sp3 CâH Borylation Catalyzed by Iridium(III) Triboryl Complex

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sp3 C-H Borylation Catalyzed by Iridium(III) Triboryl Complex: Comprehensive Theoretical Study of Reactivity, Regioselectivity, and Prediction of Excellent Ligand Rong-Lin Zhong, and Shigeyoshi Sakaki J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b01767 • Publication Date (Web): 24 May 2019 Downloaded from http://pubs.acs.org on May 24, 2019

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Journal of the American Chemical Society

sp3

Borylation Catalyzed by Iridium(III) Triboryl Complex:

Comprehensive Theoretical Study of Reactivity, Regioselectivity, and Prediction of Excellent Ligand

Rong-Lin Zhong and Shigeyoshi Sakaki* †Fukui

Institute for Fundamental Chemistry, Kyoto University, Nishi-hiraki-cho 34-4, Takano, Sakyo-ku, Kyoto 606-8103, Japan.

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Abstract: Iridium-catalyzed

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/0 borylation of THF was theoretically investigated as

example of sp3 C/H functionalization. DFT computations show -regioselective borylation occurs more easily than -regioselective one, as reported experimentally, through oxidative addition of /0 bond to iridium(III) species and reductive elimination of B/C bond. The reductive elimination is rate-determining step and regioselectivity-determining step. The lower energy transition state (TS) of the reductive elimination of -boryloxolane arises from stronger Ir··( -oxolanyl) interaction than Ir··( -oxolanyl) one at TS. The stronger Ir··( oxolanyl) interaction than the Ir··( -oxolanyl) one is a result of higher valence orbital energy of -oxolanyl group than that of -oxolanyl group due to antibonding overlap of the valence orbital with O 2p orbital, where SOMO of oxolanyl radical is taken as valence orbital hereinafter. Reactivity of substrate decreases following the order, primary ( ) /0 of ethyl ether > primary

/0 of n-pentane ~ secondary ( )

/0 of THF > secondary

/0 of

cyclopentane > secondary ( ) /0 of THF ~ secondary /0 of n-pentane > secondary ( ) /0 of ethyl ether. Primary C-H bond is more reactive than secondary one because of its smaller steric repulsion and lower energy valence orbital of primary-alkyl group.

/0 bond

of THF is more reactive than secondary C-H bond of cyclopentane, because of lower energy valence orbital of -oxolanyl group than that of cyclopentyl group. Both steric and electronic factors are important for determining reactivity of substrate. Bidentate ligand consisting of pyridine and N-heterocyclic carbene is predicted to be better than 3,4,7,8-tetramethyl-1,10phenanthroline used experimentally.


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Introduction The regioselective functionalization of nonreactive sp3

/0 bond of alkane by transition-

metal complex is attracting great interest in recent chemistry because it offers new strategy for synthesizing of functional molecules from alkanes, as reviewed recently.1-11 Though Rh,12,17-20 Pd,13-16 Ru,18,21 and Ni22 complexes have been employed as catalyst for sp3 /0 functionalization, it should be noted that iridium-catalyzed borylation of sp3 C–H bond has been recently succeeded very well1,2,18,19,23-34 and is recognized now as one of crucially important reactions in modern organic synthesis because this reaction yields organoborane compounds which are versatile starting material for constructing # 1 # 1

/

/# 1

bonds. Though several regioselective sp2 and primary sp3

and /0

functionalization reactions have been performed by incorporating various directing groups,25 regioselective functionalization of sp3

/0 bond without directing group is much more

useful but much less common and much more challenging than that with directing group even nowadays. In this regard, we are much interested in a series of iridium-catalyzed regioselective borylations of sp3 /0 bond of cyclic ether,23 cyclopropane,24 normal alkyl ether, cyclohexane, amine,28 and toluene29 reported by Hartwig group. Their reports provided new perspective for regioselective functionalization of sp3

/0 bond in the absence of

directing group.



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(NHC), as shown in Scheme 2. Through these examinations, we wish to obtain systematic and comprehensive understanding of sp3 /0 borylation catalyzed by iridium(III) triboryl complex and predict good ligand.

Computational Details All geometry optimizations were performed by the B3PW91-D3 functional35,36 in gas phase, where D3 denotes the third-generation dispersion correction by Grimme and coworkers,37 using small basis set system named BS-I; in this BS-I, the Stuttgart-DresdenBonn basis set38 was employed for valence electrons of Ir with the effective core potentials representing its core electrons and 6-31G(d) basis sets were used for other atoms. We checked if the combination of the DFT functional and the BS-I provided good geometry, as shown in Table S1 of the Supporting Information. To evaluate better potential energy, single-point calculations were performed using the I7A?J4 functional,39 where larger basis set system (BS-II) was employed; in the BS-II, two f polarization functions40 were added to Ir and 6311+G(d, p) basis sets were used for other atoms.41-43 We tested the quality of this computational method by comparing energy changes with MP4(SDQ)-calculated values, using model system; details of model calculation are described in pages S4 to S5 of the Supporting Information. Solvation effect (THF) was evaluated with polarizable continuum model (PCM)44-46 using the geometries optimized above. In this work, discussion is presented using the Gibbs energy. Thermal correction and entropy contribution to the Gibbs energy were evaluated at 298.15 K and 1 atm, where the translational entropy in solution was corrected by the method of Whiteside et al.47 The Gibbs energy of activation 5MGº‡) is defined as difference in the Gibbs energy between transition state (TS) and the resting state which is the most stable species before the TS. The Gibbs energy of reaction (MGº) is defined as difference in the Gibbs energy between the product and the resting state. All these calculations were carried out using Gaussian09 program.48 6 ACS Paragon Plus Environment

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Results and Discussion We will firstly discuss reaction mechanism, active species, and rate-determining step taking sp3

/0 borylation of THF as example, because those results are necessary for

discussion of regioselectivity; here, we will make detailed analysis of two important elementary steps, oxidative addition of sp3 /0 bond of THF and reductive elimination of B/C bond. Next, we will elucidate the origin of -regioselectivity because regioselectivity is one of the important features in sp3 C/H borylation of alkane and also the -regioselectivity is against our expectation. Then, we will make comparison of reactivity among various substrates and discuss determining factors of reactivity. In the end, theoretical prediction of better ligand for the iridium catalyst will be presented.

Overview of Catalytic Cycle. We wish to briefly discuss full catalytic cycle and important elementary steps prior to detailed discussion. In previous investigations,28,29,34,49 the reaction mechanism of the sp3 /0 borylation of THF has been proposed, as shown in Scheme 3. This reaction occurs through Ir(III)/Ir(V) catalytic cycle; the first step is oxidative addition of the sp3 /0 bond of THF to Ir(III) of (NN)Ir(Bpin)3 (NN = phenanthroline derivative) to afford iridium(V) species (NN)Ir(Bpin)3(H)(oxolanyl). The second step is reductive




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Bpin N Bpin N Ir Bpin H H

( -TS4/5) (31.3) -TS4/5 33.1

Bpin Bpin N Ir Bpin N H H

O

Bpin Bpin Ir Bpin N Bpin pinB N

Bpin N Bpin Ir N Bpin

TS1/1a 14.9 B2pin2

2 14.9

O

1a 6.1 Bpin

1 0.0 Bpin Bpin N Ir Bpin N pinB Bpin

N N

Ir

Bpin Bpin Ir Bpin N O

Bpin

( -4) (10.2) -4 10.9

N

Bpin Bpin Ir Bpin H

H

O

N N H

Bpin Bpin Ir Bpin N H H N

N

Bpin Bpin H N Bpin pinB N

Bpin Bpin H Bpin

B2pin2

TS10/11 HBpin (5.3) 1.8

9 (10.0) -8 6.5

H

N N

Bpin Bpin H

10 (3.2) -0.3

Ir O

O

-8

B

2 (12.3) 8.8

H N Bpin Ir N Bpin pinB

( -7)

Bpin (12.7) Bpin -7 Ir N H 8.7 H O B O O

O

C H oxidative addition

pinB

O

B

O

Ir

TS9/10 (16.3) 12.8

Ir

O

Bpin

Catalyst activation

( -5 ) (19.6) -5 18.9

O

O

O B

N H

Bpin Bpin H Bpin

Ir

( -TS5/6) ( -6) (28.8) (27.1) -6 -TS5/6 28.1 28.8

N

Bpin

N

O

N

3 7.7

( -TS6/7) (38.1) -TS6/7 34.6

Bpin

O

B C Isomerization reductive elimination

Bpin Bpin N Ir H N pinB Bpin

B2pin2

11 (0.7) -2.8 O O

BH

N

Bpin Ir Bpin N pinB

Catalyst regeneration

Figure 1. The Gibbs energy profile (in kcal mol-1) of iridium(III)-catalyzed sp3 /0 borylation of THF. In parenthesis is the Gibbs energy change for the

/0 borylation.

Because iridium(III) triboryl complex (phen)Ir(Bpin)3 2 (phen = 1,10-phenanthroline; HBpin = pinacolborane) is coordinately unsaturated, we investigated THF coordination to 2, B2pin2 coordination to 2 through the O atom, and oxidative addition of B2pin2 to 2, as shown in Scheme S1 in the Supporting Information, and found that the most exoergonic was the oxidative addition of B2pin2 affording iridium(V) pentaboryl complex (phen)Ir(Bpin)5 1. This oxidative addition occurs with the moderate Gibbs energy of activation ( Gº‡), as shown in Figure 1, which agrees with previous reports.49,50 Because it is likely that the oxidative addition of the /0 bond of THF to iridium(V) intermediate does not occur easily, 1 should


1 (-2.6) -6.1


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return to 2 through reductive elimination of B2pin2 before the oxidative addition of the /0 bond. THF coordinates with the Ir(III) of 2 through the O atom to afford a THF adduct (phen)Ir(Bpin)3(THF) 3, as seen in Figure 1 and Scheme S1 in the Supporting Information. However, 3 is not a good intermediate for the oxidative addition of /0 bond, because the /0 bond is distant from the Ir atom. Isomerization of 3 occurs to afford an isomer (phen)Ir(Bpin)3(THF)

4, in which the

/0 bond of THF takes a position close to the

Ir(III) atom. This isomerization is moderately endergonic by 3.2 kcal mol-1 relative to 3. Starting from

4, the oxidative addition of the

/0 bond occurs through three-center

transition state

TS4/5 to afford seven-coordinated complex (phen)Ir(Bpin)3(H)( -oxolanyl)

5. The MGº‡ is 33.1 kcal mol-1 and the MGº is 18.9 kcal mol-1 relative to 1. Though the next step is reductive elimination of B-C bond between boryl and -oxolanyl groups,

5 does not

have good geometry for this reductive elimination; details are presented in the Supporting Information Figure S6. Prior to the reductive elimination of the B/C bond, isomerization occurs through position changes of boryl groups to afford isomer mol-1. Starting from state

6 with MGº‡ of 9.2 kcal

6, the reductive elimination of the B/C bond occurs through transition

TS6/7 to yield (phen)Ir(Bpin)3(H)( -boryloxolane)

7 with MGº‡ of 34.6 kcal mol-1

and MGº of 8.7 kcal mol-1 relative to 1. After -boryloxolane dissociates from the Ir atom,

7

undergoes oxidative addition of B2pin2 to the Ir(III) atom followed by reductive elimination of HBpin to regenerate iridium(III) triboryl complex 2. The MGº‡ values for these steps are moderate (6.3 and 2.1 kcal mol-1 for the oxidative addition and the reductive elimination, respectively). These moderate MGº‡ values are similar to those of previous reports.28,29,49,50 The regenerated 2 easily reacts with one more B2pin2 to afford the resting state 1. The ratedetermining step is the reductive elimination of the B/C bond, which needs MGº‡ of 34.6 kcal mol-1 relative to the most stable species 1 in the catalytic cycle; the rather large MGº‡ is briefly discussed in the Supporting Information page S26. The

/0 borylation of THF

occurs through the same catalytic cycle, while the energetics differs moderately; the MGº‡


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(31.3 kcal mol-1) for the oxidative addition of the

/0 bond by 1.8 kcal mol-1 and the MGº‡ (38.1 kcal mol-

for the oxidative addition of the 1)

/0 bond is moderately lower than that

for the reductive elimination of the B/ bond is somewhat larger than that for the B/

bond by 3.5 kcal mol-1. These results indicate that the reductive elimination of the B/C bond is rate-determining in both the

and

THF occurs more easily than the experimentally observed

/0 borylations and the sp3

sp3 /0 borylation of

/0 borylation, which is consistent with the

-regioselectivity;23 see page S16-S17 of the Supporting

Information for details about the comparison of MGº‡ between the oxidative addition of the C/H bond and the reductive elimination of the B/C bond. It should be noted that the oxidative addition of the

/0 bond occurs with smaller MGº‡ value than that of the C/H

bond but the reductive elimination of the B/C bond needs somewhat larger MGº‡ value in the C/H borylation than in the

C/H borylation, leading to the conclusion that the -

regioselectivity is determined at the reductive elimination of the B/C bond. Here, we need to remember the deuterium kinetic isotope effect observed experimentally.23 The presence of the kinetic isotope effect suggests that the C-H bond activation is rate-determining, which is inconsistent with the present reaction mechanism seemingly because the reductive elimination of B-C bond is rate-determining in this mechanism. However, the kinetic isotope effect can be observed even when the C-H bond activation is not rate-determining if some conditions are satisfied.56 We calculated deuterium kinetic isotope effect here using (TMphen)Ir(Bpin)3 (TMphen = 3,4,7,8-tetramethyl-1,10phenanthroline) because TMphen was used experimentally. The calculated kinetic isotope effect is similar between the primary sp3 C-H borylation of ethyl ether and the

sp3 C-H

borylation of THF, as reported experimentally, while both are somewhat larger than the experimental values; we presented computational results and additional discussion of the kinetic isotope effect in pages S26-S27 and Table S10 in the Supporting Information.



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Details of Oxidative Addition of sp3 C/H bond. Though the sp3 /0 bond cleavage via oxidative addition is not rate-determining step, we wish to discuss here several important features of the sp3

/0 bond activation because the sp3

/0 bond activation is still

challenging. In the most stable THF complex 3, the C-H bond is distant from the Ir atom. Prior to the oxidative addition of the C-H bond, 3 isomerizes to S #

, 3

4 in which the

/0 bond is close to the Ir atom, as shown in Figure 2; in -4, the Ir/H distance is 2.178 Å and the /0 distance is 1.107 Å which is moderately longer than that of free THF. These geometrical features suggest that the H atom interacts with the Ir atom in a similar manner to agostic interaction of C/H bond with transition metal. This

4 is connected to

TS4/5. In

TS4/5, the /0 bond is elongated to 1.575 Å, the / and Ir-H distances (2.329 Å and 1.627 Å, respectively) are close to those of

5, and the H atom of the /0 bond seems to approach

the B1 atom of boryl group, as shown by its moderately short 7*/0 distance (2.107 Å). The intermediate

5 is seven-coordinated iridium(V) complex, in which the Ir/C and Ir/H

distances (2.182 and 1.608 Å) are normal but the 7*/0 distance becomes shorter to 1.753 Å. The moderately short B1-H distances in of the boryl group assists the

TS4/5 and

5 indicate that the empty 2p orbital

/0 bond cleavage by interacting with the H 1s orbital, as

mentioned previously in sp2 C/H borylation by iridium catalyst.49,50 The oxidative addition of the

/0 bond occurs through similar transition state

TS4/5 but with smaller Gº‡ value

than that for -TS4/5, as shown in Figure 2. Deformation/interaction energy analysis57-59 of TS4/5 (Table S3 in the SI) indicates that the deformation energy of the THF moiety is smaller in

TS4/5 than in -TS4/5. This smaller deformation energy arises from the smaller

bond energy (98.5 kcal mol-1) than the

/0

/0 one (103.8 kcal mol-1), as shown in Figure S4

in the Supporting Information.


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Details of Reductive Elimination of B/C Bond. Himo49 and Hartwig groups28,29 reported two different pathways for the reductive elimination of B-C bond, as shown in Scheme 4.

Pathway 1: Isomerization with position changes of reductive elimination of bond.a Position changes of N2 N1

B3pin B2pin Ir B pin 1 H

bonds

B1pin B pin 3 N2 Ir B2pin N1 H

O

O

-5

-TS

bonds followed by

N2 N1

5/6

Reductive elimination

B1pin B3pin Ir B2pin H

N2 N1

B1pin B3pin Ir H B2pin

O

O

-6

-TS

N2 N1

B1pin B3pin Ir H H O B2 O

O

-7

6/7

Pathway 2: Isomerization with position changes of hydride followed by reductive elimination of bond.b Reductive elimination

Hydride migration N2 N1

B3pin B2pin Ir B pin 1 H

O

-5

N2 N1

B3pin B2pin Ir H B1pin

O

N2 N1

Ir

B2pin B1pin

O

-TS -2 5/6

B3pin H

-6-2

N2 N1

B3pin H Ir B pin 1 B2pin

O

-TS -2 6/7

N2 N1

B3pin H Ir B1pin O H B2 O

-7

Scheme 4. Two pathways for isomerization followed by B/C reductive elimination.28,49 a

In -5, an approximately pentagonal plane consists of H, C of oxolanyl, N2, B3, and B1

atoms, while in -6 an approximately pentagonal plane consists of C of oxolanyl, B2, B3, B1, and N1 atoms. b In -5, hydride interacts with the B1-boryl group, while in -6-2, hydride interacts with the B3-boryl group and -6-2.


O

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According to their works, we investigated two pathways here. In one pathway (pathway 1), 5 is converted to an isomer 6 through position changes of three

/7 bonds prior to the

reductive elimination of 7/ bond, where the position of H changes little. As going from 5 to

6, the / /7: bond angle decreases and the /7: distance becomes shorter, as

shown in Figure 2, indicating that the B2-boryl group approaches the -oxolanyl moiety. The other important geometrical change is found when going from

5 to

6, as follows;

5 has distorted pentagonal bipyramidal structure, in which H, C of oxolanyl, N2, B3, and B1 atoms are on approximately pentagonal plane; remember that the d4 metal complex has pentagonal bipyramidal structure at closed-shell singlet ground state.50 On the other hand, 6 has a differently distorted pentagonal bipyramidal structure in which C of oxolanyl, B2, B3, B1, and N1 atoms are on approximately pentagonal plane. As a result of the change of the pentagonal plane, the 7:/ / angle decreases and the 7:/ distance becomes shorter in this isomerization.60 This isomerization is somewhat endoergonic by 9.2 kcal mol-1 (7.5 kcal mol-1 in in

6 than in

/0 borylation), probably because the pentagonal plane is more congested 5; note that the small hydride exists on the pentagonal plane of

more bulky boryl exists on that of

6. However,

5 but the

6 is favorable for the reductive

elimination of the 7/ bond because of the shorter 7:/ distance. This isomerization needs MGº‡ of 9.9 (9.2) kcal mol-1 relative to for the isomerization in the

5, as shown in Figure 1, where in parenthesis is MGº‡

/0 borylation hereinafter.

In the other pathway (pathway 2), isomerization occurs mainly through hydride migration, as shown in Figure 3. In

5, the hydride interacts with the B1-boryl group, while

it interacts with the B2-boryl group in

TS5/6-2,61 in which B1-boryl and B2-boryl planes

rotate around the /7* and /7: bonds to afford an intermediate an isomer of

6-2. In

6-2 which is

5, the hydride interact with the B3-boryl and the position of the B3-boryl

somewhat changes. This isomerization needs MGº‡ of 18.0 (20.4) kcal mol-1 relative to

5




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changes indicate that the B2-boryl and -oxolanyl groups approach each other. However, the B2-C distance is still much longer than the B-C covalent bond (1.560 Å) of -boryloxolane. This B-C distance and the moderately elongated Ir-C and Ir-B2 distances indicate that TS6/7 is reactant-like. In

7, the -boryloxolane coordinates with the Ir atom through the O

atom of the Bpin group, in which the /9 distance is 2.478 Å. This reductive elimination needs MGº‡ of 34.6 kcal mol-1 relative to 1, which is moderately larger than that for the C/0 bond cleavage by 1.5 kcal mol-1. The reductive elimination of the those of the

7/ bond occurs with similar geometry changes to

7/ reductive elimination. However, its MGº‡ (38.1 kcal mol-1) is somewhat

larger than that for the 7/ reductive elimination by 3.5 kcal mol-1. The MGº values for the reductive eliminations of the

and

borylations relative to -5 and

5 are -6.9 and -10.2

kcal mol-1, respectively. Accordingly, it should be concluded that the elimination occurs more easily than the

7/

reductive

one kinetically and thermodynamically, which is

consistent with experimental results that the

/0 borylation occurs but the

/0

borylation does not.23

Origin of -Regioselectivity of THF borylation. To understand well the origin of the regioselectivity, we firstly made comparison of geometry between -TS6/7 and reductive elimination of the 7/

TS6/7 of the

bond. However, significant difference was not found

between these two TSs. Then, we separated the reaction system into the (phen)Ir(H)(boryl)2 (frag-a) and oxolanyl-boryl (frag-b) moieties, as shown in Scheme 5, to make deformation/interaction analysis.57-59 As listed in Table 1, the activation barrier (Ea) for TS6/7 is smaller than that of -TS6/7 by 3.9 kcal mol-1, as shown in Table 1, where Ea is difference in potential energy between TS and the resting state 1. This difference is similar to that in the MGº‡, suggesting that the analysis of Ea provides the reason(s) why the 17 ACS Paragon Plus Environment


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frag-a N N

Bpin Bpin Ir H Bpin

H

oxolanyl radical

O

SOMO

frag-b

Scheme 5. Separation of -TS6/7 and

= -8.99 eV

oxolanyl radical SOMO = -9.19 eV

TS6/7 into two fragments, the

(phen)Ir(H)(Bpin)2 (frag-a) and oxolanyl···Bpin moieties (frag-b), and singly occupied molecular orbitals (SOMOs) of - and -oxolanyl radicals.

/0 borylation occurs more easily than the barrier Ea1 relative to 2, {Ea1= Et( -TS6/7 or

/0 one. For simplicity, the activation TS6/7) – Et(2)} was defined here to make

analysis easier, where Et represents total energy; this is not unreasonable because Ea is represented by Ea1 + {Et(2) - Et(1)} and Et(2) - Et(1) is common for - and -borylations. In this analysis, the deformation energy (EDef-a/EDef-b) of frag-a/frag-b and the interaction energy (Eint) between frag-a and frag-b are defined, as follows: EDef-a = Et(frag-a)TS – Et(frag-b)opt and EDef-b = Et(frag-b)TS – Et(frag-b)opt, where subscripts “TS” and “opt” mean the geometry in the TS and the optimized geometry, respectively. The Eint is stabilization energy provided by the interaction between frag-a and frag-b, where both have the same


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deformed geometries as those in -TS6/7 or

TS6/7; Eint = Et(TS)TS – Et(frag-a)TS – Et(frag-

b)TS. For making this analysis clearly, we compared these two transition states at almost the same position on the reaction coordinate because the comparison between late and early transition states does not provide clear difference; for instance, deformation energy and interaction energy are larger in late transition state than in early one. Here, we took the B-C distance as an approximate reaction coordinate, because this was the reductive elimination of the B-C bond, and obtained such approximate transition state geometry by IRC calculation; details are described in page S28 of the Supporting Information. The Ea1 is represented by eq. (1), as described in the Supporting Information pages S29-S30; Ea1 = E + EDef-a + EDef-b + Eint,

(1)

E = [Et{(phen)Ir(Bpin)2(H)} + Et(R-Bpin)] – [Et{(phen)Ir(Bpin)3} + Et(R-H)] (2) where R is oxolanyl group. This E term corresponds to the sum of the difference between Ir-H and Ir-Bpin bond energies and that between R-Bpin and R-H bond energies, as shown in the Supporting Information pages S31-S32; E = {BDE(Ir-H) BDE(Ir-Bpin)}

{BDE(R-Bpin)

BDE(R-H)}.

(3)

Because the sum of EDef-a and EDef-b is smaller in the -borylation than in the -one by 5.0 kcal mol-1 (Table 1), these terms are not responsible for the lower energy -TS6/7 than TS6/7. However, the Eint in

TS6/7 is more negative (more attractive) than that in -TS6/7 by

5.0 kcal mol-1 (Table 1) and the E is smaller (less repulsive) in

TS6/7 than in -TS6/7 by

3.3 kcal mol-1, indicating that these two terms are responsible for the smaller Ea of the reductive elimination in the -borylation than in the

one.



Page 20 of 45

Table 1. The Gibbs energies of activation (MGº‡(C/0) and MGº‡(B/C))a for oxidative addition of C-H bond and reductive elimination of B-C bond, activation barrier (Ea) in potential energy, deformation energies (EDef-a and EDef-b) of frag-a and frag-b,a,b interaction energy (Eint) between frag-a and frag-b in TS of reductive elimination of B-C bond.a

s-( )d p-( ) d s

MGº‡(C/0) a 31.3 33.1 33.2 32.9 33.9

MGº‡(B/C) a 38.1 34.6 43.4 30.8 35.8

Ea a 40.5 36.6 46.1 34.3 37.9

Ea1 b 12.5 9.0 18.4 6.6 10.3

E 8.9 5.6 8.8 5.9 7.3

EDef-a c 13.2 17.2 12.3 13.4 17.2

EDef-b c 59.7 60.7 58.3 71.2 60.9

Eint c -69.5 -74.7 -61.1 -84.1 -73.9

sd pd

33.5 34.4

38.7c 30.8

40.9 33.5

14.2 6.9

6.5 6.3

12.9 13.7

58.6 69.2

-63.9 -82.5

substrate THF

C/H

Ethyl ether Cyclopentane nPentane

All energy terms are in kcal mol-1 unit. a Relative b

value of TS6/7 to 1.

Relative value of TS6/7 to 2.

cE

Def-a and EDef-b are deformation energies for frag-a and frag-b and Eint

is interaction energy

between frag-a and frag-b. d

s and p represent secondary and primary C/H bonds, respectively.

The next question is why the Eint is more negative and the E is smaller in

TS6/7 than in

a-TS6/7. In eq. (3), the first term is common between - and -borylations and the difference in E arises from the second term. In this term, BDE( -oxolanyl-Bpin) is more negative than BDE( -oxolanyl-Bpin) by 8.6 kcal mol-1 but BDE( -oxolanyl-H) is more negative than BDE( -oxolanyl-H) by 5.3 kcal mol-1. Because the change in BDE(R-Bpin) by different R is largely cancelled by that in BDE(R-H) by different R, as was discussed in pioneering theoretical works of bond energy,62-68 the E does not differ very much between the - and -borylations. Thus, the difference in Ea1 mainly arises from the difference in Eint between - and -borylations. The Eint contains the Ir···Bpin and Ir···( or -oxolanyl) interactions.


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It is likely that the Ir···Bpin interaction is similar between - and -borylations when the transition state is compared at almost the same position on the reaction coordinate in - and -borylations. Accordingly, the difference in Eint mainly arises from the difference between the Ir···( -oxolanyl) and Ir···( -oxolanyl) interactions. The Ir···( -oxolanyl) and Ir···( oxolanyl) interactions in the transition sate are evaluated using an assumed eq. (4). [(phen)Ir(Bpin)2(H) ···(oxolany···Bpin)]TS [(phen)Ir(Bpin)2(H) ···(Bpin)]TS + oxolanyl radical

(4)

Though the oxolanyl radical is not involved in the borylation reaction, this eq. (4) is useful to make comparison between the Ir-( -osolanyl) and Ir-( -osolanyl) bonds. In other words, eq. (4) is used here as a model reaction for estimating the Ir-oxolanyl bonding interaction and the SOMO energy of oxolanyl radical is used as a measure of valence orbital energy; related discussion is presented in the pages S31-S32 of the Supporting Information. The covalent bond energy MEcov is approximately represented by eq (5) on the basis of the simple Hückel method, where

A

and

B

are the valence orbital energies and is the resonance integral;69-72 = (

)2 +

2

(5)

Eq. (5) indicates that the covalent bond energy increases as the energy difference between two valence orbitals increases. Because the transition metal element such as Ir generally has valence orbital at higher energy than that of alkyl group, the lower energy valence orbital of oxolanyl group leads to the formation of stronger Ir-oxolanyl bond. As shown in Scheme 5, the SOMO energy (

SOMO)

of -oxolanyl radical is lower than that of

-oxolanyl one.

Accordingly, the Ir-( -oxolanyl) interaction is stronger than the Ir-( -oxolanyl) one, which is the origin of larger Eint in -borylation than in discussion is reasonable because

-one. We wish to mention that this

-TS6/7 and -TS6/7 are reactant-like, as was described

above; for the product-like transition state, this discussion cannot be presented because the strong Ir-R interaction increases the activation barrier.



The next task is to elucidate the reason why that of

SOMO

Page 22 of 45

of -oxolanyl radical is higher than

analogue. Because the C is adjacent to the O atom, the

SOMO includes anti-

bonding overlap between the C sp3 orbital and the O lone pair orbital, which raises the SOMO energy, as shown in Scheme 5. On the other hand, the

SOMO does not include such

anti-bonding overlap with the O lone pair orbital because the C is distant from the O atom. Therefore, the -SOMO of frag-b is at lower energy than the -SOMO; in other words, the presence of O lone pair orbital is crucially important for raising SOMO energy of -oxolanyl radical. We also calculated SOMO energies of the

and -oxolanyl radicals with equilibrium

structure, as shown in Figure S8 in the Supporting Information. As is the case for distorted structure in TS, the SOMO energy (-6.65 eV) of -radical is higher than that of -radical (7.41 eV), indicating that the geometry distortion is not the reason of the SOMO energy difference between

and

oxolanyl radicals and the SOMO of the –oxolanyl radical

intrinsically exists at lower energy than that of the -one. This is general feature found in heterocycles when the hetero atom such as O and N atoms has doubly occupied p orbital, because such doubly occupied p orbital forms antibonding overlap with the SOMO at the position. This understanding suggests that the regioselectivity of

/0 borylation of

heterocycles is predicted by calculating SOMO energy of possible substrate radical.

sp3 C/H Borylation of Various Substrates. The experimental findings about sp3

/0

borylation are summarized, as follows: (i) Primary sp3 /0 borylation of n-alkyl ether and n-alkyl amine28 and secondary /0 borylation of cyclic ether and cyclic alkane have been succeeded,23,24 (ii) the secondary /0 borylation of n-aklyl ether and n-alkane could not,23 and (iii) cyclic ether is more reactive than cyclic alkane.23 To broaden the sp3

/0

functionalization by transition metal catalyst, we need the knowledge of the reactivity


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difference between primary and secondary sp3 /0 bonds and the influence of O atom on the reactivity. For this purpose, we investigated /0 borylation of various substrates such as n-pentane, ethyl ether, and cyclopentane in addition to THF. As shown in Table 1, the Gº‡ for sp3

/0 borylation becomes larger following the order, the primary ( )

/0 bond of

ethyl ether < the primary /0 bond of n-pentane ~ the secondary ( ) /0 bond of THF < the secondary

/0 bond of cyclopentane < the secondary ( )

/0 bond of THF < the

secondary /0 bond of n-pentane < the secondary ( ) /0 bond of ethyl ether; note that the oxidative addition of /0 bond is rate-determining step in the primary /0 borylation of ethyl ether and n-pentane but the reductive elimination of 7/ bond is rate-determining step for the secondary /0 borylation. This order agrees with the experimental findings of the reactivity mentioned above. It is noted that the Gº‡ significantly differs between the and

/0 borylations of THF and between the primary and secondary /0 borylations of

ethyl ether and n-pentane, indicating that the /0 borylaltions of these substrates occur in highly regioselective manner. It is likely that the higher reactivity of the primary /0 bond than that of the secondary /0 bond arises from the smaller steric repulsion of the primary C-H bond when it approaches the metal atom. However, the Gº‡(C-H) for the C-H bond activation differs little between the primary and the secondary C-H bonds, as shown by the Gº‡ values calculated for ethyl ether and n-pentane (Table 1). However, the significant difference is found at the reductive elimination of B-C bond. The Gº‡(C-H) and Gº‡(B-C) values in Table 1 clearly indicate that the smaller reactivity of the secondary C-H borylation arises from the larger Gº‡(B-C) value. This is reasonable because the steric effect is much larger in the reductive



Page 24 of 45

elimination of the B-C bond than in the oxidative addition of the C-H bond. The similar difference has been reported for the alkane borylation catalyzed by Cp*RhX(Bpin) (X = H or Bpin).12 Because the reductive elimination of the B-C bond is important process in the secondary sp3 C-H borylation, we will focus on the reductive elimination hereafter and explored this step using eqs. (1) to (3), as was discussed above. The maximum difference in E is 3.3 kcal mol-1 among these substrates (Table 1). Rather small E is reasonable because the difference in E by R is determined by the BDE(R-Bpin)

BDE(R-H) term and the

change in BDE(R-Bpin) by different R is somewhat cancelled by that in BDE(R-H), as was discussed in theoretical works.66-72 On the other hand, the maximum difference in Eint is 23 kcal mol-1 and the maximum difference in the sum of Edef-a and Edef-b values is 13 kcal mol1.

Also, the most reactive primary C-H borylation of ethyl ether suffers from the largest

deformation energy but obtains the largest stabilization energy from Eint. These results suggest that the Eint term plays crucially important role in determining the activation barrier for the B-C reductive elimination. This suggestion is supported by a good linear relationship between Ea and Eint shown in Figure S14 at page S30 of the Supporting Information. As was discussed above, the Eint value is mainly determined by the Ir···R interaction and this interaction becomes large when the SOMO energy ( against the

SOMO,

SOMO)

of R is low. We plotted Eint

as shown in Figure 4(A), where Eint is the Eint value relative to that of the

primary C-H borylation of ethyl ether (the most reactive substrate); borylation

Eint = Eint for R-H

Eint for ethyl ether borylation (note Eint is negative and Eint is positive). Almost

linear relationship (line 1) is found for linear substrates and another linear relationship (line


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2) is found for cyclic substrates; the presence of two different linear relationships is not surprising because the steric repulsion is different between linear and cyclic substrates. As expected, we also found almost the same relationship between Ea and

SOMO,

as shown in

Figure 4(B), where Ea is the difference in Ea from that of the primary C-H borylation of ethyl ether; Ea = Ea for R-H borylation

Ea for ethyl ether borylation. It is likely concluded

that Eint is crucially important for determining Ea and the

SOMO

is one of important factors

for determining Eint and Ea. As shown in Figure 4, the line 2 is below than the line 1 for the secondary /0 borylation, indicating that cyclic alkane and cyclic ether are more reactive than n-alkane and n-ether when they have the same

(SOMO).

This is reasonable because the

steric repulsion is smaller in cyclic alkane and cyclic ether than in n-alkane and n-ether, suggesting that the decrease in steric repulsion is crucially important to enhance the reactivity of the secondary

/0 bond. These results lead us to the conclusion that both steric and

electronic factors are important for determining the reactivity of alkane.



(A)

Line 1 H O

(kcal mol )

-1

H

Line 2 O

H

H

int

O

=E

H

H H

O

SOMO energy

sp3 CâH Borylation Catalyzed by Iridium(III) Triboryl Complex

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