CHEMPHYSCHEM ARTICLES DOI: 10.1002/cphc.201301204

Intramolecular Interactions of Trityl Groups Jacek S´ciebura, Agnieszka Janiak, Agata Stasiowska, Jakub Grajewski, Krystyna Gawron´ska, Urszula Rychlewska,* and Jacek Gawron´ski*[a] Trityl group, Tr, is a molecular dynamic rotor of which the conformation and helicity depend on other groups in the close vicinity. Interactions with another covalently linked Tr group and with other substituents are analyzed in terms of transfer of chirality to the trityl group. Two trityl groups in a molecule can

mutually interact at a distance of two, three, or five bonds. Despite its size, a Tr group attached to a cyclohexane or cyclopentane ring through an oxygen or nitrogen atom adopts either an axial or equatorial position, depending on additional stabilizing interactions, such as hydrogen bonding.

1. Introduction Trityl (triphenylmethyl, Tr) groups are widely used in organic synthesis as protecting devices.[1] In recent years trityl substituents have received attention as components of molecular devices[2] and chirality-reporting groups for a neighboring chiral center through induced electronic circular dichroism (ECD) spectra of the trityl chromophore,[3] because the trityl group can act as a molecular equivalent of a macroscopic rotor. A significant difference between the macroscopic rotor and the molecular trityl group rotator is that the latter is much more flexible; it can not only rotate around the bond that connects the Tr group to the molecular framework but also rotate each phenyl group. A trityl group connected to an achiral substituent R, such as Cl, Me, or NH3 + , forms a propeller (with rotating blades) with formal C3 symmetry which is dynamically racemic (I, Figure 1) and generates no circular dichroism signal. Chiral trityl ethers,[3] as well as chiral N-trityl amines,[4] form nonrigid molecular bevel gears that can transmit chiral information from a substitu-

Figure 1. Structures of trityl derivatives I–V. [a] Dr. J. S´ciebura, Dr. A. Janiak, A. Stasiowska, Dr. J. Grajewski, Dr. K. Gawron´ska, Prof. Dr. U. Rychlewska, Prof. Dr. J. Gawron´ski Department of Chemistry Adam Mickiewicz University 60-780 Poznan (Poland) Fax: (+ 48) 61829-1555 E-mail: [email protected] [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201301204.

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ent C (H, M, L) to the trityl chromophore (II, Figure 1) and generate a characteristic ECD spectrum. ECD spectroscopy thus forms a convenient and reliable method to determine the absolute configuration of a parent alcohol or amine. Analysis of the Raman optical activity and vibrational CD spectra of poly(trityl methacrylate) allows us to establish its helical screw sense.[5] There is increasing interest in the design, synthesis, and dynamic conformational study of rigid molecular bevel gears (such as triptycene derivatives) that undergo correlated rotatory motion.[6, 7] A new and unexplored challenge is observation of the interactions between two trityl groups within one molecule, apart from the bevel-gear-type interaction of a trityl group with a chiral group in the vicinity. Depending on the distance between the trityl groups, their helicity and rotation could be correlated, thus chiral information would be transferred from one trityl group to another. Installation of the trityl groups in a molecule is most readily achieved through the presence of heteroatoms, such as oxygen, nitrogen, and sulfur. By searching for suitable models of simple ditrityl molecular rotors we found that the crystal structures of ditritylmethane, ditrityl ether, and ditrityl sufide (III, Figure 1) are known[8, 9] although not analyzed in detail. Structures of individual molecules are chiral, however, the compounds are dynamically racemic (they also crystallize as racemic compounds). X-ray diffraction determined structures[8, 9] show nonbonded H···C and C···C interactions between the two trityl groups, which are partially released by widening the central bond angle C(sp3)–X–C(sp3) to the values 1208 (X = CH2, S) or 1288 (X = O). Molecules in which two trityl groups are connected by two chalcogen atoms (IV, Figure 1) are also of interest. The ditrityl peroxide molecule in the crystal shows a planar anti arrangement of the C O O C bond system (torsion angle a = 1808).[10] The molecular dynamics of a more congested bis-(hexa-tert-butyl) derivative have been described as a crankshaft motion rather than slippage.[11] Ditrityl disulfide shows a nonplanar arrangement of the C S S C bond in the crystal (torsion angle a = 1108).[12] This arrangement could bring the two trityl groups closer together, however, the increased length of the C S S C bond ChemPhysChem 2014, 15, 1653 – 1659

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system compared to the length of the C O O C bonds may ysis of all six compounds.[22] Structural data were amended by counteract this. calculations of low-energy conformers in vacuo using density Two trityl groups can be embedded into a cyclic molecule functional theory (DFT) at the B3LYP/6-311G(d,p) level and by that contains permanent chirality centers. 1,2-trans-Disubstitutspectroscopic methods (ECD, NMR). For comparison we anaed derivatives of cyclohexane and cyclopentane (V, Figure 1) lyzed the available X-ray diffraction data for the dynamically are chiral and this offers an opportunity to study their dynamic racemic ditrityl molecules III and IV mentioned earlier. structures by chiroptical methods. The degree of interaction between the two trityl groups and between the trityl groups 2. Results and Discussion and the neighboring chiral centers in V is dependent on the conformational equilibria of the molecules, including populaThe structures and relative positions of trityl groups in ditrityl tions of 1,2-diaxial and 1,2-diequatorial conformers. derivatives can be adequately described by a set of pseudoAlthough the structures and dynamic equilibria of diaxial torsion angles w1–w3 and judiciously chosen interatom distanand diequatorial conformers of 1,2-disubstituted derivatives of ces. Pseudo-torsion angles w1–w3 show the position of each chair cyclohexane were thoroughly studied,[13] little is known phenyl group relative to each other in each trityl group, as deabout structures of trityl derivatives and other derivatives with fined earlier.[3] The distance between the two trityl groups can bulky groups attached to a ring in vicinal positions via heterobe measured as the distance (l1) between the two sp3-hybridatoms. For example, silyl ethers of trans-1,2-dihydroxycyclohexized carbon atoms of the trityl groups or by any two nearest ane are known to show unusually large populations of diaxial ortho hydrogen and ortho or ipso sp2 carbon atoms of the two conformers at equilibrium. trans-1,2-Bis-triphenylsililoxycyclotrityl groups (Table 1). The distance l1 increases significantly, as hexane contains both diequatorial and diaxial conformers in soTable 1. Helicity of trityl groups (angles w) and selected intramolecular C···C and C···H contacts (l1–l7) between lution and in the crystal[14] trityl groups separated by two or three bonds, as observed in the crystal structures of the simple ditrityl whereas the analogous bis-triisomolecules. propylsililoxy derivative is diaxial in both.[15] It appears that the w(2)[a] l1[b] l2[b] l3[b] l4[b] l5[b] l6[b] l7[b] a[c] Ref. Compound w(1)[a] [8] [8] [] [] [] [] [] [] [] [8] larger the substituents on silicon, the greater the population of axTrCH2Tr[d] 72, 53, 25 76, 49, 10 2.82 3.29 3.12 3.18 2.88 2.80 2.10 – [8] [e] [16] 76, 51, 21 76, 51, 21 2.80 3.28 3.16 3.26 2.95 2.90 2.21 – [23] TrCH ially substituted conformers. 2Tr TrOTr 63, 58, 19 68, 42, 11 2.62 3.03 3.17 3.19 2.66 2.52 2.32 – [9a] In all silyl ethers derived from cyTrSTr 63, 63, 60 58, 54, 34 3.29 3.65 3.33 3.19 2.48 2.36 2.46 – [9b] clohexanediol the conformation TrOOTr 67, 61, 53 67, 61, 53 3.65 4.43 3.86 3.95 3.03 2.98 3.05 180 [10] of the Si O C H bond is TrSSTr 67, 64, 58 79, 65, 49 4.50 4.31 3.54 3.53 3.10 2.75 3.13 112 [12] syn.[14, 15] The diaxial structure [a] w(1), w(2)—pseudotorsion angles for the two trityl residues, defined as in ref. [3]. [b] Short distances bewas found by X-ray diffraction tween trityl groups: l1 - Csp3/Csp3 ; l2 - Cipso/Cipso ; l3 - Cipso/Cortho ; l4 - Cortho/Cortho ; l5 - Cipso/Hortho ; l6 - Cortho/Hortho ; l7 for a zirconium(IV) trichloride Hortho/Hortho ; distances between C and H atoms shorter than the sum of their van der Waals radii (1.7 and 1.2  for C and H, respectively) are marked in bold. [c] a = Torsion angle of the C X X C bond system. [d] Triclinic alkoxide derivative of trans-1,2crystals. [e] Monoclinic crystals. dihydroxycyclohexane.[17] Derivatives of trans-1,2-diaminocyclohexane also show preference for expected, with the number of connecting heteroatoms and on a diaxial conformation. This conformation is found by X-ray difreplacing oxygen with sulfur. The gearing effect can be expectfraction for a fully protonated cyclam-type derivative,[18] for ed for ditrityl ether (l1 2.62 ) and ditritylmethane (l1 2.82 ), as a ferrocenyl diimine derivative,[19] for an uncoordinated fragment of a calixsalen NiII complex,[20] and for the bis-triphenyldisplayed in Figure 3, and in part for ditrityl sulfide (l1 3.29 ). phosphinimine derivative.[21] In the first two cases the two trityl groups are interlocking and this is reflected in short interatomic contacts (see below) as To gain a better understanding of the structures and intrawell as in the helicities of the trityl groups (MPM or PMM), molecular interactions of ditrityl-substituted molecules of type which are different from the ideal propeller conformations V we studied a series of derivatives of cyclic chiral 1,2-diols (MMM or PPP). In the case of ditrityl sulfide, both irregular and 1,2-diamines in cyclohexane (1, 2), cyclopentane (3, 4), tet(PMM) and propeller-like structures are present in the crystal. rahydrofuran (5), and tetrahydrothiophene (6) skeletons, see Ditrityl peroxide represents a structure in which the trityl Figure 2. Suitable crystals were grown for X-ray diffraction analgroups do not interact directly (l1 3.65 ), however, their helicities are opposite (MMP and PPM) and the mutual orientation of the Tr groups is staggered, as the molecule is of Ci symmetry. Increasing the distance between the trityl groups as in ditrityl disulfide (l1 4.50 ) makes the groups structurally separated; moreover, apparently due to longer C S and S S bonds, the trityl groups behave as typical propellers of opposite helicity (MMM and PPP). Figure 2. Ditritylated diols and diamines of this study.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 4. Torsion angles b and g in diequatorial and diaxal conformers of 1 and 2.

Figure 3. Mutual disposition of Tr groups and their helicities in staggered and eclipsed conformations, as observed in the crystals of a) TrOTr and b) TrCH2Tr.

Further analysis of the X-ray diffraction data of III indicates short contacts between the trityl groups. Short contacts are listed in bold numbers in Table 1 as l2–l7 distances. The assumed van der Waals (VdW) radii are 1.20  for H and 1.70  for C. In TrCH2Tr, TrOTr, and TrSTr distances shorter than the sum of the VdW radii are observed between Hortho/Hortho atoms (2.10–2.32 ), Hortho/Cortho or Cipso atoms (2.36–2.88 ), and between carbon atoms, ipso and/or ortho (3.03–3.33 ). The molecular conformations of TrOTr and TrCH2Tr are different; in the former the trityl groups are staggered, in the latter they are eclipsed (Figure 3). Interactions between the two trityl groups are not limited to type III molecules of the formula Tr–X–Tr, in which X is CH2, O, or S. A short Hortho/Cortho contact (2.75 ) is observed in TrSSTr and a preference for heterohelicity of the trityl groups in molecules of type IV. The origin of heterohelicity of the trityl groups in ditrityl peroxide and ditrityl disulfide can be accounted for by longer Hortho–Hortho distances (and lower energies) in heterohelical conformers, compared to homohelical conformers. In the cyclic molecules 1–6 of type V, the obtained experimental X-ray diffraction data and the calculated structures indicate complexity of interactions between the various parts of the examined molecules (see Table S2 in the Supporting Information). Pseudo-torsion angles w1–w3 in the conformers of 1 and 3–6 assume either PMM or PMP (or enantiomeric MPP or MPM) values, typical for trityl bevel gears.[3] X-ray studies indicate that compounds 1 and 3–6 crystallized with two symmetry-independent molecules (Z’ = 2) whereas molecules of 2 formed solvates and crystallized with Z’ = 1 and 0.5 (2 a and 2 b, respectively). The character of the solvent molecule affects helicity of the Tr groups, which in the nonpolar environment is typical for trityl bevel gears[3] (PMP, 2 b) but changes to a less common helicity (PM0 and MMP) in CHCl3 solution (2 a), presumably due to the intermolecular NH···Cl interactions.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

To describe the relative positions of the trityl groups in molecules 1–6 we used additional torsion angles b and g, which are defined in Figure 4. Angle b changes from ( )sc in diequatorial to ac/ap in diaxial conformers. In all cases studied by X-ray diffraction, in diaxial conformers values of g are within the range corresponding to ac, while in diequatorial conformers both angles g are ap (Table 2). This result means that the trityl group tends to be situated in a position antiperiplanar or anticlinal to the C(1) C(2) bond (Figure 4). The distance between the two trityl groups in 1 and 3–6, measured as the distance between the Tr sp3 atoms, is over 5.3  in either diequatorial or diaxial conformers. In the diequatorial derivative 2 the distance between the trityl groups is shorter (5.1–5.2 ) as a result of the presence of an intramolecular NH…N hydrogen bond. Replacement of the six-membered ring by its five-membered congener results in widening of angle b and consequent greater separation of the NTr groups (the two nitrogen atoms in 4 are 0.3  further apart than in the six-membered congeners 2 a and 2 b). A greater separation of the OTr groups is combined with the introduction of a heteroatom to the ring skeleton, and leads to an anticlinal b angle in 5 and an antiperiplanar arrangement in 6. Consequently, the separation of the OTr groups increases gradually by 0.1  in the order 3 < 5 < 6. The relative orientation of the Tr groups in molecules 1, 2 a, 2 b, and 6 is staggered, as in ditrityl ether (Figures 3 a and 5 a) and the shortest distance between the ortho H atoms of the two trityl groups in these molecules exceeds the sum of the VdW radii. The data indicate that direct Tr–Tr interaction is more probable in molecules 3–5 than in 1, 2, or 6, as they have a five-membered-ring framework, in which short contacts between the ortho hydrogen atoms of the two trityl groups are found in the crystal (3: 2.28 and 2.32 ; 4: 2.32 and 2.38 ; 5: 2.18 and 2.25 ). Crystal structures of 3–5 show that the two equatorial trityloxy/tritylamine substituents attached to a five-membered ring in an envelope conformation bring about approximate Cs symmetry of this ring. The two trityl groups roughly eclipse each other, as in ditritylmethane (compare Figures 3 b and 5 b). Interestingly, in 1, 3, 5, and 6 the trityloxy group can accommodate either the equatorial or axial position; the latter is clearly dominant for ditrityl derivatives 1 and 6. In 6 the preferred conformation of five-membered ring is a half-chair, preChemPhysChem 2014, 15, 1653 – 1659

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Table 2. X-ray determined and calculated (B3LYP/6-311G(d,p) level) structural parameters of compounds 1–6 with two trityl groups separated by five bonds. X-ray determined[a] Conformation 1

diax[c]

g [8]

178

133 102 143 99 155

80 83 73 76 77

53 52 62 54 50

17 26 6 18 14

165 147 160 159 161 159

58 67 70 74 77 74

54 58 63 58 58 58

40 0 5 7 6 2

176 2

dieq[d,e]

66

2

dieq[f]

60

3

dieq[c]

72 73

4

dieq[c]

74 75

5

dieq[c]

91 89

diax[c]

178

w1

w2 [8]

Calculated[a] w3 DG [kcal mol 1]

b [8]

158 157 158 160 163 164 163 161 96

72 69 66 71 76 70 77 75 87

59 60 64 60 53 61 51 55 56

9 0 3 11 10 14 7 9 26

149 97 147

63 82 57

63 61 48

1 26 46

[%][b]

Conformation

0.0 0.02

43 42

diax[d] diax

176 179

0.6 0.0

15 100

dieq[d] dieq

61 55

0.0

46

diax[g]

120

0.29

28

dieq[g]

84

0.41

23

dieq[g]

88

0.0

70

dieq[g]

59

0.64

24

diax[g]

158

0.0

53

diax[g]

141

0.07

47

diax[d,h]

0.0

64

0.45

30

b [8]

g [8]

w1

w2 [8]

w3

140 105 140 162 159 163

77 77 76 77 68 61

55 53 57 54 57 54

17 24 20 14 2 32

162

168 150 165 165 163 156 165 167 148 171 171 151 167

78 75 79 78 73 74 65 69 67 68 72 75 72

56 58 55 57 60 54 63 56 60 58 60 58 60

20 9 23 18 13 8 16 4 1 1 5 13 5

diax[h]

169

146

83

58

31

diax[d,h]

174

160 146

73 83

58 57

7 31

6 179

[a] For the sake of simplicity the values of angles b (X-C-C-X), g (C-C-X-CTr), and w1–w2 (trityl pseudotorsion angles) are rounded to 18. [b] Conformers populated over 6 % are shown. [c] Data for two symmetry-independent molecules. [d] C2 symmetry. [e] n-Hexane solvate. [f] Chloroform solvate. [g] Envelope ring conformation. [h] Half-chair ring conformation.

Figure 5. Crystal structures of a) 2 a and b) 4 in staggered and eclipsed conformations, respectively. The staggered conformation is seen in 1, 2 a, 2 b, and 6, and the eclipsed in 3–5.

sumably due to the effect of the sulfur atom in the ring. In the diaxial structures of 1 and 6 the trityl groups in each molecule are of opposite helicity, MPM and PMP. The diequatorial structure is unequivocally favored by 2 and a less clear preference  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

for diequatorial structure is observed for 3, 4, and 5 on the basis of the obtained data. X-ray data show diequatorial structures for 3, 4, and 5, whereas DFT calculations suggest either a significant contribution (3, 4) or substantial preference (5) for diaxial conformers in the equilibrium. Calculated energy differences between the most populated conformers are small and fall within a narrow window of 0.7 kcal mol 1. The discrepancy between the calculation and the experiment may indicate that the DFT method used in this study underestimates electron correlation effects at medium to short interatomic distances, as discussed recently by Grimme and Schreiner.[24] In fact, the X-ray diffraction determined structure of five-membered ring compound 5 shows a short contact (2.18 ) between the two hydrogen atoms of the two trityl groups. The flexible nature of the diequatorial–diaxial equilibrium of ditrityl conformers has multiple origins. Firstly, oxygen-linked substituents show increased preference for diaxial conformation, as in 1 and 6, because diequatorial structure brings about repulsion of the vicinal C O bond dipoles. In contrast, X-ray diffraction data as well as calculated structures obtained for nitrogen-substituted 2 clearly show a crucial role of the intramolecular N H···N hydrogen bond in stabilizing the diequatorial conformation. We obtained crystals of 2 solvated either with ChemPhysChem 2014, 15, 1653 – 1659

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Figure 6. X-ray diffraction determined structures of a) diaxial ditrityl molecule 1 and b) solvated diequatorial ditrityl molecule 2 (CHCl3 molecule not shown).

www.chemphyschem.org R(2N) configuration and, therefore, can be regarded as a solid solution of these diastereomers. In a five-membered analog 4 both nitrogen atoms possess the same (S) configuration in the crystal. The resulting orientation of the N H bonds (anti with respect to the a carbon C H bonds) does not allow formation of an N H···N bond. The preferred conformation is diequatorial, according to both X-ray diffraction data and DFT calculations. The effect of conformational change of derivatives of trans1,2-diaminocyclohexane from diaxial to diequatorial has been previously reported as the result of coordination to a metal cation.[21] This process can be associated with breaking the symmetry of a ligand by changing the configuration of the nitrogen atoms from R,R to R,S.[25] However, no such effect was previously reported to occur in the absence of coordination to a metal cation, as we found here. To further characterize structural features of ditrityl molecules 1–6 we measured their ECD spectra and compared these with the calculated ECD spectra of their low-energy conformers discussed earlier. The method used for the calculation of the ECD spectra was time-dependent DFT (TD-DFT) at the level M06-2X/6-311G(d,p) for structures optimized at the B3LYP/6311G(d,p) level. TD B3LYP calculations of ECD spectra required inclusion of a large number of excited states and generally provided less reliable results. Measured short-wavelength Cotton effects which belong to 1B type transitions within the phenyl chromophores are shown in Table 3. Two examples of

Table 3. ECD data for ditrityl derivatives 1–6 in acetonitrile solution.

a polar solvent (chloroform, 1.0:0.7, 2 a) or with a nonpolar solvent (n-hexane, 1:1, 2 b). In both crystal structures the intramolecular NH···N hydrogen bond is formed (Figure 6) but in the former the chloroform molecule participates in stabilizing diequatorial conformation through the formation of a N H···Cl hydrogen bond (Figure 6). The nitrogen atom in the R configuration acts as a donor while the S-configured atom acts as both an acceptor and a hydrogen donor to one of the chlorine atoms of a chloroform molecule. A confirmation of the symmetry breaking of 2 in chloroform solution is provided by the 1 H NMR spectra. Four protons of the CH–NH groups give rise to two singlets at d = 2.34 and 2.46 ppm in cyclohexane-d12 and to one singlet at d = 2.42 ppm in deuterochloroform at 25 8C. Upon cooling this singlet splits at 30 8C into two and then at 60 8C into four signals (at d = 2.95, 2.87, 2.31, and 1.85 ppm, see Figure S2). This behavior corresponds to the structure shown in Figure 6 b. A molecule of 2 b adopts C2 symmetry in the crystal and therefore imposes the same helicity on the two trityl groups. However, this symmetrization is combined with disorder at the nitrogen substitution. The disorder stems from the formation of an intramolecular N H···N hydrogen bond and the requirement that the two contributing nitrogen atoms must possess an opposite chirality. This requirement obviously contradicts the positioning of the molecule on a two-fold axis. Therefore, the site is equally occupied by molecules of R(1C), R(1N), R(2C), S(2N) configuration and by molecules of R(1C), S(1N), R(2C),  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

De [nm] 1 2 3 4 5 6

21 21 53 52 33 40 37 17 17

Amplitude

(216) (215)[a] (216) (216)[a] (214) (208) (213) (215) (215)[a]

96 (201) 96 (201)[a] 102 (196) 62 (196)[a] 49 (199) 59 (194) 51 (199) 74 (200) 74 (200)[a]

Amplitude 117 117[a] 155 114[a] 82 99 88 91 91[a]

[a] In cyclohexane solution.

the ECD spectra are presented in Figure 7 to compare the results obtained for a predominantly diaxial (1) and a fully diequatorial (2) derivative. All ECD spectra are shown in Figure S1. A significant feature of all experimental and computed ECD spectra is the uniformly identical sequence of signs of Cotton effects: positive at around 215 nm and negative at around 200 nm, even though the conformer structures and conformer populations are different. This result can only be explained by the dependence of the ECD spectra on the absolute configuration of the neighboring stereogenic carbon center, which is uniformly R.[3] The differences between experimental and calculated ECD spectra of 1–6 are in the magnitudes of the Cotton effects; the ChemPhysChem 2014, 15, 1653 – 1659

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Figure 7. Calculated (c) and measured in acetonitrile (a) ECD spectra of 1 and 2.

calculated spectra are usually smaller and blue-shifted. In the case of 1 this can be rationalized by the equilibrium of participating pseudoenantiomeric trityl structures, MPM and PMP, which cause partial cancellation of the chiroptical spectra of 1. Since the trityl groups in 1–6 are situated far from each other, the amplitudes of the Cotton effects (A) are approximately twice the magnitudes of the corresponding monotritylated derivatives. For 1 and 2 the amplitudes are approximately 115 (Table 3, in cyclohexane solution) whereas the amplitudes of the Cotton effects of the corresponding monotritylated derivatives are approximately 50.[3, 4]

3. Conclusions The present study demonstrates that there are intramolecular steric interactions between trityl groups connected by two (in ditrityl ether), three (in ditrityl peroxide), and even five bonds (in 3–5). In the cases of cyclic ditrityl derivatives 1–6 the dominant intramolecular interaction is that involving the trityl group and the groups attached to an a carbon atom (see Table S2). Such an interaction generates an ECD spectrum due to the trityl group from which the absolute configuration of the a carbon atom can be readily deduced. Steric interaction in ditrityl molecules can be identified by analysis of X-ray diffraction data, and short distances between the Cortho, Cipso, and Hortho atoms of the two trityl groups fall below the sum of the corresponding vdW radii. Short C C dis 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemphyschem.org tances (below 3.40 ), C H distances (below 2.90 ), and H H distances (below 2.40 ) are found in ditritylmethane, ditrityl ether, and ditritylsulfide; in ditritylmethane they are associated with an eclipsed arrangement of two trityl residues. Such structural features can also be unveiled by calculations which, in the case of chiral structures, can be verified by comparing the calculated and the experimentally measured ECD spectra. An important structural feature of trityl groups, which identifies their interaction, is their conformation. Trityl groups not involved in steric interactions are C3-symmetrical (or nearly so) propellers and their helicity is either PPP or MMM. This is the case for ditrityl disulfide, in which the trityl groups are of opposite, propeller-type helicity. In a chiral molecular environment when the two trityl groups are at a longer separation, as in molecules 1–6, steric 1,3-interaction and weak hydrogen bonding between the trityl group and the substituent at the adjacent chiral center are the main forces of intramolecular interactions that involve trityl groups. This dominance can be detected by characteristic nonregular helicity (PMM/MPM and enantiomeric) of the trityl group and by means of the ECD spectra, due to the trityl chromophore. The spectra correctly reflect the absolute configuration of the adjacent chiral center.[3, 4] Such a nonregular helical conformation is found also in simple ditrityl molecules of the general formula Tr2X (X = CH2, O, S). Additionally, the present study shows that trans-1,2-disubstituted derivatives of cyclohexane and their five-membered ring congeners display conformational equilibria in which XTr substituents can be either equatorial or axial, depending on substituent X. When X = O, a diaxial conformer predominates (as in 1 and 6), whereas in the case of X = NH (compound 2) a diequatorial conformer is more stable due to the presence of intramolecular N H···N hydrogen bond. The bulky trityl groups attached to a small heteroatom (O, N) prefer to stay as far away from each other as possible [anti to the C(1) C(2) bond].

Acknowledgements This work was supported by UMO-2011/03/ST5/01011 research grant from National Science Center (NCN) Poland. Calculations were performed at Poznan Supercomputing and Networking Center. Keywords: circular dichroism · stereochemistry · trityl · X-ray diffraction

conformations

·

[1] a) P. J. Kocienski, Protecting Groups, Thieme, New York, 2005, pp. 269 – 274; b) T. W. Greene, P. G. M. Wats, Protective Groups in Organic Synthesis Wiley, New York, 2006, pp. 152 – 156. [2] a) V. Balzani, A. Credi, F. M. Raymo, J. F. Stoddart, Angew. Chem. Int. Ed. 2000, 39, 3348 – 3391; Angew. Chem. 2000, 112, 3484 – 3530; b) W. R. Browne, B. L. Feringa, Nat. Nanotechnol. 2006, 1, 25 – 35. [3] J. S´ciebura, P. Skowronek, J. Gawron´ski, Angew. Chem. Int. Ed. 2009, 48, 7069 – 7072; Angew. Chem. 2009, 121, 7203 – 7206. [4] J. S´ciebura, J. Gawron´ski, Chem. Eur. J. 2011, 17, 13138 – 13141. [5] a) C. Merten, A. Hartwig, Macromolecules 2010, 43, 8373 – 8378; b) C. Merten, L. D. Barron, L. Hecht, C. Johannessen, Angew. Chem. Int. Ed. 2011, 50, 9973 – 9976; Angew. Chem. 2011, 123, 10149 – 10152.

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CHEMPHYSCHEM ARTICLES [6] a) H. Iwamura, K. Mislow, Acc. Chem. Res. 1988, 21, 175 – 182; b) S. Hiraoka, E. Okuno, T. Tanaka, M. Shiro, M. Shionoya, J. Am. Chem. Soc. 2008, 130, 9089 – 9098; c) G. S. Kottas, L. I. Clarke, D. Horinek, J. Michl, Chem. Rev. 2005, 105, 1281 – 1376; d) R. E. Kay, D. A. Leigh, F. Zerbetto, Angew. Chem. Int. Ed. 2006, 46, 72 – 191; Angew. Chem. 2006, 119, 72 – 196; e) A. Carella, J.-P. Launay, R. Poteau, G. Rapenne, Chem. Eur. J. 2008, 14, 8147 – 8156; f) S. Ogi, T. Ikeda, R. Wakabayashi, S. Shinkai, M. Takeuchi, Chem. Eur. J. 2010, 16, 8285 – 8290; g) C.-Y. Kao, Y.-T. Hsu, H.-F. Lu, I. Chao, S.-L. Huang, Y.-C. Lin, W.-T. Sun, J.-S. Yang, J. Org. Chem. 2011, 76, 5782 – 5792. [7] D. K. Frantz, A. Linden, K. K. Baldridge, J. S. Siegel, J. Am. Chem. Soc. 2012, 134, 1528 – 1535. [8] J. F. Blount, A. Gutierrez, K. Mislow, J. Org. Chem. 1982, 47, 4359 – 4361. [9] a) C. Glidewell, D. C. Liles, Acta Crystallogr. Sect. B 1978, 34, 696 – 698; b) G. A. Jeffrey, A. Robbins, Acta Crystallogr. Sect. B 1980, 36, 1820 – 1826. [10] C. Glidewell, D. C. Liles, D. J. Walton, G. M. Sheldrick, Acta Crystallogr. Sect. B 1979, 35, 500 – 502. [11] T.-A. V. Khuong, G. Zapeda, C. N. Sanrame, H. Dang, M. D. Bartberger, K. N. Houk, M. A. Garcia-Garibay, J. Am. Chem. Soc. 2004, 126, 14778 – 14786. [12] M. Ostrowski, J. Jeske, P. G. Jones, W. W. du Mont, Chem. Ber. 1993, 126, 1355 – 1359. [13] E. L. Eliel, S. H. Wilen, M. P. Doyle, Basic Organic Stereochemistry, Wiley-Interscience, New York, 2001, Chap. 11.4. [14] C. H. Marzabadi, J. E. Anderson, J. Gonzalez-Outeirino, P. R. J. Gaffney, C. G. H. White, D. A. Tocher, L. J. Todaro, J. Am. Chem. Soc. 2003, 125, 15163 – 15173.

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemphyschem.org [15] H. Yamada, K. Okajima, H. Imagawa, T. Mukae, Y. Kawamura, M. Nishizawa, Bull. Chem. Soc. Jpn. 2007, 80, 583 – 585. [16] E. L. Eliel, H. Satici, J. Org. Chem. 1994, 59, 688 – 698. [17] B. Galeffi, M. Simard, J. D. Wuest, Inorg. Chem. 1990, 29, 955 – 958. [18] I. Alfonso, C. Astorga, F. Rebolledo, V. Gotor, S. Garcia-Granda, A. Tesouro, J. Chem. Soc. Perkin Trans. 2 2000, 899 – 904. [19] H. Wçlfle, H. Kopacka, K. Wurst, K.-H. Ongania, H.-H. Gçrtz, P. Preishuber-Pflgl, B. Bildstein, J. Organomet. Chem. 2006, 691, 1197 – 1215. [20] Z. Li, C. Jablonski, Inorg. Chem. 2000, 39, 2456 – 2461. [21] M. T. Reetz, E. Bohres, R. Goddard, Chem. Commun. 1998, 935 – 936. [22] CCDC 969033 (1), 969034 (2 a), 969035 (2 b), 969036 (3), 969037 (4), 969038 (5) and 969039 (6) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif. [23] W. Winter, A. Moosmayer, A. Rieker, Z. Naturforsch. B 1982, 37, 1623 – 1628. [24] S. Grimme, P. R. Schreiner, Angew. Chem. Int. Ed. 2011, 50, 12639 – 12642; Angew. Chem. 2011, 123, 12849 – 12853. [25] E. Rafii, B. Dassonneville, A. Neumann, Chem. Commun. 2007, 583 – 585.

Received: December 19, 2013 Revised: February 26, 2014 Published online on April 1, 2014

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