Kinetic and mechanistic studies of the formal (1+2) -and (1+4)-Cycloadditions of germylenes to conjugated dienes

Article abstract of DOI:10.1021/om900791e

Fast kinetic studies of the reactions of isoprene and 2,3-dimethyl-l,3- butadiene (DMB) with diphenylgermylene (GePh₂, 2a) and of isoprene with a series of diarylgermylenes bearing polar ring substituents (GeAr ₂, 2b-g) have been car

Full text of DOI:10.1021/om900791e

Organometallics 2009, 28, 6777–6788 6777
DOI: 10.1021/om900791e
Kinetic and Mechanistic Studies of the Formal (1þ2)- and
(1þ4)-Cycloadditions of Germylenes to Conjugated Dienes
Lawrence A. Huck and William J. Leigh*
Department of Chemistry and Chemical Biology, McMaster University, Hamilton ON L8S 4M1, Canada
Received September 10, 2009
Fast kinetic studies of the reactions of isoprene and 2,3-dimethyl-1,3-butadiene (DMB) with
diphenylgermylene (GePh2, 2a) and of isoprene with a series of diarylgermylenes bearing polar ring
substituents (GeAr2, 2b-g) have been carried out in hexanes solution. Though the major stable products
of the reactions with isoprene are the corresponding 1,1-diarylgermacyclopent-3-ene derivatives, the
results indicate that the major initial products are the corresponding transient 1,1-diaryl-2-vinylgermir-
anes (6a-g) resulting from formal (1þ2)-cycloaddition to the less-substituted CdC bond of the diene.
These compounds are formed reversibly and with rate constants in excess of 109 M-1 s-1, and appear as
discrete reaction intermediates exhibiting λmax = 285 nm and lifetimes of 2-670 μs depending on the
identity of the germylene and the diene. The variations in the lifetimes with aryl substituents are shown to
be most consistent with a stepwise mechanism for vinylgermirane fgermacyclopent-3-ene isomerization,
involving (reversible) dissociation to the free germylene and diene followed by (irreversible) (1þ4)-
cycloaddition. The bimolecular rate constants for (1þ4)-cycloaddition to isoprene, calculated from the
data on the basis of this model, vary over the range of 5 ꢀ 106 to 3 ꢀ 108 M-1 s-1 depending on the aryl
ring substituent(s). The variation in the rate and equilibrium constants for reaction of 2a-g with isoprene
indicates that the germylene plays the role of an electrophile in both the (1þ2)- and (1þ4)-cycloaddition
processes and demands the involvement of a polarized steady-state intermediate in the (1þ2)-addition
reaction. The temperature dependence of the experimental rate and equilibrium constants for reaction of
GePh2 with isoprene, and of the rate coefficient for decay of the corresponding vinylgermirane, allows
most aspects of the potential energy surface to be defined quantitatively.
Introduction
with 2,3-dimethylbutadiene (DMB),1,2,7,8 less highly substi-
tuted alkyl- and phenyl-substituted acyclic dienes also afford
products derived from addition of two molecules of the diene
to GeMe2, in addition to the 1:1 cycloadduct.9 Analogous
(1:2)-cycloadducts are also known to be formed upon reac-
tion of GeMe2 with styrene10,11 and were proposed to result
from reaction of a second molecule of alkene with the
corresponding (transient) germirane, formed by (1þ2)-cy-
cloaddition in the initial step. The corresponding trappable
intermediate in the reaction with dienes would thus be a
transient vinylgermirane, whose usual fate is conversion to
the isomeric germacyclopent-3-ene in high yield (eq 2). The
mechanism of the isomerization has been alternatively for-
mulated as a formal [1,3]-sigmatropic rearrangement and a
One of the best known and most frequently used trapping
reactions of reactive germylene derivatives is the formal
(1þ4)-cheletropic addition of conjugated dienes, which af-
fords the corresponding germacyclopent-3-ene derivative
in high chemical yields (eq 1).1,2 Early studies of the reac-
tions of thermally generated dimethylgermylene (GeMe2)
with dienes showed that the reaction proceeds stereo-
specifically,3-6 which led to the proposal that it proceeds
via a concerted (1þ4)-cycloaddition mechanism.
(7) Bobbitt, K. L.; Lei, D.; Maloney, V. M.; Parker, B. S.; Raible,
J. M.; Gaspar, P. P. In Frontiers of Organogermanium, -tin and -lead
Chemistry. Proceedings of the VIIth International Conference on the
Organometallic and Coordination Chemistry of Germanium, Tin and
Lead, Riga, 1993; Lukevics, E., Ignatovich, L., Eds.; Latvian Institute of
Organic Synthesis: Riga, 1993; pp 41-53.
While the (1þ4)-cycloadduct is the only product observed
from reaction of GeMe2 and many other germylene derivatives
*Corresponding author. E-mail: leigh@mcmaster.ca.
(1) Neumann, W. P. Chem. Rev. 1991, 91, 311.
(8) Neumann, W. P.; Weisbeck, M. P.; Wienken, S. Main Group Met.
Chem. 1994, 17, 151.
(2) Tokitoh, N.; Ando, W.; Moss, R. A.; Platz, M. S.; Jones, M., Jr. In
Reactive Intermediate Chemistry; John Wiley & Sons: New York, 2004;
pp 651-715.
(3) Schriewer, M.; Neumann, W. P. Angew. Chem., Int. Ed. Engl.
1981, 20, 1019.
(4) Ma, E. C. L.; Kobayashi, K.; Barzilai, M. W.; Gaspar, P. P.
(9) Neumann, W. P.; Michels, E.; Koecher, J. Tetrahedron Lett. 1987,
28, 3783.
(10) Koecher, J.; Neumann, W. P. Organometallics 1985, 4, 400.
(11) Shusterman, A. J.; Landrum, B. E.; Miller, R. L. Organometal-
lics 1989, 8, 1851.
J. Organomet. Chem. 1982, 224, C13.
(5) Schriewer, M.; Neumann, W. P. J. Am. Chem. Soc. 1983, 105, 897.
(6) Koecher, J.; Neumann, W. P. J. Am. Chem. Soc. 1984, 106, 3861.
(12) Leigh, W. J.; Huck, L. A.; Held, E.; Harrington, C. R. Silicon
Chem. 2005, 3, 139.
(13) Nag, M.; Gaspar, P. P. Organometallics 2009, 28, 5612.
r
2009 American Chemical Society
Published on Web 11/17/2009
pubs.acs.org/Organometallics

6778 Organometallics, Vol. 28, No. 23, 2009
Huck and Leigh
stepwise cycloreversion/(1þ4)-cycloaddition process.1,12,13
Thus, the faster-formed (1þ2)-cycloadduct is either a neces-
sary intermediate in the formation of the stable (1þ4)-
cycloadduct or a reversibly formed bystander that serves
mainly as a mediator in the overall reaction. The two
mechanisms are kinetically indistinguishable.
Detailed kinetic information on the reactions of isoprene
and DMB with transient silylenes and germylenes such as
SiMe2,22 SiMePh,23 SiPh2,22 GeMe2,24 GeMePh,25 and
26
GePh2 under ambient conditions in hexanes solution has
been reported, and in all cases the measured absolute rate
constant for consumption of the free metallylene by the diene
approaches the diffusion-controlled limit. The silylenes are
each slightly more reactive than the germylene of homolo-
gous structure, as is also the case for the dimethyl- and
dihydrometallylenes in the gas phase,27 but in both series
of metallylenes the rate constants for reaction with iso-
prene vary by only a factor of ca. 2, in the order MMe2 ∼
MMePh > MPh2.23-25 The results for the germylenes show
that the main component of their reactions with isoprene
are reversible on the microsecond time scale and are char-
acterized by equilibrium constants ranging from ca.
6000 M-1 for GePh2 to ca. 20 000 M-1 for GeMe2.24-26
The kinetic behavior of GeMe2 and GeMePh in the presence
of DMB is also consistent with fast, reversible reaction in
both cases; accurate values of the equilibrium constants
could not be measured, but in the case of GeMePh it is
clearly considerably smaller than that for reaction with
isoprene.24,25 In contrast, the results for the silylenes are
consistent with equilibrium constants in considerable ex-
cess of 105 M-1 in all cases, such that in fast time-resolved
experiments the primary reactions appear to be irrever-
sible.22,23
While the corresponding 1-silacyclopent-3-enes are also
the major products of reaction of dienes with dimethylsily-
lene (SiMe2) under high-temperature conditions, their for-
mation was proposed to result from secondary thermal
isomerization of the corresponding vinylsiliranes even in
the earliest studies of the reaction.14-18 The reaction of
aliphatic dienes with Si(tBu)219 and with diarylsilylenes such
as SiMes2 (Mes = 2,4,6-trimethylphenyl),20 Si(Mes)Tbt
(Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl),21 and
22
SiPh2 at ambient temperatures affords the corresponding
vinylsiliranes as the major products, stereospecifically in the
case of SiMes2.20 Those derived from SiMes2 and Si(Mes)Tbt
are stable indefinitely at room temperature, but isomerize
stereospecifically to the corresponding silacyclopent-3-enes
upon heating,20,21 while that from SiPh2 does so over a
number of hours in solution at room temperature.22 Tokitoh
and co-workers were able to show that the isomerization of
the (1þ2)-cycloadduct from Si(Mes)Tbt and isoprene pro-
ceeds via the stepwise pathway involving (reversible) disso-
ciation to the free silylene and diene followed by direct (1þ4)-
cycloaddition; thus, in the case of heavily hindered systems at
least, the formation of the formal (1þ2)- and (1þ4)-cycload-
dition products occurs competitively.21 A very recent com-
putational study of the reactions of SiMe2 and GeMe2 with
1,3-butadiene, and of SiPh2 and GePh2 with alkylated
dienes, indicates that in all these cases this mechanism
provides a significantly lower energy pathway to the corres-
ponding metallacyclopent-3-ene than the one involving [1,3]-
sigmatropic rearrangement of the vinylmetallirane
intermediate.13
The phenylated metallylenes react with these dienes to
form a long-lived transient product exhibiting a UV/vis
spectrum centered at λmax ≈ 285 nm, which has been assigned
to the corresponding vinylmetallirane derived from the
(1þ2)-addition reaction. The putative vinylgermiranes ex-
hibit first-order lifetimes of 500-670 μs in hexanes contain-
ing 0.01-0.05
M
isoprene at 25 °C,25,26 reflecting
(presumably) the rate of isomerization to the corresponding
1-germacyclopent-3-ene derivative.12,25,26 On the other
hand, the corresponding species from reaction of SiPh2 with
DMB is sufficiently stable to be detectable in the crude
reaction mixture by 1H NMR spectroscopy.22
The role of electronic factors associated with either the
metallylene or the diene in these reactions has not been
extensively explored, save for the early studies of the
reactions of thermally generated GeMe2 with various
mono- and disubstituted dienes by W. P. Neumann and co-
workers.1,3,5,6,9 These studies led to the conclusion that the
reaction is LUMO-diene controlled, the germylene thus
playing the role of a nucleophile in the transition state of
the rate-determining step.1,6 This seems surprising, given
the high degree of electrophilicity that we have come to
associate with GeMe2 and other transient germylenes we
have studied, not to mention its nearly diffusion- or colli-
sion-controlled reactivity toward 1,3-butadiene28 and al-
kyl-substituted diene derivatives24,29 in the gas phase and
in solution. Other mechanistic details of the reaction also
remain to be elucidated experimentally, such as the possi-
ble role of prereaction complexes or other intermediates,
activation and thermodynamic parameters, and the me-
chanism of the vinylgermirane f germacyclopent-3-ene
isomerization. The goal of the present work was to explore
(14) Atwell, W. H.; Weyenberg, D. R. J. Am. Chem. Soc. 1968, 90,
3438.
(15) Hwang, R. J.; Conlin, R. T.; Gaspar, P. P. J. Organomet. Chem.
1975, 94, C38.
(16) Lei, D.; Hwang, R. J.; Gaspar, P. P. J. Organomet. Chem. 1984,
271, 1.
(17) Lei, D.; Gaspar, P. P. Chem. Commun. 1985, 1149.
(18) Clarke, M. P.; Davidson, I. M. T. Chem. Commun. 1988, 241.
(19) Kroke, E.; Willms, S.; Weidenbruch, M.; Saak, W.; Pohl, S.;
Marsmann, H. Tetrahedron Lett. 1996, 37, 3675.
(20) Zhang, S.; Conlin, R. T. J. Am. Chem. Soc. 1991, 113, 4272.
(21) Takeda, N.; Tokitoh, N.; Okazaki, R. Chem. Lett. 2000, 622.
(22) Moiseev, A. G.; Leigh, W. J. Organometallics 2007, 26, 6277.
(23) Moiseev, A. G.; Coulais, E.; Leigh, W. J. Chem.;Eur. J. 2009,
15, 8485.
(24) Leigh, W. J.; Lollmahomed, F.; Harrington, C. R. Organome-
tallics 2006, 25, 2055.
(25) Leigh, W. J.; Dumbrava, I. G.; Lollmahomed, F. Can. J. Chem.
2006, 84, 934.
(26) Leigh, W. J.; Harrington, C. R. J. Am. Chem. Soc. 2005, 127,
5084.
(27) Becerra, R.; Walsh, R. Phys. Chem. Chem. Phys. 2007, 9, 2817.
(28) Becerra, R.; Boganov, S. E.; Egorov, M. P.; Lee, V. Y.; Nefedov,
O. M.; Walsh, R. Chem. Phys. Lett. 1996, 250, 111.
(29) Lollmahomed, F.; Leigh, W. J. Organometallics 2009, 28, 3239.

Article
Organometallics, Vol. 28, No. 23, 2009 6779
these mechanistic details for the specific case of GePh2,
through a study of the effects of polar aryl ring substituents
on the kinetics and thermodynamics of its reaction with
isoprene.
the others (4c, 4d, 4f, and 4g) were tentatively identified
on the basis of their mass spectra and GC retention
times.
Results
The transient germylenes studied in this work (2a-g) were
prepared and detected directly by laser flash photolysis of the
1,1-diaryl-3,4-dimethylgermacyclopent-3-ene
derivatives
The course of the photolysis of 1a in the presence of
1
isoprene was studied in greater detail by H NMR spec-
1a-g (eq 3). Six of these compounds have been reported
previously,30-33 while the seventh (1e) was prepared,
purified, and characterized by similar methods to those
employed for the others.30-33 Its photochemical behavior
was established by photolysis (254 nm) of the compound
as a 0.02 M solution in cyclohexane-d12 containing
methanol (MeOH; 0.2 M) and hexamethyldisilane (2
mM; internal standard), with periodic monitoring of the
photolysate over the 0-10% conversion range by 1H
NMR spectroscopy. This led to the formation of
DMB and a single germanium-containing product, which
was tentatively identified as diarylmethoxygermane 3e (eq
4) on the basis of comparisons to those obtained in similar
experiments with the other compounds in the series.30-33
The photolysis was carried out in parallel with that
of a similar solution of 1a as actinometer.30 The experi-
ment afforded a quantum yield of Φ = 0.62 ( 0.09 for
the formation of 3e and DMB from 1e, indicating that
the presence of the ring substituent has no discernible
effect on the efficiency of germylene photoextrusion, as
we found previously for 1b-d,f,g.31-33 Further details of
these experiments are provided in the Supporting Infor-
mation.
troscopy. Spectra recorded at periodic intervals during
photolysis (254 nm; 25 °C) of a solution of 1a (0.032 M)
in cyclohexane-d12 containing isoprene (0.2 M) and Si2Me6
(2 mM) confirmed the formation of 4a and DMB as the
major products.30 The relative slopes of concentration vs
time plots constructed for the two products over the 0 to ca.
12% conversion range in 1a indicate them to be formed in
relative yields of [4a]:[DMB] = (0.74 ( 0.08):1.0 (see eq 6
and Figure S2, Supporting Information). The NMR spec-
tra also showed broad baseline absorptions in the aro-
matic and aliphatic regions, which increased in intensity
as the photolysis progressed, indicative of the accompa-
nying formation of oligomeric material. The plot of
oligomer concentration (calculated in terms of GePh2
equivalents) vs time exhibited a slope of 0.29 ( 0.05
relative to DMB and exhibited slight positive curvature,
indicating the yield relative to the other products in-
creases with increasing light exposure. A MALDI mass
spectrum of the crude photolysate showed evidence of at
least 10 germanium-containing compounds with molecu-
lar weights in the range 500-900 Da, of which the major
product exhibited a series of isotopic molecular ions
consistent with the formula Ge2Ph4(C5H8)4 (see Figure
S3, Supporting Information). A second experiment,
carried out under identical conditions but at a reaction
temperature of 51 ( 2 °C, afforded 4a, DMB, and
oligomeric material in relative yields of (0.70
(
0.06):1.0:(0.48 ( 0.07), indicating that the yield of the
oligomeric material also increases with increasing reac-
tion temperature. Unfortunately, the change in concen-
tration of 1a with time could not be monitored in these
experiments because of overlap of its NMR signals with
those due to 4a, and thus material balances could not be
determined.
Photolysis of deoxygenated hexanes solutions of 1b-g
(0.02 M) containing isoprene (0.05 M) led to the formation
of the corresponding 1,1-diaryl-3-methylgermacyclo-
pent-3-enes (4b-g) as the only products detectable
by GC/MS up to ca. 40% conversion of the starting
materials. Compounds 4b33 and 4e were identified by
comparison of their mass spectra and retention times to
those of independently prepared authentic samples, while
Laser flash photolysis of a flowed, deoxygenated solu-
tion of the new compound 1e (ca. 0.003 M) in anhydrous
hexanes, using the pulses from a KrF excimer laser (∼20
ns, ∼100 mJ, 248 nm) for excitation, resulted in the
prompt formation of a transient exhibiting absorption
bands centered at λmax ∼300 nm and ∼490 nm, which
decayed over 2-3 μs with the concomitant growth of a
substantially longer lived species (τ ≈ 20 μs) exhibiting
absorptions centered at λmax = 440 nm (see Figure 1). The
behavior is closely analogous to that exhibited by 1a-d,f
under similar conditions,30,31 and we thus assign the two
(30) Leigh, W. J.; Harrington, C. R.; Vargas-Baca, I. J. Am. Chem.
Soc. 2004, 126, 16105.
(31) Huck, L. A.; Leigh, W. J. Organometallics 2007, 26, 1339.
(32) Leigh, W. J.; Potter, G. D.; Huck, L. A.; Bhattacharya, A.
Organometallics 2008, 27, 5948.
(33) Lollmahomed, F.; Huck, L. A.; Harrington, C. R.; Chitnis, S. S.;
Leigh, W. J. Organometallics 2009, 28, 1484.

6780 Organometallics, Vol. 28, No. 23, 2009
Huck and Leigh
Figure 1. Transient absorption spectra recorded by laser flash
photolysis of a 0.003 M solution of 3,4-dimethyl-1,1-bis-
[3-fluorophenyl]germacyclopent-3-ene (1e) in deoxygenated
hexanes, recorded 96-128 ns (O) and 2.78-2.81 μs (Δ) after
the laser pulse. The inset shows transient absorption vs time
profiles recorded at 500 and 440 nm.
Figure 2. Transient absorption spectra recorded by laser flash
photolysis of a 0.003 M solution of 3,4-dimethyl-1,1-diphenyl-
germacyclopent-3-ene (1a) in deoxygenated hexanes containing
1.5 mM isoprene, recorded 48-54 ns (O) and 1.74-1.76 μs (Δ)
after the laser pulse. The inset shows transient absorption vs
time profiles recorded at 500, 440, and 290 nm.
species to germylene 2e and tetraaryldigermene 5e, res-
pectively (eq 7).
Figure 2 shows representative transient spectra and decays
recorded with a solution of 1a in hexanes containing 1.5 mM
isoprene. Under these conditions the lifetime of germylene 2a
is reduced to ca. 100 ns, the growth of the digermene
absorption is completely quenched due to the competing
reaction of the germylene with the diene, and the presence of
the major product of the latter reaction is evident from the
strong long-lived absorption centered at λmax = 285 nm;
much of the expected growth of the latter absorption is
masked by strong sample fluorescence that obscures
the short-wavelength region of the spectrum in the first ca.
50 ns after excitation. Similar long-lived absorptions were
observed with each of the compounds studied in this work in
the presence of the diene, and we assign them to the corre-
sponding vinylgermiranes, 6a-g (eq 10; vide infra). The UV/
vis spectral maxima of these compounds are red-shifted by
ca. 20 nm relative to those of 1a-g and 4a-g (see Supporting
Information).
Flash photolysis of hexanes solutions of 1a-f containing
isoprene (0.1-2.0 mM) afforded behavior similar to that
reported by us previously in our initial study of 1a in the
presence of this substrate.26 The germylene decays assumed a
bimodal form in the presence of the diene, consisting of a fast
initial decay and a much slower decaying residual absorp-
tion. The initial decay accelerated and the contribution of the
residual component of the overall transient decay profile
decreased with increasing isoprene concentration, consistent
with reversible reaction with the diene.24-26 The fast initial
decay is due to the pseudo-first-order approach to equilibri-
um, for which the first-order rate coefficient is given by eq 8,
where kQ is the second-order (“forward”) rate constant for
the reversible reaction with the germylene and k-Q is the
first-order rate constant for the reverse process. The long-
lived residual absorption is due to free germylene remaining
after equilibrium has been attained; its decay is due predo-
minantly to dimerization to the corresponding digermene,
which (as evidenced by growth decay/profiles recorded at its
absorption maximum of 440 nm) is formed in successively
decreasing overall yields and with a decreasing time constant
as the diene concentration is increased.26 At a given diene
concentration [Q], the intensity of the residual signal at the
end of the fast initial decay (ΔAres,Q) is related to the
equilibrium constant for reaction by eq 9, where ΔA0 is the
initial transient absorbance in the absence of diene.
Owing to the modest overlap between the spectra of the
germylenes and corresponding digermenes, the determina-
tion of meaningful values of kdecay and ΔAres,Q from data
recorded at various concentrations of diene required correc-
tion of the raw 500 nm decay profiles, subtracting the minor
contribution due to the corresponding digermene. At each
diene concentration ([Q]) that was studied, corrected tran-
sient absorbance data for the germylene (ΔAcorr,t) were
calculated by scaled subtraction of a growth/decay trace
recorded at the digermene absorption maximum of 440 nm
from that for the germylene at 500 nm, employing a scaling
factor of 0.15, the ratio of the extinction coefficients at 500
and 440 nm in the UV/vis absorption spectrum of digermene
5a;26 the scaling factor varies only slightly from compound to
kdecay ≈ k-Q þ kQ½Qꢁ
ð8Þ
ð9Þ
ðΔAo=Ares,QÞ ¼ 1 þ Keq½Qꢁ

Article
Organometallics, Vol. 28, No. 23, 2009 6781
Figure 3. Plots of (a) kdecay vs [Q] and (b) (ΔAo/ΔAres,Q) vs [Q], for the reaction of germylene 2b with isoprene in deoxygenated hexanes
at 25.0 °C. The solid lines are the linear least-squares fits of the data to eqs 8 and 9, respectively.
Table 1. Second-Order Rate Constants (kQ) and Equilibrium Constants (Keq) for the Reactions of Diarylgermylenes 2a-g with Isoprene
and First-Order Rate Coefficients for Decay of the Corresponding Vinylgermiranes 6a-g (k6-decay) and the Products Keqk6-decay in
Deoxygenated Hexanes Solution at 25 °C (except where otherwise noted, errors are reported as (2σ)
kQ/109 M-1 s-1
Keq/103 M-1
k6-decay/103 s-1a
Keqk6-decay/106 M-1 s-1
2a
2b
2c
2d
2e
2f
H
5.2 ( 0.5
3.4 ( 0.5
4.3 ( 0.5
6.6 ( 1.4
4.8 ( 1.6
2.5 ( 0.4
1.6 ( 0.7b
6.0 ( 1.5
3.9 ( 0.5
3.3 ( 0.6
1.7 ( 0.3
4.0 ( 1.0
5.0 ( 0.5
0.6 ( 0.3b
2.52 ( 0.18
1.34 ( 0.01
1.82 ( 0.07
7.43 ( 0.07
9.54 ( 0.54
14.5 ( 0.6
487 ( 3
15.1 ( 4.6
5.2 ( 0.7
7.8 ( 1.7
12.6 ( 2.3
38 ( 12
3,4-Me2
4-Me
4-F
3-F
4-CF3
3,5-(CF3)2
72 ( 9
2g
290 ( 145
a Decay rate constants were determined by nonlinear least-squares analysis of decays recorded in the presence of 30-50 mM isoprene, and are each the
average of several determinations; errors are reported as ( 1 standard deviation of the mean. b Mean ( 1 standard deviation of three independent
determinations.
compound within the series. Representative examples of raw
and corrected decay profiles for germylene 2b (from 1b) in
the presence of 0, 0.32, and 0.82 mM isoprene are shown in
Figure S4 of the Supporting Information. The corrected
decay profiles obtained with isoprene concentrations at
the upper end of the concentration range studied ([Q] >
0.5 mM), where the decay of the residual absorption was
substantially slower than the initial fast decay, were analyzed
as single-exponential decays according to eq 11. Those
recorded at the lower end of the concentration range were
generally analyzed as two-exponential decays, in which case
kdecay was taken as the decay rate coefficient of the fast
component, and an approximate ΔAres,Q value was obtained
by visual extrapolation of the slow decay component back
to the “end” of the fast initial decay (see Supporting
Information). The resulting (ΔAo/ΔAres,Q) values were
then corrected for minor screening of the excitation light
by the diene, which absorbs weakly at the laser wavelength
(ε248nm = 81 ( 8 M-1 cm-1).
consider the Keq values to have an error of ca. 25%. Figure 3
shows representative plots of this type from experiments with
germylene 2b, and Table 1 lists the values of kQ and Keq
for the reactions of 2a-f with isoprene in hexanes solution at
25 °C. Those obtained for germylene 2a are in excellent
agreement with the earlier reported values.26
The kinetic behavior of germylene 2g, from photolysis of
hexanes solutions of 1g containing isoprene, was more
difficult to analyze than the others because growth/decay
profiles recorded for the corresponding digermene (between
420 and 460 nm) all showed an initial short-lived decay
component superimposed on the slow growth of the diger-
mene signal.32 This precluded the use of the correction
procedure that was employed for the other derivatives for
determination of kdecay values for the germylene and re-
quired that they be determined from the raw decay profiles,
treating the digermene component as a nondecaying residual
absorption. This leads to a distorted estimate of the decay
rate coefficient at each diene concentration, which is ulti-
mately transmitted to the kQ value determined from the plot
of kdecay vs [isoprene]; a raw value of kQ = (2.7 ( 0.7) ꢀ 109
M-1 s-1 was obtained from the average of three independent
experiments. The correction factor that needs to be applied
to this value was estimated by analyzing the raw decays from
the experiments with 1b, 1d, and 1f and comparing the
resulting raw kQ values to the true ones. This afforded an
average correction factor of 0.6 ( 0.1, which in turn affords a
value of kQ=(1.6 ( 0.7)ꢀ109 M-1 s-1 for the rate constant
for reaction of isoprene with germylene 2g. A value of
Keq = 600 ( 300 M-1 s-1 was determined for the equilbrium
ΔAcorr,t ¼ ΔA0 expð -kdecaytÞ þ ΔAres
ð11Þ
Plots of the pseudo-first-order rate coefficients for ap-
proach to equilibrium (kdecay) vs isoprene concentration
exhibited excellent linearity in each case, allowing the sec-
ond-order rate constants (kQ) for reaction of 2a-f with the
diene to be determined by analysis of the data according to eq
8. Excellent linearity was also exhibited by the corresponding
plots of (ΔAo/ΔAres,Q) vs [isoprene], although in several cases
the best-fit value of the intercept was significantly greater
than the value of unity predicted by eq 9; as a result, we

6782 Organometallics, Vol. 28, No. 23, 2009
Huck and Leigh
constant using the same procedure as was employed for the
others, as the presence of the fast initial decay in the
digermene signals does not affect the validity of the correc-
tion procedure required to estimate the residual absorbances
due to the germylene after equilibrium has been attained.
Transient UV/vis spectra recorded with solutions of 1a-g
in hexanes containing 15-60 mM isoprene verified that
the vinylgermiranes (6a-g) are the only species detectable
at high concentrations of the diene; in each case, the
species exhibited λmax = 285 nm and decayed with first-
order kinetics. The lifetimes varied with the aryl ring sub-
stituent, from τ ≈ 2 μs in the case of 6g to τ ≈ 740 μs for 6b;
representative transient spectra and decay profiles, recorded
with hexanes solutions of 1b and 1f in the presence of 50 mM
isoprene at 25 °C, are shown in Figure S5 of the Supporting
Information. In each case, there was no detectable variation
in lifetime over the 15-60 mM concentration range in added
isoprene. Table 1 includes the first-order decay rate coeffi-
cients (k6-decay) of the seven compounds, measured in hex-
anes containing 50 mM isoprene at 25 °C, along with the
products of the vinylgermirane decay coefficients and the
corresponding equilibrium constants for their formation
(Keqk6-decay). The significance of the latter parameters will
be discussed later in the paper.
Rate and equilibrium constants for the reactions of iso-
prene with germylenes 2a, 2c, and 2f were also determined at
two or three additional temperatures over the 14-61 °C
temperature range. Figure 4 shows the resulting Arrhenius
and van’t Hoff plots of the data obtained for germylene 2a,
while the corresponding plots for 2c and 2f are shown in
Figure S6 of the Supporting Information; the result-
ing activation and thermodynamic parameters are listed in
Table 2. The Arrhenius plot for 2a shows distinct curvature,
but was analyzed as a straight line because of the modest
number of points. The rate constants obtained for the other
two compounds also did not vary substantially with temp-
erature and were also analyzed as straight lines.
Figure 4. Arrhenius (O) and van’t Hoff (0) plots for the reac-
tion of GePh2 (2a) with isoprene in deoxygenated hexanes.
Table 2. Arrhenius and van’t Hoff Parameters for the Reactions
of Germylenes 2a, 2c, and 2f with Isoprene in Deoxygenated
Hexanes Solutiona
2a (H)
2c (4-Me)
2f (4-CF3)
Ea/kcal mol-1
log(A/M-1 s-1
-0.9 ( 2.4
þ9 ( 2
þ1.5 ( 0.9
þ11 ( 1
þ3.7 ( 4.2
þ12 ( 3
)
ΔG/kcal mol-1
ΔH/kcal mol-1
ΔS/cal K-1 mol-1
-5.2 ( 0.2
-9.7 ( 1.1
-15 ( 4
-4.8 ( 0.1
-10.5 ( 0.9
-19 ( 3
-5.0 ( 0.2
-12 ( 6
-21 ( 20
a The standard state is a 1M hexanes solution at 298.15 K; see Table
S4 of the Supporting Information for the corresponding values employ-
ing the gas phase at 1 bar and 298.15 K as standard state. Errors are
quoted as (2σ.
First-order decay coefficients for vinylgermirane 6a were
also determined as a function of temperature in hexanes
containing 50 mM isoprene. Both the lifetime of the species
and its initial absorption intensity were found to decrease
with increasing temperature between 11 and 52 °C, as the
spectra of Figure 5 illustrate. The figure also shows the
resulting Arrhenius plot of the first-order decay coefficients
(k6a-decay), from which were obtained values of Ea = 11.8 (
0.5 kcal mol-1 and log(A/s-1) = 12.0 ( 0.4.
In contrast to the behavior observed for 1a in the presence
of isoprene, addition of DMB to hexanes solutions of the
GePh2 precursor resulted in behavior consistent with fast but
relatively inefficient reaction of 2a with the diene, and
relatively high concentrations of the substrate were required
in order to elicit discernible effects on the germylene signals.
Decay traces recorded on longer time scales appeared merely
to be reduced in overall signal intensity with increasing
concentrations of diene, and to an extent that was barely
differentiable from that predicted on the basis of competing
Figure 5. Transient absorption spectra of vinylgermirane 6a,
recorded by laser photolysis of 1a in deoxygenated hexanes
containing 50 mM isoprene at 27 °C (b) and 51 °C (O). The
insets show transient decay profiles recorded at 290 nm at the
two temperatures (top) and the Arrhenius plot of the first-order
decay coefficients over the 11-52 °C temperature range.
absorption of the excitation light by the substrate (ε248nm
=
161 ( 15 M-1 cm-1). Bimodal decays could be detected over
the 3-7.3 mM concentration range in added diene, but
required the use of relatively fast time scales in order to
after correction for screening of the excitation light by the
diene (see Figure S16, Supporting Information). A spectrum
recorded in the presence of 30 mM DMB showed a single
transient absorption, also centered at λmax = 285 nm, which
we assign to vinylgermirane 7a; it decays with clean first-
order kinetics and lifetime τ = 8.3 μs. The spectrum is quite
resolve the initial fast decay component. A value of kQ
=
(2.3 ( 1.0) ꢀ 109 M-1 s-1 was estimated from a three-point
plot of kdecay vs [DMB], while a value of Keq = 150 ( 20 M-1
was obtained from analysis of the residual signal intensities

Article
Organometallics, Vol. 28, No. 23, 2009 6783
similar to that obtained after photolysis of 1a in a 3-methyl-
pentane matrix at 78 K, which was also assigned to this
compound.12
the equilibrium constants for Lewis acid-base complexation
of GeMe2 and diarylgermylenes with O- and N-donors
indicate that GeMe2 is a significantly weaker Lewis acid
than GePh2.24,26,33,36 Furthermore, the log Keq values for
Lewis acid-base complexation of diarylgermylenes with
O-donors such as tetrahydrofuran and ethyl acetate corre-
late well with Hammett σ-values,31 while those for reaction
of 2a-g with isoprene do not (vide infra).
Discussion
The structural assignments of the long-lived transient
products observed in laser photolysis experiments with
1a-g in the presence of isoprene and (for 1a) with DMB,
to the corresponding vinylgermiranes 6a-g and 7a, respec-
tively, are supported by several independent observations.
First, the formation of transient products with similar
spectral and kinetic characteristics has been found to be a
general feature of the reactions of both GePh2 (2a) and
GeMePh with terminal alkenes and dienes; for example,
reaction of GePh2 with 4,4-dimethyl-1-pentene (DMP)
yields a transient product exhibiting λmax = 275 nm (τ =
1.2 ms),26 while GeMePh affords transient products exhibit-
ing λmax = 285 nm (τ = 670 μs) and λmax = 275 nm (τ = 2.6
ms) in the presence of isoprene and DMP, respectively.25
Second, photolysis of 1a in a hydrocarbon matrix at 78 K
results in the formation of a species exhibiting an essentially
identical UV/vis spectrum to that obtained in solution phase
experiments with 1a in the presence of 30 mM DMB; the
species disappears rapidly upon warming the matrix above
its softening point.12 The formation of 7a on photolysis of 1a
under these conditions has been proposed to be due to
the rigid solvent cage preventing diffusive separation of the
germylene and its diene coproduct; between this and the low
temperature of the medium, the equilibrium mixture is
forced essentially entirely in the direction of the vinylgermi-
rane.12 Third, the major product of reaction of SiPh2 with
DMB, whose early identification as vinylsilirane 834 has been
more recently confirmed by NMR evidence,22 exhibits a very
similar UV/vis spectrum (λmax = 280 nm) to those of the
transient products from GePh2 with DMB and isoprene;22
the same compound is also formed as the major product of
photolysis of 1-silacyclopent-3-ene 9 in solution (eq 13).12
Consistent with the significantly greater thermodynamic
stabilities of siliranes compared to germiranes,35 vinylsili-
rane 8 exhibits a lifetime of several hours, isomerizing slowly
in the dark at room temperature to generate 9.22 The
similarity between the UV/vis spectra of 8 and those assigned
to 6a and 7a provides reasonable evidence against a possible
assignment of the latter spectra to other reactive intermedi-
ates that might be considered, such as a germylenerdiene π-
complex. The π-complex assignment is also mitigated
against by the higher Keq value estimated for the reaction
of GeMe2 with isoprene24 compared to that for reaction of
GePh2 with the same diene, to the extent that variations in
the magnitude of Keq for complex formation with germylene
structure can be expected to mirror germylene Lewis acidity;
Two regioisomers are of course possible for the vinylger-
mirane derived from reaction of GePh2 with isoprene (6a and
6a0), depending on which of the two CdC bonds in the diene
(if either) is the preferred site of reaction. The structures of
the 1:2 adducts isolated by Neumann and co-workers from
the reaction of thermally generated GeMe2 with isoprene
indicate that the less-substituted CdC bond is the preferred
site of reaction with this germylene;9 we have found no
evidence for the formation of the analogous products from
2a at room temperature, however, with isoprene concentra-
tions as high as 0.2 M. Our identification of 6a as the
preferred regioisomer is based on a comparison of the rate
and equilibrium constants for its formation to those for
formation of 7a from GePh2 and DMB, for which only a
single structure is possible. The difference in Keq values is
particularly significant, with that for formation of 7a being
ca. 40 times smaller than that for formation of the isoprene
adduct. This is consistent with a free energy difference Δ(ΔG)
≈ 2.2 kcal mol-1, which is significantly larger than what one
would reasonably expect for regioisomer 6a0 given that the
only difference between it and 7a is the methyl group at the 1-
position of the vinyl substituent. We had hoped there might
be a detectable difference between the UV/vis spectra of the
two vinylgermirane derivatives that would lend additional
support to the assignment, but none is evident.
Equilibrium constants have now been measured or esti-
mated for the reactions of a number of transient germylenes
with isoprene, the trends in which provide some indication of
the variation in the thermodynamic stabilities of the corre-
sponding vinylgermirane derivatives as a function of sub-
stitution at germanium. Those for reaction of isoprene with
GeMe2 (Keq ≈ 20 000 M-1; ΔG ≈ -5.9 kcal mol-1),24
GeMePh (Keq ≈ 15 000 M-1; ΔG ≈ -5.7 kcal mol-1),25
and GePh2 (Keq = 6000 M-1; ΔG = -5.2 kcal mol-1
)
indicate that aryl substitution at germanium results in a
modest decrease in thermodynamic stability, relative to
dimethyl substitution. Substituents in the meta- and/or
para-positions of the aryl rings have only small effects on
ΔG, and there is no systematic variation with Hammett
substituent constants (see Figure 6b). This is also true of
ΔH, where the measured values for the 4-Me and 4-CF3
(34) Tortorelli, V. J.; Jones, M., Jr.; Wu, S.; Li, Z. Organometallics
1983, 2, 759.
(35) Horner, D. A.; Grev, R. S.; Schaefer, H. F., III. J. Am. Chem.
(36) Leigh, W. J.; Lollmahomed, F.; Harrington, C. R.; McDonald,
J. M. Organometallics 2006, 25, 5424.
Soc. 1992, 114, 2093.

6784 Organometallics, Vol. 28, No. 23, 2009
Huck and Leigh
Figure 6. Hammett plots of the (a) rate and (b) equilibrium constants for reaction of diarylgermylenes 2a-g with isoprene in hexanes
solution at 25 °C.
derivatives (6c and 6f, respectively) are the same within
experimental error as that of the parent compound (ΔH =
-9.6 ( 1.1 kcal mol-1). Interestingly, it appears that
these effects are dwarfed by those of substitution at the
germirane ring-carbons (vide supra); increasing steric bulk
at germanium can also be anticipated to exert a substantial
stabilizing effect, judging from the results noted in the
Introduction for 1,1-diaryl-2-vinylsilirane derivatives.20-22
Quantifying these effects are the subject of further work in
our laboratory.
Hammett plots of the rate and equilibrium constants for
reaction of isoprene with GePh2 and 2b-g are shown in
Figure 6. As mentioned above, there is no systematic varia-
tion in Keq with Hammett σ-values (see Figure 6b); this
indicates that the transient products of the reactions have
no appreciable polar (or dipolar) character, hence support-
ing the vinylgermirane assignment. On the other hand, the
corresponding plot for the (forward) reaction rate constants
isomers of the π-complex. The first step in the sequence is
essentially a Lewis-acid-base reaction and, hence, would be
expected to exhibit a positive Hammett F-value, as it should
be assisted by increased electrophilicity in the germylene.31
The second step involves nucleophilic attack of the germa-
nium lone pair at carbon within the complex; it should
exhibit a negative Hammett F-value since it proceeds with
neutralization of formal negative charge at germanium.33,38
The observed Hammett behavior is consistent with step 1
being rate-determining in those derivatives bearing substit-
uents more electropositive than 4-fluoro (H, 4-Me, 3,4-Me2)
and step 2 being rate-determining in those bearing substitu-
ents more electronegative than 4-fluoro (3-F, 4-CF3,
3,5-(CF3)2). The change in the overall activation energy to
(small) positive values for 2c and 2f, which are situated on
either side of the apex in the Hammett plot of Figure 6a, is
also consistent with this interpretation.
(kQ; Figure 6a) displays a smooth but distinctly curved
P
correlation with σ, with the maximum rate occurring with
the para-F derivative (2d) and decreasing as the substituents
are made more electron-donating or electron-withdrawing
relative to this substituent. The behavior is consistent with a
two-step mechanism in which the rate-determining step
changes with substituent and where the individual rate
constants for the two steps are affected in opposite ways by
polar substituents.37 The limiting slopes defined by the two
extremes of the plot (F = þ0.42 ( 0.07 on the donor side and
F = -0.42 ( 0.07 on the acceptor side; the 4-F derivative was
included in both analyses) indicate a significant dipolar
character for the intermediate in the reaction. The curved
Arrhenius plot and resulting negative average activation
energy that is exhibited by the parent compound are also
consistent with a two-step reaction mechanism, in which the
identity of the slower step changes as a function of tempera-
ture, from the first step at low temperatures to the second
step at the other extreme.
The recent computational study of Nag and Gaspar on the
reaction of GeMe2 with 1,3-butadiene provides mixed sup-
port for the involvement of a reactive π-complex as an
intermediate.13 A reactive complex was found to exist as a
discrete intermediate at the B3LYP/6-31G(d,p)-6-311G(d,p)
level of theory, separated by a free energy barrier of 3.0 kcal
mol-1 from the corresponding vinylgermirane, but ca. 5.7
kcal mol-1 higher in free energy than the isolated reactants;
the complex is more stable than the free reactants by 1.2 kcal
mol-1 at a higher level of theory (CCSD(T)/cc-pVTZ//
B3LYP/6-31G(d,p)-6-311G(d,p)), but the (free energy) bar-
rier for its conversion to the vinylgermirane disappears. The
results of the higher level calculations are in better agree-
ment with experimental kinetic data for the reactions of
GeMe2 with aliphatic dienes in the gas phase and in solution,
which are consistent with a very small but significant
barrier to reaction; they proceed with rate constants close
to, but clearly less than, the diffusional (or collisional) limit
at 25 °C.24,27-29 The involvement of reactive π-complexes
as intermediates in the addition of GeH2 and GeMe2 to
A mechanism consistent with this behavior involves the
initial formation of a polarized π-complex as a steady-state
intermediate, which undergoes competing dissociation to the
free reactants and collapse to vinylgermirane, the latter also
reversibly. Equation 14 illustrates the proposed mechanism
for formation of 6, showing one of the two possible regio-
(37) Anslyn, E. V.; Dougherty, D. A., Modern Physical Organic
Chemistry; University Science Books: New York, 2005; pp 259-296.
(38) Eaborn, C.; Singh, B. J. Organomet. Chem. 1979, 177, 333.

Article
Organometallics, Vol. 28, No. 23, 2009 6785
ethylene and other alkenes is a common feature of all of the
various theoretical studies of these reactions that have been
reported39-45 and is consistent with experimental kinetic
studies of the reaction of GeH2 with ethylene in the gas
phase.27,41
Our data also provide information on the factors affecting
the lifetime of vinylgermirane 6a, which was monitored in the
presence of sufficiently high concentrations of isoprene
(50-60 mM) to make it the only species detectable by laser
photolysis methods. The species was found to decay with
clean pseudo-first-order kinetics and a lifetime of τ = 400 (
25 μs at 25 °C, in reasonable agreement with the value
reported earlier.26 The lifetime did not vary significantly
over the 15-60 mM concentration range in isoprene, indi-
cating that it does not react with a second molecule of the
diene at rates exceeding ca. 104 M-1 s-1. The lifetime was
found to decrease by a factor of ca. 15 as the temperature was
increased over the 10.6-52.0 °C temperature range, leading
to a linear Arrhenius plot (Figure 5) and overall activation
parameters of Ea = 11.8 ( 0.5 kcal mol-1 and log(A/s-1) =
12.0 ( 0.4, the latter corresponding to a value of ΔS q = -5.6
( 1.8 cal K-1 mol-1 for the entropy of activation at 25 °C.
The maximum transient absorbance due to 6a also decreased
significantly with increasing temperature (see Figure 5),
roughly in the manner expected from the variation in Keq
over the same temperature range. For example, the intensity
ratio of the absorbances at 285 nm at 27 and 51 °C (ca. 1.65
from the data of Figure 5) is in reasonable agreement with the
value of 1.82 predicted from the ratio of the equilibrium
constants at the two temperatures, determined by interpola-
tion of the van’t Hoff plot of Figure 4.
We assign the first-order decay of vinylgermirane 6a to its
thermal conversion to the corresponding germacyclopent-3-
ene derivative, 4a, and thus the measured activation para-
meters to those associated with this process. Two mechan-
isms have been discussed for the isomerization, which is
common to both vinylgermirane and vinylsilirane deriva-
tives: a direct, single-step [1,3]-rearrangement pathway and a
two-step mechanism proceeding via metallylene extrusion
followed by concerted (1þ4)-cycloaddition (eq 15).13 Put
another way, the first mechanism is the one that would hold
if the formation of 4 from the free germylene and isoprene
requires the intermediacy of vinylgermirane 6, while the
second is the one that would hold if the two products
are formed competitively, and 6 acts merely as an unstable
spectator in mobile equilibrium with the free germylene and
diene; the latter mechanism is the one predicted to be correct
on the basis of computational studies of the reaction of both
silylenes and germylenes with aliphatic dienes.13 Experimen-
tally, the two mechanisms are kinetically indistinguishable
from the context of either the free germylene or the vinyl-
germirane. Thus, our kinetic data for the reactions of 2a with
Figure 7. Arrhenius plot of the hypothetical second-order rate
constants for (1þ4)-cycloaddition of GePh2 (2a) and iso-
prene, calculated from the first-order decay coefficients of
vinylgermirane 6a and interpolated values of the equilibrium
constants for formation of 6a as a function of temperature
(k(1þ4) = Keqk6a-decay).
isoprene and DMB, combined with those for the subsequent
decay of the corresponding vinylgermiranes, indicate only
that formation of the thermodynamically stable products of
these reactions (the corresponding germacyclopent-3-enes,
4a and 1a, respectively) proceeds substantially more slowly
than formation of the corresponding vinylgermiranes (6a
and 7a, respectively). A value of k(1þ4) = (1.5 ( 0.5) ꢀ 107
M-1 s-1 can be calculated for the bimolecular rate constant
of the putative (1þ4)-cycloaddition reaction in the case of
GePh2 and isoprene at 25 °C, from the first-order rate
coefficient for decay of 6a and the Keq value for its formation.
This is roughly 350 times slower than the rate constant for the
(1þ2)-cycloaddition process that yields 6a (kQ in Table 1),
corresponding to a difference in free energy of activation of
ca. 3.5 kcal mol-1. Similar treatment of the rate coefficients
for decay of 6a over the ca. 11-52 °C temperature range
with interpolated values of Keq (from the van’t Hoff plot
of Figure 4) leads to the Arrhenius plot of Figure 7,
from which is obtained values of Ea = þ2.2 ( 0.5 kcal
mol-1 and log(A/M-1 s-1) = 8.8 ( 0.4, and hence
ΔSq = -20 ( 2 cal K-1 mol-1 at 25 °C. The treatment thus
affords an enthalpy of activation for (1þ4)-cycloaddition
that is ca. 3 kcal mol-1 higher than the (average) ΔHq for
(1þ2)-cycloaddition. The entropies of activation for the two
pathways are the same within experimental error and in the
range expected for cycloaddition processes.46
(39) Sakai, S. Int. J. Quantum Chem. 1998, 70, 291.
(40) Su, M. D.; Chu, S. Y. J. Am. Chem. Soc. 1999, 121, 11478.
(41) Becerra, R.; Boganov, S. E.; Egorov, M. P.; Faustov, V. I.;
Promyslov, V. M.; Nefedov, O. M.; Walsh, R. Phys. Chem. Chem. Phys.
2002, 4, 5079.
(42) Su, M. D. Chem.;Eur. J. 2004, 10, 6073.
(43) Birukov, A. A.; Faustov, V. I.; Egorov, M. P.; Nefedov, O. M.
Russ. Chem. Bull. Int. Ed. 2005, 54, 2003.
(44) Boganov, S. E.; Egorov, M. P.; Faustov, V. I.; Krylova, I. V.;
Nefedov, O. M.; Becerra, R.; Walsh, R. Russ. Chem. Bull. Int. Ed. 2005,
54, 483.
The resulting free energies of activation, calculated from
q
the experimental rate constants, are ΔG (1þ2) = þ4.2 ( 0.1
kcal mol-1 and ΔG (1þ4) = þ7.7 ( 0.1 kcal mol-1 in hexanes
q
(45) Joo, H.; Kraka, E.; Quapp, W.; Cremer, D. Mol. Phys. 2007, 105,
2697.
(46) Isaacs, N. S. Physical Organic Chemistry, 2nd ed.; Prentice Hall:
New York, 1995.

6786 Organometallics, Vol. 28, No. 23, 2009
Huck and Leigh
Figure 8. Hammett plots of (a) the first-order rate coefficients for decay of vinylgermiranes 6a-g (k6-decay) and (b) the calculated rate
constants for (1þ4)-cycloaddition of 2a-g with isoprene (k(1þ4) = Keqk6-decay), in hexanes solution at 25 °C.
q
solution at 298 K, corresponding to values of ΔG
=
(k6-decay) shows a strong but significantly scattered positive
correlation of the rates with substituent constants, as shown
in Figure 8a. The correlation improves quite considerably
when the equilibrium constants are included in the analysis
(Figure 8b). Thus, consideration of the pseudo-first-order
decays of the vinylgermiranes in terms of the stepwise
dissociation/(1þ4)-cycloaddition mechanism, which requires
that k6-decay = k(1þ4)/Keq, provides a significantly better and
more easily rationalizable explanation of the trends in reactiv-
ity as a function of substitution than is possible in the context
of the sigmatropic rearrangement mechanism.
The Hammett correlation of Figure 8b (F = þ0.77 ( 0.04)
suggests there to be significant negative charge development
at germanium in the transition state of the rate-determining
step for (1þ4)-cycloaddition of GePh2 to isoprene and is
consistent with a concerted process driven largely by inter-
actions between the germylene LUMO and diene HOMO.
The indication of charge polarization during the rate-deter-
mining step could be the result of asynchronous bonding in
the transition state, as Nag and Gaspar found in their
computational study of the reaction of GeMe2 with 1,3-
butadiene.13 The behavior is also consistent with a stepwise
mechanism proceeding via electrophilic addition of the
germylene to one of the termini of the diene substrate,
leading to a zwitterionic intermediate that undergoes rapid
collapse to the (1þ4)-cycloadduct (eq 16); if such an inter-
mediate is involved, however, it must be different than the
one involved in the formal (1þ2)-addition process, as the
latter reaction proceeds more than 2 orders of magnitude
faster. The involvement of such an intermediate in the
reaction of GeMe2 with electron-rich dienes requires that
the lifetime of the intermediate be too short to allow bond
rotations in the carbon framework of the species, to be
compatible with the fact that the (1þ4)-cycloaddition of
GeMe2 to electron-rich dienes proceeds stereospecifically;4
no such information is yet available for GePh2, to our
knowledge.
(1þ2)
þ6.1 ( 0.1 kcal mol-1 and ΔG (1þ4) = þ9.6 ( 0.1 kcal mol-1
employing the gas phase at 1 bar and 298.15 K as standard
state. These are both ca. 9 kcal mol-1 smaller than those
computed by Nag and Gaspar for the same reaction path-
ways at the B3LYP/6-31G(d,p)-6-311G(d,p) level of theory,
but as they pointed out, B3LYP significantly overestimates
the stability of the free germylene relative to that indicated at
higher levels of theory.13 Interestingly, the computed
(B3LYP) free energy difference between the transition state
for (1þ4)-cycloaddition and the most stable conformer of 6a,
which eliminates the errors specifically associated with the
computed energy of the free germylene as much as possible, is
q
ΔGq = 10.4 kcal mol-1 13
This is in quite good agreement
.
with the value calculated from the experimental (first-order)
q
rate coefficient for the decay of 6a at 298 K in hexanes (ΔG =
12.8 kcal mol-1). The difference in the calculated (B3LYP) free
energies of activation for the (1þ2)- and (1þ4)-cycloadditions
of GePh2 and isoprene (Δ(ΔGq) = 3.6 kcal mol-1 13
is in
)
outstanding agreement with the experimentally derived value.
The effects of substituents in both the diene and the
germylene on the rate and equilibrium constants lend more
concrete experimental support for the dissociation/(1þ4)-
cycloaddition mechanism. The first-order rate coefficient for
decay of vinylgermirane 7a, from the reaction of GePh2 with
DMB, is close to 50 times larger than that of 6a, a rather large
and unexpected difference in reactivity in the context of a
direct [1,3]-sigmatropic rearrangement mechanism.47 This
difference all but disappears, however, if the rate coefficient
is considered in the context of the dissociation/cycloaddition
mechanism. The product of the k7a-decay and Keq values
affords k(1þ4) = (1.8 ( 0.3) ꢀ 107 M-1 s-1 for the rate
constant for (1þ4)-cycloaddition, which is roughly 100 times
smaller than the rate constant for (1þ2)-cycloaddition to the
same substrate, but the same within experimental error as
that calculated above for the formation of 4a via (1þ4)-
cycloaddition of 2a to isoprene. It thus appears that the key
difference between the reactions of GePh2 with the two
dienes lies in the equilibrium constant for vinylgermirane
formation, which is 40 times smaller for DMB than for
isoprene; the individual free energy barriers for formation
of the two competing products appear to be quite similar
for these two dienes. Similarly, a Hammett plot of the first-
order rate coefficients for the decay of vinylgermiranes 6a-g
In any event, the electronic effects on the two competing
cycloaddition pathways in GePh2 have dramatic conse-
quences on the relative rate constants for the two processes
(47) Baldwin, J. E. Chem. Rev. 2003, 103, 1197.

Article
Organometallics, Vol. 28, No. 23, 2009 6787
and 2,3-dimethyl-1,3-butadiene (DMB) in hexanes solution
indicate that the major stable products of the reaction, the
corresponding 1,1-diarylgermacyclopent-3-enes, are formed
by a (1þ4)-cycloaddition mechanism, which is slow relative
to the reversible formation of the kinetic product, the
corresponding 1,1-diaryl-2-vinylgermirane derivative, via
formal (1þ2)-cycloaddition. The results are in broad agree-
ment with the conclusions of a recent theoretical study of the
reactions of GeMe2 and GePh2 with 1,3-butadiene and
isoprene, respectively, and in addition show that the rate
constants for both reactions exhibit a marked sensitivity to
the presence of polar substituents on the germylene and
to alkyl substitution in the diene. (1þ2)-Cycloaddition,
which favors formation of the vinylgermirane corresponding
to addition to the less-substituted of the two CdC bonds in
the diene, proceeds 2 orders of magnitude more rapidly than
(1þ4)-addition and is rapidly reversible at ambient tempera-
tures; in the reaction of GePh2 with isoprene, the correspond-
ing vinylgermirane is more stable than the free germylene
and diene by only ΔG = -5.2 kcal mol-1, corresponding
to an enthalpy difference of ΔH = -9.6 kcal mol-1. The
difference is reduced significantly with DMB as the diene
rather than isoprene.
Substituent and temperature effects are consistent with a
two-step reaction mechanism for vinylgermirane formation,
involving a polarized steady-state intermediate such as the
corresponding germylenerdiene π-complex; the Arrhenius
parameters indicate a small negative overall activation en-
ergy for the reaction (Ea ≈ -0.9 kcal mol-1) and a moder-
ately large, negative entropy of activation (ΔSq ≈ -20
cal K-1 mol-1) for the reaction of the parent diarylgermylene
(GePh2) with isoprene. The corresponding vinylgermiranes
are readily detectable by time-resolved UV/vis spectroscopy,
as long-lived transients exhibiting λmax = 285 nm; the spect-
rum is insensitive to substitution on either the aryl rings or at
(one of) the germirane ring carbons. On the other hand, the
lifetime is markedly sensitive to substituents, decreasing
substantially with increasing electron-withdrawal at germa-
nium or with methyl-substitution at the germirane ring
carbon. These results are best accommodated by a mechan-
ism for vinylgermirane decay that involves (reversible) dis-
sociation to the free germylene and diene, followed by slow
(1þ4)-cycloaddition to yield the thermodynamically stable
product, the corresponding 1,1-diarylgermacyclopent-3-ene.
In the case of GePh2 and isoprene, the latter reac-
tion proceeds with activation parameters of Ea = þ2.2 kcal
mol-1 and ΔSq ≈ -20 cal K-1 mol-1. It too shows a marked
sensitivity to polar substituents, the rate constant increasing
dramatically with increasing germylene electrophilicity in a
manner consistent with a LUMO-germylene/HOMO-diene
controlled process. As germylene electrophilicity increases
past a certain point, the rate of (1þ4)-cycloaddition increases
at the expense of that of (1þ2)-cycloaddition because the
polar intermediate involved in the latter process becomes a
bottleneck in the reaction pathway.
Figure 9. Hammett plots of the rate constants for competing
(1þ2)- (O) and (1þ4)-cycloaddition (0) of diarylgermylenes
2a-g with isoprene in hexanes at 25 °C.
as the electronic characteristics of the germylene are varied.
This is illustrated in Figure 9, which combines the individual
Hammett plots of Figure 6a (for (1þ2)-cycloaddition) and
Figure 8b (for (1þ4)-cycloaddition) in a single graph. (1þ2)-
Cycloaddition to isoprene proceeds ca. 350 times more
rapidly than (1þ4)-cycloaddition in the case of GePh2, and
this difference increases (modestly) as germylene electrophi-
licity is decreased by donor substituents on the aryl rings. In
contrast, increasing germylene electrophilicity results in a
much more dramatic reduction in this difference, to the point
where, with germylene 2g, the rate constants for the two
competing reactions are very nearly the same. This presum-
ably occurs mainly because the germylenerdiene π-complex
becomes a bottleneck in the reaction pathway leading to the
(1þ2)-cycloadduct as germylene electrophilicity is increased.
The indication that GePh2 plays the role of an electrophile
in its (1þ4)-cycloaddition reaction with isoprene is opposite
Neumann and co-workers’ early conclusion for GeMe2.5,6,9
However, the latter was based on relative product yields
obtained in competition experiments with a variety of sub-
stituted dienes, which show simply that the (1þ4)-cycload-
ducts of GeMe2 with electron-poor dienes are formed more
efficiently than those from electron-rich dienes. Given the
complexities in the reaction that are now apparent, as well as
the fact that GeMe2 is even more reactive than GePh2 toward
the electron-rich dienes that have been studied in the present
work,24,33 it is clear that such a statement is an overgener-
alization at the very least. What could not be revealed by
the early competition kinetics studies is the way in which the
unstable (1þ2)-cycloadducts, which are undoubtedly
formed much faster than the isolable ((1þ4)-cycloaddition)
products and whose stabilities are now known to be acutely
sensitive to substituents on the diene, control the overall
relative product yields in a competition experiment involving
two different dienes. It seems likely to us that the mechanistic
details of the reactions of transient germylenes with dienes
and other unsaturated systems will prove to vary quite
significantly with the electronic characteristics of the sub-
strate.
Further mechanistic studies of this and other reactions of
transient germylenes and their silicon homologues are in
progress.
Experimental Section
Summary and Conclusions
Germacyclopent-3-enes 1a-d,f,g were synthesized and pur-
ified according to the previously reported procedures;30-33 the
synthesis and characterization of 1e and 4e and the basic
Kinetic and thermodynamic results for the reactions of
GePh2 and various aryl-substituted derivatives with isoprene

6788 Organometallics, Vol. 28, No. 23, 2009
Huck and Leigh
photochemical behavior of 1e are described in the Supporting
Information. Hexanes (EMD OmniSolv) for laser flash photo-
lysis experiments was dried by passage through activated neutral
alumina (250 mesh; Purifry Co., Gramercy LA) under nitrogen
using a Solv-Tek solvent purification system. Isoprene and
DMB (both Aldrich) were distilled or passed as neat liquids
through a short column of silica gel prior to use.
solutions. Transient absorbance-time profiles at each concen-
tration of scavenger are the signal-averaged result of 7-40 laser
shots. Decay rate coefficients were calculated by nonlinear least-
squares analysis of the transient absorbance-time profiles using
the Prism 5.0 software package (GraphPad Software, Inc.) and
the appropriate user-defined fitting equations, after importing
the raw data from the Luzchem mLFP software and applying
the necessary corrections to remove the minor contributions
from the corresponding digermenes at low substrate concentra-
tions.26,31 Rate constants were calculated by linear least-squares
analysis of decay rate-concentration data (generally 4-7
points) that spanned as large a range in transient decay rate as
possible. Errors are quoted as twice the standard error obtained
from the least-squares analyses. Rate constants determined at
temperatures other than 25 °C were corrected for thermal
solvent expansion.48
Nanosecond laser flash photolysis experiments were carried
out using the pulses from a Lambda-Physik Compex 120
excimer laser, filled with F2/Kr/Ne (248 nm; ca. 20 ns; 100 (
5 mJ), and a Luzchem Research mLFP-111 laser flash photo-
lysis system, modified as described previously.30 Solutions were
prepared in a calibrated 100 mL reservoir, fitted with a glass frit
to allow bubbling of argon through the solution for at least
30 min prior to and then throughout the duration of each
experiment. Concentrations were such that the absorbance at
the excitation wavelength was between ca. 0.7 and 0.9. The
solutions were pumped from the reservoir through Teflon
tubing connected to a 7 ꢀ 7 mm Suprasil flow cell using a
Masterflex 77390 peristaltic pump. The glassware, sample cell,
and transfer lines were dried in a vacuum oven (65-85 °C)
before use. In experiments carried out at 25 °C, solution
temperatures were measured with a Teflon-coated copper/con-
stantan thermocouple inserted into the thermostated sample
compartment in close proximity to the sample cell; those in
which the solution temperature was varied were carried out
using a flow cell that allowed insertion of the thermocouple
directly into the sample solution. Reagents were added directly
to the reservoir by microliter syringe as aliquots of standard
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council (NSERC) of Canada for
financial support and for a graduate scholarship to L.A.
H. We also thank Teck-Cominco Metals Ltd. for a
generous gift of germanium tetrachloride, Mr. Saurabh
Chitnis for the synthesis of 1b, and Professor P. P. Gaspar
(Washington University, St. Louis) for helpful discus-
sions and an advance copy of ref 13.
Supporting Information Available: Details of the preparation
and characterization of compounds; additional kinetic data
determined from laser flash photolysis experiments. This ma-
terial is available free of charge via the Internet at http://pubs.
acs.org.
(48) Density of Solvents as a Function of Temperature. In CRC
Handbook of Chemistry and Physics, Internet Version, 87th ed.; Lide, D.
R., Ed.; CRC Press/Taylor and Francis: Boca Raton, FL, 2007; pp 15-25.