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Highly Selective Allylic Alkylation with a Carbon Nucleophile at the More Substituted Allylic Terminus Catalyzed by an Iridium Complex An Efficient Method for Constructing Quaternary Carbon Centers.

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COMMUNlCATlONS
[I] a ) D. H . Harris, M. F. Lappert, .
I
Chem. Soc. Chem. Commun. 1974.895; b) T.
Fieldberg. H. Hope, M . F. Lappert, P. P. Power. A. J. Thorne. ibid. 1983. 639.
[2] H Braunschweig. R. W. Chorley, P. B. Hitchcock, M. F Lappert. J Chen?.
Suc Chmi. C'onnnun. 1992, 1311.
[3] H Braunschweig, P. B. Hitchcock. M. F. Lappert. L. J. -M. Pierssens, Angew.
Chem. 1994. 104. 1243, Angels. Chem. Int. Ed. Engl. 1994, 33. 1156.
141 A. W. Duff. P. B. Hitchcock, M F. Lappert, R. G Taylor, J. A. Segal. J
Ur~urromrr Chmi. 1985, 293, 271.
[5] J. Pfeiffer. M Maringelie, M. Noltemeyer, A. Meller, Cliem. Ber. 1989, 122,
245.
[6] H. Braunschwrig. B. Gehrhus, P. B. Hitchcock. M. F. Lappert. 2. Anorg. ,411..
Chtnr.1995. 611. 1922
[7] R . W. Chorleq. P B. Hitchcock, B. S. Jolly, M. F. Lappert, G . A. Lawless. J.
Chcni Sot. Chenr. Conimun 1991, 1302.
[XI Crystal data 1 : C,,H,,N,Si,Sn,,
M =738.4, triclinic, space group Pi (110.2).
u = 9.X14(1). h = 12.101(4). c =15.288(3) A, x =73.11(2), p = 89.57(1), ;=
7 7 6 3 2 ) . V = 1694.2A'.F(000)=744.Z= 2,pcxIcd
= 1 . 4 0 g ~ m - ~ ; p ( M o , ,=
)
16.3 cm I. crystal dimensions 0.5 x 0.2 x 0.15 mm, 5979 unique reflections for
2 < 0 < 2 5 . R1 = 0.063 for 3532 reflections with ( F 2 j > 2 0 ( F Z ) .Ris = 0070,
M = 1802.2. monoclinic, space group P2,,n
S = I X. 2: C,,H,,N,Si,Sn,,
(non-standard no. 14), u =17.388(9), b = 20.370(7). c = 20.869(9) A.
p = 95 59(4) . V=7357(6)A3. F(OO0) = 3560, Z = 4, pEnlcd~ 1 . 6 g3 ~ m - ~ ;
p(MoK,) = 2 17 mm-I. crystal dimensions 0.4 x 0.3 x 0.25 mm, 8974 unique
reflections for 2 < 8 < 2 2 , R1 = 0.085 for 6764 reflections with jF21> 2 0 ( F 2 ) ,
W R 2 = 0.245 for all data. S = 1.036. For both 1 and 2: T = 293 K, EnrafNonius CAD-4 diffractometer. absorption correction, structural solution by
heavy atom methods, full-matrix least-square refinement on F using EnrafNonius SDP programs (for I ) or o n F Z using SHELXL-93 (for 2) with non-hydrogen atoms misotropic. Crystallographic data (excluding structure factors)
for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC179-137 Copies of the data can be obtained from The Director, CCDC, 12
Union Road. Cambridge CB2 1EZ. UK (Telefax: Int. code Int. code +(1223)
336-031: r-rn;iiI. depositkr chemcrys.cam.ac.uk).
[9] See L. R . Sita. .4r/i.. Orgunontef. ChtJm.1995, 38. 189.
[lo] See M. Veith. Chem. Rev. 1990, 90. 3.
1111 M. S. Gordon, K . A. Nguyen, M. T. Carroll, Polyhrdron, 1991, 10, 1247.
[12] M Weidenbruch, J Schlaefke. A. Schafer, K. Peters, H. G. von Schnering and
H. Marsmann, Angew. Chem. 1994, 104, 1938; Angew Chem. I n / . Ed. Engl.
1994, 33. 1846.
[13] See R . Taylor, E/erfroplirlic Aromuric Suhstrturion. Wiley, Chichester 1990,
Chapter 5.
[14] J. Nagy. P. Hencsei. E. Zimonyi-Hegedus, Hung. Period. Polytech. Chem. Eng.,
1974. 1 8 l 4 J ,175.
Highly Selective Allylic Alkylation with a
Carbon Nucleophile at the More Substituted
Allylic Terminus Catalyzed by an Iridium
Complex: An Efficient Method for
Constructing Quaternary Carbon Centers**
Ryo Takeuchi* and Mikihiro Kashio
Iridium complexes have been studied as models for intermediates in catalytic reactions. For example, the oxidative addition
reaction of [IrCI(CO)(PPh,),J is well known."] Since Crabtree
reported that a cationic iridium complex is a highly active catalyst for hydrogenation of alkenes,''] iridium complexes have
received much attention as potential hydrogenation catalysts.
Recently an iridium complex catalyzed asymmetric hydrogenation was reported.l3I In contrast, the development of carbon[*] Prof. Dr R. Takeuchi, M. Kashio
[**I
Department of Environmental Science
Faculty of Science. Yokohama City University
Kanazawa-ku. YOkOhdmd 236 (Japan)
Fax. Int. code +(45)7872218
e-mail: rtakeuchir Yokohama-cu.ac.jp
This work was wpported by the Ciba-Geigy Foundation (Japan) for the Promotion of Science.
A n g i w Clwm. In? Ed Engl. 1997, 36, No. 3
G
carbon bond forming reactions catalyzed by iridium complexes
lags far behind.14] Synthetically useful stereo- and regiocontrolled carbon-carbon bond forming reactions are unexplored.
We report here on highly selective allylic alkylations with carbon
nucleophiles that are catalyzed by an iridium complex [Eq. (a)].
a:
3
2
1
R' = npr, R2= R3= R4= H; b: R'
=
Ph, R'= R3= R4= H
c: R' = Me, R2 = R3 = R4 = H; d: R' = R
' = Me, R3 = R4 = H
e: R'
=
R 2 = H, R3= R 4 = Me; f: R' = R2= H, R3= Me, R4= nBu
g: R' = R2 = H, R3 = Me, R4 = CH2CH2CH=CMe,
Transition metal complex catalyzed allylic alkylation with
carbon nucleophiles is an essential method for constructing
complex organic molecules. Palladium,['] nickel,I6] molybdenum,['I iron,[*] tungsten,[g1and ruthenium1"] complexes can
serve as catalysts. Oxidative addition of the allylic substrate to
a low-valent transition metal complex produces a x-ally1 complex, which is then attacked by the nucleophile to give the final
product. The control of regioselectivity in this reaction is important. One of the most extensively studied transition metals in
allylic alkylation is palladium. Generally palladium complexes
direct attack at the less substituted allylic terminus. In contrast,
molybdenum, tungsten, and ruthenium complexes tend to lead
to preferential attack at the more substituted allylic terminus.
Complete regiocontrol of the nucleophilic attack at the more
substituted allylic terminus is a challenging problem.
The reaction of (E)-2-hexenyl acetate (la, X = Ac) with diethyl malonate anion in the presence of [Ir(cod)Cl], gave a mixture of 2a and 3a. Product 2a arose from alkylation at the more
substituted allylic terminus and 3a from alkylation at the less
substituted allylic terminus. The addition of phosphorus ligands
had a significant effect on the selectivity and yield of products.
The results are summarized in Table 1. P(OPh), was found to be
Table 1. Effect of the phosphorus ligand on the iridium complex catalyzed allylic
alkylation of 1 a. X = Ac, with the anion of diethyl malonate [a]
Entry
Ligand
Conditions
Yield(oA][b] 2a:3a[c]
1
2
3
4
5
P(OPh),
P(OEt),
P(OiPr),
PPh,
PnBu,
room
THF,
THF,
THF,
THF,
89
81
44
6
0
temperature, 3 h
reflux, 3 h
reflux, 9 h
reflux, 16 h
reflux, 16 h
96:4
59:41
53~47
24~76
[a] A mixture of I a. X = Ac (2 mmol), NaCH(CO,Et), (4 mmol). [ I r ( ~ o d ) C l ] ~
(0.04mmol), the phosphorus ligand (0.16mmol), and T H F (10 mL) was stirred
under Ar. [b] Yield of isolated product. [c] Determined by gas chromatography
the most effective ligand. The reaction was complete in three
hours at room temperature to give an excellent yield of products. Compound 2a was obtained in 96% selectivity (entry 1);
the alkylation was highly selective at the more substituted allylic
terminus. The reaction using P(OEt), as a ligand gave a comparable yield of products with much less selectivity (entry 2). When
a more electron-donating phosphorus ligand such as PPh, and
P(nBu), was used, products were obtained in poor yields (entries 4 and 5).
We examined the reaction of a series of allylic compounds.
The results summarized in Table 2 indicate that alkylation at the
VCH Verlugsgesellsrhu/~mbH, 0.69451 Wemhebn. 19Y7
OS70-0833jU7;3603-0263S 15.00+ .25'0
263
COMMUNICATIONS
Table 2. Iridium complex catalyzed allylic aikylation with carbon nucleophilies
following Equation (a) [a].
Entry
1
2[d,e]
3
4
5[d,e]
6
7
8
9
10
ll[e]
12[e,g]
13[e]
14[e,gl
1, X
t [h]
l a , Ac
l a , Ac
l a , Ac
l a . C0,Me
la,H
l b , Ac
Ic, Ac
Id, COCF,
l e , Ac
l e , Ac
1f.Ac
If, Ac
l g , Ac
l g , Ac
3
8
3
1
2
3
3
5
2
2
16
16
18
18
Nu
NaCH(CO,Et),
NaCHAcC0,Et
NaCH(CO,Me),
NaCH(CO,Et),
NaCH(CO,Et),
NaCH(CO,Et),
NaCH(CO,Et),
NaCH(CO,Et),
NaCH(CO,Et),
NaCH(CO,Me),
NaCH(CO,Et),
NaCH(CO,Me),
NaCH(CO,Et),
NaCH(CO,Me),
Yield [YO][b] 2:3 [c]
89
85
90
94
100
98
77
70
80
70
80
78
85
80
96:4
93[f]:7
97:3
97:3
9614
99.1
97:3
95:s
0: 100
0:lOO
0: 100
0: 100
0: 100
0:lOO
(0.04 mmol),
[a] A mixture of 1 (2 mmol), nucleophile (4 mmol), [Ir(~od)Cl]~
P(OPh), (0.16 mmol), and TH F (10 mL) was stirred under Ar at room temperature.
[b] Yield of isolated product. [c] Determined by gas chromatography. [d] Heated
at reflux. [el 6 mmol Nucleophile. [f] 1 :1 Mixture of diastereomers. [g] 0.08 mmol
[Ir(cod)CI],, 0.32 mmol P(OPh),
more substituted allylic terminus occurred with high regioselectivity. The reactions proceeded smoothly at room temperature
except in the case of allylic alcohol la, X = H. The sodium salt
of ethyl acetoacetate also could be used as carbon nucleophile
(entry 2). The reactivity of carbonate la, X = C0,Me (entry 4),
was comparable to that of acetate la, X = Ac (entry 1). Allylic
esters are generally used as substrates because of their high
reactivity in oxidative additions with low-valent transition
metal complexes. The palladium complex catalyzed reaction of
allylic alcohols with carbon nucleophiles is reported to be nonregioselective and requires extreme reaction conditions.[”] With
iridium catalysis, allylic alcohol la, X = H, underwent regioselective alkylation at the more substituted allylic terminus under
mild conditions (entry 5 ) . The reaction of I-hexen-3-yl acetate,
a regioisomer of la, X = Ac, gave products 2a and 3a in 85%
yield under the same reaction conditions with a ratio of 95:5.
The results strongly suggest the intermediacy of a n-ally1 iridium
complex. Acetates l b and lc were readily alkylated at the more
substituted allylic terminus (entries 6 and 7). 3-Methyl-2butenyl acetate failed to react. The replacement of the acetyl
leaving group with a trifluoroacetyl group enhanced the reactivity of the allylic substrate considerably. Product 2d was obtained with 95 YOselectivity (entry 8). The most unique aspect
of the iridium complex catalyzed reactions is demonstrated in
the reactions of le- lg. Regiospecific alkylation at the tertiary
allylic terminus resulted in the construction of a quaternary
carbon center (entries 9-14), a feature in numerous natural
products.
The regioselectivity of the allylic alkylation is controlled by
three factors: 1) the steric interaction between the incoming
nucleophile and allylic terminus, 2) the charge distribution of
the n-ally1 ligand on the metal center, and 3) the stability of the
resulting alkene-metal complex as the initial product. The results described here indicate that the first is not important. With
regards to the third factor, it is clear that nucleophilic attack at
the more substituted allylic terminus results in formation of the
more stable terminal alkene complex[* as the initial product.
P(OPh), plays a crucial role with regards to the second factor.
Since it is a better n-acceptor,[131it promotes carbonium ion
character at the more substituted allylic terminus in the n-ally1
and directs the nucleophilic attack to this position.
In agreement with this, when a ligand with weaker acceptor
264
0 V C H Verlagsgesellschaft mhH, 0-69451 Weinhelm, 1997
properties was used such as P(OEt), , the reaction was much less
selective (Table 1, entry 2).
The stereochemistry of the reaction was also examined. (Z)-5(Methoxycarbonyl)-2-cyclohexen-l-ylmethyl carbonate was
treated with the sodium salt of dimethyl malonate [Eq. (b)].
+
THF,A, 3 3 h
E = C02Me
NaCHE2
Unlike the reactions with acyclic systems, this reaction was quite
sluggish and did not go to completion. The product was obtained in 37 YOyield; the starting material was recovered in 40 YO
yield. The product with net retention of configuration was obtained exclusively, which suggests that the reaction proceeds by
a double inversion mechanism.[”. 1 6 ]
In conclusion, iridium complexes were found to be new and
efficient catalysts for the allylic alkylation of acyclic systems.
The regioselectivity is opposite to that achieved with palladium
catalysts. Currently we are testing synthetic applications of this
reaction in asymmetric synthesis.
Experimental Section
Table 2, entry 13: A solution of Ig (393 mg, 2.0mmol), P(OPh), (49.6 mg,
0.16 mmol), and [Ir(cod)Cl], (26.9 mg, 0.04 mmol) was stirred in 5.0 mL of THF
under Ar atmosphere. In a separate flask, diethyl malonate (961 mg, 6.0 mmol) was
added to a slurry of sodium hydride (144 mg, 6.0 mmol) in 5.0 mL of THF. The
resulting clear solution was added to the first solution by syringe, and the mixture
was stirred at room temperature for 18 h. The progress of the reaction was monitored by gas chromatography. After complete consumption of Ig the reaction mixture was partitioned between ether and water. The organic layer was separated and
the aqueous layer was extracted with ether. The combined organic layers were
concentrated in vacuo. The residue was purified by column chromatography (n-hexane/ethyl acetate 98/2) to give 3g (505 mg, 85%).
Received: July 18, 1996 [Z93471E]
German version: Angeir. Chem. 1997, 109, 268-270
Keywords: alkylations . ally1 complexes
iridium regioselectivity
-
*
C-C coupling
-
[I] L. Vaska. Arc. Chem. Res. 1968, I, 335.
[2] R. H. Crabtree, Acc. Chem. Res. 1979, 12, 331
[3] K. Tani. J. Onouchi. T. Yamagata. Y Kataoka, Chem. Lett. 1995, 955; X.
Zhang, T. Taketomi, T. Yoshizumi, H. Kumobayashi, S. Akutagawa, K.
Mashima. H. Takaya. J. Am. Chem. Soc. 1993,115, 3318.
141 Examples: N. Chatani, S. Yamaguchi. Y Fukumoto, S. Mural. Organometa/lics 1995, 14,4418; N. Chatani, S. Ikeda, K. Ohe, S. Mural, J Am. Chem. Soc.
1992, 114. 9710; R. P. Patil, A. A. Kelkar, R. V. Chaudhari, J. Moi. Cural.
1992, 72, 153; A. Behr, E. Herdtweck, W. A. Herrmann, W. Keim, W. Kipshagen, OrgunometaNies 1987,6, 2307; A. J. Kunin, R. Eisenberg, J. Am. Chem.
Soc. 1986,108, 535, W. H. Champbell, P. W. Jennings, OrganometaNics 1983,
2, 1460; R. L. Pruett, R. T. Kacmarcik, ihid. 1982, 1, 1693; W. H. Champbell,
P. W. Jennings, ihid. 1982, 1. 1071.
[ S ] P J. Harrington in Comprehensive Organometallic Chemistry I I . Vol. 12 (Eds.:
E. W. Abel, F. G. Stone, G. Wilkinson), Elsevier, Oxford, 1995, p. 797, and
references therein.
[6] G. Consiglio, A. Indolese, OrgunometaNics 1991, f0,3425.
[7] B. M. Trost. C. A. Medic, J. Am. Chcm. Soc. 1990, 112.9590; J. W. Faller, C.
Lambert. M. R. Mazzieri, J Organomet. Chem. 1990, 383, 161
[8] Y. Xu, 8. Zhou, J. 01%.Chem. 1987, 52, 974.
[9J G. C L .Jones. A. Pfaltz, Angets. Chem. 1995, 107,534; Angebc. Chem. Inr. Ed.
Engl. 1995,34,462: B. M. Trost. M.-H. Hung, J Am. Chem. Soc. 1983, 105.
7757.
[lo] T. Kondo, H. Ono, N . Satake, T. Mitsudo, Y. Watanabe, Organometalhcs 1995,
14, 1945; S:W.
Zhang, T. Mitsudo, T. Kondo, Y Watdnabe, J. Organomel
Clzrm. 1993, 480, 197.
0570-0833/97/3603-0264$/5.00+ ,2510
Angew. Chem. I n f . Ed. Engl. 1997,36, N o . 3
COMMUNICATIONS
[3 I] D. E. Bergbreiter, D. A. Weatherford, J1 Chem. SOC.Chem. Commun. 1989,
883. Recently the palladium complex catalyzed reaction of allylic alcohols
with CH-active methylene compounds under CO, pressure was described:
M. Sakamoto, I.Shimidzu, A. Yamamoto, Bull. Chem. SOC.Jpn. 1996, 69,
1065.
[I21 I-Alkene complexes are more stable than complexes with substituted double
bonds; see R. Cramer, J Am. Chem. SOC.1967,89, 4623.
[I31 A. Van Rooy, E. N. Oriji, P. C. J. Kramer, P. W. N. M. Van Leeuwen,
Organometallics 1995, 14, 34.
[14] B. iikermdrk, S. Hansson. B. Krakenberger, A. Vitagliano, K. Zetterberg,
Organometallics 1984, 3, 679.
[IS] B. M. Trost, T. R. Verhoeven, ,
I
Am. Chem. SOC.1980, fU2,4730.
[I61 A mechanism with net double retention of configuration cannot be ruled out;
see J. W. Faller. D. Linebarrier, Orgunometullics 1988, 7, 1670.
Dihydrogen Formation in a Trihydride
Metallocene and Its Elimination,
Both Assisted by Lewis Acids:
The [Cp,NbH,] BH3 System**
+
Santiago Camanyes, Feliu Maseras, Miquel Moreno,
Agusti Lledos," JosC M. Lluch," and Juan Bertran
Before 1984 hydrogen coordination to a transition metal was
assumed to involve breaking of the H - H bond to produce two
separate hydride ligands.['] Nowadays the existence of coordinated molecular hydrogen in certain transition metal complexes
is well established.['] The number of new dihydrogen complexes
discovered has not ceased to grow, and the reinvestigation of
polyhydride systems is opening new routes to dihydrogen complexes. These two structures are indeed closely related.l3] It has
been shown that trihydride metallocene complexes possess a
thermally accessible dihydrogen state.[41The presence of such a
state has been used to explain the abnormally large and temperature-dependent NMR JHH
coupling constants, a phenomenon
associated with the quantum mechanical exchange of a pair of
h y d r i d e ~ .51~Recent
~,
studies have revealed that the replacement
of one hydride by a n-acceptor ligand in niobocene and tantalocene trihydride complexes enables the observation and characterization of the first stable dihydrogen complexes of
Group 5.16]It has been suggested that the formation of an adduct between the trihydride complex and a Lewis acid could
similarly stabilize the dihydrogen species by decreasing the electron density around the metal.[71These arguments have been
used to explain the exchange couplings observed in Lewis acid
adducts of niobocene trihydride~.~~.
The most common reaction exhibited by these trihydride species is the loss of molecular hydrogen on heating.['] It is interesting to note that addition of a HBR, Lewis acid clearly lowers the
temperature for hydrogen evolution.["] In this communication
we present, for the first time. ab initio DFT calculations indicating that the formation of an adduct between a trihydride com[*] Prof. Dr. A. Lledos, Prof. Dr. J. M. Lluch, Dip1.-Chem. S. Camanyes,
Dr. M. Moreno, Prof. Dr. J. Bertrin
Unitat de Quimica Fisica, Departament de Quimica
Universitat Autonoma de Barcelona
E-08193 Bellaterra, Barcelona (Spain)
Fax: Int. code +(3)581 2920
Dr. F. Maseras
Laboratoire de Structure et Dynamique des Systemes Moleculaires et Solides
Universite de Montpellier I1 (France)
[**I
Financial support is acknowledged from the Spanish Direccion General de
Ensefianza Superior (DGES) under projects PB95-0637 and PB95-0639.
Angew. Chem. In 1. Ed. Engl. 1991, 36, N o . 3
plex and a Lewis acid converts the dihydrogen structure into a stable complex
and, in addition, so assists
the loss of molecular hydrogen that it becomes quite
easy. To this end we have
performed an ab initio density functional theory['']
(DFT) study on the
[Cp,NbH,] + BH,
SYStem.['2-171 BH, has been
chosen as the simplest modFigure I . Energies [kcalrnol-'1 of the
srationary points located. Compounds
el of a Lewis acid.
1,2a, Zb,and 4 correspond to minimum
f?igure1 the energies
energy structures; 3 is a transition state.
of all the stationary points
(that is, points of zero gradient) are presented schematically. The corresponding geometries are sketched in Figure 2. Structure l is for the
[Cp,NbH,] trihydride.
BH, can interact with any of the
three hydrides of [Cp,NbH,]. This
interaction leads to two different
minimum energy adducts (2a and
2b). Geometrical analysis of 2a
shows a strong interaction between
the BH, and the central hydride that
lengthens the distance between Nb
3
4
Figure 2. Geometries of the stationary points 1-4. Bond distances are given in A.
and the central H by 0.08 A with respect to that in 1. Furthermore, the BH, fragment, which is planar when isolated, has
already adopted a nonplanar geometry that begins to resemble
the tetrahedral BH, anion. This affinity of the Lewis acids for
the hydride ligands of metallocene complexes had already been
observed experimentally for [Cp,MH,] (M = Nb, Ta)"sdl and
[Cp,Nb(CO)H] .[18b1 Adduct 2a involves a strong interaction
between the two fragments as indicated by the large stabilization of this complex with respect to the separated partners
(14.6 kcalmol- ', see Figure 1). However, this interaction is
weaker than that between NH, and BH,, which is
32.4 kcal mol- ' at our level of calculation. For comparison, an
0 VCH ~rlugsgesells~huft
mbH. 0-6945S Weinheim. 1997
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265
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