Absolute Reactivity of (N-Methyl-3-pyridinium)chlorocarbene - The

Apr 7, 2015 - (N-Methyl-3-pyridinium)chlorocarbene tetrafluoroborate (MePyr+CCl BF4–, 4) is generated by laser flash photolysis (LFP) of the corresp...
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Absolute Reactivity of (N‑Methyl-3-pyridinium)chlorocarbene Hui Cang, Robert A. Moss,* and Karsten Krogh-Jespersen* Department of Chemistry & Chemical Biology, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08903, United States S Supporting Information *

ABSTRACT: (N-Methyl-3-pyridinium)chlorocarbene tetrafluoroborate (MePyr+CCl BF4−, 4) is generated by laser flash photolysis (LFP) of the corresponding diazirine (5) and reacted with tetramethylethylene, cyclohexene, 1-hexene, 2-ethyl-1-butene, methyl acrylate, and acrylonitrile. Absolute rate constants are measured for these carbene−alkene addition reactions, and activation parameters are obtained for additions of MePyr+CCl BF4− to tetramethylethylene, cyclohexene, and 1-hexene. MePyr+CCl BF4− is computed to be a highly reactive, electrophilic, singlet carbene, and experiments are in accord with expectations. Its activation parameters are compared with those of CF3CCl, CCl2, CClF, and CF2. In all cases, enthalpy−entropy compensation is observed, with ΔH‡ and ΔS‡ decreasing in tandem as carbenic stability decreases. A qualitative explanation is offered for this phenomenon.

1. INTRODUCTION

2. EXPERIMENTAL AND COMPUTATIONAL DETAILS 2.1. Experimental Details. 3-Chloro-3-(N-methyl-3pyridinium)diazirine tetrafluoroborate (5)1 was prepared by quaternization of 3-chloro-3-(3-pyridyl)diazirine (6)5 with trimethyloxonium tetrafluoroborate. Solid trimethyloxonium tetrafluoroborate (0.554 g, 3.74 mmol) was added to a solution of diazirine 6 (0.721 g, 4.68 mmol) in 50 mL of dry dichloromethane (DCM). (An excess of diazirine was maintained to avoid possible methylation of the diazirine nitrogens.) The resulting suspension was stirred under nitrogen for 6 h at ambient temperature. The filtered solid product (5) was washed with DCM, pentane, and ether to remove unreacted 6. The residue was recrystallized from methanol to give white crystals of diazirine salt 5, 0.89 g, 75% yield. 1H NMR (300 MHz, D2O, δ): 8.80−8.82 (m, 2H), 8.14−8.17 (m, 1H), 8.01−8.06 (m, 1H), 4.38 (s, 3H). 13C NMR (75 MHz, D2O, δ): 145.6, 143.7, 142.0, 136.8, 127.7, 48.6, 43.1. UV: (λmax, dichloroethane, DCE): 351 nm, absorption from 340 to 370 nm.

Some years ago, we described the alkene addition reactions of several pyridylhalocarbenes and pyridiniumhalocarbenes.1 We found that the 2-pyridylhalocarbenes (1) and the 3pyridylhalocarbenes (2) behaved as ambiphiles in additions to a series of alkenes of varying electronic character. Additionally, we reported that the corresponding N-methylpyridiniumhalocarbenes 3 and 4 (as BF4− salts) added to isobutene, affording the appropriate cyclopropanes.1

More recently, Cozens and co-workers elaborated on these findings.2,3 Laser flash photolysis (LFP)-mediated determinations of rate constants for the additions of 2-, 3-, and 4pyridylhalocarbenes (hal = F, Cl, Br) to a series of alkenes reaffirmed the ambiphilicity of this family of carbenes while enabling a detailed examination of the electronic effects of pyridine nitrogen location and halogen identity on carbenic reactivity.2 LFP studies of pyridiniumcarbenes 4, however, were complicated by the difficulty of obtaining absolute rate constants for the olefin additions. Instead, relative reactivities were measured with the aid of carbene-pyridine ylides. The results indicated that carbenes 4 were strongly reactive electrophilic species.3 Our interest in the activation parameters attending alkene additions of voraciously electrophilic carbenes (cf. the global electrophilicities in Table 1 and chlorotrifluoromethylcarbene4) impelled us to reexamine the addition reactions of carbene 4 (X = Cl, Y = BF4). We are pleased to report here that LFP experiments afforded both absolute rate constants and activation parameters for these reactions. © 2015 American Chemical Society

LFP experiments employed a XeF2 excimer laser emitting at 351 nm. The system is described in detail in ref 4. Samples for irradiation consisted of 1.67 mM diazirine 5 in 2 mL of DCE in Received: February 20, 2015 Revised: March 30, 2015 Published: April 7, 2015 3556

DOI: 10.1021/acs.jpca.5b01729 J. Phys. Chem. A 2015, 119, 3556−3562

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The Journal of Physical Chemistry A quartz cuvets. For alkene addition studies, 10−100 μL of an alkene/DCE solution was added so as to provide measurable quenching of the carbene signal. Precise temperatures (±0.1 K) were measured at the instant of LFP by a thermocouple immersed in the photolysis solution. 2.2. Computational Details. Electronic structure calculations, based on wave function (HF, MP2, CCSD(T)) or density functional theory (DFT), were carried out with the Gaussian 09 suite of programs.6 We employed MP27 theory and cc-pVTZ basis sets8,9 in geometry optimizations of the lowest singlet and triplet states of MePyr+CCl and MePyr+CCl BF4−. The stationary points located on the potential energy surfaces were characterized further by normal-mode analysis, and the (unscaled) vibrational frequencies formed the basis for the calculation of vibrational zero-point energy (ZPE) corrections. Standard thermodynamic corrections (based on the harmonic oscillator/rigid rotor approximations and ideal gas behavior) were made to convert from purely electronic energies to enthalpies (ΔH°, ΔZPE included, T = 298.15 K) and Gibbs free energies (ΔG°; T = 298.15 K, P = 1 atm). The triplet states of MePyr+CCl and MePyr+CCl BF4− suffer from spin contamination, and we have hence used projected MP2 (PMP2) energies in the determination of singlet−triplet gaps.10 Improved electronic energies were obtained from single-point calculations at the CCSD(T)/cc-pVTZ level.11 To generate the carbene reactivity data in Table 1, geometry optimizations were specifically executed at the MP2/6-31G(d,p)7,12,13 or B3LYP/6-311++G(2d,p)14−19 levels to conform with prior practice.

3. RESULTS AND DISCUSSION 3.1. Diazirine 5. In previous studies, pyridiniumdiazirine 5 was prepared from pyridyldiazirine 6 by a two-step sequence; diazirine 6 was first quaternized with methyl iodide, and then, the iodide anion was replaced by tetrafluoroborate using either AgBF4/MeCN or ion-exchange chromatography.1,3 Traces of unremoved iodide, however, can hinder LFP experiments by reacting with carbene 4 (X = Cl). In the present experiments, direct preparation of diazirine 5 by quaternization with Me3O+BF4− obviates the iodide problem and shortens the synthesis. 3.2. Some Computational Results. We first ascertained the ground-state spin multiplicity, singlet (S) or triplet (T), of 4 (X = Cl) in an idealized gas phase and in a polar DCE solution. Gas-phase calculations at the MP2/cc-pVTZ level on the “naked” MePyr+CCl species predict a singlet ground state and a modest singlet−triplet separation of ΔES−T = 4.9 kcal/mol (ΔH°S−T = 6.8 kcal/mol, ΔG°S−T = 6.0 kcal/mol; P = 1 atm; T = 298 K); a similar S−T separation, ΔES−T = 4.3 kcal/mol, was obtained at the CCSD(T)/cc-pVTZ level. Embedding of the MePyr+CCl cation in a polar solvent preferentially favored the singlet state, and we found ΔES−T = 9.7 kcal/mol at the MP2/ cc-pVTZ level with the CPCM solvation model and DCE model solvent. It seems likely that the MePyr+CCl cation would associate with one or more BF4− anions in a polar solvent. Although we have not performed an exhaustive search for the minimumenergy MePyr+CCl BF4− ion pair, several initial guesses for plausible MePyr+CCl BF4− low-energy singlet structures did converge upon geometry optimization to the type of structure illustrated in Figure 1. This intimate MePyr+CCl BF4− ion pair

Table 1. Quantitative Measures of Carbene Reactivity carbene +

MePyr CCl CF3CCl CCl2 CClF CF2

εLUa −0.16 0.57 1.00 1.74 2.74

εHOa d

−10.58 −11.30 −10.91 −11.71 −12.85

ωb d

1.38 1.21 1.03 0.92 0.82

ΔEstabc d

30.2e 21.3 45.5 56.1 70.9

εLU and εHO are the LUMO and HOMO energies in eV, computed at the HF/6-31G(d,p)//MP2/6-31G(d,p) level, see ref 24. bIn eV; ω = global electrophilicity = (εLU + εHO)2/8(εLU - εHO), see refs 22−24. cIn kcal/mol. ΔEstab is defined as the negative of the reaction energy for CH2 + CH3X + CH3Y → CXY + 2CH4, see ref 25. Computed here at the B3LYP/6-311++G(2d,p) level; see ref 26. dCalculated as the MePyr+CCl BF4−(S) species, see Figure 1 and also ref 26. eCalculated for the “naked” MePyr+CCl species. a

Figure 1. Intimate MePyr+CCl BF4− ion pair (MP2/cc-pVTZ, singlet optimized structure shown). (a) Side view; (b) front view. The optimized structure for the triplet MePyr+CCl BF4− ion pair is similar.

Calculations of electronically excited-state properties (transition wavelengths (λ) and oscillator strengths (f)) were performed at the optimized ground-state geometries using the time-dependent DFT formalism20 and the B3LYP hybrid exchange−correlation functional17−19 (TD-B3LYP/cc-pVTZ// MP2/cc-pVTZ). The polarizable conductor self-consistent reaction field model (CPCM) was chosen to incorporate general bulk solvent effects;21 Gaussian 09 default parameters were applied for the DCE solvent. Assignment of a particular electronic excitation was based on consideration of the largest transition amplitudes for the excitation and by visualization of the contributing MOs.

shows the anion coordinating perpendicular to the MePyr+CCl plane above the positively charged nitrogen center (∠BNC(ring or methyl) ≈ 90°) with distances N−B = 3.08 Å and N− F = 2.77, 2.85, and 3.02 Å in the singlet state; corresponding distances in the triplet state are N−B = 3.14 Å and N−F = 2.82, 2.88, and 3.13 Å, suggesting slightly stronger anion−cation interactions in singlet MePyr+CCl BF4−. Indeed, singlet MePyr+CCl is preferentially stabilized by complexation with BF4−, and the energy difference between MePyr+CCl BF4−(S) and MePyr+CCl BF4−(T) is increased to ΔES−T = 9.1 kcal/mol in favor of MePyr+CCl BF4−(S) in the idealized gas phase (MP2/cc-pVTZ). General solvent effects appear to increase the S−T separation slightly further, and we find ΔES−T = 10.7 kcal/ mol for the MePyr+CCl BF4− ion pair in simulated DCE 3557

DOI: 10.1021/acs.jpca.5b01729 J. Phys. Chem. A 2015, 119, 3556−3562

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The Journal of Physical Chemistry A

Figure 2. (a) Calibrated31 UV−vis spectrum of MePyr+CCl in DCE 200 ns after LFP of diazirine 5. (b) Computed UV−vis spectrum of the intimate singlet MePyr+CCl BF4− ion pair (cf. Figure 1) in simulated DCE.

In previous work, the kinetics of addition of MePyr+CCl to alkenes were followed via the pyridine ylide of the carbene, which absorbs strongly at 600 nm.3 Reactivities of MePyr+CCl toward a series of alkenes, relative to kpyr for its reaction with pyridine, were estimated by Platz’s Stern−Volmer method.33 Given that kpyr itself was not determined, absolute rate constants for the MePyr+CCl−alkene additions were not obtained.3 In our studies, we were able to directly follow the pseudofirst-order decay of the (uncalibrated) 308 nm absorption of MePyr+CCl, generated by LFP of 5 (X = Cl), as a function of the concentrations of added alkenes. We thus obtained absolute rate constants for the additions of MePyr+CCl to tetramethylethylene (TME), cyclohexene, 1-hexene, 2-ethyl-1-butene, methyl acrylate (MeAcr), and acrylonitrile (AcrCN). These values appear in Table 2; see the Supporting Information for

solution. Thus, the computational support for 4 (X = Cl) possessing a singlet state is substantial, and our ensuing analysis of MePyr+CCl chemistry in terms of only MePyr+CCl(S) appears fully justified. Table 1 collects computed parameters for carbene 4 and four other electrophilic carbenes, CF3CCl, CClF, CCl2, and CF2.4,22−29 Singlet MePyr+CCl has the lowest-lying LUMO30 and the highest global electrophilicity22,23 of the carbenes in Table 1, so that it should be the most electrophilic. Although MePyr+CCl is computed to be more stable than CF3CCl by ∼9 kcal/mol (relative to CH2),25,26 it is predicted to be considerably less stable than the iconic electrophiles CCl2 and CF2 (cf. Table 1). MePyr+CCl is thus expected to be a highly reactive electrophilic carbene toward alkenes, in agreement with the previous analysis.3 For the singlet MePyr+CCl cation in simulated DCE solution, strong absorption is calculated at 278 nm ( f = oscillator strength = 0.28); similarly, for the singlet MePyr+CCl BF4− ion pair (Figure 1), we calculate strong absorption at 287 nm (f = 0.25); compare Figure 2b. The electronic transition is π → π* in nature and comprised of π-orbitals delocalized over the aromatic ring, the carbene center, and the chloride (see the Supporting Information for contour plots of relevant MOs). Absorption is also predicted to occur at longer wavelengths (see the Supporting Information for details), but these transitions carry intensities that are at most 1% that of the intense 280−290 nm transition. 3.3. Kinetics. Cozens et al. reported “no visible transient absorption signal above 300 nm” when diazirine 5 was subjected to LFP in DCM at 355 nm.3 However, we have observed the absorption of MePyr+CCl peaking at 308 (uncalibrated spectrum) and 284 nm (calibrated spectrum) upon LFP of diazirine 5 in DCE at 351 nm; see Figure 2a.31 We do not know why this absorption was not seen by Cozens et al. Perhaps, in their LFP installation, the absorption fell below their lower observational limit of 300 nm. The λmax of the carbene’s absorption agrees well with the computed values; see above, Figure 2, and ref 3.32 We note, however, that if diazirine 5 is not completely methylated at the pyridine nitrogen (i.e., some diazirine 6 is present), then the carbene can react with 6 at its pyridine N, forming an ylide that absorbs at ∼450 nm;1 compare Figure S-4 (Supporting Information).

Table 2. Rate Constants for Additions of MePyr+CCl to Alkenes alkene TME c-C6H10 1-hexene 2-Et-1-butene MeAcr AcrCN

k (M−1 s−1)a 1.84 6.28 2.36 3.47 1.66 7.57

± ± ± ± ± ±

0.03 0.06 0.10 0.08 0.05 0.06

× × × × × ×

9

10 108 108 108 108 107

k (M−1 s−1)b

kCCl2 (M−1 s−1)c

2.0 × 10

5.1 × 108

4.7 × 109 6.4 × 107 1.8 × 107

3.3 × 108 2.0 × 108

5.9 × 105 4.9 × 105

9

a

This work, in DCE at 298 K. Values are averages of two determinations; errors are average deviations. bFrom ref 3, in DCM at 295 K, assuming that kpyr = 7.5 × 109 M−1 s−1. cAbsolute rate constants for additions of CCl2 in pentane at 297 K; see ref 34.

graphical displays of the kinetics data. In Table 2, our values are compared with those reported by Cozens, who assumed that kpyr for the reaction of MePyr+CCl with pyridine was 7.5 × 109 M−1 s−1.3 For additional comparisons, absolute rate constants for alkene additions of CCl2, a classic electrophilic carbene, are also included in Table 2.34 Our absolute rate constants for the additions of MePyr+CCl are in reasonable agreement with the estimated values derived by Cozens.3 The electrophilic reactivity pattern of MePyr+CCl toward the alkenes of Table 2 is clear,27−29 so that its characterization as a very reactive electrophilic carbene seems 3558

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The Journal of Physical Chemistry A Table 3. Activation Parameters for Carbene Additionsa carbene +

c

MePyr CCl CF3CCld CCl2e CClFe CF2f MePyr+CClc CF3CCld CCl2e CClFe CF2f MePyr+CClc CF3CCld CCl2e CClFe CF2f

alkeneb

Ea

log A

ΔH‡

ΔS‡

−TΔS‡

TME TME TME TME TME c-C6H10 c-C6H10 c-C6H10 c-C6H10 c-C6H10 1-hex 1-hex 1-hex 1-hex 1-hex

−3.1 −2.1 −1.2 0.9 3.0 0.92 2.6 3.8 5.6 6.9 2.1 3.5 4.7 6.0 8.0

7.0 7.8 8.8 9.7 11.0 9.5 9.3 10.9 11.5 12.3 9.9 9.8 10.7 11.5 12.4

−3.7 −2.6 −1.8 0.3 2.5 0.36 2.1 3.3 5.0 6.3 1.5 2.9 4.1 5.4 7.4

−28.5 −25 −20 −16 −10 −17.2 −17.8 −10.5 −7.8 −4.3 −15.2 −15.5 −11.5 −7.8 −3.9

8.5 7.4 6.0 4.7 3.0 5.1 5.3 3.1 2.3 1.3 4.5 4.6 3.4 2.3 1.1

ΔG‡ 4.8 4.8 4.2 5.0 5.5 5.5 7.4 6.4 7.3 7.6 6.0 7.5 7.5 7.7 8.6

(0.5) (0.1) (0.2) (0.2) (0.3) (0.2) (0.6) (0.4) (0.4) (0.5) (0.3) (0.5) (0.4) (0.3) (0.1)

a Units are kcal/mol for Ea, ΔH‡, −TΔS‡, and ΔG‡; M−1 s−1 for log A, and cal-deg/mol for ΔS‡. ΔH‡ is calculated at 283 K; ΔG‡ is calculated at 298 K. Errors are 0.2−0.3 kcal/mol or less in Ea; errors in ΔS‡ are ∼1 eu; errors are shown in parentheses for ΔG‡. bTME = tetramethylethylene; cC6H10 = cyclohexene; 1-hex = 1-hexene. cThis work. dFrom ref 4. eFrom ref 36. fFrom ref 37.

warranted. As for reactivity, MePyr+CCl displays a rate constant range of only ∼24 over the alkenes in Table 2, whereas the range for CCl2 is ∼9600!34 The electrophilicity of MePyr+CCl BF4− (4) also follows from theoretical considerations. Computed differential orbital HO LU HO energies ΔEE(εLU 4 − εCC = p − π) and ΔEN(εCC − ε4 = π* − σ) correspond respectively to the electrophilic and nucleophilic transition state interactions of MePyr+CCl with alkenes.28,29 Cozens found that ΔEE was lower (i.e., more favorable) than ΔEN for the alkene additions of MePyr+CCl,3 and so do we.30,35 MePyr+CCl is therefore both predicted and observed to be electrophilic toward the alkenes of Table 2. 3.4. Activation Parameters. Activation parameters for the additions of MePyr+CCl to TME, cyclohexene, and 1-hexene were obtained from measurements of kaddn at five temperatures (±0.1 K) between 280 and 308 K. The activation parameters were derived from the slopes and intercepts of Arrhenius correlations between ln kaddn and 1/T. The correlations were of good quality, with r = 0.99 in each case. Graphical displays of the kinetics data and the Arrhenius correlations appear in the Supporting Information. The activation parameters appear in Table 3, where they are compared to analogous data for CF3CCl,4 CCl2,36 CClF,36 and CF2.37 We have previously commented on the negative activation energies and enthalpies exhibited by the strongly electrophilic and reactive carbenes CF3CCl and CCl2 in additions to TME.4,36 The values of Ea = −3.1 kcal/mol and ΔH‡ = −3.7 kcal/mol observed for the reaction of MePyr+CCl with TME are the most negative yet observed for carbene additions to TME. We believe that in these highly exothermic additions, ΔH continually decreases along the reaction coordinate, leading to negative values of the corresponding activation parameters (Ea, ΔH‡).38−40 Barriers to these additions appear in ΔG‡, generated by the very negative values of ΔS‡.38−40 Nearly 50 years ago, Skell and Cholod applied Hammond principles to dichlorocarbene−alkene addition reactions.41,42 Their predictions can be paraphrased as follows. (1) In reactions with olefins, strongly electrophilic carbenes will traverse looser, reactant-like transition states, and their activation parameters will be entropy-dominated. Conversely, weakly electrophilic carbenes will traverse tighter transition

states, affording enthalpy-dominated activation parameters. (2) Reactions of carbenes with more reactive olefins will traverse transition states with larger carbene/olefin separations, “weaker-binding, lower-frequency vibrations along the contracting reaction axis, more closely spaced vibrational states, a greater use of those states in the transition state saddle, and consequently a more positive ΔS‡”.42 Less reactive olefins will traverse tighter transition states with wider spaced, more sparsely occupied vibrational levels and a less positive (more negative) ΔS‡. (3) A “remarkable feature would be that the transition between ΔS‡- and ΔH‡-dominated reaction types occurs with compensations that maintain” good Arrhenius correlations.42 Skell and Cholod attached a proviso: the Arrhenius equation must be able to effectively separate ΔS‡ and ΔH‡ terms, especially in cases where Ea or ΔH‡ is very small.42 Although the Skell analysis was originally offered for the additions of a single carbene (CCl2) to a large set of alkenes of varying reactivity, we will apply it here to five carbenes of differing electrophilicity and reactivity, adding to three alkenes of varied nucleophilicity and reactivity. Consider Table 3, in which the five carbenes (MePyr+CCl, CF3CCl, CCl2, CClF, and CF2) are arranged in order of decreasing electrophilicity (ω) and increasing stability (ΔEstab), except for the inversion in ΔEstab at MePyr+CCl and CF3CCl; cf. Table 1. The three alkenes (TME, cyclohexene, and 1-hexene) are arranged in order of decreasing reactivity toward electrophilic carbenes;43 compare Table 2. Skell’s prediction (1) appears to hold. With each of the alkenes, Ea and ΔH‡ increase as carbenic electrophilicity decreases, and the dominance of ΔS‡ (i.e., −TΔS‡ > ΔH‡) gradually shifts through equality toward dominance by ΔH‡. These trends are very well illustrated by the reactions of the carbenes with cyclohexene and 1-hexene, but with the most reactive alkene (TME), ΔH‡ never dominates −TΔS‡, even with the least electrophilic and most stable carbene, CF2.44 To get some idea of the range of ΔH‡/TΔS‡ changes in Table 3, compare the most reactive pairing (MePyr+CCl/ TME) with the least reactive pairing (CF2/1-hexene); the ΔH‡/TΔS‡ balance (in kcal/mol) changes from −3.7 versus 8.5 3559

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The Journal of Physical Chemistry A Table 4. Activation Parameters for Carbene Additionsa (Alkene Variation) carbene c

MePyrCCl MePyrCClc MePyrCClc CF3CCld CF3CCld CF3CCld CCl2e CCl2e CCl2e CClFe CClFe CClFe CF2f CF2f CF2f

alkeneb

Ea

log A

ΔH‡

ΔS‡

−TΔS‡

TME c-C6H10 1-hex TME c-C6H10 1-hex TME c-C6H10 1-hex TME c-C6H10 1-hex TME c-C6H10 1-hex

−3.1 0.92 2.1 −2.1 2.6 3.5 −1.2 3.8 4.7 0.9 5.6 6.0 3.0 6.9 8.0

7.0 9.5 9.9 7.8 9.3 9.8 8.8 10.9 10.7 9.7 11.5 11.5 11.0 12.3 12.4

−3.7 0.36 1.5 −2.6 2.1 2.9 −1.8 3.3 4.1 0.3 5.0 5.4 2.5 6.3 7.4

−28.5 −17.2 −15.2 −25 −17.8 −15.5 −20 −10.5 −11.5 −16 −7.8 −7.8 −10 −4.3 −3.9

8.5 5.1 4.5 7.4 5.3 4.6 6.0 3.1 3.4 4.7 2.3 2.3 3.0 1.3 1.1

ΔG‡ 4.8 5.5 6.0 4.8 7.4 7.5 4.2 6.4 7.5 5.0 7.3 7.7 5.5 7.6 8.6

(0.5) (0.2) (0.3) (0.1) (0.6) (0.5) (0.2) (0.4) (0.4) (0.2) (0.4) (0.3) (0.3) (0.5) (0.1)

a Units are kcal/mol for Ea, ΔH‡, −TΔS‡, and ΔG‡; M−1 s−1 for log A, and cal-deg/mol for ΔS‡. ΔH‡ is calculated at 283 K; ΔG‡ is calculated at 298 K. Errors are 0.2−0.3 kcal/mol or less in Ea; errors in ΔS‡ are ∼1 eu; errors are shown in parentheses for ΔG‡. bTME = tetramethylethylene; cC6H10 = cyclohexene; 1-hex = 1-hexene. cThis work. dFrom ref 4. eFrom ref 36. fFrom ref 37.

to 7.4 versus 1.1, that is, from marked dominance by TΔS‡ to marked dominance by ΔH‡, in accord with Skell’s prediction. However, the second Skell prediction is not upheld by the data; increasing olefinic reactivity does not yield a more positive ΔS‡. Just the opposite is observed; more reactive olefins exhibit more negative ΔS‡ values in their reactions with the carbenes. This trend is illustrated in Table 4, where the data of Table 3 are recast so as to stress the variation of the alkenes. With few exceptions, ΔS‡ is less positive (more negative) with the more reactive alkene. What is the origin of this counterintuitive, contra-Skell behavior? Why does the most reactive carbene/ alkene pairing (MePyr+CCl/TME) exhibit such a negative ΔS‡ (−28.5 eu) compared to the least reactive pairing (CF2/1hexene), where ΔS‡ is only −3.9 eu? Although Skell’s prediction (3) appears valid, namely, the transition between ΔS‡- and ΔH‡-dominated reaction types occurs with compensations that maintain good Arrhenius correlations (cf. Supporting Information), what if his proviso is breached? What if the Arrhenius equation cannot effectively separate ΔS‡ and ΔH‡ terms in this set of 15 reactions, where Ea never exceeds 8 kcal/mol? To begin to answer the questions raised here, we turn to an admittedly speculative consideration of the adequacy of transition state theory and the possible importance of dynamics in reactions with very low activation energies and enthalpies. 3.5. Compensation. We have noted that another sort of “compensation” appears to operate between ΔH‡ and ΔS‡ in additions of halocarbenes to alkenes, where both parameters increase in tandem, compare Table 3.4,26,45 The effect is for −TΔS‡ to counter the influence of ΔH‡ on ΔG‡. As ΔH‡ and ΔS‡ increase, −TΔS‡ becomes more negative and buffers the increase in ΔG‡. Thus, from top to bottom of Table 3, ΔH‡ increases by 11.1 kcal/mol, and ΔS‡ increases by 24.6 eu, but ΔG‡ increases by only 3.8 kcal/mol. We suggested that the contra-Skell simultaneous increase of ΔH‡ and ΔS‡ might be profitably analyzed by a consideration of dynamic effects in carbene−alkene additions and studies of reaction trajectories instead of (or in addition to) classical transition state theory.45 Suppose, for the sake of argument, that the activation energies for additions of MePyr+CCl to TME and 1-hexene are both zero. In this regime, the success of

reaction will be best understood in terms of dynamics. There will be many trajectories of attack for the carbene on the alkene. Because of the greater shielding of the double bond in the case of (carbene + TME) than (carbene + 1-hexene), there will be fewer successful trajectories in the former case, addition will be less probable, and the entropy of activation will be more negative. Therefore, ΔS‡ will become more positive as we move from (MePyr+CCl + TME) to (MePyr+CCl + 1-hexene). Now imagine that there is a zero (or negative) activation energy for (MePyr+CCl + TME) but a small positive activation energy for (MePyr+CCl + 1-hexene). If we are still in a regime where dynamics trumps transition state analysis, then the above considerations hold, and we will observe an increase in both Ea (ΔH‡) and ΔS‡ as we move from (MePyr+CCl + TME) to (MePyr+CCl + 1-hexene). This will afford the appearance of compensation, although there may46 or may not47 be a deeper significance. As the carbene is made increasingly stable, activation energies for the additions will increase, and transition states will become tighter and move toward products. At some point, transition state theory will become appropriate. In such regimes, traditional Hammond principles, as applied to carbene/alkene additions by Skell, should be valid.42 Now, increasing activation energies will be associated with increasing steric interactions in tighter transition states and more negative values of ΔS‡. In this regime, compensation will not be observed, and ΔS‡ should become more negative as ΔH‡ increases. The question is where the transition between compensation and “normal” behavior may be found in carbene/alkene additions. Experimentally, the additions of more stabilized carbenes to alkenes seem a good place to look. These reactions will have appreciable activation energies and may show a normal (inverse) relation between ΔH‡ and ΔS‡. However, it remains to be seen whether such a regime is experimentally accessible within the realm of carbene−alkene addition reactions.

4. CONCLUSION MePyr+CCl BF4−, generated by LFP of diazirine 5, absorbs at 284 (calibrated UV spectrum) or 308 nm (uncalibrated). Following the decay of the latter absorption as a function of the 3560

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(12) Ditchfield, R.; Hehre, W. J.; Pople, J. A. Self-Consistent Molecular Orbital Methods. IX. An Extended Gaussian-Type Basis for Molecular Orbital Studies of Organic Molecules. J. Chem. Phys. 1971, 54, 724−728. (13) Hariharan, P. C.; Pople, J. A. Accuracy of AHn Equilibrium Geometries by Single Determinant Molecular Orbital Theory. Mol. Phys. 1974, 27, 209−214. (14) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. SelfConsistent Molecular Orbital Methods. XX. A Basis Set for Correlated Wavefunctions. J. Chem. Phys. 1980, 72, 650−654. (15) McLean, A. D.; Chandler, G. S. Contracted Gaussian-Basis Sets for Molecular Calculations. I. Second Row Atoms, Z=11−18. J. Chem. Phys. 1980, 72, 5639−5648. (16) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. v. R. Efficient Diffuse Function-Augmented Basis Sets for Anion Calculations. III. The 3-21+G Basis Set for First-Row Elements, Li−F. J. Comput. Chem. 1983, 4, 294−301. (17) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (18) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle− Salvetti Correlation Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785−789. (19) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. Ab Initio Calculation of Vibrational Absorption and Circular Dichroism Spectra Using Density Functional Force Fields. J. Phys. Chem. 1994, 98, 11623−11627. (20) Casida, M. E. Time-Dependent Density Functional Response Theory of Molecular Systems: Theory, Computational Methods, and Functionals. Recent Developments and Applications of Modern Density Functional Theory; Seminario, J. M., Ed.; Elsevier: Amsterdam, The Netherlands, 1996; pp 391−439. (21) Barone, V.; Cossi, M. Quantum Calculation of Molecular Energies and Energy Gradients in Solution by a Conductor Solvent Model. J. Phys. Chem. A 1998, 102, 1995−2001. (22) Parr, R. G.; Szentpaly, L. v.; Liu, S. Electrophilicity Index. J. Am. Chem. Soc. 1999, 121, 1922−1924. (23) Perez, P. Theoretical Evaluation of the Global and Local Electrophilicity Patterns of Singlet Carbenes. J. Phys. Chem. A 2003, 107, 522−525. (24) Moss, R. A.; Krogh-Jespersen, K. Carbenic Philicity and the ‘Intrinsic Reactivity Index.’. Tetrahedron Lett. 2013, 54, 4303−4305. (25) Rondan, N. G.; Houk, K. N.; Moss, R. A. Transition States and Selectivities of Singlet Carbene Cycloadditions. J. Am. Chem. Soc. 1980, 102, 1770−1776. (26) Zhang, M.; Moss, R. A.; Thompson, J.; Krogh-Jespersen, K. Evolution of Structure and Reactivity in a Series of Iconic Carbenes. J. Org. Chem. 2012, 77, 843−850. (27) Moss, R. A. Carbene Philicity. In Carbene Chemistry; Bertrand, G., Ed.; FontisMedia, S. A. and Marcel Dekker: New York, 2002; pp 57−101. (28) Moss, R. A. Carbenic Selectivity in Cyclopropanation Reactions. Acc. Chem. Res. 1980, 13, 58−64. (29) Moss, R. A. Carbenic Reactivity Revisited. Acc. Chem. Res. 1989, 22, 15−21. (30) In ref 3, the HOMO and LUMO energies of MePyr+CCl are given as −10.10 and −7.73 eV, respectively, from calculations carried out at the B3LYP/6-31G(d) level on the “naked” cation. The reported LUMO energy (−7.73 eV) is exceedingly negative and unrealistic in the context of providing an estimate for the electron affinity of the carbene. We find that physically reasonable orbital energies for MePyr+CCl can only be derived from calculations on a species that is electrically neutral overall, that is, a counterion must be explicitly included. In particular, in order to obtain HOMO/LUMO energy values compatible with those of the other carbenes of Table 1, our calculations were made at the HF/6-31G(d,p) level on the conformer representing the MePyr+CCl BF4− ion pair shown in Figure 1. (31) Calibration corrects the intensities of the observed absorptions for wavelength-dependent variations in sample absorptivity, xenon monitoring lamp emission, and detector response from 244 to 804 nm.

concentration of added alkene enabled us to obtain absolute rate constants for the additions of MePyr+CCl to a series of alkenes. Computational studies indicated that the carbene should possess a singlet ground state and be highly reactive and electrophilic toward alkenes. These expectations were fulfilled. Activation parameters were determined for additions of the carbene to TME, cyclohexene, and 1-hexene. From comparisons to the corresponding parameters of CF3CCl, CCl2, CClF, and CF2, we found that activation enthalpies decreased as carbenic stability decreased and as alkene reactivity increased. Moreover, activation entropies decreased in tandem with activation enthalpies. A qualitative explanation was offered for this linkage.

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ASSOCIATED CONTENT

S Supporting Information *

Figures for all spectra, kinetics, and activation parameter data and computed geometries and energetics. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the National Science Foundation and to Rutgers University for financial support. REFERENCES

(1) Moss, R. A.; Jang, E. G.; Kim, H.-R.; Ho, G.-J.; Baird, M. S. Pyridylhalocarbenes and Pyridiniumhalocarbenes. Tetrahedron Lett. 1992, 33, 1427−1430. (2) Moya-Barrios, R.; Cozens, F. L.; Schepp, N. P. Absolute Reactivity of Halo(pyridyl)carbenes. J. Org. Chem. 2009, 74, 1148− 1155. (3) Moya-Barrios, R.; Fregeau, B. M.; Cozens, F. L. Reactivity of Halo(pyridinium)carbenes. J. Org. Chem. 2009, 74, 9126−9131. (4) Moss, R. A.; Wang, L.; Krogh-Jespersen, K. Reactivity of Chlorotrifluoromethylcarbene: Activation Parameters for Halocarbene−Alkene Addition Reactions. J. Org. Chem. 2013, 78, 11040− 11044. (5) Baird, M. S.; Bruce, I. Generation and Trapping of 2- and 3Pyridylhalogenocarbenes from Diazirines. J. Chem. Res., Synop. 1989, 374−375; J. Chem. Res., Miniprint 1989, 2852−2871. (6) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, revision D.01; Gaussian, Inc.: Wallingford, CT, 2009. (7) Head-Gordon, M.; Pople, J. A.; Frisch, M. J. MP2 Energy Evaluation by Direct Methods. Chem. Phys. Lett. 1988, 153, 503−506. (8) Dunning, T. H., Jr. Gaussian Basis Sets for Use in Correlated Molecular Calculations. I. The Atoms Boron through Neon and Hydrogen. J. Chem. Phys. 1989, 90, 1007−1023. (9) Dunning, T. H., Jr.; Woon, D. E. Gaussian Basis Sets for Use in Correlated Molecular Calculations. III. The Second Row Atoms, Al− Ar. J. Chem. Phys. 1993, 98, 1358−1371. (10) Schlegel, H. B. Potential Energy Curves Using Unrestricted Møller−Plesset Perturbation Theory with Spin Annihilation. J. Chem. Phys. 1986, 138, 221−227. (11) Pople, J. A.; Head-Gordon, M.; Raghavachari, K. Quadratic Configuration Interaction. A General Technique for Determining Electron Correlation Energies. J. Chem. Phys. 1987, 87, 5968−5975. 3561

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The Journal of Physical Chemistry A Calibration also alters the λmax of strong absorptions. For details, see Figures S-1−S-3 in the Supporting Information. (32) Triplet MePyr+CCl and MePyr+CCl BF4− are computed to absorb at shorter wavelengths, namely, 267 nm (f = 0.30) for MePyr+CCl and 266 nm ( f = 0.27) for MePyr+CCl BF4− (see the Supporting Information). (33) Wang, J.-L.; Toscano, J. P.; Platz, M. S.; Nikolaev, V.; Popik, V.; Dicarbomethoxycarbene, A. Laser Flash Photolytic Study. J. Am. Chem. Soc. 1995, 117, 5477−5483. (34) Moss, R. A.; Zhang, M.; Krogh-Jespersen, K. Latent Nucleophilicity of Dichlorocarbene. Org. Lett. 2009, 11, 1947−1950. (35) To convert Cozens’s values for ΔEE (ref 3, Table 4) to values that employ εLU(MePyr+CCl) = −0.16 eV (cf. ref 30), add 7.57 eV to Cozens’s values. (36) Moss, R. A.; Wang, L.; Zhang, M.; Skalit, C.; Krogh-Jespersen, K. Enthalpy versus Entropy in Chlorocarbene/Alkene Addition Reactions. J. Am. Chem. Soc. 2008, 130, 5634−5635. (37) Moss, R. A.; Wang, L.; Krogh-Jespersen, K. A New Synthesis of Difluorodiazirine and the Absolute Reactivity of Difluorocarbene. J. Am. Chem. Soc. 2009, 131, 2128−2130. (38) Houk, K. N.; Rondan, N. G.; Mareda, J. Theoretical Studies of Halocarbene Cycloaddition Selectivities. A New Interpretation of Negative Activation Energies and Entropy Control of Selectivity. Tetrahedron 1985, 41, 1555−1563. (39) Houk, K. N.; Rondan, N. G.; Mareda, J. Are π-Complexes Intermediates in Halocarbene Cycloadditions? J. Am. Chem. Soc. 1984, 106, 4291−4293. (40) Houk, K. N.; Rondan, N. G. Origin of Negative Activation Energies and Entropy Control of Halocarbene Cycloadditions and Related Fast Reactions. J. Am. Chem. Soc. 1984, 106, 4293−4294. (41) Hammond, G. S. A Correlation of Reaction Rates. J. Am. Chem. Soc. 1955, 77, 334−338. (42) Skell, P. S.; Cholod, M. S. Reactions of Dichlorocarbene with Olefins. Temperature Dependence of Relative Reactivities. J. Am. Chem. Soc. 1969, 91, 7131−7137. (43) Values of επ (from the ionization potentials) of TME, cyclohexene, and 1-hexene are (respectively) −8.30, −8.95, and −9.46 eV, see: Ionization Potentials for Common Industrial Gases. www. indsci.com/docs/manuals/VX500_IP.pdf (2015). (44) We note in Table 3 that ΔS‡ becomes more positive as the carbenes become less electrophilic. This could be construed as a violation of Skell’s prediction (1), except that the less electrophilic carbenes also happen to be smaller and, toward a given alkene, might encounter fewer steric and entropic problems, even in tighter transition states. (45) Moss, R. A. “Carbon Dichloride”: Dihalocarbenes Sixty Years After Hine. J. Org. Chem. 2010, 75, 5773−5783. (46) Leffler, J. E.; Grunwald, E. Rates and Equilibria of Organic Reactions; Wiley: New York, 1963; pp 315−402. (47) Cornish-Bowden, A. Enthalpy−Entropy Compensation: A Phantom Phenomenon. J. Biosci. 2002, 27, 121−126.

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