martens, 1992; Joshi et al., 1989). The 1,3,2-oxazaborolidine derived from the (S)-α,α-diphenyl prolinol, (Scheme), known as the Corey, Bakshi and Shibata (CBS) catalyst, (I), is the most efficient for the reduction of aromatic ketones (Corey et al., 1987; Mathre et al., 1991). The CBS reagent is reported to be extremely sensitive to air and moisture and difficult to purify. Therefore, it is usually prepared in situ, without characterization, and used as a reagent or catalyst in many enantioselective transformations (Corey & Helal, 1998; Deloux & Srebnik, 1993; Singh, 1992; Yadav et al., 2000; Garrett et al., 2002). Pure CBS oxazaborolidine was isolated and characterized by Corey and coworkers (Corey et al., 1987), who prepared the reagent by refluxing the corresponding chiral amino alcohol with an excess of borane–tetrahydrofuran (THF) complex under a closed argon–borane atmosphere system. However, 11B NMR and FT–IR spectroscopic analyses indicated the presence of an additional unidentified dimeric species [δTHF(0.17 M) 7.6 p.p.m., d, JBH = 130 Hz, νBH,THF(0.1 M) 2413 cm−1]. On the other hand, Mathre and co-workers (Mathre et al., 1993) have attempted to prepare the CBS reagent by the same method without success. Moreover, they did not observe the dimmer reported by Corey and co-workers, but instead proposed a different dimeric complex, (II). In addition, further dimeric forms or aggregates for different oxazaborolidines have been obtained (Lang et al., 1997) and studied by quantum molecular modelling calculations (Nevalainen, 1994). Thus, further experimental study is needed to corroborate the structure of the possible dimeric forms of the CBS reagent to establish the true nature of the catalyst. In our effort to prepare the CBS reagent for a subsequent reaction, we isolated a remarkably stable crystalline sample, m.p. 477–483 K, which was identified by 11BNMR(δ, CDCl3, 10.5 and –14.4 p.p.m) and FT–IR (νBH 2321 cm−1) as complex (II) (Mathre et al., 1993). The structure of this complex has now been confirmed by single-crystal X-ray diffraction analysis. The structure shows very interesting features and is presented in this paper.

graphic file with name nihms160417f2.jpg

The bond lengths in the oxazaborolidine rings of (II) (Table 1) are mostly in accord with interatomic distances in similar compounds. However, the B–N distances are shorter than expected. The B1–N1 distance [1.590 (4) Å] is the second shortest reported B–N distance in structurally characterized oxazaborolidines. The only shorter one [1.486 or 1.489 Å] was reported for (S)-diphenylprolinolmethyl-borolidine–borane (Corey et al., 1992; Mathre et al., 1993). This is also the only oxazaborolidine-containing N-boryl group, and thus it is suitable for comparison with the title compound, (II). Oxazaborolidines with H instead of a boryl group have significantly longer B–N distances [1.626–1.660 Å; Rettig & Trotter, 1977; Mathre et al., 1991). The B1–N2 distance in (II) [1.636 (4) Å] its into this range well.

Table 1.

Selected geometric parameters (Å, °).

B1–O2 1.433 (3) O2–C10 1.420 (3)
B1–O1 1.436 (3) N1–C4 1.501 (3)
B1–N1 1.590 (4) N2–C9 1.499 (3)
B1–N2 1.636 (4) C4–C5 1.571 (3)
B2–N1 1.618 (3) C9–C10 1.543 (4)
O1–C5 1.426 (3)
O2–B1–O1 111.84 (19) O1–B1–N2 111.8 (2)
O2–B1–N1 116.4 (2) N1–B1–N2 110.32 (18)
O1–B1–N1 104.25 (19) C4–N1–B1 104.39 (19)
O2–B1–N2 102.41 (19) C9–N2–B1 103.60 (18)
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On the other hand, B–NH distances from all three known structures are shorter than the B–N distances for compounds with monosubstituted N in nonconjugated oxazaborolidine rings, which span the range 1.643–1.693 Å (Trujillo et al., 1998; Hopfl et al., 1997, 1998; Rico et al., 1999). If no substituent is located on the N, typical B–N distances are longer [1.607–1.658 Å; Low et al., 2000; Trujillo et al., 1998; Rettig & Trotter, 1973, 1974, 1976; Hopfil et al., 1997; Cynkier & Hope, 1978; Gravelle & Bott, 1995; Cox & Wardell, 2002). If two are present, the range is 1.670–1.744 Å (Rettig & Trotter, 1983; Huskens et al., 1998; Hopfl et al., 1997, 1998; Ebeling et al., 1989; Short & Masamune, 1989).

The B–O distance in oxazaborolidine seems to be less sensitive to the character of the N or B in the ring. Typical B–O distances span the range of 1.450–1.547 Å, with the exception of (S)-diphenylprolinolmethylborilidine–borane [1.349 or 1.335 Å; Corey et al., 1992; Mathre et al., 1993]. Both B–O distances in (II) are identical to within the s.u. [1.433 (3) and 1.436 (3) Å] and apparently not affected by the different N environment. The N–B–O angles seem to be sensitive to the oxazaborolidine substituents. While compounds with a nonconjugated oxazaborilidine ring have N–B–O angles in the range 95.9–100.7° and do not show dependence on N nor on B substituents, pyrrolooxazaboroles with H on N show smaller N–B–O angles [98.5–99.8°] than (S)-diphenylprolinolmethylborilidine–borane [110.3 or 111.3°]. Values for the N–B–O angles of (II) are not sensitive towards N substituents [102.41 (19) and 104.25 (19)°] and are similar to those found in other oxazaborilidine dimers (Lang et al., 1997; Kim et al., 1999; Chi et al., 1999, 2001). The observed B2–N1 distance of 1.618 (3) Å falls into the range of typical B–N distances [1.591–1.649 Å].

Both oxazaborolidine rings in (II) are distorted from planarity, as expected, and form a dihedral angle of 87.81 (12)°. Molecules of (II) do not show any intra- or intermolecular hydrogen bonds. The closest intermolecular contacts were found between H atoms (2.23–2.47 Å). The closest contacts between H and non-H atoms were found in the range 2.84–2.99 Å.

Experimental

A solution of (S)-(–)-α,α-diphenyl-2-pyrrolidinemethanol (3.95 mmol, 1.00 g) in dry tetrahydrofuran (THF; 10 ml) was added slowly (20 min) to a stirred solution of BH3–THF (10.3 ml, 1.0 M, 10.3 mmol) at ambient temperature. The colourless mixture was stirred overnight at 298 K and concentrated under vacuum leaving a white powder, which was then heated gradually from 313 K to 403–408 K in an oil bath for 45 min. After dissolving the white solid residue in dry THF (15 ml) and cooling the mixture to 195 K, a solution of n-BuLi (1.56 ml, 2.85 M in hexane, 4.46 mmol) was added dropwise. The blue mixture was stirred overnight at 298 K. After a mild acid hydrolysis with solid ammonium chloride at 273 K for 3 h, the solid was removed using a Schlenk filter and the filtered solution was concentrated under vacuum. The white solid was heated in an oven at 453 K and 0.8 mmHg (1 mm Hg = 133.322 Pa) and then the solid in the distillation flask was partially dissolved in dry CH2Cl2. After evaporation of the solvent, a colourless powder with a hard crystalline material appropriate for X-ray diffraction study was obtained.

Crystal data

C34H38B2N2O2 Dx = 1.191 Mg m−3
Mr = 528.28 Mo Kα radiation
Monoclinic, P21 Cell parameters from 911
a = 11.455 (2) Å   reflections
b = 9.1008 (17) Å θ = 2.7–27.9°
c = 14.779 (3) Å μ = 0.07 mm−1
β = 106.993 (3)° T = 298 (2) K
V = 1473.4 (5) Å3 Prism, colourless
Z = 2 0.40 × 0.31 × 0.12 mm
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Data collection

Bruker SMART 1 K CCD area-
  detector diffractometer		 3774 independent reflections
2974 reflections with I > 2σ(I)
φ and ω scans Rint = 0.025
Absorption correction: multi-scan θmax = 28.1°
   (XPREP; Sheldrick, 1990) h = −15 → 12
   Tmin = 0.974, Tmax = 0.991 k = −12 → 10
10 262 measured reflections l = −18 → 19
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Refinement

Refinement on F2 w = 1/[σ2(Fo2) + (0.063P)2
R[F2 > 2σ(F2)] = 0.044    + 0.1498P]
wR(F2) = 0.126     where P = (Fo2 + 2Fc2)/3
S = 1.10 (Δ/σ)max < 0.001
3774 reflections Δρmax = 0.16 e Å −3
362 parameters Δρmin = −0.19 e Å −3
H-atom parameters constrained
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Compound (II) is chiral, but its absolute configuration could not be determined because of a lack of heavier atoms in the molecule. Friedel pairs were merged before the final refinement. However, the absolute configurations of atoms C4 and C9 can be assigned as (S), based on the (S)-α,α-diphenylprolinol used as a starting material. The H atom on N was located from a difference map, fixed at an N–H distance of 0.89 Å, refined and then fixed. Other H atoms were positioned geometrically and treated as riding, with C–H distances in the range 0.93–0.98 Å and B–H distances of 0.96 Å, and with Uiso(H) = 1.2Ueq(C) or 1.5Ueq(B).

Data collection

SMART-NT (Bruker, 1998); cell refinement: SMART-NT; data reduction: SAINT-Plus (Bruker, 1999); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL-NT (Bruker, 1998); software used to prepare material for publication: SHELXTL-NT.

Supplementary Material

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Figure 1.

Figure 1

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A view of (II) with the atomic numbering scheme. Displacement ellipsoids are drawn at the 30% probability level.

Acknowledgments

Financial support from the National Institutes of Health through their MBRS–SCoRE (GM 08216) and NIH–AREA (GM 59829) grants is greatly appreciated. The support of the NSF–MRI (01–07) for the acquisition of a Bruker ADVANCE 400 MHz NMR spectrometer is also gratefully acknowledged.

Footnotes

Supplementary data for this paper are available from the IUCr electronic archives (Reference: FR1462). Services for accessing these data are described at the back of the journal.

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Supplementary Materials

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