Quantitative determination of the regioselectivity of nucleophilic addition to η3-propargyl rhenium complexes and direct observation of an equilibrium between η3-propargyl rhenium complexes and rhenacyclobutenes

Article abstract of DOI:10.1021/om800739j

PMe₃ adds selectively to the central carbon of the η³-propargyl complex [C₅Me₅(CO) ₂Re(η³-CH₂C=CCMe₃)][BF ₄] (1-t-Bu) to form the metallacyclobutene [C

Full text of DOI:10.1021/om800739j

Organometallics 2009, 28, 123–131
123
Quantitative Determination of the Regioselectivity of Nucleophilic
Addition to η3-Propargyl Rhenium Complexes and Direct
Observation of an Equilibrium between η3-Propargyl Rhenium
Complexes and Rhenacyclobutenes
Charles P. Casey,* Timothy M. Boller, Joseph S. M. Samec, and John R. Reinert-Nash
Department of Chemistry, UniVersity of Wisconsin-Madison, Madison, Wisconsin 53706
ReceiVed July 31, 2008
PMe3 adds selectively to the central carbon of the η3-propargyl complex [C5Me5(CO)2Re(η3-
CH2CtCCMe3)][BF4] (1-t-Bu) to form the metallacyclobutene [C5Me5(CO)2Re(CH2C(PMe3)d
CCMe3)][BF4] (7). The rate of rearrangement of the metallacyclobutene 7 to η2-alkyne complex
[C5Me5(CO)2Re(η2-Me3PCH2CtCCMe3)][BF4] (8) is independent of phosphine concentration, consistent
with a dissociative mechanism proceeding via η3-propargyl complex 1-t-Bu. The rate of this rearrangement
is 480 times slower than the rate of exchange of PMe3 with the labeled metallacyclobutene 7-d9. This
rate ratio provides an indirect measurement of the regioselectivity for addition of PMe3 to the central
carbon of η3-propargyl complex 1-t-Bu to give 7 compared to addition to a terminal carbon to give 8.
The addition of PPh3 to 1-t-Bu gives the metallacyclobutene [C5Me5(CO)2Re(CH2C(PPh3)dCCMe3)][BF4]
(11). Low-temperature 1H NMR spectra provide evidence for an equilibrium between metallacyclobutene
11 and η3-propargyl complex 1-t-Bu (Keq ≈ 44 M-1 at -46 °C and ∆G°(0 °C) ) -1.2 ( 0.2 kcal
mol-1).
Scheme 1
Introduction
η3-Propargyl transition metal complexes, the triple-bond
analogues of the very well studied η3-allyl complexes, are
becoming increasingly important in synthetic chemistry.1 The
first stable η3-propargyl complex, [(Me3P)4Os(η3-PhCt
CCdCHPh)][PF6], was reported by Werner in 1985,2 and the
first isolated η3-propargyl complex not having an exo-double
bond, [(C6Me5H)Mo(CO)2(η3-CH2CtCH)][BF4], was reported
by Kryvich in 1991.3
η1-propargyl or η1-allenyl metal halide complexes,5 reaction of
metal halides with propargyl nucleophiles,6 reaction of propargyl
ether complexes with Lewis acids,7 rearrangement of η1-
homopropargyl metal complexes,8 and oxidative addition of
propargyl halides or tosylates to metal complexes.6 9
,
1H and 13C NMR spectroscopic data and X-ray crystal-
lographic structures suggest that η3-propargyl complexes are best
represented as a combination of η3-propargyl and η3-allenyl
resonance structures (Figure 1).1 Crystallographic data show that
all three carbons are within bonding distance of the metal
(2.2-2.5 Å), with the central carbon often nearest to the metal
center. The CR2dC bond length is substantially longer (1.34-1.40
We first synthesized η3-propargyl rhenium complexes by
hydride abstraction from alkyne complexes and later developed
a regioselective synthesis via protonation of propargyl alcohol
complexes (Scheme 1).4 Other methods for the synthesis of
isolable η3-propargyl complexes include halide abstraction from
(5) (a) Blosser, P. W.; Schimpff, D. G.; Gallucci, J. C.; Wojcicki, A.
Organometallics 1993, 12, 1993. (b) Huang, T.-M.; Chen, J.-T.; Lee, G.-
H.; Wang, Y. J. Am. Chem. Soc. 1993, 115, 1170. (c) Huang, T.-M.; Hsu,
R.-H.; Yang, C.-S.; Chen, J.-T.; Lee, G.-H.; Wang, Y. Organometallics
1994, 13, 3657. (d) Ogoshi, S.; Tsutsumi, K.; Kurosawa, H. J. Organomet.
Chem. 1995, 493, C19. (e) Baize, M. W.; Blosser, P. W.; Plantevin, V.;
Schimpff, D. G.; Gallucci, J. C.; Wojcicki, A. Organometallics 1996, 15,
164.
* Corresponding author. E-mail: casey@chem.wisc.edu.
(1) Reviews: (a) Chen, J.-T. Coord. Chem. ReV. 1999, 190-192, 1143.
(b) Kurosawa, H.; Ogoshi, S. Bull. Chem. Soc. Jpn. 1998, 71, 973. (c)
Doherty, S.; Corrigan, J. F.; Carty, A. J.; Sappa, E. AdV. Organomet. Chem.
1995, 37, 39. (d) Wojcicki, A. New J. Chem. 1994, 18, 61.
(2) Gotzig, J.; Otto, H.; Werner, H. J. Organomet. Chem. 1985, 287,
247.
(6) (a) Blosser, P. W.; Gallucci, J. C.; Wojcicki, A. J. Am. Chem. Soc.
1993, 115, 2994. (b) Gordon, G. J.; Whitby, R. J. J. Chem. Soc., Chem.
Commun. 1997, 1045.
(3) (a) Krivykh, V. V.; Taits, E. S.; Petrovskii, P. V.; Struchov, Y. T.;
Yanovski, A. I. MendeleeV Commun. 1991, 103. (b) Gusev, O. V.; Krivykh,
V. V.; Rubezhov, A. Z. Organomet. Chem. USSR 1991, 4, 233.
(4) (a) Casey, C. P.; Yi, C. S. J. Am. Chem. Soc. 1992, 114, 6597. (b)
Casey, C. P.; Selmeczy, A. D.; Nash, J. R.; Yi, C. S.; Powell, D. R.; Hayashi,
R. K. J. Am. Chem. Soc. 1996, 118, 6698.
(7) Weng, W.; Arif, A. M.; Gladysz, J. A. Angew. Chem., Int. Ed. Engl.
1993, 32, 891.
(8) Stang, P. J.; Crittell, C. M.; Arif, A. M. Organometallics 1993, 12,
4799.
(9) Nishida, T.; Ogoshi, S.; Tsutsumi, K.; Fukunishi, Y.; Kurosawa, H.
Organometallics 2000, 19, 4488.
10.1021/om800739j CCC: $40.75
2009 American Chemical Society
Publication on Web 11/24/2008

124 Organometallics, Vol. 28, No. 1, 2009
Casey et al.
Scheme 3
Figure 1. Resonance structures for η3-propargyl complexes.
Å) than in free allenes, and the CtCR bond length is longer
than in free acetylenes. The CR2-CtCR angle varies from 145°
to 155°, implying substantial strain about the central carbon.
This strain may be responsible for the highly reactive nature of
η3-propargyl complexes.
carbon to give an allene complex was prevented by steric
hindrance (Scheme 3).
Scheme 2
In some cases, nucleophilic additions to η3-propargyl com-
plexes have led to η3-allyl complexes, consistent with initial
kinetic attack at the center carbon of the η3-propargyl ligand
followed by protonation of the metallacyclobutene intermediate.12,6
In other cases, nucleophilic additions to η3-propargyl complexes
have led to η3-trimethylenemethane complexes, consistent with
initial kinetic attack at the center carbon of the η3-propargyl
ligand followed by protonation of the metallacyclobutene
intermediate and deprotonation at the atom R to the central allyl
carbon (Scheme 4).13
Scheme 4
In contrast to η3-allyl metal complexes, which are usually
attacked by nucleophiles at a terminal carbon, η3-propargyl metal
complexes are selectively attacked by nucleophiles at the center
carbon.1 Nucleophiles have been observed to attack each of the
three carbon centers of η3-propargyl rhenium complexes. We
found that while kinetic addition of nucleophiles occurs at the
center carbon of the η3-propargyl ligand to give rhenacy-
clobutene complexes, this addition is sometimes reversible and
can be followed by addition at either of the other two metal-
boundcarbonatomstogiveη2-alleneorη2-alkynecomplexes.4a,10,11
The methyl-substituted η3-propargyl complex [C5Me5(CO)2Re(η3-
CH2CtCMe)][BF4] (1-Me) reacted irreversibly with carbon
nucleophiles such as malonates and acetylides to give metal-
lacyclobutenes (Scheme 2); protonation of the metallacy-
clobutenes led to η3-allyl complexes. At low temperature,
nitrogen nucleophiles added to the central propargyl carbon to
produce the metastable metallacyclobutene complexes. Upon
warming, the nitrogen nucleophile dissociated from the rhena-
cyclobutenes and then readded to the resulting η3-propargyl
complex at one of the terminal sites to give either an η2-allene
or an η2-alkyne complex. For example, 1-Me reacted with
pyridine at -40 °C to give rhenacyclobutene 3, which rear-
ranged to η2-allene complex 4 at room temperature (Scheme
2). 4-Dimethylaminopyridine added to the central carbon of the
tert-butyl-substituted 1-t-Bu below -40 °C to give the rhena-
cyclobutene complex 5, which rearranged to η2-alkyne complex
6 at room temperature; apparently, attack at the t-Bu-substituted
Tsuji pioneered the development of palladium-catalyzed
reactions for addition of two nucleophiles to propargyl sub-
strates.14 For example, the reactions of carbon nucleophiles with
propargyl carbonates produce double nucleophilic addition
products where nucleophiles have added to both the central and
a terminal carbon of the propargyl unit. We have proposed that
these double nucleophilic additions occur by oxidative addition
to produce an η3-propargyl intermediate, which then undergoes
addition of the first nucleophile to the central propargyl carbon
to produce a metallacyclobutene; this is followed by protonation
of the metallacyclobutene to generate an η3-allyl complex that
is attacked by a second nucleophile (Scheme 5).10
Scheme 5
(10) Casey, C. P.; Nash, J. R.; Yi, C. S.; Selmeczy, A. D.; Chung, S.;
Powell, D. R.; Hayashi, R. K. J. Am. Chem. Soc. 1998, 120, 722.
(11) The Pd-catalyzed reaction of propargyl halides with thiols gives
mixtures of propargyl and allenyl sulfides. These reactions have been
proposed to occur by attack of nucleophiles at the terminal carbons of an
η3-propargyl intermediate. However, kinetic attack at the central propargyl
carbon might be reversible. Tsutsumi, K.; Yabukami, T.; Fujimoto, K.;
Kawase, T.; Morimoton, T.; Kakiuchi, K. Organometallics 2003, 22, 2996.
Ohno has imaginatively constructed bicyclic heterocycles by
palladium-catalyzed domino cyclization of propargyl bro-
mides.15 Yoshida has devised a clever palladium-catalyzed three-
component coupling of propargylic oxiranes, phenols, and

Nucleophilic Addition to η3-Propargyl Re Complexes
Organometallics, Vol. 28, No. 1, 2009 125
Scheme 6
Indirect Determination of Regioselectivity of Attack of
Phosphines. The relative rates of nucleophilic addition to the
center and terminal carbons can be measured indirectly by
comparing the rates of exchange of nucleophiles with the
metallacyclobutene adducts and of rearrangement of the met-
allacyclobutene to an η2-alkyne complex (eqs 1-4, Scheme 7,
and Figure 2). The difference between the barriers for exchange
and rearrangement is also the difference between the barriers
for attack at the central and terminal carbons. This method
assumes a dissociative mechanism for nucleophilic exchange
with the metallacyclobutene that proceeds via an η3-propargyl
intermediate.
Scheme 7
carbon dioxide.16 Both of these processes involve double
nucleophilic addition to propargyl substrates (Scheme 6).17
Because rhenacyclobutenes are the only observed kinetic
product of nucleophilic attack on η3-propargyl rhenium com-
plexes, it was not possible to quantitatively measure the
regioselectivity from product ratios. Here we report an indirect
method for the quantitative determination of this regioselectivity.
We also report the first direct observation of an equilibrium
between an η3-propargyl metal complex and a metallacy-
clobutene.
Rate(exchange) ) k-1[7]
(1)
Rate(rearrangement) ) k-1[7](k2[PMe3] ⁄ {k1[PMe3] +
k2[PMe3]} ) k-1[7](k2 ⁄ {k1 + k2}) (2)
Rate(rearrangement) ⁄ Rate(exchange) ) k2 ⁄ {k1 + k2} (3)
Rate(rearrangement) ⁄ Rate(exchange) ≈ k2 ⁄ k1 when k1 . k2
(4)
Results
We chose to study the exchange and rearrangement kinetics
of [C5Me5(CO)2Re(CH2C(PMe3)dCCMe3)][BF4] (7) since this
metallacyclobutene is relatively stable and had been fully
characterized by X-ray crystallography.10 In addition, 7 had been
found to slowly rearrange at room temperature to a single
product, the η2-alkyne complex [C5Me5(CO)2Re(η2-Me3-
Both the rate and the regioselectivity of the addition of
nucleophiles to the central carbon of η3-propargyl rhenium
complexes are so high that it is not possible to directly measure
how much faster attack at the center carbon is than attack at
one of the terminal carbons. Since the rhenacyclobutene is the
only observed initial product and neither an η2-alkyne complex
nor an η2-allene complex is initially seen, estimates of >20 can
be placed on the regioselectivity. In some cases, the initially
formed metallacyclobutene rearranged to a thermodynamically
more stable η2-alkyne complex (∆G° e -2.3 kcal mol-1). The
reaction was complete before the cold sample was inserted into
the precooled NMR probe; this requires a t1/2 of less than a
minute at -50 °C and ∆Gq(0.02 M ligand) e 14.7 kcal mol-1
∆Gq(1.0 M ligand) e 12.9 kcal mol-1
,
.
(12) Tsai, F.-Y.; Hsu, R.-H.; Huang, T.-M.; Chen, J.-T.; Lee, G.-H.;
Wang, Y. J. Organomet. Chem. 1996, 520, 85.
(13) (a) Baize, M. W.; Furilla, J. L.; Wojcicki, A. Inorg. Chim. Acta
1994, 223, 1. (b) Baize, M. W.; Plantevin, V.; Gallucci, J. C.; Wojcicki, A.
Inorg. Chim. Acta 1995, 235, 1. (c) Su, C.-C.; Chen, J.-T.; Lee, G.-H.;
Wang, Y. J. Am. Chem. Soc. 1994, 116, 4999. (d) Plantevin, V.; Blosser,
P. W.; Gallucci, J. C.; Wojcicki, A. Organometallics 1994, 13, 3651.
(14) (a) For reviews, see: Tsuji, J.; Mandai, T. Angew. Chem., Int. Ed.
Engl. 1995, 34, 2589. (b) Ohno, H. Chem. Pharm. Bull. 2005, 53, 1211.
(c) Labrosse, J.-R.; Lhoste, P.; Sinou, D. J. Org. Chem. 2001, 66, 6634.
(15) Ohno, H.; Okanao, A.; Kosaka, S.; Tsukamoto, K.; Ohata, M.;
Ishihara, K.; Maeda, H.; Tanaka, T.; Fujii, N. Org. Lett. 2008, 10, 1171.
(16) Yoshida, M.; Murao, T.; Sugimoto, K.; Ihara, M. Synlett 2007, 575.
(17) Another interesting synthetic application that involves propargyl
complexesisthepalladium-catalyzeddeoxygenationofpropargylformates.Radinov,
R.; Hutchings, S. D. Tetrahedron Lett. 1999, 40, 8955.
Figure 2. Free energy diagram for nucleophilic addition to
η3-propargyl complex 1-t-Bu.

126 Organometallics, Vol. 28, No. 1, 2009
Casey et al.
The deuterium-labeled metallacyclobutene [C5Me5(CO)2Re-
(CH2C[P(CD3)3]dCCMe3)][BF4] (7-d9) was prepared by addi-
tion of P(CD3)3 to 1-t-Bu. The kinetics of the exchange of a
4.5-fold excess of P(CH3)3 with 0.1 M 7-d9 were studied by 1H
NMR spectroscopy at 10 °C in CD3CN by monitoring the
appearance of the P(CH3)3 resonance of the exchanged metal-
lacyclobutene 7 (Figure 4). Data analysis gave kexch ) (4.9 (
0.5) × 10-5 s-1 and ∆Gq
) 22.4 kcal · mol-1
.
Since exchange of phosphine with 7 is much faster than
rearrangement to 8, it was not possible to measure both rates at
the same temperature. We therefore measured the rate of
10 °C
1
rearrangement of 7 to 8 by H NMR spectroscopy in CD3CN
Figure 3. X-ray structure of [C5Me5(CO)2Re(η2-Me3PCH2Ct
CCMe3)]+.
between 35 and 65 °C. A linear Eyring plot of ln[k/T] versus
1/T gave ∆Hq ) 30.1 ( 0.9 kcal · mol-1, ∆Sq ) 16.0 ( 0.9 eu,
and ∆Gq
) 25.6 kcal · mol-1. Extrapolation of the rear-
PCH2CtCCMe3)][BF4] (8),10 which was further characterized
in the current work by X-ray diffraction (Figure 3). The
rearrangement product, η2-alkyne complex 8, results from
nucleophilic addition to the less crowded terminal CH2 carbon
of the intermediate η3-propargyl complex 1-t-Bu and not to the
more sterically hindered tert-butyl-substituted carbon to form
an η2-allene isomer.
Since our method for determining relative regioselectivity
depends on the assumption that rearrangement of metallacy-
clobutene 7 to η2-alkyne complex 8 proceeds by phosphine
dissociation to generate an η3-propargyl intermediate, we needed
to establish the rate law for the rearrangement. This was
particularly necessary since associative mechanisms are con-
ceivable. For example, phosphine attack might occur at the
terminal carbon of 7 to give intermediate A, which could then
undergo loss of phosphine to generate 8 (Scheme 8). Alkyl and
vinyl CpRe(CO)2R- anions are well-known stable species.18 The
dissociative mechanism of Scheme 7 and the associative
mechanism of Scheme 8 are readily distinguished by their
kinetic independence or dependence on [PR3].
10 °C
rangement rate to 10 °C gave krearr ) 1.02 × 10-7 s-1
.
The rate of exchange of PMe3 with metallacyclobutene 7 is
therefore 480 times faster than the rearrangement of 7 to 8 at
10 °C (kexch/krearr ) [4.9 × 10-7 s-1] ÷ [1.0 × 10-7 s-1] )
480). Inspection of Figure 2 reveals that this is also the rate
difference between attack of PMe3 at the center carbon and the
CH2 terminal carbon of η3-propargyl complex 1-t-Bu. This rate
difference corresponds to a 3.2 kcal · mol-1 difference in free
energy of activation (∆∆Gq
) 25.6 - 22.4 kcal · mol-1).
10 °C
Regioselectivity of addition of PMePh2 to 1-t-Bu was also
determined indirectly by comparing the rates of rearrangement
and exchange of rhenacyclobutene [C5Me5(CO)2Re(CH2-
C[PMePh2]dCCMe3)][BF4] (9). Addition of PMePh2 to η3-
propargyl complex 1-t-Bu at -88 °C led to complete conversion
1
to the rhenacyclobutene 9, which had characteristic H NMR
resonances at δ 0.11 (d, J ) 12.6 Hz, CHH), 1.49 (d, J ) 12.2
Hz, CHH), and 2.39 (d, JPH ) 12.2 Hz, PCH3). The reaction of
excess PMe3 with PPh2Me-substituted metallacycle 9 led to
exchange and formation of the more stable PMe3-substituted
metallacycle 7. The rate constant for exchange at -40 °C was
Scheme 8
1
determined by H NMR spectroscopy: k ) 5.85 × 10-4 s-1
,
∆Gq ) 16.9 kcal · mol-1
.
Upon warming to room temperature, 9 rearranged to
η2-alkyne
complex
[C5Me5(CO)2Re(η2-Ph2MePCH2Ct
1
CCMe3)][BF4] (10), which had characteristic H NMR reso-
nances at δ 2.45 (d, JPH ) 13.1 Hz, PCH3), 4.15 (t, J ) 16.1
Hz, CHH), and 4.72 (t, J ) 15.8 Hz, CHH). The rate of
rearrangement of 9 to 10 at -15 °C was measured by 1H NMR
spectroscopy: k ) 3.27 × 10-5 s-1, ∆Gq(-15 °C) ) 20.3
kcal · mol-1. The difference between this barrier for rearrange-
ment and the barrier for exchange (∆∆Gq ) 20.3 - 16.9 ) 3.4
kcal · mol-1) is similar to the difference in barriers for exchange
and rearrangement of PMe3-substituted metallacycle 7 and
implies a regioselective preference of 960 for addition of
PMePh2 to the center propargyl carbon of 1-t-Bu.
The rate of rearrangement of metallacyclobutene complex 7
was studied in the presence and absence of added PMe3. Two
yellow CD3CN solutions19 of 7 were prepared in resealable
NMR tubes, and excess PMe3 was condensed into one. The rate
of rearrangement of 7 to 8 was monitored by 1H NMR
spectroscopy at 64 °C. First-order rate law fits were obtained.
The rate in the absence of added PMe3 (krearr ) (8.2 ( 0.1) ×
(7.9 ( 0.1) × 10-4 s-1) were very similar. The failure to observe
a rate enhancement in the presence of added PMe3 rules out an
associative mechanism for the rearrangement of metallacy-
clobutene complex 7 to η2-alkyne complex 8.20
10-4 s-1) and in the presence of 0.29 M added PMe3 (krearr
)
More Rapid Dissociation of PPh3 than PMe3 from
Metallacyclobutenes. Previously, we had reported that the
addition of PPh3 to 1-t-Bu gave the metallacyclobutene complex
[C5Me5(CO)2Re(CH2C(PPh3)dCCMe3)][BF4] (11), which sub-
sequently rearranged to alkyne complex [C5Me5(CO)2Re(η2-
Ph3PCH2CtCCMe3)][BF4] (12). In repeating this work, we
monitored the reaction of 1-t-Bu (0.030 M) with PPh3 (0.063
M) in CD2Cl2 by 1H NMR spectroscopy and found that
metallacycle 11 was formed at -78 °C with characteristic low-
frequency doublets for the metal-bound CH2 group at δ 0.14
and 1.1 (2J ) 12 Hz). When the sample was warmed to -20
°C, the conversion of metallacycle 11 to alkyne complex 12
(18) (a) Casey, C. P.; Vosejpka, P. C.; Askham, F. R. J. Am. Chem.
Soc. 1990, 112, 3713. (b) Casey, C. P.; Vosejpka, P. C.; Gavney, J. A., Jr.
J. Am. Chem. Soc. 1990, 112, 4083.
(19) Phosphine reactions in CD2Cl2 were problematic at elevated
temperatures due to nucleophilic attack of PMe3 on the solvent.
(20) The intriguing possibility of an associative mechanism for rear-
rangement of a metallacyclobutene was explored by studying rearrangements
involving 1,2-bis(diphenylphosphino)ethane ligands, where the associative
mechanism would be more likely with the possibility of intramolecular attack
by a second phosphine. Even with diphos ligands, the dissociative
mechanism is strongly favored. See Supporting Information.
1
was observed by H NMR spectroscopy to be 80% complete
within 2 h (∆Gq ≈ 18.9(5) kcal · mol-1). Upon warming to 25

Nucleophilic Addition to η3-Propargyl Re Complexes
Organometallics, Vol. 28, No. 1, 2009 127
Figure 4. Elapsed time 1H NMR spectra for exchange at 10 °C in CD3CN: 7-d9 + P(CH3)3 approaches equilibrium with 7 + P(CD3)3. Peak
t
4, P(CH3)3 of 7. Other peaks: peak 2, Cp* of 7; 5, CH of 7; 6, Bu of 7; 7, excess P(CH3)3; 1, toluene standard; 3, CD3CN.
Scheme 9
metallacyclobutene [C5Me5(CO)2Re(CH2C(PPh3)dCCH3)][BF4]
does not rearrange to either an η2-alkyne or η2-allene complex
even upon extended heating at 50-60 °C. Only eventual
decomposition is observed.
Observation of Equilibrium between η3-Propargyl
Complex 1-t-Bu and PPh3-Substituted Metallacyclobutene
1
11. Upon close examination of the low-temperature H NMR
spectra, it became apparent that the formation of metallacy-
clobutene 11 did not go to completion and that an equilibrium
was established with the η3-propargyl complex 1-t-Bu. After
PPh3 (0.063 M in CD2Cl2) was added to a yellow solution of
η3-propargyl complex 1-t-Bu (0.030 M in CD2Cl2) at -78 °C,
1H NMR spectroscopy at -88 °C showed a tiny resonance at δ
3.41 (CHH) characteristic of 1-t-Bu in addition to intense
resonances for the metallacyclobutene 11 (δ 0.14, CHH). Upon
increasing the temperature to -66 °C, the amount of η3-
propargyl complex 1-t-Bu increased and a 9.6:1 equilibrium
mixture of 11/1-t-Bu was observed (Keq ) [11]/[ 1-t-Bu][PPh3]
≈ 280 M-1). At -56 °C, the amount of the η3-propargyl
complex increased further, and a 4.5:1 equilibrium mixture of
11/1-t-Bu was observed (Keq ≈ 100 M-1). When the sample
was recooled to -66 °C, the ratio of 11: 1-t-Bu reverted to
9.6:1. At -46 °C, the ratio of 11:1-t-Bu dropped to 1.9:1 (Keq
≈ 44 M-1). When the temperature was lowered to -66 °C, the
ratio of 11:1-t-Bu increased to 10.8:1 (Keq ≈ 310 M-1). Thus,
as the temperature increases, the equilibrium shifts toward η3-
propargyl complex 1-t-Bu and free PPh3 at the expense of the
adduct rhenacyclobutene 11 (Figure 5).
1
°C, only η2-alkyne complex 12 was observed by H NMR
spectroscopy (Scheme 9). Note that this rate of rearrangement
conflicts with the preViously reported slow rearrangement of
the metallacycle 11 to η2-alkyne complex 12 oVer 2 days at
room temperature in CD2Cl2.10
The much more rapid rearrangement of the PPh3-substituted
rhenacyclobutene 9 than the PMe3-substituted rhenacyclobutene
7 (∆∆Gq ≈ 6.7(5) kcal · mol-1) implies a correspondingly more
rapid dissociation of phosphine from their respective rhenacy-
clobutenes and can be attributed in part to the greater leaving
group ability of the much less nucleophilic PPh3 group. In
addition, the particularly unfavorable steric interaction between
the large PPh3 and the bulky tert-butyl group is relieved upon
PPh3 dissociation. The much less congested methyl-substituted
When the sample was warmed to -34 °C, the ratio of 11:
1-t-Bu fell to 1.1:1 (Keq ≈ 25 M-1) and some conversion
to η2-alkyne complex [C5Me5(CO)2Re(η2-Ph3PCH2Ct
CCMe3)][BF4] (12) (16% after 10 min) was seen (∆Gq ≈ 17.5(5)
kcal · mol-1).21 At -23.5 °C, a 1:2 ratio of 11:1-t-Bu (Keq
≈
11 M-1) was seen along with 56% conversion to the thermo-
dynamically stable η2-alkyne complex 12 after 10 min (∆Gq ≈

128 Organometallics, Vol. 28, No. 1, 2009
Casey et al.
Figure 5. Ratio of rhenacyclobutene 11 to η3-propargyl complex 1-t-Bu as temperature is raised and lowered.
17.8(5) kcal mol-1).21 After 1 h at room temperature, complete
conversion to 12 was seen. When this solution of η2-alkyne
complex 12 was recooled to -66 °C, no change in the NMR
spectrum was seen, demonstrating that the formation of the
thermodynamically stable η2-alkyne complex is irreversible.
kcal · mol-1).22 When the solution was recooled to -66 °C, the
ratio reverted to 0.43:1. When the solution was warmed to -2
°C, the ratio dropped to 0.2:1 (Keq ≈ 50 M-1, ∆G°(-2 °C) ≈
-2.1 kcal · mol-1) and some rearrangement to the thermody-
namically stable η2-alkyne complex 10 was observed (6% after
5 min). Upon further increasing the temperature to 9 °C, the
ratio of 9:1-t-Bu dropped to 0.1:1 (Keq ≈ 25 M-1, ∆G°(9 °C)
≈ -1.8 kcal · mol-1). After 15 min at 9 °C, all the PMePh2
was consumed and only propargyl complex 1-t-Bu and η2-
alkyne complex 10 were observed. A plot of ∆G versus
temperature allowed extrapolation to ∆G°(0 °C) ≈ -2.0
kcal · mol-1.
Additional measurements of the equilibrium between the
rhenacyclobutene and the η3-propargyl complex carried out
using lower concentrations of the PPh3 (0.042 M in CD2Cl2)
and the same concentration of η3-propargyl complex 1-t-Bu
(0.03 M in CD2Cl2) gave similar values for the equilibrium
constants. At -66 °C, a 4.5:1 equilibrium mixture of rhenacy-
clobutene 11:η3-propargyl complex 1-t-Bu was observed by 1H
NMR spectroscopy (Keq ≈ 260 M-1). At -46 °C, a 1:1 mixture
of 11/1-t-Bu was seen (Keq ≈ 35 M-1). A van’t Hoff plot of all
the equilibrium data gave ∆H° ) -8.0 ( 1 kcal · mol-1, ∆S°
) -25 ( 3 eu, and ∆G°(0 °C) ) -1.2 ( 0.2 kcal · mol-1
.
Equilibrium between η3-Propargyl Complex 1-t-Bu and
Other Phosphine-Substituted Metallacyclobutenes. The more
nucleophilic phosphine PMePh2 formed the more stable met-
allacyclobutene 9, and the less nucleophilic phosphine P(C6H4-
p-F)3 formed the less stable metallacyclobutene 13.
Addition of PMePh2 (0.05 M in CD2Cl2) to η3-propargyl
complex 1-t-Bu (0.030 M in CD2Cl2) at -88 °C led to complete
conversion to rhenacyclobutene 9. Neither η3-propargyl complex
1-t-Bu nor the stable η2-alkyne complex 10 was observed by
1H NMR spectroscopy below -24 °C. However, upon warming
Figure 6. Thermodynamics of rhenacyclobutene formation.
1
Addition of the less nucleophilic phosphine P(C6H4-p-F)3
(0.05 M in CD2Cl2) to η3-propargyl complex 1-t-Bu (0.030 M
to -13 °C, H NMR spectroscopy showed small amounts of
the η3-propargyl complex 1-t-Bu (8%) and of η2-alkyne complex
10 (17% after 10 min and increasing amounts at longer times).
When the temperature was raised to 8 °C, the amount of η3-
propargyl complex 1-t-Bu remained relatively constant (8%)
while the amount of rhenacyclobutene complex 9 decreased and
the amount of η2-alkyne complex 10 increased.
At lower than stoichiometric concentrations of PMePh2 (0.011
M in CD2Cl2) compared to 1-t-Bu (0.030 M in CD2Cl2), a
relatively constant 0.43:1 ratio of rhenacyclobutene complex 9
to η3-propargyl complex 1-t-Bu was seen between -88 and -56
°C. Too little free PMePh2 was present to be observed. When
the temperature was increased to -24 °C, some dissociation of
PMePh2 from the metallacycle occurred and the ratio of 9:1-t-
Bu decreased to 0.32:1 (Keq ≈ 80 M-1, ∆G(-24 °C) ≈ -2.2
in CD2Cl2) at -80 °C led to slow conversion (∆Gq (metallacycle
21
formation) ≈ 14.2(8) kcal mol-1
)
to the rhenacyclobutene
[C5Me5(CO)2Re(CH2C[P(C6H4-p-F)3]dCCMe3)][BF4] (13), which
had characteristic H NMR resonances at δ 0.05 (CHH) and
1
0.70 (CMe3). After 2 h, a 1.3:1 equilibrium mixture of 13/1-t-
Bu was observed (Keq ) [13]/[1-t-Bu][PAr3] ≈ 38 M-1). When
the solution was warmed to -70 °C, the ratio dropped to 0.56:1
(Keq ≈ 14 M-1). Upon further increasing the temperature to
-60 °C, the ratio of 13:1-t-Bu dropped to 0.2:1 (Keq ≈ 4.6 M-1).
Conversion to the η2-alkyne complex [C5Me5(CO)2Re[η2-(p-F-
C6H4)3PCH2CtCCMe3][BF4] (14) began at -30 °C and was
complete at -20 °C (∆Gq ≈ 17.2(8) kcal · mol-1).21
(22) Because the free phosphine was difficult to observe directly, its
concentration was estimated by subtracting the concentration of rhenacy-
clobutene complex and η2-alkyne complex from the initial phosphine
concentration. Errors in estimation are considerable, and only very ap-
proximate equilibrium constants were obtained.
(21) These approximate ∆Gq values are obtained from the approximate
rates. If rate estimates are off by a factor of 3 at 0 °C, ∆Gq would be off
by 0.6 kcal · mol-1
.

Nucleophilic Addition to η3-Propargyl Re Complexes
Organometallics, Vol. 28, No. 1, 2009 129
Additional measurements of the equilibrium between the 13
and 1-t-Bu were carried out using a higher concentration of the
P(C6H4-p-F)3 (0.10 M in CD2Cl2), and the same concentration
of 1-t-Bu (0.03 M in CD2Cl2) gave similar values for the
equilibrium constants. [At -80 °C, 4.3:1 13:1-t-Bu, Keq ≈ 54
M-1; at -70 °C, 1.7:1 13:1-t-Bu, Keq ≈ 20 M-1; at -60 °C,
0.7:1 13:1-t-Bu, Keq ≈ 7.6 M-1.] A van’t Hoff plot gave ∆H°
) -7.2 ( 0.5 kcal · mol-1, ∆S° ) -30 eu ( 4, and ∆G°(0 °C)
) -1.0 ( 0.2 kcal · mol-1
.
The equilibrium between PMe3-substituted metallacycle 7 and
η3-propargyl complex 1-t-Bu lay too far on the side of the
metallacycle to be directly observable. A limit on the equilibrium
constant can be estimated from the fact that none of 1-t-Bu was
detectable by 1H NMR spectroscopy of 0.1 M solutions of 7, and
2% of 1-t-Bu would have been readily detected. This requires Keq
g 25 000 M-1 and ∆G°(0 °C) e -5.5 kcal · mol-1. Figure 5
summarizes the thermodynamic information of the equilibrium
between rhenacyclobutenes and η3-propargyl complex 1-t-Bu.
Figure 7. Free energy diagram for reaction of PMe3 with 1-t-Bu.
Discussion
Discussions of nucleophilic addition to η3-propargyl complexes
usually involve comparisons with well-studied η3-allyl complexes.
Transition metal η3-allyl complexes usually undergo nucleo-
philic addition to a terminal carbon,23 although an increasing
number of cases of attack at the central carbon have been
reported. The most direct evidence for attack at the central
carbon of η3-allyl complexes comes from isolation of metalla-
cyclobutanes.24 Formation of cyclopropanes in the reaction of
nucleophiles with η3-allyl complexes is best explained by attack
at the central carbon followed by reductive elimination (Scheme
10).25 η3-Allyl complexes having a leaving group such as Cl or
OR at the central carbon sometimes react with nucleophiles to
give products in which the substituent at the central carbon is
replaced by a nucleophile and another nucleophile adds to the
Thermodynamics of Rhenacyclobutene Formation. It is
surprising that the metallacyclobutene complexes and η3-propargyl
complex 1-t-Bu are so similar in thermodynamic stability. For PPh3,
PPh2Me, and P(C6H4-p-F)3, both species were observable in
solution and Keq could be measured directly. For PMe3, η3-
propargyl complex 1-t-Bu was not directly observable, and a limit
of ∆G° e -5.5 kcal · mol-1 was placed on the stability of the
metallacycle. The stability of the metallacycles follows the same
order as the nucleophilicity of the phosphines. Steric effects are
also important in determining the stability of the metallacycles, as
shown by the fact that the metallacycle formed by addition of PPh3
to Me-substituted η3-propargyl complex 1-Me is much more
kinetically stable than the metallacycle 11 formed by addition of
PPh3 to t-Bu-substituted 1-t-Bu.
Kinetics of Exchange and Rearrangement. The kinetics of
exchange of phosphines with the metallacycles proceed by a
kinetically first-order dissociative mechanism and were mea-
sured only for metallacycles 7 and 9. The barrier for exchange
of PMe3 with PPh2Me-substituted metallacycle 9 was 5.5 kcal
lower than the barrier for PMe3 exchange with deuterium-labeled
metallacycle 7-d9. This activation energy difference parallels
the thermodynamic stability of the metallacycles and the leaving
group ability of the phosphines.
(23) (a) Trost, B. M.; Verhoeven, T. R. In ComprehensiVe Organome-
tallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.;Perga-
mon: Oxford, UK, 1982; Vol. 8, Chapter 57. (b) Tsuji, J. Tetrahedron 1986,
42, 4361. (c) Trost, B. M.; Van Vranken, D. L. Chem. ReV. 1996, 96, 395.
(d) Trost, B. M.; Crawley, M. L. Chem. ReV. 2003, 103, 2921. (e) Trost,
B. M. J. Org. Chem. 2004, 69, 5813.
(24) (a) Ephritikhine, M.; Green, M. L. H.; Mackenzie, R. E. J. Chem.
Soc., Chem. Commun. 1976, 619. (b) Ephritikhine, M.; Francis, B. R.; Green,
M. L. H.; Mackenzie, R. E.; Smith, M. J. J. Chem. Soc., Dalton Trans.
1977, 1131. (c) Adam, G. J. A.; Davies, S. G.; Ford, K. A.; Ephritikhine,
M.; Todd, P. F.; Green, M. L. H. J. Mol. Catal. 1980, 8, 15. (d) Periana,
R. A.; Bergman, R. G. J. Am. Chem. Soc. 1984, 106, 7272. (e) McGhee,
W. D.; Bergman, R. G. J. Am. Chem. Soc. 1985, 107, 3388. (f) Periana,
R. A.; Bergman, R. G. J. Am. Chem. Soc. 1986, 108, 7346. (g) Tjaden,
E. B.; Stryker, J. M. J. Am. Chem. Soc. 1990, 112, 6420. (h) Wakefield,
J. B.; Stryker, J. M. J. Am. Chem. Soc. 1991, 113, 7057. (i) Tjaden, E. B.;
Stryker, J. M. Organometallics 1992, 11, 16. (j) Tjaden, E. B.; Schwiebert,
K. E.; Stryker, J. M. J. Am. Chem. Soc. 1992, 114, 1100. (k) Schwiebert,
K. E.; Stryker, J. M. Organometallics 1993, 12, 600. (l) Tjaden, E. B.;
Stryker, J. M. J. Am. Chem. Soc. 1993, 115, 2083. (m) Review of
metallacyclobutenes: Jennings, P. W.; Johnson, L. L. Chem. ReV. 1994,
94, 2241.
(25) (a) Hegedus, L. S.; Darlington, W. H.; Russell, C. E. J. Org. Chem.
1980, 45, 5193. (b) Carfagna, C.; Maniani, L.; Musco, A.; Sallese, G.; Santi,
R. J. Org. Chem. 1991, 56, 3924. (c) Carfagna, C.; Galarini, R.; Musco,
A.; Santi, R. Organometallics 1991, 10, 3956. (d) Hoffmann, H. M. R.;
Otte, A. R.; Wilde, A. Angew. Chem., Int. Ed. Engl. 1992, 31, 234. (e)
Wilde, A.; Otte, A. R.; Hoffmann, H. M. R. J. Chem. Soc., Chem. Commun.
1993, 615. (f) Formica, M.; Musco, A.; Pontellini, R.; Linn, K.; Mealli, C.
J. Organomet. Chem. 1993, 448, C6. (g) Carfagna, C.; Galarini, R.; Linn,
K.; Lo´pez, J. A.; Mealli, C.; Musco, A. Organometallics 1993, 12, 3019.
(h) Hoffmann, H. M. R.; Otte, A. R.; Wilde, A.; Menzer, S.; Williams,
D. J. Angew. Chem., Int. Ed. Engl. 1995, 34, 100. (i) Satake, A.; Nakata,
T. J. Am. Chem. Soc. 1998, 120, 10391. (j) Satake, A.; Koshino, H.; Nakata,
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4, 707.
Crude measurements of the rates of rearrangement of all the
metallacycles to η2-alkyne complexes were made for all the
metallacycles. The barriers for rearrangement also paralleled
the thermodynamic stability of the metallacycles and the leaving
group ability of the metallacycles [∆Gq for PMe3 (25.6) >
PPh2Me (20.3) > PPh3 (∼18) > P(C6H4-p-F)3 (∼17)].
Regioselectivity of metallacyclobutene formation was too
high to be measured directly for all of the phosphines studied.
The regioselectivity of PMe3 addition to 1-t-Bu was carefully
determined by an indirect method: comparison of the rate of
exchange of PMe3 with deuterium-labeled metallacycle 7-d9 with
the rate of rearrangement of metallacycle 7 to η2-alkyne complex
8 (Figure 7). The regioselectivity for addition of PMe3 to the
central carbon of 1-t-Bu was 480:1 (∆∆Gq ) 3.2 kcal · mol-1).
Similarly, a similar regioselectivity (∆∆Gq ) 3.4 kcal · mol-1
)
for addition of PPh2Me to the central carbon of 1-t-Bu was
estimated from the rate of exchange of PMe3 with rhenacy-
clobutene 9 and from the rate of rearrangement of 9 to 10.
DFT computational studies of rhenium η3-propargyl
complexes were undertaken to better understand the regiose-
lective addition of nucleophiles to the central propargyl carbon.

130 Organometallics, Vol. 28, No. 1, 2009
Casey et al.
Scheme 10
Scheme 11
terminal carbon (Scheme 11).26 These reactions have been
suggested to proceed by an unusual addition of a nucleophile
to the central carbon followed by elimination of the leaving
group from the central carbon to give a substituted η3-allyl
complex, which is then attacked at a terminal carbon by a second
nucleophile. However, an alternative mechanism involves
elimination of HX from the η3-allyl complex to produce an η3-
propargyl complex, which then undergoes “normal” addition
of a nucleophile to the central carbon to produce a metallacy-
clobutene; protonation of the metallacyclobutene can then
produce an η3-allyl complex, which is then attacked at a terminal
carbon by a second nucleophile.
Figure 8. HOMO, LUMO, and LUMO+1 of [C5H5(CO)2Re(η3-
CH2CtCH)]+ (B).
Several computational studies of η3-allyl compounds have
addressed the regioselectivity issue and provided insight into the
importance of ligand substitution, conformational preference, and
the question of orbital control versus charge control.27 Most studies
have focused on η3-allyl palladium and platinum complexes.28
center carbon. Wojcicki and Bursten reported30 calculations of
the model η3-propargyl complex [(PH3)2Pt(η3-CH2CtCH)]+
using the Fenske-Hall (FH) molecular orbital method and later
DFT calculations31 and concluded that nucleophilic addition to
the central η3-propargyl carbon was charge controlled.
Since no analogous theoretical studies of mid-transition metal
η3-propargyl compounds were available, we carried out DFT
calculations on the model complex [C5H5(CO)2Re(η3-
CH2CtCH)]+ (B) using B3LYP/LANL2DZ,32 available in the
Gaussian 9433 and Gaussian 9834 packages. The molecular
orbitals of the η3-propargyl fragment may be described as the
simple combination of π-systems for its allyl and ethylene
fragments. DFT calculations gave an accurate picture of the
frontier molecular orbitals (FMOs) for B and provided insight
into the regioselectivity of nucleophilic additions. The energy
diagram and the molecular orbitals of the HOMO, LUMO, and
LUMO+1 of B are shown in Figure 8.
If kinetic addition of nucleophiles to B were orbital controlled,
the LUMO of B should have a significant coefficient on the central
carbon. Since the LUMO calculated for B has a node at the center
carbon and is concentrated on the terminal carbons, the observed
nucleophilic attack at the central carbon is not orbital controlled.
If nucleophilic attack at the η3-fragment were charge controlled,
Green has reported extended Hu¨ckel molecular orbital
calculations (EHMO) on the neutral η3-propargyl molybdenum
complex C5H5(η2-HCtCH)Mo(η3-CH2CtCH).29 Although the
regiochemistry of nucleophilic addition was not commented
upon, the EHMO results indicated a significant difference in
calculated atomic charges between the center (positive) and
terminal (negative) carbons, which might form the basis of a
charge-controlled explanation of addition of nucleophiles to the
(26) (a) Ohe, K.; Matsuda, H.; Morimoto, T.; Ogoshi, S.; Chatani, N.;
Murai, S. J. Am. Chem. Soc. 1994, 116, 4125. (b) Castan˜o, A. M.; Aranyos,
A.; Szabo´, K. J.; Ba¨ckvall, J.-E. Angew. Chem., Int. Ed. Engl. 1995, 34,
2551. (c) Tsai, F.-Y.; Chen, H.-W.; Chen, J.-T.; Lee, G.-H.; Wang, Y.
Organometallics 1997, 16, 822. (d) Aranyos, A.; Szabo´, K. J.; Castan˜o,
A. M.; Ba¨ckvall, J.-E. Organometallics 1997, 16, 1058. (e) Organ, M. G.;
Miller, M. Tetrahedron Lett. 1997, 38, 8181. (f) Organ, M. G.; Miller, M.;
Konstantinou, Z. J. Am. Chem. Soc. 1998, 120, 9283. (g) Kadota, J.; Komori,
S.; Fukumoto, Y.; Murai, S. J. Org. Chem. 1999, 64, 7523. (h) Kadota, J.;
Katsuragi, H.; Fukumoto, Y.; Murai, S. Organometallics 2000, 19, 979. (i)
Organ, M. G.; Arvanitis, E. A.; Hynes, S. J. J. Org. Chem. 2003, 68, 3918.
(27) (a) Suzuki, T.; Fujimoto, H. Inorg. Chem. 1999, 38, 370. (b) Sakaki,
S.; Takeuchi, K.; Sugimoto, M.; Kurosawa, H. Organometallics 1997, 16,
2995. (c) Aranyos, A.; Szabo´, K. J.; Castan˜o, A. M.; Ba¨ckvall, J.-E.
Organometallics 1997, 16, 1058. (d) Ward, T. R. Organometallics 1996,
15, 2836. (e) Carfagna, C.; Galarini, R.; Linn, K.; Lopez, J. A.; Mealli, C.;
Musco, A. Organometallics 1993, 12, 3019. (f) Curtis, M. D.; Eisenstein,
O. Organometallics 1984, 3, 887. (g) Davies, S. G.; Green, M. L. H.;
Mingos, D. M. P. Tetrahedron 1978, 34, 3047.
(30) Graham, J. P.; Wojcicki, A.; Bursten, B. E. Organometallics 1999,
18, 837.
(31) Hall, M. B.; Fenske, R. F. Inorg. Chem. 1972, 11, 768.
(32) Density functional theory (DFT) has been established as an excellent
method for calculations involving organometallic compounds. (a) Ziegler,
T. Chem. ReV 1991, 91, 651. (b) Ziegler, T. Can. J. Chem. 1995, 73, 743.
(33) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.;
Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G. A.;
Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.;
Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara,
A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.;
Adres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.;
Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon, M.;
Gonzalez, C.; Pople, J. A. Gaussian 94, ReVision E.2; Gaussian, Inc.:
Pittsburgh, PA, 1995.
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Nucleophilic Addition to η3-Propargyl Re Complexes
Organometallics, Vol. 28, No. 1, 2009 131
had a node at the central carbon, and that nucleophilic attack at
the central carbon involved interaction with LUMO+1. Their
conclusion that regioselective attack at the central carbon
resulted from a combination of charge control and the avail-
ability of a low-lying orbital centered on the middle propargylic
carbon is similar to that reported here.
Experimental Section
Rate of Rearrangement of 7 to 8. A CD3CN solution of 7 (26
mg, 0.04 mmol, 0.085 M) containing toluene as an internal
integration standard (10 µL, 8.5 µmol) was monitored by 1H NMR
spectroscopy at 64.5 °C over 1.5 h. A plot of ln [7] versus time
was linear to over 3 half-lives (R2 > 0.98) and gave krearr ) 7.25 ×
10-4 s-1, t1/2 ) 16 min, and ∆Gq ) 24.7 kcal · mol-1
.
Figure 9. Walsh orbital diagram for approach of PH3 to the center
carbon of B.
Measurements were repeated on similarly prepared samples of
7 over the temperature range 35-55 °C, and the resulting rates of
rearrangement were used in an Eyring plot of kobs versus 1/T: kobs(35
°C) ) 9.2 × 10-6 s-1; kobs(44 °C) ) 3.1 × 10-5 s-1; kobs(54 °C) )
1.7 × 10-4 s-1 [see Figure 1-S in the Supporting Information for
the charge on the central carbon of the η3-propargyl unit should
be more positive than that on the terminal carbons. The natural
charges computed by the natural population analysis35subset of the
DFT calculation show a greater positive charge on the center carbon
and support the hypothesis that the observed kinetic addition to
the central carbon is charge controlled (Figure 8).36
Eyring plot]. These data provided ∆Hq ) 30.1 ( 0.9 kcal · mol-1
,
∆Sq ) 16.0 ( 0.9 eu, and were used to calculate ∆Gq10 °C ) 25.6
kcal · mol-1, krearr ) 1.02 × 10-7 s-1, t1/2 ≈ 78 days.
[C5Me5(CO)2Re(CH2C[P(CD3)3]dCCMe3)][BF4] (7-d9). An
excess of P(CD3)3 was condensed onto a frozen CH2Cl2 solution
of 1-t-Bu (28.2 mg, 0.05 mmol) at 77 K. A pale yellow-green
solution was obtained upon brief mixing at room temperature.
Volatile material was evaporated to give 7-d9 in quantitative yield
The attack of a nucleophile at the central carbon of an η3-
propargyl complex can be envisioned as attack at LUMO+1,
which has a major contribution from the central carbon. Thus,
an incoming nucleophile could interact with the slightly higher
energy LUMO+1 orbital to avoid steric repulsions at the C1
and C3 positions, resulting in a preference for central attack. A
linear synchronous transit calculation (LST) was performed to
approximate addition of the model nucleophile PH3 to the central
propargyl carbon of B to give the model metallacyclobutene
complex [C5H5(CO)2Re(CH2C(PH3)dCH)]+. Along the reaction
coordinate, a rapid decrease in energy of the HOMO (lone pair
of PH3) was observed as the lone pair develops a bonding
interaction, and an inversion of the LUMO and LUMO+1
orbitals was observed as the nucleophile approaches (Figure 9).
This implies that while charge controls the site of nucleophilic
attack, the accessible energy and nodal properties of LUMO+1
allow the attack to proceed smoothly. An exclusive LUMO
orbital control argument can be made for the observed η2-alkyne
and η2-allene rearrangement products arising from nucleophilic
attack at the terminal carbons.
1
as an off-white flaky powder. H NMR (CD3CN, 360 MHz, -20
°C): δ 0.086 (d, J ) 12.2 Hz, CHH), 1.01 (s, CMe3), 1.40 (dd, J
) 12.3, 1.1 Hz, CHH), 1.85 (s, C5Me5).
Rate of Exchange of P(CH3)3 with 7-d9. Excess P(CH3)3 was
condensed onto a frozen (77 K) CD3CN solution of 7-d9 (30 mg, 0.047
mmol, 0.107 M) containing toluene (10 µL, 8.5 µmol) as an internal
standard for NMR integration. The sample was melted, thoroughly
mixed at -78 °C, and transferred to a NMR spectrometer probe
precooled to -20 °C. Integration showed that its initial concentration
of P(CH3)3 was 0.55 M (5 equiv relative to 7-d9). The NMR probe
was warmed to 10 °C (thermocouple calibration), and the appearance
of the P(CH3)3 1H NMR resonance of 7 (δ 1.79) was monitored versus
toluene internal standard. The reaction was monitored for 1.5 half-
lives, with a late data point acquired after 3 half-lives [see Figure 2-S
in the Supporting Information for kinetic plot]. Exponential fit of the
concentration versus time data (R2 > 0.98) gave a calculated equilib-
rium concentration for 7 (0.0979 M) and kexch ) (4.9 ( 0.5) × 10-5
s-1, t1/2 ) 3.9 h, and ∆Gq ) 22.1 kcal · mol-1.
A similar DFT study of [(PH3)2Pd(η3-CH3CHCtCH)]+ was
performed by Delbecq, Sinou, and co-workers.37 Similar to the
results reported here for a rhenium cation, they found that the
central propargylic carbon was most positive, that the LUMO
Acknowledgment. Financial support was provided by the
National Science Foundation (CHE-0209476). Grants from the
NSF (CHE-9629688) and NIH (I S10 RR04981-01) for the
purchase of NMR spectrometers are acknowledged. We thank
Dr. Douglas R. Powell for assistance with X-ray crystal-
lography, Dr. Charles Fry for assistance with NMR spectrom-
etry, and Professor Frank A. Weinhold for assistance with
computations.
(34) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.,
Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.;
Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.;
PiskorzP.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith,
T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.;
Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.;
Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian
98, ReVision A.9; Gaussian, Inc.: Pittsburgh, PA, 1998.
(35) (a) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter,
J. E.; Weinhold, F. NBO 4.0; Theoretical Chemistry Institute, University
of Wisconsin: Madison, WI, 1996. (b) Reed, A. E.; Weinstock, R. B.;
Weinhold, F. J. Chem. Phys. 1985, 83, 735.
(36) The Mulliken charges show a similar trend, with the central carbon
being most positive: CH2(+.091)sC(+0.357)sCH(+0.005).
(37) Labrosse, J.-R.; Lhoste, P.; Delbecq, F.; Sinou, D. Eur. J. Org.
Chem. 2003, 2813.
Supporting Information Available: General experimental
information, experimental procedures, additional free energy dia-
grams for reactions of phosphines with 1-t-Bu3, information on
rearrangement of diphosphine-substituted metallacyclobutenes,
selected computational output for [C5H5(CO)2Re(η3-CH2CtCH)]+
(B), X-ray crystallographic data for 8-Cl and {C5Me5(CO)2Re[η2-
(Ar2PCH2CH2PPh2)CH2CtCCMe3]}[BF4] · 2CH2Cl2 (18-CH2Cl2).
This material is available free of charge via the Internet at
http://pubs.acs.org.
OM800739J