Asymmetric Ni-H insertion reaction cooperatively catalyzed by rhodium and chiral spiro phosphoric acids

Article abstract of DOI:10.1002/anie.201105485

So near so SPA: Achiral dirhodium(II) carboxylates and chiral spiro phosphoric acids (SPA) cooperatively catalyzed asymmetric Ni-H insertion reactions with high enantioselectivity, high yields, and fast reaction rates at low catalyst loading (see scheme;

Full text of DOI:10.1002/anie.201105485

DOI: 10.1002/anie.201105485
Asymmetric Catalysis
ꢀ
Asymmetric N H Insertion Reaction Cooperatively Catalyzed by
Rhodium and Chiral Spiro Phosphoric Acids**
Bin Xu, Shou-Fei Zhu,* Xiu-Lan Xie, Jun-Jie Shen, and Qi-Lin Zhou*
Nitrogen-containing organic compounds, such as a-amino
acids and alkaloids, are important biologically active com-
pounds, thus the development of efficient and enantioselec-
tive methods for the construction of carbon–nitrogen bonds is
a fundamental goal in modern organic synthesis.[1] Transition-
ꢀ
metal-catalyzed carbene insertion into N H bonds is one of
the most efficient methods to construct carbon–nitrogen
bonds[2] and the development of asymmetric versions of the
ꢀ
N H insertion reaction has attracted considerable atten-
tion.[3] In initial studies, chiral dirhodium catalysts were tested
in intramolecular[4] and intermolecular[5] N H insertion
reactions, however, only low to modest enantioselectivities
ꢀ
Scheme 1. Proposed mechanism for chiral phosphoric acid induced
ꢀ
asymmetric N H insertion.
(< 50% ee) were achieved. Since these reports, other tran-
sition metals including copper and silver have been used as
catalysts, and gave enantioselectivities up to 48% ee.[6]
It is generally accepted that the rhodium-catalyzed N H
ꢀ
ꢀ
Recently, we reported a highly enantioselective N H inser-
insertion most likely proceeds via an ylide intermediate
(Scheme 1A).[2a] We speculated that the subsequent proton-
transfer step could be facilitated by a chiral phosphoric acid[12]
species via a seven-membered-ring transition state, and that,
consequently, chiral induction could be accomplished in this
step (Scheme 1B). The groups of Yu[13] and Platz[14] have
reported that either water or alcohols can assist proton
tion reaction (up to 98% ee) using a copper complex with
chiral spiro bisoxazoline ligands.[7] Subsequently, two other
types of chiral copper catalysts have been developed, one with
a planar chiral bipyridine ligand[8] and the other with a binol-
derivative ligand,[9] and both of these catalysts give high
ꢀ
enantioselectivities in N H insertion reactions.
ꢀ
ꢀ
Although progress on copper-catalyzed asymmetric N H
transfer in O H insertion reactions, as indicated by density
functional theory calculations and ultrafast time-resolved IR
spectroscopy studies. These studies stimulated our interest in
insertion reactions has been substantial, they still have serious
ꢀ
limitations. For instance, all the copper-catalyzed N H
ꢀ
insertion reactions require high catalyst loading (5–
10 mol%) for satisfactory yields and enantioselectivities,
thus more-efficient chiral catalysts are highly desirable.
Because the activity of dirhodium(II) catalysts is usually
exploring asymmetric N H insertion in the presence of a
proton-transfer catalyst. As part of our ongoing work on the
development of asymmetric carbene insertion reactions,[15] we
ꢀ
report herein the asymmetric N H insertion reaction coop-
ꢀ
superior to that of copper catalysts in nonenantioselective N
eratively catalyzed by dirhodium(II) carboxylates and chiral
spiro phosphoric acids (SPAs).[16] Excellent reactivity and
high enantioselectivity (up to 95% ee) were achieved in the
presence of as little as 0.1 mol% of catalyst.
H insertion reactions,[10] the possibility of using dirhodium
ꢀ
catalysts to achieve highly enantioselective N H insertion
reactions is an intriguing one. Recently, Saito et al.[11]
reported that dirhodium(II) carboxylates and cinchona alka-
In our initial study, we carried out the insertion of methyl
ꢀ
ꢀ
loids cooperatively catalyze the asymmetric N H insertion
a-diazo-a-phenylacetate (3a) into the N H bond of tert-butyl
reactions of a-diazo-a-arylacetates with anilines. The com-
bined catalysts exhibit excellent reactivity but only modest
enantioselectivity (up to 71% ee).
carbamate (BocNH2) in CHCl3 at 258C using 1 mol% of
[Rh2(OAc)4] and 10 mol% of chiral SPAs 1 as the catalysts
(Table 1). SPAs 1 were prepared by a simple condensation of
P(O)Cl3 with 6,6’-disubstituted-1,1’-spirobiindane-7,7’-diols 2,
followed by hydrolysis (Scheme 2).[17] Diols 2 were synthe-
sized from spinol (1,1’-spirobiindane-7,7’-diol),[18] as de-
scribed previously.[19] In the presence of (R)-1a, the N H
[*] B. Xu, Prof. S.-F. Zhu, X.-L. Xie, J.-J. Shen, Prof. Q.-L. Zhou
State Key Laboratory and Institute of Elemento-organic Chemistry
Nankai University, Tianjin 300071 (China)
ꢀ
insertion reaction proceeded within 5 minutes to afford the
insertion product in excellent yield with 11% ee (Table 1,
entry 2). Control experiments showed that the SPAs alone did
not promote the insertion reaction.
A range of SPAs with various substituents at the 6 and 6’
positions were evaluated (Table 1, entries 3–9). All the tested
E-mail: qlzhou@nankai.edu.cn
[**] We thank the National Natural Science Foundation of China and the
National Basic Research Program of China (2011CB808600), and
the “111” project (B06005) of the Ministry of Education of China for
financial support.
ꢀ
SPAs afforded high yields in the N H insertion reaction. SPA
(R)-1h, which bears a 6,6’-di(naphth-2-yl) group, afforded the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201105485.
Angew. Chem. Int. Ed. 2011, 50, 11483 –11486
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11483

Communications
slowed the reaction and lowered both the yield and the
ee value (Table 1, entry 10). [Rh2(TFA)4], which has electron-
deficient carboxylate ligands, improved the enantioselectivity
to 74% ee (Table 1, entry 13). When we used [Rh2(TPA)4],
which contains bulky carboxylate ligands, the enantioselec-
tivity was markedly improved to 92% ee with no accompany-
ing decrease in reactivity or yield (Table 1, entry 14). Chang-
ing the rhodium/SPA ratio from 1:10 to 1:1 had almost no
impact on either the reaction time or the enantioselectivity
(Table 1, compare entries 14 and 15). Interestingly, even when
the amount of SPA was reduced to 0.1 equivalents relative to
[Rh2(TPA)4], good yield and high enantioselectivity were still
obtained (Table 1, entry 16). The reactivity of the cooperative
catalysts was so high that 0.1 mol% of catalyst was sufficient
to achieve a satisfactory outcome (entry 17). Thus, our
preliminary results indicated that the combination of [Rh2-
(TPA)4] and (R)-1h is an efficient catalytic system for the
Scheme 2. Synthesis of chiral spiro phosphoric acids.
ꢀ
Table 1: N H insertion reactions catalyzed by dirhodium(II) carboxy-
lates and chiral spiro phosphoric acids: Optimization of reaction
conditions.[a]
ꢀ
asymmetric N H insertion reaction. For comparison, we also
tested chiral phosphoric acid (R)-5,[12] which has a binaphthol
scaffold, however, under identical reaction conditions only
modest enantioselectivity was obtained (Table 1, entry 18).
This result clearly shows that the spirobiindane backbone of
the SPAs plays a crucial role in the chiral induction step.
Under the optimized reaction conditions, various a-aryl-
a-diazoacetate substrates were investigated (Table 2,
entries 1–12). Impressively, all the reactions were complete
Entry [Rh]
1
x/y
[mol%]
t [min] Yield [%][b] ee [%][c]
1
[Rh2(OAc)4] none
–
5
5
89
90
86
94
90
94
94
91
92
70
78
76
85
94
94
84
94
97
–
11
ꢀ9
50
18
6
2
3
4
5
6
7
8
9
[Rh2(OAc)4] (R)-1a 1:10
[Rh2(OAc)4] (R)-1b 1:10
[Rh2(OAc)4] (R)-1c 1:10
[Rh2(OAc)4] (R)-1d 1:10
[Rh2(OAc)4] (R)-1e 1:10
[Rh2(OAc)4] (R)-1 f 1:10
[Rh2(OAc)4] (R)-1g 1:10
[Rh2(OAc)4] (R)-1h 1:10
5
5
5
5
ꢀ
Table 2: N H insertion reactions of various substrates catalyzed by
[Rh2(TPA)4] and chiral spiro phosphoric acid (R)-1h.[a]
5
5
5
30
1
1
1
1
1
10
49
67
26
56
51
74
92
92
80
87
ꢀ30
10
11
12
13
14
15
16
17
18
[Rh2(cap)4]
[Rh2(oct)4]
[Rh2(piv)4]
[Rh2(TFA)4] (R)-1h 1:10
[Rh2(TPA)4] (R)-1h 1:10
[Rh2(TPA)4] (R)-1h 1:1
[Rh2(TPA)4] (R)-1h 1:0.1
[Rh2(TPA)4] (R)-1h 0.1:0.1
(R)-1h 1:10
(R)-1h 1:10
(R)-1h 1:10
Entry
R1, R2
Product
Yield [%]
ee [%]
1
2
3
4
5
6
7
8
9
C6H5, Me (3a)
4a
4b
4c
4d
4e
4 f
4g
4h
4i
94
88
90
92
90
94
97
85
92
93
92 (R)
90
90 (R)
92 (R)
94
92
91
91
95
4-MeOC6H4, Me (3b)
4-MeC6H4, Me (3c)
4-ClC6H4, Me (3d)
4-PhC6H4, Me (3e)
3-MeOC6H4, Me (3 f)
3-MeC6H4, Me (3g)
3-ClC6H4, Me (3h)
2-MeOC6H4, Me (3i)
2-ClC6H4, Me (3j)
1
1
1
[Rh2(TPA)4] (R)-5
1:1
[a] Reaction conditions: [Rh]/1/3/BocNH2 =0.002x:0.002y:0.2:0.2
(mmol) in 3 mL of CHCl3 at 258C. [b] Yield of the isolated product.
[c] Determined by HPLC with a Chiralcel OJ-H column. Boc=tert-
butyloxycarbonyl, naph=naphthyl.
10
4j
91 (R)
11
, Me (3k)
4k
94
90
12
2-Naphthyl, Me (3l)
Me, Bn (3m)
4l
4m
89
99
91
50 (R)
13[b]
[a] The reaction conditions and analysis method were the same as those
described in Table 1, entry 15. All reactions were complete within 1 min.
[b] (R)-1g, instead of (R)-1h, was used. Bn=benzyl.
highest enantioselectivity (67% ee; Table 1, entry 9). Differ-
ent dirhodium complexes were then studied with (R)-1h as a
co-catalyst (Table 1, entries 10–14). Both the electronic and
the steric properties of the ligands of the dirhodium com-
plexes affected the reactivity and the enantioselectivity of the
within 1 min under the mild reaction conditions, and the
ꢀ
corresponding N H insertion products were obtained in high
yields (85–97%) with high enantioselectivities (90–95% ee).
Compared to the chiral copper catalysts developed for the
same reaction, the combination of [Rh2(TPA)4] and (R)-1h
ꢀ
N H insertion reactions. Carboxamidate ligated [Rh2(cap)4]
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Angew. Chem. Int. Ed. 2011, 50, 11483 –11486

has obvious advantages, including lower catalyst loading and
higher yields. Additionally, in the reaction of a-diazopropi-
onate 3m, the copper catalyst gives 1,2-H migration rather
[8]
ꢀ
than N H insertion, whereas [Rh2(TPA)4]/(R)-1g afforded
ꢀ
the N H insertion product almost quantitatively, albeit with
only moderate enantioselectivity (Table 2, entry 13).
ꢀ
Similar to the N H insertion reaction of BocNH2, the
reaction of CbzNH2 exhibited high yield and high enantiose-
lectivity under the optimized reaction conditions (Scheme 3).
ꢀ
In contrast, the N H insertions of acetamide and 4-methyl-
benzenesulfonamide afforded racemic products.
ꢀ
Scheme 4. Proposed N H insertion mechanism.
Scheme 3. Cbz=benzyloxycarbonyl.
into the precise reaction mechanism are underway in our
laboratory.
We performed additional experiments to improve our
understanding of the mechanism of the chiral induction. First,
a sodium salt of (R)-1h was prepared and used to catalyze the
ꢀ
In summary, an asymmetric N H insertion reaction
catalyzed cooperatively by dirhodium(II) complexes and
chiral SPAs with high yield and high enantioselectivity was
ꢀ
N H insertion reaction of 3a with BocNH2. The insertion
ꢀ
reaction proceeded smoothly in 96% yield, however, only
7% ee was achieved. This result demonstrated that the acidic
hydrogen of SPA was critical for chiral induction. In 1992
McKervey et al.,[20] and Pirrung and Zhang[21] reported the
synthesis and use of a chiral dirhodium(II) phosphate
developed. This entirely new type of enantioselective N H
insertion catalyst system is expected to have further applica-
ꢀ
tions in asymmetric X H bond insertion reactions.
complex for asymmetric carbene transformations. To rule Experimental Section
ꢀ
A
typical procedure for asymmetric N H insertion: [Rh2-
out the formation of a chiral rhodium(II) phosphate species
by a simple ligand exchange in our system, a 31P NMR study
was performed. The 31P NMR spectrum of a CDCl3 solution
of (R)-1h did not change when 0.5 equivalents of [Rh2(TPA)4]
was introduced at room temperature, thus indicating that
there was no coordination of (R)-1h to rhodium. The addition
of 0.5 equivalents of (R)-1h to a CDCl3 solution of BocNH2
had no effect on the 13C NMR and FTIR signals of the
carbonyl group of BocNH2. This result implies that the
activation of BocNH2 through the formation of a hydrogen
bond with (R)-1h was unlikely.
(Ph3CCO2)4]·0.5CH2Cl2 (2.7 mg, 0.002 mmol, 1 mol%) and (R)-1h
(1.2 mg, 0.002 mmol, 1 mol%) were introduced into an oven-dried
Schlenk tube in an argon-filled glovebox. After CHCl3 (2 mL) was
injected into the Schlenk tube, the mixture was stirred at 258C. A
solution of methyl a-diazo-a-phenylacetate 3a (35.2 mg, 0.2 mmol)
and BocNH2 (23.4 mg, 0.2 mmol) in CHCl3 (1 mL) was added to the
mixture in one portion by a syringe. The color of the diazo compound
disappeared immediately after the addition. The reaction mixture was
purified by flash chromatography on silica gel (petroleum ether/ethyl
acetate = 8:1) to give methyl 2-(tert-butoxycarbonylamino)-2-phenyl-
acetate 4a as a white solid. Yield: 94%, white solid, m.p.: 104–1058C.
92% ee [HPLC conditions: Chiralcel OJ-H column, n-hexane/i-
propanol = 90:10, flow rate = 1.0 mLminꢀ1
, wavelength = 220 nm,
On the basis of the aforementioned experiments and
28
ꢀ
analysis, we propose a preliminary N H insertion reaction
tR = 9.78 min (S), tR = 12.93 min (R)]. ½aꢁD ¼ꢀ113 (c = 1.0, CHCl3);
1H NMR (400 MHz, CDCl3): d = 7.38–7.30 (m, 5H, ArH), 5.54 (brs,
1H, NH), 5.32 (d, J = 7.2 Hz, 1H, CH), 3.72 (s, 3H, OCH3), 1.43 ppm
(s, 9H, C(CH3)3).
mechanism shown in Scheme 4. First, treatment with the
dirhodium complex results in decomposition of the diazo
compound to generate a rhodium carbene intermediate (I),
which reacts with the BocNH2 to generate an ylide (II). The
SPA assists proton transfer via a seven-membered-ring
Received: August 3, 2011
Published online: October 4, 2011
ꢀ
intermediate (III) to form the N H insertion product, and
the dirhodium catalyst and chiral SPA are simultaneously
regenerated. The chiral induction occurs in the proton-
transfer step. The dirhodium catalyst is most likely involved
in the proton-transfer step, considering its strong influence on
the enantioselectivity of the reaction (see Table 1, entries 9–
14). According to this mechanism, the chiral SPA acts as a
proton-transfer shuttle. We believe this new model for
enantioselective proton-transfer catalysis will help the
design of additional reactions. More detailed investigations
Keywords: amino acids · asymmetric catalysis · carbenes ·
chiral phosphoric acids · cooperative catalysis
.
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Angew. Chem. Int. Ed. 2011, 50, 11483 –11486