Short I · · · O Interactions in the Crystal Structures of Two 2-Iodo-Phenyl Methyl-Amides as Substrates for Radical Translocation Reactions

: Radical translocation reactions are ﬁnding various uses in organic synthesis, in particular the stereospeciﬁc formation of complex natural products. In this work, the syntheses and single-crystal structures of two substituted 2-iodo-phenyl methyl-amides are reported, namely cyclo -propane carboxylic acid (2-iodo-phenyl)-methyl-amide, C 11 H 12 INO ( 1 ), and cyclo -heptane carboxylic acid (2-iodo-phenyl)-methyl-amide, C 15 H 20 INO ( 2 ). In each case, the methyl-amide group has a syn conformation, and this grouping is perpendicular to the plane of the benzene ring: these solid-state conformations appear to be well setup to allow an intramolecular hydrogen atom transfer to take place as part of a radical translocation reaction. Short intermolecular I · · · O halogen bonds occur in each crystal structure, leading to [010] chains in 1 [I · · · O = 3.012 (2) Å] and isolated dimers in 2 [I · · · O = 3.024 (4) and 3.057 (4) Å]. The intermolecular interactions are further quantiﬁed by Hirshfeld surface analyses.


Introduction
Radical translocation reactions (radical generation by photolysis, heat, or reaction with an initiator, followed by an intramolecular H atom shift) have found various uses in organic synthesis, from forming simple carbocycles [1] to complex natural products [2,3]. These reactions rely on the translocation (i.e., H atom migration) of an initially generated radical to a remote site, usually four [4] to seven [5] atoms away. Figure 1 illustrates the general principle involved.

Introduction
Radical translocation reactions (radical generation by photolysis, heat, or reaction with an initiator, followed by an intramolecular H atom shift) have found various uses in organic synthesis, from forming simple carbocycles [1] to complex natural products [2,3]. These reactions rely on the translocation (i.e., H atom migration) of an initially generated radical to a remote site, usually four [4] to seven [5] atoms away. Figure 1 illustrates the general principle involved. The driving force behind the rearrangement is assumed to be the formation of a more stable radical after translocation; typically, the initially formed radical is an aryl or vinyl species, perhaps formed by photolytic cleavage of a C-X (X = halogen) bond in a benzene ring, which is unstable and highly reactive [6]. The high reactivity and lack of stability of these radicals is presumed to be a result of the lone electron occupying an σ-orbital that The driving force behind the rearrangement is assumed to be the formation of a more stable radical after translocation; typically, the initially formed radical is an aryl or vinyl species, perhaps formed by photolytic cleavage of a C-X (X = halogen) bond in a benzene ring, which is unstable and highly reactive [6]. The high reactivity and lack of stability of these radicals is presumed to be a result of the lone electron occupying an σ-orbital that is orientated perpendicular to the aromatic/conjugated π system, and therefore, stabilization of the radical by delocalization is not possible.
In order for the intramolecular translocation reaction to occur, it has been determined that the molecule must adopt a cis conformation [7], in which the two components (the initial radical and the C-H bond to supply the transferrable H atom) face each other, and the use of a suitable N-bonded substituent attached to the aromatic ring can provide a 'conformational lock', thereby optimizing the likelihood of translocation and subsequent cyclization [8]. In order to help further understand this process, the related compounds cyclo-propane carboxylic acid (2-iodo-phenyl)-methyl-amide (alternative name: N-(2-iodophenyl)-N-methylcyclopropanecarboxamide), C 11 H 12 INO (1), and cyclo-heptane carboxylic acid (2-iodo-phenyl)-methyl-amide (alternative name: N-(2-iodophenyl)-Nmethylcycloheptanecarboxamide), C 15 H 20 INO (2), were prepared, and their crystal structures were determined.

Synthesis of 1
Cyclopropane carbonyl chloride (1.04 g, 10.4 mmol) was added dropwise to a solution of 2-iodoaniline (2.00 g, 9.13 mmol) and N,N-diisopropylethylamine (Hünig's base or DIPEA) (1.53 g, 11.9 mmol) in tetrahydrofuran (THF) (20 mL) at 0 • C under nitrogen ( Figure 2). The solution was then allowed to warm to room temperature and stirring was continued for a further four hours. The reaction mixture was then diluted with diethyl ether (50 mL) and washed with brine (2 × 30 mL) and then water (30 mL). The ether layer was then dried with MgSO 4 and filtered, and the solvent was removed at reduced pressure. The product was purified by recrystallization from the mixed solvents of dichloromethane and hexane, yielding cyclopropane carboxylic acid (2-iodo-phenyl)-amide (3) (Figure 1) Figure 2. Synthesis schemes for 1 and 2 via intermediates 3 and 4 (see the text for abbreviations).

Synthesis of 2
The 2-iodoaniline (3.00 g, 13.7 mmol) was added to a solution of cycloheptane carboxylic acid (2.4 g, 17 mmol) in dichloromethane (DCM) (20 mL). A solution of di-cyclohexylcarbodiimide (DCC) (3.17 g, 15.4 mmol) in DCM (10 mL) was then added dropwise at 0 °C and 4-(dimethylamino)pyridine (DMAP) (0.17 g, 0.14 mmol) was added as a catalyst. The resulting solution was stirred for 30 min at room temperature then cooled in ice, and the solid was filtered off and washed with DCM. The filtrate was collected and washed with 2 N HCl solution (3 × 30 mL), and then with saturated NaHCO3 solution (3 A solution of 3 (1.00 g, 3.48 mmol) in THF (10 mL) was added dropwise to a suspension of sodium hydride (502 mg, 4.53 mmol) in dry THF (20 mL) at 0 • C. Once hydrogen evolution had ceased, iodomethane (497 mg, 3.48 mmol) was added to the solution and was allowed to stir overnight. The reaction was quenched with ammonium chloride solution (2 mL) and diluted with diethyl ether (50 mL). The ethereal solution was washed with brine (2 × 30 mL) and then water (30 mL). The ethereal solution was then dried with MgSO 4 , filtered, and solvent was removed at reduced pressure. The crude product was purified by column chromatography, eluting with hexane/ethyl acetate (4:1), yielding 1 as a colorless solid (942 mg, 90%); m.p. 97-99 • C; HRMS: found MH + , 302.0032 C 11

Synthesis of 2
The 2-iodoaniline (3.00 g, 13.7 mmol) was added to a solution of cycloheptane carboxylic acid (2.4 g, 17 mmol) in dichloromethane (DCM) (20 mL). A solution of di-cyclohexylcarbodiimide (DCC) (3.17 g, 15.4 mmol) in DCM (10 mL) was then added dropwise at 0 • C and 4-(dimethylamino)pyridine (DMAP) (0.17 g, 0.14 mmol) was added as a catalyst. The resulting solution was stirred for 30 min at room temperature then cooled in ice, and the solid was filtered off and washed with DCM. The filtrate was collected and washed with 2 N HCl solution (3 × 30 mL), and then with saturated NaHCO 3 solution (3 × 30 mL) and water (30 mL). The organic layer was collected and dried over MgSO 4 , filtered, and the solvent was removed at reduced pressure. The crude product was then purified by column chromatography, eluting with hexane/ethyl acetate (3:1), yielding cycloheptane-carboxylic acid (2-iodo-phenyl)-amide (4) as a white solid (  A solution of 4 (500 mg, 1.45 mmol) in dry THF (5 mL) was added dropwise to a suspension of sodium hydride (45 mg, 1.89 mmol) in dry THF (10 mL) at 0 • C. Once hydrogen evolution had ceased, iodomethane (227 mg, 1.60 mmol) was added and the solution was allowed to stir overnight. The reaction was quenched with ammonium chloride solution (2 mL) and taken up in diethyl ether (50 mL). The ethereal solution was washed with brine (2 × 30 mL) and water (30 mL). The ethereal solution was then dried over MgSO 4 , filtered, and then reduced to yield the crude product, which was purified by column chromatography, eluting with hexane/ethyl acetate

X-ray Data Collection and Refinement
The intensity data for 1 were collected on a Bruker SMART1000 CCD diffractometer at 293 K, and the corresponding data for 2 were collected on an Enraf-Nonius KappaCCD diffractometer at 120 K. Empirical (SADABS multi-scan) absorption corrections were applied at the data reduction stage and the structures were routinely solved by direct methods with SHELXS-97, while the atomic models were completed and optimized by refinement against |F| 2 with SHELXL-2018. One of the cyclo-heptyl rings in 2 is disordered over two orientations for atoms C26, C28, and C29 and their attached H atoms in a 0.60 (3):0.40 (3) ratio. The H atoms were mostly located in difference maps and relocated to idealized locations (C-H = 0.93-0.97 Å), and they were further refined as riding atoms with the constraint U iso (H) = 1.2U eq (C) or 1.5U eq (methyl C) applied. The methyl groups were allowed to rotate, but not to tip, to best fit the electron density. Full details are provided in the deposited cifs.
Crystal Compound 1 crystallizes in the monoclinic space group P2 1 with one molecule in the asymmetric unit ( Figure 3). The dihedral angle between the mean planes of the C1-C6 benzene ring and the C7/C8/N1/O1 methyl-amide grouping is 86.09 (14) • . The bondangle sum at N1 of 360.0 • implies the expected sp 2 hybridization for this atom, and its un-hybridized 2p orbital is therefore well-aligned to interact with the π system of the adjacent C=O group, as reflected in the typical amide C8-N1 bond length of 1.357 (4) Å. However, this 2p orbital lies almost perpendicular to the delocalized π system of the benzene ring, and therefore, the C6-N1 bond length of 1.435 (4) Å is essentially that of a single bond. The conformation of the methyl-amide group is syn (C7-N1-C8-O1 torsion angle = 2.5 (5) • ), as is the conformation of the C6-N1-C8-C9 grouping (2.6 (5) • ). The dihedral angle between the C7/C8/N1/O1 grouping and the C9/C10/C11 cyclo-propyl ring is 86.3 (3) • , and the dihedral angle between the benzene and cyclo-propyl rings is 58.7 (3) • . The cyclo-propyl ring is, of course, strictly planar, and the terminal C10-C11 bond length of 1.471 (6) Å is notably shorter than the other two bonds (1.500 (5) and 1.502 (5) Å), which is normal when an unsaturated substituent is attached to the methine group [9]. The C10-C9-C11 bond angle of 58.7 (3) • is notably smaller than the C9-C10-C11 and C9-C11-C10 angles (60.7 (3) and 60.6 (3) • , respectively). The overall conformation of the molecule of 1 could be described as V-shaped, in which the C9-H9 bond (i.e., the methine group of the cyclo-propyl ring) faces the C1-I1 bond in the aromatic ring (H9···C1 = 2.94 Å, C9-H9···C1 = 113 • ), in what appears to be a very favorable orientation for a 1,5-translocation reaction to occur, assuming that the solid-state conformation is maintained in solution. Key geometrical data for 1 are summarized in Table 1. , in what appears to be a very favorable orientation for a 1,5-translocation reaction to occur, assuming that the solid-state conformation is maintained in solution. Key geometrical data for 1 are summarized in Table 1.  For molecule 2B, see Figure 5 for the equivalent atom designations. See the text for further discussion of the torsion angles ϕ, ξ, and ψ. The acceptor O and C atoms in 1 are generated by the symmetry operation 1-x, y-½, −z.  −165.5 (7) I· · · O=C-N −87.6 (4) −82.3 (7) −85.2 (7) For molecule 2B, see Figure 5 for the equivalent atom designations. See the text for further discussion of the torsion angles ϕ, ξ, and ψ. The acceptor O and C atoms in 1 are generated by the symmetry operation 1-x, y-1 2 , −z.
In the crystal of 1, a very short C1-I1· · · O1 i (i = 1-x, y-1 2 , −z) contact or 'halogen bond' [10] with an I···O separation of 3.012 (2) Å occurs, which is some 0.49 Å shorter than the expected Bondi [11] van der Waals' separation of about 3.50 Å for these two atoms. One way to interpret this directional contact is in terms of an electrostatic attraction between the Lewis base (the lone pair bearing an O atom of the carbonyl group) and an 'σ hole' [12] on the Lewis acid (the iodine atom), which has close parallels with the way that hydrogen bonds can be envisaged [13]. The C-I· · · O grouping is almost linear (bond angle = 171.78 (9) • ), which is quite typical for this type of interaction, and the I· · · O=C bond angle is 135.3 (2) • . This I···O halogen bond leads to C(6) chains [14] of molecules propagating in the [010] direction in the crystal of 1 (Figure 4), with adjacent molecules related by the operation of the 2 1 screw axis. There are no π-π stacking interactions in 1, with the shortest separation between the benzene-ring-centroids of nearby molecules in the crystal being greater than 5.6 Å. Thus far, as the H atoms of the cyclo-propyl rings are concerned, the shortest intermolecular H···H contacts are H9···H5 (2.42 Å), H10a···H11a (2.56 Å), H10b···H10a (2.60 Å), H11a···H10a (2.56 Å), and H11b···H10a (2.37 Å). Only the last of these is slightly shorter than the expected van der Waals' radius sum of 2.40 Å for two H atoms; thus, we may conclude that van der Waals (dispersion) forces are most important in determining the packing. This is further quantified by the Hirshfeld surface analysis described below.
[10] with an I⋅⋅⋅O separation of 3.012 (2) Å occurs, which is some 0.49 Å shorter than the expected Bondi [11] van der Waals' separation of about 3.50 Å for these two atoms. One way to interpret this directional contact is in terms of an electrostatic attraction between the Lewis base (the lone pair bearing an O atom of the carbonyl group) and an 'σ hole' [12] on the Lewis acid (the iodine atom), which has close parallels with the way that hydrogen bonds can be envisaged [13]. The C-I⋯O grouping is almost linear (bond angle = 171.78 (9)°), which is quite typical for this type of interaction, and the I⋯O=C bond angle is 135.3 (2)°.
This I⋅⋅⋅O halogen bond leads to C(6) chains [14] of molecules propagating in the [010] direction in the crystal of 1 (Figure 4), with adjacent molecules related by the operation of the 21 screw axis. There are no π-π stacking interactions in 1, with the shortest separation between the benzene-ring-centroids of nearby molecules in the crystal being greater than 5.6 Å. Thus far, as the H atoms of the cyclo-propyl rings are concerned, the shortest intermolecular H⋅⋅⋅H contacts are H9⋅⋅⋅H5 (2.42 Å), H10a⋅⋅⋅H11a (2.56 Å), H10b⋅⋅⋅H10a (2.60 Å), H11a⋅⋅⋅H10a (2.56 Å), and H11b⋅⋅⋅H10a (2.37 Å). Only the last of these is slightly shorter than the expected van der Waals' radius sum of 2.40 Å for two H atoms; thus, we may conclude that van der Waals (dispersion) forces are most important in determining the packing. This is further quantified by the Hirshfeld surface analysis described below.
In the crystal of 2, the asymmetric molecules associate into dimers with approximate local C2 symmetry, linked by pairs of C-I⋯O interactions with a slight asymmetry between the I⋯O separations (3.024 (4) and 3.057 (4) Å) and C-I⋯O angles (171.71 (17) and 175.98 (16)°), which could possibly be ascribed to packing effects (see Table 1 for the full geometrical details). Otherwise, no directional intermolecular interactions beyond normal van der Waals contacts could be identified: the shortest contact between hydrogen atoms is H15⋅⋅⋅H28b at 2.38 Å.

Hirshfeld Surface Analyses
In order to further quantify the intermolecular interactions in these crystals, their Hirshfeld surfaces were generated using CrystalExplorer [15] following the methodology described by Tan et al. [16]. The Hirshfeld surface of 1 ( Figure 6) shows intense red spots in the vicinity of atoms O1 and I1, which clearly correlate the halogen bond described above. Otherwise, the surface is blue, indicating contacts at the expected van der Waals' distance or greater. In the crystal of 2, the asymmetric molecules associate into dimers with approximate local C 2 symmetry, linked by pairs of C-I· · · O interactions with a slight asymmetry between the I· · · O separations (3.024 (4) and 3.057 (4) Å) and C-I· · · O angles (171.71 (17) and 175.98 (16) • ), which could possibly be ascribed to packing effects (see Table 1 for the full geometrical details). Otherwise, no directional intermolecular interactions beyond normal van der Waals contacts could be identified: the shortest contact between hydrogen atoms is H15···H28b at 2.38 Å.

Hirshfeld Surface Analyses
In order to further quantify the intermolecular interactions in these crystals, their Hirshfeld surfaces were generated using CrystalExplorer [15] following the methodology described by Tan et al. [16]. The Hirshfeld surface of 1 ( Figure 6) shows intense red spots in the vicinity of atoms O1 and I1, which clearly correlate the halogen bond described above. Otherwise, the surface is blue, indicating contacts at the expected van der Waals' distance or greater.
The percentage contributions of the different types of interactions identified in twodimensional fingerprint plots [17] are listed in Table 2.  The percentage contributions of the different types of interactions identified in twodimensional fingerprint plots [17] are listed in Table 2. These data indicate that the H⋅⋅⋅H contacts are the most important in both structures, with a significantly higher percentage for 2 than 1, although this is not consistent with the atom percentages of hydrogen in the structures (44% H in 2 versus 46% H in 1). The O⋅⋅⋅H/H⋅⋅⋅O contacts in 1 contribute almost three times as much to the surface as in 2, whereas the H⋅⋅⋅C/C⋅⋅⋅H contacts in 2 are almost double those in 1. Despite their presumed importance in establishing the packing, the I⋅⋅⋅O interactions only contribute a very modest percentage to the surfaces. The fingerprint plot for the I⋅⋅⋅O contacts for 1 (Figure 7) shows distinctive 'crescent' shapes with the tips at di + de ≈ 3.0 Å, obviously corresponding to the I⋅⋅⋅O separation established in the crystal structure. These data indicate that the H···H contacts are the most important in both structures, with a significantly higher percentage for 2 than 1, although this is not consistent with the atom percentages of hydrogen in the structures (44% H in 2 versus 46% H in 1). The O···H/H···O contacts in 1 contribute almost three times as much to the surface as in 2, whereas the H···C/C···H contacts in 2 are almost double those in 1. Despite their presumed importance in establishing the packing, the I···O interactions only contribute a very modest percentage to the surfaces. The fingerprint plot for the I···O contacts for 1 (Figure 7) shows distinctive 'crescent' shapes with the tips at d i + d e ≈ 3.0 Å, obviously corresponding to the I···O separation established in the crystal structure.

Comparison with Related Structures
The syn orientation of the methyl-amide group is common to all three molecules (1, 2A, and 2B) and is by far the most common geometry for this grouping: a survey of the Cambridge Structural Database [18] yielded the scatterplot shown in Figure 8, which com-

Comparison with Related Structures
The syn orientation of the methyl-amide group is common to all three molecules (1, 2A, and 2B) and is by far the most common geometry for this grouping: a survey of the Cambridge Structural Database [18] yielded the scatterplot shown in Figure 8, which compares the C1-C6-N1-C8 (ϕ) and C7-N1-C8-O1 (ξ) torsion angles (using the atom numbering in this paper) for some 120 different structures. The methyl-amide torsion angles are heavily clustered in the range −10 • < ξ < 10 • , with one or two outliers with |ξ| > 160 • (i.e., corresponding to an anti-conformation for the C-N-C-O grouping), which might be attributable to severe steric strain. The φ angle (equivalent to the torsion angle between the benzene ring and the methyl-amide group) shows clustering around ϕ = ±90 • , i.e., a near-perpendicular arrangement in all cases.

Conclusions
We have prepared and structurally characterized the related compounds C11H12INO (1) and C15H14INO (2) as possible precursors for radical cyclization translocation reactions: the donor H atom and the pre-radical C-I bond appeared to be well-aligned in the solid state for this to occur. The crystals of both compounds featured short C-I⋅⋅⋅O halogen bonds, which generated chains in 1 and dimers in 2. A survey of the Cambridge Structural Database showed that almost all methyl-amide groups adopted a syn confirmation, and when this grouping was bonded to a benzene ring, the moieties were orientated approximately normal to each other. The Hirshfeld surfaces indicated that the I⋅⋅⋅O contacts made a modest percentage contribution.

Conclusions
We have prepared and structurally characterized the related compounds C 11 H 12 INO (1) and C 15 H 14 INO (2) as possible precursors for radical cyclization translocation reactions: the donor H atom and the pre-radical C-I bond appeared to be well-aligned in the solid state for this to occur. The crystals of both compounds featured short C-I···O halogen bonds, which generated chains in 1 and dimers in 2. A survey of the Cambridge Structural Database showed that almost all methyl-amide groups adopted a syn confirmation, and when this grouping was bonded to a benzene ring, the moieties were orientated approximately normal to each other. The Hirshfeld surfaces indicated that the I···O contacts made a modest percentage contribution.

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Data Availability Statement: CCDC 2254542 and 2254543 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via: www.ccdc.cam.ac.uk/ data_request/cif (accessed on 2 May 2023), by e-mailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44-1223336033.