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Fischer Carbene Complexes Facilitate the Intramolecular Pauson-Khand Reaction.

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Experimental Procedure
3: Salt 2 [lo] (0.60 g, 1.58 mmol), suspended in CH,CI, (30 mL) under N,, was
cooled to - 40°C. Compound 1 (151 (0.46mL. 1.58 mmol), dissolved in
CH,CI, (10 mL), was added dropwise. After the addition was complete, the
cooling bath was removed and the heterogeneous mixture was allowed to warm
to room temperature, turning into a clear, slightly yellow solution. By adding
hexane under vigorous stirring, a yellowish oil separated out and was washed
with hexane twice to remove tri-n-butyltin cyanide. Crystallization occurred by
adding the oil, dissolved in a small amount of CH,CI,, to vigorously stirring
hexane. After decantation of the solvent, the precipitate was dried in vacuo,
yielding 0.45 g (1.18 mmol; 75%) of white microcrystalline 3. M.p. 32°C. 'H
NMR (300MHz. CD,CI,): 6 = 6 . 2 2 (dd, JH.,,H.,=5.2, J H . I , H . 3 = 1 5 . 0 H ~ .
1 H; H-l), 6.95 (dd, JH.I,H.I= 5.2, JH.Z,H-J
= 6.8 Hz, 1 H; H-2, f r a s to I), 7.20
(dd,J,.,,H.,=6.8.JH.,.H.I = 15Hz,IH;H-3,cistoI),7.57(t,J=7.8Hz,2H;
H,,,,), 7.75 ( t , J =7.3 Hz, 1 H ; H,,,,), 7.99 (d, J = 8.4 Hz, 2 H ; Ha,,,,J I3C
NMR (75 MHz, CD,CI,): d = 109.5 ( s ; =CH), 110.1 ( s ; C,,,,), 120.5 (4,
Jc,F= 320 Hz; CF,), 132.5, 133.2, 136.6 (3s; CHs,om),133.4 (s; =CH,). 1R 1161:
v^ [cm-']= 3087, 3072, 3004, 2946, 1588, 1568, 1474, 1446, 1379, 1253, 1223,
1165, 1156, 1025, 992, 733. MS(FAB): m / z 231 (Me), High-resolution MS:
calcd for C,H,F,IO,Se, 230.96707; found, 230.96655.
4 [17]: To 3 (0.84 g, 2.21 mmol), dissolved in benzonitrile, was added 10 mol %
of silver triflate (0.06 g). After the mixture was stirred for 3 d, colorless 4 (0.06 g,
0.34mmol) was distilled off in 1 5 % yield. B.p. 80°C. 'H NMR (300 MHz,
= 3.7, JH.2,H.l
= 5.5Hz; H-2, trans to OTf),
CD,CN): 6 = 5.14 (dd, JH.2,H.3
5.36 (dd, JH.3,H.z
=3.7, JH.3,H.,
= 1 3 . 0 H ~ .1 H ; H-3, cis to OTf), 6.84 (dd,
= 5.5, JH.,.H.3= 13.1 Hz, 1 H ; H-I). "C NMR (75MHz, CD,CN):
6 = 106.4 ( s : =CH,), 119.4 (4,Jc,F= 319 Hz; CF,), 143.5 ( s ; =CH). IR: B
[cm-') = 3111, 2994, 1646. 1428, 1309, 1250, 1215. 1145, 1086, 963, 798;
MS(E1): nijz 176 (Me). High-resolution MS: calcd for C,H,F30,Se,
175.97550; found, 175.97532.
5 : [M(PPh,),(CO)CI] [I81 (13 mg, M = Rh: 15 mg, M = Ir; 0.019 mmol) was
stirred with 3 (8.7 mg, 0.023 mmol) in C,H, (20 mL) at room temperature for
30 min (for 5 a) or 5 h (for 5 b) under N, and with protection from light. Precipitation with hexane afforded 5 containing small impurities of decomposed 3.
After recrystallization from CH,Cl,/hexane (or etherihexane), 5 a is obtained as
off-white crystals in 84 % yield and 5 b as yellow crystals in 75 % yield. 5b: M.p.
178-210°C (dec). 'H NMR (300 MHz, CD,CI,): 6 = 4.43 (dm, JH.I,H.l
= 16.4 Hz, 1 H ; H-3, cis to Ir), 5.41 (dm. JH.z,H.I
= 8.2 Hz, 1 H; H-2, trans to
Ir),6.67(dd.JH.,.,.,=8.3,J".,,H.3=16.5Hz,1H;H-l).3'PNMR(121 MHz,
CD,CI,): 6 = - 8.3. 1R [16]: C [cm-'1 3061, 2063, 1579, 1484, 1436, 1315.
1265,1230,1202,1093,1004,744,693. MS(FAB): m/z 807 [Me - OTfl. Highresolution MS: calcd for C,,H,,CIIrOP~, 807.13280; found 807.13285. 5 b:
M.p. 55-120°C (dec). 'H NMR (300 MHz, CD,CI,): 6 = 4.40 (dm, JH.3,H.l
16.4 Hz, 1 H; H-3,cistoRh).4.97(m, 1 H ; H-2,transtoRh), 6.67(m, 1 H ; H-I).
7.39-7.82 (m, 30H; H,,,,). ,IP NMR (121 MHz, CD,CI,): 6 = 15.1 (d,
= 88.5 Hz). IR [16]: t[cm-'] = 3069,2088, 1574, 1482, 1436, 1324, 1259,
1233, 1204, 1171. 1029, 1016, 746,692,685. MS(FAB): m/r 627 [Rh(PPh,)y].
High-resolution MS: calcd for C3,H,,P,Rhe, 627.08778; found, 627.08708.
Received: April 29, 1991 [Z 4597 IE]
Publication delayed a t authors' request
German version: Angew. Chem. 103 (1991) 1549
[I] For a review see: T. Ohnuma, Yuki Gosei Kagaku Kyokaishill(l983) 768;
Chem. Abstr. 99 (1983) 157 378 w. Recent papers include: N. P. Lebedeva,
1. V. Kalaus, Khim Geterotsikl. Soedin. 1989,856; Chem. Absrr. 112(1990)
178801 f; V. D. She1udyakov.V. 1. Zhun, M. I. Shumilin, V. N. Bochkarev,
T. F. Slyusarenko, Zh. Obshch. Khim. 58 (1988) 1583; Chem. Abstr. 110
(1989) 212906q; K. Tamao, K. Maeda, T.Yamaguchi, Y. Ito, J. A m . Chem.
Suc. 111 (1989) 4984.
(21 a) B. A. Trofimov, S. E. Korostova, S. G. Shevchenko, E. A. Polubentsev,
A. I. Mikhaleva, Zh. Org. Khim. 26 (1990) 1110; J. Org. Chem. USSR Engl.
Trunsl. 26 (1990) 956; b) B. V. Trzhtsinskaya, N. D. Abramova, E. V.
Rudakova, A. V. Afonin, V. V. Keiko. Izv. Akad. Nauk SSSR, Ser. Khim.
1988, 1882; Chem. Absrr. 110 (1989) 231 516d.
[31 See, for example: a) F. L. Wang, W. Ueda, Y. Morikawa, T. Ikawa, Chem.
Lett. 1989, 281; b) K. Karabelas, A. Hallberg, J. Org. Chem. 53 (1988)
4909; c) A. Arcadi, E. Bernocchi, A. Burini, S. Cacchi, F. Marinelli, B.
Pietroni, Tetruhedron Lett. 30 (1989) 3465, and references cited therein.
[4] P. J. Stang, M. Hanack, L. R. Subramanian. Synthesis f982, 85.
[5] a) P. J. Stang, Z. Rappoport, M. Hanack, L. R. Subramanian: Vinvl
Cations. Academic Press, New York 1979; b) P. Vogel: Carbucarion Chemistry, Elsevier, Amsterdam 1985.
[6] J. M. Brown, N. A. Cooley. Chem. Rev. 88 (1988) 1031, and references
[A For some recent papers, see: a) P. 0. Stoutland, R. G. Bergman, J Am.
Chem. Sot. 110 (1988) 5732; b) J. Martinez, J. B. Gill, H. Adams, N. A.
Bailey, J. M. Saez, G. J. Sunley, P. M. Maitlis, J. Organornet. Chem 394
(1990) 583.
VerlagsgeselischaftmbH, W-6940 Weinheim, 1991
[8] Some recent examples: a) P. 0. Stoutland, R. G. Bergman, J. Am. Chem.
Soc. 107 (1985) 4581 ;b) T. W. Bell, D. M. Haddleton, A. McCanley, M. G.
Partridge, R. N. Perutz, H. Wilber, ibid. 112 (1990) 9212.
[9] The formation of bis(etheny1)phenyliodonium hexachlorostannate from
trichloro(viny1)tinand iodosobenzene dichloride in 5 % yield was reported
previously. A. N. Nesmeyanov, T. P. Tolstaya, A. V. Petrakov, Dokl.
Akad. Nauk SSSR 197 (1971) 1337; Dokl. Chem. Engl. Transl. 197 (1971)
343. Several other counterions were introduced by anion exchange.
[lo] Compound 2, a mild iodonium transfer reagent, was developed recently:
V. V. Zhdankin, C . M. Crittell, P. J. Stang, N. S . Zefirov, TetrahedronLeu.
31 (1990) 4828. It offers an elegant way to prepare several new classes of
alkynyl(pheny1)iodonium triflates [I 11.
[I I] a) P. J. Stang, V. V. Zhdankin, J. Am. Chem. SOC.112 (1990) 6437; ibid. 113
(1991) 4571; b) P. J. Stang. J. Ullmann, Synthesis 1991, in press.
[12] a) J. P. Collman, C . T. Sears, Inorg. Chem. 7 (1968) 27; b) J. M. Jenkins,
B. L. Show, J. Chem. SOC.1965,6789.
[13] F. R. Hartley, S. Patai (Eds.): The Chemistry of the Mela1 Carbon Bond,
Wiley. London 1985.
1141 a) Z. Rappoport, R e d . Trav. Chim. Pays-Bas f04 (1985) 309; b) Z. Rappoport, Acc.Chem. Res. 14(1981)7;c)P. J.Stang,A. K.Datta,J. Am.Chem.
Suc. 111 (1989) 1358.
[l5J D. Seyferth, F. G. A. Stone, J. Am. Chem. Soc. 79 (1957) 515.
[16] Powder, dispersed between NaCl plates.
[17] The parent vinyl nonaflate was obtained by reacting nonafluorobutanesulfonic acid with acetylene. E. Eckes, L. R. Subramanian, M. Hanack, Tetrahedron Lett. 1973, 1967. No yield was reported.
[18] a) D. Evans, J. A. Osborn, G. Wilkinson, lnorg. Synth. 1f (1968) 99; b) K.
Vrieze, J. P. Collman, C . T. Sears, M. Kubota, ibid. 11 (1968) 101.
Fischer Carbene Complexes Facilitate
the Intramolecular Pauson-Khand Reaction **
By Francisco Camps, Josep M . Moreto,* Susagna Ricart,
and Josep M . Viiias
The [Co,(CO),]-mediated carbonylative cycloaddition of
acetylenes and olefins (Pauson-Khand reaction) is one of the
most straightforward methods for preparation of cyclopentenone derivatives.['] This procedure has been successfully
applied to the synthesis of a variety of bioactive natural
products.[*] Recently, the first efficient enantioselective synthesis of a hirsutene precursor has also been reported.[31
However, there are still some unsolved drawbacks that
limit the scope of application of the Pauson-Khand reaction.
For example, low yields are often obtained owing to the high
temperatures required to further activate the alkyneCo,(CO), intermediate complex, which, in addition, may
preclude the presence of certain thermally labile moieties in
the starting reagents. To overcome these problems, different
modifications have been put forth such as the use of silica
gel,'41 nitrogen oxides,['' sonication,'61 or UV irradiation!']
Yields can be substantially improved in intramolecular reactions.[2-5' *'As an example of its potential practical importance, an efficient catalytic synthesis of jasmone by application of this reaction has been recently pubIished.["
In connection with our current studies of the applications
of Fischer carbene complexes to organic synthesis, we have
proved, in agreement with results described by other authors,"" that a triple bond conjugated to a transitionmetal
carbene moiety becomes highly polarized, resulting in easy
[*I Dr. J. M. Moreto, Prof. Dr. F. Camps, Dr. S. Ricart, Dr. J. M. Vifias
Departament de Quimica Organica Biologica
Consell Superior d'lnvestigacions Cientifiques
Centre d'lnvestigacio i Deseuvolupament
J. Girona. 18-26, E-08034 Barcelona (Spain)
[**I This work was supported by Direccion General de Investigacion Cientifica
y T&nica (DGICYT) (Project No. PB87-0201-C-03-03). J. M. V thanks
the Spanish Ministry of Education and Science for a postdoctoral fellowship.
0570-0833/91/1111-1470 3 3.50+.25/0
Angew. Chem. Int. Ed. Engl. 30 (1991) No. I 1
cycloadditions and Michael additions with a variety of substrates." '] We anticipated that this approach might be used
to facilitate the intramolecular Pauson-Khand reaction by
activation of the corresponding acetylenic moiety in a similar
strategy to that reported for analogous olefin complexes in
Diels-Alder cycloadditions.['
When alkynyl allylamino carbene complexes 1 a-d were
treated with one equivalent of [Co,(CO),] in dry THFfor a
.few hours at room temperature, the expected cyclopentenone
derivatives 2a-d were obtained in 70-75% yields. Moderate yields of tricyclic derivative 2e were obtained by performing the reaction with cyclohexenylamine complex 1e
(Table 1).
K O (CO) 1
A (CO),M=
perature and the [Co,(CO),]-amide complex 5 was obtained
instead of the expected cycloadduct 6.
A few remarkable points deserve further comment. Good
yields of cycloaddition products are obtained despite the fact
that disubstituted acetylenes bearing electron-withdrawing
substituents (the metal-carbene group may be considered to
fall into this category) usually give mainly linear diene adducts and low yields of cycloadducts.[" 131 Furthermore, the
reaction occurs at room temperature with high stereoselectivity (in cases 2d and 2e the formation of only one stereoisomer was detectedtl4I). This stereoselectivity control might be
expected from the defined coordination geometries around
both metal centers.
In conclusion, we have proved that intramolecular Pauson-Khand reactions can be efficiently performed on a
metal-carbene-activated enyne derivative in a stereoselective
manner under very mild conditions. The resulting cycloadducts still bearing the Fischer carbene functionality may be
used to carry out further reactions (such as benzannulation['51 or cyclopropanation["jl).
Experimental Procedure
Table 1. Pauson-Khand reaction at alkynyl allylamino carbene complexes 1 to
give cyclopentenone derivatives 2.
Enyne complex
Reaction time [h]
Yield [YO]
Surprisingly, unlike the report of previous authors on high
yields of Pauson-Khand cycloadducts from trimethylsilylacetylenic derivatives," 2b3',gl carbene complex 1 f afforded
only [Co,(CO),]-alkyne complex 3 in good yields. We attribute this failure mainly to the steric encumbrance of the
trimethylsilyl group, which should push the olefin moiety
towards coordination with the transition metal of the carbene.
The activating role of the carbene-metal functionality was
made clear by an independent experiment in which one
equivalent of the enyne amide was stirred with one equivalent of [Co,(CO),] in dry T H F for three days at room tem-
[Co,(CO),] (1 mmol) was added to the allylamino carbene complex ( I mmol) in
25 mL of dry TH F and the reaction mixture was left at room temperature for
2-12 h (Table 1). The reaction course was monitored by thin layer chromatography (hexanelethyl acetate 2/1). After the starting product had completely
disappeared, the solvent was removed and the residue passed through a flash
chromatography column using the mentioned eluent. The cyclopentenone complexes crystallize upon concentration. All compounds (2a-e, 3) gave satisfactory spectroscopic data and elemental analysis. Those for 2 d and 2 e are given as
representative: 2d: IR (CHCI,): F [cm- '1 = 2058(m), 1989(w), 1943(vs),
1925(sh), 1755(m), 1725(m); 'H NMR (300 MHz, CDCI,): 6 = 9.05 (br. s, 1 H),
7.45 (m, SH), 4.12 (ddd, J = 2.1, 8.4, 11.4 Hz, 1 H), 3.53 (dd, J = 8.1, 11.4 Hz,
1H), 3.25 (ddd, J = 4.2, 8.4, 8.1 Hz, 1H), 2.54 (dq, J =7.3, 4.2 Hz, 1 H), 1.39
(d, J =7.3 Hz, 3H); ',C NMR (75.4 MHz, CDCI,): 6 = 232.9, 209.7, 202.7,
198.2, 178.1, 143.0,,, 128.5, 59.6,51.4,47.6, 13.7.2e:
IR (CHCI,): P [cm-'1 = 2060(m), 1970(w), 1920(vs), 1705(m); 'H NMR
(80 MHz, (CD,),CO): 6 = 9.02 (br.s, 1 H), 7.45 (m,5H), 4.25 (dd, J = 8,
16 Hz, 1 H),3.5(t,J = 8 Hz, 1 H), 3.0(dd, J = 8,16 Hz, 1 H), 1.1-2.8(m,6H);
I3C NMR (75.4 MHz, CDCI,): 6 = 363.3, 210.9, 201.9, 197.5, 174.9, 145.0,
130.3, 130.2, 129.5, 128.4, 63.8, 45.7, 43.1, 31.2, 25.8, 20.0.
Received: June 3, 1991 [Z 4683 IE]
German version: Angew. Chem. 103 (1991) 1540
[CO (CO) 1
,( c o ) , w = c <
Angew. Chem. Int. Ed. Engl. 30 (1991) No. 11
CAS Registry numbers:
l a , 136658-86-1; 1 b, 136658-87-2; l c , 136675-75-7; Id, 136658-88-3; l e ,
136658-89-4; If, 136658-98-7; 2a, 136658-91-8; 2b, 136658-92-9; 2c, 13665893-0; 2d, 136658-94-1; 2e, 136675-76-8; 3, 136658-95-2; [Co,(CO),], 1021068-1.
[I] a) P. L. Pauson, Tetrahedron 41 (1985) 5855; b) N. E. Schore, Chem. Rev.
88 (1988) 1081.
[2] a) N. E. Schore, E. G. Rowley, 1 Am. Chem. SOC.110 (1988) 5224; b) P.
Magnus, L. M. Principe, M. J. Slater, J. Org. Chem. 52 (1987) 1483; c) P.
Magnus, C. Exon, P. Albaugh-Robertson, Tetrahedron 41 (1985) 5861 ;
d) D. C. Billington, P. L. Pauson, Orgunometallics I (1982) 1560; e) D. C.
Billington, Tetrahedron Lett. 24 (1983)2905; f) J. Mulzer, K. D. Graske, B.
Kirste, Liebigs Ann. Chem. 1988,891; g) P. Magnus, D. P. Becker, J. Am.
Chem. SOC.109 (1987) 7945; h) H. Jaffer, P. L. Pauson, J. Chem. Res. (5')
1983, 244.
[3] J. Castro, H. Sorensen, A. Riera, C. Morin, A. Moyano, M. A. Pericas,
A. E. Greene, J. Am. Chem. SOC.112 (1990) 9388.
(41 S. 0.Symonian, W. A. Smit, A. S . Gybin, A. S. Shashkov, G. S. Mikaelian, V. A. Tarasov, I. I. Ibragimov, R. Caple, D. E. Froen, Tetrahedron
Lett. 27 (1986) 1245.
VerlagsgesellschafrmbH. W-6940 Weinheim. 1991
[5] S. Shambayati, W. E. Crowe, S. L. Schreiber. Trrruhedron Lert. 31 (1990)
[6] D. C. Billington, I. M. Helps, P. L. Pauson, W. Thomson. D. Willison, 1
Orgunomer. Chem. 354 (1988) 233.
[7] S. C. Brown, P. L. Pauson, 1 Chem. Sor. Perkin Trans. I 1990, 1205.
[S] N. E. Schore, M. C. Croudace. J; Org. Chem. 46 (1981) 5436.
[91 V. Rautenstrauch. P. Megdrd, J. Conesa. W Kuster. Angew. Chem. 102
(1990) 1441; .4ngew. Chem. Inr. Ed. Engl. 29 (1990) 1413.
[lo] W. D . Wulff, K. S. Chan. J. A m . Chem. SOC. I08 (1986) 5229.
[I I ] a) F. Camps, J. M. Moreto. S. Ricart. J. M. Vifias, E. Molins, C. Mirdvitlles, J; Chrm. So<.Chem. Commun. 1989.1560; b) F. Camps, A. Llebaria,
J. M. Moreto, S. Ricart, J. M. Visas, Tefruhedron Leu. 31 (1990) 2479;
c) K. L. Faron, W. D. Wulff, J. Am. Chem. Soc. 110 (1988) 8727; d) E. 0.
Fischer. H. J. Kalder. J. Organomer. Chem. 131 (1977) 57; e) F. Camps, A.
Llebaria, J. M. Moreto, S. Ricart. J. M. Vifias, J. Ros, R. Yafiez, ibid. 401
(1991) C 17; f ) H. Fischer. T. Meisner, J. Hofmann, Chem. Ber. 123 (1990)
[I21 K. H. Dotz, R. Noack, K . Harms, G. Muller, Tixruhedron46 (1990) 1235.
1131 M. E. Krafft, C. A. Juliano. 1. L. Scott. C. Wright, M. D. McEachin. J.
Am. Chem. SOC.113 (1991) 1693.
[14] The structural assignment has been made on basis of N M R data as well as
structural and mechanistic considerations. Particular significance has been
given to the different coupling constants for 2 d and 2e for the bridgehead
proton with the vicinal =-ketonic proton. These values ( J = 4.2 Hz for 2 d
and J = 8 Hz for 2e). although slightly higher than those reported by
Schore et al. for related organic structures (N. E. Schore, M. J. Kundsen,
J. Org. Client. 52 (1987) 569). are complementary and in the same order of
assignment (the larger for the cis-vicinal coupling constant and the smaller
for the corresponding fruns).Furthermore, the stereoisomers thus obtained
are those which are to be expected starting from a /ran.? olefin (E-crotylamine) and a c i . ~one (2-cyclohexenylamine) in a stereospecific process.
[IS] K. H . Dotz. Angew. Chem. 96 (1984) 573; Angew. Chrm. I n [ . Ed. Engl. 23
( I 984) 587.
[16] A. Wienandt. H. U. Reissig. Orgunomerullics 9 (1990) 3133. and references
Crown Ethers with a Lewis Acidic Center:
A New Class of Heterotopic Host Molecules**
By Manfred 7: Reetz,* Christof M . Miemeyer,
and Klaus Harms
Many cation-selective crown ethers and cryptands are
known,"' but only a few host compounds bind anions selectively. Naturally, the latter are electronically inverse macrocycles that usually contain suitably positioned protonated or
quaternary nitrogen functions.c21Here we report a new class
of anion-selective receptors, 2, which in addition to a conventional crown ether moiety for complexation of cations,
contain a o-bonded Lewis acidic metal center-in this case,
boron- for complexation of anions. Our concept is based
on the expectation that, for example, potassium salts, KX
(X = F, CI, Br, I, SCN, CN, OCH,) should be able to bind
to the host 2 in either a monotopic (3) or a heterotopic (4)
fashion. The selectivity would then be governed by quite
diverse factors, such as the strength of the B-X bond in the
ate complex 4, the lattice energy of the salt KX, Coulomb
and van der Waals interactions, and solvent effects.
Since potassium salts were to be investigated first, we
chose the 21-membered crown ether 2, which contains six
oxygen atoms. The aryl bromide 1 a was used for its synthes ~ s . ' The
~ ] corresponding aryllithium compound was borylated with B(OCH,), and the crude product was hydrolyzed to
Prof. Dr. M. T, Reetz ['I, DipLChem. C . M. Niemeyer, Dr. K. Harms
Fachbereich Chemie der Universitlt
Hans-Meerwein-Strasse, W-3550 Marburg (FRG)
[ '1 New address: Max-Planck-Institut fur Kohlenforschung
Kaiser-Wilhelm-Platz. W-4330 Mulheim a.d. Ruhr (FRG)
[**I This work was supported by the Deutsche Forschungsgemeinschaft (Leibniz-Program) and the Fonds der Cheniischen Industrie.
VCH Verlugsgesellschafi mbH, W-6940 Weinheim, 1991
1. tBuLi
la, R = Br
lb. R = H
CI; c, X = B r ; d, X = I
e, X = SCN; f, X = CN; g, X = O C H , ;
a, X = F; b,
give the arylboronic acid.151Treatment of the acid with catechol finally led to the target compound 2 (> 95 % overall
yield), which is stable and easy to handle under an inert
In the first complexation experiment, host 2 was added to
a suspension of dry K F (excess) in dichloromethane at room
temperature. A stoichiometric amount of the otherwise insoluble salt dissolved within 4 h with quantitative formation
of adduct 4a. This is noteworthy since potassium-specific
crown ethers such as [18]~rown-6[~]
or 1,3-xyly1[21]crown-6
(1 b)"] complex at most catalytic amounts of KF.I*] In the
case of 4a, the strength of the B-F bondcg1is decisive. The
usual interaction between K0 and the oxygen atom of the
crown ether acts synergistically, since a control experiment
with K F and the catechol ester of phenylboronic acid afforded no adduct."'. ' ' ]
The host-guest compound 4 a was investigated in solution
particularly by "B and I3C N M R spectroscopy. The "B
N M R spectrum (CD,CI,) at room temperature with BF,ether as external standard shows a single signal at 6 = 10.0,
which means a shift of A6 = 20 to higher field compared
with the signal of the uncomplexed host 2 (6 = 30). This is
characteristic of compounds with tetracoordinated boron.['21The participation of K Q as guest is shown by a comparison of the ',C N M R spectra of the free host 2 and the
adduct 4a (Fig. 1). The low-field shift of the signals of the
benzylic C atoms (C7 and C18) and the high-field shift of the
other ether C atoms are definite indications of cation binding.c'31The fact that the spectra of mixtures of compounds
2 and 4a exhibit sharp signals shows that, under these conditions, rapid exchange processes d o not occur on the N M R
time scale. The "F N M R (6 = - 124.4; CFCI, as external
standard) and the 'H N M R spectrum are also consistent
with structure 4a.
Figure 2 shows the result of an X-ray structure analysis of
4a.[I4]The inclusion of K 0 and F' is clearly evident (Fig. 2,
top), as are intermolecular interactions between the units of
the host-guest compound (Fig. 2, bottom). The "unsymmetrical" structure (Fig. 2, top) could be due to a crystal packing
effect, since the ',C N M R spectrum of 4a in solution does
not show a double set of signals in the range of 40°C to
- 80 "C.
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intramolecular, reaction, carbene, khanh, complexes, pauson, fischer, facilitates
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