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Article Cite This: J. Org. Chem. 2018, 83, 3293−3298

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Stereoelectronic Interactions Exhibited by 1JC−H One-Bond Coupling Constants and Examination of the Possible Existence of the Intramolecular α‑Effect in Six-Membered Oxygen-Containing Heterocycles Eusebio Juaristi*,†,‡ and Rafael Notario*,§ †

Departamento de Química, Centro de Investigación y de Estudios Avanzados, Av IPN No. 2508, 07360 Ciudad de México, Mexico El Colegio Nacional, Luis González Obregón No. 23, Centro Histórico, 06020 Ciudad de México, Mexico § Instituto de Química Física “Rocasolano”, CSIC, c/Serrano 119, 28006 Madrid, Spain ‡

S Supporting Information *

ABSTRACT: For more than five decades since its original conception, the α-effect has been advocated with arguments based on kinetic reactivity data. The present study was undertaken with the aim of gathering theoretical information on thermodynamic bond energy data in systems that could in principle give rise to intramolecular α-effects. In particular, oxygen-containing six-membered rings oxa-, 1,2-dioxa-, 1,3-dioxa-, 1,2,4trioxa-, and 1,2,4,5-tetraoxacyclohexane were optimized at the B3LYP/aug-cc-pVTZ level of theory, and the magnitude of all C−H one-bond coupling constants was determined. Furthermore, hyperconjugative interactions were evaluated with Natural Bond Orbital analysis. Analysis of the collected information leads to the conclusion that ether oxygens are apparently better donors than peroxide oxygens; that is, the n(O) → σ*(C−Hax) two-orbital/two-electron interaction seems to be stronger than the n(O−O) → σ*(C−Hax) two-orbital/two-electron interaction, and this finding is contrary to expectations in terms of the αeffect.

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INTRODUCTION

In this regard, a very recent theoretical study of several fluorine-containing six-membered heterocycles found that the magnitude of the O−C−F anomeric effect relative to the O−O− C−F anomeric effect is contrary to expectations in terms of the α-effect, which would suggest increased donor ability of the peroxide [n(O−O) → σ*(C−Fax)] relative to an ether analogue [n(O) → σ*(C−Fax)]. This observation implies that the lone electron pairs in a pair of directly connected heteroatoms are not raised in energy to become stronger donors, but rather it would appear that the key n(O−O) → σ*(C−F) interactions in the “αsystems” are weaker than n(O) → σ*(C−F) interactions in “normal” systems.8 In this context, significant experimental and theoretical evidence has shown that two-orbital/two-electron stereoelectronic interactions that weaken an acceptor C−H bond result in an attenuation of the Fermi contribution to the associated one-bond 13C−1H coupling constant. Particularly relevant in this regard is the observation by Perlin and Casu that the magnitude of the one-bond coupling constant for an axial C− H bond adjacent to oxygen or nitrogen is 8−10 Hz smaller than 1 JC−H for the geminal equatorial C−H bond; that is, 1JC−Heq is larger than 1JC−Haxthe so-called “Perlin effect”.9 Indeed, empirical observations and extensive theoretical modeling in cyclohexane and six-membered heterocycles have confirmed the

Among emblematic reactions in organic chemistry, bimolecular nucleophilic substitution occupies a central place. By the middle of the 1950s, a particular group of nucleophiles had been found to react more rapidly than expected in terms of their intrinsic polarity. This group of unusually reactive nucleophiles includes hydroxylamine, hydrazine, the anions of peroxides, and others.1 It was appreciated that the common feature among these nucleophiles is the presence of an electronegative atom containing one or more pairs of unshared electron pairs adjacent to the nucleophilic atom.2 In 1962, Edwards and Pearson proposed that the excess reactivity exhibited by this class of reagents be called the “α-effect”.3 In the most accepted explanation of the α-effect, it is considered that the ground state of the nucleophile is destabilized by lone-pair/lone-pair electron repulsion, which leads to a high energy HOMO and, therefore, increased nucleophilicity.4 The αeffect is conceptually reasonable and rather attractive and has been indeed widely accepted by organic chemists.5,6 Nevertheless, in 1985, Hoz and Buncel7 criticized what they deemed a rather superficial way in which the α-effect had been advanced in the literature and pointed out that a systematic determination of thermodynamic data was missing. In particular, Hoz and Buncel recommended the examination of chemical equilibria that might exhibit “thermodynamic α-effects” in order to complement the available kinetic data. © 2018 American Chemical Society

Received: January 24, 2018 Published: February 22, 2018 3293

DOI: 10.1021/acs.joc.8b00220 J. Org. Chem. 2018, 83, 3293−3298

Article

The Journal of Organic Chemistry importance of n(X) → σ*(C−Hax) (X = O or N), σ(C−Hax) → σ*(C−Hax), σ(C−S) → σ*(C−Heq), and other two-electron/ two-orbital stereoelectronic interactions.10,11 Aiming to gather additional information regarding the putative existence of the α-effect in endocyclic O−O segments, we undertook the theoretical examination [with Natural Bond Orbital (NBO) theorythe method of choice for analysis of stereoelectronic interactions]12,13 of several six-membered heterocycles where an acceptor σ C−Hax bond can be used as a probe. In particular, the increased donor ability anticipated in O−O α-segments should lead to larger Perlin effects (weaker C− Hax bond, and therefore smaller 1JC−Hax coupling constants) relative to derivatives with a single heteroatom, i.e., ethers (Scheme 1).

Table 1. Calculated 1JC−H One-Bond Coupling Constant for the Axial and Equatorial C−H Bonds in Cyclohexanea

calculated C−Hax C−Heq a

B3LYP/aug-cc-pVTZ

ref 10c

experimental (in CS2)19

126.9 131.3 (4.4)

119.5 124.2 (4.7)

122.4 126.4 (4.0)

ΔJeq/ax = 1JC−Heq − 1JC−Hax values are shown in parentheses.

molecule, the smaller 1JC−H value for the axial bond is explained in terms of “double-bond/no-bond” σ(C−Hax) ↔ σ*(C−Hax) hyperconjugation that weakens C−Hax bonds.10g To facilitate the analysis of the collected data, Chart 1 (Supporting Information) summarizes the calculated (at the B3LYP/aug-cc-pVTZ level of theory) one-bond 13C−1H coupling constants (in Hz) for cyclohexane and the oxygen-containing heterocycles studied in this the present study. By comparison with cyclohexane, oxacyclohexane exhibits a much more sizable Perlin effect at the methylenic C−H bonds adjacent to the heteroatom (Table 2). Indeed, ΔJeq/ax is

Scheme 1. According to the Generally Accepted Interpretation of the α-Effect, Increased Electron Donation from the Peroxide O−O “α” System (Top) Should Lead to Smaller 1JC−Hax Coupling Constants Relative to the “Normal” Ether System (Bottom)

Table 2. Calculated 1JC−H One-Bond Coupling Constants for the Axial and Equatorial C−H Bonds in Oxacyclohexanea

B3LYP/aug-cc-pVTZ

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C2(6)−Hax C2(6)−Heq C3(5)−Hax C3(5)−Heq C4−Hax C4−Heq

COMPUTATIONAL METHODS

The package of Gaussian 09 programs was used to carry out all calculations.14 Molecular structures of minimum energy were fully optimized at the B3LYP/aug-cc-pVTZ level of theory. 1JC−H one-bond coupling constants were obtained using the gauge-independent atomic orbital (GIAO) method,15 requesting a two-step spin−spin coupling calculation16 (NMR = mixed keyword in Gaussian 09). All calculations were performed for isolated molecules in the gas phase, and no corrections for solvent effects were made. In this regard, a potential shortfall of the B3LYP functional in reproducing experimental JC−H values in some oxygen-containing tricyclic ring systems with stereoelectronic effects was reported very recently.17 Potential hyperconjugative interactions were evaluated with Natural Bond Orbital analysis, NBO program (version 3.1).18

a

139.3 150.0 (10.7) 130.9 129.7 (−1.2) 127.2 134.4 (7.2)


calculated to be +10.7 Hz at the B3LYP/aug-cc-pVTZ level of theory. As indicated in the Introduction, this large normal Perlin effect is interpreted in terms of n(O) → σ*(C−Hax) hyperconjugation that weakens the axial C−H bond. Also peculiar is the slightly negative Perlin effect, which is the slightly negative difference in 1JC−H one-bond coupling constants for the equatorial and axial C3,5−H bonds, Δ1JC−Heq/ax = −1.2 Hz at the B3LYP/aug-cc-pVTZ level of theory (Table 2). This negative Perlin effect is apparently the consequence of the counterbalancing stereoelectronic interactions σ(O1−C2,6) → σ*(C3,5-Heq) that weaken the equatorial C(3,5)-H bonds and σ(C−Hax) → σ*(C−Hax) hyperconjugation that weakens C− Hax bonds.10g By contrast, ΔJeq/ax at C(5) exhibit a normal Perlin effect, 1 JC−Heq − 1JC−Hax = 7.2 Hz, at the B3LYP/aug-cc-pVTZ level of theory (Table 2). This observation is indicative of the wellestablished σ(C−Hax) → σ*(C−Hax) hyperconjugation that weakens C−Hax bonds.10g Dramatically, the 1,2-dioxane molecule incorporating a peroxide α-segment exhibits negative ΔJeq/ax differences in

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RESULTS AND DISCUSSION A. Calculated 1JC−H Coupling Constants. Table 1 presents the calculated values of 1JC−H one-bond coupling constants (in Hz) for the axial and equatorial C−H bonds in cyclohexane. It can be appreciated that B3LYP/aug-cc-pVTZ calculation reproduces the relative magnitude of the experimental axial and equatorial C−H one-bond coupling constants observed in cyclohexane;19 that is, the normal Perlin effect worth +4.0 Hz (positive JC−Heq − JC−Hax, ΔJ values correspond to normal Perlin effects, where hyperconjugation leads to weaker axial C−H bonds and therefore smaller 1JC−Hax one-bond coupling constants relative to 1JC−Heq in the equatorial bond). In the cyclohexane 3294


Article

The Journal of Organic Chemistry coupling constants worth −3.7 Hz at the B3LYP/aug-cc-pVTZ level of theor, for the methylenic C(3,6)−H bonds (Table 3).

Table 4. Calculated 1JC−H One-Bond Coupling Constants (in Hz) for the Axial and Equatorial C−H Bonds in 1,3Dioxacyclohexanea

Table 3. Calculated 1JC−H One-Bond Coupling Constants (in Hz) for the Axial and Equatorial C−H Bonds in 1,2Dioxacyclohexanea

calculated B3LYP/aug-cc-pVTZ C3(6)−Hax C3(6)−Heq C4(5)−Hax C4(5)−Heq a

C2−Hax C2−Heq C4(6)−Hax C4(6)−Heq C5−Hax C5−Heq

148.7 145.0 (−3.7) 131.1 131.8 (0.7)


a

This observation is of course contrary to expectations in terms of the α-ef fect, since increased donor ability by the peroxide donor [n(O−O) → σ*(C−Hax) hyperconjugation] relative to the ether oxygen donor [n(O) → σ*(C−Hax) hyperconjugation] should afford a weaker C−Hax, and therefore a smaller 1JC−Hax coupling constant relative to 1JC−Heq. Evaluation of the relative magnitude of all two-orbital/two-electron stereoelectronic interactions in 1,2-dioxacyclohexane (see section B) exhibited a dominant σ(C3,6-Heq) → σ*(O−O) hyperconjugative interaction that weakens the C(3,6)−Heq bonds, counterbalancing the anomerictype n(O1,2) → σ*(C3,6−Hax) hyperconjugative interaction. As observed in oxacyclohexane, the small Perlin effect at C(4,5) is a consequence of a balance between σ(O1,2−C3,6) → σ*(C4,5−Heq) that weakens the equatorial C(4,5)-H bonds and σ(C3,6−Hax) → σ*(C4,5−Hax) hyperconjugation that weakens the C4,5−Hax bonds.10g Calculations for 1,3-dioxacyclohexane show the anticipated normal (and quite sizable) Perlin effect for the methylenic axial and equatorial C−H bonds at C(2), that is, +15.6 Hz as estimated by the B3LYP/aug-cc-pVTZ level of theory (Table 4). This very substantial Perlin effect at C(2) can be understood by considering that, here, two rather than one n(O) → σ*(C−Hax) stereoelectronic interactions are taking place. On the other hand, the negative Perlin effect at C(5), ΔJeq/ax = −6.8 Hz at the B3LYP/aug-cc-pVTZ level of theory is a consequence of “homoanomeric” n(O1,3) → β−σ*(C5−Heq) interactions.10b,g,20,21 Table 5 summarizes the calculated 1JC−H one-bond coupling constants for 1,2,4-trioxacyclohexane. This heterocycle is particularly interesting since it incorporates both a peroxide O−O segment that can in principle give rise to an α-effect at the methylenic C(3)−H and C(6)−H bonds as well as an ether oxygen that should originate a normal Perlin effect at the methylenic C(5)−Heq and C(5)−Hax. As it turns out, the calculated 1JC−H one-bond coupling constants for the equatorial and axial C−H bonds at C(5) give evidence of a medium-size normal Perlin effect worth 5.1 Hz from B3LYP/aug-cc-pVTZ level of theory (Table 5). By contrast, ΔJeq/ax for the methylenic C−H bonds at C(6), which are adjacent to the peroxide segment, exhibit a rather large negative Perlin effect, worth −10.6 Hz, as estimated by the B3LYP/aug-cc-pVTZ level of theory. Again, this is contrary to anticipation in terms of the α-effect and is due to a counterbalancing σ(C6−Heq) → σ*(O1−O2) hyperconjugative interaction that weakens the equatorial C(6)-H bond (see

B3LYP/aug-ccpVTZ

ref 10c

158.7 174.3 (15.6) 140.1 153.6 (13.5) 135.3 128.5 (−6.8)

152.8 167.6 (14.8) 132.5 145.0 (12.5) 128.3 122.0 (−6.3)

experimental (in CD2Cl2)10c 158.6 167.5 (8.9) 143.6 145.0 (1.4) 128.9 128.9 (0.0)


Table 5. Calculated 1JC−H One-Bond Coupling Constants (in Hz) for the Axial and Equatorial C−H Bonds in 1,2,4Trioxacyclohexanea

B3LYP/aug-cc-pVTZ C3−Hax C3−Heq C5−Hax C5−Heq C6−Hax C6−Heq a

170.4 168.1 (−2.3) 145.1 150.2 (5.1) 153.9 143.3 (−10.6)


section B) and dominates over the “anomeric-type” n(O1) → σ*(C6−Hax) hyperconjugative interaction. Finally, it is clear that the methylenic C−H bonds at C(3) exhibit a balance of a normal Perlin effect induced by the oxygen ether O4 that weakens C(3)− Hax and a negative Perlin effect induced by the peroxide αsegment, that is the σ(C3−Heq → σ*(O2−O1) interaction that weakens C(3)−Heq. Thus, ΔJeq/ax for C(3)−H is calculated to be −2.3 Hz according to B3LYP/aug-cc-pVTZ calculations (Table 5). On the other hand, 1,2,4,5-tetraoxacyclohexane incorporates two peroxide α-segments. In line with observations with 1,2dioxacyclohexane and 1,2,4-trioxacyclohexane (see above), a large and negative Perlin effect worth −18.3 Hz is calculated for ΔJeq/ax according to the B3LYP/aug-cc-pVTZ level of theory (Table 6). This finding is contrary to expectations in terms of the α-effect, meaning that the axial C−H bonds are stronger than the equatorial C−H bonds, and this observation seems to be a consequence of dominant σ(C3,6-Heq) → σ*(O−O) hyperconjugation that weakens the equatorial C(3,6)−Heq bonds. B. Interaction Energies E(2) and Donor/Acceptor Orbital Gaps ΔE. A particular feature of NBO analysis is that it provides an estimate of the magnitude of the interaction 3295


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The Journal of Organic Chemistry Table 6. Calculated 1JC−H One-Bond Coupling Constants (in Hz) for the Axial and Equatorial C−H Bonds in 1,2,4,5Tetraoxacyclohexane.a

Table 8. Interaction Energies [E(2)] and the Corresponding Energy Gaps (ΔE) for the Main Hyperconjugative Interactions in 1,2-Dioxacyclohexanea

B3LYP/aug-cc-pVTZ C3(6)−Hax C3(6)−Heq a

182.2 163.9 (−18.3)


energies between participating donor and acceptor orbitals. Tables 7−11 collect the interaction energies [E(2)] and the Table 7. Interaction Energies [E(2)] and the Corresponding Energy Gaps (ΔE) for the Main Hyperconjugative Interactions in Oxacyclohexanea

interaction

E(2) (kcal/mol)

ΔE (hartrees)

nOax → σ*(C2−Hax) nOax → σ*(C2−Heq) nOeq → σ*(C2−Heq) nOeq → σ*(C2−Hax) σ(C3−Heq) → σ*(C2−O) σ(C6−Heq) → σ*(O−C2) σ(C2−Hax) → σ*(C3−Hax) σ(C3−Hax) → σ*(C2−Hax) σ(C3−Hax) → σ*(C4−Hax) σ(C4−Hax) → σ*(C3−Hax)

6.39

0.65

2.50 0.82 3.92 3.52 2.80 2.84 2.88 2.91

0.95 0.92 0.79 0.80 0.89 0.85 0.88 0.88

interaction

E(2) (kcal/mol)

ΔE (hartrees)

nOax → σ*(C−Hax) nOax → σ*(C−Heq) nOeq → σ*(C−Heq) nOeq → σ*(C−Hax) nOeq → σ*(O−C) σ(C−Heq) → σ*(O−O) σ(O−O) → σ*(C−Heq) σ(C−Heq) → σ*(C−O)

5.06

0.69

1.23 0.68 1.87 4.93 1.33 3.65

1.06 1.06 0.96 0.60 1.09 0.78

a

All calculations were carried out at the B3LYP/aug-cc-pVTZ level of theory.

Hax) hyperconjugation in the ether analogue oxacyclohexane (6.39 kcal/mol, cf. Table 7). Equally important is the observation that the interaction energy E(2) for σ(C3,6−Heq) → σ*(O−O) hyperconjugation is rather large, 4.93 kcal/mol. This finding helps explain the small coupling constant calculated for the equatorial C(3,6)−H bonds that leads to a negative Perlin effect for these methylenic C−H bonds (cf. Table 7). Finally, it is worth pointing out the rather weak donor ability of the σ O−O bond toward the antiperiplanar σ*[C(3,6)−H] orbitals, that is E(2) = 1.33 kcal/mol for a putative σ(O−O) → σ*(C3,6−Heq) . It can be appreciated in Table 9 that the large Perlin effect observed at C(2) in 1,3-dioxacyclohexane (Jeq − Jax > 10 Hz, Table 9. Interaction Energies [E(2)] and the Corresponding Energy Gaps (ΔE) for the Main Hyperconjugative Interactions in 1,3-Dioxacyclohexanea

a


corresponding energy gaps (ΔE) for the main hyperconjugative interactions in oxacyclohexane, 1,2-dioxacyclohexane, 1,3dioxacyclohexane, 1,2,4-trioxacyclohexane, and 1,2,4,5-tetraoxacyclohexane, respectively. According to expectations, an inverse relationship between donor/acceptor energy gap and the magnitude of the twoelectron/two-orbital hyperconjugative interaction is seen. As anticipated,10b,g most relevant are the “anomeric-type” nOax → σ*(C2,6−Hax) hyperconjugative interactions that weaken the axial C(2,6)−H bonds, resulting in smaller one-bond coupling constant relative to the equatorial C(2,6)−H bondsthe normal Perlin effect (cf. Table 2). Also important are the stereoelectronic interactions between C(3,5)−Heq and the antiperiplanar σ* C−O bonds that weaken the equatorial carbon−hydrogen bonds. Finally, antiperiplanar σ(C−Hax) → σ*(C−Hax) hyperconjugative interactions that weaken the C− Hax bonds are also evidenced by the calculations (cf. Tables 7−11).10g Most interesting is the observation that, contrary to expectations in terms of an α-effect, the interaction energy E(2) for nOax → σ*(C3,6−Hax) hyperconjugation in this peroxide system is calculated to be smaller (5.06 kcal/mol) than nOax → σ*(C−

interaction

E(2) (kcal/mol)

ΔE (hartrees)

nO1ax → σ*(C2−Hax) nO1ax → σ*(C2−Heq) nO1ax → σ*(C6−Hax) nO1ax → σ*(C6−Heq) nOeq → σ*(C2−Heq) nOeq → σ*(C2−Hax) nOeq → σ*(C6−Heq) nOeq → σ*(C6−Hax) nOeq → σ*(C−O) σ(C2−Heq) → σ*(O−C6) σ(C6−Heq) → σ*(O−C2) σ(C5−Heq) → σ*(C6−O) σ(C6−O) → σ*(C2−Heq)

5.36

0.64

5.96

0.66

2.55 1.15 2.43 0.97 11.63 3.70 3.16 3.61 1.09

0.92 0.92 0.96 0.93 0.60 0.80 0.81 0.79 1.18

a


3296


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The Journal of Organic Chemistry Table 4) is the result of the sizable nO1ax → σ*(C6−Hax) and nO1ax → σ*(C2−Hax) stereoelectronic interactions E(2) = 5.96 and 5.36 kcal/mol, respectively, that weaken the axial C(2)−H bonds, resulting in a smaller coupling constant relative to the equatorial C(2)−H bonds. In this regard, σ(C4,6−O) → σ*(C2− Heq) is comparatively rather small, E(2) = 1.09 kcal/mol. Also quite significant is the confirmation of the strong nOeq → σ*(C−O) hyperconjugative interaction, E(2) = 11.63 kcal/mol, as well as the stereoelectronic interaction between equatorial C− H bonds antiperiplanar to O−C bonds (Table 9). It is interesting to contrast the n(O1) → σ*(C−O3) stereoelectronic interaction in 1,3-dioxacyclohexane [E(2) = 11.63 kcal/mol, cf. Table 9] with the n(O1−O2) → σ*(C−O4) stereoelectronic interaction present in 1,2,4-trioxacyclohexane [E(2) = 12.39 kcal/mol, Table 10]. While the stereoelectronic

Scheme 2. Comparison of the Interaction Energies in O−C− O versus O−O−C−O in 1,3-Dioxacyclohexane and 1,2,4Trioxacyclohexane, Respectively

Table 10. Interaction Energies [E(2)] and the Corresponding Energy Gaps (ΔE) for the Main Hyperconjugative Interactions in 1,2,4-Trioxacyclohexanea

Table 11. Interaction Energies [E(2)] and the Corresponding Energy Gaps (ΔE) for the Main Hyperconjugative Interactions in 1,2,4,5-Tetraoxacyclohexanea

interaction

E(2) (kcal/mol)

ΔE (hartrees)

nO1ax → σ*(C6−Hax) nO1ax → σ*(C6−Heq) nO1eq → σ*(C6−Heq) nO1eq → σ*(C6−Hax) nO2ax → σ*(C3−Hax) nO4ax → σ*(C3−Hax) nO4ax → σ*(C5−Hax) nO2eq → σ*(C3−O4) nO2eq → σ*(O1−C6) nO4eq → σ*(C3−O2) σ(C3−Heq) → σ*(O−O) σ(C6−Heq) → σ*(O−O) σ(O−O) → σ*(C3−Heq) σ(O−O) → σ*(C6−Heq) σ(C3−Heq) → σ*(O4−C5) σ(C5−Heq) → σ*(O4−C3) σ(C5−Heq) → σ*(C6−O1) σ(C6−Heq) → σ*(C5−O4)

5.26

0.69

1.13

1.07

4.42 5.30 6.14 12.39 1.81 11.78 5.00 4.72 1.37 1.42 3.57 3.10 3.27 3.29

0.67 0.66 0.67 0.62 0.96 0.59 0.60 0.60 1.08 1.08 0.81 0.82 0.79 0.80

interaction

E(2) (kcal/mol)

ΔE (hartrees)

nOax → σ*(C−Hax) nOeq → σ*(C−O) nOeq → σ*(O−C) σ(C−Heq) → σ*(O−O) σ(O−O) → σ*(C−Heq) σ(C−Hax) → σ*(C−O)

4.39 12.45 1.60 4.76 1.45 0.53

0.69 0.60 0.98 0.61 1.08 0.81

a


Most interestingly, the nOax → σ*(C−Hax) stereoelectronic interactions that weaken the axial C−H bonds at C(3,6), E(2) = 4.39 kcal/mol, are counterbalanced by σ(C−Heq) → σ*(O−O) stereoelectronic interactions E(2) = 4.76 kcal/mol that weaken the equatorial C−H bonds at C(3,6). This explains the negative Perlin effect at the methylenic C−H bonds in 1,2,4,5tetraoxacyclohexane (cf. Table 6).

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CONCLUSIONS In summary, calculated 1JC−H one-bond coupling constants can provide essential information regarding the relative strength of carbon−hydrogen bonds taking part of stereoelectronic interactions. Furthermore, NBO calculations help in the quantification and understanding of such hyperconjugative interactions. In the present study, analysis of stereoelectronic interactions involving oxygen as an electron donor in “anomerictype” hyperconjugation with antiperiplanar acceptor C−H bonds suggests that ether oxygens are apparently better donors than peroxide oxygens; that is, n(O) → σ*(C−Hax) two-orbital/twoelectron interaction seems to be stronger than n(O−O) → σ*(C−Hax) two-orbital/two-electron interaction, and this finding is contrary to expectations in terms of the α-effect.

a


interaction involving the peroxide donor segment is slightly larger than the same interaction in the ether analogue [E(2) = 11.63 kcal/mol, Table 9], the difference is relatively small and probably within the error margin of the calculations (Scheme 2). In this context, the “anomeric-type” nO1 → σ*(C6−Hax) involving the α-peroxide donor is calculated to be actually smaller [E(2) = 5.26 kcal/mol] than the same interaction involving an ether donor nO4 → σ*(C5−Hax) [E(2) = 6.14 kcal/ mol]. This result is contrary to expectations in terms of the αeffect. The relevance of σ(C3−Heq) → σ*(O−O) [E(2) = 5.00 kcal/ mol] and σ(C6−Heq) → σ*(O−O) [E(2) = 4.72 kcal/mol] is also confirmed (Table 10). As already discussed above, these stereoelectronic interactions seem to be responsible for the negative Perlin effect observed at C(3) and C(6) (cf. Table 5).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.8b00220. 3297


Article

The Journal of Organic Chemistry

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B3LYP/aug-cc-pVTZ-optimized Cartesian coordinates of the compounds studied in this work (PDF)

AUTHOR INFORMATION

Corresponding Authors

* E-mail: [email protected]. * E-mail: [email protected]. ORCID

Eusebio Juaristi: 0000-0003-0936-7020 Rafael Notario: 0000-0003-2957-8183 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from Consejo Nacional de Ciencia y Tecnologia,́ CONACYT grant number 220945, is gratefully acknowledged. We are also grateful to Igor V. Alabugin and Gabriel dos Passos Gomes, Florida State University, for helpful comments.

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REFERENCES

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