Electrophilicity and Nucleophilicity of Boryl Radicals - The Journal of

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Electrophilicity and Nucleophilicity of Boryl Radicals Chao Wu, Xiufang Hou, Yuheng Zheng, Pengfei Li, and Dongmei Lu J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b02849 • Publication Date (Web): 22 Feb 2017 Downloaded from http://pubs.acs.org on February 23, 2017

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The Journal of Organic Chemistry

Electrophilicity and Nucleophilicity of Boryl Radicals Chao Wu1, Xiufang Hou2, Yuheng Zheng1, Pengfei Li1, Dongmei Lu3* 1. Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi’an 710054, China; 2. College of Chemistry and Chemical Engineering, Yan’an University, Yan’an, 716000, China; 3. Department of Applied Chemistry, School of Science, Xi’an Jiaotong University, Xi’an 710049, China. KEYWORDS: boryl radicals, electrophilicity, nucleophilicity, density functional calculations.

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ABSTRACT We carried out a survey of the relative reactivity of a collection of 91 neutral boryl radicals using density functional calculations. Their reactivities were characterized by four indices, i.e., the global electrophilicity, global nucleophilicity, local electrophilicity, and local nucleophilicity. Particularly, the global electrophilicity and nucleophilicity indices span over a moderately wider range than those of carbon radicals, indicating their potentially broader reactivity. Thus boryl radicals may be utilized in electrophilic radical reactions, while traditionally they are only considered for nucleophilic radical reactions. In contrast, the local electrophilicity and nucleophilicity indices at the boron center show a different reactivity picture than their global counterparts. The inconsistency is rooted in the low and varying spin density on boron (for most radicals, less than 50%) in different boryl radicals, which is a combinative result of radical stabilization via electron delocalization and the low electronegativity of boron (compared to carbon). In short, the boron character in boryl radicals may be weak and their reactivity is not reflected by the local indices based on boron but by the global ones.


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Introduction Neutral boryl radials with the boron atom covalently bonded to two substitution groups (R) are called 3-center-5-electron (3c-5e, BR2•) type boryl radicals. However, as the central boron atom is highly electron deficient, 3c-5e type boryl radicals are extremely unstable and thus have not been detected. To relieve the electron deficiency problem, Lewis bases (LBs) are introduced to complex with the boron center and 3c-5e boryl radicals are converted to more stable 4-center-7electron (4c-7e, [LB→BR2]•) type boryl radicals. Traditionally, LBs, substitution groups, and the boron center are viewed as structurally independent moieties. For example, until a few years ago, the development of boryl radicals was solely focused on finding more stabilizing LBs (Fig. 1a), including amine- and phosphine-based LBs studied a few decades ago,1 carbenes (acridinederived carbenes;2, 3 azole-derived N-heterocyclic carbenes, NHCs;4-8 cyclic (alkyl)(amino)carbenes, CAACs;9, 10), and N-heterocycles (pyridine-11-13 and azole-derived cyclic amines14). Recently, the concerted stabilization brought by a conjugated cyclic backbone incorporating the boron center, substituents, and/or LBs has been employed to afford novel boryl radical frameworks (Fig 1b). For examples, the framework of borolyl radicals can be a borole ring15 or its derivatives like benzo[1,3,2]dioxaborole16, where the boron atom and two stabilizing substituents are fused into a ring, whose conjugation provides extra stabilization for the unpaired π electron. Furthermore, combining a LB, a substitution group, and the boron atom in a conjugated backbone like the 1,3,2-diazaborinine framework17 or the naphthalene framework18 also produces isolable boryl radicals, whose structures enforce the intramolecular LB→B interaction. Just in last year, even complex 3D structures of carborane clusters (C2B10H12)19 have been employed to generate the boron-centered carboranyl (o-C2B10H11•) radicals.


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(a) Stabilizing LBs amine/phosphine-based N R

H N

R R N

P R

carbene-based

R

R

N R

R

R N

R

R

R

N

R

N R N

R R

R

acridine-derived azole-derived carbenes NHCs

amine/phosphine pyridine-derived azole-derived -derivatives N-heterocycles N-heterocycles

R

R

CAACs

(b) Conjugated cyclic frameworks R

R

R

B

R

B

LB

R

borole-based

R

N

R

N

F

R

1,3,2-diazaborinine-based

R P

B R

R

naphthalene-based

Figure 1. Structural diversity of boryl radicals. (a) Experimentally identified Lewis bases for efficient stabilization of boryl radicals. (b) Experimentally developed cyclic boryl radicals. LB = Lewis base. R = functional groups. For simplicity, the same R's may represent different groups. Dashed arrows describe the unpaired electron delocalization. As a class of emerging radical species, boryl radicals possess distinctive electronic properties, 20-23

which offer unprecedented opportunity for radical reactions. For examples, amine- and

phosphine-stabilized boryl radicals have been utilized for decades in so-called "polarity-reversal catalysis", as their nucleophilic nature ensures faster hydrogen abstraction than the electrophilic alkyl radicals.1 Very recently, the 4-cyanopyridine-stabilized boryl radicals have been shown to be able to catalytically reduce inert chemicals like azo-compounds to hydrazine derivatives and sulfoxides to sulfides.13 However, there is a lack of a systematic way to measure the difference in the reactivity of various boryl radicals. In general, electrophilicity and nucleophilicity are known


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for a long time as two fundamental descriptors to denote a reactant's reactivity.24 For common organic non-radical nucleophiles and electrophiles, the reactivity scale has been developed by the Mayr group using experimental kinetics data.25, 26 For radicals, systematic experimental measurements can be very challenging. Fortunately, theoretical descriptions of electrophilicity and nucleophilicity indices for radicals have been proposed and applied to analyze C-, N-, O-, S-, and halogen-radicals recently.27, 28 In search of an understanding of the relative reactivity of boryl radicals, in this work, we carried out a theoretical investigation on the electrophilicity and nucleophilicity of neutral boryl radicals. We first employed the density functional calculations to obtain necessary parameters for computing both the global and local electrophilicity and nucleophilicity indices for 91 4c-7e boryl radicals (Fig. 2), which were designed based on experimental and theoretical studies. We then discussed the trends of the indices and compare them with that of representative carbon- and fluorine-based radicals. Next, we compared the computed indices with the experimentally measured reactivity of a group of boryl radicals reported in literature. Further, we inspected the relationships among the global and local electrophilicity and nucleophilicity indices. Finally, we tried to rationalize the relationship between the spin population on the boron atom and the global electrophilicity and nucleophilicity indices, which helps us understand the characteristics of boryl radicals as radicals and their reactivity as reactants. Theoretical Calculations Here we briefly presented the definitions of the four indices of boryl radicals, including the global electrophilicity, the local electrophilicity, the global nucleophilicity, and the local nucleophilicity. The global indices quantitatively label the degree of the electrophilicity and


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nucleophilicity of the whole radical and the local indices reflect the electrophilicity and nucleophilicity of the reaction center pinpointing the reacting atom. For the electrophilicity index (ω), we used Parr’s classical definition (eq. 1),29 which is related to the global chemical potential (µ)30 and the global chemical hardness of radicals (η)31,

ω = µ 2 / 2η

(1)

The latter two factors are defined as µ ≈ −( I + A) / 2 and η ≈ ( I − A) ,27 where I and A correspond to the ionization potential and the electron affinity of the boryl radical, respectively. I and A are further approximated by the energies of the highest occupied molecular orbital (HOMO) of the α α ) and the lowest unoccupied molecular orbital (LUMO) of the β spin spin states ( I = − E HOMO β ) of the radical.28 states ( A = − E LUMO

The global nucleophilicity index (N) of boryl radicals is defined according to Domingo and Pérez28, α α N = E HOMO − E HOMO (·F )

(2)

where the fluorine radical (·F) is taken here as the reference radical. As ·F is one of the most electrophilic radicals, by setting its global nucleophilicity index to zero, we hoped that the global nucleophilicity indices for the considered boryl radicals can be made positive, which will facilitate comparison. The local electrophilicity ( ω B ) and nucleophilicity ( N B ) indices for radical are defined as below,

ω B = ω PB

(3)

N B = NPB

(4)

where ω and N are their global counterparts defined before, and PB is the local radical Parr function.27, 29 Again, the proposal of Domingo and Pérez 28 was used, which directly equates the


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Mulliken spin density at the boron atom ( ρs,B ) of a boryl radical with the local radical Parr function, PB = ρ s , B . Density functional theory (DFT) based calculations were carried out by using the Gaussian 09 package32 at the (U)MPW1K33/6-31+G(d)34 level for all molecular species under the ideal gas assumption. This model chemistry has been proven to be able to accurately describe the geometries of zwitterions (i.e., LB stabilized boryl radicals).21-23,

35-37

For each species, a

geometry optimization followed by a frequency calculation using the harmonic oscillator model was employed to identify stable minimum energy structure. Results and Discussion We studied 91 4c-7e type boryl radicals, covering many experimental identified and theoretically proposed structures (Figure 2). In order to have a systematic comparison, we included reported radical series, e.g., radicals stabilized by cyclic amines11-14 and azole-derived NHC carbenes.4-8 However, our collection did not intend to be exhaustive as the field is developing fast. Missed structures like CAAC-boryl radicals and boron-centered carboranyl radicals are left for future study. Still, the variation in chemical composition and structure is substantial among these radicals. The boron atom is positioned in either acyclic or cyclic frameworks. Most of the latter are five/six-membered rings either conjugated or unconjugated. In the substitution groups, boron-connecting atoms include common elements of H, C, N, and O. Stabilizing LBs include amine/phosphine-derivatives and carbenes. We hoped that through an investigation of this collection of boryl radicals, useful information on their reactivity can be extracted. All four kinds of reactivity indices have been calculated and the results are summarized in Table 1 (more details, see Supporting Information Table S1). The global reactivity descriptors (defined in eqs. 1 and 2) are believed to provide intermolecular reactivity


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trends whereas local counterparts represent the site-specified reactivity (defined in eqs. 3 and 4), which is chosen to be the boron atom.


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Figure 2. Boryl radicals studied in this work. They are numbered according to the descending order of their global electrophilicity indices (ω). Radicals with red numbers had been experimentally studied in literature. Table 1. Four types of indices (the global electrophilicity ω, global nucleophilicity N, local electrophilicity ωB, and local nucleophilicity NB) for the 91 boryl radicals are listed. Radicals are numbered according to the descending sequence of ω. All the units are eV. No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

ω

ω 26.86 25.28 25.21 24.52 22.60 22.17 21.81 21.05 20.20 19.90 19.55 18.84 16.94 16.71 16.05 15.85 15.46 13.74 13.55 13.34 13.20 12.09 11.86 10.69 10.60 10.44 10.03 9.83 8.75 7.91 7.73 7.48 7.14

N 5.58 5.80 5.74 5.99 6.00 5.79 6.21 6.14 5.98 6.74 6.03 6.31 6.25 6.54 6.96 6.92 7.16 7.34 6.79 7.51 6.90 7.24 7.29 7.49 8.06 7.29 7.24 7.57 7.79 7.78 7.70 7.42 7.69

ωB 6.30 7.77 6.63 7.43 7.94 5.75 7.60 3.09 6.02 6.72 6.75 3.11 6.78 3.96 5.32 4.50 6.19 6.07 3.71 1.70 4.13 4.21 4.77 3.34 3.75 3.46 3.94 4.83 3.95 2.13 3.35 1.94 3.57

NB 1.31 1.78 1.51 1.82 2.11 1.50 2.16 0.90 1.78 2.28 2.08 1.04 2.50 1.55 2.31 1.97 2.86 3.24 1.86 0.96 2.16 2.52 2.93 2.34 2.85 2.42 2.85 3.72 3.52 2.09 3.34 1.93 3.84

No. 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79

ω

ω 5.10 5.08 4.98 4.95 4.77 4.64 4.60 4.57 4.50 4.18 4.11 4.02 4.00 3.94 3.94 3.93 3.93 3.88 3.86 3.85 3.80 3.77 3.72 3.59 3.49 3.45 3.44 3.41 3.37 3.36 3.35 3.00 2.96

N 8.10 8.18 8.21 8.07 8.43 8.35 8.51 8.29 8.02 8.37 7.44 8.14 8.29 8.18 8.47 8.45 8.36 8.33 8.33 8.35 8.52 8.35 8.16 8.50 8.32 8.15 8.57 8.63 8.54 8.57 8.49 8.41 9.06

ωB 2.54 2.67 2.97 1.95 2.57 2.36 1.72 2.19 2.06 2.29 2.80 1.70 0.23 2.32 2.13 1.45 1.20 1.46 0.31 2.06 1.90 1.51 -0.15 1.59 1.73 2.04 0.05 1.66 0.14 1.98 1.67 2.35 0.63

NB 4.03 4.31 4.89 3.17 4.55 4.24 3.17 3.97 3.68 4.57 5.07 3.44 0.48 4.80 4.58 3.11 2.56 3.14 0.67 4.48 4.26 3.34 -0.32 3.76 4.13 4.82 0.13 4.20 0.36 5.05 4.23 6.60 1.94


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34 35 36 37 38 39 40 41 42 43 44 45 46

6.92 6.85 6.72 6.57 6.36 6.35 6.14 6.07 5.49 5.44 5.41 5.39 5.32

7.79 7.96 7.69 7.73 7.52 7.68 7.59 7.55 7.73 7.91 7.90 8.33 8.23

3.66 1.54 2.31 2.72 1.77 3.13 2.38 2.20 2.05 1.36 1.29 0.51 2.11

4.12 1.79 2.65 3.21 2.09 3.78 2.94 2.74 2.88 1.98 1.89 0.79 3.25

80 81 82 83 84 85 86 87 88 89 90 91

2.94 2.84 2.71 2.05 1.94 1.87 1.59 1.57 0.97 0.92 0.77 0.68

8.70 8.57 8.61 9.04 9.10 9.24 9.26 9.24 8.96 9.51 8.80 8.85

0.12 1.29 1.23 0.40 0.37 0.32 0.07 0.20 1.17 1.46 0.82 0.84

0.35 3.90 3.92 1.78 1.75 1.56 0.40 1.18 10.84 15.12 9.30 10.95

We first examined the computed global electrophilicity (ω) and nucleophilicity (N) indices of the boryl radicals (Figure 3). To have a better understanding, we also computed ω and N for a few representative radicals (SI, Table S2), including some most electrophilic (fluorine radical and dicyanomethyl radical), most nucleophilic (2-hydroxyprop-2-yl radical and 2-aminoprop-2yl radical), and intermediate ones (phenyl radical and trifluoromethyl radical). As expected, fluorine radical has the highest global electrophilicity and the lowest global nucleophilicity. Both types of global indices of the selected carbon radicals exhibit low, medium, and high values. Moreover, boryl radicals show even moderately wider ranges of both global electrophilicity and nucleophilicity than the carbon radicals, implying their broader reactivity tunability. The most electrophilic boryl radicals (e.g. No. 1 to 15) have similar or even larger ω than the dicyanomethyl radical. The common feature of their structure is the five π-electron conjugated ring including the boron atom and two neighboring nitrogen atoms plus other stabilizing electron withdrawing groups. In addition, the stabilizing LB is pyridine or the unsubstituted NHC (imidazole-based), both of which provide strong σ-donating capability and at the same time exert minimum steric effect to allow the entire complex to stay in plane. The spin population on boron for these radicals are lower than 0.4 e, which are usually lower than boryl radicals with smaller ω values (SI, Table S1). In fact, the concerted stabilization mechanism of strong σ-donating LBs and strong π-accepting substituents also suggests that these radicals may have the potential to efficiently accommodate and delocalize the obtained electron density during


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electrophilic reactions. 22, 23 However, the most electrophilic boryl radicals studied here have not been synthesized yet, thus their nucleophilic image will still prevail.1 In comparison, the least electrophilic boryl radicals (e.g. No. 80 to 91) have similar or even smaller ω than the 2-aminoprop-2-yl radical. Their structures either do not have a conjugated ring with boron fused in or the boron-containing conjugated ring already has seven π-electrons (e.g. No. 86), evidently unfavorable for taking more electron density. It is worth noting that the least electrophilic boryl radicals of No. 88 to 91 have (intramolecular) nonaromatic amines as the stabilizing LBs, which do not help delocalize spin density at all (SI, Table S1).

Figure 3. The global electrophilicity (ω) and nucleophilicity (N) indices of the 91 boron radicals and six other representative radicals according to the decreasing order of ω. Blue bar stands for ω, red bar for N, and cyan bar is ω of the labeled carbon and fluorine radicals. The least electrophilic radicals (e.g. No. 80 to 91) are the most nucleophilic. Particularly, the amine-stabilized boryl radical without any substitution, No. 90, is a typical nucleophile and has been observed in experiment to have very high reactivity towards Micheal reaction with


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methylacrylate (MA).11,12,14 Its addition rate constants with MA is 13×107 M-1s-1,11 much larger than boryl radicals stabilized by pyridine-derived LBs (No. 32, 36, 38, 40, 50, 57, 70 Table 2). Their global nucleophilicity indices linearly correlate well with the MA addition reaction rate constants (Figure 4a), which demonstrates that the global nucleophilicity indices can describe the reactivity of nucleophilic reactions of a series of nucleophiles with similar structures, as they are expected to go through the same reaction mechanism. In comparison, their global electrophilicity indices do not linearly correlate so well with the rate constants (Figure 4b). Moreover, the boronhydrogen bond dissociation energies (BDEs) does not linearly correlate well with either ω or N. BDEs are usually linked to boron's spin population, which is a better descriptor for radicalradical reaction. This group of boryl radicals also illustrate the indirect effect of substitution at LB on the reactivity. For examples, radicals No. 41, 50 and 70 have the global electrophilicity values of 6.07, 4.95, and 3.59 eV, respectively. The morpholine and piperidine groups in No. 50 and 70 clearly exhibit their electron-donating nature, which is a result of the N resonance with the pyridine ring. Therefore, their global electrophilicity values are lowered. The oxygen atom in morpholine evidently reduces the electron-donating ability of morpholine compared with the piperidine group, resulting in a bigger electrophilicity value for No. 50. Additionally, their global nucleophilicity indices are in the reversed order of their global electrophilicity values, i.e., 7.55, 8.07, and 8.50 eV for No. 41, 50 and 70, respectively. However, in table 2 there are two exceptions, No. 55 and 71 (stabilized by pyrazole-based LBs, not shown in Figure 4), who have extraordinarily high MA addition rate constants (40×107 and 39×107 M-1s-1) and very small reaction barrier (less than 1 kcal mol-1).14 According to the nucleophilicity indices (either global or local), even though they are strong nucleophiles, they are expected to be weaker than No. 90. It might be that the MA addition mechanism for boryl radicals stabilized by pyrazolyl LBs is different from those stabilized by pyridine-derivatives. Yet, the real reason is unclear and this definitely calls for more study. Table 2. Calculated bond dissociation energy of a series of pyridine-derived boranes; measured rate constants of pyridine-stabilized boryl radicals with methylacrylate, and computed global electrophilicity and nucleophilicity indices.


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Boryl radical No.

B-H BDE[a]

kadd [b]

ω[c]

N[c]

32

67.311

0.0511

7.48

7.42

36

79.714

114

6.72

7.69

38

70.814

0.0314

6.36

7.52

40

71.911

0.111

6.14

7.59

50

81.912

2.412

4.95

8.07

55

84.814

4014

6.14

7.59

57

90.711

0.7511

4.11

7.44

70

82.212

1412

3.59

8.50

71

85.814

3914

3.49

8.32

90

101.111

1311

0.77

8.80

[a]. B-H bond dissociation energy in kcal mol-1. Calculated at the level of B3LYP/6–31+ G*. [b]. Reaction rate constants with methylacrylate (MA) at room temperature in acetonitrile/di-tertbutylperoxide solvent, 107 M-1 S-1. Data of [a] and [b] are taken from refs. 11, 12, and 14. [c]. Computed global electrophilicity (ω) and nucleophilicity (N) in eV.

Figure 4 Correlations between the indices of the global nucleophilicity (a) and electrophilicity (b) with the MA addition reaction rate constants for a series of boryl radicals stabilized by pyridine derivatives. Rate constants are taken from refs. 11, 12, and 14, details see Table 2.


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If the stabilizing LB of a boryl radical is kept constant, then the substitution groups neighboring the boron atom will determine the values of its global electrophilicity and nucleophilicity. This can be exemplified by a series of boryl radicals (No. 26, 35, 43 and 44) , all stabilized by pyridine. The conjugated borole ring in No. 26 has five π-electrons and would readily hold an additional electron to form the favorable six π-electrons configuration. The partial conjugation in the six-membered ring of No. 35 also has five π-electrons, but due to the broken conjugation at C4 (the C opposite to B), its electrophilicity (6.85 eV) is much smaller than that of No. 26 (10.44 eV). No. 43 and 44 do not have conjugation at the boron part at all, thus their electrophilicity indices (5.44 and 5.41 eV) are both smaller than that of No. 35. In comparison, their global nucleophilicity indices are qualitatively reversed (values are 7.29, 7.96, 7.91 and 7.90 eV for No. 26, 35, 43 and 44). The close values of nucleophilicity indices of No. 35, 43 and 44 is the result of the much narrower distribution of

nucleophilicity than

electrophilicity. In other words, they are essentially similar nucleophiles. The numerical values of the global nucleophilicity (N) indices are much lower than that of the global electrophilicity (ω) and they are approximately inversely proportional to each other like carbon radicals (Figure 5a), a result of their definition.27 Most synthesized boryl radicals are in the tail part of the electrophilicity plot, whose strong nucleophilic feature is often labeled to all boryl radicals. Data in Figure 3 can help us select proper boryl radicals to act as the nucleophilic initiator to accelerate the hydrogen abstraction reaction that is traditionally initiated by the electrophilic alkoxyl radicals.1


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ω B = 0.2784ω + 0.5174

N = −0.1313ω + 8.8814 10

R 2 = 0.81

2

R = 0.90

8

9 6

ωB

N

8 7

4 2

6 0 5

0

5

10

15

20

25

0

30

5

10

15

ω

ω

(a)

(b)

16

20

25

30

16

12

12

8

NB

NB

4

8 4

0

0

6

7

8

9

10

0

2

4

6

8

0.8

1.2

ωB

N

(c)

(d)

10

25

9

20 8

15

N/eV

ω/eV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10 5

7 6

0

5

0.0

0.4

0.8

1.2

1.6

0.0

B spin density

(e)

0.4

1.6

B spin density

(f)

Figure 5 Correlations among the indices of electrophilicity and nucleophilicity calculated for the boryl radicals in Table 1. (a) The global nucleophilicity (N) versus the global electrophilicity (ω).


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(b) ω versus the local electrophilicity (ωB). (c) N versus the local nucleophilicity (NB) (d) NB versus ωB. (e) ω versus the spin density on boron. (f) N versus the spin density on boron. Next we compared the global indices with their local counterparts. The global electrophilicity (ω) shows a much weaker linear correlation with the local electrophilicity (ωB). The deviation from a good linear relation naturally rises as ωB of the chosen boron atom represents only a site of the molecule and due to electron (spin) delocalization, its value is only a fraction of ω. The scattered points indicate that the degree of spin delocalization away from the boron atom is different in each radical. Moreover, the pair of global (N) and local (NB) nucleophilicities do not show linear correlation at all. NB's of No. 88, 89, and 91 are much larger than the corresponding N's. This uncommon relative magnitude is also observed for their global and local electrophilicity indices. This is the result of the abnormally high Mulliken spin population (> 1.0 e, details see Supporting Information, Table S1) over boron in these radicals, whose structures feature an unusual and unconjugated double-ring backbone formed by the intramolecular amine stabilization (Figure 2). Furthermore, the local indices of electrophilicity and nucleophilicity lack evident linear correlation (Figure 4d), which indicates that the capability to accept or lose electron is not well described by the property of boron. Underneath the local indices is the low value of spin density on boron, which again does not correlate to either ω or N (Figure 5e and 5f). The discrepancy among the local and global indices indicate the high and inconsistent degrees of spin density delocalization in various boryl radicals. Borrowing Pearson's hard soft acid base concept, where soft acid or base is highly polarizable, we could tentatively and qualitatively describe the highly delocalized boryl radicals as "soft" radicals. For instance, radicals No. 12, 20, 45, etc., with extremely low spin population on boron (< 0.2 e) belong to this category. Naturally the less


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delocalized boryl radicals can be treated as "hard" radicals. Still, where to draw the exact demarcation line and more critically how the reactivity match with the types of radicals are left to upcoming studies. Boryl radicals actually exhibit two seemingly contradictory features. As radicals, their boryl character is small due to both the high degree of spin delocalization required by radical stability and boron's low electronegativity (e.g. 2.04 for B vs. 2.55 for C in the Pauling scale, causing its low capability to hold electrons). As reactants, the low spin population on boron may result into the change of reacting site to other atoms other than boron. For example, pyridine stabilized boryl radical No. 43, as shown by Zipse et al.21 may react at C2 or C4 of pyridine, whose spin populations are substantially bigger. This explains that the local property descriptors (local indices and spin density) are not good descriptors of the reactivity of boryl radicals.22 Similar trend has been observed in closely related borane radical anions, where even the B-H BDE has been found not to linearly correlate with the spin population on boron due to the complex spin delocalization.38

Conclusions We have characterized the relative reactivities of an emerging family of 91 neutral boryl radicals. These radicals exhibit wide distributions of the global indices of electrophilicity ω and nucleophilicity N, even moderately wider than those of carbon radicals. Therefore, boryl radicals should not only be viewed as nucleophilic and some of them may be utilized for electrophilic radical reactions. Local indices of electrophilicity ωB and nucleophilicity NB are chosen on the boron atom, however, their values in general do not linearly correlate with their global counterparts, which is a result of highly delocalized and yet differently distributed spin density


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among boryl radicals. Consequently, boryl radicals with low spin population on boron may react at positions other than the boron atom, losing its "boryl" feature.21 Thus the reactivity of boryl radicals is best described by the global indices rather than the local ones only focusing on boron. ASSOCIATED CONTENT

Supporting Information. The full data to compute the four indices of 91 boryl radicals. This material is available free of charge via the Internet at http://pubs.acs.org.. AUTHOR INFORMATION

Corresponding Author Dongmei Lu: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT We acknowledge use of computational resources at the Materials Physics Center and the Center of Microstructure Science of FIST. The National Science Foundation of China (grant number 21477096, 21406175) has provided financial support for this work. REFERENCES 1. Robert, B. P. Chem. Soc. Rev. 1999, 28, 25-35. 2. Chiu, C.-W.; Gabbaï, F. P. Angew. Chem. Int. Ed. 2007, 46, 1723-1725. 3. Chiu, C.-W.; Gabbaï, F. P. Angew. Chem. Int. Ed. 2007, 46, 6878-6881. 4. Matsumoto, T.; Gabbaï, F. P. Organometallics 2009, 28, 4252-4253. 5. Ueng, S.-H.; Solovyev, A.; Yuan, X.; Geib, S. J.; Fensterbank, L.; Lacôte, E.; Malacria, M.; Newcomb, M.; Walton, J. C.; Curran, D. P. J. Am. Chem. Soc. 2009, 131, 11256-11262.


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Electrophilicity and Nucleophilicity of Boryl Radicals - The Journal of

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