1,5-Asymmetric induction during nucleophilic additions to arenetricarbonylchromium complexes: tricarbonyl(eta(6)-1-methyl-4-{spiro[(1R,2S)-1,7,7-trimethylbicyclo[2.2.1]heptane-3,2'-1,3-dioxolan]-2-yloxy}benzene)chromium.

The tricarbonylchromium unit bound to the arene ring of the chiral title complex, [Cr(C(19)H(26)O(3))(CO)(3)], is rotated by ca 25 degrees in agreement with the proposed mechanism for 1,5-asymmetric induction of nucleophilic attack.

Comment

Arenes (Kündig & Pape, 2004 ▶, and references therein) can be converted to cyclo­hexenes and cyclo­hexenones by the one-pot sequence of nucleophilic addition to the corresponding arenetricarbonyl­chromium complex, followed by treatment of the resulting dienylchromium complex with a suitable electrophile and then decomplexation, leading to methodology that finds applications in the synthesis of natural products (Schmalz et al., 2004 ▶). When alkoxy­arenetricarbonyl­chromium complexes are used in this sequence, carbon nucleophiles add predominantly to the meta position, ultimately yielding5-substituted cyclo­hexenones. [For examples of application of cyclo­hexenone synthesis, see Pearson et al. (2004 ▶).] This reaction can be adapted to give an asymmetric synthesis of 5-substituted cyclo­hexenones by using chiral alkoxy­arene­tri­carbonyl­chromiums, as reported earlier from our laboratories (Pearson et al., 1996 ▶; Pearson & Gontcharov, 1998 ▶; Gontcharov, 1997 ▶) and by Semmelhack & Schmalz (1996 ▶). Nucleophilic addition to chiral complexes of type (I) presents an interesting example of 1,5-asymmetric induction (Hollowood et al., 2003 ▶; Evans et al., 2003 ▶; O’Malley & Leighton, 2001 ▶; Castelot-Deliencourt et al., 2001 ▶), rare in the chemical literature, but the mechanism of asymmetric induction in these reactions is not completely understood.

A plausible mechanism for chirality transmission is by a small rotation of the tricarbonyl­chromium tripod on the arene ligand. This would lead to a distortion of the complex lowest unoccupied molecular orbital (LUMO), so that its coefficients at the diastereotopic meta positions are unequal, thereby promoting preferential addition at one site (Pearson et al., 1995 ▶). In contrast, in simple methoxy­arene complexes [(I), with R* = CH3], the tricarbonyl­chromium Cr—C bonds eclipse the ipso and both meta C atoms of the arene (Semmelhack, 1991 ▶). As part of an effort to determine the extent to which the chiral alk­oxy group causes this rotation, and therefore whether it might be a causative factor in asymmetric induction, the crystal structures of several alkoxy­arenetricarbonyl­chromium complexes are being studied. We have already reported some of these results for complexes having a trimethyl­silyl substituent [(I), with R′ = Si(CH3)3] para to the chiral alk­oxy group, all of which structures support our hypothesis (Paramahamsan et al., 2008 ▶). In order to show that such rotation of the Cr(CO)3 group is not specific to the nature of the R′ substituents, we have also more recently determined the structure the title complex, (II), a complex bearing a simple alkyl group. The structure of this compound, prepared as described previously (Pearson & Gontcharov, 1998 ▶), is reported in the present paper.

As shown in Fig. 1 ▶, there is indeed a significant rotation of the Cr(CO)3 group (ca 25°) [the torsion angles with respect to the centroid (X) of the arene ring are C20—Cr1—X—C13 = 25.5 (2)°, C21—Cr1—X—C15 = 25.9 (2)° and C22—Cr1—X—C17 = 24.1 (2)°], and the direction of rotation is in agreement with the sense of asymmetric induction that we have previously reported during nucleophilic additions to this complex (Pearson & Gontcharov, 1998 ▶). The rotation of the Cr(CO)3 unit also removes the mirror symmetry of the arene; for example, the aromatic bonds to atom C16 differ by 0.018 Å, in agreement with the distortion of the LUMO referred to above. Other geometrical parameters are unremarkable.

Figure 1

A view of the title complex, (II), projected on to the plane of the arene ring. Displacement ellipsoids are shown at the 50% probability level.

Experimental

The title complex, (II), was prepared as described previously (Pearson & Gontcharov, 1998 ▶), and crystals were obtained by recrystallization from 1:1 hexane/dichloro­methane.

Crystal data

[Cr(C19H26O3)(CO)3]

M = 438.43

Monoclinic,

a = 7.9484 (7) Å

b = 13.6135 (11) Å

c = 9.5596 (8) Å

β = 93.026 (3)°

V = 1032.96 (15) Å3

Z = 2

Mo Kα radiation

μ = 0.59 mm−1

T = 120 (1) K

0.25 × 0.15 × 0.05 mm

Data collection

Bruker SMART 6000 CCD area-detector diffractometer

Absorption correction: multi-scan (SADABS; Sheldrick, 1997 ▶) T min = 0.908, T max = 1.000 (expected range = 0.882–0.971)

8946 measured reflections

4556 independent reflections

4224 reflections with I > 2σ(I)

R int = 0.026

Refinement

R[F 2 > 2σ(F 2)] = 0.040

wR(F 2) = 0.090

S = 1.09

4556 reflections

263 parameters

1 restraint

H-atom parameters constrained

Δρmax = 0.63 e Å−3

Δρmin = −0.42 e Å−3

Absolute structure: Flack (1983 ▶), 1898 Friedel pairs

Flack parameter: 0.041 (18)

Data collection: SMART (Bruker, 1997 ▶); cell refinement: SAINT (Bruker, 1997 ▶); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ▶); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ▶); molecular graphics: SHELXTL (Sheldrick, 2008 ▶); software used to prepare material for publication: SHELXTL.

Crystal structure: contains datablocks I, global. DOI: 10.1107/S0108270108004666/tr3032sup1.cif

Structure factors: contains datablocks I. DOI: 10.1107/S0108270108004666/tr3032Isup2.hkl