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Crystal structures of 3,6-diallyltetracyclo[6.3.0.04,11.05,9]undeca-2,7-dione and 1,7-diallylpentacyclo[5.4.0.02,6. 03,10.05,9]undecane-8,11-dione: allylated caged compounds

The crystal structures of two allylated caged molecules and the correlation of bond distances and feasibility of ring-closing metathesis is discussed.


Chemical context
Caged molecules are much sought after chemical entities due to their diverse applications such as high-energy materials, drug intermediates and starting materials for complex natural products (Marchand, 1989a,b;Mehta & Srikrishna, 1997). The intricacies involved in the structural frame of caged molecules, such as deformation of ideal C-C bond angle and other unusual structural features, make them challenging synthetic targets (Olah, 1990;Osawa & Yonemitsu, 1992). Caged molecules are strained due to the rigid geometrical features and they exhibit interesting properties (Von et al., 1986): the high negative heat of combustion and elevated positive heat of formation for caged compounds reveal the strain involved in their molecular architecture.

Structural commentary
The caged carbon skeleton of (1), Fig. 3, can be described as a fusion of four five-membered rings and one six-membered ring, the latter having a boat conformation. All four fivemembered rings exhibit envelope conformations, with atoms C3, twice C17, and C11 as the flap atoms of the various rings. Compound (1) is symmetrically substituted with two allyl groups at atoms C5 and C10. The few crystal structures of PCUD compounds that are recorded in Cambridge Structural Database (Groom & Allen, 2014) show no bridging route through the substituents that link the C-atoms [e.g. C1 to C9, Fig. 3]. These compounds are substituted at C1 and/or C9 so that these molecules form the open mouth of the cage. The tetracyclic compound (1) shows symmetrical substitution with keto moieties at atoms C1 and C9.
The C-C strained bond angles in (1) vary from 95.31 (10) to 125.21 (14) , deviating from the ideal tetrahedral angle of 109.5 . Previous studies showed that PCUD caged compounds normally display C-C bond lengths which deviate from expected value of 1.54 Å (Bott et al., 1998;Flippen-Anderson et al., 1991;Linden et al., 2005;Kruger et al., 2005). The structure of (1) also exhibits unusual Csp 3 -Csp 3 single-bond lengths ranging from 1.5092 (19) Å to 1.5935 (19) Å . The bond C2-C10, which is parallel and immediately adjacent to C1-C9 axis, was found to be longer, with a value of 1.5935 (19) Å . The increase in bond length can be the result of stretching strain commenced by the open mouth of the cage formed by carbonyls bearing carbon atoms, i.e. C1 and C9. Similar observations were made in compound (2), i.e. 1.597 (4) Å for C5-C10.

Figure 3
A view of the molecular structure of compound (1), with atom labelling. Displacement ellipsoids are drawn at the 50% probability level.
C7-C2 is the most strained with the smallest angle of 88.77 (17) and the C15-C7-C8 bond angle of 119.6 (2) is the largest one, again showing considerable deviation from the standard value of 109.5 .
It was anticipated that the two allyl groups present in (1) would undergo RCM to generate a new pentacyclic system (3) (Fig. 1). However, it was observed that even under forcing reaction conditions, (1) did not generate the expected RCM product, whereas compound (2) underwent an RCM sequence smoothly to give (4) in good yield (Fig. 1). It was found that the allyl-bearing carbon atoms in tetracyclic system (1) are too far apart [C5-C13 = 2.9417 (17) Å ] and we believe that due to this reason, the RCM protocol failed. When these carbon atoms are bonded, the distance between them was found to be smaller. Thus in (2), the distance between the bonded atoms C2-C7 is 1.611 (3) Å .

Figure 5
A view along the b axis of the crystal packing of compound (1). Hydrogen bonds are shown as dashed lines (see Table 1 for details; only the H atoms involved in these hydrogen bonds are shown).

Figure 6
A view along the c axis of the crystal packing of compound (2). Hydrogen bonds are shown as dashed lines (see Table 2 for details; only the H atoms involved in these hydrogen bonds are shown).
preferred over RCM, and when the distance is smaller, the RCM product is predominant over the CM product.
The conclusion is that, as the C5-C13 separation in (1) is large [2.9417 (17) Å ], the carbon atoms bearing the allyl groups are far apart in this tetracyclic system, and the expected ring-closing metathesis (RCM) protocol failed to give the ring-closing product (3), Fig. 1. When these carbon atoms are connected by a C-C bond as in (2), the C2-C7 bond distance was found to be much smaller [1.611 (3) Å ], and consequently the RCM process was successful giving the diallyl compound (4), Fig. 1.

Supramolecular features
In the crystal of (1), molecules are linked via C-HÁ Á ÁO hydrogen bonds, forming sheets lying parallel to (010); see Fig. 5

Synthesis and crystallization
Compounds (1) and (2) were prepared by the procedures reported in the literature (Kotha et al., 1999 andKotha et al., 2006, respectively) and their melting points were compared with the reported values. In addition, their identity was confirmed by NMR spectroscopic data.
Compound (2): The crude compound (2) was obtained after reaction work-up and was purified using silica gel column chromatography (5% EtOAc/petroleum ether). Colourless crystals were isolated when the solvent was allowed to evaporate (m.

Refinement
Crystal data, data collection and structure refinement details of compounds (1) and (2) are summarized in the Table 3. For both the compounds all H atoms were placed in geometrically calculated positions and refined using a riding model, with C-H = 0.95-1.00 Å and with U iso (H) = 1.2U eq (C).  SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008), PLATON (Spek, 2009) and publCIF (Westrip, 2010). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.20 e Å −3 Δρ min = −0.15 e Å −3 Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.  (2) Geometric parameters (Å, º)