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Iridium-Catalyzed Borylation of SecondaryBenzylic C-H Bond Directed by Hydrosilane Seung Hwan Cho, and John F. Hartwig J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja403462b • Publication Date (Web): 17 May 2013 Downloaded from http://pubs.acs.org on May 18, 2013
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Journal of the American Chemical Society
Iridium-Catalyzed Borylation of Secondary Benzylic C‒H Bond Directed by Hydrosilane Seung Hwan Cho, and John F. Hartwig* Department of Chemistry, University of California, Berkeley, California 94270, United States
Supporting Information Placeholder ABSTRACT: Most functionalizations of C‒H bonds by
main group reagents occur at aryl or methyl groups. We describe a highly regioselective borylation of sec-‐ ondary benzylic C‒H bonds catalyzed by an iridium precursor and 3,4,7,8-‐tetramethylphenanathroline as ligand. The reaction is directed to the benzylic position by a hydrosilyl substituent. This hydrosilyl directing group is readily deprotected or transformed to other functional groups after the borylation reaction, provid-‐ ing access to diverse set of secondary benzylboronate esters by C‒H borylation chemistry.
opment of an iridium-‐catalyzed borylation of the second-‐ ary benzylic C‒H bonds directed by a hydrosilyl group. In contrast to a heteroaryl directing group, the hydrosilyl group can be traceless, or can be converted to a series of functional groups.10 a. Previous secondary (sp3) C-H borylation Bpin
O n
X
DG R
Bpin
Bpin
N
Bpin R
n
(ref 9a)
(ref 9b)
(ref 9c)
(1)
(ref 9d)
b. This Work: Silyl-directed secondary (sp3) C-H borylation H
Alkylboron reagents are useful synthetic intermediates in a variety of organic transformations, and methods for their preparation have attracted much attention.1 Such compounds are typically prepared by the reaction of or-‐ ganometallic nucleophiles, such as Grignard or organo-‐ lithium reagents, with boron electrophiles2 or by the hy-‐ droboration of alkenes.1b,3 More recently, the cross-‐ coupling of boron reagents with alkyl electrophiles have been reported.4 Transition-‐metal catalyzed functionalization of C‒H bonds with bis(pinacolato)diboron (B2pin2) has become a practical method for the direct formation of boronate esters.5 Significant progress has been made on the boryla-‐ tion of sp2 C−H bonds,6-‐7 but the borylation of sp3 C‒H bonds is less developed. Compared to the borylation of sp2 C‒H bonds, the borylation of sp3 C‒H bonds typically requires high catalyst loadings and excess of the saturated substrates. The regioselectivity of the borylation of sp3 C‒ H bonds strongly favors reaction at primary C‒H bonds,8 making the functionalization of secondary C‒H bonds challenging. We recently disclosed a highly reactive catalytic system derived from the combination of [Ir(COD)OMe]2 (COD = cyclooctadiene) and phenanthroline derived ligands that functionalizes the secondary C‒H bonds of cyclic ethers9a and cyclopropanes.9b Sawamura and co-‐workers also re-‐ ported the borylation of secondary C‒H bonds of cyclic amides9c and alkyl pyridines9d catalyzed by an immobi-‐ lized-‐catalyst system (eq 1). Here, we describe the devel-‐
N
R1 cat. [Ir]/ligand B2pin2 SiR2H likely via :
R1
H N Ir N Bpin Si R R Bpin
Bpin R1
(2)
SiR2H
We previously showed that hydrosilyl groups (-‐SiR2H) direct the sp2 C‒H borylation of arenes11a and N-‐ heteroarenes.11b In these cases, the formation of a five-‐ membered bisboryl monosilyl complex was proposed to lead to the observed regioselectivity. If a ring of the same size could be generated from an alkylarylsilane, then the product would result from borylation at a benzylic posi-‐ tion (eq 2). Because of the scarcity of borylations of ben-‐ zylic C‒H bonds,12 it was unclear if this sequence would lead to reaction at a secondary C‒H bond over an aryl group. Thus, we conducted experiments to test the feasi-‐ bility of this directed borylation process. To evaluate potential catalysts for this transformation, we investigated the borylation of 2-‐propylphenyl dime-‐ thylsilane (1a). We allowed 1a to react with B2pin2 in THF at 80 °C in the presence of the combination of [Ir(COD)OMe]2 and a series of nitrogen ligands (Table 1). The reaction catalyzed by [Ir(COD)OMe]2 and 4,4’-‐di-‐ tert-‐butyl-‐2,2’-‐bipyridine (dtbpy), which is the ligand most frequently used for the borylation of aryl C‒H bonds,5 proceeded to form the benzylic boronate ester 2a with 90% conversion and 83% yield, as determined by NMR spectroscopy (Table 1, entry 1). The reaction oc-‐
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curred selectively at the benzylic position; no borylation of the sp2 bonds of the arene was observed. This result indicated that the hydrosilane could redirect the boryla-‐ tion reaction from the typically reactive aryl position to the benzylic position. Reactions conducted with ligands that should be less electron donating than dtbpy (entry 1) occurred to mod-‐ erate conversion and yield (entry 2).13 In contrast, reac-‐ tions conducted with a series of ligands derived from 1,10-‐ phenanthroline (entries 3-‐5) showed that the catalyst generated from 3,4,7,8-‐tetramethyl-‐1,10-‐phenanthroline (Me4phen) is highly active for the conversion of 1a to the benzylboronate 2a in 93% yield (entry 5)..
Table 1. Effect of the Bipyridine Ligands on the Catalytic Borylation of Benzylic C‒H bonds a H Me + SiMe2H
O
O B B
O
THF, 80 oC
O
Me SiMe2H 2a
B2Pin2
1a
Bpin
[Ir(COD)OMe]2 (0.5 mol %) Ligand (1.0 mol %)
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masked aldehyde (2j), or diol (2k). The present reaction occurred equally well on a 5 mmol scale as on the smaller 0.3 mmol scale (2g). Variation of the electronic properties of the arene had little influence on the yield (2l, 2m and 2p). Substrates bearing fluoro (2n), chloro (2o and 2r), silyl ether (2q) groups, and a protected catechol (2s) were compatible with this catalytic system. Finally, the heteroaromatic benzothiazole derivative formed product 2t in 58% yield. The hydrosilyl group can be removed or used for diver-‐ sification of the arene. Such transformations of the hy-‐ drosilyl directing group are shown in Scheme 1. The reac-‐ tion of 2c and 2g with the combination of TMSI (generat-‐ ed in situ) and water resulted in cleavage of the hydrosilyl group, affording desilylated benzylboronate ester (3a and 3b) or deuterated analogues (3a-‐d) in almost quantitative yields at room temperature.15 Alternatively, the hy-‐ drosilane was cleaved by reaction with AgF and H2O (see Supporting Information). This cleavage of the C-‐Si bond occurred while keeping the benzylic-‐Bpin linkage intact. a,b
Ligand
Conversion (%)
Yield (%)
1
L1
90
83
2
L2
80
61
3
L3
90
85
4
L4
94
89
5
L5
100
97 (93)
c
L5
15
9
6 R
R
N
N
R'
R'
R = t-Bu, L1 R R = H, L2
R N
N
Table 2. Substrate Scope
b
Entry
R = R' = H, L3 R = H, R' = Me, L4 R = R' = Me, L5
H R2 R1 SiMe2H
+ B2Pin2
[Ir(COD)OMe]2 (0.5 mol %) Me4phen (1.0 mol %) THF, 80
oC,
18 h
Bpin R2 R1 SiMe2H
1
2 Bpin
Bpin
Me
Bpin Me
Me
SiMe2H 2b, 73%c,f
SiMe2H 2c, 80%
SiMe2(O-iPr) 2d, 70%d
Bpin
Bpin
Bpin OMe
Conditions: 1a (0.3 mmol), B2pin2 (0.3 mmol), [Ir(COD)OMe]2 (0.5 mol %), and ligand (1.0 mol %) in THF o b 1 at 80 C for 18 h. Determined by H NMR analysis with 1,1,2,2-‐tetrachloroethane as an internal standard. Numbers in c parenthesis correspond to isolated yield. HBpin (0.6 mmol) was used in place of B2pin2 as a boron reagent.
With an active catalyst in hand, we investigated the scope of the hydrosilyl-‐directed, benzylic borylation (Ta-‐ ble 2). Simple alkylarenes (2b, 2c and 2d) underwent the borylation reaction in high yield. Analytically pure 2d was isolated after conversion of the hydrosilane to the silyl ether with 2-‐propanol in the presence of 0.5 mol % [Ru(p-‐ cymene)Cl]2.14 Substrates containing alkyl chains substi-‐ tuted with phenyl and cyclopropyl substituents, which were previously shown to undergo borylation at the C‒H bonds of these groups by the Ir/Me4phen catalytic system, gave the corresponding benzylboronate esters (2e and 2f) in good yields.6, 9a This selectivity for the benzylic C‒H bonds over the aryl and cyclopropyl C‒H bonds under-‐ scores the impact of the hydrosilyl directing group. The directed borylation reaction also proceeded in high yield at the benzylic position to form products 2g, 2h and 2i from ether-‐containing substrates. Moreover, this process led to the benzylic borylation of alkyl chains containing a
SiMe2H
SiMe2H 2e, 71%
a
Bpin
2g, 95% (85%e)
Bpin O
OMe
Bpin Ph
O
SiMe2H
O
SiMe2H 2i, 70%c
2h, 77% Me Me
Bpin O
2k, 63%c,g Bpin
Bpin
SiMe2H
Me CF3
Bpin MeO
Me
SiMe2H
TBSO
Me SiMe2H
SiMe2H
X = F, 2n, 70%f X = Cl, 2o, 68%f Bpin Cl
SiMe2H 2m, 73% Bpin
2l, 81%
Me
2p,
60%c,f
2q, 66%c,f
Bpin Me
SiMe2H 2r, 64% a
O
Me Me
SiMe2H
SiMe2H 2j, 74%f
Bpin
O
X
SiMe2H
2f, 85%c
O O
Me SiMe2H 2s, 50%c,g
SiMe2H Bpin S 2t, 58%c,g
Me
Conditions: 1 (0.3 mmol), B2pin2 (0.3 mmol), [Ir(COD)OMe]2 (0.5 mol %), and ligand (1.0 mol %) in THF o b c at 80 C for 18 h. Isolated yields. 1.5 equiv. of 1 with respect to B2pin2 (0.3 mmol); the yield was determined based on d B2pin2. Isolated yield after conversion of the hydrosilane to
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the silyl ether with 2-‐propanol (See Supporting Information). e f g Conducted on a 5 mmol scale. Run for 24 h. Run for 36 h.
In addition, the aryl-‐hydrosilyl unit was transformed in-‐ to an aryl iodide without affecting the benzylic-‐Bpin link-‐ age, after Ru-‐catalyzed conversion of the hydrosilane to a hydrosilyl ether with 2-‐propanol. Treatment of the result-‐ ing silyl ether with AgF and NIS formed the aryl iodide. This sequence converted 2c into 4 in 67% yield over 2 steps.10 Both of hydrosilane and Bpin functionality can be con-‐ verted to the corresponding hydroxyl groups. For exam-‐ ple, reaction of 2c and 2g in the presence of an excess of NaOH (aq) and H2O2 in THF gave the corresponding 2-‐ (α-‐hydroxylalkyl)phenols 5a and 5b in excellent yield. The benzylboronate unit can also be used for C-‐C bond formation. The desilylated secondary benzylboronate es-‐ ter 3a underwent one-‐carbon homologation with bromo-‐ chloromethane and n-‐BuLi to produce the corresponding homobenzylic boronate ester 6 in 67%, respectively. In addition, Suzuki-‐Miyaura cross-‐coupling of 3a with two representative aryl iodides in the presence of a catalytic amount of Pd2(dba)3 and PPh3 and the combination of Ag2O and K2CO3 as base at 70 oC in THF formed the ary-‐ lated products 7a and 7b in good yields (70% and 81%, respectively).16
Scheme 1. Functionalization of Hydrosilane and Boronate ester Bpin
Bpin
R
a
c,d
H 3a, R = Et, 97% 3b, R = OMe, 94%
I R
Bpin Me
Me
Bpin
SiMe2H 2
b
4, 67% (2 steps) OH R e
D
OH 5a, R = Et, 95% 5b, R = OMe, 90%
3a-d, 98% Bpin
Scheme 2. Proposed Mechanism R N
HBpin
N 1
Ir
SiMe2
Bpin Bpin (C)
N N
Bpin
N
Bpin Bpin (A)
N
Ir
Ir
R
Bpin H
Bpin Bpin
N N
(B)
Ir Bpin
SiMe2 Bpin
(D) 2 B2Pin2
R N N
Ir Bpin
Si H
Bpin (E)
In summary, we have developed an iridium-‐catalyzed borylation of secondary benzylic C‒H bonds directed by a hydrosilane to give benzylboronate esters. This reaction constitutes a rare example of secondary C‒H borylation and an example of directed borylation of secondary C‒H bonds with a directing group that can be removed or used for further transformations. The formation of a bisboryl monosilyl complex is proposed to trigger the benzylic C‒ H activation through a five-‐membered metallacycle. Fur-‐ ther studies to expand the scope of the borylation of satu-‐ rated C‒H bonds and development of an asymmetric ver-‐ sion of this transformation are ongoing.
ASSOCIATED CONTENT
Ar Me
6, 67%
goes reaction with 1 to afford iridium silyl bisboryl com-‐ plex C via unsaturated complex B. Addition of the ben-‐ zylic C‒H bond would then form five-‐membered metal-‐ lacycle D, and coupling of the benzyl group with a Bpin ligand would give silyliridium hydride complex E. Reduc-‐ tive elimination of silane and oxidative addition of B2pin2 would produce the borylated product and regenerate tris-‐ boryl iridium intermediate (B).
f
3a
g
Me 7a, Ar = 4-OMe-C6H4, 70% 7b, Ar = 3-OMe-C6H4, 81%
Conditions: (a) TMSCl (1.2 equiv), KI (1.2 equiv), H2O (1.2 o equiv), CH3CN at 25 C (b) TMSCl (1.2 equiv), KI (1.2 equiv), o D2O (1.2 equiv), CH3CN at 25 C (c) [Ru(p-‐cymene)Cl]2, (0.5 o mol %) 2-‐propanol (2.0 equiv) at 0 C. (d) AgF (4.0 equiv), o NIS (4.0 equiv), THF at 25 C. (e) NaOH/H2O2 (excess), THF o o at 0 C to 25 C. (f) BrCH2Cl (2 equiv), n-‐BuLi (2 equiv), THF o o at -‐78 C to 25 C (g) Pd2(dba)3 (5 mol %), PPh3 (40 mol %), Ag2O (1.5 equiv), K2CO3 (1.5 equiv), ArI (2.0 equiv), THF at 80 o C.
A proposed mechanism for the silyl-‐directed benzylic borylation is presented in Scheme 2. The borylation of aryl C‒H bonds has been shown to occur through a tris-‐ boryl complex, such as complex A,13 containing a chelat-‐ ing nitrogen ligand. We suggest that complex A under-‐
Supporting Inform ation Experimental procedures and spectra for all new com-‐ pounds. This material is available free charge of via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author
[email protected] Notes The authors declare no competing financial interest.
A CKNOWLEDGMENT We thank the NSF (CHE-‐1156496) and a T. J. Park postdoctoral fellowship to S.H.C for funding of this work. We thank Johnson Matthey for a gift of
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[Ir(COD)OMe]2, and AllyChem for a gift of B2pin2. S.H.C. thanks Dr. Qian Li and Carl Liskey for helpful discussions.
REFERENCES (1) (a) Boronic Acids: Preparation and Applications in Organic Synthesis and Medicine (Ed.: D. G. Hall), Wiley-‐VCH, Wein-‐heim, 2005; (b) Crudden, C. M.; Edwards, D. Eur. J. Org. Chem. 2003, 4695. (2) (a) Brown, H. C. Organic Synthesis via Organoboranes; Wiley Interscience, New York, 1975. (b) Brown, H. C.; Cole, T. E. Organometallics 1983, 2, 1316. (3) (a) Carroll, A. M.; O’Sullivan, T. P.; Guiry, P. J. Adv. Synth. Catal. 2005, 347, 609. (b) Burgess, K.; Ohlmeyer, M. J. Chem. Rev. 1991, 91, 1179. (c) Noh, D.; Chea, H.; Ju, J.; Yun, J. Angew. Chem., Int. Ed. 2009, 48, 6062. (4) (a) Yang, C.-‐T.; Zhang, Z.-‐Q.; Tajuddin, H.; Wu, C.-‐C.; Liang, J.; Liu, J.-‐H.; Fu, Y.; Czyzewska, M.; Steel, P. G.; Marder, T. B.; Liu, L. Angew. Chem., Int. Ed. 2012, 51, 528. (b) Ito, H.; Kubota, K. Org. Lett. 2012, 14, 890. (c) Dudnik, A. S.; Fu, G. C. J. Am. Chem. Soc. 2012, 134, 10693. Transition-‐metal free approach, see: (d) Li, H.; Wang, L.; Zhang, Y.; Wang, J. Angew. Chem., Int. Ed. 2012, 51, 2943. (5) For recent reviews, see: (a) Mkhalid, I. A. I.; Barnard, J. H.; Marder, T. B.; Murphy, J. M.; Hartwig, J. F. Chem. Rev. 2010, 110, 890. (b) Hartwig, J. F. Chem. Soc. Rev. 2011, 40, 1992. (c) Hartwig, J. F. Acc. Chem. Res. 2012, 45, 864. (6) For examples of non-‐directed sp2 C−H borylations, see: (a) Cho, J.-‐Y.; Iverson, C. N.; Smith, M. R., III. J. Am. Chem. Soc. 2000, 122, 12868. (b) Tse, M. K.; Cho, J.-‐Y.; Smith, M. R., III. Org. Lett. 2001, 3, 2831. (c) Ishiyama, T.; Takagi, J.; Ishida, K.; Miyaura, N.; Anastasi, N. R.; Hartwig, J. F. J. Am. Chem. Soc. 2002, 124, 390. (d) Cho, J.-‐Y.; Tse, M. K.; Holmes, D.; Maleczka, R. E., Jr.; Smith, M. R. III. Science 2002, 295, 305. (e) Takagi, J.; Sato, K.; Hartwig, J. F.; Ishiyama, T.; Miyaura, N. Tetrahedron Lett. 2002, 43, 5649. (f) Ishiyama, T.; Takagi, J.; Hartwig, J. F.; Miyaura, N. Angew. Chem., Int. Ed. 2002, 41, 3056. (g) Ishiyama, T.; Nobuta, Y.; Hartwig, J. F.; Miyaura, N. Chem. Commun. 2003, 2924. (h) Ishiyama, T.; Takagi, J.; Yonekawa, Y.; Hartwig, J. F.; Miyaura, N. Adv. Synth. Catal. 2003, 345, 1103. (i) Paul, S.; Chotana, G. A.; Holmes, D.; Reichle, R. C.; Maleczka, R. E., Jr.; Smith, M. R., III. J. Am. Chem. Soc. 2006, 128, 15552. (j) Mkhalid, I. A. I.; Coventry, D. N.; Albesa-‐Jove, D.; Batsanov, A. S.; Howard, J. A. K.; Perutz, R. N.; Marder, T. B. Angew. Chem., Int. Ed. 2006, 45, 489. (k) Iwadate, N.; Suginome, M. J. Organomet. Chem. 2009, 694, 1713. (7) For examples of directed sp2 C−H borylations, see: (a) Ka-‐ wamorita, S.; Ohmiya, H.; Hara, K.; Fukuoka, A.; Sawamura, M. J. Am. Chem. Soc. 2009, 131, 5058. (b) Ishiyama, T.; Isou, H.; Kiku-‐ chi, T.; Miyaura, N. Chem. Commun. 2010, 46, 159. (c) Yamazaki, K.; Kawamorita, S.; Ohmiya, H.; Sawamura, M. Org. Lett. 2010, 12, 3978. (d) Itoh, H.; Kikuchi, T.; Ishiyama, T.; Miyaura, N. Chem. Lett. 2011, 40, 1007. (e) Ros, A.; Estepa, B.; Loṕez-‐Rodríguez, R.; Álvarez, E.; Fernańdez, R.; Lassaletta, J. M. Angew. Chem., Int. Ed.
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2011, 50, 11724. (f) Kawamorita, S.; Miyazaki, T.; Ohmiya, H.; Iwai, T.; Sawamura, M. J. Am. Chem. Soc. 2011, 133, 19310. (g) Dai, H.-‐X.; Yu, J.-‐Q. J. Am. Chem. Soc. 2012, 134, 134. (8) (a) Chen, H.; Hartwig, J. F. Angew. Chem., Int. Ed. 1999, 38, 3391. (b) Iverson, C. N.; Smith, M. R. III. J. Am. Chem. Soc. 1999, 121, 7696. (c) Chen, H.; Schlecht, S.; Semple, T. C.; Hartwig, J. F. Science 2000, 287, 1995. (d) Lawrence, J. D.; Takahashi, M.; Bae, C.; Hartwig, J. F. J. Am. Chem. Soc. 2004, 126, 15334. (e) Hartwig, J. F.; Cook, K. S.; Hapke, M.; Incarvito, C. D.; Fan, Y.; Webster, C. E.; Hall, M. B. J. Am. Chem. Soc. 2005, 127, 2538. (f) Murphy, J. M.; Lawrence, J. D.; Kawamura, K.; Incarvito, C.; Hartwig, J. F. J. Am. Chem. Soc. 2006, 128, 13684. (g) Ohmura, T.; Torigoe, T.; Suginome, M. J. Am. Chem. Soc. 2012, 134, 17416. (9) (a) Liskey, C. W.; Hartwig, J. F. J. Am. Chem. Soc. 2012, 134, 12422. (b) Liskey, C. W.; Hartwig, J. F. J. Am. Chem. Soc. 2013, 135, 3375. (c) Kawamorita, S.; Miyazaki, T.; Iwai, T.; Ohmiya, H.; Sawamura, M. J. Am. Chem. Soc. 2012, 134, 12924. (d) Kawamori-‐ ta, S.; Murakami, R.; Iwai, T.; Sawamura, M. J. Am. Chem. Soc. 2013, 135, 2947. (10) Chernyak, N.; Dudnik, A. S.; Huang, C.; Gevorgyan, V. J. Am. Chem. Soc. 2010, 132, 8270. (11) (a) Boebel, T. A.; Hartwig, J. F. J. Am. Chem. Soc. 2008, 130, 7534. (b) Robbins, D. W.; Boebel, T. A.; Hartwig, J. F. J. Am. Chem. Soc. 2010, 132, 4068. (12) For transition metal catalyzed benzylic borylation reac-‐ tions, see: (a) Shimada, S.; Batsanov, A. S.; Howard, J. A. K.; Marder, T. B. Angew. Chem., Int. Ed. 2001, 40, 2168. (b) Ishiyama, T.; Ishida, K.; Takagi, J.; Miyaura, N. Chem. Lett. 2001, 30, 1082. (c) Boebel, T. A.; Hartwig, J. F. Organometallics 2008, 27, 6013. (13) For recent reports on the electronic effects of C−H boryla-‐ tion of arenes, see: (a) Boller, T. M.; Murphy, J. M.; Hapke, M.; Ishiyama, T.; Miyaura, N.; Hartwig, J. F. J. Am. Chem. Soc. 2005, 127, 14263. (b) Dang, L.; Lin, Z.; Marder, T. B. Chem. Commun. 2009, 3987. (c) Liskey, C. W.; Wei, C. S.; Pahls, D. R.; Hartwig, J. F. Chem. Commun. 2009, 5603. (d) Chotana, G. A.; Vanchura, B. A., II.; Tse, M. K.; Staples, R. J.; Maleczka, R. E., Jr.; Smith, M. R., III. Chem. Commun. 2009, 5731. (e) Vanchura, B. A., II.; Preshlock, S. M.; Roosen, P. C.; Kallepalli, V. A.; Staples, R. J.; Maleczka, R. E., Jr.; Singleton, D. A.; Smith, M. R., III. Chem. Commun. 2010, 46, 7724. (14) Rayment, E. J.; Summerhill, N.; Anderson, E. A. J. Org. Chem. 2012, 77, 7052. (15) Radner, F.; Wistrand, L.-‐G. Tetrahedron Lett. 1995, 36, 5093. (16) Imao, D.; Glasspoole, B. W.; Laberge, V. S.; Crudden, C. M. J. Am. Chem. Soc. 2009, 131, 5024.
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E+ H R2 R1 SiMe2H
Ir(I)/ligand B2Pin2 THF, 80 oC
Bpin
R2 R1 E (E = H, D or I)
R2 R1 SiMe2H 20 examples (50-95% yield)
OH [O]
R2 R1 OH
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