ARTICLE pubs.acs.org/JPCA
Computational Insight into the Carbenic Character of Nitrilimines from a Reactivity Perspective Heidi M. Muchall Centre for Research in Molecular Modeling and Department of Chemistry and Biochemistry, Concordia University, 7141 Sherbrooke Street West, Montreal, Quebec, Canada H4B 1R6
bS Supporting Information ABSTRACT: Nitrilimines (R—CNN—R) can be described through a carbenic valence bond structure, and although intermolecular carbenic reactions from nitrilimines are unknown, intramolecular reaction products from ortho-vinyl MeCOO— CNN—Ph (1) and Ph—CNN—Ph (2) that seem to have followed two typical carbene reaction mechanisms, [1+2] cycloaddition and C—H insertion, have been reported. This study sheds light on whether such reaction mechanisms are tenable, using the electron density and its Laplacian (QTAIM), natural bond orbital (NBO) descriptions, and reaction profiles. It is shown that the reaction in 1 is distinctly different from the [1+2] cycloadditions of ethene with the typical singlet carbene CF2 and the nitrilimine F—CNN—F with its large carbenic character, and the formal [1+2] cycloaddition product from 1 is in fact not the primary product. Similarly, it is shown that the reaction in 2 is fundamentally different from the C—H insertions of CF2, F—CNN—F, and even H—CNN—H with its small carbenic character into methane, and again the formal C—H insertion product from 2 is not the primary product. The small model reactions from CF2, F—CNN—F, and H—CNN—H were analyzed using B3LYP, MP2, and B2PLYP with the aug-cc-pVTZ basis set, whereas the full study was performed with B3LYP/6-31+G(2d,2p), as it was shown to be sufficient.
’ INTRODUCTION Do nitrilimines (R1—CNN—R2) exhibit carbenic reactivity? The possibility is certainly there, with a general nitrilimine described in the literature14 by a total of six different valence bond structures as given in Scheme 1. The most prominent reaction of nitrilimines is the [3+2] (or 1,3-dipolar) cycloaddition. From a synthetic point of view, it offers a convenient entry to a variety of nitrogen-containing, fivemembered heterocycles (Scheme 2a).5 Though usually this cycloaddition is an intermolecular reaction between a nitrilimine as the 1,3-dipole and an unsaturated system as the dipolarophile, the latter can also be tethered to the nitrilimine functionality, thus allowing for an intramolecular reaction.6 For sufficiently long tethers, both as C- or N-substituent on the nitrilimine, intramolecular [3+2] cycloadditions have been observed (Scheme 2b).7,8 In Scheme 2a,b the nitrilimine is therefore represented as a 1,3dipole. A short tether, though, does not allow for the necessary flexibility of the geometry to achieve a [3+2] transition state, and other reaction channels become prominent. Thus, the addition of the nitrilimine carbon to the terminal carbon of a tethered CdC double bond with formation of seven-membered rings has been observed, as have cyclopropanes (Scheme 3).2,9,10 This formation of a cyclopropyl ring from a nitrilimine is particularly intriguing, because formally it represents a [1+2] cycloaddition as would be observed from a carbene, a reaction that in its intermolecular version has not yet been observed experimentally r 2011 American Chemical Society
for a nitrilimine, and the question arises on how to represent the electronic structure of said nitrilimine. Scheme 3 presents the nitrilimine as a carbene.2 In the absence of a dipolarophile, a different type of cyclization, arising from a formal insertion of the nitrilimine carbon into a C—H bond, has been reported for nitrilimines carrying aromatic substituents (Scheme 4).1113 As C—H insertion is again a reaction observed from carbenes, Scheme 4 also presents the nitrilimine as a carbene.12 Although the reactions in Schemes 3 and 4 have previously been associated with carbenic reactivity from the CNN moiety,2,10,12 other descriptions can be found in the literature. Thus, the formation of a seven-membered ring has been classified as a 1,7electrocyclization,6,9 the C—H insertion as an electrophilic substitution11 or a 1,5-electrocyclization,6 and the nitrilimines were represented with their propargylic structure.6 Previous efforts using computational chemistry to shed light on cycloaddition reactions from nitrilimines seem to have been focused almost entirely on their [3+2] cycloadditions. Thus, regioselectivity has been probed through a comparison of transition state energies,14,15 and an understanding has been sought using reactivity indices.16 Transition state energies have Received: September 3, 2011 Revised: October 3, 2011 Published: October 05, 2011 13694
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Scheme 3
Scheme 4 Scheme 2
been compared to rationalize an observed stereoselectivity in enantiomerically pure tethered nitrilimines.8 The one computational study on [1+2] cycloadditions from nitrilimines presents an analysis of the shared potential energy surface for [3+2] and [1+2] addition of H—CNN—H to C2H4 and a rationalization for the observed product formation,17 even though the presence of a carbenic resonance form for H—CNN—H has been questioned.18 As expected from the kinetic instability of nitrilimines, activation energies are in general small. Thus, the activation enthalpy for the intermolecular [3+2] cycloaddition of H—CNN—H and C2H4 was reported at 7.2 kcal mol1 from CBS-QB3, compared to 4.5 kcal mol1 from B3LYP/cc-pVTZ.19 With PBE0/6-311+ +G(2df,pd), the corresponding zero-point vibrational corrected activation energy was reported at 8.4 kcal mol1.17 Addition of MeCOO—CNN—Ph to a CdS moiety required a B3LYP/631G(d) gas phase activation energy around 3.5 kcal mol1 for attack from the electron rich sulfur onto the electrophilic CNN carbon atom.15 The regioisomeric C 3 3 3 C interaction, on the other hand, was determined to be a relatively high-energy process, with a barrier of about 21 kcal mol1.15 In contrast, for the [3+2] cycloaddition of MeCOO—CNN—Me to indenone determined with B3LYP/6-311G(d), the transition states with their C 3 3 3 C interactions leading to the two regioisomers are closer in energy, 5.4 and 9.0 kcal mol1.14 For the intramolecular [3+2] cycloaddition of RCOO—CNN—Ph, where R = (S)CH2dCH—CHMe, relative energies for the two diastereomeric transition states from MP2/cc-PVDZ are 4.6 and 5.9 kcal mol1 and drop to as low as 2.7 kcal mol1 for a single point energy calculation with the aug-cc-PVDZ basis set.8 The activation energy for [1+2] cycloaddition to C2H4 depends largely on the nitrilimine substituents. Thus, with PBE0/6-311++G(2df,pd), H—CNN—H needs the relatively high activation energy of
12.5 kcal mol1, whereas F—CNN—F requires only 1.7 kcal mol1.17 An analysis of the electronic structure of the smallest nitrilimine, H—CNN—H, from PBE0/6-311++G(2df,pd) within natural resonance theory,2022 gave a resonance description of 22% propargylic, 18% allenic, 33% 1,3-dipolar, and 27% carbenic valence bond contributors.23 In fact, the PBE0/6-311++G(2df, pd) potential energy surface for H—CNN—H addition to C2H4 demonstrates an initial carbene-like interaction, as expected in a [1+2] reaction and in accord with a carbenic component to the electronic structure of H—CNN—H, with the NC 3 3 3 CC torsional angle close to 90°. In later stages of C 3 3 3 C approach, this torsional angle decreases to 0°, which is followed by closure to the five-membered ring expected from a [3+2] reaction.17 In accord with this, it was suspected early that the lack of [3+2] reactivity in nitrilimines with short alkene tethers could be attributed to geometrical constraints.10 In this paper, we explore with computational methods whether the description of reactions of ortho-vinyl MeCOO— CNN—Ph 1 (intramolecular CdC addition) and Ph—CNN— Ph 2 (intramolecular C—H insertion) in terms of carbene-type reactivity as alluded to in the literature is appropriate. To this end, we first analyze a typical singlet carbene, CF2, and its transition state for addition with ethene and compare the results to those obtained for the nitrilimines F—CNN—F and H—CNN—H, whose carbenic character has been reported,23 and the [1+2] transition states for their additions to ethene. This is followed by an analysis of the insertion of CF2 into a C—H bond of methane, again with the aim to demonstrate a typical singlet carbene case, and comparisons to insertions for F—CNN—F and H—CNN—H. The analyses are preformed on the electron densities within the quantum theory of atoms in molecules,24 and on the molecular orbitals within the natural bond orbital approach,20,25 and the results are compared to those for 1 and 2. For 1 and 2, the reaction paths are discussed as well.
’ COMPUTATIONAL DETAILS All geometry optimizations were performed using the Gaussian 09 suite of programs.26 The full study was performed with the Becke327Lee, Young and Parr (LYP)28 hybrid density 13695
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Table 1. Total and Gibbs Free Energies (kcal mol1), Bond Angles (deg) on CNN Atoms, and C 3 3 3 C Distances (pm) in the Transition States, from B3LYP/6-31+G(2d,2p) Etot
G298
CF2
237.719715
237.737097
104.5
CF2-TSb CF2-TSinsd
316.304730 278.190567
316.272919 278.165970
105.3a 108.7a
RCN
CNN
NNR
C3 3 3C
a
185.9 (248.0)c 214.5 (113.8)e
FCNNF
347.124570
347.137606
115.4
156.6
108.1
FCNNF-TSb
425.722693
425.687140
113.1
131.9
107.9
216.1 (266.7)c
387.620958
387.591809
112.2
135.7
109.7
222.0 (122.6)e
FCNNF-TSins
d
HCNNH
148.716595
148.708295
129.7
169.2
109.4
HCNNH-TSb
227.298157
227.241612
125.3
142.3
108.3
199.8 (252.4)c
189.173186
189.123742
121.2
135.1
109.5
216.9 (119.1)e
685.095233 685.093224
684.949252 684.947030
138.6 139.6
170.6 170.5
117.9 118.0 119.7
HCNNH-TSins 1a 1b
d
1c
685.088492
684.942374
139.2
169.8
1d
685.086631
684.940194
140.5
169.8
119.3
1-TS
685.077959
684.929111
133.8
155.1
117.8
2
610.874175
610.720650
177.9
172.5
117.1
2-TS
610.835525
610.679962
130.8
127.6
109.8
229.0 (277.3)c 220.0 (202.2)e
The carbene carbon atom. The transition state for [1+2] cycloaddition to ethene (Etot 78.601382, G298 78.572660 au). The longer C 3 3 3 C distance (missing the bond path) in parentheses. d The transition state for insertion into methane (Etot 40.526937, G298 40.501964 au for C1 symmetry). e C 3 3 3 H distance in parentheses. a
b
functional (B3LYP)29 and the 6-31+G(2d,2p) basis set. We chose this basis set for its flexibility with respect to the description of the CNN atoms as well as for hydrogen atoms in the various transition states. We checked the performance of this model chemistry against that of B3LYP with the larger aug-cc-PVTZ basis set,30,31 as well as against second-order MøllerPlesset perturbation theory (MP2)32 with the aug-cc-pVTZ basis set for the small transition states (all geometries optimized). Considering that B3LYP overestimates and MP2 underestimates barriers to reaction, we also employed the highly praised B2PLYP (where B is Becke’s 1988 exchange functional,33 the 2 “indicates the number or parameters and the order of perturbation theory and PLYP denotes perturbative correlation with the LYP functional”),34 again with the aug-cc-pVTZ basis set. B2PLYP has been shown to outperform both B3LYP and MP2 for activation enthalpies as judged against CBS-QB3,19,35 including for the H—CNN—H plus ethene [3+2] cycloaddition.19 The nature of the B3LYP/6-31+G(2d,2p) stationary points was probed through vibrational analyses within the harmonic oscillator approximation. Intrinsic reaction coordinate (IRC) calculations using mass-weighted Cartesian coordinates were performed, again for B3LYP/6-31+G(2d,2p) only, to confirm the relationship between transition states and reactants and products. The IRC reaction paths for the small model systems (addition and insertion reactions of CF2, F—CNN—F, and H— CNN—H) are displayed in Figure S1 of the Supporting Information. Additional transition states and minima were not located on the singlet surface. All results presented used restricted wave functions; lower energy open shell and in particular diradical solutions were not obtained. All B3LYP/6-31+G(2d,2p) electronic and Gibbs free energies (at 298 K) are summarized in Table 1; the aug-cc-pVTZ energies from the various theories are listed in Table S1 of the Supporting Information. Basis set superposition errors were disregarded for all transition states, as the two target molecules (1 and 2) exhibit intramolecular reactions.
c
Atomic charges were determined using the natural bond orbital (NBO) approach,20,25 as implemented in Gaussian 09, and the quantum theory of atoms in molecules24 (QTAIM) through the program AIMAll.36 Orbital interactions were obtained from the NBO analyses. Delocalization indices were determined, and molecular graphs and contours and isosurfaces of the negative of the Laplacian of the electron density were plotted, from AIMAll. In the molecular graphs, small red spheres indicate bond critical points, small yellow spheres ring critical points. QTAIM and NBO can be considered complementary analytical methods for the evaluation of structures and reactivities, as well as for the relationship between these properties, and the literature has numerous examples for either method of which only a few are referenced here that employ the Laplacian of the electron density within QTAIM,3740 and the lone pair occupancy within NBO.41
’ RESULTS AND DISCUSSION 3.1. Model Bimolecular Addition and Insertion Reactions. Before we turn to a description of the interactions within the potentially intricate systems 1 and 2, we present QTAIM and NBO analyses for two typical reactions from singlet carbenes: addition to CdC and insertion into C—H bonds, both of which are concerted reactions.42 Considered are CF2, a typical electrophilic carbene, F—CNN—F, a nitrilimine with large carbenic character, and H—CNN—H, a nitrilimine with less carbenic character (as determined from NRT).23 The purpose is to show, with CF2 and F—CNN—F, what might be considered “standard” data that are easy to interpret, and to compare the data for H—CNN—H to these. Table 1 lists energies and bond angles. From molecular orbital theory, a singlet carbene, such as CF2, and its reactions are described through an empty p-orbital on carbon, p(C), perpendicular to the molecular plane and lending it electrophilic properties, and an electron lone pair on carbon, 13696
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Table 2. Charges (au) on CNN Atoms, Value of r2G(r) (au) at the Nonbonded Charge Concentration of the CNN Carbon Atom, and Delocalization Index (au), from B3LYP/6-31+G(2d,2p) NBO q(C)
q(N)
QTAIM q(N)
0.679b
CF2 e
CF2-TS
0.641
b
0.477 0.522
b
q(C)
q(N)
q(N)
r2F
DI(C,C1)a
DI(C,C2)a
0.543b
0.377b
g
0.655h
1.65b,d
1.258b,c b
1.46b
b
0.542
1.089 0.211
0.253
1.065 1.268
0.669
0.320
1.41 1.33
FCNNF-TSe
0.501
0.232
0.194
1.123
0.587
0.242
1.36
0.321
0.159
FCNNF-TSinsf
0.335
0.196
0.221
1.002
0.500
0.234
1.26
0.323g
0.529h
CF2-TSins FCNNF
f
b
HCNNH
0.055
0.053
0.515
0.720
0.835
0.385
0.75
HCNNH-TSe
0.053
0.124
0.526
0.663
0.782
0.421
0.77
0.423
0.167
HCNNH-TSinsf
0.272
0.130
0.431
0.486
0.619
0.347
0.92
0.365g
0.596h
1a
0.140
0.001
0.253
0.756
0.826
0.383
0.69
1b 1c
0.146 0.142
0.001 0.001
0.254 0.257
0.761 0.755
0.830 0.821
0.381 0.384
0.67 0.69
1d
0.152
0.006
0.263
0.767
0.824
0.387
0.66
(0.033)i
(0.013)i
1-TS
0.169
0.055
0.254
0.756
0.816
0.416
0.67
0.260
0.059
2
0.331
0.011
0.343
0.828
0.926
0.453
2-TS
0.264
0.179
0.272
0.632
0.780
0.472
0.75
0.349g
0.072h
a
Delocalization index for the weak interactions with the C1dC2 atoms: C,C1 short distance; C,C2 long distance. b The carbene carbon atom. c 1.552 from HF/6-31+G(d,p), taken from ref 40. d 1.91 au from HF/6-31+G(d,p), taken from ref 40. e The transition state for [1+2] cycloaddition to ethene. f The transition state for insertion into CH4. g Delocalization index for the C 3 3 3 C interaction. h Delocalization index for the C 3 3 3 H interaction. i DI values for the corresponding atoms in the reactant.
n(C), in the molecular plane, which is responsible for its nucleophilic properties.42 QTAIM. Although evidence for electron lone pairs is not present in the electron density, F(r), within QTAIM it is uncovered through the Laplacian of the electron density, r2F(r), and in particular through a maximum in the valence shell charge concentration (VSCC). The position of such a nonbonded maximum often coincides with the position of a lone pair in the molecular orbital approach.24,39 The value of these nonbonded maxima on carbon for CF2, F—CNN—F, and H—CNN—H are given in Table 2. Because the Laplacian values within the VSCC are negative, the most pronounced maximum possesses the most negative value. Figure 1a depicts this (large) nonbonded maximum in the VSCC (solid lines) of the carbene CF2, for the carbon atom and in the molecular plane.40 Possibly more importantly for our study, Figure 1b shows a similar result for the carbon atom in F—CNN—F, which, from PBE0/6-311++G(2df,pd), exhibits a carbon bond angle of 117° and 57% carbene character.23 The slightly smaller nonbonded charge concentration is found again in the FCN plane, where an electron lone pair on an sp2 hybridized carbon atom would be expected. A small nonbonded charge concentration is located in H—CNN—H (Figure 1c), for which a carbon bond angle of 133° and 27% carbene character were determined (from PBE0/6-311++G(2df,pd)).23 The electron density equivalent of an empty p-orbital is a minimum in the VSCC.24,39 Though this can be shown through contour maps of r2F(r), Figure 2 depicts the r2F = 0 isosurface instead. The large hole in the VSCC on carbon in CF2 and the somewhat smaller hole in F—CNN—F are readily apparent. In contrast, the carbon atom in H—CNN—H does not possess such a hole, in agreement with the small (least negative) value of its VSCC maximum, as well as its much smaller positive charge as compared to carbon atoms in CF2 and F—CNN—F (Table 2).
NBO. Within the NBO analysis, the QTAIM nonbonded charge concentration on the carbon atom is represented by an electron lone pair, n(C), the hole in the VSCC by a partially occupied p-orbital, p(C). Table 3 lists the values of these parameters for CF2 and F—CNN—F, but not for H—CNN—H: in accord with the decreasing maxima (less negative r2F(r) values) from Table 2, n(C) decreases in Table 3 and is not located for H—CNN—H. This is also in accord with the changing resonance description of the two nitrilimines: going from F—CNN—F to H—CNN—H, both propargylic and 1,3-dipolar contributions to the electronic structure, which are small in F—CNN—F, increase (by 16 and 14%, respectively) at the expense of the carbenic contribution,23 which is reflected in a triple bond for CN in H—CNN—H and the lack of n(C) and p(C). To better understand the NBO description of H— CNN—H, its CN π-bond contributions were compared to those from H2CdNH (an imine with its regular CdN) and HCNH2 (a carbene with its ylidic CdN). The data are reported in Table S2 of the Supporting Information. Briefly, when the sum of the atomic coefficients for a bond orbital is set to 100%, for a regular π(CN) the coefficient on nitrogen is larger than that on carbon, yet stays below 60%. In the carbene HCNH2, in agreement with its CdN viewed as an n(N) f p(C) donation, the coefficient on nitrogen is as large as 80%. Finally, in the nitrilimine, the size of the nitrogen coefficient for the perpendicular π(CN) lies between those of the imine and the carbene and, more importantly, the in-plane π-bond completing the triple bond does not possess pure p-character on carbon but exhibits a large s-contribution (18%), coupled with a disproportionately large coefficient on carbon. One could argue that the NBO description therefore also recovers the small “lone pair” on carbon in H—CNN—H, in agreement with the small maximum in r2F(r). 13697
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Figure 2. Plot of the 0 au isosurface of r2F(r) of CF2, F—CNN—F, and H—CNN—H, from B3LYP/6-31+G(2d,2p). The arrow points to the hole in the VSCC of carbon in CF2 and F—CNN—F.
Figure 1. Contour plots of r2F(r), from B3LYP/6-31+G(2d,2p), (a) in the molecular plane of CF2, (b) in the FCN plane of F—CNN—F and (c) in the HCN plane of H—CNN—H. The solid contour lines depict regions of local charge concentration (negative values). Each arrow points to the nonbonded maximum in the VSCC of carbon.
3.2. Addition of CF2, F—CNN—F, and H—CNN—H to C2H4. QTAIM. The contour plots for r2F(r) of the transitions
states for [1+2] cycloaddition are given in the CCC plane in Figures 3ac. The three plots are qualitatively similar: In Figure 3a, the newly forming C—C bond has progressed further than in Figure 3b,c (Table 1), which moves the bond critical point for this interaction into the VSCC. The values of r2F(r) for the nonbonded charge concentration on the carbenic or CNN carbon atom in a transition state are again listed in Table 2. For CF2, the maximum becomes substantially smaller (less negative) upon formation of the transition state (CF2-TS), whereas those for the nitrilimines do not change. The tilt angle of the two formal planes (the undistorted, ideal C2H4 plane and the RCR plane in the carbene or the nitrilimine) against one another is 48° (Figure 3a), 48.5° (Figure 3b), and 66°
(Figure 3c). Thus, the approach of the nucleophilic ethene onto CF2 does not follow an acute angle with the C—F bonds, as was discussed from the location of the hole earlier,40 and it is apparent from the tilt angle and Figure 3a that the minimum in the VSCC of the CF2 carbon atom is not aligned with the bond path for the newly forming C—C bond. This is true for all three transition states in Figure 3. Neither is the maximum aligned. In fact, the bond path bisects the angle formed by a carbon nucleus and the maximum and minimum in its VSCC. For CF2 and F—CNN—F with their carbenic character, this is of course in line with the molecular orbital notion that both an electron lone pair and an empty p-orbital on carbon can interact with the π-system of ethene with its π* and π orbitals, respectively. With the missing hole in the VSCC, QTAIM establishes a qualitative difference for H—CNN—H that is reflected in the larger tilt angle. Parts dm of Figure 3 illustrate that descriptions with the larger basis set (aug-cc-pVTZ) qualitatively yield the same analysis, with two exceptions. With MP2 (Figure 3gi), which tends to underestimate weak interactions, only the CF2-plusethene transition state resembles those from B3LYP. B2PLYP (Figure 3km), which compensates for both B3LYP overestimation and MP2 underestimation of weak interactions, confirms the B3LYP description of the transition states, thus rendering the MP2 results for the two nitrilimines (Figure 3h,i) outliers. In other words, the results from the smaller model chemistry (B3LYP/6-31+G(2d,2p)) are representative for the type of systems studied here, and its use for 1 and 2 is justified. Total energies, C 3 3 3 C distance in the transition states, RCR angles, and r2F(r) values for the nonbonded charge concentration for the larger model chemistries are given in Table S1 of the Supporting Information. Finally, it is worth commenting on the concertedness of the cycloaddition. The orbital-controlled geometry of the transition state for cycloaddition of a singlet carbene was established early.43 Thus, the transition state is not symmetric (often called asymmetric) with respect to the midpoint of the CdC bond and the reaction therefore not synchronous. Figure 3 clearly illustrates the expected displacement for CF2, yet the sole bond path for one weak C 3 3 3 C interaction might lead to the conclusion that the reaction is in fact not concerted, but stepwise. Apart from the IRC connecting this transition state to the cyclopropane without further minima or transition states (Figure S1 of the Supporting Information), the concertedness can be established from the electron density through the delocalization index (DI or δ),44 i.e., the average number of shared electrons that the CF2 carbon possesses with each of the carbon atoms of the CdC bond. For a single bond in which electrons are equally 13698
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Table 3. NBO Occupancies of n- and p-Orbitals on the Carbenic or CNN Carbon and of σ-Bonds Forming in Transition States, and Energies (kcal mol1) for Selected Orbital Interactions in Transition States, from B3LYP/6-31+G(2d,2p) n(C)
p(C)
CF2
1.996
0.331
CF2-TSa CF2-TSinsc
1.709 d
0.584 e
FCNNF
1.706
f
FCNNF-TSa
1.650
0.583
d
e
HCNNH
f
f
HCNNH-TSa
f
f
FCNNF-TSins
c
HCNNH-TSins 1-TS 2-TS
c
d
e
f f
f f
σ(CC)
σ(CH)
1.919
n(C)/π*(CdC)
π(CdC)/p(C)
61.7 (42.7)b
175.6
21.7 (9.4)b
55.8
21.1, 66.7g
19.0, 31.9g
0.5, 6.0g
9.0, 10.1g
1.874
1.660
1.712
1.663
1.740
1.605
a
The transition state for [1+2] cycloaddition to ethene. b The p(C)/π*(CdC) interaction in parentheses. c The transition state for insertion into methane. d Missing due to new σ(CH) description in the NBO analysis. e Missing due to new σ(CC) description in the NBO analysis. f Missing due to π(CN) description in the NBO analysis. g The corresponding nitrilimine π(CN) and π*(CN) orbitals as described in the text.
shared between two atoms, as is the case for, e.g., H2, the DI is 1 and thus recovers the one shared electron pair.44,45 From the values in Table 2 it can be seen that, as expected, in the transition state for CF2 addition to ethene electrons are delocalized between the carbenic carbon and both carbon atoms of the CdC bond to a similar extent, with the C 3 3 3 C1 interaction that shows the bond path in Figure 3 further progressed. For the transition state of F—CNN—F [1+2] cycloaddition to ethene (FCNNF-TS), both DI values are reduced to about the same extent, whereas the imbalance of the two values is pronounced in the H—CNN—H transition state (HCNNH-TS; Table 2). NBO. Table 3 lists n(C) and p(C) values in the transition states for [1+2] cycloaddition. The NBO results reflect those obtained from the analysis of r2F(r) above. First, the occupancy of n(C) in CF2 is greater than that in F—CNN—F. Second, whereas the value for n(C) on CF2 decreases upon formation of the transition state, that for F—CNN—F is more or less unchanged. Furthermore, and corresponding to the bisecting bond path for the weak interaction, the NBO analysis reveals that both n(C) and p(C) are involved in intermolecular interactions in the transition state (Table 3). As expected from the electrophilic nature of CF2 and the relatively large carbenic character of F—CNN—F,23 the dominant orbital interaction is π(CdC)/p(C), but substantial “charge donation” into π*(CdC) is found from n(C) and the partially occupied p(C). These findings are thus in accord with the direction of charge transfer in the transition states for [1+2] cycloaddition determined earlier from the chemical potentials of carbenes and alkenes, as well as with the mutual electron donation from the two partners determined from a perturbational approximation within the hardsoft acidbase approach.46 For the H—CNN—H transition state, however, the direction of charge transfer is reversed, and the dominant orbital interaction is π(CNN)/π*(CdC), where π(CNN) is the in-plane orbital (described in the NBO analysis as a π*(CN) that is partially occupied from an interaction with an n(N) on the adjacent nitrogen atom, and thus mimicking the NBO description of the highest occupied π-type orbital of the allyl anion, which is presented in Table S3 of the Supporting Information). This reversal of charge transfer agrees well with the higher percentage of carbon octet structures in the resonance description for H—CNN—H,23 which render the carbon atom much less electrophilic than its F—CNN—F analogue.
To conclude on this part, both QTAIM and NBO recover the qualitative molecular orbital picture of singlet carbene addition to ethene, for CF2, but also for F—CNN—F with its large carbenic character. This is encouraging, as these should be the two clearcut cases. For H—CNN—H, though, with its diminished carbenic character, the interpretation of the data is not as straightforward. Although some evidence for a lone pair on carbon remains in both analyses, this particular feature is not very prominent and is definitely not expressed as a lone pair in the NBO analysis. Evidence for its counterpart, an unoccupied p-orbital, is missing in both analyses. The differences of H— CNN—H from both CF2 and F—CNN—F are further apparent in the transition states for cycloaddition that of course follow from the electronic structures of the electrophilic species. HCNNH-TS possesses the largest tilt angle of its two “planes”, with as much as 72.5° from B2PLYP, in full accord with a diminished electrophilic character of the H—CNN—H carbon that would be expected from a greater proportion of electronoctet valence bond structures in the resonance description. This again is reflected in the imbalance of the two DI values, as well as in the predominantly nucleophilic description of this TS from the NBO analysis. 3.3. Insertion of CF2, F—CNN—F, and H—CNN—H into CH4. QTAIM. The contour plots for r2F(r) of the transitions states for insertion are given in the CCC plane in Figure 4ac for B3LYP/6-31+G(2d,2p) and in Figure 4dm for the larger model chemistries. The plots for CF2, F—CNN—F, and H—CNN—H are qualitatively similar, within and between model chemistries, in that they show the C—H insertion to be initiated by a C 3 3 3 H interaction; the conformational changes from the methyl fragment are not under discussion here. Similarly, the additional weak interactions from the N-substituent in the nitrilimine plots lend stability to the transition states but are not further discussed here. The distances between the carbenic or CNN carbon and the carbon and hydrogen atoms of the reacting bond, C 3 3 3 C and C 3 3 3 H, given in Table 1 and in Table S1 of the Supporting Information, are similar to those reported earlier from MeCCl and PhCCl insertions into an axial C—H bond of cyclohexane,47 and from NH2CCN insertion into methane.48 This last transition state, because of its short C 3 3 3 H distance to the carbene carbon atom, has been designated as being “late”.48 Late transition states are also found for all three species here. 13699
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Figure 3. Contour plots of r2F(r) in the CCC plane of the transition states for [1+2] cycloaddition of ethene and (a) CF2, (b) F—CNN—F, and (c) H—CNN—H, from B3LYP/6-31+G(2d,2p); (d)(f) from B3LYP/aug-cc-pVTZ, (g)(i) from MP2/aug-cc-pVTZ, and (k)(m) from B2PLYP/ aug-cc-pVTZ. The solid contour lines depict regions of local charge concentration (negative values). The molecular graph in (i) has been truncated.
This is particularly prominent in Figure 4a, where the newly forming C—H bond has progressed further than in Figure 4b,c (Table 1), and consequently the bond critical point for the weakened methane C—H bond is almost moved out of the VSCC. The values of r2F(r) for the nonbonded charge concentration on the carbenic or CNN carbon atom are again listed in Table 2 and in Table S1 of the Supporting Information. There is again a reduction in size (less negative values) for the nonbonded maxima in the transition states for insertion of CF2 (CF2-TSins) and F—CNN—F (FCNNF-TSins); in contrast, for H—CNN—H the maximum is most prominent in the transition state for insertion (HCNNH-TSins), in agreement with the substantially decreased bond angle on the CNN carbon (Table 1). As for the cycloaddition transition states, the new bond path again bisects the angle formed by the carbenic or CNN carbon nucleus and the maximum and minimum in its VSCC.
Finally, again, the concertedness of the insertion of a singlet carbene was established early.49 From the molecular graphs, it is clear that C—H insertion is initiated by a C 3 3 3 H interaction from a carbene or CNN carbon atom. Again, this one bond path could suggest a stepwise rather than a concerted insertion. Once more, though, the IRC connects the transition states to the ethanes without further minima or transition states (Figure S1 of the Supporting Information), and, accordingly, from the delocalization indices (Table 2) between the CF2 carbon atom and the carbon and hydrogen atoms of the reacting C—H bond in the transition state, electrons are delocalized to a similar extent, with the C 3 3 3 H interaction that shows the bond path in Figure 4 further progressed, which is not unlike the transition state for addition to ethene. In the transition states for nitrilimine insertion, both DI values are reduced, with this effect somewhat more dominant for delocalization between the two carbon atoms (Table 2), once more not unlike in the cycloaddition. 13700
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Figure 4. Contour plots of r2F(r) in the CHC plane of the transition states for insertion into methane of (a) CF2, (b) F—CNN—F, and (c) H— CNN—H, from B3LYP/6-31+G(2d,2p); (d)(f) from B3LYP/aug-cc-pVTZ, (g)(i) from MP2/aug-cc-pVTZ, and (k)(m) from B2PLYP/aug-ccpVTZ. The solid contour lines depict regions of local charge concentration (negative values). A dashed bond path signifies an electron density at its bond critical point of 0.01 au.
NBO. Whereas in the transition states for [1+2] cycloaddition discussed in section 3.2 both reactants are still separate and only weak interactions between the two partners are found from an NBO perspective, in the transition states for insertion hydrogen transfer has already taken place and the newly forming C—H bond is recognized as such (Table 3), in agreement with the C 3 3 3 H bond path in the electron density from Figure 4. Furthermore, the newly forming C—C bond is also recognized (Table 3), which agrees with the substantial DI values between the two carbon atoms that were identified from the electron density. To conclude on this part, both QTAIM and NBO agree on a concerted, asynchronous, C 3 3 3 H initiated insertion reaction. The transition states for insertion of the two nitrilimines resemble that for insertion of the carbene much more than is the case for cycloaddition involving these species. In particular, HCCNHTSins does not stand out as HCNNH-TS does. 3.4. CdC Addition in Nitrilimine 1. Four nitrilimine conformers, differing in relative orientation of CNN and CdC moieties, are identified for 1 (Figure 5). In all four conformers 1a1d, whereas the CNN unit is linear with 170° as was found for H—CNN—H, the CNN carbon atom possesses a bond angle of 140°, larger even than that of H—CNN—H (Table 1) and thus even less carbene-like. Only the highest-energy conformer 1d, 56 kcal mol1 above the most stable conformer 1a,
Figure 5. Reaction profile for intramolecular addition in 1, from B3LYP/6-31+G(2d,2p). Relative Gibbs free energies (kcal mol1) are given in parentheses.
has the CNN carbon and CH2 terminus of the CdC bond prealigned for cyclization, with a distance of 322.0 pm. 13701
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Figure 6. Plots of the 0.6 au isosurface of r2F(r) of (a) 1d and (b) 1TS, from B3LYP/6-31+G(2d,2p). A solid arrow points to a nonbonded maximum in the VSCC of carbon; the dashed arrow, to the bond critical point of the newly forming bond. The dashed bond path signifies an electron density at its bond critical point of 0.01 au.
As for H—CNN—H, the NBO analysis provides a propargylic description for 1, with a CN triple bond. Accordingly, the charge distribution should be CN+N, as has been proposed.6 Yet from NBO, the carbon atom carries the positive charge instead, as is expected for a 1,3-dipole (Scheme 2), a distribution that was suggested in an early report on the corresponding ethyl esters of 1.10 This discrepancy clearly illustrates the complex electronic structure of this (and all) nitrilimines and supports the notion that multiple resonance contributors must be considered.23 An analysis of the charges determined from the electron density distribution gives C+NN (Table 2), which is not in accord with any of the possible valence bond structures for nitrilimines (Scheme 1). This situation, different charge distributions from NBO and QTAIM approaches, is somewhat unexpected, as we found a good correspondence previously, albeit for a different, and less complex, electronic system (R—NSO).50 Both NBO and QTAIM agree, though, that the positive charge on the CNN carbon of 1 is much reduced from its value in the two species with large carbenic character, CF2 and F—CNN—F (Table 2). In accord with the bend on the CNN carbon, r2F(r) exhibits a maximum in its VCSS on carbon, but it is the smallest (least negative) maximum in the series (Table 2), with little variation for the four conformers. It is depicted for 1d in Figure 6a (shown within an isosurface rather than through a contour plot). As for H —CNN—H, a minimum in the VSCC for the CNN carbon is not located. Because of the CtN description, the NBO shows neither an n(C) nor a p(C); the discussion on the nature of the two π-bonds given above for H—CNN—H holds here as well, yet the s-component (5%) on carbon to the in-plane π-orbital is
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small (raw data provided in Table S2 of the Supporting Information), and therefore the CtN description is more “normal”. But one could argue that a lone pair on the CNN carbon is recovered in one way or another by both methods. Two mechanisms were originally proposed for the formation of the tricyclic cyclopropa[c]cinnoline given in Scheme 3.2 The concerted mechanism would be a [1+2] cycloaddition as it would be observed from a carbene, the stepwise mechanism would show initial formation of a seven-membered ring, followed by its collapse to the three-membered ring.2,10 Figure 5 clearly illustrates that the intramolecular reaction for 1 with its vinyl moiety is stepwise, in that the transition state 1-TS yields a sevenmembered ring that can stabilize itself by further cyclization to a three-membered ring. The CNN angle in 1-TS is closed to 155° to allow for the best possible relative orientation of the CNN carbon and the CdC system, though the ideal alignment as in the bimolecular transition states in section 3.2 is still not achieved. The bond angle on carbon in 1-TS is decreased from that in 1d but remains larger than that in HCNNH-TS (Table 1). The small change in the bond angle on carbon results in similarly sized maxima in the VSCC of 1 and 1-TS (Table 2). In 1-TS, the nonbonded maximum is located out of the CCN plane and away from the newly forming bond (shown in Figure 6b within an isosurface), thus resembling the situation in the transition states for [1+2] cycloaddition. As in Figure 3 for the bimolecular reactions, Figure 6b exhibits only one bond path between the reacting moieties in 1-TS. But in contrast to the bimolecular reactions, the DI for the longer C 3 3 3 C distance is only 20% of the DI for the shorter C 3 3 3 C distance with its bond path in the electron density (Table 2, with DI values for 1d provided for comparison). Even in the atypical (with respect to the other two [1+2] transition states) HCNNH-TS, this value is as large as 40% (Table 2). Consequently, 1-TS does not ring-close to a [1+2] product, but rather to a seven-membered ring, not unlike a 7-endotrig cyclization.51 According to the NBO analyses, like 1, 1-TS does not exhibit an electron lone pair on its CNN carbon atom, in agreement with H—CNN—H and its transition state for [1+2] cycloaddition to ethene. Not unlike H—CNN—H and 1, 1-TS is described with a CN triple bond (Table S2 of the Supporting Information), but interactions between the CNN carbon, as described through the two π-bonds, and the CdC moiety are small in comparison to those in HCNNH-TS (Table 3). In their original rationalization of the observed reaction products, the authors proposed a nucleophilic attack of CdC onto the nitrilimine carbon atom on the basis of their 1,3-dipolar charge description of the CNN unit,10 which is in accord with the NBO finding of larger energy values for the π(CdC)/π*(CdN) interactions in 1-TS (Table 3). To conclude here, 1-TS does not achieve the optimal geometry for a concerted [1+2] addition reaction, and C 3 3 3 C distances are larger than in the respective bimolecular transition states. The corresponding delocalization indices are thus naturally smaller. That this is not an electron density argument for a concerted addition reaction to the CdC bond, though, is clearly illustrated by the formation of the seven-membered reaction intermediate. It thus seems that this transition state is indeed better described as one for the previously suggested 1,7-electrocyclization,6 whereas the “telltale” three-membered ring is formed in a subsequent cyclization step. 3.5. C—H Insertion in Nitrilimine 2. In contrast to 1, only one minimum is identified on the reactant side of the potential 13702
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Figure 7. Reaction profile for cyclization in 2, from B3LYP/6-31+G(2d,2p). Relative Gibbs free energies (kcal mol1) are given in parentheses.
Figure 8. Plot of the 0.6 au isosurface of r2F(r) of 2-TS, from B3LYP/6-31+G(2d,2p). The solid arrow points to the nonbonded maximum in the VSCC of carbon; the dashed arrow, to the bond critical point of the newly forming bond.
energy surface for 2 (Figure 7). Except for terminal-N-substitution, its CNN unit is linear, even on carbon (Table 1). Again, and not surprisingly, the NBO gives a progargylic description of the CNN moiety, and the overall distribution of charges on the CNN atoms is as in 1, but charges on the CNN carbon atom are somewhat more positive. The adaptability of the CNN geometry to reaction requirements was alluded to even in the early stages of this field of research52,53 and was more recently supported by high-level CBS-QB3 calculations on distortion energies in series of dipoles.54 Consequently, in the transition state for cyclization 2-TS, bond angles on both carbon and central nitrogen atoms decrease to about 130° (Table 1). The bond angles in 2-TS are quite dissimilar from those of H—CNN—H and F—CNN—F in their transitions states for insertion into methane (Table 1), and (with particular interest to this study) the bond angle on carbon is much larger. With respect to the expected concertedness of any C—H insertion from a singlet carbene, Figure 7 illustrates that 2-TS leads to a primary reaction product in which the hydrogen atom remains on the original carbon atom. A subsequent 1,2-hydrogen shift is required for formation of the experimentally observed product. And although stepwise C—Cl insertions from singlet carbenes are known, they proceed with Cl-abstraction, not C-addition.55 From the reaction path in Figure 7 alone it is clear that the overall reaction should not be described as a C—H insertion.
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Naturally, and judging from the linear geometry around its CNN carbon atom, 2 does not possess a maximum in the VSCC of carbon. In 2-TS with its bent geometry on carbon, a small nonbonded charge concentration is recovered (Figure 8 and Table 2), resembling that in H—CNN—H but even smaller (less negative) than in HCNNH-TSins (Table 2). In accord with the stepwise reaction given in Figure 7, 2-TS possesses only one weak interaction between the reacting moieties, that between the CNN and ortho carbon atoms (Figure 8). This is in obvious contrast to the bimolecular insertions (Figure 4), where a bond path between the carbenic or CNN carbon and the moving hydrogen atom is found instead. Furthermore, from the DI values in Table 2, which show an insubstantial delocalization of electrons between CNN carbon and ortho-hydrogen atoms, it is clear that 2-TS is not a transition state for C—H insertion. According to the NBO analyses, neither 2 nor 2-TS exhibit an electron lone pair on their CNN carbon atom, again in agreement with H—CNN—H and its transition states. As H—CNN—H and all four conformers of 1, 2 is described with a CN triple bond, whereas 2-TS possesses a CN double bond and its newly forming σ-bond (Table S2 of the Supporting Information and Table 3, respectively). The NBO analysis of 2-TS reveals several interactions for the relevant ortho C—H bond, including one with π(CN), yet all are uniformly small (>3 kcal mol1), in accord with both the reaction profile and the sole weak interaction in the electron density (Figure 8). Finally, in general, carbenes do not insert into C—H bonds on aromatic rings, with benzene often being the solvent of choice in carbene reactions. Again it seems that this transition state for formal insertion is better described as one for the previously suggested 1,5-electrocyclization:6 C 3 3 3 C bond formation with distinct rehybridization of the participating ortho carbon atom to give an intermediate from an addition reaction.
’ CONCLUSIONS The singlet carbene CF2 and the nitrilimines F—CNN—F, H—CNN—H, ortho-vinyl MeCOO—CNN—Ph, and Ph— CNN—Ph as well as their reactions have been analyzed using the electron density and its Laplacian (QTAIM), and natural bond orbitals (NBO). As expected, the two important features for CF2 are recovered: an electron lone pair and an empty p-orbital on carbon, or their equivalents in the electron density. Only for the nitrilimine F—CNN—F, with its electronic structure to about two-thirds a carbene, are these features recovered by both analyses. Accordingly, it has been shown that the prototypical singlet carbene reactions, [1+2] cycloaddition to ethene and insertion into methane, from CF2 and F—CNN—F are described well by both analyses. In particular, addition is characterized by large orbital interactions between the two carbon-centered orbitals, n(C) and p(C), and both π and π* orbitals on the alkene (NBO description), and large delocalization indices between the carbene carbon and the two alkene carbon atoms, with one new bond path formed in the transition state (QTAIM). Insertion is characterized through large populations of both newly formed bonds, C—H and C—C, to the carbene or CNN carbon atom (NBO) and again large delocalization indices between those atoms, with the new C—H bond path formed in the transition state (QTAIM). It is unclear whether the addition of H—CNN—H, with less than a third of its resonance description given as a carbene, to ethene should be described as a carbene-type cycloaddition. The, apart from the larger tilt angle 13703
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The Journal of Physical Chemistry A between the HCN and alkene planes, geometric similarity of its transition state to those from CF2 and F—CNN—F is certainly a positive argument, yet the high asymmetry of the transition state as deduced from the delocalization indices (QTAIM) and the differing orbital description (NBO) are definitely negatives. The insertions of H—CNN—H, F—CNN—F, and CF2 into methane are certainly highly similar in all respects. The observed intramolecular reaction product from orthovinyl MeCOO—CNN—Ph possessing a three-membered ring could in principle have arisen from a [1+2] cycloaddition reaction, yet the reaction profile exhibits an initial seven-membered ring intermediate that cyclizes to the formal [1+2] cycloaddition product. The formation of the intermediate is in line with only one reasonable, yet still large, distance between the interacting atoms in the transition state and the lone significant delocalization index to the terminal vinyl carbon (QTAIM). This first transition state therefore does not resemble those from the bimolecular model [1+2] cycloaddition reactions. Similarly, the observed intramolecular reaction product from Ph—CNN—Ph could have arisen from a C—H insertion reaction, yet the reaction profile reveals an initial intermediate in which the hydrogen atom remains on the original carbon atom. The formation of this intermediate agrees well with the fact that in the transition state only the C 3 3 3 C interaction shows an appropriate distance, with its bond path and associated delocalization index (QTAIM). Thus, in summary, in the past it seemed that intramolecular reaction products from ortho-vinyl MeCOO—CNN—Ph and from Ph—CNN—Ph might be indicators of carbenic reactivity of these nitrilimines. Yet, a close analysis of their transition states and reaction profiles, and a comparison to electron density and natural bond orbital data from related bimolecular reactions of carbenes and nitrilimines with large carbenic character reveal that these products are not formed from the two typical carbene-type reactions, [1+2] cycloaddition and C—H insertion.
’ ASSOCIATED CONTENT
bS
Supporting Information. Tables for energies, distances, angles, and Laplacian values from the aug-cc-pVTZ basis set; NBO contributions for the CdN bond and the allyl anion. Figure of IRC reaction paths for CF2, FCNNF, and HCNNH addition and insertion. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
Tel: 514 8482424 ext 3342. Fax: 514 8482868. E-mail: muchall@ alcor.concordia.ca.
’ ACKNOWLEDGMENT Calculations were performed at the Centre for Research in Molecular Modeling (CERMM), which was established with the financial support of the Concordia University Faculty of Arts and ducation du Quebec (MEQ), and the Science, the Ministere de l’E Canada Foundation for Innovation (CFI). Additional computational resources were provided by the Reseau Quebecois de Calcul de Haute Performance (RQCHP) and Compute Canada. This work was supported by a research grant from the Natural Sciences and Engineering Research Council (NSERC) of Canada.
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