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13C NMR chemical shifts of the nitrile carbon in cyclohexanecarbonitriles directly correlate with the configuration of the quaternary, nitrile-bearing stereocenter. Comparing 13C NMR chemical shifts for over 200 cyclohexanecarbonitriles reveals that equatorially oriented nitriles resonate 3.3 ppm downfield, on average, from their axial counterparts. Pairs of axial/equatorial diastereomers varying only at the nitrile-bearing carbon consistently exhibit downfield shifts of δ 0.4–7.2 for the equatorial nitrile carbon, even in angularly substituted decalins and hydrindanes.
Cyclic nitriles are versatile synthetic intermediates2 that are readily transformed into an array of bioactive targets.3 Cylcohexanecarbonitriles in particular have a long and distinguished history as versatile synthetic intermediates,2,4 partly because of the prevalence of 6-membered rings in nature5 and partly because of the nitrile's small steric demand.6 The miniscule steric demand of the nitrile group, a mere 0.2 kcal mol−1,6 allows efficient installation of hindered quaternary carbons through alkylations7 and conjugate cyanations,8 even in cases where other methods fail.9
Assigning the configuration of nitrile-bearing, quaternary centers is challenging.10 X-ray crystallography provides an excellent method for crystalline nitriles whereas axial and equatorial orientations of liquid cyclohexanecarbonitrile diastereomers can be differentiated by their infrared CN bond intensity.11 In contrast, using mechanistic arguments to assign the configuration of cyclic nitriles resulting from conjugate cyanations12 or alkylations13 is not always reliable. An illustrative case is the alkylation of 1 in which deprotonations with i-PrMgBr or LDA/BuLi and methylation leads to different nitrile diastereomers (Scheme 1, 1→2→3 and 1→4→5). The divergent stereoselectivity is consistant with alkylations via C- or N-metalated cyclohexanecarbonitriles 2 and 4, respectively.14
During the synthesis of several diastereomeric cyclohexanecarbonitriles, the 13C chemical shift of the nitrile carbon was found to correlate with the orientation of the nitrile group (Scheme 1, compare 3 and 5). The attraction of using a simple NMR method to assign the stereochemistry of quaternary nitrile-bearing carbons stimulated an extensive examination of 13C NMR shifts in a diverse population of substituted cyclohexanecarbonitriles. Comparison of the chemical shifts revealed distinct regions depending on the configuration at the quaternary center. Equatorially oriented nitriles on fully substituted carbons typically resonate downfield, between δ 126.8-124.4, whereas the axially oriented counterparts resonate further upfield, between δ 124.6-118.6. Equatorial cyclohexanecarbonitrile diastereomers consistently resonate downfield from their axial counterparts, even for angular nitriles embedded within cis-and trans-decalins. The 13C NMR chemical shift provides a rapid, useful method for assigning the configuration of quaternary, nitrile-bearing centers in cyclohexanecarbonitriles.
Searching the Chemical Abstracts Service database15 was performed using the cyclohexanecarbonitrile16 substructure 6 (Figure 1).17 The search results were refined by specifying that the structures were single components, with reported stereochemistry, and with one or more references to articles in English. The references were limited to "journals" or "letters" appearing after 1985 from which time onwards the reporting of 13C NMR data was relatively routine. These search criteria returned 1008 references.
Nitriles without secure configurational assignments18 in the chair conformation were removed and duplications noted. Cis-fused cyclohexanecarbonitriles with nitrile substitution in the angular position were removed unless a matching trans-diastereomer was present. Using these criteria, comparative 13C NMR chemical shifts of the nitrile carbon, acquired in CDCl3, were obtained for 211 structures.19 Structures were categorized into three structural types: equatorially oriented nitriles other than angularly-substituted nitriles in cis-fused polycycles, axially oriented nitriles including angularly substituted nitriles in trans-fused polycycles, and pairs of nitrile diastereomers differing solely in the configuration of the angularly substituted nitrile (Tables 1, 2, and 3 respectively in the Supporting Information).
Using the 13C NMR nitrile chemical shift to assign configurations in cyclohexanecarbonitriles is attractive because the nitrile carbon resonates in a narrow chemical shift range,20 has a diagnostic signal intensity,21 and occupies a spectral window with minimal overlap by other functionalities.22 Nitriles influence the chemical shifts of adjacent nucleii23 through π-electron deshielding24 and through a polar inductive effect.25 The reverse anisotropic influence on the nitrile carbon chemical shift appears to only have been used in assigning the E/Z geomertry in a series of alkenenitriles.26 The consistent downfield shift of the 13C nitrile resonance in E-alkenenitriles provides a valuable assignment method that hints at a greater role for the nitrile 13C NMR resonance in reporting the local stereochemistry.
Cyclohexanecarbonitriles preferentially adopt slightly flattened chair conformations in solution27 and the solid state.14,28 The 13C NMR shifts of axial and equatorial substituents on regular chair conformations depend on the complex interplay between steric effects,29 electron motion and hyperconjugation.30 Analyzing the cyclohexanecarbonitrile 13C chemical shifts for the nitrile carbon reveals two distinct populations: equatorial nitrile carbons resonate downfield (δ 126.8-124.4 ppm), and axial nitrile carbons resonate further upfield (δ 124.6-118.6 ppm). A third group of cyclohexanecarbonitriles bearing adjacent heteroatoms or carbonyls exhibit no clear correlation of chemical shift with stereochemistry (Table 4, Supporting Information).31 These electron withdrawing groups induce their own anisotropy32 and create a complex interplay between the adjacent σ- and π- electrons33 that overrides the local magnetic field.
The two populations of equatorially and axially oriented nitriles have a region of chemical shift overlap for the nitrile carbon resonances of only 0.2 ppm! Only 9 out of 129 axial or equatorial cyclohexanecarbonitrile resonances lie in the same chemical shift region (Tables 1 and 2, Supporting Information). The graphical representation of this distribution shows the different chemical shift regions for these two populations and the small region where the two populations overlap (Figure 2). The visual picture also highlights the greater population of axially oriented nitriles (red), reflecting their facile formation by hydrocyanation of enones8 and through alkylations of N-metalated nitriles.2a The average chemical shift of the equatorially oriented nitriles is 3.3 ppm further downfield than the axial counterparts (125.7 ppm, σ = 0.7 and 122.4 ppm, σ = 1.5, respectively).
The correlation between the chemical shift and the configuration of the nitrile-bearing stereocenter is more pronounced in diastereomeric pairs of cyclohexanecarbonitriles (Table 3, Supporting Information). In 24 pairs of nitrile diastereomers, the equatorial nitriles consistently resonate downfield from their axial counterparts by 0.4–7.2 ppm. The diastereomeric nitriles 8e and 8a illustrate the general trends and the types of functionality present in the data set (Figure 3). Hydroxylation has a minimal effect on the nitrile chemical shift when present either within alkyl substituents or incorporated within the cyclohexane ring. Olefinic and aromatic substituents are tolerated as ring substitutents. Carbonyl groups two, or more, carbons removed are tolerated but their strong anisotropy overides the chemical shift correlation when adjacent to the nitrile-bearing carbon.32
Remarkably, the chemical shift trends are maintained in cis-and trans-decalins that differ only in the stereochemistry of an angular nitrile! In cis-decalins the angular nitrile is equatorial with respect to one cyclohexane ring (9e Figure 4, black) and axial with respect to the other (9e Figure 4, magenta). In contrast, the angular nitrile of the trans-decalin counterpart is axial with respect to two rings (compare 9e with 9a in Figure 4). Despite this seemingly small structural difference the "equatorial" nitrile carbon in cis-decalins resonates 0.4–4.6 ppm further downfield (Table 3, Supporting Information). The only instance in which the trend is not observed is with a conformationally mobile cis-decalin.34 This underscores the requirement for the cyclohexanecarbonitrile to be in a "locked" chair conformation. The largest chemical shift differences were for angularly substituted cis and trans-hydrindanes (Δδ = 4.2–7.6 ppm) which show the same respective equatorial downfield and axial upfield chemical shift movements as the cis- and trans-decalins.
An extensive literature search located more than 200 examples of 1-alkylcyclohexanecarbonitriles with assignable 13C NMR chemical shifts for the nitrile carbon. Equatorially oriented cyclohexanecarbonitriles, in which the nitrile-bearing carbon is fully substituted, typically resonate between δ 126.8-124.4 whereas the corresponding axial diastereomers resonate further upfield, between δ 124.6-118.6. Only 9 out of 129 resonances lie in the same chemical shift region with the average equatorially oriented nitrile resonating 3.3 ppm downfield from the axial counterpart.
Diastereomeric pairs of cyclohexanecarbonitriles that vary only at the nitrile-bearing carbon exhibit consistent downfield shifts for the equatorial nitrile carbon provided that the chair conformation is not fluxional. The 13C NMR chemical shift of the nitrile carbon provides a rapid method of assigning the configuration of quaternary nitrile-bearing carbons in diverse cyclohexanecarbonitriles.
Financial support of nitrile-based research from NIH (2R15AI051352) and NSF (CHE 0808996) is gratefully acknowledged.