The Essential Role of Bond Energetics in CâH ... - ACS Publications

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The Essential Role of Bond Energetics in C−H Activation/ Functionalization Xiao-Song Xue,‡ Pengju Ji,† Biying Zhou,‡ and Jin-Pei Cheng*,†,‡ †

Center of Basic Molecular Science (CBMS), Department of Chemistry, Tsinghua University, Beijing, 100084, China State Key Laboratory of Elemento-organic Chemistry, Collaborative Innovation Center of Chemical Science and Engineering, Nankai University, Tianjin, 300071, China

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‡

ABSTRACT: The most fundamental concepts in chemistry are structure, energetics, reactivity and their inter-relationships, which are indispensable for promoting chemistry into a rational science. In this regard, bond energy, the intrinsic determinant directly related to structure and reactivity, should be most essential in serving as a quantitative basis for the design and understanding of organic transformations. Although C−H activation/functionalization have drawn tremendous research attention and flourished during the past decades, understanding the governing rules of bond energetics in these processes is still fragmentary and seems applicable only to limited cases, such as metal− oxo-mediated hydrogen atom abstraction. Despite the complexity of C−H activation/ functionalization and the difficulties in measuring bond energies both for the substrates and intermediates, this is definitely a very important issue that should be more generally contemplated. To this end, this review is rooted in the energetic aspects of C−H activation/functionalization, which were previously rarely discussed in detail. Starting with a concise but necessary introduction of various classical methods for measuring heterolytic and homolytic energies for C−H bonds, the present review provides examples that applied the concept and values of C−H bond energy in rationalizing the observations associated with reactivity and/or selectivity in C−H activation/functionalization.

CONTENTS 1. Introduction 1.1. Background 1.2. Scope 2. Briefs on Methods of Bond Energy Determination 2.1. C−H BDE by Gas-Phase Methods 2.1.1. Pyrolysis 2.1.2. Halogen Radical Kinetics 2.1.3. Photoionization Mass Spectrum (PIMS) 2.1.4. Negative Ion Cycles (NIC) 2.2. C−H BDE by Solution Method via the Thermodynamic Cycle 2.3. C−H Bond Heterolysis Energy in Solution (pKa of Carbon Acid) 2.4. Bond Cleavage Energy of Radical Ions in Solution via the Thermodynamic Cycle 2.5. About C−M and M−H BDE 2.5.1. Experimental Measurement of Relative C−M BDE 2.5.2. Computation of Absolute/Relative C−M BDE 2.5.3. Measurement of M−H BDE via the Thermodynamic Cycle 3. Understanding C−H Activation Based on Bond Energetics 3.1. Reactivity Related to C−H BDE in C−H Activations Mediated by Metal−Oxo Compounds © 2017 American Chemical Society

3.1.1. Reactivity in C−H Activation Mediated by Group VI Metal−Oxo Complexes 3.1.2. Reactivity in C−H Activation Mediated by Group VII Metal−Oxo Complexes 3.1.3. Reactivity in C−H Activation Mediated by Group VIII Metal−Oxo Complexes 3.1.4. Reactivity in C−H Activation Mediated by Group IX Metal−Oxo Complexes 3.1.5. Reactivity in C−H Activation Mediated by Group X Metal−Oxo Complexes 3.2. Reactivity Related to C−H BDE in C−H Activations Mediated by Metal Hydroxo Compounds 3.3. Reactivity Related to C−H BDE in C−H Activations Mediated by Metal Methoxide Compounds 3.4. Reactivity Related to C−H BDE in C−H Activations Mediated by Metal Peroxo, Superoxo, Acylperoxo, and Acetate Compounds 3.5. Reactivity Related to C−H BDE in C−H Activations Mediated by Metal Imido Complexes

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8628 Special Issue: CH Activation Received: September 29, 2016 Published: March 10, 2017

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DOI: 10.1021/acs.chemrev.6b00664 Chem. Rev. 2017, 117, 8622−8648

Chemical Reviews 3.6. Reactivity Related to C−H BDE in C−H Activation Mediated by Metal Nitrido and Sulfilamido Complexes 3.7. Site Selectivity Governed by Substrate’s C− H BDE 4. Conclusion and Outlook Author Information Corresponding Author ORCID Author Contributions Notes Biographies Acknowledgments References

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simply helpless for analyzing problems raised in C−H activation. Many reasons could be responsible for ending up in such a situation. For instance, it may be because (1) there is nowhere to find the bond energies needed for energetic analysis, even with the help of the newly established iBonD databank;10 (2) the energetic contributions of the C−M or M− L bond(s) (M = metal or metal complex, L = ligand) involved in reaction coordination may have been overlooked or misinterpreted; (3) factors reflecting steric hindrance and/or an innate strain in the transition states have not been properly quantified and counted; and so on. Nevertheless, no matter how complex the actual situation may look, it is our opinion that it will definitely be worthwhile to apply as many of the available quantitative tools as you can to untangle the problems observed in C−H activation, so as to accumulate new insights into current understandings to help promote this field of research to steadily become more and more rational. To this end, we in this review tried to sum up the research reflecting the roles of bond energy found in certain C−H activations/ functionalizations, on the basis of the remarkable endeavors devoted in past decades in this connection.

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1. INTRODUCTION 1.1. Background

Most chemical substances known today (∼108 species) are C− H-bond-bearing compounds derived from nature or from the enormous chemical transformations conducted in recent decades. These organic substrates are normally stable under nonextreme conditions due largely to the high strength of their peripheral C−H bonds which, in a sense, act as a hurdle to shield relatively tender inner bonds from direct attack in reactions. Hence, reorganization of C−H and C−C bonds in hydrocarbons into target molecules via C−H bond activation/ functionalization is inherently a highly difficult task,1,2 especially when exploring selective and efficient conversions of readily available compounds with multiple C−H bonds to value-added species, so it has been viewed as the Holy Grail in synthetic chemistry.3,4 From an energetic point of view, however, chemistry should basically be the science concerning bond reorganization through bond breaking and formation, which are governed intrinsically by the energies of relevant bonds and externally by reaction conditions adjusted to overcome kinetic barriers.5 Naturally, the rational design of desired reactions with good comprehension of these governing factors would be the Holy Grail in chemistry by this logic. Although many important guidelines in this regard (Marcus theory,6 Hammett-type equations,7 etc.) that have been soundly established in the past have profoundly affected our current understandings of chemistry and promoted this discipline to become a more rational science, the knowledge necessary to guide the practices in this staggeringly booming field of C−H activation/ functionalization is, however, in severe vacancy, and the field still has to rely largely on the trial-error mode for exploration. This might be understandable because of the great complexities embedded in C−H activation, as manifested by its varying modes of mechanisms8 featuring multifaceted interplays of the structures and properties of substrates and catalyst, the solvent and additives, the temperature and pressure, and so on.9 Nevertheless, though the actual situation could be quite diverse, the “first thing to know” for chemists to understand a reaction should still be the energetics of the bonds being broken and formed.5 As will be seen in this review, this logic is indeed workable in some cases of C−H activation/ functionalization, for example, when direct hydrogen atom transfer (HAT) is energetically and kinetically favorable. It should be pointed out, however, that in the majority of other cases, one may just find that the bond energy criteria are

1.2. Scope

It is necessary to mention that, among the sheer volume of excellent reviews on C−H activation (>1200 pieces under the subject “C−H activation”, among them >800 in past decade alone11) that have appeared in past, we failed to find one with a primary focus on the advances directly associated with bond energy. That said, it is actually an unrealistic task to encompass all of the achievements related, more or less, to this respect from the oceans of existing publications. Hence, for the sake of space and time, we chose to include in this account only those that applied the concept and values of bond energy in addressing their main observations associated with reactivity and/or selectivity in C−H activation/functionalization. Although, at present time, there seem not too many systematic practices, like the HAT reaction, where a certain governing trend has indeed been realized,12,13 it does not necessarily rule out the possibility of finding much more in the future in other reaction systems, especially when the bond energetics for both the starting and resulting species of the reaction in both the activation and the follow-up functionalization stages are taken into consideration. In addition, remember that the hitherto phrased “C−H activation/functionalization” (hereafter expressed as “C−H activation” for short) may not bear exactly all the critical elements as in their original definitions described by respective leading experts of the field.14−30 It is actually noted that the initial definition of C−H activation1,3,14,31,32 has been substantially expanded to become much more inclusive nowadays than when it was coined. Though the authors of this review may have a preference toward the early definition featured by differentiating the initial bond activation from its follow-up functionalization,33,34 this account has, however, made no attempt to differentiate them because the focus here is on bond energetics, which should be applicable for analyzing both the activation and functionalization steps despite that they may be defined differently. We also made no efforts to summarize the works that reflect, though may be partially, the role of bond energy on C−H activation by means of theoretical calculation. For readers who may be interested, recent works of Eisenstein and co-workers,35 Ess and co-workers,36 Fu and co-workers,37 and Klein et al.38 8623


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Table 1. Representative BDE Values of Typical C−H Bonds40

and a review by Eisenstein and co-workers17 may be recommended. To help readers to get more acquainted with the basic knowledge of bond energy and thus to better understand the topic, besides the above-mentioned scope of the survey, we also provide a very brief introduction on some primary concepts and selected experimental approaches commonly used for deter-

mining various modes of bond cleavage energies in the section below.

2. BRIEFS ON METHODS OF BOND ENERGY DETERMINATION The three major modes of C−H bond scission are shown in eqs 1−3, where bond dissociation energy (BDE), pKa, and ΔHhydride denote homolytic bond dissociation enthalpy (in 8624


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Table 2. Bond Energies of Representative Radical Ions versus Parents in DMSO (BDE in kcal mol−1) C−H bond toluene Ph2CH2 Ph3CH 9,10-dihydroanthrancene xanthene PhCH2CN fluorene 1-naphthylacetonitrile pentamethylcyclopentadiene (Cp*) cyclopentadiene (Cp) 9-anthrylacetonitrile 9-cyanofluorene X−H bond pKa(X−H)10 phenol 4-nitrophenol aniline 4-nitroaniline carbazole thiophenol PhCH2SH a

18.0 10.8 30.6 20.9 19.9 10.3 15.4

pKa(C−H)10

pKa(C−H•+)

∼43 32.2 30.6 30.1 30.0 21.9 22.6 20.85 26.1 18.0 19.8 8.3

−23 −25a −18a −24a −18a,b −32b −17a,b −18.5b −6.5c −17c −13b −25b

BDE(C−H)10,40

BDE(C−H•+)

88.0 82.0 81.0 78.0 75.0 82.0 79.5 81.3 77.1 81.2 78.8 74.7

a

a

pKa(X−H•+)

BDE(X−H)10,40

−8.1 −18.0e 6.4f 2.0f 1.5f −11.7g 2.4g e

48 36a 45a 31a 41a 47.5b 56a

BDE(C−H•−) cyclohexene > cumene > ethylbenzene > toluene, which is similar to the case of C−H bond oxidation by metal−oxo complexes.180 A linear correlation was observed between log k′ and BDE of the C−H bond in the substrate (Figure 34b), supporting a hydrogen atom abstraction mechanism.211 In 2012, Cundari, Warren, and their co-workers investigated the reactivity of the nickel−imide [Me3NN]NiNAd (64) in C−H functionalization, which underwent hydrogen atom

Figure 33. Plot of log k2′ versus C−H BDEs of substrates. Reproduced with permission from ref 209. Copyright 2016 American Chemical Society.

Figure 34. (a) C−H bond amidation by [RuVI(TMP)(NSO2R)2] and [RuVI(Por)(NSO2R)2]. (b) Plots of log k′ versus BDE for reactions of [RuVI(TMP)(NNs)2] and [RuVI(F20-TPP)(NTs)2] with hydrocarbons. Reproduced with permission from ref 210. Copyright 2005 American Chemical Society. 8639


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Figure 35. Kinetic data for the reaction between [Me3NN]NiNAd (64) and R−H compounds via a hydrogen atom abstraction mechanism. Reproduced with permission from ref 212. Copyright 2012 American Chemical Society.

abstraction reactions with benzylic substrates R−H (indane, ethylbenzene, toluene).212 Kinetic study showed that the rates for hydrogen atom abstraction reactions between 64 and these C−H substrates decrease with increasing of C−H bond strength (Figure 35), which is consistent with a rate-limiting hydrogen atom abstraction mechanism. Recently, Che and co-workers found that the sevencoordinate complex [Fe(qpy)(MeCN)2](ClO4)2 (66) (qpy = 2,2′:6′,2″:6″,2‴:6‴,2″″-quinquepyridine) is a highly active nonheme iron catalyst for intra- and intermolecular amination of C(sp3)−H bonds.213 This complex effectively catalyzed the amination of not only benzylic and allylic C(sp3)−H bonds in hydrocarbons but also the C(sp3)−H bonds of cyclic alkanes and cycloalkane/linear alkane moieties in sulfamate esters. A detailed kinetic study showed that the second-order rate constants of the ferric complex catalyzed amination of DHA, fluorene, cumene, ethylbenzene, cyclooctane, and cyclohexane, with PhINTs as the additive, correlate with the C−H BDEs of these hydrocarbons, suggesting the involvement of electrophilic metal−imide/nitrene intermediates via hydrogen atom abstraction. It should be noted that although the hydrogen atom abstraction mechanism was proposed for the above C−H activations mediated metal imido complexes, concerted C−H insertion mechanism may not be ruled out.15,214

Figure 36. Plot of log k2′ vs C−H BDEs for the reaction of hydrocarbons with 67a in CH3CN at 298.0 K. Reproduced with permission from ref 215. Copyright 2010 American Chemical Society.

In 2012, Lau and co-workers reported kinetic and mechanistic studies of the intermolecular C−H bond activation of a number of alkanes by a well-characterized (salen)ruthenium(VI)−nitrido complex, [RuVI(N)(L)(MeOH)]PF6 (68) [L = N,N′-bis(salicylidene)-o-cyclohexyldiamine dianion] (Figure 37).216 It was found that this complex does not react with alkanes with a C−H BDE higher than that of DHA (78.0 kcal mol−1). However, in the presence of 0.1 M pyridine, the rate for oxidation of DHA by 68 was accelerated by over 2 orders of magnitude. More significantly, the coupled system 68/pyridine could activate C−H bonds as strong as those in cyclohexane (95.4 kcal mol−1). In particular, a linear correlation between the rate constant and the C−H BDE of alkanes is identified, suggesting that these reactions occur via an initial HAT step.

3.6. Reactivity Related to C−H BDE in C−H Activation Mediated by Metal Nitrido and Sulfilamido Complexes

In 2010, Lau at al. reported two (salen)ruthenium(IV) sulfilamido species, [RuIV(NH)(SR)(L)(NCCH3)](PF6) (67) [R = tBu, 67a; R = Ph, 67b; L = N,N′-bis(salicylidene)-ocyclohexyldiaminedianion)], that are able to abstract hydrogen atoms from hydrocarbons with weak C−H bonds.215 These hydrogen atom abstraction reactions occur with large deuterium KIEs; the KIE values for the oxidation of DHA, 1,4-cyclohexadiene, and fluorene by 67a are 51, 56, and 11, respectively. A plot of log k2′ vs C−H BDE of the hydrocarbons shows a good linear relationship for xanthene, CHD, DHA, and fluorene (Figure 36). However, the hydrogen abstraction of bulky diphenylmethane and triphenylmethane by the ruthenium(IV) complex was much slower than expected from their BDEs. Molecular modeling shows that there is substantial steric hindrance for HAT by 67a and 67b from these substrates.

3.7. Site Selectivity Governed by Substrate’s C−H BDE

In 2010, Zhang and co-workers developed a cobalt(II) complex of 3,5-DitBu-IbuPhyrin, [Co(P1)] (70), for the selective intramolecular C−H amination of a wide range of sulfamoylazides under neutral and nonoxidative conditions.217 Excellent 8640


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yield cyclic sulfoamides in excellent yields with high regio- and stereoselectivity. In order to shed light on the origin of the remarkable catalytic capacity of the Co(II)-based system toward the selective amination of C−H bonds in these electron-deficient compounds, they also performed a direct competition experiment between the secondary C−H bonds in electron-deficient and -rich compounds under similar steric environments by choosing N-n-butyl sulfamoylazide ketone as the amination substrate. Under the standard catalysis of [Co(P1)], the reaction of 75 afforded two products, 76 and 77, in 80% and 12% yields, respectively (Scheme 2). The high Scheme 2. Regioselective Intramolecular Amination of C−H Bonds in the Electron-Deficient Compound by [Co(P1)] (70) Figure 37. Plot of log k2′ against C−H BDE of alkane in (CH2Cl)2 at 296.0 K. Reproduced with permission from ref 216. Copyright 2012 John Wiley and Sons.

regioselectivity, as well as high diastereoselectivity and stereospecificity, was observed with the cobalt(II)-based catalytic system. The C−H amination by a Co(II)/azidebased catalytic system that involves an unusual Co(III)−nitrene radical intermediate undergoing a stepwise radical abstraction− substitution pathway was proposed for these transformations. To gain insights into the suggested radical mechanism, sulfamoylazide 71, which contains a cyclopropyl unit, was designed and synthesized as a radical probe substrate for [Co(P1)]-catalyzed amination. The major product of the C−H amination reaction of 71 was the six-membered cyclic sulfamide 72, which results from 1,6-C−H nitrene insertion into the primary C−H bond (ca. 100 kcal mol−1; Figure 38).40 The potential amination product 74 from the more electron-rich but stronger secondary C−H bonds (ca. 106 kcal mol−1)40 was not observed. This result indicates that a key metallonitrene intermediate performs radical hydrogen atom abstraction rather than electrophilic C−H activation and that the observed high selectivity for 72 is due to the relative low C−H BDE in the substrate. Zhang and co-workers reported in 2012 that the cobalt(II) complex of 3,5-DitBu-IbuPhyrin, [Co(P1)] (70), is an effective catalyst for the intramolecular amination of C−H bonds in electron-deficient compounds. The C−H bonds that are adjacent to electron-withdrawing CO2R, C(O)NR2, C(O)R, and CN groups were activated by the cobalt(II) complex to

selectivity was attributed to the fact that the C−HA bond (92 kcal mol−1) is significantly weaker than the C−HB bond (98 kcal mol−1), and thus, the hydrogen atom abstraction by the key Co(III)−nitrene radical intermediate is expected to be more facile for HA than HB, demonstrating the C−H BDE as a fundamentally important factor in controlling and differentiating the reactivity and selectivity of the C−H bonds.218 In 2014, Lu, Zhang, and their co-workers reported the cobalt(II) complex of 3,5-DitBu-IbuPhyrin, [Co(P1)] (70), for intramolecular amination of propargylic C(sp3)−H bonds of Nbishomopropargylic sulfamoylazides 78 (Scheme 3). [Co(P1)] Scheme 3. Synthesis of Functionalized Propargylamine from [Co(P1)] (70) Catalyzed Chemoselective Amination of Propargylic C−H Bond in a Sulfamoylazide

was found to be highly efficient for the chemoselective amination of different types of propargylic C−H bonds with excellent regioselectivity. The chemoselectivity was explained to

Figure 38. Mechanism proposed on the basis of cyclopropyl-ring opening for the cobalt(II)-catalyzed intramolecular C−H amination; note that the regioselectivity of the reaction is very sensitive toward the C−H BDEs in the substrate. Reproduced with permission from ref 217. Copyright 2010 John Wiley and Sons. 8641


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over, C−H pKa has also been found to be an important quantity for addressing reactivity or site selectivity in some C− H activation reactions where palladium (or copper) catalyst is involved in a concerted metalation−protonation transition state.223−227 This, together with that mentioned above, indicates that reactivity and selectivity of C−H activation in different systems could be dictated by different types of bond energies under respective mechanisms. It is realized, however, that the available data (especially M−C BDEs) are still very limited. Despite the remarkable achievements that Jones104,106,108−113 and others105,107,114,222 have made on quite some relative M−C bond energies, substantial endeavors are avidly expected in this respect, especially for accurate and absolute C−M BDEs from both experiments and computations, with the former as the first priority. New or refined experimental approaches for determining various modes of bond cleavage energies and future experimental re-evaluations of related data are also highly desirable.

be caused by the lower BDEs of the propargylic C−H bonds (ca. 85 kcal mol−1) compared to those of the aliphatic C−H bonds (ca. 98 kcal mol−1).219 These metal-complex-catalyzed intramolecular cyclizations reported by Zhang et al. provide characteristic examples of how the BDE of a C−H bond acts as a dictating factor for the regioselectivity in the C−H activation process.

4. CONCLUSION AND OUTLOOK The studies on C−H activation that applied the knowledge of bond strength to interpret the observed linear correspondences between the C−H bond energy and reactivity or selectivity of reactions are summarized in this review. The reactions demonstrating such features are often associated with a reaction mode commonly denoted as hydrogen atom transfer (HAT) or hydrogen atom abstraction, which, as evident from this review, relies on the use of high-valent transition-metal− oxo complexes to promote reactions. Thus, the metal−oxo species capable of mediating an apparent HAT can also be regarded as oxidant due to its high tendency to remove an electron, but the actual role of the oxidants in initiating C−H activations has, nevertheless, not been clearly addressed in terms of detailed mechanistic analyses. In this regard, and based on our knowledge on bond energetics of C−H radical cations (cf. section 2.4), we anticipate that the observed HAT in C−H activation would most likely proceed via a multistep process, i.e., the e−H+ path, in which a CH+• radical cation is first generated by a metal−oxo upon removal of an electron from substrate, followed by immediate deprotonation to form a carbon radical, giving an overall hydrogen atom abstraction. Normally, the two sequential steps are not separable due to the huge energetic driving force for CH+• to collapse to C· and H+, as obvious from the ΔpKa(CH−CH+•) gaps in Table 2. In some photoredox-catalyzed bond activations, this reaction path has already been recognized.92−94 Although in thermal processes an unambiguous single-electron transfer (SET) may not always be detected, interactions between a metal−oxo complex and a substrate would still make the C−H bond quasi-activated, the trend of which to undergo reaction can be expected to follow the same pattern. However, this does not necessarily rule out other pathways, like e−H· or e−H+−e (ends up in a H− transfer) under some conditions, and thus offers a chance for rational manipulations of certain key factors toward desired reactions. We envision that the research to explore/design more efficient and selective C−H bond activation in this respect will receive ever-increasing attention. As the chemistry regarding application of bond energies in C−H activation is insufficiently developed compared to methodologies, future work should evoke more joint endeavors among physical organic and synthetic chemists toward untangling puzzles found in this area from this particular angle. Generally speaking, weaker C−H bonds should be activated in preference to those that are stronger; indeed, such a rule of reactivity and selectivity is frequently observed in metal−oxo-complex-mediated C−H bond activations reviewed herewith. However, the opposite trend has also been observed in C−H activation, especially for transition-metal-based systems, where metals activate a stronger but not a weaker C−H bond.15,85,103−115,220−222 This could, again, be rationalized on the grounds of bond energy knowledge; that is, the stronger C−H bond interacting with a suitable metal could form an even stronger M−C bond.15,85,103−115,220−222 More-

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Xiao-Song Xue: 0000-0003-4541-8702 Jin-Pei Cheng: 0000-0001-8822-1577 Author Contributions

X.-S.X. and P.J. contributed equally to this work. Notes

The authors declare no competing financial interest. Biographies Xiao-Song Xue was born in 1983 in Chongqing, China. He received his Ph.D. in organic chemistry from Nankai University in 2013 under the supervision of Prof. Jin-Pei Cheng. In the same year, he joined Nankai University as an assistant professor. His current research interests are chemical bonds and physical organofluorine chemistry. Pengju Ji graduated from the University of Huddersfield (Huddersfield, UK) with a Ph.D. degree in physical organic chemistry (with Profs. Mike Page and John Atherton) in 2011. After a 2-year postdoc with Prof. Jin-Pei Cheng at Tsinghua University, Beijing, China, he became an assistant professor at the Center of Basic Molecular Science of the Chemistry Department, Tsinghua University. His research interests mainly cover the solvation and solvent effects of nonaqueous solvents, such as liquid ammonia and more recently ionic liquids. He is also interested in the energetic aspects of bond transformations and organic reaction mechanisms in these solvents. Biying Zhou was born in Heilongjiang China, in 1993. She received her B.Sc. in chemistry from Northeast Normal University (Changchun, China) in 2015 and began her graduate study at Nankai University in the same year under the supervision of Prof. Jin-Pei Cheng. Her current research is focused on the thermodynamics of fluorinating reagents and the mechanism of fluorination reaction. Jin-Pei Cheng, born in Tianjin, China, received his M.Sc. in 1981 with Chen-Heng Kao at Nankai University and his Ph.D. in 1987 with Fred Bordwell at Northwestern University, Evanston, Ill., where he learned more details on chemical bonds. He then worked with Ned Arnett as a postdoctoral fellow at Duke University. In 1988, he started his academic career first as a lecturer and then associate professor (1988) and professor (1990) at Nankai University, where he also acted as vice president for research and international relations in 1995−2000. After 8642


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a service as vice minister of S&T (for basic research), he resumed his full-time research on bond energetics and its applications at the State key Lab on Elemento-organic Chemistry, Nankai University. In 2012, he founded the Center of Basic Molecular Science of Tsinghua University, and he serves as director and professor. He is a member of CAS (2001 to present) and TWAS (2001 to present), and a fellow of RSC (2007 to present). In 2016, he with his team built iBonD (an Internet-based bond-energy databank) for free access to BDE data. He has been also one of the contributors to CRC Handbook of Chemistry and Physics (for BDEs) since 2012.

ACKNOWLEDGMENTS The authors thank National Natural Science Foundation of China (NSFC) for continuous support on their bond energy research (Grant Nos. 21390401, 20832004, 20902091, 21172112, 21172118, 21402099, and 21672124). Partial financial support from Tsinghua University, the State Key Laboratory on Elemento-organic Chemistry of Nankai University, and Tianjin Collaborative Innovation Center of Chemical Science and Engineering is also appreciated. J.-P.C. dedicates this review to his Ph.D. mentor, the late Prof. Frederick G. Bordwell, for his profound impact on education on the 100th anniversary of his birth. X.-S.X and P.J want to express their sincere gratefulness to the late Dr. Yu-Ran Luo for his meticulous mentoring and encouragement to work on bond energetics. The authors are also very appreciative of the very valuable comments and suggestions from the reviewers. REFERENCES (1) Labinger, J. A.; Bercaw, J. E. Understanding and Exploiting C−H Bond Activation. Nature 2002, 417, 507−514. (2) Li, J. J. C−H Bond Activation in Organic Synthesis; CRC Press: Boca Raton, FL, 2015. (3) Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.; Peterson, T. H. Selective Intermolecular Carbon-Hydrogen Bond Activation by Synthetic Metal Complexes in Homogeneous Solution. Acc. Chem. Res. 1995, 28, 154−162. (4) Yu, J.-Q.; Ding, K. C−H Bond Functionalization: The Holy Grail of Chemistry. Huaxue Xuebao 2015, 73, 1223−1224. (5) Blanksby, S. J.; Ellison, G. B. Bond Dissociation Energies of Organic Molecules. Acc. Chem. Res. 2003, 36, 255−263. (6) Marcus, R. A. Chemical and Electrochemical Electron-Transfer Theory. Annu. Rev. Phys. Chem. 1964, 15, 155−196. (7) Hammett, L. P. The Effect of Structure upon the Reactions of Organic Compounds. Benzene Derivatives. J. Am. Chem. Soc. 1937, 59, 96−103. (8) Boutadla, Y.; Davies, D. L.; Macgregor, S. A.; PobladorBahamonde, A. I. Mechanisms of C−H Bond Activation: Rich Synergy between Computation and Experiment. Dalton Trans. 2009, 5820−5831. (9) Yu, J. -Q. Catalytic Transformations via C−H Activation; Science of Synthesis; Thieme Medical Publishers, 2016; Vols. 1 and 2. (10) A comprehensive Internet-based bond energy databank, iBonD, was established by us recently. It includes more than 30 000 equilibrium acidity constants (pKas) for about 17 000 compounds in various molecular solvents and about 6500 BDEs of various important chemical bonds. For details, see the following: http://ibond.chem. tsinghua.edu.cn or http://ibond.nankai.edu.cn. (11) ACS SciFinder: https://scifinder.cas.org. (12) Fokin, A. A.; Schreiner, P. R. Selective Alkane Transformations via Radicals and Radical Cations: Insights into the Activation Step from Experiment and Theory. Chem. Rev. 2002, 102, 1551−1594. (13) Newhouse, T.; Baran, P. S. If C−H Bonds Could Talk: Selective C−H Bond Oxidation. Angew. Chem., Int. Ed. 2011, 50, 3362−3374. (14) Shilov, A. E.; Shul’pin, G. B. Activation of C−H Bonds by Metal Complexes. Chem. Rev. 1997, 97, 2879−2932. 8643


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