Iron-Catalyzed Enantioselective Si-H Bond Insertions

Article abstract of DOI:10.1021/acs.orglett.8b02868

The first iron-catalyzed enantioselective Si-H bond insertion reaction of α-diazoesters was developed. A new chiral spiro-bisoxazoline ligand has proven to be an optimal ligand for the asymmetric reaction to give versatile chiral α-silyl esters in good yi

Full text of DOI:10.1021/acs.orglett.8b02868

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Iron-Catalyzed Enantioselective Si−H Bond Insertions
Haorui Gu,† Zhao Han,† Hujun Xie,*,‡ and Xufeng Lin*,†
†Department of Chemistry, Zhejiang University, Hangzhou 310027, China
‡Department of Applied Chemistry, Zhejiang Gongshang University, Hangzhou 310018, China
S
* Supporting Information
ABSTRACT: The first iron-catalyzed enantioselective Si−H
bond insertion reaction of α-diazoesters was developed. A new
chiral spiro-bisoxazoline ligand has proven to be an optimal
ligand for the asymmetric reaction to give versatile chiral α-
silyl esters in good yields with high enantioselectivities. The
mechanism and detailed stereochemical models for enantio-
selective induction were elucidated by DFT calculations,
suggesting that the reaction proceeds via a concerted quintet
transition state.
hiral organosilanes have attracted increasing attention in
recent years due to widespread applications in organic
asymmetric reactions are still elusive. Herein, we report the
first iron-catalyzed asymmetric Si−H insertion of α-diazoesters
to afford highly enantioenriched α-silyl esters using a new type
of chiral hexamethyl-1,1′-spirobiindane-based bisoxazoline
ligands (HMSI-BOX). The mechanism and stereochemical
models for enantioselective induction were elucidated by DFT
calculations.
The chiral HMSI-BOX was synthesized from industrially
available Bisphenol C as a starting material. Hexamethyl-1,1′-
spirobiindane-6,6′-diol 5 (6,6′-HMSIOL) was obtained in 92%
yield on a 20 g scale via acid-catalyzed rearrangement by our
previous report.16 To obtain enantiomerically pure 5, we
developed an efficient and practical method for the resolution
of 5, as shown in Scheme 1. We studied the inclusion
C
synthesis,1 materials science,2 and medicinal chemistry.3
Remarkable methodological advances have been made in the
catalytic enantioselective synthesis of chiral organosilanes,
including asymmetric hydrosilylation,4 desymmetrization,5 and
asymmetric addition.6 Despite a growing focus toward
synthetic efforts, the development of new strategies for
efficient catalytic enantioselective synthesis of chiral organo-
silanes remains highly desirable and the development of new
methods continues to be an active area of research. In fact, the
asymmetric Si−H bond insertion of an α-diazoesters approach
could represent an ideal route for this purpose.7 Although
Kramer and Wright first developed the insertion reaction of
carbene species into the Si−H bond in 1963,8 the first
asymmetric attempts of rhodium(II)-catalyzed Si−H bond
insertion of α-diazoesters were reported by Doyle and Moody
in 1996.9a Recently, notable advances have been achieved in
this field of transition-metal-catalyzed asymmetric Si−H bond
insertion reactions of diazo compounds, where a series of
rhodium(I)/(II),9 copper(I)/(II),10 iridium(III),11 and
ruthenium(II)12 complexes have shown to be very promising.
To the best of our knowledge, the efficient, earth-abundant,
and lower cost iron catalysts for enantioselective Si−H bond
insertion of α-diazoesters have not been well explored, except a
more recent racemic example developed by Ollevier and co-
worker.13
Scheme 1. Synthesis and Resolution of 6,6′-HMSIOL 5
crystallization17 of racemic 5 with (8S,9R)-(−)-N-benzylcin-
chonidinium chloride 10 and found that toluene was an
effective solvent. When chiral resolving reagent 10 and racemic
5, in a molar ratio of 0.6:1, were heated under reflux as a
solution in toluene, a solid inclusion complex could be
obtained at room temperature containing the spirobiindane-
6,6′-diol and the alkaloid in a 1:1 ratio by 1H NMR
spectroscopic analysis. The decomposition of the solid
Iron catalysis has been very attractive in the field of synthetic
chemistry due to its low price and environmentally benign
character.14 Although a large variety of highly intriguing
specific iron-catalyzed reactions are springing up and reflecting
the increasing demand for green and sustainable chemistry,
chiral iron catalysis is quite challenging and only a limited
number of iron-catalyzed asymmetric reactions have exhibited
excellent enantioselectivity (>90% ee).15 Moreover, the origins
of the stereocontrol in chiral iron complexe-catalyzed
Received: September 7, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02868
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
inclusion complex with 1 N HCl afforded (R)-(+)-5 in 90%
yield (based on one enantiomer) with >99% ee. The other (S)
enantiomer of 5 could be obtained in 93% ee from the mother
liquor, and further recrystallization from ethyl acetate−hexane
afforded enantiomerically pure (S)-(−)-5 with >99% ee in 70%
overall yield.
With the enantiomerically pure 6,6′-HMSIOL 5 in hand, we
then developed a practical method for the synthesis of HMSI-
BOX ligands, as shown in Scheme 2. The subsequent Duff
Scheme 2. Synthesis of HMSI-BOX Ligands 1
Figure 1. X-ray crystal structure of (Ra,S,S)-1b.
optimally provided the desired product (95% yield and 91%
ee, Table 1, entries 1−4). Furthermore, we found that the
chirality at the spiro backbone of 1 had a significant impact on
the reactivity and asymmetric induction of the iron catalysis
after screening of these new chiral ligands (Table 1, entries 1−
8). The combination of an Ra configuration of the spiro
backbone and the S configuration of the oxazoline moiety was
revealed as the well matched case. The opposite diastereomer
of the ligands 1a−d accordingly lowered the yields and ee’s,
and the yield is lower due to incomplete conversion.
As a comparison, we examined the known three analogous
bisoxazoline ligands 11−1319 in the iron-catalyzed Si−H
insertion reaction to show varying degrees of decrease of the
reaction enantioselectivity (Table 1, entries 9−11). A series of
other iron precursors were also evaluated, but no improved
results could be obtained in terms of both reactivity and
enantioselectivity (Table 1, entries 12−18). It is worth
pointing out that Fe(III) precursors, such as FeCl3 and
FeCl3·6H2O, also enabled the desired Si−H bond insertion
into carbene species with good yields and high enantiose-
lectivities (Table 1, entries 16−17). We found that a poor yield
and enantioselectivity was obtained without NaBArF as the
additive (Table 1, entry 19), suggesting that NaBArF could
reaction of (R)-(+)-5 with hexamethylenetetramine (HMTA)
and esterification with trifluoromethane sulfonic anhydride
(Tf2O) afforded diester (R)-7 in 78% yield for two steps. The
Pd-catalyzed selective reduction with triethylsilane furnished
dialdehyde (R)-8 in 88% yield. The diacid (R)-9 was obtained
in 97% yield by oxidation of (R)-8. The diacid (R)-9 was
further transformed to the target chiral HMSI-BOX ligands
1a−d in 74−91% yield in three steps by a reaction with thionyl
chloride and then a variety of enantiopure amino alcohols,
followed by a final cyclization using the typical method that is
purification-free of intermediates.18 The absolute configuration
of (Ra,S,S)-1b was determined by X-ray crystallographic
analysis (Figure 1; CCDC 1812886). The other diastereomeric
HMSI-BOX ligands 1a−d with Sa configuration of the spiro
backbone were also prepared by using another enantiomer (S)-
(−)-5.
These new ligands were tested in the iron-catalyzed
asymmetric Si−H insertion of α-diazoesters. For optimizing
the reaction conditions, initial experiments began with the
model reaction of methyl α-diazophenylacetate (2a) and
triethylsilane (3a) in dichloromethane in the presence of 5 mol
% of Fe(OTf)2 and 6 mol % of NaBArF with 6 mol % of
various bisoxazoline ligands. The key results of the reaction
optimization are summarized in Table 1. To our delight, the
iron-catalyzed Si−H bond insertion was successfully achieved
to afford the desired α-silyl ester (4a) in 62% yield with 83% ee
when the chiral ligand (Ra,S,S)-1a was used (Table 1, entry 1).
We screened the chiral HMSI-BOX ligands with different
substituents on the oxazoline rings and found that the aryl
substituents on the oxazoline ring gave good yields and
significant enantioselectivity, and the ligand (Ra,S,S)-1b
−
provide the larger and noncoordinating ion BArF to give the
reactive catalyst Fe(OTf)(BArF)[(Ra,S,S)-1b] in the current
reaction. We also tried a lower temperature (25 °C); however,
the reaction hardly worked (Table 1, entry 20). Other solvents
were also tested, including toluene and 1,4-dioxane, and poor
results were obtained (Table 1, entries 21−22). When the
catalyst loading was decreased to 1% from 5%, only traces of
the desired product were detected (Table 1, entry 23).
With the optimized reaction conditions in hand (Table 1,
entry 2), we then evaluated the substrate scope of the iron-
catalyzed asymmetric Si−H insertion reaction. The results are
summarized in Scheme 3, and it is noted that all substrates
require 48 h to react for complete conversion. A variety of
methyl α-diazoarylacetates bearing different substituents on
the aromatic ring were examined in the reaction with Et3SiH
under the optimal reaction conditions. In all cases, the reaction
ran smoothly to produce the corresponding α-silylesters 4 in
high yields with excellent enantioselectivities (4b−d, 90−96%
ee). Methyl α-diazo-(2-naphthyl) acetate was also successfully
applied in the insertion reaction to give the corresponding
product 4e in 98% yield with 90% ee. The substrates benzyl or
isobutyl α-diazophenylacetates afforded the desired α-
B
DOI: 10.1021/acs.orglett.8b02868
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
a
Table 1. Reaction Optimization
Scheme 3. Substrate Scope
a
Reactions conditions: 2 (0.1 mmol), 3 (0.4 mmol), Fe(OTf)2 (5 mol
%), (Ra, S, S)-1b (6 mol %), and NaBArF (6 mol %) in 2 mL of
CH2Cl2 at 40 °C under N2 for 48 h. Yields are of isolated product.
Enantioselectivity was determined by chiral HPLC. Products 4f, 4m,
4p, and 4r−t were obtained with (Ra,4S,4′S,5S,5′S)-1d as the ligand.
b
On a 1.0 mmol scale, 48 h, yield of 4a = 75%, ee of 4a = 90%.
Scheme 4. Asymmetric Insertion with α-Diazoalkylacetatesa
a
Reactions conditions: 2a (0.1 mmol), 3a (0.4 mmol), [Fe] (5 mol
%), ligand (6 mol %), and NaBArF (6 mol %) in 2 mL of CH2Cl2 at
b
c
40 °C under N2 for 48 h. Isolated yields. Determined by chiral
Additionally, we have performed the kinetic isotope effect
(KIE) experiment for the chiral iron carbene insertion reaction
between dimethylphenylsilane and 2a to gain some insight into
the mechanism, as illustrated in Scheme 5. This KH/KD value is
HPLC analysis. NaBArF: sodium tetrakis(3,5-bis(trifluoromethyl)-
phenyl)borate. No NaBArF. At 25 °C. With toluene as a solvent.
With 1,4-dioxane as a solvent. With [Fe] (1 mol %), ligand (1.2 mol
d
e
f
g
h
%), and NaBArF (1.2 mol %).
Scheme 5. Kinetic Isotope Effect Experiment
silylesters in good yields with excellent enantioselectivities (4f:
95% yield, 91% ee; 4g: 99% yield, 96% ee). Moreover, when
dimethylphenylsilane were applied in this transformation, a
variety of α-diazoarylacetates bearing either an electron-
donating or -withdrawing substituent on the aromatic ring
afforded the desired products with good yields and excellent
enantioselectivities (4h−o and 4q−u, 84−99% yield, 90−96%
ee), except for 4p.
Moreover, we tested two α-diazoalkyl compounds (2′a and
2′b) with Et3SiH (3a) or dimethylphenylsilane (3b) in the
asymmetric Si−H insertion reaction, but no reaction happened
under the presently developed chiral iron catalyst system, as
shown in Scheme 4.
determined to be 1.33, which reveals no significant kinetic
isotope effect and is in accord with the reported value of other
transition-metal-catalyzed carbenoid Si−H inser-
tions,9d,10a,11a,13,20 suggesting that the Si−H bond cleavage
cannot be the rate-determining step and the Fe(OTf)2/1b
complex-catalyzed insertion reaction may proceed via Si−H
bond oxidative addition concerted with a hydride insertion
state.21
C
DOI: 10.1021/acs.orglett.8b02868
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
We next explored the origins of ligand-controlled
enantioselectivity by density functional theory (DFT)
calculations using α-diazophenylacetate (2a) and triethylsilane
(3a) as the model substrates and Fe(OTf)2/(Ra,S,S)-1b
complex as the chiral catalyst (see the Supporting Information
(SI) for computational details). Our previous calculations
demonstrated that the Si−H bond oxidative addition
concerted with hydride insertion determines the product
selectivity in Pd- and Ni-catalyzed allene hydrosilylation.22
Herein, we further examined the relative Gibbs free energy
barriers for two transition states of alternative spin states, as
shown in Figure 2. We found that the quintet-state structures
application of the current chiral spiro-bisoxazoline ligand to
expand chiral iron catalysis for other challenging chemical
transformations are underway in our laboratory.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02868.
Experimental procedures and compound characteriza-
tion (PDF)
Accession Codes
CCDC 1812886 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: hujunxie@gmail.com (X.H.)
*E-mail: lxfok@zju.edu.cn (L.X.)
ORCID
Hujun Xie: 0000-0002-0035-2634
Xufeng Lin: 0000-0003-4611-0507
Notes
The authors declare no competing financial interest.
Figure 2. Relative Gibbs free energy barriers for two transition states
with different spin states.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(21572200) and the Fundamental Research Funds for the
Central Universities (2017QNA3013) for financial support.
are lower in energy than singlet- and triplet-state structures,
and the transition states of quintet of the FeII species are more
stable than those of the singlet and triplet. The calculations
(see Figure S1 in SI) showed that the spin densities of two
quintet transition states are mainly localized at the central Fe
atom. The optimal ligand (Ra,S,S)-1b, TSa, which leads to the
favorable insertion product (R)-4a, is 2.4 kcal/mol for the
quintet more favorable than the competing TSb. This
enantioselectivity via DFT calculations agrees well with the
experimental observations (entry 2, Table 1). Moreover, the
geometries of the two enantioselectivity-determining transition
states are illustrated in Figure 2 and indicate that the phenyl
group has obvious steric repulsions with the ester group in TSb
(quintet), leading to the exceptional enantioselectivity.
In summary, the first iron-catalyzed asymmetric Si−H
insertion reaction has been developed. This mild and user-
friendly protocol provides rapid access to valuable chiral α-silyl
esters in high yields with a high level of enantiocontrol from α-
diazoarylacetates and silanes. Critical to the successful
optimization of the protocol is the development of HMSI-
BOX, a newly designed chiral spiro-bisoxazoline ligand based
on the hexamethyl-1,1′-spiro biindane backbone, exhibiting
excellent levels of enantiocontrol in this transformation. The
mechanism and stereochemical models for the novel
asymmetric iron-catalzed Si−H insertion were investigated by
deuterium-labeling kinetic isotope effect experiments and DFT
calculations, suggesting that the reaction proceeds via a
concerted quintet transition state. Efforts on further
REFERENCES
■
(1) For reviews, see: (a) Chan, T. H.; Wang, D. Chiral
Organosilicon Compounds in Asymmetric Synthesis. Chem. Rev.
1992, 92, 995−1006. (b) Masse, C. E.; Panek, J. S. Diastereoselective
Reactions of Chiral Allyl and Allenyl Silanes with Activated C: X.pi.-
Bonds. Chem. Rev. 1995, 95, 1293−1316. (c) Fleming, I.; Barbero, A.;
Walter, D. Stereochemical Control in Organic Synthesis Using
Silicon-Containing Compounds. Chem. Rev. 1997, 97, 2063−2192.
(d) Denmark, S. E.; Regens, C. S. Palladium-Catalyzed Cross-
Coupling Reactions of Organosilanols and Their Salts: Practical
Alternatives to Boron- and Tin-Based Methods. Acc. Chem. Res. 2008,
41, 1486−1499. (e) Nakao, Y.; Hiyama, T. Silicon-Based Cross-
Coupling Reaction: An Environmentally Benign Version. Chem. Soc.
Rev. 2011, 40, 4893−4901. (f) Jensen, K. L.; Dickmeiss, G.; Jiang, H.;
Albrecht, Ł.; Jørgensen, K. A. The Diarylprolinol Silyl Ether System: A
General Organocatalyst. Acc. Chem. Res. 2012, 45, 248−264.
(g) Zhang, H. J.; Priebbenow, D. L.; Bolm, C. Acylsilanes: Valuable
Organosilicon Reagents in Organic Synthesis. Chem. Soc. Rev. 2013,
42, 8540−8571. (h) Komiyama, T.; Minami, Y.; Hiyama, T. Recent
Advances in Transition-Metal-Catalyzed Synthetic Transformations of
Organosilicon Reagents. ACS Catal. 2017, 7, 631−651. (i) Cui, Y.-M.;
Lin, Y.; Xu, L.-W. Catalytic Synthesis of Chiral Organoheteroatom
Compounds of Silicon, Phosphorus, and Sulfur via Asymmetric
Transition Metal-Catalyzed C-H Functionalization. Coord. Chem. Rev.
2017, 330, 37−52.
(2) (a) Patai, S.; Rappoport, Z.; Ojima, I. The Chemistry of Organic
Silicon Compounds; Wiley: Chichester, 1989. (b) Brook, M. A. Silicon
D
DOI: 10.1021/acs.orglett.8b02868
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
in Organic, Organometallic, and Polymer Chemistry; Wiley: New York,
1999.
Heterocyclic Carbenes. J. Am. Chem. Soc. 2010, 132, 2898−2900.
(g) Zhao, K.; Loh, T. P. Asymmetric Conjugate Addition of Grignard
Reagents to 3-Silyl Unsaturated Esters for the Facile Preparation of
Enantioenriched β-Silylcarbonyl Compounds and AllylicSilanes.
Chem. - Eur. J. 2014, 20, 16764−16772. (h) Kitanosono, T.; Zhu,
L.; Liu, C.; Xu, P.; Kobayashi, S. An Insoluble Copper(II)
Acetylacetonate-Chiral Bipyridine Complex that Catalyzes Asymmet-
ric Silyl Conjugate Addition in Water. J. Am. Chem. Soc. 2015, 137,
15422−15425. (i) Zhang, Y.; Huang, J.; Guo, Y.; Li, L.; Fu, Z.;
Huang, W. Access to Enantioenriched Organosilanes from Enals and
β-SilylEnones: Carbene Organocatalysis. Angew. Chem., Int. Ed. 2018,
57, 4594−4598.
(3) (a) Bains, W.; Tacke, R. Silicon Chemistry as a Novel Source of
Chemical Diversity in Drug Design. Curr. Opin. Drug Discovery Dev.
2003, 6, 526−543. (b) Pooni, P. K.; Showell, G. A. Silicon Switches of
Marketed Drugs. Mini-Rev. Med. Chem. 2006, 6, 1169−1177.
(c) Gately, S.; West, R. Novel Therapeutics with Ehanced Biological
Activity Generated by the Strategic Introduction of Silicon Isosteres
into Known Drug Scaffolds. Drug Dev. Res. 2007, 68, 156−163.
(d) Franz, A. K.; Wilson, S. O. Organosilicon Molecules with
Medicinal Applications. J. Med. Chem. 2013, 56, 388−405. (e) Min,
́
G. K.; Hernandez, D.; Skrydstrup, T. Efficient Routes to Carbon-
Silicon Bond Formation for the Synthesis of Silicon-Containing
Peptides and Azasilaheterocycles. Acc. Chem. Res. 2013, 46, 457−470.
(4) (a) Jensen, J. F.; Svendsen, B. Y.; la Cour, T. V.; Pedersen, H. L.;
Johannsen, M. Highly Enantioselective Hydrosilylation of Aromatic
Alkenes. J. Am. Chem. Soc. 2002, 124, 4558−4559. (b) Shimada, T.;
Mukaide, K.; Shinohara, A.; Han, J. W.; Hayashi, T. Asymmetric
Synthesis of 1-Aryl-1,2-ethanediols from Arylacetylenes by Palladium-
Catalyzed Asymmetric Hydrosilylation as a Key Step. J. Am. Chem.
Soc. 2002, 124, 1584−1585. (c) Gibson, S. E.; Rudd, M. The Role of
Secondary Interactions in the Asymmetric Palladium-Catalysed
Hydrosilylation of Olefins with Monophosphane Ligands. Adv.
Synth. Catal. 2007, 349, 781−795. (d) Ito, J.-i.; Nishiyama, H.
Bifunctional Phebox Complexes for Asymmetric Catalysis. Top.
Organomet. Chem. 2011, 37, 185−205. (e) Guo, J.; Shen, X.; Lu, Z.
Regio-and Enantioselective Cobalt-Catalyzed Sequential Hydro-
silylation/Hydrogenation of Terminal Alkynes. Angew. Chem., Int.
Ed. 2017, 56, 615−618. (f) Cheng, B.; Lu, P.; Zhang, H.; Cheng, X.;
Lu, Z. Highly Enantioselective Cobalt-CatalyzedHydrosilylation of
Alkenes. J. Am. Chem. Soc. 2017, 139, 9439−9442. (g) Gribble, M.
W.; Pirnot, M. T.; Bandar, J. S.; Liu, R. Y.; Buchwald, S. L.
Asymmetric Copper Hydride-Catalyzed Markovnikov Hydrosilylation
of Vinylarenes and Vinyl Heterocycles. J. Am. Chem. Soc. 2017, 139,
2192−2195.
(5) (a) Shintani, R.; Moriya, K.; Hayashi, T. Palladium-Catalyzed
Enantioselective Desymmetrization of Silacyclobutanes: Construction
of Silacycles Possessing a Tetraorganosilicon Stereocenter. J. Am.
Chem. Soc. 2011, 133, 16440−16443. (b) Shintani, R.; Otomo, H.;
Ota, K.; Hayashi, T. Palladium-Catalyzed Asymmetric Synthesis of
Silicon-Stereogenic Dibenzosiloles via Enantioselective C-H Bond
Functionalization. J. Am. Chem. Soc. 2012, 134, 7305−7308.
(c) Shintani, R. Recent Advances in the Transition-Metal-Catalyzed
Enantioselective Synthesis of Silicon-Stereogenic Organosilanes. Asian
J. Org. Chem. 2015, 4, 510−514. (d) Zhang, Q.; An, K.; Liu, L. C.;
Zhang, Q.; Guo, H.; He, W. Rhodium-Catalyzed Intramolecular C-H
Silylation by Silacyclobutanes. Angew. Chem., Int. Ed. 2016, 55, 6319−
6323. (e) Su, B.; Harting, J. F. Ir-CatalyzedEnantioselective,
Intramolecular Silylation of Methyl C-H Bonds. J. Am. Chem. Soc.
2017, 139, 12137−12140.
(7) For reviews, see: (a) Keipour, H.; Carreras, V.; Ollevier, T.
Recent Progress in the Catalytic Carbene Insertion Reactions into the
Silicon-Hydrogen Bond. Org. Biomol. Chem. 2017, 15, 5441−5456.
For first biocatalysis, see: (b) Kan, S. B. J.; Lewis, R. D.; Chen, K.;
Arnold, F. H. Directed Evolution of Cytochrome C for Carbon-
Silicon Bond Formation: Bringing Silicon to Life. Science 2016, 354,
1048−1051.
(8) Kramer, K. A. W.; Wright, A. N. The Reaction of Diazoalkanes
with Group IV Hydrides. J. Chem. Soc. 1963, 3604−3608.
(9) Selected examples, see: (a) Davies, H. M. L.; Hansen, T.;
Rutberg, J.; Bruzinski, P. R. Rhodium(II) (S)-N-(Arylsulfonyl)-
prolinate Catalyzed Asymmetric Insertions of Vinyl- and Phenyl-
carbenoids into the Si-H Bond. Tetrahedron Lett. 1997, 38, 1741−
1744. (b) Doyle, M. P.; Hu, W.; Phillips, I. M.; Moody, C. J.; Pepper,
A. G.; Slawin, A. M. Z. Reactivity Enhancement for Chiral
Dirhodium(II) Tetrakis(Carboxamidates). Adv. Synth. Catal. 2001,
343, 112−117. (c) Ge, M.; Corey, E. J. A Method for the Catalytic
Enantioselective Synthesis of 6-Silylated 2-Cyclohexenones. Tetrahe-
dron Lett. 2006, 47, 2319−2321. (d) Sambasivan, R.; Ball, Z. T.
Metallopeptides for Asymmetric Dirhodium Catalysis. J. Am. Chem.
Soc. 2010, 132, 9289−9291. (e) Chen, D.; Zhu, D.-X.; Xu, M.-H.
Rhodium(I)-Catalyzed Highly Enantioselective Insertion of Carbe-
noid into Si-H: Efficient Access to Functional Chiral Silanes. J. Am.
Chem. Soc. 2016, 138, 1498−1501.
(10) (a) Dakin, L. A.; Schaus, S. E.; Jacobsen, E. N.; Panek, J. S.
Carbenoid Insertions into the Silicon Hydrogen Bond Catalyzed by
Chiral Copper(I) Schiff Base Complexes. Tetrahedron Lett. 1998, 39,
8947−8950. (b) Dakin, L. A.; Ong, P. C.; Panek, J. S.; Staples, R. J.;
Stavropoulos, P. Speciation and Mechanistic Studies of Chiral
Copper(I) Schiff Base Precursors Mediating Asymmetric Carbenoid
Insertion Reactions of Diazoacetates into the Si-H Bond of Silanes.
Organometallics 2000, 19, 2896−2908. (c) Zhang, Y.-Z.; Zhu, S.-F.;
Wang, L.-X.; Zhou, Q.-L. Copper-Catalyzed Highly Enantioselective
Carbenoid Insertion into Si-H Bonds. Angew. Chem., Int. Ed. 2008, 47,
́
8496−8498. (d) Hyde, S.; Veliks, J.; Liegault, B.; Grassi, D.; Taillefer,
M.; Gouverneur, V. Copper-Catalyzed Insertion into Heteroatom-
Hydrogen Bonds with Trifluorodiazoalkanes. Angew. Chem., Int. Ed.
2016, 55, 3785−3789.
(6) (a) Chang, K. J.; Rayabarapu, D. K.; Yang, F. Y.; Cheng, C. H.
Unusual Palladium-Catalyzed Silaboration of Allenes Using Organic
Iodides as Initiators: Mechanism and Application. J. Am. Chem. Soc.
2005, 127, 126−131. (b) Shintani, R.; Okamoto, K.; Hayashi, T.
Carbon-Carbon Bond-Forming Enantioselective Synthesis of Chiral
Organosilicon Compounds by Rhodium/Chiral Diene-Catalyzed
Asymmetric 1,4-Addition Reaction. Org. Lett. 2005, 7, 4757−4759.
(c) Balskus, E. P.; Jacobsen, E. N. α,β-Unsaturated β-Silyl Imide
Substrates for Catalytic, Enantioselective Conjugate Additions: A
Total Synthesis of (+)-Lactacystin and the Discovery of a New
Proteasome Inhibitor. J. Am. Chem. Soc. 2006, 128, 6810−6812.
(d) Ohmura, T.; Taniguchi, H.; Suginome, M. Palladium-Catalyzed
Asymmetric Silaboration of Allenes. J. Am. Chem. Soc. 2006, 128,
13682−13683. (e) Walter, C.; Auer, G.; Oestreich, M. Rhodium-
Catalyzed Enantioselective Conjugate Silyl Transfer: 1,4-Addition of
Silyl Boronic Esters to Cyclic Enones and Lactones. Angew. Chem., Int.
Ed. 2006, 45, 5675−5677. (f) Lee, K. S.; Hoveyda, A. H.
Enantioselective Conjugate Silyl Additions to Cyclic and Acyclic
Unsaturated Carbonyls Catalyzed by Cu Complexes of Chiral N-
(11) (a) Yasutomi, Y.; Suematsu, H.; Katsuki, T. Iridium(III)-
Catalyzed Enantioselective Si-H Bond Insertion and Formation of an
Enantioenriched Silicon Center. J. Am. Chem. Soc. 2010, 132, 4510−
4511. (b) Wang, J.-C.; Xu, Z.-J.; Guo, Z.; Deng, Q.-H.; Zhou, C.-Y.;
Wan, X.-L.; Che, C.-M. Highly Enantioselective Intermolecular
Carbene Insertion to C-H and Si-H Bonds Catalyzed by a Chiral
Iridium(III) Complex of a D4-Symmetric HaltermanPorphyrin
Ligand. Chem. Commun. 2012, 48, 4299−4301.
(12) Nakagawa, Y.; Chanthamath, S.; Fujisawa, I.; Shibatomi, K.;
Iwasa, S. Ru(II)-Pheox-Catalyzed Si-H Insertion Reaction: Con-
struction of Enantioenriched Carbon and Silicon Centers. Chem.
Commun. 2017, 53, 3753−3756.
(13) Keipour, H.; Ollevier, T. Iron-Catalyzed Carbene Insertion
Reactions of α-Diazoesters into Si-H Bonds. Org. Lett. 2017, 19,
5736−5739.
(14) For reviews, see: (a) Bolm, C.; Legros, J.; Le Paih, J.; Zani, L.
Iron-Catalyzed Reactions in Organic Synthesis. Chem. Rev. 2004, 104,
6217−6254. (b) Bolm, C. A. New Iron Age. Nat. Chem. 2009, 1, 420.
(c) Gopalaiah, K. Chiral Iron Catalysts for Asymmetric Synthesis.
E
DOI: 10.1021/acs.orglett.8b02868
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Chem. Rev. 2013, 113, 3248−3296. (d) Darcel, C.; Sortais, J.-B. In
Iron Catalysis II; Bauer, E., Ed.; Springer: Berlin, 2015. (e) Bauer, I.;
Alternative to Oxidative Addition. Organometallics 2012, 31, 1300−
1314. (c) Jiang, Y. Y.; Li, Z.; Shi, J. Mechanistic Origin of
Regioselectivity in Nickel-Catalyzed Olefin Hydroheteroarylation
through C-H Activation. Organometallics 2012, 31, 4356−4366.
(d) Hong, X.; Wang, J.; Yang, Y.-F.; He, L.; Ho, C.-Y.; Houk, K. N.
Computational Exploration of Mechanism and Selectivities of
(NHC)Nickel(II)hydride-Catalyzed Hydroalkenylations of Styrene
with α-Olefins. ACS Catal. 2015, 5, 5545−5555. (e) Tang, S.;
Eisenstein, O.; Nakao, Y.; Sakaki, S. Aromatic C-H σ-Bond Activation
by Ni0, Pd0, and Pt0 Alkene Complexes: Concerted Oxidative
Addition to Metal vs Ligand-to-Ligand H Transfer Mechanism.
Organometallics 2017, 36, 2761−2771. (f) Singh, V.; Nakao, Y.;
Sakaki, S.; Deshmukh, M. M. Theoretical Study of Nickel-Catalyzed
Selective Alkenylation of Pyridine: Reaction Mechanism and Crucial
Roles of Lewis Acid and Ligands in Determining the Selectivity. J.
Org. Chem. 2017, 82, 289−301. (g) Gao, Y.; Houk, K. N.; Ho, C.-Y.;
Hong, X. The Mechanism and Regioselectivities of (NHC)Nickel-
(II)hydride-Catalyzed Cycloisomerization of Dienes: a Computa-
tional Study. Org. Biomol. Chem. 2017, 15, 7131−7139. (h) Zell, D.;
̈
Knolker, H.-J. Iron Catalysis in Organic Synthesis. Chem. Rev. 2015,
115, 3170−3387. (f) Ollevier, T. Iron Bis(oxazoline) Complexes in
Asymmetric Catalysis. Catal. Sci. Technol. 2016, 6, 41−48.
(15) For selected highly enantioselective iron-catalyzed reactions,
see: (a) Corey, E. J.; Imai, N.; Zhang, H.-Y. Designed Catalyst for
Enantioselective Diels-Alder Addition from a C2-Symmetric Chiral
Bis(oxazoline)-Iron(III) Complex. J. Am. Chem. Soc. 1991, 113, 728−
729. (b) Suzuki, K.; Oldenburg, P. D., Jr.; Que, L. Iron-Catalyzed
Asymmetric Olefin cis-Dihydroxylation with 97% Enantiomeric
Excess. Angew. Chem., Int. Ed. 2008, 47, 1887−1889. (c) Zhu, S.-F.;
Cai, Y.; Mao, H.-X.; Xie, J.-H.; Zhou, Q.-L. Enantioselective Iron-
Catalysed O-H Bond Insertions. Nat. Chem. 2010, 2, 546−551.
(d) Egami, H.; Matsumoto, K.; Oguma, T.; Kunisu, T.; Katsuki, T.
Enantioenriched Synthesis of C2-Symmetric BINOLs: Iron-Catalyzed
Cross-Coupling of 2-Naphthols and Some Mechanistic Insight. J. Am.
Chem. Soc. 2010, 132, 13633−13635. (e) Williamson, K. S.; Yoon, T.
P. Iron Catalyzed Asymmetric Oxyamination of Olefins. J. Am. Chem.
Soc. 2012, 134, 12370−12373. (f) Shen, J.-J.; Zhu, S.-F.; Cai, Y.; Xu,
H.; Xie, X.-L.; Zhou, Q.-L. Enantioselective Iron-Catalyzed Intra-
molecular Cyclopropanation Reactions. Angew. Chem., Int. Ed. 2014,
53, 13188−13191.
Bursch, M.; Muller, V.; Grimme, S.; Ackermann, L. Full Selectivity
̈
Control in Cobalt(III)-Catalyzed C-H Alkylations by Switching of the
C-H Activation Mechanism. Angew. Chem., Int. Ed. 2017, 56, 10378−
10382.
(22) (a) Xie, H.; Zhao, L.; Yang, L.; Lei, Q.; Fang, W.; Xiong, C.
Mechanisms and Origins of Switchable Regioselectivity of Palladium-
and Nickel-Catalyzed Allene Hydrosilylation with N-Heterocyclic
Carbene Ligands: A Theoretical Study. J. Org. Chem. 2014, 79, 4517−
4527. (b) Xie, H.; Kuang, J.; Wang, L.; Li, Y.; Huang, L.; Fan, T.; Lei,
Q.; Fang, W. A DFT Study on Palladium and Nickel-Catalyzed
Regioselective and Stereoselective Hydrosilylation of 1,3-Disubsti-
tuted Allenes. Organometallics 2017, 36, 3371−3381.
(16) (a) Sun, W.; Gu, H.; Lin, X. Synthesis and Application of
Hexamethyl-1,1′-spirobiindane-Based Phosphine-Oxazoline Ligands
in Ni-Catalyzed Asymmetric Arylation of Cyclic Aldimines. J. Org.
Chem. 2018, 83, 4034−4043. (b) Shan, H.; Zhou, Q.; Yu, J.; Zhang,
S.; Hong, X.; Lin, X. Rhodium-Catalyzed Asymmetric Addition of
Organoboronic Acids to Aldimines Using Chiral Spiro Mono-
phosphite-Olefin Ligands: Method Development and Mechanistic
Studies. J. Org. Chem. 2018, 83, 11873.
(17) (a) Toda, F.; Tanaka, K. Optical Resolution of Binaphthyl and
Biphenanthryl Diols by Inclusion Crystallization with N-Alkylcincho-
nidium Halides. Structural Characterization of the Resolved Materials.
J. Org. Chem. 1994, 59, 5748−5751. (b) Ding, K.; Wang, Y.; Yun, H.;
Liu, J.; Wu, Y.; Terada, M.; Okubo, Y.; Mikami, K. Highly Efficient
and Practical Optical Resolution of 2-Amino-2’-hydroxy-1,1’-
binaphthyl by Molecular Complexation with N-Benzylcinchonidium
Chloride: A Direct Transformation to Binaphthyl Amino Phosphine.
Chem. - Eur. J. 1999, 5, 1734−1737. (c) Zhang, J.-H.; Liao, J.; Cui, X.;
Yu, K.-B.; Zhu, J.; Deng, J.-G.; Zhu, S.-F.; Wang, L.-X.; Zhou, Q.-L.;
Chung, L. W.; Ye, T. Highly Efficient and Practical Resolution of 1,1’-
Spirobiindane-7,7’-diol by Inclusion Crystallization with N-Benzylcin-
chonidinium Chloride. Tetrahedron: Asymmetry 2002, 13, 1363−
1366.
(18) Zhang, W.; Adachi, Y.; Hirao, T.; Ikeda, I. Highly
Diastereoselective ortho-Lithiation of 1,1’-Bis-(oxazolinyl)ferrocene
Directed to C2-symmetric Chiral Ligands. Tetrahedron: Asymmetry
1996, 7, 451−460.
(19) (a) Szczepura, L. F.; Maricich, S. M.; See, R. F.; Churchill, M.
R.; Takeuchi, K. J. Synthesis, Characterization, and Reactivity of an
Oxoruthenium(IV) Complex Containing a Bis(oxazoline) Ligand.
Crystal Structure of [Ru((S)-bpop)(Cl)(trpy)](BF4). Inorg. Chem.
1995, 34, 4198−4205. (b) Imai, Y.; Zhang, W.; Kida, T.; Nakatsuji,
Y.; Ikeda, I. Novel Chiral Bisoxazoline Ligands with a Biphenyl
Backbone: Preparation, Complexation, and Application in Asym-
metric Catalytic Reactions. J. Org. Chem. 2000, 65, 3326−3333.
(c) Liu, B.; Zhu, S.-F.; Wang, L.-X.; Zhou, Q.-L. Preparation and
Application of Bisoxazoline Ligands with a Chiral Spirobiindane
Skeleton for Asymmetric Cyclopropanation and Allylic Oxidation.
Tetrahedron: Asymmetry 2006, 17, 634−641.
(20) Landais, Y.; Parra-Rapado, L.; Planchenault, D.; Weber, V.
Mechanism of Metal-carbenoid Insertion Into the Si-H Bond.
Tetrahedron Lett. 1997, 38, 229−232.
(21) (a) Balcells, D.; Clot, E.; Eisenstein, O. C-H. Bond Activation
in Transition Metal Species from a Computational Perspective. Chem.
́
Rev. 2010, 110, 749−823. (b) Guihaume, J.; Halbert, S.; Eisenstein,
O.; Perutz, R. N. Hydrofluoroarylationof Alkynes with Ni Catalysts.
C-H Activation via Ligand-to-Ligand Hydrogen Transfer, an
F
DOI: 10.1021/acs.orglett.8b02868
Org. Lett. XXXX, XXX, XXX−XXX