Amine attack on coordinated alkenes: An interconversion from anti-Markovnikoff to Markovnikoff products

Article abstract of DOI:10.1021/ja0469939

A sequence of alkene complexes of platinum, PtCl₂(PPh ₃)(alkene) (alkene = ethylene, propene, 1-butene, cis-2-butene, 1-hexene, 1-octene, and 1-decene), has been prepared. These complexes are characterized by NMR spectroscopy, including assignment of each proton, and X-ray crystal structures of the 1-propene and 1-hexene complexes. Each complex was reacted with diethylamine. For the 1-hexene, 1-octene, and 1-decene complexes, the amine displaces the alkene. For the smaller alkenes, the diethylamine nucleophilically attacks the coordinated alkene. For propene and 1-butene, the low-temperature addition leads to the anti-Markovnikoff nucleophilic attack, which slowly converts at room temperature to the Markovnikoff product. The transformation from anti-Markovnikoff to Markovnikoff addition occurs without diethylamine dissociation.

Full text of DOI:10.1021/ja0469939

Published on Web 09/14/2004
Amine Attack on Coordinated Alkenes: An Interconversion
from Anti-Markovnikoff to Markovnikoff Products
Ruslan Pryadun, Dinesh Sukumaran, Robert Bogadi, and Jim D. Atwood*
Contribution from the Department of Chemistry, UniVersity at Buffalo,
State UniVersity of New York, Buffalo, New York 14260
Received May 21, 2004; E-mail: jatwood@buffalo.edu
Abstract: A sequence of alkene complexes of platinum, PtCl2(PPh3)(alkene) (alkene ) ethylene, propene,
1-butene, cis-2-butene, 1-hexene, 1-octene, and 1-decene), has been prepared. These complexes are
characterized by NMR spectroscopy, including assignment of each proton, and X-ray crystal structures of
the 1-propene and 1-hexene complexes. Each complex was reacted with diethylamine. For the 1-hexene,
1-octene, and 1-decene complexes, the amine displaces the alkene. For the smaller alkenes, the
diethylamine nucleophilically attacks the coordinated alkene. For propene and 1-butene, the low-temperature
addition leads to the anti-Markovnikoff nucleophilic attack, which slowly converts at room temperature to
the Markovnikoff product. The transformation from anti-Markovnikoff to Markovnikoff addition occurs without
diethylamine dissociation.
Nucleophilic attack on an alkene coordinated to a transition
metal is arguably the most important application of organome-
tallic chemistry. Theoretical studies1,2 and reviews3 summarize
the basics of coordination and activation. Depending on the
attack site, Markovnikoff or anti-Markovnikoff products may
result.
accounts of amine attack on a simple alkene were those of
Orchin et al.,4
trans-PtCl2(C2H4)(Py) + C5D5N f
trans-PtCl2(C2H4C5D5N)(py) (1)
(py ) C5H5N), and Panunzi et al.,5
An often-studied nucleophilic attack involves amines and
palladium or platinum alkene complexes.4-10 Among the earliest
cis-PtCl2(alkene)PR3 + NHR′2 f
cis-PtCl2(CH2CH2NR′3)PR3 (2)
(1) Eisenstein, O.; Hoffmann, R. J. Am. Chem. Soc. 1981, 103, 4308.
(2) Fujimoto, H.; Yamasaki, T. J. Am. Chem. Soc. 1986, 108, 578.
(3) Hartley, F. R. In ComprehensiVe Organometallic Chemistry 6; Wilkinson,
G., Stone, F. G. A., Abel, E. A., Eds.; Pergamon: New York, 1982; p
664.
(4) Kaplan, P. D.; Schmidt, P.; Orchin, M. J. Am. Chem. Soc. 1968, 90, 4175.
(5) Panunzi, A.; DeRenzi, A.; Palumbo, R.; Paiaro, G. J. Am. Chem. Soc. 1969,
91, 3879.
(6) (a) Panunzi, A.; DeRenzi, A.; Paiaro, G. J. Am. Chem. Soc. 1970, 92, 3488.
(b) Palumbo, R.; DeRenzi, A.; Panunzi, A.; Paiaro, G. J. Am. Chem. Soc.
1969, 91, 3874. (c) DeRenzi, A.; Paiaro, G.; Panunzi, A.; Paolillo, L. Gazz.
Chim. Ital. 1972, 102, 281. (d) DeRenzi, A.; Paiaro, G.; Panunzi, A.;
Ramano, V. Chim. Ind. 1973, 55, 248. (e) DeRenzi, A.; Paiaro, G.; Panunzi,
A. Gazz. Chim. Ital. 1972, 102, 413.
(7) (a) al-Najjar, I. M.; Green, M. J. Chem. Soc., Chem. Commun. 1977, 926.
(b) Al-Najjar, I. M.; Green, M., J. Chem. Soc., Dalton Trans. 1979, 1651.
(c) Al-Najjar, I. M.; Green M.; Kerrison, S. J. S.; Sadler, P. J. J. Chem.
Soc., Chem. Commun. 1979, 311. (d) Green, M.; Sarkan, J. K. K. Inorg.
Chim. Acta 1980, 45, L31. (e) Al-Najjar, I. M.; Green, M.; Sarkan, J. K.
K. Inorg. Chim. Acta 1980, 44, L213. (f) Al-Najjar, I. M.; Green, M.;
Sarkan, J. K. K.; Ismail, I. M.; Sadler, P. J. Inorg. Chim. Acta 1980, 44,
L187. (g) Green, M.; Sarkan, J. K. K.; Al-Najjar, I. M. Organometallics
1984, 3, 520. (h) Sarkan, J. K. K.; Green, M.; Al-Najjar, I. M., J. Chem.
Soc., Dalton Trans. 1984, 771. (i) Green, M.; Sarkan, J. K. K.; Al-Najjar,
I. M. J. Chem. Soc., Dalton Trans. 1981, 1565.
(8) (a) Kawatsara, M.; Hartwig, J. F. J. Am. Chem. Soc. 2000, 122, 9546. (b)
Nettekoven, U.; Hartwig, J. F. J. Am. Chem. Soc. 2002, 124, 1166. (c)
Pawlas, J.; Nakao, Y.; Kawatsura, M.; Hartwig, J. F. J. Am. Chem. Soc.
2002, 124, 3669. (d) Kawatsura, M.; Hartwig, J. F. J. Am. Chem. Soc.
2003, 125, 14286.
(9) (a) Utsunomiya, M.; Kuwano, R.; Kawatsura, M.; Hartwig, J. F. J. Am.
Chem. Soc. 2003, 125, 5608. (b) Beller, M.; Eichberger, M.; Frauthwein,
H. Angew. Chem., Int. Ed. Engl. 1997, 36, 2225. (c) Beller, M.; Frauthwein,
H.; Eichberger, M.; Breindl, C.; Mhller, J. R. Eur. J. Inorg. Chem. 1999,
1121.
(10) Hahn, C.; Morvillo, P.; Herdtweck, E.; Vitagliano, A. Organometallics 2002,
21, 1807.
(alkene ) C2H4, C3H6, 1-C4H8; PR3 ) PPh3, PnBu3; NHR′2 )
NHMe2, NHEt2, NHBu2). The Panunzi group published exten-
sively on related reactions around 1970.6
Green and colleagues published a series of manuscripts
around 1980, further describing the electronic and steric effects
involved in amine attack on coordinated alkenes, cis-PtCl2-
(alkene)L, with L ) a variety of amine ligands, DMSO, etc.7
In a number of cases, azaplatinacyclobutane complexes were
formed, and their stability was evaluated.7c,f-i In only one case,
cis-PtCl2(3-methylbut-1-ene)(P(OMe)3), was anti-Markovnikoff
addition reported.7i
Interest in such nucleophilic attack on coordinated alkenes
revived around 2000, primarily as a result of Hartwig’s work
on palladium-catalyzed hydroaminations of vinylarenes.8 These
proceed through an η3-arylethyl complex and give Markovnikoff
products. However, a rhodium complex provided anti-Mark-
ovnikoff hydroamination through a metallacyclic intermediate.9
Markovnikoff addition of amines to alkenes coordinated to a
dicationic platinum complex was reported to be facile.10
A
summary of recent developments emphasizing the importance
of hydroamination was published.11 A DFT theoretical study
(11) Nobis, M.; Driessen-Ho¨lscher, B. Angew. Chem., Int. Ed. 2001, 46, 3983.
9
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J. AM. CHEM. SOC. 2004, 126, 12414-12420
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Amine Attack on Coordinated Alkenes
A R T I C L E S
of complexes M(Cl)(PH3)2 evaluated the hydroamination process
for different metals, concluding that nucleophilic attack of amine
is thermodynamically and kinetically favorable for Group 10
metals.12 A review of aminations summarizes the research prior
to 1998.13
In re-examining reactions of cis-PtCl2(alkene)(PPh3), we have
discovered that anti-Markovnikoff addition is kinetically con-
trolled, but Markovnikoff addition is thermodynamically con-
trolled. Herein, we report the synthesis of seven alkene
complexes and reactions with NHEt2.
given the synthesis from a mixture of PtCl2, alkene, and PPh3,
cis-PtCl2(alkene)(PPh3) appears to be thermodynamically fa-
vored, maximizing the bonding by placing the strong trans-
influence ligands cis.17b
The new procedure for synthesizing the ethylene complex
has proven to be invaluable, because synthetic methods de-
scribed in the literature require several steps and long times,14
resulting in both smaller yields and decreased purity of the
product. Utilizing DMF in a “one-pot” synthesis allowed easy
separation of the product and a significant increase in yield (from
40% to 95%) based on the starting Pt(II) complex. The success
of the procedure may arise from the reaction conditions, in
which bridged platinum species were broken into monomers
by association with triphenylphosphine, yielding an open
coordination site blocked by DMF long enough to not coordinate
another phosphine ligand but to allow coordination of ethylene.
Therefore, DMF played a crucial role in formation of the
product. A similar reaction run in dichloromethane resulted in
formation of the bistriphenylphosphine complex, indicating that
the solvent was not coordinating enough. However, when
DMSO was used, solvent complex, PtCl2(DMSO)(PPh3), was
produced, presumably due to greater coordinating power of the
solvent over ethylene. These results were also confirmed by an
independent experiment in which ethylene-bound complex was
dissolved in DMSO-d6; evolution of the gas was immediate and
PtCl2(DMSO-d6)(PPh3) was formed. Therefore, the choice of
solvent plays an important role in the synthesis, yield, and purity
of the ethylene complex. The ethylene complex cis-PtCl2(C2H4)-
(PPh3) provides an excellent precursor to the other alkene
complexes.
Results and Discussion
Synthesis. Simple alkenes coordinate to platinum(II), yielding
stable species not prone to air oxidation. However, the synthetic
procedures documented for syntheses of such alkene complexes
are quite complex and require long incubation times,5,14 which
result in both time dependency and overall decrease in yield.
We have established a new synthetic procedure for the synthesis
of cis-PtCl2PPh3(alkene) (alkene ) ethylene, propylene, 1-butene,
cis-butene 1-hexene, 1-octene and 1-decene) with an overall
yield >80% based on the starting platinum complex. PtCl2,15
Amine Addition. The alkene complexes were reacted
independently with diethylamine. Each reaction was performed
at -75 °C, since the addition is exothermic and the lower
temperature should shift the equilibrium toward formation of
the products.5-7 To achieve greater yields, excess amine was
used. In the case of the ethylene adduct, only a stoichiometric
amount of amine was required for reaction. However, with the
longer chain alkene adducts, no reaction was observed with 1
equiv of diethylamine, possibly due to increased steric hindrance
about the olefin. Temperature control has proven to be essential
for obtaining alkyl derivatives, and experiments resulted in
failure when the reaction mixture was brought to room tem-
perature too rapidly, causing ligand substitution rather than
nucleophilic attack. Even at low temperatures, the alkene was
displaced if it was larger than four carbons.
Alkyl species resulting from the addition of diethylamine onto
a bound alkene are shown in Figure 1. For propylene and
1-butene, Markovnikoff or anti-Markovnikoff addition is pos-
sible. Due to some discrepancies in the literature with regard
to the latter and former conventions, we consider a Markovnikoff
species to result from attack of a nucleophile onto an internal
carbon of the bound alkene, thus resulting in a bond between a
metal and a carbon carrying two hydrogens.
dissolved in DMF, was reacted with ethylene in the presence
of triphenylphosphine. The product was precipitated from DMF
by addition of triply distilled water, and the white solid was
recrystallized from chloroform. To obtain species with higher
olefins, the ethylene complex was dissolved in chloroform and
placed in the presence of the desired alkene. The solubility of
cis-PtCl2PPh3(alkene) complexes in chloroform decreases sig-
nificantly with increase in the hydrocarbon chain length of the
bound alkene.
Based on the results of our investigation, alkene addition to
PtCl2PPh3 takes place cis to triphenylphosphine. These results
are supported by the body of work reported in the literature,
especially that by Green et al.7 and Panunzi et al.5,6 and our
own X-ray crystal structures (reported herein). Coordination cis
to triphenylphosphine is not expected based on steric factors.
Whereas coordination of smaller alkenes does not sterically
hinder rotation of triphenylphosphine, coordination of 1-hexene
and larger olefins can, in theory, pose some steric hindrance
between the alkyl chains and phenyl rings, thereby lowering
the Eact barrier for dissociation of the respective alkene, and
thus the larger alkene complexes are less stable.16
cis-PtCl2(PPh3)C2H4NHEt2. The 1H NMR spectrum (Figure
2) of product I shows a triplet of doublets in the region of δ
0.47 ppm and a triplet around δ 4.2 ppm, corresponding to a
and b protons, respectively. The phosphorus atom caused an
unexpected splitting of the triplet at 0.47 ppm; cis-coupling is
A kinetic preference for trans geometry might be expected,
given the strong trans effect of PPh3 and alkene.17a However,
(12) Senn, H. M.; Blo¨chl, P. R.; Togni, A. J. Am. Chem. Soc. 2000, 122, 4098.
(13) Mu¨ller, T. E.; Beller, M. Chem. ReV. 1998, 98, 675.
(14) Boag, N. M.; Ravetz, M. S. J. Chem. Soc., Dalton Trans. 1995, 3473.
(15) Kerr, G. T.; Schweizer, A. E. Inorg. Synth. 1980, 20, 48.
(16) (a) Ashley-Smith, J.; Douek, Z.; Johnson, B. F. G.; Lewis, J. J. Chem.
Soc., Dalton Trans. 1974, 128. (b) Albright, T. A.; Hoffman, R.; Thibeault,
J. C.; Thorn, D. L. J. Am. Chem. Soc. 1979, 101, 3801.
(17) (a) Basolo, F.; Pearson, R. G. Prog. Inorg. Chem. 1964, 4, 381. (b) Basolo,
F. AdV. Chem. Ser. 1965, 49, 81.
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A R T I C L E S
Pryadun et al.
with original selection of the doublet at δ 0.11 ppm, corre-
sponding to the methyl group. Results of the experiment (Figure
5) showed that four separate resonances in the 1H NMR
correspond to the protons on the propyl chain. Integration of
the peaks establishes the pattern of 1:1:1:3. However, unprec-
2
195Pt-CH2
edented J
coupling of 70 Hz is difficult to explain. A
platinum decoupling experiment (Figure 6) shows the disap-
pearance of the platinum satellites. Two-dimensional 195Pt-1H
HSQC (Figure S1) shows a definitive coupling interaction
between the 195Pt and the methyl group. Both e and d protons
1
are coupled to the same carbon, as shown in H-13C HSQC
(Figure S2). The chemical shift of the carbon atom carrying
those protons is indicative of the presence of bound nitrogen.
Moreover, an upfield shift of the carbon bonded to the metal
can be rationalized in terms of shielding effects imposed by
the presence of the metal nucleus.
Contrary to the literature precedent,5-7 we found that the anti-
Markovnikoff adduct, initially formed, rearranges in chloroform
at room temperature to form the Markovnikoff addition species.
The appearance of the doublet at δ 1.11 ppm and disappearance
of the doublet at δ 0.11 ppm in the 1H NMR spectrum (Figure
S3) indicated formation of the new species. The progress can
Figure 1. Proton assignments for the platinum alkyl complex resulting
from diethylamine attack on coordinated alkene.
usually too small to be observed. Upon decoupling phosphorus
(1H{31P} NMR), the triplet of doublets was transformed into a
triplet resulting from splitting by adjacent b protons, as shown
in Figure 3. The broad satellites arise from coupling of the
methylene protons to the 195Pt nucleus (33% abundant), which
1
be monitored by H NMR spectroscopy. However, the time
frame required for complete rearrangement is measured in
months. Slow emergence of new resonances was accompanied
by a decrease in the intensity of the resonances, corresponding
to the anti-Markovnikoff species. A striking feature in the
spectral characterization of the new alkyl derivative was the
absence of platinum satellites and significant shift on the NMR
scale of 1 ppm for f protons. Methylene protons e and d were
also shifted upfield due to shielding effects caused by the
presence of an electropositive nucleus. Methyne proton c was
shifted upfield, and the connectivity of the alkyl chain was
possesses a spin of 1/2. The coupling 2J
has a magnitude
195Pt-CH2
of 80 Hz. The b protons are significantly more downfield,
around δ 4.1 ppm, due to the deshielding effects of the nitrogen
atom. The decrease in 3J
coupling of the 4.1 ppm
195Pt-CH2
resonance is attributed to an increase in spatial separation
between interacting nuclei. Methyl and methylene hydrogens
of the ethyl groups on nitrogens showed complex splitting
patterns due to overlap of the bound and unbound diethylamine.
Therefore, no precise assignments in those regions were made
for this and the other alkyl derivatives.
cis-PtCl2(PPh3)C3H6NHEt2. The 1H NMR spectrum of
complex II is characterized by the presence of several multiplets
in the aliphatic region of the spectrum. No coherent assignment
for the protons of the aliphatic region is present in the literature
due to the complexity of the 1H spectrum (Figure 4). To establish
the connectivity within the alkyl chain, a TOCSY was performed
1
established through TOCSY NMR (Figure S4) and 2D H-
13C HSQC (Figure S2). Changes in the relative positions of the
carbon atoms in the 13C NMR are also indicative of the
rearrangement and provide further support for the presence of
a σ bond between the primary carbon and platinum.
cis-PtCl2(PPh3)C4H8NHEt2. Synthesis of alkyl derivative IV
was challenging due to rapid dissociation of the cis-2-butene
1
Figure 2. Partial H spectrum of cis-PtCl2(PPh3)C2H4NHEt2 in CDCl3.
9
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Amine Attack on Coordinated Alkenes
A R T I C L E S
1
1
1
Figure 3. Partial H{31P} NMR spectrum of cis-PtCl2(PPh3)C2H4NHEt2 in CDCl3. (bottom, H NMR spectrum; top, H{31P} NMR spectrum).
1
Figure 4. Partial H spectrum of cis-PtCl2(PPh3)C3H6NHEt2 in CDCl3.
and formation of the dimeric platinum species. The coupling
pattern within the alkyl chain of IV was established by TOCSY
NMR with selective mixing times (Figure S5). A doublet at δ
0.11 ppm in the 1H NMR spectrum showed platinum satellites
(Figure S6), and a multiplet at δ 3.95 ppm exhibited numerous
activated methyl group to be shifted upfield of an inactivated
methyl carbon due to formation of the partial three-center, four-
electron bond in a fluxional manner with n hydrogens.19
A
TOCSY experiment confirmed the connectivity within the
carbon chain.
1
couplings. Based on the chemical shift in the H NMR, the
cis-PtCl2(PPh3)-1-C4H8NHEt2. Complex III was character-
1
multiplet at δ 3.95 ppm is most likely from carbon coordinated
to diethylamine. A multiplet at δ 0.9 ppm is consistent with m,
and the doublet (1.05 ppm) results from coupling of p with o.
Satellites for n are a result of interaction with platinum (3JPt-H).18
To confirm the preliminary assignment of resonances in the 1H
ized in the same way as the other alkyl derivatives. The H
NMR analysis (Figure S8) allowed distinguishing between
several possible isomers. Multiplets around δ 3.9 ppm and δ
4.4 ppm are similar to those observed in the propyl complex
and may be assigned to g and h from both chemical shift and
1
NMR spectrum, a 2D H-13C HSQC was taken (Figure S7).
1
2D H-13C HSQC (Figure S9), which shows that both are
The carbon atom bonded to p is shifted downfield to around δ
70 ppm, indicative of the bond to nitrogen. The m proton is
bonded to the carbon shifted upfield above δ 0 ppm due to
coordination to platinum. The carbon at δ 20 ppm carries n,
while that at δ 18 ppm is bonded to o. One would expect an
coupled to the same carbon. However, it is difficult to
distinguish g from h without quantum mechanical calculations.
The identification of the resonances corresponding to the anti-
Markovnikoff species IIIa was complicated by significant
1
overlap in the H NMR. Thus, a TOCSY NMR with selective
9
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A R T I C L E S
Pryadun et al.
Figure 5. TOCSY 1D NMR spectrum of cis-PtCl2(PPh3)C3H6NHEt2 in CDCl3.
1
Figure 6. Partial H{195Pt} NMR spectrum of cis-PtCl2(PPh3)C3H6NHEt2 in CDCl3.
mixing was taken (Figure S10), and j and k were separated
and identified at δ 1.3 ppm and δ 1.5 ppm, an area obscured
by methyl protons of bound and unbound diethylamine. Methyl
protons in both IIIa and IIIb were located at 0.3 and 0.7 ppm,
respectively, and showed a peculiar coupling pattern: both
appear as triplets, although methylene protons j and k are
inequivalent. A series of irradiation experiments resulted in
disappearance of the triplets and appearance of broad doublets.
It is possible that the Jj-n and Jk-n are of a similar magnitude,
giving the observed splitting pattern. The i proton was deter-
mined to be underneath the resonances of j and k.
Multiple studies on addition of a nucleophile to a Pt(II)-bound
alkene have been performed, but few dealt with precise
assignment of the stereochemistry of addition.6a-d It is very
difficult to unequivocally conclude which type of addition is
more favorable, because many factors come into play, such as
the nature of the amine, cone angle of the phosphine ligand,
temperature of addition, and subtle electronic effects.5,7b,16,20
Azaplatinacyclobutane has been suggested7 from reaction
between secondary amine and Pt(II)-bound alkenes. Our NMR
analyses, detailed above, show no evidence for formation of
azaplatinacyclobutanes for our reactions. Microanalyses indicate
two Cl- for each platinum, consistent with the NMR results.
An amine can displace the alkene or attack the alkene inter-
or intramolecularly. Based on calculations by Sieghban,20
intermolecular attack provides a lower energy barrier for an OH-
attack onto a bound ethylene of PdCl2(C2H4). Dewar and Ford21
calculated the lowest energy pathway for addition of an amine
to a bound olefin. Their conclusions indicate that there is a
significant difference in energy between intermolecular and
intramolecular modes of attack. Nucleophilic addition can be
established either with the terminal, least hindered, or with the
internal, most hindered, carbon of the alkene. Since the
temperature of the addition was low, the product with the lowest
activation energy should be favored. The presence of anti-
Markovnikoff product exclusively for II and III at low
temperature indicates the preference for attack on the least
substituted carbon.
If II and III are left in solution for an extended period of
time, they rearrange to form the Markovnikoff addition products.
(18) Ananikov, V. P.; Mitchenko, S. A.; Beletskaya, I. P. J. Organomet. Chem.
2000, 604, 290.
(19) Brookhart, M.; Green, M. L. H.; Pardy, R. B. H. J. Chem. Soc., Chem.
Commun. 1983, 691.
(20) Sieghban, P. E. M. J. Am. Chem. Soc. 1995, 117, 5409.
(21) Dewar, M. J. S.; Ford, G. P. J. Am. Chem. Soc. 1979, 101, 783.
9
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Amine Attack on Coordinated Alkenes
A R T I C L E S
All attempts at making Markovnikoff species by direct addition
have failed, because when amine addition takes place at
temperatures above -30 °C, olefin displacement takes precedent
over amine addition and no alkyl derivatives could be isolated
for either II or III. The rearrangement from the anti-Markovni-
koff product to the Markovnikoff takes place quantitatively with
no loss of alkene. Given the ready displacement of alkene under
the conditions where the rearrangement occurs, any mechanism
involving a free alkene complex would be precluded. Allyl
complexes could form22 and have been suggested for isomer-
ization reactions; however, those involve 1,3 shifts,23 not the
1,2 shift observed here. A synchronous formation of Pt-C and
C-N bonds, while weakening of the existing Pt-C and C-N
bonds, seems most consistent with the observations. Calculations
may clarify this unusual rearrangement.
After the rearrangement takes place, a doublet corresponding
to f along with the satellites disappears, and emergence of the
Markovnikoff product IIb is accompanied by the appearance
of a sharp doublet at δ 1.11 ppm, indicative of the f protons. A
1H NMR shift of 1 ppm indicates a decrease in electron density
surrounding the f protons. Disappearance of platinum satellites
signifies the absence of interaction between methyl protons and
the metal ion. It is very peculiar that the platinum interaction
takes place only when f protons are on the carbon adjacent to
the carbon bonded to platinum. Thus, the distance between the
methyl protons must be significant. A 13C decoupled spectrum
Figure 7. ORTEP diagram of PtCl2(PPh3)C3H6 (50% thermal ellipsoids).
13C-1H
shows J
coupling close to normal, but the platinum
satellites in the 1H spectrum of IIa indicate interactions between
the metal ion and f protons, with interaction ceasing to exist
after the rearrangement to IIb. Species III exhibits the same
type of rearrangement, although no platinum satellites were
observed for either Markovnikoff or anti-Markovnikoff products,
confirming that the methyl group has to be adjacent to the carbon
bonded to the metal.
Structures. Crystals of cis-PtCl2(PPh3)C3H6 (Figure 7) and
cis-PtCl2(PPh3)C6H12 (Figure 8) were grown in an H tube by
slow diffusion of pentane into 1,1,2,2-tetrachloroethane. X-ray
crystal structure analysis showed cis coordination of the olefin
relative to the triphenylphosphine. Selected bond distances and
angles are given in Tables 1 and 2. The geometry of both is a
distorted square-planar complex with the alkene perpendicular
to the plane. The distances and angles are about the same,
although the average Pt-C(alkene) bond distance is slightly
longer for 1-hexene, consistent with the decreased stability. The
rest of the olefin-bound complexes also showed cis coordination,
based on the inequivalency of the respective protons stemming
from the perpendicular and parallel modes of coordination
relative to the square-planar geometry of the molecule. Com-
plexes of 1-butene and cis-butene readily decomposed in
solution to form platinum dimer and respective gases.
Figure 8. ORTEP diagram of cis-PtCl2(PPh3)(1-hexene) (50% thermal
ellipsoids).
Table 1. Selected Bond Lengths (Å) and Angles (°) for
PtCl2(PPh3)C3H6
Pt(1)-C(1)
Pt(1)-C(2)
Pt(1)-P(1)
Pt(1)-Cl(1)
2.146(3)
2.188(3)
2.2610(7)
2.3195(8)
Pt(1)-Cl(2)
C(1)-C(2)
C(2)-C(3)
2.3623(8)
1.394(5)
1.497(6)
C(1)-Pt(1)-C(2)
C(1)-Pt(1)-P(1)
C(2)-Pt(1)-P(1)
C(1)-Pt(1)-Cl(1) 164.29(11)
C(2)-Pt(1)-Cl(1) 158.11(11)
P(1)-Pt(1)-Cl(1) 87.35(3)
C(1)-Pt(1)-Cl(2) 88.13(10)
37.52(14)
95.58(10)
91.06(10)
C(2)-Pt(1)-Cl(2)
P(1)-Pt(1)-Cl(2)
93.87(10)
175.05(3)
Cl(1)-Pt(1)-Cl(2) 88.21(3)
C(2)-C(1)-Pt(1)
C(1)-C(2)-C(3)
C(1)-C(2)-Pt(1)
C(3)-C(2)-Pt(1)
72.9(2)
127.1(4)
69.6(2)
113.2(3)
Orchin et al.24 implied that the magnitude of JPt-CH is
2
representative of the distance between the metal ion and the
respective protons. Coupling with a magnitude of 70 Hz, as
observed for II and IV, is quite large and unprecedented for a
(location of the resonance for f protons in the 1H NMR spectrum
and location of the respective carbon atom in 2D 1H-13C
HSQC), we believe that direct interaction takes place between
the methyl protons and the Pt center. This interaction can take
place due to proximity of the methyl group in both II and IV
and results in direct spin-spin coupling between the f and o or
n protons with the platinum atom. If C-H activation takes place,
it occurs in a fluxional manner in which all methyl protons
become equivalent on the NMR time scale due to the rapid
1
three-bond separation. Based on the 2D H-195Pt spectrum
(22) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and
Applications of Organotransition Metal Chemistry; University Science
Books: Mill Valley, CA, 1987; pp 175-180.
(23) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis: The Applications
and Chemistry of Catalysis by Soluble Transition Metal Complexes, 2nd
ed.; John Wiley and Sons: New York, 1992; pp 14-22.
(24) Kaplan, P. D.; Orchin, M. Inorg. Chem. 1964, 4, 1393.
9
J. AM. CHEM. SOC. VOL. 126, NO. 39, 2004 12419

A R T I C L E S
Pryadun et al.
Table 2. Selected Bond Lengths (Å) and Angles (°) for
PtCl2(PPh3)(1-hexene)
Concluding Remarks
Pt(1)-C(1)
Pt(1)-C(2)
Pt(1)-P(1)
2.144(5)
2.204(5)
2.2517(8)
Pt(1)-Cl(1)
Pt(1)-Cl(2)
C(1)-C(2)
2.3149(9)
2.3514(9)
1.405(8)
To the best of our knowledge, we report the first example of
facile synthesis for PtCl2PPh3(alkene). Complexes were thor-
oughly characterized, and crystal structures indicate cis coor-
dination with respect to triphenylphosphine. Each species was
reacted with diethylamine, and alkenes with a hydrocarbon chain
longer than four carbons were displaced rather than undergoing
nucleophilic addition. All alkyl derivatives were extensively
characterized by NMR, and each proton was assigned on the
alkyl chain. Unprecedented coupling was shown to take place
between the methyl protons of the carbon adjacent to the carbon
bonded to platinum and the metal nucleus.
C(1)-Pt(1)-C(2)
C(1)-Pt(1)-P(1)
C(2)-Pt(1)-P(1)
C(1)-Pt(1)-Cl(1) 164.6(2)
C(2)-Pt(1)-Cl(1) 157.7(2)
P(1)-Pt(1)-Cl(1)
37.7(2)
98.0(2)
92.3(1)
C(1)-Pt(1)-Cl(2)
C(2)-Pt(1)-Cl(2)
P(1)-Pt(1)-Cl(2)
Cl(1)-Pt(1)-Cl(2) 88.27(4)
Cl(2)-C(1)-Pt(1)
Cl(1)-C(2)-Pt(1)
87.8(2)
95.5(1)
172.1(4)
73.4(3)
68.9(3)
84.77(3)
rotation around the C-C σ bond. Based on the results by
Eisenstein et al.,25 if C-H activation takes place, the JC-H should
have a magnitude of 100 Hz or below, whereas in all other cases
it is around 125 Hz for a methyl group. The 13C spectrum
showed a coupling constant of 123 Hz for the carbon carrying
f and n or o protons, which is normal for methyl carbon. Thus,
while the 1H chemical shift and JPt-H indicate significant
interactions of the methyl of II and one methyl of IV with
platinum, the JC-H does not indicate C-H activation. Therefore,
the Pt-H interaction must not cause significant C-H elongation,
consistent with the equivalency of the three hydrogens of the
methyl. The carbon carrying f protons in IIb has an almost
identical chemical shift and JC-H coupling constant. Although
Diethylamine addition to the 1-propene and 1-butene com-
plexes at low temperature leads to the anti-Markovnikoff
product, which slowly rearranges at room temperature to the
Markovnikoff product. The mechanism for this rearrangement
is not established, but probably does not involve amine loss to
give the platinum-alkene complex. This rearrangement could
be very important in catalysis directed toward anti-Markovnikoff
products.
Supporting Information Available: Experimental Section,
additional NMR data, further X-ray crystallographic data, and
CIF files. This material is available free of charge via the Internet
at http://pubs.acs.org.
1
e and d of IIb are inequivalent in the H spectrum, they split
carbon carrying them into a triplet in the 1H-coupled 13 C NMR
(Figure S11). Thus, the JC-H coupling constant of each of the
protons is similar enough to provide the appearance of a triplet.
Moreover, the C-H coupling constant of 139 Hz for both
Markovnikoff and anti-Markovnikoff species indicates partial
sp2 hybridization26 on the carbon attached to the metal, which
is peculiar because the former is primary and the latter is
secondary.
JA0469939
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Organometallics 2002, 21, 545.
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12420 J. AM. CHEM. SOC. VOL. 126, NO. 39, 2004