Rhodium-catalyzed alkene hydrosilylation via a hydride shuttle process by diene ligands: Dramatic enhancement of regio- and diastereoselectivity

Article abstract of DOI:10.1002/ejoc.201402742

A cooperative ligand-assisted, Rh-catalyzed intramolecular alkene hydrosilylation of homoallylic silyl ethers (1) was developed to provide 1,3-trans-oxasilacyclopentanes (trans-2) in a highly regio- and diastereoselective manner. The modification of metal-ligand architecture employing an inner-sphere functional diene ligand (1,3-cyclohexadiene) and a supporting phosphine ligand (BINAP) was identified as responsible for dramatic enhancement of selectivities. Mechanistic details of a diene ligand-mediated hydride shuttle process are presented as the potential mechanistic driving force behind the high level of the selectivities.

Full text of DOI:10.1002/ejoc.201402742

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201402742
Rhodium-Catalyzed Alkene Hydrosilylation via a Hydride Shuttle Process by
Diene Ligands: Dramatic Enhancement of Regio- and Diastereoselectivity
Yuanda Hua,[a] Hiep H. Nguyen,[a] Gabriela Trog,[a] Adam S. Berlin,[a] and Junha Jeon*[a]
Dedicated to Professor Chan-Mo Yu (Sungkyunkwan University) on the occasion of his 60th birthday
Keywords: Homogeneous catalysis / Rhodium / Hydrosilylation / Hydride shuttle / Diene ligands / Cyclization
A cooperative ligand-assisted, Rh-catalyzed intramolecular
alkene hydrosilylation of homoallylic silyl ethers (1) was de-
veloped to provide 1,3-trans-oxasilacyclopentanes (trans-2)
in a highly regio- and diastereoselective manner. The modifi-
cation of metal-ligand architecture employing an inner-
sphere functional diene ligand (1,3-cyclohexadiene) and a
supporting phosphine ligand (BINAP) was identified as re-
sponsible for dramatic enhancement of selectivities. Mechan-
istic details of a diene ligand-mediated hydride shuttle pro-
cess are presented as the potential mechanistic driving force
behind the high level of the selectivities.
Introduction
Transition metals ligated with precisely designed li-
gands[1] are versatile platforms for a wide array of catalytic
processes.[2] Readily modifiable metal-ligand architectures
often render unprecedented reactivity and selectivity to
molecular catalysis by controlling electronic and steric
properties of the catalyst through engaging favorable coor-
dination.[3] Our laboratory recently disclosed ligand-con-
trolled, norbornene-mediated, regio- and diastereoselective
rhodium-catalyzed intramolecular alkene hydrosilylation re-
actions[4–6] (Scheme 1, a).[7] The strained cycloalkene nor-
bornene (nbe) acts as a functional ligand in the RhI com-
plex 4, in conjunction with a choice of a supporting phos-
phine ligand (BINAP or dpph), to impact regio- and dia-
stereoselectivity via a “hydride shuttle” process.[8] Although
the proposed hydride shuttle process was intriguing, we
sought to expand our initial findings into a more compre-
hensive understanding of the process and further enhance
Scheme 1. Ligand-controlled rhodium-catalyzed intramolecular
alkene hydrosilylation.
the regio- and diastereoselectivity. Herein, we report a co- Results and Discussion
operative functional diene and supporting phosphine li-
We envisioned that steric and electronic alteration of the
gand-mediated, highly regio- and diastereoselective Rh-cat-
alyzed intramolecular alkene hydrosilylation and present
mechanistic details concerning the diene-mediated hydride
shuttle process (Scheme 1, b).
metal complex by substitution of ligands, from nbe and
chloride to 1,3-cyclodienes[9] and larger halides, could posi-
tively impact reactivity and selectivity. Toward this end, we
devised a new rhodium complex 5, which holds functional
[1,3-cyclohexadiene (1,3-CHD)] ligand and supporting (BI-
NAP) ligand (Scheme 2). Simultaneous incorporation of
the 1,3-CHD and bromide ligands to the metal was
achieved via a sequence of oxidative addition of 3-bromo-
cyclohexene to the RhI dimer 6a/6b (to 7a/7b), β-hydride
elimination (to 8a/8b), and reductive elimination of HX (to
5). When [Rh(rac-binap)Br]2 was treated with 3-bromo-
[a] Department of Chemistry and Biochemistry, University of
Texas at Arlington, Planetarium Place, Chemistry Research
Building,
Arlington, Texas 76019, USA
E-mail: jjeon@uta.edu
http://www.uta.edu/faculty/jjeon/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201402742.
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Rh-Catalyzed Alkene Hydrosilylation via a Hydride Shuttle Process
cyclohexene, the rhodium hydride intermediate 8b and 1,3- (Ͼ 24 h)], signifying that the hydrosilylation occurs faster
CHD were immediately observed via 1H NMR spec- when using a strained diene ligand. Acyclic diene donors
troscopy. The rhodium hydride signal appeared as doublet (obtained from isoprenyl and geranyl bromide, L8 and L9,
of triplets at δ = –14.7 ppm (JRh-H = 20.7, JP-H = 14.7 Hz), respectively, entries 8–9) also exhibited high selectivity, al-
indicating a cis orientation of the rhodium hydride in rela- beit with diminished yields compared to their cyclic coun-
tion to the phosphine ligand.[10] We anticipated this ap- terparts. These results support that incorporation of the
proach would minimize the impact of competitive alkene functional diene ligand via β-H elimination is the key to the
hydrosilylation by 6a/6b.
preparation of the active catalyst, which is responsible for
amplification of both regio- and diastereoselectivity.
Table 1. Effect of functional diene ligands.[a]
Entry
Ligand[b]
Yield [%][c]
2a/3a[d]
2a (trans/cis)[d]
1
2
3
4
5
6
7
8
9
L1
L2
L3
L4
L5
L6
L7
L8
L9
53
90
43
52
55
90
78
52
63
4.9:1
24:1
6.1:1
4.9:1
4.1:1
18:1
21:1
8.8:1
18:1
5.1:1
Ͼ 20:1
10.3:1
2.6:1
4.5:1
13:1
15:1
16:1
20:1
BINAP = 2,2Ј-bis(diphenylphosphanyl)-1,1Ј-binaphthyl
[a] Conditions: silane 1a (0.1 mmol), THF (0.025 m). [b] 10 mol-%
of a diene donor ligand was used. [c] Determined by 1H NMR
spectroscopy utilizing an internal standard (DMF). [d] Determined
Scheme 2. Preparation of a reactive catalyst 5.
1
by GC/MS analysis and H NMR spectroscopy.
We commenced the investigation by determining whether
1,3-dienes could serve as functional ligands to improve re-
gio- and stereoselectivity in Rh-catalyzed, intramolecular
alkene hydrosilylation. As shown in Table 1, we proceeded
to examine a series of cyclic “diene” donors (10 mol-%) (en-
tries 1–9). The hydrosilylation of 1a[11] employing [Rh(rac-
binap)Cl]2 and 3-bromocyclohexene (L2, entry 2) resulted
in the formation of trans-2a with both excellent regio- and
diastereoselectivity.[7] Chloro-, iodo-, acetate, and carb-
amate congeners (entries 1 and 3–5) provided significantly
lower selectivities.[12] We subsequently explored the “allyl”
donors [e.g. 3-(pseudo)halo-1-propenes], which also proved
to be less selective.[13] These results demonstrate that both
diene and bromide ligands are essential to achieve high
levels of selectivities.[14] Other cyclic and acyclic diene
donors were investigated as potential functional ligands in
order to better identify characteristics thereof. 3-Bromo-
cyclopentene L6 and 3-bromocycloheptene L7 provided ex-
cellent selectivities (entries 6–7, cf. entry 2). We recognized
a substantial difference of reaction rates among the cyclic
While norbornene-mediated hydride shuttle processes
have been described for Pd-catalyzed reactions, they are not
well-explored in the context of Rh catalysis.[7,8] To better
understand the catalytic process, the hydrosilylation of L2
(20 mol-%) and silyl ether 9, which does not bear an alkene
moiety, was investigated (Scheme 3). Isolation of a mixture
of 10a and 10b (ca. 3%) suggested that 1,3-CHD likely re-
sides within a coordinative sphere of a rhodium-substrate
complex.[15] Furthermore, reductive elimination at room
temperature leading to 10a/10b is viable within this metal
diene donors [L6 (tcompletion = 1 h) Ͼ L2 (6 h) Ͼ L7 Scheme 3. Hydrosilylation of 1,3-cyclohexadiene.
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Y. Hua, H. H. Nguyen, G. Trog, A. S. Berlin, J. Jeon
SHORT COMMUNICATION
Scheme 4. Cross-over experiment.
complex when migratory insertion of a double bond is not studied H/D scrambling in substrate 1a-D during the reac-
available.[16]
tion (Scheme 5, bottom). When homoallylic silyl ether 1a-
A cross-over experiment using 1a-D and 1f-H under the D was used, fast H/D exchange (12%) in 1a-H/D was ob-
standard reaction condition confirmed that the hydro- served at 20% conversion of 1a-D to trans-2a. Interestingly,
silylation proceeds via intramolecular pathway (Scheme 4). the H/D exchange remained constant (12%) until full con-
Further evidence of the hydride shuttle process in the sumption of 1a-D,[17] implying that an additional H/D
hydrosilylation was obtained by the hydrogen-deuterium scrambling step would be necessary to reach the total
scrambling study with L2 (Scheme 5, top). Both hydrogen hydrogen incorporation (16%) in trans-2a. In sharp con-
and deuterium were incorporated at the terminal methyl trast, when saturated silyl ether 12-D was subjected to iden-
group of trans-2a (the deuterium was only found in the ter- tical reaction conditions, only minor H/D exchange (ca.
minal methyl position), as evidenced by an integration value 2%) in 12-H/D was observed at 50% conversion of 1a-D.
of 2.16 in 1H NMR spectroscopy. In addition, partially deu- Therefore, these experiments suggest that the H/D scram-
terated 1,3-CHD, benzene, and cyclohexene, generated by bling only readily occurs in the presence of an alkene, the
disproportionation of 1,3-CHD,[9a] were observed in GC/ proximity of which would facilitate the hydrorhodation of
MS. In detail, the syn-migratory insertion of 1,3-CHD into 1,3-CHD[18] (i.e., an initiation step for a hydride shuttle pro-
Rh-D within 11a generates 11b having an open coordina- cess) to furnish an alkene-bound complex and subsequently
tion site, which then undergoes β-H elimination to afford undergo the C–Si bond forming cyclization.
H/D scrambled 11c. Of note, the reductive elimination of
A plausible mechanism of the diene ligand-mediated in-
11b to give a net cis adduct such as 10a/10b (Scheme 3) is tramolecular alkene hydrosilylation is depicted in Scheme 6.
seemingly not amenable to the presence of an alkene moiety Evidence suggests that 1,3-CHD functions uniquely as a
within the substrate. This isotope-labeling study also estab- catalytic vehicle; the “hydride shuttle” process exclusively
lishes that reductive elimination to trans-2a after silyl olefin renders the modified Chalk–Harrod pathway, specifically
insertion (diastereoselective and putative turnover-limiting via a sequential proximal alkene-assisted syn-hydrorhod-
step) occurs faster than β-hydride elimination. We also ation,[18] silylrhodation, and β-hydride elimination (13a[19]
Scheme 5. H/D scrambling studies.
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Scheme 6. Plausible mechanism.
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Rh-Catalyzed Alkene Hydrosilylation via a Hydride Shuttle Process
Scheme 7. Putative stereochemical model.
Table 2. Substrate scope.[a,b]
[a] Conditions: silane 1 (0.2 mmol), THF (0.025 m). [b] Regioisomeric ratio (rr) and diastereomeric ratio (dr) were determined by GC/
MS analysis and 1H NMR spectroscopy. [c] [Rh(rac-binap)Cl]2 (0.5 mol-%) was used (72 h). [d] Isolated yields of diacetates 15. [e] Isolated
yields of 1,3-anti-diols 14.
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Y. Hua, H. H. Nguyen, G. Trog, A. S. Berlin, J. Jeon
SHORT COMMUNICATION
to 13d).[20–22] We speculate that the added inner-sphere ste- phine ligand (BINAP) permits selective access to trans-2.
ric and electronic properties of cyclohexenyl and bromide Notably, mechanistic details into the hydride shuttle process
ligands within the RhIII complex 13b [the requisite interme- exploiting 1,3-cyclohexadiene as a catalytic vehicle was pro-
diate for the initial H/D scrambling (13a to 13b)] influence vided.
the olefin migratory insertion step (13b to 13c). These ef-
fects are responsible for significant enhancement of the ob-
served regio- and diastereoselectivity of trans-2. An ad-
Experimental Section
ditional H/D scrambling would be feasible via 13c to 13d.
General Procedure for Preparation of 1,3-trans-Oxasilacyclopent-
The resulting RhIII hydride species 13d then undergoes re- anes (trans-2): [Rh(nbd)Cl]2 (2.3 mg, 0.005 mmol, 2.5 mol-%) and
rac-BINAP (9.34 mg, 0.0075 mmol, 7.5 mol-%) were added to a
flame-dried, septum-capped vial. The mixture was dissolved with
THF (0.50 mL) and was stirred for 1 h at room temp. Volatiles
including norbornadiene (nbd) were removed in vacuo (the absence
of nbd was confirmed by GC/MS spectrometry), and the resulting
mixture was kept under vacuum for 0.5 h. 3-Bromocyclohexene
(2.3 μL, 0.020 mmol, 10 mol-%) and THF (8 mL, 0.025 m) were
added to [Rh(binap)Cl]2. Homoallylic silyl ether (0.2 mmol) was
added in one portion to the reaction mixture via a syringe. The
septum on the vial was replaced by a screw cap with a Teflon®
ductive elimination to provide trans-2 and regenerate the
RhI complex 5.
The putative stereochemical model of the hydrosilylation
is shown in Scheme 7. The boat-like conformation TS-1
within the octahedral geometry reduces the transannular in-
teractions between the pseudo-axial phenyl moiety on the
silyl group and the alkene terminus, as well as a non-bond-
ing interaction between a diphenylphosphino moiety of the
ligand and the alkene terminus. Although TS-2 adopts a
chair-like conformation, it experiences the destabilizing in- liner, and the mixture was stirred for 24 h at room temp. The reac-
tion progress can be monitored by GC/MS spectrometry. The yield
of the reaction was determined by 1H NMR spectroscopy by an
addition of DMF (16 μL, 0.20 mmol) as an internal standard after
the volatiles were removed in vacuo. The regio- and diastereomeric
ratios were determined by 1H NMR spectroscopy and GC/MS
spectrometry. For an analytical purpose, the product was purified
by medium pressure liquid chromatography (MPLC, hexanes/
EtOAc = 80:1, 7 mL/min, retention time 10–20 min).
teraction between the diphenylphosphino moiety and the
pseudo-equatorial alkene terminus, leading to the minor
diastereomer (cf. substrate-controlled hydrosilylative cycli-
zations typically produce cis-oxasilacyclopentanes, con-
trolled by minimizing allylic strains).[4,5] We were able to
determine the absolute stereochemistry when the enantiose-
lective desymmetrizing hydrosilylation of diallylic silyl ether
1i was carried out employing (S)-BINAP.[23] TS-3 shows an
interaction of the pseudo-equatorial phenyl moiety of the
silyl group and the diphenylphosphino moiety, leading to
formation of the minor enantiomer.
Supporting Information (see footnote on the first page of this arti-
cle): Spectroscopic characterization data and procedures for prepa-
ration of all new compounds.
The utility of this reaction is enhanced by a reasonably
wide substrate scope, comprised of both electronically and
sterically diverse aliphatic and aromatic substrates
(Table 2). Both regio- and diastereoselectivity among all
substrates were greatly enhanced vis-à-vis the nbe-mediated
reactions.[7] We were able to lower the catalyst loading to
0.5 mol-% without severely impacting the diastereoselectiv-
ity in trans-2a (24:1 rr, 18:1 dr); the reaction simply took
longer to complete (72 h). Electron-deficient aromatic sub-
Acknowledgments
The authors thank UT Arlington for support of the scientific pro-
gram. The National Science Foundation (NSF) (CHE-0234811 and
CHE-0840509) is acknowledged for partial funding of the pur-
chases of the NMR spectrometers used in this work.
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Conclusion
In summary, we have developed a cooperative ligand-me-
diated, Rh-catalyzed intramolecular alkene hydrosilylation
to provide 1,3-trans-oxasilacyclopentanes (trans-2) with
high regio- and diastereoselectivity. Alteration of the metal-
ligand architecture employing an inner-sphere functional
diene ligand (1,3-cyclohexadiene) and a supporting phos-
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Rh-Catalyzed Alkene Hydrosilylation via a Hydride Shuttle Process
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[19] The cross-over experiment depicted in Scheme 4 confirmed
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[13] For detailed information see the Supporting Information.
[14] To further establish the function of L2 in the hydrosilylation,
several control experiments were undertaken. In particular,
counterion identity was an important factor for diastereoselec-
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and diastereoselectivity. For detailed information see the Sup-
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when the cyclization is not available.
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Received: June 11, 2014
Published Online: August 21, 2014
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