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Isolation and X-Ray Crystal Structure of a Palladacyclobutane Insight into the Mechanism of Cyclopropanation.

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\
d'
-
N
, -Pd
philes
The isreaction
of general
of (~~3-allyl)palladium
synthetic and fundamental,
complexes
mechanistic
with nucleointerest."] We report here on the preparation, isolation, and full
characterization of a palladacyclobutane, the structure of which
affords insight into the mechanism of cyclopropane formation.
-TmA*do-\ok
HCI. -15'C
ON-
lb
2
1
Scheme 1. R = H, aryl. alkyl: R = H. aryl. nor alkyl. LDA = lithium diisopropylamide. TMEDA = N,N,.V',N'-tetrarnethylethylenediamine.
Pdo + TMEDA
2) TMEDA is the most successful ligand and cosolvent-other
bidentate N.N-donors (e.g., sparteine) provide lower yields;
3) added carbon monoxide accelerates and facilitates cyclopropane formation; and 4) the reaction temperature must be
carefully optimized and depends on the pK, and reactivity of the
attacking nucleophile (e.g.. deprotonated esters, amides, sulfones, fluorene, nitriles, and deprotonated ketones). The electrophilicity of the (q3-allyl)palladium complex also varies depending on the structure of the ally1 ligand. The optimum
temperature is in the range - 60 "C to - 15 "C.
It has been suggested that "reductive elimination of cyclopropane is a fast process, as no palladacyclobutane has been
Indeed, attempts to isolate palladacyclobutanes
from the reaction of q3-allyl complex 1 a and benzenologous
ester enolate 3 at - 15 "C were not successful (Scheme 2). However, we assumed metallacyclobutane 4 to be the reactive intermediate, since addition of concentrated aqueous HCI at - 15 'C
caused an immediate precipitation of palladium black. On
workup olefin 5 was isolated. This product is thought to arise as
a result of monoprotonation of a carbon-palladium o bond in
4 and b-hydride elimination. We then tried to prepare palladacyclobutanes with two secondary rather than two primary (cf. 4)
alkylpalladium bonds; this would allow us to take advantage of
the "aryl effect".[4]
The reaction of (q3-l ,3-diphenylallyl)palladium with deprotonated isobutyronitrile in THF/TMEDAC6]at - 15 "C
(Scheme 3) gave a yellow solution. which, on warming to room
temperature, furnished a yellow powder. This was isolated, pu[*] Prof. H. M. R. Hoffrnann, Dr. A. R. Otte. Dr. A. Wilde
Department of Organic Chemistry. University of Hnnnover
Schneiderberg 1 B. D-30167 Hannover (Germany)
Te1ef;ix: Int. code. (51 1)762-3011
+
Dr. S. Menzer, Dr. D. I. Williams
Chemical Crystallography Laboratory, Imperial College of Science
Technology and Medicine. South Kensington. London, SW7 2AY (UK)
[**I
We thank Prof. H. Duddeck for advice with the N M R spectra. Our work was
supported by the Deutsche Forschungsgemeinschaft and Degussa by a generous gift of palladium salts. The Fonds der Chernischen Industrie provided a
PhD fellowship to A. Wilde.
100
((3 VCH
Verlugsge.sullr~Iiu/lmhH, 0-69451 W<~inlidm.
I995
Scheme 3
rified and identified by chemical and spectroscopic means. A
cyclopropane product was not observed. The compound was
kinetically inert at room temperature, and also stable to air and
water. When carbon monoxide was passed over a dichloromethane solution of these crystals, palladium precipitated and
the substituted cyclopropane 2a was formed (by reductive elimination). H and "C NMR data suggested that palladacyclobutane 6 was the isolated intermediate (Scheme 3, Table 1).
Table 1. Physical and spectroscopic data for 6 .
M.p. i28.C (decornp.): IR: (CHCI,): ;[cm-'] = 3000, 2972. 2912, 2228, 1586.
1484, 1460; ' H N M R (200 MHz. CD,CI,): b =7.49 (d. J = 7 Hz, 4H. H,,,,), 7.12
(t, J = 7 . 7 Hz, 4 H . H,,,,), 6.86 (tt, J =7 Hz. I Hz, 2H. H,,,,). 3.22 (t, J = 9.5 Hz.
iH.CH).2.36(s.6H.ZNCH,).2.55~2.40,2.17-2.02(m.4H.CH,-CH,).i.56(d.
J = 9.5 Hz, 2H. PhCH), 1.44 (s, 6H. 2 NCH,), 0.98 [s, 6H. 2 (CH,),CCN]
I3C N M R [a]: (CD,CI,): 6 =154.47 ( 7 ) . 128.35 (I), 127.58 (5). 126.34 ( t ) .121.44
48.66 (I). 47.71 (11%
45.20 (T). 24.51 (1). 5.21 (1); MS
(70 eV): in;= (YO):
261 (4). 193 (76), 115 (100). 91 (30)
(1). 63.23 (1). 60.05 (TI.
[a] APT (uttached proton rest): spin echo based selection of multiplicities of
" C NMR signals. Quaternary C and CH, carbon atoms give positive signals (t).
while CH and CH, give negative signals ( 5 ) .
Slow evaporation of a dichloromethane solution of 6 at room
temperature provided single crystals suitable for X-ray crystallography (Fig. I)."] The molecule is virtually symmetric and
has an average noncrystallographic C, symmetry about the
plane perpendicular to the coordination plane and bisecting the
C(6) -C(7) bond (Fig. 2, top). The two phenyl rings are approximately coplanar (5" between their mean planes) and oriented
orthogonally (89") with respect to the C(2)-Pd-C(4) plane. This
arrangement of the two phenyl rings optimizes orbital overlap
of the two Pd-C (5 bonds with the aromatic n electron systems.
+
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Angrw. Chum. hi.Ed. Engl. 1995. 34. N o . 1
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The ligands around the Pd atom adopt a distorted square-planar geometry with internal ligand angles at Pd o1'69.8(2) within
the palladacyclobutane moiety and 81.7(2) within diazapalladacyclopentane. The palladacyclobutane ring is not flat,
but slightly folded at a fold angle of 161.4'' about the C(2)-C(4)
diagonal (i.e., 18.6' out of plane). As a consequence of this slight
folding,[81there is unfavorable near-eclipsing of the substituents
attached to C ( 2 ) and C(3) and of those at C(3) and C(4)
(Fig. 3).[9.l o ] The Pd-C bond lengths [Pd-C(4) 2.048(4), PdC(2) 2.060(5) A] are within the normal range. However, the
Pd-N bonds [2.230(4) and 2.233(4) A]. which are trans to a
Pd-C sp3 bond, are unusually long; this is consistent with a
trans effect." 'I
1811
Y
C1311
Fig. I . Cryhtal51i iicture o f 6 Hydrogen atoms only shown at C(2). C(3). and C(4).
The numhering of thc atonis does not conform to IUPAC conventions. Important
bond lengths [A] and angles [ I : Pd-C(2) 2.060(5). Pd-C(4) 2.048(4). Pd-N(5)
2.230(41. Pd-N(X) 2 233(4), C(2)-C(3) 1.535(6), C(3)-C(4) 1.532(6); C(Z)-Pd-C(4)
69.X('). N(i)-Pd-N(X) 81 7(2). Pd-C(2)-C(3) 93.33). Pd-C(Z)-C(21) 115.3(3).
C(21 )-C(2)-C(39) 1 IY.0(4). C(2)-C(3)-C(4) 100.1(3). C(2)-C(3)-C(31) 1 16.5(4).
(.(4)-<VI-C(.311 I I6 3(4). C(3)-C(4)-Pd93.7(3), C(3)-C(4)-C(41) 117.3(4). C(41)C(4)bPd Il4..3(3)
b w
Cl2ll
Fig. 3. Selected torsion angles
Fig. 2 . Crystal structure of' 6 (top view). Top: ball-and-stick representation. Bottom: .rpacc-lilling model.
in
6.
The distance between the two ortho aromatic hydrogens
pointing inwards is small (2.57 A); the palladium atom is therefore screened from attack from the bottom face of the molecule.
However, the top face is comparatively accessible (Fig. 2, bottom), and reaction of the palladium center with carbon monoxide is thus possible.[12]8-Hydride eliminations are generally fast
reactions, but require a synperiplanar conformation. In the case
at hand, the torsion angles are not far from 90 (Pd-C(4)-C(3)H(3) 107" and Pd-C(2)-C(3)-H(3) - 107'). and aikene formation is thus effectively blocked.
The NMR spectrum (solvent CD,CIJ of 6 is temperaturedependent. The '%signals of the four N(CH,), carbon atoms
are not equivalent and appear as two singlets; this is consistent
with the 'H NMR spectrum (two singlets, 6 H ; cf. ref. [9]). At
-80°C the two 13C signals of the N(CH,), carbons split into
four signals (T, e 233'K; AG: = 10.5 0.5 kcalmol-'). and
the singlet corresponding to the CH,CH, group is transformed
into two singlets (T, =z 208'K; AG: = 10.4 0.5 kcalmol-I).
Moreover, there are two PdCHPh singlets (T, % 218°K;
AG: = 10.3 f 0.5 kcalmol-I), and the singlet due to the geminal dimethyl carbon atoms (C(CH,),CN) splits into two singlets. However, the singlets assigned to the quaternary
(C(CH,),CN), cyano, and ring (C(3))carbon atoms remain
sharp down to -90-C. These results are consistent with the
following conformational fluctuations : The molecule loses its
"dynamic C, symmetry"; instead, two equally populated enantiomers ( C , symmetry) appear, which interconvert (racemize)
(Scheme 4).[13]
At room temperature the aromatic carbons appear as four
discrete signals (2 x @so. 4 x ortho. 4 x m'tu. 2 x para). At
- 80 ' C signals of the ipso and the pura C atoms are each split
into two, as expected for the hindered racemization discussed
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racemizatton, CD~CI, Me
the growing class of metallacyclobutanes, which
AG;
Me
= 10.4 kml ml-'
Me'
Schetnc 4. Racenuration o f 6 in CD,C-I, ( t h e broken c~i-cledenotes the Pd i t t o i n :
other iitwn\ o m i t t c d ) . Sc1ieiii;itic rcpt-ezcntatton: The dcviatton of the methylone
c;it-hon\ li-om the N-Pd-N plane i s only ithout 0.026 0.05 A
are of general
relevance in cata1ysis.l'"'
Rccci\cd: .luly4. 1904 [Z 70Y5 IE]
Gernim ~ r s i o n 4iiq,11. C'liiwi, 1995. 107. 7
Keywords: cyclopropanation . palladacyclobutaiies * palladium
compounds
above. However, signals corresponding to the orilio and mc'itr C
atoms each split not into two, but four signals. Thus, all twelve
aromatic carbon atoms become inequivalent. Hindered racemizntion is accompanied by a freezing of the phenyl group rotation. We assume that the barrier to rotation of the phenyl group
is comparable to that of racemizatioii. The barrier to rotation
about the C - Ph (ACT = 10 kcalinol-I) indicates a significant
steric hindrance. Each phenyl group is jammed by one opposing
N-methyl group.["]
The crystal structures of 6 and the final product 2a1'"] allow
us to map out the reaction pathway of ($-I .3-diphenylallyl)palladium complex 1 b with deprotonated isobiityronitrile
in the presence of TMEDA. The bulky nucleophile does not
attack the electron-rich metal center, but instead the central
carbon atom of the ally1 ligand, and palladacyclobutane 6 is
formed. Because of the mutual repulsion between the two
phenyl groups in 6, the spoiitaiieous reductive elimination to the
cis-diphenyl cyclopropane 2 a froin palladacyclobutane 6 is iiiipeded. However. cyclopropanation is accelerated, and yields are
generally improved by passing carbon inonoxide over the reaction solution.['. '.I Carbon monoxide is not only sterically acccssible, but also a strong K acid. By attacking the 16-electron
metallacycle froin the top face (Fig. 2. bottom) carbon nionoxide reinows electron density from palladium and thus facilitates
reductive elimination and conversion of the 1.3-propanediyl ligand into the corresponding cyclopropane. I t seeins likely that
the transformation of 6 to 2 a in the presence of carbon monoxide proceeds via an 18-electron transition state or intermediIn any event, the transannular C ( 2 ) . ' C(4) distance in 6
(2.35
must be contracted substantially to approximately
1.55 A in 2a. corresponding to the new C-C bond.[141
The question arises as to whether the nucleophilic enolate
initially attacks the r/3-allyl complex at palladium through its
heteroatoiii (ix..oxygen). and the palladacyclobutane is subsequently formed by ;I favorable intrainolecular pathway. We
have found that the anion of fluorene (pK, = 23). which is
devoid of all heteroatoms, is also capable of inducing cyclopropanation (Scheme 5 ) . Thus direct attack of the hindered nucleophilic carbon at the central position of the ally1 ligand seems
to be the more likely mechanism.
A)
Scheme 5
In summary, we have established the experimental conditions
for converting a range of r!'--allyl Pd complexes into cyclopropanes. Palladacyclobutane is a key intermediate, the structure of which permits an in-depth analysis of the reaction pathway. With the synthesis and full characterization of the title
heterocycle. we have contributed a new type of metallacycle to
[I?]The ~ t r i ~ c ~ uanalysis
raI
of Za i s currently in progress.
[l 51 Thc conwrsion of isolated pall;id3cyclohutanc.icyci~~h~tt~ine
6 into Za h a s idso been brought
.tbout by reaction with isocIcctroinc NO' (from N O ' PF, i t 1 CH>C12)and hy
oxitl;itive ;iddillon oliodine (or tnelhyl iodide). pimumably via ii Pd" intermedtatc.
[ lh] Mct~illac)-clohut;in~~
of carly ti-.iiis~t~on
metills iis reactive intcrniedialc< i n
olclin mctathesir . J. Feldmnn. R. R Schrock. Pr(i,q.c it or^^. Chcvii. 1991. 39.
i i h o E B.Tlxicti. G L Carty. .I.M . Stiq ker. .I. ,4111.C ' l w i i t S ( K 1993.
11.5, 9x14. K. Ohc. H. M a l \ u d i t . T. Morimoto. S. O p a h t . t
i Chatant. S.
Moriit, t h i d 1994. I i 6 . 425.
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crystals, structure, isolation, mechanism, insights, palladacyclobutane, cyclopropanation, ray
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