sp³ C–H Borylation Catalyzed by Iridium(III) Triboryl Complex: Comprehensive Theoretical Study of Reactivity, Regioselectivity, and Prediction of Excellent Ligand

Abstract

Iridium-catalyzed C–H borylation of THF was theoretically investigated as example of sp3 C–H functionalization. DFT computations show that β-regioselective borylation occurs more easily than does α-regioselective, as reported experimentally, through oxidative addition of C–H bond to iridium­(III) species and reductive elimination of B–C bond. The reductive elimination is both a rate-determining step and a regioselectivity-determining step. The lower energy transition state (TS) of the reductive elimination of β-boryloxolane arises from the Ir···(β-oxolanyl) interaction at TS being stronger than the Ir···(α-oxolanyl) one. The Ir···(β-oxolanyl) interaction being stronger than the Ir···(α-oxolanyl) one is a result of the valence orbital energy of the α-oxolanyl group being higher than that of the β-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 (β) C–H of ethyl ether > primary C–H of n-pentane ∼ secondary (β) C–H of THF > secondary C–H of cyclopentane > secondary (α) C–H of THF ∼ secondary C–H of n-pentane > secondary (α) C–H of ethyl ether. The primary C–H bond is more reactive than the secondary one because of its smaller steric repulsion and lower energy valence orbital of the primary alkyl group. The β-C–H bond of THF is more reactive than the secondary C–H bond of cyclopentane because of valence orbital energy of the β-oxolanyl group being lower than that of the 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-1,10-phenanthroline used experimentally.

Ligand-Driven Advances in Iridium-Catalyzed sp3 C–H Borylation: 2,2′-Dipyridylarylmethane

“…Computational treatments of sp 3 C-H borylation also support an oxidative addition mechanism for C-H cleavage, 14,[23][24] though a σbond metathesis mechanism has not been rigorously excluded. 25 The resulting iridium diboryl monohydride C is presumed to react with B2pin2 to regenerate A and extrude HBpin.…”

“…This species has been identified as the catalyst resting state in arene borylation 19 and is believed to react with arene substrates via rate-limiting oxidative addition to give a formal Ir(V) trisborylhydridoaryl B. 19,22 Computational treatments of sp 3 C-H borylation also support an oxidative addition mechanism for C-H cleavage, 14,23,24 though a -bond metathesis mechanism has not been rigorously excluded. 25 The resulting iridium diboryl monohydride C is presumed to react with B 2 pin 2 to regenerate A and extrude HBpin.…”

O2activation by core–shell Ru13@Pt42particles in comparison with Pt55particles: a DFT study

“…This eqn (1) has been employed to discuss the relative bond strengths of various M-R bonds (M ¼ transition metal element and R ¼ alkyl, silyl, etc.). [83][84][85][86][87] Because the M-R bond is understood to be a polarized covalent bond, it is likely that this eqn (1) can be applied to the present discussion. here we wish to mention that higher energy dvalence band-top relates to higher energy d-band center in many cases, indicating that the d-band center is also useful for discussion.…”

One-Pot Iridium Catalyzed C–H Borylation/Sonogashira Cross-Coupling: Access to Borylated Aryl Alkynes

“…The regiochemistry of iridium-catalyzed CHB of arenes is traditionally governed by sterics [3,7,8]; often complementing regiochemical outcomes of electrophilic aromatic substitution and directed ortho metalation. Since its discovery [9], methods to expand regiocontrol (ortho, meta, and para) [10][11][12], sp 3 borylation protocols [13][14][15][16][17][18][19][20][21] and one-pot reactions [22][23][24][25][26][27] have been developed.…”

“…-NMR (CDCl 3 , 300 MHz): δ 7.78 (dd, J = 1.5, 1.0 Hz, 1 H), 7.70 (dd, J = 2.2, 1.0 Hz, 1 H), 7.52 (dd, J = 2.2, 1.5 Hz, 1 H), 1.34 (br s, 12 H, 4 CH 3 of Bpin), 0.23 (s, 9 H, 3 CH 3 of TMS); 13 C NMR { 1 H} (CDCl 3 , 125 MHz): δ 136.5 (CH), 134.6 (CH), 134.2 (CH), 133.9 (C), 124.6 (C), 103.6 (C), 95.8 (C), 84.5 (2 C), 25.0 (4 CH 3 of Bpin), 0.0 (3 CH 3 of TMS); 11 B-NMR (CDCl 3 , 96 MHz): δ 30.6; FT-IR (neat) extractions were washed w being concentrated under subjected to column chrom 3.4. Analytical data of produc 3-(Phenylethynyl)-5-(4,4,5,5-From Sonogashira cou the borylated version of 3 acetylene (121 μL, 112 mg, 1 was concentrated and pass product as orange yellow o From one-pot CHB/S bromobenzotrifluoride (279 HBpin (436 μL, 384 mg, 3.0 out with phenyl acetylene chromatography (pentane: desired product as orange y 1 H NMR (CDCl3, 300 M H), 7.42-7.30 (m, 3 H), 1.37 131.8 (2 CH), 130.8 (q, 3 JC-F = (CH), 128.5 (2 CH), 124.1 (q (4 CH3 of Bpin); 11 B-NMR (C ῦ max: 2980, 1601, 1493, (EI) m/z (% relative intensi [(M + ); Calcd for C21H20BF3O 3-(Trimethylsilylethynyl)-5-(4 From Sonogashira cou the borylated version of 3-b (156 μL, 108 mg, 1.10 mmo (pentane/ether 9:1, Rf 0.8) fu From one-pot CHB/S bromotoluene (122 μL, 171 (218 μL, 192 mg, 1.50 mmol trimethylsilyl acetylene (15 (pentane/ether 9:1, Rf 0.8) fu 1 H NMR (CDCl3, 500 M 12 H, 4 CH3 of Bpin), 0.22 (s (CH), 135.5 (CH), 135.2 (CH 0.2 (3 CH3 of TMS); 11 B-NM max : 2978, 2166, 1562, 1352, 1143, 966, 927, 844, 760, 702 cm −1 ; GC-MS (EI) m/z (% relative intensity): M + 334 (8), 320 (100), 219 (10); HRMS (FAB): m/z 335.1407 [(M + ); Calcd for C 17 H 25 BO 2 SiCl: 335.14055].3-(Phenylethynyl)-5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)-toluene(15) …”