C,C-diacetylenic phosphaalkenes as heavy diethynylethene analogues

Article abstract of DOI:10.1021/jo901942u

(Chemical Equation Presented) Aseries of C,C-diacetylenic phosphaalkenes 1b-e has been prepared from 1-chloropenta-1,2-dien-4-ynes 6b-e in a reaction with Mes*PCl₂ (Mes* = 2,4,6-(^tBu) ₃Ph) in the presence of LDA. Under ide

Full text of DOI:10.1021/jo901942u

pubs.acs.org/joc
C,C-Diacetylenic Phosphaalkenes as Heavy Diethynylethene Analogues
†
†
†
†
†
€
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Elisabet Oberg, Bernhard Schafer, Xue-Li Geng, Jenny Pettersson, Qi Hu,
Mikael Kritikos,‡ Torben Rasmussen,§ and Sascha Ott*,†
†
˚
€
Department of Photochemistry and Molecular Science, Angstrom Laboratories, Uppsala University,
Box 523, 75120 Uppsala, Sweden, ‡Department of Physical, Inorganic and Structural Chemistry, Arrhenius
Laboratory Stockholm University, 106 91 Stockholm, Sweden, and National Supercomputer Centre,
Linko€ping University, 581 83 Linko€ping, Sweden
§
sascha.ott@fotomol.uu.se
Received September 8, 2009
A series of C,C-diacetylenic phosphaalkenes 1b-e has been prepared from 1-chloropenta-1,2-dien-4-
ynes 6b-e in a reaction with Mes*PCl2 (Mes* = 2,4,6-(tBu)3Ph) in the presence of LDA. Under
identical conditions, isomeric butadiyne-substituted phosphaalkenes 2c-f can be obtained from
3-chloropenta-1,4-diynes 5c-f. The title compounds represent rare examples of diethynylethenes in
which a constituting methylene has been replaced by a phosphorus center. The formation of both
isomers can be rationalized by a common pathway that involves isomeric allenyllithium species.
Spectroscopic, electrochemical, and theoretical investigations show that the phosphorus heteroa-
toms are an intrinsic part of the compounds’ π-systems and lead to decreased HOMO-LUMO gaps
compared to those in all-carbon-based reference compounds.
Introduction
light-emitting diodes and field effect transistors4-6 or as
potential unimolecular electronics components such as
molecular diodes7 and wires.8,9 The inclusion of heteroaro-
matics such as pyridines, thiophenes, or furans into these
π-conjugates alters their electronic properties and, in addi-
tion, offers coordination sites for Lewis acids.10 In contrast
to the plethora of acetylenic (hetero)aromatic compounds,
oligoacetylenes in which sp- or sp2-hybridized carbon centers
of the one-dimensional backbone are replaced by heavier
The art to prepare monodisperse oligoacetylenes of high
complexity has reached a considerable level of sophistication
over the last decades.1-3 The combination of highly unsatu-
rated acetylenes and aromatic units has produced a multi-
tude of carbon-rich, π-conjugated compounds with potential
applications in organic electronic devices such as organic
(1) Diederich, F.; Stang, P. J.; Tykwinski, R. R. Acetylene Chemistry;
Wiley-VCH: Weinheim, 2005.
€
(7) Elbing, M.; Ochs, R.; Koentopp, M.; Fischer, M.; Hanisch, C. v.;
(2) Nielsen, M. B.; Diederich, F. Chem. Rev. 2005, 1837–1868.
(3) Kivala, M.; Diederich, F. Acc. Chem. Res. 2009, 42, 235–248.
(4) Bunz, U. H. F. Chem. Rev. 2000, 100, 1605–1644.
(5) Haley, M. M.; Tykwinski, R. R. Carbon-Rich Compounds; Wiley-
VCH: Weinheim, 2006.
Weigend, F.; Evers, F.; Weber, H. B.; Mayor, M. Proc. Natl. Acad. Sci. U.S.
A. 2005, 102, 8815–8820.
(8) Frampton, M. J.; Anderson, H. L. Angew. Chem., Int. Ed. 2007, 46,
1028–1064.
(9) Robertson, N.; McGowan, C. A. Chem. Soc. Rev. 2003, 32, 96–103.
(10) James, D. K.; Tour, J. M. Top. Curr. Chem. 2005, 257, 33–62.
(6) Anthony, J. E. Chem. Rev. 2006, 5028–5048.
DOI: 10.1021/jo901942u
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Published on Web 11/13/2009
J. Org. Chem. 2009, 74, 9265–9273 9265
2009 American Chemical Society

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Oberg et al.
JOCFeatured Article
SCHEME 1. Synthesis of Alcohols 4b-f, Chloropentadiynes
5b-f, and Allenes 6b-ea
FIGURE 1. Conceivable diacetylenic (I-III) and peracetylenic
(IV) phosphaalkenes.
main group elements are essentially unknown. Considering
the analogy between carbon and its diagonal neighbor in the
periodic table, phosphorus,11 it seems conceivable to incor-
porate phosphorus heteroatoms in the form of phosphaalk-
enes into fully π-conjugated oligoacetylenes.12 The interest in
acetylenic phosphaalkenes (APAs) as alternative organo-
phosphorus π-conjugated materials13,14 is further fuelled
by recent findings that phosphole-containing π-systems ex-
hibit relatively small HOMO-LUMO gaps. This effect
arises from the pyrimidalization of the phosphorus centers
which are thus only partly involved in the conjugation and
can act as n-dopants.15-21 The chemistry of phosphaalkenes
has matured continuously over the last decades and is nowa-
days well established. In a material science context, it is
noteworthy that phosphaalkenes can be employed in a living
anionic polymerization to afford phosphorus-containing
polymers where the phosphorus centers are saturated.22,23
Unsaturated, π-conjugated polymers with intact phosphaal-
kenes were realized in poly(phenylenephosphaalkene)s.24-26
Phosphaalkenes can be combined with acetylenes in a
number of ways. Three different diacetylenic phosphaalk-
enes (A2PA) I-III and peracetylenic phosphaalkenes
(A3PA) IV are challenging synthetic targets (Figure 1).
Furthermore, they represent attractive building blocks
which should allow for the preparation of more elaborate
oligomeric and cyclic architectures once they can be ac-
cessed. Apart from our own exploratory study toward I,12
I-IV are unknown which is presumably a result of the
general instability of phosphaalkenes. Stabilization of I-IV
can be expected from the incorporation of PdC into a
conjugated framework as is the case in phosphinine,27 from
aKey: (i) (1) BuLi, -30 °C; (2) ethyl formate, -78 to -30 °C, 2 h, 4b
(54%), 4c (66%), 4d (76%), 4e (73%), 4f (75%); (ii) SOCl2, Et2O or
CH2Cl2, rt, 12 h or reflux 2 h, 5b (82%), 5c (84%), 5d (66%), 5e (71%);
(iii) SOCl2, Et2O, 2 drops of DMF, 2 h, 5e (31%), 6e (53%).
a complexation of PdC to metal fragments28 or from a
kinetic stabilization by large substituents at the P-termi-
nus.29 In this report, we utilized the latter stabilization to
target C,C-A2PA I and to study its electronic properties by
spectroscopic, electrochemical and computational techni-
ques. Particular focus is put on comparisons with known
all-carbon-based literature compounds.
Results and Discussion
Synthesis of Diacetylenic Phosphaalkenes. A2PAs of type I
can potentially be synthesized by a number of different
synthetic approaches. In analogy to the preparation of
ethynylethenes, metal-mediated cross-coupling reactions of
C-bromo-functionalized phosphaalkenes with monosubsti-
tuted acetylenes could be considered. It has, however, been
shown in the literature that such a strategy will fail due to a
rearrangement that follows the insertion of the metal into the
carbon-halogen bond that ultimately leads to the formation
of phosphaalkynes.30 We have therefore devised an alter-
native route that relies on the formation of the PdC bond in
the last step of the synthetic sequence by reacting Mes*PCl2
with a 3-chloropenta-1,4-diyne 5 in the presence of LDA.31
The latter was envisaged to become accessible from the
corresponding propargylic alcohol 4.
(11) Dillon, K. B.; Mathey, F.; Nixon, J. F. Phosphorus: The Carbon
Copy; John Wiley & Sons: Chichester, 1998.
€
(12) Schafer, B.; Oberg, E.; Kritikos, M.; Ott, S. Angew. Chem., Int. Ed.
€
2008, 8228–8231.
ꢀ
(13) Baumgartner, T.; Reau, R. Chem. Rev. 2006, 106, 4681–4727.
(14) Gates, D. P. Top. Curr. Chem. 2005, 250, 107–126.
ꢀ
(15) Hay, C.; Fischmeister, C.; Hissler, M.; Toupet, L.; Reau, R. Angew.
Chem., Int. Ed. 2000, 39, 1812–1815.
(16) Baumgartner, T.; Neumann, T.; Wirges, B. Angew. Chem., Int. Ed.
2004, 43, 6197–6201.
(17) Matano, Y.; Nakashima, M.; Imahori, H. Angew. Chem., Int. Ed.
2009, 48, 4002–4005.
Alcohols 4b-f were synthesized from the respective acetyl-
enes and ethyl formate following a published procedure.32
Treatment of 4b-f with thionyl chloride in refluxing CH2Cl2
afforded 3-chloropenta-1,4-diynes 5b-f in reasonable to
good yields (Scheme 1).33 The chloromethyl protons of
5b-e feature as singlets between 5.22 and 5.28 ppm in
ꢀ
ꢀ
1
(18) Dienes, Y.; Durben, S.; Karpati, T.; Neumann, T.; Englert, U.;
ꢀ
their respective H NMR spectra, whereas that of phenyl-
Nyulaszi, L.; Baumgartner, T. Chem.;Eur. J. 2007, 13, 7487–7500.
ꢀ
substituted 5f can be observed at 5.78 ppm. Two signals are
visible in the acetylene region of the 13C NMR spectra,
confirming the symmetric structures of 5b-f. During
chlorination of 4b-e, it was found that a side product is
(19) Crassous, J.; Reau, R. Dalton Trans. 2008, 6865–6876.
(20) Fadhel, O.; Szieberth, D.; Deborde, V.; Lescop, C.; Nyulaszi, L.;
ꢀ
ꢀ
Hissler, M.; Reau, R. Chem.;Eur. J. 2009, 15, 4914–4924.
(21) Hay, C.; Hissler, M.; Fischmeister, C.; Rault-Berthelot, J.; Toupet,
ꢀ
ꢀ
L.; Nyulaszi, L.; Reau, R. Chem.;Eur. J. 2001, 7, 4222–4236.
(22) Noonan, K. J. T.; Gates, D. P. Angew. Chem., Int. Ed. 2006, 45,
7271–7274.
(23) Noonan, K. J. T.; Patrick, B. O.; Gates, D. P. Chem. Commun. 2007,
3658–3660.
(24) Wright, V. A.; Gates, D. P. Angew. Chem., Int. Ed. 2002, 41, 2389–
2392.
(25) Wright, V. A.; Patrick, B. O.; Schneider, C.; Gates, D. P. J. Am.
Chem. Soc. 2006, 128, 8836–8844.
(28) Floch, P. L. Coord. Chem. Rev. 2006, 250, 627–681.
(29) Yoshifuji, M. J. Organomet. Chem. 2000, 611, 210–216.
(30) Jun, H.; Young, V. G.; Angelici, R. J. Organometallics 1994, 13,
2444–2453.
(31) Toyota, K.; Kawasaki, S.; Yoshifuji, M. J. Org. Chem. 2004, 69,
5065–5070.
(26) Smith, R. C.; Protasiewicz, J. D. J. Am. Chem. Soc. 2004, 126, 2268–
2269.
€
(27) Markl, G. Angew. Chem. 1966, 78, 907–908.
(32) Tallman, K. A.; Roschek, B.; Porter, N. A. J. Am. Chem. Soc. 2004,
126, 9240–9247.
(33) Bohlmann, F. Chem. Ber. 1953, 86, 657–665.
9266 J. Org. Chem. Vol. 74, No. 24, 2009

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SCHEME 2. Synthesis of the Acetylenic Phosphaalkenes 1 and 2 from 5 and/or 6a
aKey: (vi) Mes*PCl2, LDA, THF, 2 h, -100 to -20 °C, 1b (40%) from 5b, 1c (11%) and 2c (32%) from a mixture of 5c and 6c (ca. 5:1), 1d (12%) and 2d
(32%) from a mixture of 5d and 6d (ca. 5:1), 1e (59%) from neat 6e, 2e (57%) from neat 5e, 2f (48%) over two steps from 4f.
formed in addition to 5b-e with a maximum yield of 20%
depending on reaction time and temperature. 1H NMR
spectra of the concomitantly formed products exhibit a
singlet that is slightly downfield shifted by ca. Δδ = 0.2
ppm compared to that of the chloromethyl proton of the
corresponding 5. 13C NMR spectra show a diagnostic peak
around δ = 210 ppm in addition to four peaks in the
customary acetylene region between 100 and 80 ppm. Based
on this analysis and in analogy with analytical data of a
previously reported bromoallene,34 the side product was
identified as chloroallene 6b-e. Whereas TMS-terminated
5e and 6e could be separated by column chromatography,
purification of 5b-d and 6b-d was unsuccessful. The pro-
portion of allene 6e could be increased dramatically until it
was obtained as the major product by adding two drops of
DMF to the reaction. No allene was observed during the
chlorination of phenyl-terminated 4f.
With purified 5e,f in hand, their reaction with supermesityl
phosphonous dichloride (Mes*PCl2) in the presence of LDA
was investigated.31 Although these reactions yielded phos-
phaalkenes, we were surprised to find that an isomeriza-
tion of the acetylene framework had occurred and that
butadiyne-substituted phosphaalkenes 2e,f had formed
(Scheme 2). Disappointed by the failure of the initial strat-
egy, we turned our focus to the reactivity of chloroallene side
products 6. Exposing chloroallene 6e to equivalent reaction
conditions (Mes*PCl2, LDA), we were delighted to find that
the desired A2PA 1e was formed selectively and in acceptable
yields. When mixtures of 5c,d and 6c,d are used in the
phosphaalkene preparation, product mixtures of 1c,d and
2c,d are obtained in ratios that reflect the relative propor-
tions of the starting materials 5c,d and 6c,d. Conveniently,
1c,d can be separated from 2c,d by careful column chroma-
tography. Noteworthy is that the reaction of 5c-f produces
exclusively 2c-f, but none of the isomer where Mes* is trans
to the butadiyne. The only exception to the observed trend
in reactivity is 5b and 6b with the bulky TIPS substituents
which both result in the formation of diacetylenic phos-
phaalkene 1b. Isomers 1 and 2 can be distinguished by
the chemical shift of the PdC carbon in their respective
13C NMR spectra. Whereas this signal features as a doublet
beyond δ = 160 ppm in isomer 2, it is shifted upfield to less
than δ = 140 ppm in 1. In addition, the acetylene carbons
in 1 resonate at higher chemical shift than those of 2. Further
proof for the structural assignment could be deduced from
X-ray crystallography (vide infra).12
With the exception of 5 and 6b, it emerges that chloroal-
lenes 6 always give rise to A2PAs 1 whereas butadiynes 2 are
formed in the reaction of 3-chloropentadiynes 5. The reac-
tions are thus regioselective and proceed at the β-carbon
relative to the chloride. The reactivity of 5c-f with a con-
comitant 1,2 shift of the acetylene bears a resemblance to that
of chromium(0) alkynylcarbenes.35 Although it is possible to
postulate a related C5 carbene intermediate that is formed
from 5 by R-elimination,36-38 it is appealing to propose a
common and more general mechanism to rationalize the
formation of 1 and 2 from 6 and 5, respectively. Further-
more, reactions of nonstabilized carbenes such as a C5
carbene are known to exhibit poor regioselectivity.36 The
reaction of allene 6 to form A2PA 1 suggests that allene
intermediates may also be present in the reaction of 5. Our
mechanistic proposal thus starts with the abstraction of the
relatively acidic protons in 5 and 6 by LDA (Scheme 3). It has
previously been shown that propargyllithium species are in
equilibrium with allenyllithium and that this equilibrium lies
far toward the latter when silyl substituents are present.39,40
In analogy to these reports, we postulate a tautomerization
of 5-P to the corresponding 5-A. Support for this hypothesis
was obtained from an experiment where 5e was first exposed
to LDA at -78 °C and then quenched by the addition of
an aqueous solution of NH4Cl. 13C NMR analysis of the
reaction products revealed a diagnostic signal at δ = 211
ppm together with four signals between 110 and 90 ppm.
These results are consistent with the formation of an acet-
ylenic allene which, as expected, is different from 6e. It is
important to note that 5-A is a regioisomer of deprotonated 6
ꢀ ꢀ
(35) Barluenga, J.; Garcıa-Garcıa, P.; Saa, D. d.; Fernandez-Rodrıguez,
´ ´ ´
ꢀ
ꢀ
M. A.; Rua, R. B. d. l.; Ballesteros, A.; Aguilar, E.; Tomas, M. Angew.
Chem., Int. Ed. 2007, 46, 2610–2612.
(36) Hori, Y.; Noda, K.; Kobayashi, S; Taniguchi, H. Tetrahedron Lett.
1969, 40, 3563–3566.
(37) Bowling, N. P.; McMahon, R. J. J. Org. Chem. 2006, 71, 5841–5847.
(38) Bowling, N. P.; Halter, R. J.; Hodges, J. A.; Seburg, R. A.; Thomas,
P. S.; Simmons, C. S.; Stanton, J. F.; McMahon, R. J. J. Am. Chem. Soc.
2006, 128, 3291–3302.
(39) Reich, H. J.; Holladay, J. E. Angew. Chem., Int. Ed. 1996, 35, 2365–
2367.
(34) An, Y.-Z.; Ellis, G. A.; Viado, A. L.; Rubin, Y. J. Org. Chem. 1995,
60, 6353–6361.
(40) Reich, H. J.; Holladay, J. E.; Walker, T. G.; Thompson, J. L. J. Am.
Chem. Soc. 1999, 121, 9769–9780.
J. Org. Chem. Vol. 74, No. 24, 2009 9267

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SCHEME 3. Proposed Mechanism43 for the Preparation of Isomers 2 and 1 from 5 and 6, Respectively
(6-A). Compounds 5-A and 6-A attack Mes*PCl2 in a nucleo-
philic substitution,41 leading to isomeric 3-chloroallenyl-
1-phosphines. Lithium-halogen exchange at the isomeric
allene portions42 establishes new allenyllithium 1LiCl and
2LiCl which eliminate lithium chloride to afford isomers 1
and 2 (Scheme 3).43 In the case of 2LiCl, it is this last
elimination step that determines the relative position of the
substituents across the PdC double bond in 2c-f. To mini-
mize steric repulsion between Mes* and R2, the isomer in
which the butadiyne is cis to Mes* is formed in all instances.
After successful synthesis and purification of 1b-e and
2c-f, the electronic properties of these rather different
π-system were studied in more detail. Particular focus was
given to the effect of the phosphorus heteroatom. For
comparison and a more complete evaluation, we have in-
cluded the previously reported 2g,h and octatetrayne 7 in this
study (Figure 2).12
the phenyl group in the overall π-conjugation. The 31P NMR
chemical shifts are even sensitive to perturbation of the
remote phenyl groups at the butadiyne terminus. The elec-
tron-withdrawing substituent at the phenyl group in 2g
causes a downfield shift compared to 2f, whereas the elec-
tron-donating dimethylamino group in 2h leads to a shield-
ing of the phosphorus center.
X-ray Crystallography. The crystal structures of 2g and 2h
are depicted in Figure 3 and 4. Most importantly, both
structures feature the butadiyne moieties cis to the Mes*
group, giving further support to the structural assignments
of the synthetically obtained isomers.
The bond lengths in the PC5 backbone are in the usual
region for PdC double, CtC triple, and C(sp2)- C(sp) and
C(sp)-C(sp) single bonds. The dihedral angle between the
plane defined by the phenyl ring at the PdC carbon and the
PC5 scaffold is very small (6° in 2g and -1° in 2h), pointing
toward a sizable contribution of the phenyl ring in the overall
π-conjugation in the solid state.
Electronic Absorption Spectroscopy and Cyclic Voltamme-
try. Comparing the longest wavelength absorption maxima
of silyl-terminated diacetylenic phosphaalkenes 1c-e with
those of 2c-e, it emerges that the end absorptions of the
latter are slightly shifted toward lower energies (Table 2).
Since the absorption arises from similar πfπ* transitions
(vide infra), this finding suggests a lower degree of π-delo-
calization in cross-conjugated isomer 1 compared to that in
linear conjugated 2. The introduction of a phenyl group at
the phosphaalkene carbon in 2f-h and 7 gives rise to larger
shifts of the end absorptions which in addition are strongly
dependent on the substituent at the peripheral butadiyne
terminus. Significant alterations of the participating frontier
molecular orbitals are necessary to account for the observed
shifts of the longest wavelength absorption maxima that
range from 379 nm (2f) over 407 (2g) and 441 (2h) to 484 nm
in octatetrayne 7.
FIGURE 2. Nitrophenyl (2g) and N,N-dimethylaniline (2h) substi-
tuted 1-phosphahex-1-en-3,5-diyne and bis-phosphaalkene end-
capped octatetrayne 7.12 Mes* = 2,4,6-(tBu)3Ph.
As already deducible from the differences in 31P NMR
chemical shifts in Table 1, it emerges that the phosphorus
heteroatoms in 1, 2, and 7 are an intrinsic part of the entire
π-conjugated system in all compounds. It is interesting to
note that the 31P chemical shifts of the bis-silyl substituted
C,C-A2PAs 1c-e are lower than those of the 1-phosphahex-
1-ene-3,5-diynes 2c-e. Exchanging the silyl substituent at
the PdC in 2c-e by a phenyl group in 2f-h results in an
upfield shift of the 31P resonance, indicating participation of
An all-carbon analogue of 1e with a corresponding
1,10-diethynylethene skeleton, namely 3-methylene-1,5-
bis(trimethylsilyl)penta-1,4-diyne,44 has a reported end
absorption of 247 nm which is at considerably higher
energy than that of 1e. A second carbon-based ana-
logue (4-phenyl-2-phenylethynylbut-1-en-3-ynyl)benzene,
€
(41) Markl, G.; Pflaum, S.; Maack, A. Tetrahedron Lett. 1992, 33, 1981–
1984.
(42) Jux, N.; Holczer, K.; Rubin, Y. Angew. Chem., Int. Ed. 1996, 35,
1986–1990.
(43) In principle, lithium-halogen exchange could also occur at the
P-bound chloride. This species would, however, give rise to the same product
after elimination.
(44) Alberts, A. H. J. Am. Chem. Soc. 1989, 111, 3093–3094.
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TABLE 1. 31P NMR (ppm) Chemical Shifts of C,C-Diacetylenic
Phosphaalkenes 1 and 1-Phosphahex-1-en-3,5-diynes 2
diacetylenic
phosphaalkenes
(isomer 1)
1-phosphahex-
1-en-3,5-diyne
(isomer 2)
substituents
entry
R1
TIPS
TBDMS TBDMS
R2
31P NMR
31P NMR
b
c
d
e
f
g
h
7
TIPS
331
340
339
346
372
367
364
311
319
304
331
TES
TES
TMS
Ph
TMS
Ph
PhNO2
Ph
PhNMe2 Ph
Ph
FIGURE 4. ORTEP drawing (at 30% probability level) of 2h. For
clarity, only one of the two crystallographic independent molecules
is shown, and hydrogen atoms are omitted. Selected bond lengths
(A) and angles (deg): P1-C19 1.695(5), C19-C26 1.409(7),
C26-C27 1.203(7), C27-C28 1.377(8), C28-C29 1.207(7). Angles:
C1-P1-C19 100.8(2), P1-C19-C26 123.9(4), P1-C19-C20
120.3(4). Dihedral angle: C1-P1-C19-C20 178.6(5).
TABLE 2. Absorption Band Maxima and Molar Extinction Coeffi-
cients from UV/vis Spectroscopic Measurements in CH2Cl2 at 25 °C.
Electrochemical Datafor 1 mM Solutions (0.1 M NBu4PF6), ν = 100 mV/s
(All Potentials Are Given vs Fcþ/0
)
compd
λ
max[nm] (ε [103 M-1 cm-1])
Ep,c (V)
Ep,a (V)
1b
1c
1d
1e
2c
2d
2e
2f
349 (18.5)
347 (17.0)
343 (12.0) 355 (12.5)
347 (11.5)
346 (13.5) 359 (13.0)
346 (12.5) 359 (12.5)
343 (13.0) 358 (13.0)
379 (17.0)
332 (21.5) 407 (15.5)
328 (46.5) 441 (26.5)
286 (52.0) 336 (22.0)
389 (22.5) 443 (15.0) 484 (10.0)
-2.07
1.15
1.12
1.14
1.16
1.18
1.19
1.18
1.05
-2.12b
-2.12b
-2.07
-2.21
-2.19
-2.17
-1.98c
FIGURE 3. ORTEP drawing (at 30% probability level) of 2g.
Hydrogen atoms are omitted for clarity. Selected bond lengths
(A) and angles (deg): P1-C1 1.686(3), C1-C2 1.420(3), C2-C3
1.195(3), C3-C4 1.364(4), C4-C5 1.194(4). Angles: C1-P1-C18
101.0(1), P1-C1-C2 123.0(2), P1-C1-C12 121.0(2). Dihedral
angle: C12-C1-P1-C18 -175.9(2).
2g
2h
7
-1.40c, -1.84c 1.08
-2.04c
0.47, 1.07
-1.62c, -1.96c 1.06
aShoulder. bReversible at scan rates higher than 1 V/s. cElectrochemi-
cally reversible E1/2 = (Ep,c þ Ep,a)/2.
λmax = 340 nm)45 with additional phenyl rings at both
acetylene termini that increase the π-system features an end
absorption that is still at higher energy compared to that of
1 and 2. It thus seems that the inclusion of a phosphorus
heteroatom in 1 and 2 causes a sizable decrease of their
HOMO-LUMO gaps. This effect becomes even more
apparent when comparing the longest wavelength absorp-
tion maximum of 7 with that of dodeca-1,11-diene-3,5,7,9-
tetrayne46 which is shifted by 80 nm toward higher energy.
The reduction of the HOMO-LUMO gap in APAs thus
appears to be a general effect that is caused by the λ3-σ2
phosphorus heteroatoms. Although different orbitals are
involved, the effect of the λ3-σ2 phosphorus in phosphaalk-
enes on the HOMO-LUMO gaps of appended π-systems is
thus similar to that of the λ3-σ3 phosphorus lone pair in
phospholes.15-21
The cyclic voltammograms (CVs) of 1b-e and 2c-e
feature two electrochemically irreversible processes, an oxi-
dation between 1.12 and 1.19 V (all potentials are vs Fcþ/0
)
and a reduction between -2.07 and -2.21 V (see Table 2). In
analogy to the optical spectroscopy, the situation changes
dramatically when phenyl substituents are introduced. First,
the oxidations of 2f-h and 7 are shifted to milder potentials
by ca. 100 mV, pointing toward a participation of the PdC
phenyl ring in the HOMO. The first oxidation that is
observed at 0.47 V in the CV of 2h can be assigned to
an isolated process on the N,N-dimethylaniline.47 Second,
the reductions of 2f-h and 7 become electrochemically
(45) Hwang, G. T.; Son, H. S.; Ku, J. K.; Kim, B. H. J. Am. Chem. Soc.
2003, 125, 11241–11248.
(47) Nielsen, M. B.; Utesch, N. F.; Moonen, N. N. P.; Boudon, C.;
€
Gisselbrecht, J.-P.; Concilio, S.; Piotto, S. P.; Seiler, P.; Gunter, P.; Gross,
M.; Diederich, F. Chem.;Eur. J. 2002, 8, 3601–3613.
(46) Zhao, Y.; McDonald, R.; Tykwinski, R. R. J. Org. Chem. 2002, 67,
2805–2812.
J. Org. Chem. Vol. 74, No. 24, 2009 9269

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TABLE 3. Relative Energies (kJ/mol) of the Three Isomers 1-3 of a-g,
As Calculated at the B3LYP/6-311þþG**//B3LYP/6-31G* Level of
Theorya
entry
Ar
Ph
Mes*b
isomer 1 isomer 2 isomer 3
a
b
c
d
e
f
R1 = R2 = H
R1 = R2 = TIPS
47.4
0
6.0
6.8
0
0
74.1
28.9
23.8
25.3
22.1
R1 = R2 = TBDMS Mes*b
6.0
1.3
4.5
13.6
R1 = R2 = TES
R1 = R2 = TMS
R1 = R2 = Ph
Mes*b
Mes*b
Mes*b
0
0
0
FIGURE 5. Three possible isomers with an unsaturated PC5 frame-
work: C,C-A2PA (isomer 1), cis-1-phosphahex-1-en-3,5-diyne
(isomer 2), and trans-1-phosphahex-1-en-3,5-diyne (isomer 3).
aThe isomer of lowest energy was set to 0 kJ/mol, and the energies of
the other two are relative to this value. bMes* = (tBu)3Ph.
quasi-reversible. The potentials that are associated with the
reduction of the π-system are highly dependent on the
substituent on the peripheral phenyl ring and become in-
creasingly more negative with increasing donor strength of
the phenyl terminus. Owing to the dimeric character of
octatetrayne 7, a second reduction can be observed that
is separated from the first by ΔE = 340 mV, indicating
a sizable communication between the two phosphaalkenes.
The communication is very similar to that observed in
related bis(diphosphene) systems where the coupling is
mediated through a ferrocene or p-phenylene spacer.48,49
The first reduction of 2g at -1.40 V is assigned to an isolated
process on the nitrophenyl group.50
π-delocalization to a greater extent and is thus responsible
for the lower energy of 3a. The corresponding dihedral angle
in 1a is 45.4°, and the participation of its phenyl ring in the
overall π-conjugation is thus comparable to that in 2a. The
calculations, however, show that isomer 1a is greatly dis-
favored by 41.4 kJ/mol compared to 2a. This energy differ-
ences thus have to be attributed to the isomeric PC5 units,
which are arranged in a cross-conjugated fashion in 1a
compared to the linear geometry in 2a and 3a. The PC5 system
thus follows a similar trend as all-carbon based π-conjugates
in that structures with linear conjugation are usually lower in
energy than isomeric cross-conjugated systems.51,52
DFT Calculations. In addition to 1 and 2 which were
synthesized in this study, the unsaturated PC5 unit can
potentially be arranged in a third isomeric form 3 in which
the butadiyne is trans to the P-substituent (Figure 5). DFT
calculations at the B3LYP/6-311þþG**//B3LYP/6-31G*
level of theory were performed on all three isomers to identify
the lowest energy isomer and to gain insight into factors
that may explain the synthetic preference for their formation.
Conformers of each isomer used in the DFT study were
obtained from a conformational search with the OPLS2005
force field.
A comparative study for isomers 1-3 using the two
functionals BLYP and BH&HLYP was performed in order
to validate our method of choice. These calculations showed
that the relative energies of the constitutional isomers varied
somewhat, but the order was maintained over the different
functionals (see the Supporting Information). Furthermore,
the results obtained with the two hybrid functionals (B3LYP
and BH&HLYP) were very similar. Table 3 summarizes the
calculated energies of the lowest energy conformation that
was found of each isomer. In every entry, the energies of the
isomers are relative to the energy of the lowest energy isomer.
In the absence of any bulky substituents at the acetylene
termini and a phenyl as P-substituent (entry a), steric factors
are kept to a minimum and the results of the calculations can
be interpreted from a conjugation perspective. Butadiyne-
substituted phosphaalkenes 2a and 3a are lowest in energy of
the a-series with the latter being lower by 6 kJ/mol. This
difference is easily explained by looking at the dihedral angle
between the benzene ring and the plane defined by the
remaining molecule in 2a and 3a which is 50.4° and 31.7°,
respectively. The phenyl ring in 3a is able to participate in
In the presence of the synthetically important Mes* group
and substituents R1 and R2 in isomers 1b-3f, the dihedral
angle between Mes* and the PC5 framework is close to 90°
and communication between the two units becomes negligi-
ble. Most importantly, however, the energetic order of the
compounds changes drastically. In all cases b-f, isomer 3
becomes the most energy-rich one due to steric clashes
between Mes* and R1 that are directly attached to the
PdC double bond in a cis relationship. The high energy
of 3b-f correlates well with the observation that isomer 3
is never found synthetically. It thus seems that the steric
arguments that make 3b-f highest in energy also raise the
energies of the transition states that would lead to their
formation.
A further interesting correlation between the calculated
energies and the reactivity of the chlorides can be observed
when comparing entry b with c-f. In the latter cases, isomer
2 is always lowest in energy whereas isomer 1 has the lowest
energy of the b series. Since all silyl groups can be expected to
be electronically very similar, the difference in energetic
preference has to be caused by steric factors. From a
synthetic viewpoint, it thus seems that the vast steric bulk
of the TIPS groups that destabilizes 2b also prevents the
allenyllithium species of 5b to react in the customary fashion
and a reaction of the propargyllithium occurs instead.
The calculations strongly indicate that the stability of
isomer 2 is a result of a favorable π-delocalization term. A
similar stabilization is operating in isomer 3 but is obscured
by strongly disfavoring steric clashes between Mes* and R1
which are in a cis relationship across the PdC double bond.
The acetylene spacer in isomer 1 and the butadiyne in 2
increase the spatial separation between the two units and
thus lead to a reduction of the strain. Although one would
anticipate that the longer butadiyne in 2 would lead to less
(48) Nagahora, N.; Sasamori, T.; Watanabe, Y.; Furukawa, Y.; Tokitoh,
N. Bull. Chem. Soc. Jpn. 2007, 80, 1884–1900.
(49) Dutan, C.; Shah, S.; Smith, R. C.; Choua, S.; Berclaz, T.; Geoffroy,
M.; Protasiewicz, J. D. Inorg. Chem. 2003, 42, 6241–6251.
(50) Hilger, A.; Gisselbrecht, J.-P.; Tykwinski, R. R.; Boudon, C.;
€
Schreiber, M.; Martin, R. E.; Luthi, H. P.; Gross, M.; Diederich, F. J. Am.
Chem. Soc. 1997, 119, 2069–2078.
€
(51) Bruschi, M.; Giuffreda, M. G.; Luthi, H. P. Chem.;Eur. J. 2002, 8,
4216–4227.
(52) Giuffreda, M. G.; Bruschi, M.; Luthi, H. P. Chem.;Eur. J. 2004,
5671–5680.
€
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FIGURE 7. Calculated HOMO and LUMO with relative orbital
energies for 1e and 2e-h at the B3LYP/6-311þþG**//B3LYP/
6-31G* level.
FIGURE 6. Graphic representation of 1b,e and 2b,e at the B3LYP/
6-311þþG**//B3LYP/6-31G* level of theory. Highlighted is the
void in the center of the P-Mes* group that can accommodate an
acetylene substituent in 1b,e and the steric demands at its periphery
that lead to clashes with the butadiyne substituent in 2b,e. Mes* =
(tBu)3Ph.
the spectroscopic and voltammetric experiments, the
HOMO of 2f has a contribution from the phenyl substituent
at the PdC carbon. On the other hand, the phenyl group at
the butadiyne terminus is not participating in the FMOs.
This situation changes as electron-withdrawing and -donat-
ing substituents are introduced in 2g and 2h, respectively. A
molecular orbital with large contributions from the periph-
eral nitrophenyl group becomes the new LUMO of 2g. It is
interesting to note that the calculated LUMOþ1 is almost
identical to the LUMO of 2f (see the Supporting Infor-
mation). The calculations are thus consistent with the elec-
trochemically obtained results that the first reduction is a
localized process on the extended nitrophenyl group in 2g.
Similarly, a new orbital with large coefficients on the
extended aniline portion features as the HOMO in 2h and
the HOMO-1 is analogous to the HOMO of 2f,g. The
calculations are thus again in agreement with the electro-
chemical experiments where an additional oxidation is
observed at milder potential than that associated with the
oxidation of the PC5-based π-conjugated system. Due to the
partial spatial separation of HOMO and LUMO in 2g,h,
the lowest energy optical transitions can be expected to
exhibit some charge-transfer character.
steric constraints compared to those in 1, it is interesting to
note that the opposite is the case and that the energy
difference between 1c-f and 2c-f (1.3-13.6 kJ/mol) is
relatively small compared to that between 1a and 2a
(41.4 kJ/mol). In other words, there seems to be a factor
that destabilizes isomer 2 more than 1 and leads to a situation
where the two isomers are closer in energy than accounted
for by the π-conjugation term. We believe to have found the
reason for this unexpected effect in the special nature of
the Mes* group which is sterically very demanding at the
peripheral ortho and para positions, but is rather uncon-
gested in its center. As visible from the graphical representa-
tions of the structure optimizations for 1b,e and 2b,e in
Figure 6, the acetylene spacer in isomer 1 places the silyl
substituents in a position where they can extend into the void
of the Mes* center. In contrast, the butadiyne in 2 forces
the silyl substituents into a position where they clash with
the para-tBu group of Mes*. As a result, the repulsive
Mes* R1 interaction in isomer 2 is greater than that in 1.
In case of the most bulky TIPS substituents, the Mes* R1
3 3 3
3 3 3
Conclusions
repulsion in 2b dominates even over the stabilization
from π-delocalization and renders 1b the lowest energy
isomer of the b series.
We have identified an unexpected isomerization during
the chlorination of alcohols 4b-e which leads to the forma-
tion of 3-chloropenta-1,4-diyne 5b-f and 1-chloropenta-1,2-
dien-4-yne 6b-e. Different regioisomeric acetylenic phos-
phaalkenes are formed in the LDA-promoted reaction with
Mes*PCl2 from both starting chlorides. The reactions pro-
ceed selectively at the β-carbon relative to the chloride and
lead to C,C-A2PAs 1b-e from 6b-e and to butadiyne-
substituted phosphaalkenes 2c-f from 5c-f. Both products
are proposed to be formed in a similar fashion from isomeric
allenyllithium species. DFT calculations on the three poten-
tially attainable isomers 1-3 reveal that a delicate interplay
between electronic and steric factors determines the energetic
order of the different isomers. The calculations suggest that
steric constraints influence the reaction outcome and the
preference for the formation of a certain isomer. The phos-
phaalkenes are an intrinsic part of the π-conjugated system
in all compounds and are responsible for a lowering of their
HOMO-LUMO gaps compared to those in all-carbon
Calculated Frontier Molecular Orbitals. As representa-
tively shown for structures 1e and 2e-h in Figure 7, the
calculated frontier molecular orbitals (FMOs) are almost
exclusively of π-character and for 1b-e and 2c-e, both
HOMO and LUMO are localized over the same atoms.
The lowest energy absorptions in the UV/vis spectra can
therefore be expected to result from πfπ* transitions.
Furthermore, the calculations show that the phosphaalkenes
are an intrinsic part of the π-delocalized systems and present
in all FMOs, except for the LUMO of 2g. The calculated
HOMO-LUMO gaps are in good to excellent agreement
with the values obtained for the lowest energy absorption
maxima in the UV/vis absorption experiments. For example,
for 2e,f,h, the calculated HOMO-LUMO separations of
3.45, 3.25, and 2.85 eV correlate very well with the experi-
mentally determined 3.46 (358 nm), 3.27 (379 nm), and
2.81 eV (441 nm), respectively. As already anticipated from
J. Org. Chem. Vol. 74, No. 24, 2009 9271

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Oberg et al.
JOCFeatured Article
based reference compounds. The reliable access to C,C-
A2PAs, in particular bis-TMS protected 1e, offers great
potential for the preparation of elaborate oligomeric and
cyclic structures, since the in situ removal of the silyl protect-
ing group and preceding coupling protocols can both be
conducted under mild basic conditions.53 Efforts in these
directions are the subject of ongoing investigations.54
(s, 6H, SiCH3), -0.18 (s, 6H, SiCH3). 13C NMR (CDCl3, 100
1
MHz): δ =153.5, 150.4, 141.5 (d, JPC = 35 Hz), 134.7 (d,
=
1JPC = 58 Hz), 122.4, 105.7 (d, JPC = 10 Hz), 105.4 (d, JPC
26 Hz), 103.8 (d, JPC = 19 Hz), 99.6 (d, JPC = 15 Hz), 38.2,
35.0, 33.0 (d, 4JPC = 6 Hz), 31.4, 26.2, 26.1, 17.0, 16.5, -4.7,
-4.8. 31P NMR (CDCl3, 162 MHz): δ = 339.6. EI MS (70
eV): m/z 566.2 (55) [Mþ], 509.3 (12) [Mþ - tert-butyl], 275.2
(100) [Mes*Pþ - H]. Anal. Calcd for (C35H59PSi2): C, 74.14;
H, 10.49. Found: C, 74.35; H, 10.39.
2c. Yellow solid. Yield: 0.341 g (0.60 mmol, 32%). 1H NMR
(CDCl3, 400 MHz): δ = 7.41 (s, 2H, CH-Mes*), 1.47 (s, 18H,
t-Bu-Mes*), 1.34 (s, 9H, t-Bu-Mes*), 0.97 (s, 9H, t-Bu-Si), 0.87
(s, 9H, t-Bu-Si), 0.27 (s, 6H, SiCH3), 0.04 (s, 6H, SiCH3). 13C
NMR (CDCl3, 100 MHz): δ = 163.1 (d, 1JPC = 78 Hz), 152.6,
150.2, 138.9 (d, 1JPC = 68 Hz), 122.1, 96.4 (d), 95.2 (d, JPC = 11
Hz), 89.2 (d, JPC = 9 Hz), 77.3 (d), 37.8, 34.9, 33.2 (d, 4J (P,C) =
6 Hz), 31.4, 26.8, 26.1, 18.4, 16.8, -4.7, -4.9 (d, 3JPC = 10 Hz).
31P NMR (CDCl3, 162 MHz): δ = 371.5. EI MS (70 eV): m/z
567.26 (10) [Mþ þ H], 398.1 (90) [Mþ - (3 tert-butyl) þ 3Hþ],
397.1 (100) [Mþ - (3-tert-butyl) þ 2Hþ], 275.2 (66) [Mes*Pþ-
H]. Anal. Calcd for (C35H59PSi2): C, 74.14; H, 10.49. Found: C,
74.03; H, 10.31.
General Procedure for the Preparation of the Carbinols 4b-f.
n-Butyllithium (12 mL of a 2.5 M solution in hexanes) was added
to a solution of the acetylene (30 mmol) in THF (40 mL) at
-30 °C. The colorless solution was stirred for 15 min and cooled
to -78 °C before ethyl formate (1.2 mL, 15 mmol) was added via
syringe. The reaction was stirred for 2 h, during which time it
was allowed to warm to -30 °C. After the yellow solution was
quenched at this temperature with a mixture of aqueous satu-
rated NH4Cl (50 mL) and 1 M HCl (10 mL), the phases were
separated and the organic phase extracted with EtOAc (3 ꢀ
40 mL). The combined organics were washed with brine
(40 mL), dried over Na2SO4, and concentrated in vacuo. Pur-
ification by column chromatography (5% EtOAc in pentane).
1,5-Bis(triisopropylsilyl)penta-1,4-diyn-3-ol (4b). From n-bu-
tyllithium (10.5 mL of a 2.5 M solution in hexanes),
(triisopropylsilyl)acetylene (5.2 g, 28.5 mmol), and ethyl for-
mate (0.93 g, 12.5 mmol). Colorless oil. Yield: 1.67 g (6.77 mmol,
54%). 1H NMR (300 MHz, CDCl3): δ = 5.11 (d, 3J = 7.8 Hz,
Experimental Section
Computational Methods. Conformational searches were per-
formed for all three isomers of the investigated compounds
using the OPLS2005 force field in the MacroModel program.55
Geometry optimizations were in turn performed on typically
two to six representative low energy conformers using gas-phase
density functional theory (DFT) calculations with the B3LYP
hybrid functional56 and employing the 6-31G* basis set. Single-
point calculations were performed with the larger 6-311þþG**
basis set on the geometries obtained at the B3LYP/6-31G* level.
Molecular orbitals were computed using the B3LYP functional
and the 6-311þþG** basis set. All calculations were performed
using the Jaguar 07 program package. For details, see the
Supporting Information.
Materials and General Methods. Chemicals were used as
received. THF and Et2O were distilled from sodium/benzophe-
none. CH2Cl2 was distilled from calcium hydride. All reactions
were performed under an inert atmosphere of N2 or Ar. Column
chromatography was performed on silica gel SI-60 A (35-70).
For characterization of 1b, 2g,h, and 7 see ref 12. For
characterization of 1d,e, 2d-f, 4c-f, and 5c-f, see the Support-
ing Information.
General Procedure for the Preparation of Phosphaalkenes
1b-e and 2c-f. To a solution of 2,4,6-tri-tert-butylbromoben-
zene in THF (20 mL) was added dropwise n-butyllithium (2.5 M
solution in hexanes) at -78 °C. After 0.5 h of stirring, PCl3
(3 equiv) was added quickly in one portion at -78 °C. The
solution was warmed to room temperature and refluxed for 2-
3 h. The volatiles were removed in vacuo. The white solid was
dissolved in THF, and 3-chloropenta-1,4-diyne 5 and/or
1-chloro-1,2-pentadien-4-yne 6 were added. LDA (2.2 equiv)
was added at -98 °C, and after 15 min, the red solution was
allowed to warm to -30 °C over 2 h under stirring. The reaction
mixture was quenched by aqueous saturated NH4Cl, diluted
with hexane, washed with H2O and brine, dried over Na2SO4,
and concentrated in vacuo. Purification and isolation of 1 and 2
was achieved by column chromatography (silica, 1% EtOAc in
pentane). Recrystallizations were made from MeOH or MeOH/
CH2Cl2.
(1,5-Bis(tert-butyldimethylsilyl)penta-1,4-diyn-3-ylidene)-
(2,4,6-tri-tert-butylphenyl)phosphine (1c) and (E)-(1,5-Bis(tert-
butyldimethylsilyl)penta-2,4-diynylidene)(2,4,6-tri-tert-butyl-
phenyl)phosphine (2c). From 1,5-bis(tert-butyldimethylsilyl)-
3-chloropenta-1,4-diyne 5c and 1-chloro-1,5-bis(tert-butyl-
dimethylsilyl)-1,2-pentadien-4-yne 6c (5:1 mixture: 0.715 g,
1.90 mmol), Mes*Br (0.700 g, 2.15 mmol), n-butyllithium
(0.87 mL of a 2.5 M solution in hexanes), PCl3 (0.56 mL
6.45 mmol), and LDA (4.30 mmol). 1c. Yellow solid. Yield:
0.121 g (0.21 mmol, 11%). 1H NMR (CDCl3, 400 MHz): δ =
7.44 (s, 2H, CH-Mes*), 1.49 (s, 18H, t-Bu-Mes*), 1.32 (s, 9H,
t-Bu-Mes*), 0.98 (s, 9H, t-Bu-Si), 0.73 (s, 9H, t-Bu-Si), 0.17
3
1H, CH), 2.16 (d, J = 7.8 Hz, 1H, OH), 1.09-1.02 (m, 42H,
i-Pr). 13C NMR (100 MHz, CDCl3): δ = 104.3, 85.6, 53.0,
18.5, 11.1.
General Procedures for the Preparation of Chloropentadiynes
5b-f. In Et2O. Alcohol 4 was dissolved in Et2O and deaerated
with N2 for 15 min. The solution was cooled to -20 °C, and
SOCl2 was added dropwise. The yellow solution was warmed to
rt and stirred for 12 h. The solution was diluted with Et2O,
poured on ice, and neutralized with saturated aqueous KHCO3.
The phases were separated, the aqueous phase extracted with
Et2O (3 ꢀ 40 mL), and the combined organics washed with brine
(40 mL), dried over Na2SO4, and concentrated in vacuo. Pur-
ification by column chromatography (pentane).
In CH2Cl2. Alcohol 4 was dissolved in CH2Cl2 (20 mL) and
deaerated with N2 for 15 min. The solution was cooled to 0 °C,
and SOCl2 was added dropwise. The yellow solution was
warmed to rt and refluxed for 2 h. The solution was diluted
with CH2Cl2, poured on ice, and neutralized with saturated
aqueous NaHCO3. The phases were separated, extracted with
Et2O (3 ꢀ 40 mL), and the combined organics washed with brine
(40 mL), dried over Na2SO4 ,and concentrated in vacuo. Pur-
ification by column chromatography (pentane).
1. 5-Bis(triisopropylsilyl)-3-chloropenta-1,4-diyne (5b). From
1,5-bis(triisopropylsilyl)penta-1,4-diyn-3-ol (2.17 g, 5.5 mmol),
SOCl2 (5.5 mL, 75 mmol), and Et2O (8 mL). Colorless oil. Yield:
1.85 g (4.50 mmol, 82%). 1H NMR (400 MHz, CDCl3): δ = 5.27
(s, 1H, CH), 1.08 (m, 42H, i-Pr). 13C NMR (100 MHz, CDCl3):
δ = 101.0, 88.1, 35.6, 18.5, 11.1.
(53) Pak, J. J.; Weakley, T. J. R.; Haley, M. M. J. Am. Chem. Soc. 1999,
121, 8182–8192.
(54) Geng, X. L.; Ott, S. Chem. Commun. 2009, DOI: 10.1039/B916640H.
(55) Mohamadi, F.; Richards, N. G. J.; Guida, W. C.; Liskamp, R.;
Lipton, M.; Caufield, C.; Chang, G.; Hendrickson, T.; Still, W. C. J. Comput.
Chem. 1990, 11, 440.
(56) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1987, 37, 785–789.
9272 J. Org. Chem. Vol. 74, No. 24, 2009

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1-Chloro-1,5-bis(trimethylsilyl)-1,2-pentadien-4-yne (6e). 1,5-
Bis(trimethylsilyl)penta-1,4-diyn-3-ol (900 mg, 4 mmol) was
dissolved in Et2O (20 mL), SOCl2 (2.2 mL, 30 mmol) was added
dropwise at rt, and two drops of DMF were added. The reaction
was allowed to stir for 2-3 h before it was poured into a mixture
of ice-water (30 mL) and hexane (80 mL). The organic layer was
separated and washed with saturated aqueous KHCO3, water,
and brine and then dried over Na2SO4 and concentrated
in vacuo. Purification by column chromatography (hexane).
1-Chloro-1,5-bis(trimethylsilyl)-1,2-pentadien-4-yne (6e) was
obtained as a colorless oil. Yield: 540 mg (2.1 mmol, 53%)
accompanied by 1,5-bis(trimethylsilyl)-3-chloropenta-1,4-diyne
(5e, 0.30 g (1.2 mmol, 31%). 1H NMR (400 MHz, CDCl3): δ =
NMR (100 MHz, CDCl3): δ = 212.0, 100.5, 99.6, 96.0, 79.9,
-0.2, -2.3.
Acknowledgment. This work was supported by the Swed-
€
ish Research Council, the Goran Gustafsson Foundation,
the Carl Trygger Foundation, and Uppsala University
through the U3MEC molecular electronics priority initiative.
Supporting Information Available: Analytical and crystal-
lographic details; comparative computational study with differ-
ent functionals; calculated FMO energies and Cartesian
coordinates of the lowest energy conformations of 1-3. This
material is available free of charge via the Internet at http://
pubs.acs.org.
5.55 (s, 1H, CH), 0.23 (s, 9H, SiCH3), 0.20 (s, 9H, SiCH3). 13
C
J. Org. Chem. Vol. 74, No. 24, 2009 9273