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DBU and DBN are Strong Nucleophiles X-Ray Crystal Structures of Onio- and Dionio-Substituted Phosphanes.

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very similar trifluoroacetic acid favors the formation of the
(S)-isomer (entry 2). Evidently, the lactone is protonated by
two different reaction mechanisms. Thus, this is a further
example of a reversal of stereoselectivity on changing from a
methyl to a trifluoromethyl group in a reaction partner."'
The results can be explained if it is assumed that owing to
the nucleophilicity of the carbonyl oxygen atom, the acetic
acid forms a dative bond with the boron atom. This adduct
transfers the acidic proton to the enol in a cyclic transition
state, for example 5, whereby the (R)-isomer is formed. A
similar cyclic transition state was also postulated by Brown
et al. for the protolysis of B-crotyl-9-borabicyclo[3.3.I]n0nane.[~I
In contrast, trifluoroacetic acid (TFA) protonates the enolate without prior binding at the boron atom. The reason for
the reduced capability for coordination of TFA is the reduced nucleophilicity of the carbonyl oxygen atom due to
the accepting ability of the trifluoromethyl group. Thus, in
contrast to the IR spectra of trifluoroacetoxydibutylborane,
the IR spectra of acetoxydibutylborane indicate an intramolecular coordination of the carbonyl oxygen atom to
the boron atom.['*] Interestingly, the reaction rates for the
protolytic cleavage of the B-C bond are less influenced by
the acidity of the acid than by its nucleophilicity. Thus, the
protolyses with trifluoro- and trichloroacetic acid occur extremely slowly. whereas carbonic acids which can coordinate
to the boron atom cause cleavage very rapidly.[' Indeed,
after the protonation of 4 with trichloroacetic acid, (S)-2 is
enriched even more (entry 3).
Evidently, noncoordinating acids directly attack the enol
b-C atom of 4 preferably from the less shielded si side. In
contrast, proton sources capable of coordinating at the
boron atom form an adduct with 4, from whose re side the
proton is transferred. Thus, different protonation mechanisms can be used in such a way that with the same chiral
auxiliary opposite stereoselectivities are achieved." '1
Sterically less demanding alcohols (methanol, trifluoroethanol) protonate 4 unselectively. The stopping of the reaction with acetic acid and with TFA both produce the same
result-a racemic mixture (entries 4,5). Thus, the protonation has occurred before the addition of the acids. The same
applies for the more acidic trifluoroethanol (entry 6). In contrast, the bulky rert-butyl alcohol is inert towards 4.This i s
revealed by the change in configuration on addition of TFA
and AcOH (entries 7,8). Thus, if a reaction is stopped with
acetic or trifluoroacetic acid this allows a simple examination of the effect of a proton source. Stereoselective protonation of 4 is observed with P-dicarbonyl compounds. Strangely, the configuration of the product changes with the transfer
from acetylacetone ((S)-2,46% ee) to acetoacetic ester ((R)2, 19 YOP P , en tries 10,1I ).
The experiments with amines (entries 12-22) reveal that
primary and secondary amines influence the stereoselective
protonation considerably (tertiary amines have no or only a
weak effect), without functioning as a proton source. The
different effects of the antipodes of chiral amines indicate
that amines are inserted-probably via noncovalent interactions-in the transition state. Direct coordination of amines
with the boron enolate 4, however, seems ruled out: ParticuAngc'ii. ( % ~ n r . In/. Ed. End. 1993, 32, No. 3
larly highly shielded 2,2,6,6-tetramethylpiperidine (TMP)
shows the strongest effects (76% re, entry 19). For the protonation of 3 with acetic acid, under the addition of TMP.
(R)-1 could be enriched to almost 80% ee! A reaction with
chlorodiisopinocampheylborane 5 in excess cannot serve as
an explanation for the amine effect. It is already known that
5 does not add sterically hindered amines.I6]
A further optimization of the auxiliary-induced stereoselective protonation via boron enolates can be expected from
the variation of the chiral boron moiety. Thus, the conformatively flexible boron diisopinocampheyl group could be
substituted by cyclic, chiral, C , symmetrical boron ligands,
as has already proved successful for stereoselective aldol rea c t i o n ~ . '1[ ~ ~ ,
Protonation of the boron enolates 4 applied can be described as an auxiliary-induced stereoselective reaction. Normally. in such reactions the auxiliary is only removed in a
subsequent reaction. Thus, for example, in the previously
mentioned aldol addition13] the boron-containing auxiliary
is first seized by the alcoholate function formed and subsequently removed by hydrolysis.
The protonation described herein, however, belongs to the
rare examples in which the construction of the new stereocenter is coupled with the removal of the auxiliary- an important step of enantioselective catalytic synthesis.
Received: November 19. 1992 125691 IE]
German version: A n p i . Chrm. 1993. I05. 443
[I] H. Waldmann. Nuchr. Chem. Tech. Lab. 1991, 39. 413-418.
[2] M. W. Andersen, B. Hildebrandt, G. Dahmdnn, R. W. Hoffmdnn. Chem.
5er. 1991, 124. 2127-2139.
[3] a) I. Paterson, J. M. Goodman, M. A. Lister, S. M. Riseman. I. Shinkai,
Tetruhedron 1990, 46. 4663-4684; b) S. Masdmune, T. Sato, B M. Kim.
T. A. Wollmann, J. Am. Chem. Suc. 1986, 108,8279 -8281: c) M. T. Reetz.
F. Kunisch. P. Heitmann, Te/ruhedron Leit. 1986. 27, 4721 -4724.
[4] H. C. Brown, R. K. Dhar, K. Ganesan, B. Singaram, J. Org. Clwm. 1992,
57, 27 16- 2721
[5] We thank Prof. Dr. Wrackmeyer, UniversitPt Bayreuth, for enabling us to
carry out the measurements and for his help with the interpretation.
[b] H. C. Brown. J. Chdndrasekharan, V. Ramachandrdn, J. A m . Chem. Soc.
1988, 110, 1539-1546.
171 H. C. Brown, R. K. Dhdr. K. Gdnesan, B. Singdram. J. Urg. Clirm. 1992,
[8] M. Gautschi, D. Seebach, Angew. Chem. 1992, 104, 1061-1062: Angni
Chem. In!. Ed. EngL 1992. 31. 1083-1085.
191 G. W. Kramer, H. C. Brown, J. Orgunomef. Chem. 1977, 132.9-27.
1101 L. A. Duncanson. W. Gerrdrd. M. F. Lappert, H. Pyszora, R. Shafferman.
J. Chem. Sot. 1958, 3652-3656.
I l l ] L. H. Torporcer, R. E. Dessy, S. I. E. Green, J. Am. Clirm. Sor. 1965.87,
1236- 1240.
[12] D. Seebach. Angew. Chem. 1990. 102, 1363-1409; Angew Chew /nt. Ed.
Engl. 1990, 29, 1320-1367.
DBU and DBN are Strong Nucleophiles:
X-Ray Crystal Structures of Onio- and
Dionio-Substituted Phosphanes**
By Robert Reed, Rkgis R ~ a uFrangoise
Dahan, and
Guy Butrand*
Base-induced intra- or intermolecular dehydrohalogenation reactions play a key role in organic and inorganic chemistry. Since the 1970s the efficiency of DBU (1,8-diazabi[*I Dr. G. Bertrand, Dr. R. Reed, Dr. R. Reau, Dr. F. Dahan
Labordtoire de Chimie de Coordination du CNRS
205. route de Narbonne, F-31077 Toulouse Cedex (France)
[**I This work was supported by the Centre National de la Recherche Scientifique.
C VCH firlugsgesell.~cIiu~
mhH, W-6940 Weinheim. 1993
0570-0833/9310303-1)399 si 10.00+ .25/0
cyclo[5.4.0]undec-7-ene)and DBN (1,5-diazabicyclo[4.3.0]non-Sene) has been widely demonstrated,['] although many
authors have reported unexplained phenomena."' In this
paper we report that these bicyclic amidines, known as "nonnucleophilic strong bases"". 31 can behave as strong nucleophiles ; X-ray crystal structures of onio- and dionio-substituted phosphanes (Weiss-type compounds)[41are presented.
In dichloromethane solution chlorobis(diisopropy1amino)phosphane (1) and DBN or DBU are in equilibrium
with the cationic phosphanes Za or 3 a . This equilibrium is
shifted towards the products in acetonitrile, or by exchanging the chloride for hexafluorophosphate to yield 2 b and
3 b.IS1
n = 1 DBN
R =iPr
n = 2 DBU
imize the steric interactions. The P1 -N3 and P1 -N4 bond
lengths [1.666(4) and 1.661(4) A] are classical, longer than
those observed in phosphanylium ions where 2pn-3pn interaction between the phosphorus and the nitrogens atom takes
place.171Of particular interest are the very long P1 -N1 bond
[1.796(3) A] and the almost equal C1 -Nl [1.322(5) A] and
Cl-N2 [1.298(5) A] bond lengths. The structure of this
cationic phosphane is comparable to that of borylium ions in
which a cationic bivalent boron atom is stabilized by an
electron pair donor and the positive charge is delocalized on
the cluster.'']
As expected, a similar ionic adduct 2c can be obtained by
treating bis(diisopropylamino)phosphanylium tetrachloroaluminate['] with DBN. In addition, the free bis(diisopropy1amino)phosphanylium salt can be cleanly regenerated by the
addition of one equivalent of borontrifluoride-etherate complex to 2 b.
Most importantly, the nucleophiiic behavior of DBU and
DBN is very general (Table 1) and also allows the preparation of dionio-substituted phosphanes 6-9, which formally
result from the interaction of two DBU or DBN molecules
with monocoordinated phosphorus dications (RP").
Table 1. Spectroscopic data of the compounds (R'R*R'P)'PF,
(R'R2R3P)2'2PF; 6b-9b [al.
The 31PNMR chemical shifts observed for these compounds [6 = 108 (Zb), 107 (3b)l are consistent with triaminophosphanes; furthermore in the 3C NMR spectra,
phosphorus coupling to the bridgehead carbons ['JC, =
31.I (2 b) and 21.1 Hz (3 b)] confirms that the bicyclic amidines are bound to the phosphorus. To establish the exact
nature of the interaction between the amidine and the phosphorus atom, a single crystal X-ray diffraction study was
performed on 2b.16] The ORTEP view (Fig. 1 ) reveals the
phosphorus atom in a pyramidal environment in which the
three substituents are arranged in a propeller fashion to min-
Fig. 1. Structure of cation 2 b (ORTEP). Selected bond lengths [A] and angles
1.796(3), P L N 3 1.661(4). P1-N4 1.666(4). Nl-Cl 1.322(5), C L N 2
1.298(5); NI-Pl-N3 101.6(2), Nl-Pl-N4 99.1(2), N3-Pl-N4 109.3(2), P1-NlC1 120.3(3). Pl-Nl-C7 124.0(3), C7-Nl-Cl 114.9(3), N1-Cl-C2 123.7(4), N2C1-C2 110.0(4), Nl-C1-N2 126.2(4). Cl-N2-C5 123.7(4), Cl-N2-C4 115.3(3),
C4-N2-C5 120.9(4).
Verlagsgesellschafl mbH, W-6940 Weinhrim. 1993
Cy2N [d]
Cy,N (d]
S3'P [b]
[ O h ]
2b-5b and
Yield M.p.
[c] [ C l
[a] The spectroscopic data of compounds 2a-9a (isolated as viscous oils, CI- as
counter anion) and 2 c (AICI, as counter anion) are essentially identical to 2 b 9 b
(PF; as counter anion). [b] Spectra of cations in CDCI, solution. dications in
CD,CN solution. Coupling constants are in Hz. [c] Yields after recrystallization.
The yields determined by "P NMR spectroscopy are essentially quantitative; no
other phosphorus products were observed. (dl Cy = cyclohexyl. [el Compound not
The X-ray crystal structure of 6 b is shown in Figure 2.['01
The PI -N1 and PI -N3 bonds are rather long [1.766(5) and
1.752(4) A], and the C-N bond lengths of the NCN moieties
[I .339(6), 1.304(8) and 1.334(7), 1.290(7) A] are between
single and double bonds. Here also, the positive charges are
strongly delocalized.
This new understanding of the behavior of DBU and
DBN towards halogenated derivatives of main group elements" 'I led us to reconsider the mechanism of dehydrohalogenation in the presence of these cyclic amidines. For
example, addition of DBU to a mixture of chlorodiphenylphosphane and diphenylphosphane gives the tetraphenyldiphosphane. In fact, no reaction between diphenylphosphane and DBU is observed by 3 1 P N M R spectroscopy,
whereas adduct 5 a reacts with diphenylphosphane to yield
the diphosphane quantitatively.
In contrast to what is generally admitted, these results
show that DBU and DBN can act as strong nucleophiles.
This new understanding will help to clarify the superior utility and frequently observed unexplained behavior of these
highly popular reagents in "base-induced'' dehydrohalogenation reactions.
3 10.00f .25/0
Angen. Chem. Int. Ed. Engl. 1993. 32, N o . 3
er). Thieme, Stuttgart, 1990, S. 129-148; A. H. Cowley. M. C. Kemp,
Chem. Rev. 1985.85, 367-382.
[XI P. Kolle, H. Noth, Chem. Rev. 1985, 85, 399-418.
[9] A. H. Cowley, M. C. Cushner. J. S. Szobota, J. Am. Chem. Soc. 1978, IOO.
[lo] Crystal data for 6b: Space group P2,/n, a =17.341(2), b = 9.715(1),
c = 24.032(2) A, p =109.99(1)", V = 3805(1) A3, Z = 4; 5950 measured
reflections, 3294 independent reflections observed [Fi > 3o(F:)], 289 refined parameters, R(F,) = 0.046 (R, = 0.048). Further details of the crystal structure investigation may be obtained from the Fachinformationszentrum Karlsruhe, Gesellschaft fur wissenschaftlich-technischeInformation mbH, D-W-7514 Eggenstein-Leopoldshafen 2 (FRG). on quoting the
depository number CSD-56813, the names of the authors, and the journal
[I 11 Preliminary experiments show that DBU and DBN can act as nucleophiles
towards a variety of halogenated compounds of main group elements.
Fig. 2. Structure of cation 6 b (ORTEP). Selected bond lengths [A] and angles
[ I : PUN1 1.766(5). PGN3 1.752(4), P1-N5 1.628(5), N1-Cl 1.339(6). C1-N2
1.304(8). N3-CX 1.334(7), C8-N4 1.290(7); Nl-Pl-N3 95.9(2), Nl-PI-NS
104.7(2). N3-Pl-NS 106.5(2), P1-N1-Cl 117.2(4), PI-NI-C7 125.1(3). Cl-NIC7 116.3(5). Nl-Cl-N2 123.9(5), Nl-CbC2 125.6(6), C2-CI-NZ 110.5(5). C1N2-C4 113.6(5). Cl-N2-C5 125.3(4), C4-N2-C5 121.1(5).
Perspirocyclopropanated I31Rotane-A Section
of a Carbon Network Containing
Spirocyclopropane Units?**
By Sergei I . Kozhushkov, Thomas Haumann, Roland Boese,
and Armin de Meijere*
Dedicated to Professor Emanuel Vogel
on the occasion of his 65th birthday
Experimental Procedure
Representative synthesis of compounds 2a-9b, example 2 a and 2b: An acetonitrile solution (20 mL) of DBN (5.6 mL, 45 mmol) was added dropwise at
room temperature to an acetonitrile solution (20 mL) of chlorobis(diisopropy1am1no)phosphane 1 (12.0 g, 45 mmol) to afford 2 a as an extremely hygroscopic. viscous oil. , ' P NMR (32 MHz, CH,CN, 35°C): 6 =106.7; 13CNMR
(50MHz. CDCI,, 25°C): 6=18.8 (s, CH,), 19.5 (s. CH,), 23.8 (t.
'J(C,P) = 7 Hz. CH,), 31.2 (d. 'J(C.P) = 27 Hz, CH,), 42.5 (s, CH,), 43.6 (s,
CH2).47.5(d, '4C.P) =12 Hz, CH), 53.8(s,CH2), 165.1 (d, 'J(C,P) = 31 Hz.
NCN). This solution was added slowly at 0°C to an acetonitrile solution
(20 mL) of KPF, (8.5 g, 46 mmol) and stirred for 24 hrs. The precipitated KCI
was filtered off. and the solvent removed under vacuum. Compound 2b was
crystallized at -40°C from CH,CI,/Et20 (14.5 g, 64%). "P NMR (32 MHz,
CDCI,. 35'C): b =108.4, -143.6 (sept, 'J(P.F) =709 Hz); 13C NMR
(50 MHz, CDCI,, 25°C): 6 = 18.7(d, 'J(C,P) = 4 Hz, CH,), 19.3 (s, CH,), 23.8
(t. 3J(C.P) = 7 Hz. CH,), 31.2 (d, 2J(C,P) = 27 Hz, CH,), 42.5 (d,
'4C.P) = 3 Hz, CH,). 43.1 (s, CH,), 47.9 (d. 'J(C,P) =14 Hz, CH). 53.5 (s,
CH,), 165.4 (d. 'J(C,P) = 31 Hz, NCN).
Twenty years ago the synthesis[', '1 of [3]rotane
have been seen as the end of the development of highly
strained cyclopropane derivatives with unusual struct ~ r a l [ ~ - and
* ] spectroscopic properties.[g1However, more
like 2" - 13]
recent reports on branched [n]triang~lanes['~]
and 3['2.131containing a [3]rotane subunit show that 1 is
actually the starting point of a larger group of star-shaped,
branched hydrocarbons with nothing but spirocyclic linkages between three-membered rings. Accordingly, the perspirocyclopropanated [3]rotane 4 ) I 4 I a [lO]triangulane['O1
with D,,symmetry, which we describe here, is also just another peak on a possible route to higher summits.
Received: November 6, 1992 [Z 5669 IE]
German version: Angeu. Chem. 1993, 105.464
H. Oedidiger, F. Moiler, K. Eiter, S-ynthesis 1972, 591-598; I. Hermecz,
Ad]'.Hrterocycl. Chem. 1987.42,83; A. H. Cowley, Arc. Chem. Res. 1984,
17. 386-392.
C. N. Smit, Dissertation, Vrije Universiteit te Amsterdam, 1987; T. C.
1978, fO0,4886Klebach, R. Lourens, F. Bickelhaupt, J. Am. Chem. SOC.
4888; A. H. Cowley, J. E. Kilduff. J. G. Lasch, S. K. Mehrotra, N. C.
Norman. M. Pakulski, B. R. Whittlesey, J. L. Atwood, W. E. Hunter, Inorg. Chem. 1984, 23. 2582-2593; J. Escudie, C. Couret, H. Ranaivonjatovo. J. Satge. J. Chem. SOC.Chem. Commun. 1984, 1621-1622; T. R.
Juneja, D. K. Garg, W. Schdfer, Terrahedron 1982, 38, 551-556; L. L.
McCoy, D. Ma1.J. Org. Chem. 1981,46, 1016-1018; H. 0. House, M. B.
de Tar, D. Von Derveer, ibid. 1979,44,3793-3800; T. A. Van Der Knapp,
F. Bickelhaupt, Phosphorus Sulfirr 1984.21.227-236; S . Kim, H. Chang,
Bull. Chem. SIC.
Jpn. 1985, 58, 3669-3670.
H. A Muathen, J Org. Chem. 1992, 57, 2740-2743; P. Wolkoff, ibid.
1982. 47. 1944-1948.
R . WeiO. S. Engel. Synthesis 1991, 1077-1079; R. WeiR. S. Engel, Angew.
Chem. 1992. 104, 239-240; Angen'. Chem. Int. Ed. Engl. 1992, 31, 216217.
I t should be noted that chlorophosphane 1 is inert towards triethylamine,
and even towards 4-dimethylaminopyridine.
Crystal data for 2b: space group P2,/n, a =7.877(1), b = 9.638(1),
c = 34.066(3) A. p = 92.62(1)", V = 2583.5(8)
Z = 4; 4540 measured
reflections, 2852 independent reflections observed [F: > 3a(F:)], 280 refined parameters, R(F,) = 0.044 (R, = 0.045).
M. Sanchez. M. R. Maziere, L. Lamande, R. Wolf in Multiple Bonds and
L O NCoordination in Phosphorus Chemistry (Eds.: M. Regitz, 0. J. ScherAngeu. Chem. In!. Ed. Engl. 1993, 32, No. 3
The starting material for the successful route['51 to 4
(Scheme 1, Table 1) was the recently described bi(dispir0[2.0.2.l]heptylidene) 5.["] Cyclopropanation of 5 with two
[*I Prof. Dr. A. de Meijere, Dr. S. I. Kozhushkov
Institut fur Organische Chemie der Universitat
Tammannstrasse 2, D-W-3400 Gottingen (FRG)
Priv. Doz. Dr. R. Boese, DipLChem. T. Haumann
Institut fur Anorganische Chemie der Universitat-Gesamthochschule
This work was supported by the Fonds der Chemischen Industrie. We
thank the companies BASF, Bayer, Hoechst, and Degussa AG for gifts of
chemicals. S.I.K. is grateful to the Alexander von Humboldt Foundation
for a research fellowship.
Vrrlogsgesellschaji mbH, W-6940 Weinheim, 1993
0570-0833/93/0303-040l $10.00+ ,2510
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crystals, dbn, structure, dionio, dbu, strong, phosphanes, onion, substituted, ray, nucleophilic
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