Synthesis and ligand properties of stable five-membered-ring allenes containing only second-row elements

Article abstract of DOI:10.1002/anie.200801176

(Chemical Equation Presented) Push-push substitution patterns render allenes so flexible that the usually linear C=C=C fragment can be accommodated in a five-membered ring composed of only nitrogen and carbon atoms (see structure; C black, N blue, O red). These cyclic allenes appear to be strong donor η¹ ligands for alkali and transition metals.

Full text of DOI:10.1002/anie.200801176

Angewandte
Chemie
DOI: 10.1002/anie.200801176
Bent Allenes
Synthesis and Ligand Properties of Stable Five-Membered-Ring
Allenes Containing Only Second-Row Elements**
Vincent Lavallo, C. Adam Dyker,* Bruno Donnadieu, and Guy Bertrand*
Allenes are compounds with two contiguous carbon–carbon
double bonds. They consequently feature a linear CCC
skeleton with orthogonal pairs of substituents.[1] Because of
their rigidity,allene frameworks can only be accommodated
into a ring with difficulty,and calculations predict that the
ring strain increases considerably with each successive
removal of a carbon atom from the backbone of cyclic allenes
[from 1,2-cyclooctadiene (12 kcalmolÀ1) to 1,2-cyclobuta-
diene (90 kcalmolÀ1)].[2] Even the low-temperature NMR
spectroscopic characterization of all-carbon cyclic allenes is
limited to those containing more than seven carbon atoms;[3]
the only exception is the 1,2,4,6-cycloheptatetraene A2,which
observed C-C-C angles: 166,161,and 156 8,respectively;
Mes = mesityl),and the recently isolated five-membered
halfnium-containing allenoid A7 (C-C-C angle: 1568)[8] are
the most bent cyclic allenes isolated to date. Because of the
presence of the heavier elements,the allene fragments in A4–
A7 are not significantly more distorted than in the eight-
membered ring A3. More-strained cyclic allenes,such as
1,2-cyclopentadiene (A1; calculated C-C-C angle: 1148),are
only known as reaction intermediates,in line with calculations
that predict that they have a strong diradical character.[2]
We showed recently[9] that a push–push substitution
pattern[10] can induce a severe deviation from the classical
geometry of acyclic allenes. Indeed,a single-crystal X-ray
diffraction study revealed that allene A8 has a C-C-C bond
angle of 134.88,with the two NCN planes not perpendicular to
one another but twisted by 698. Moreover,theoretical studies
by Tonner and Frenking[11] on tetraaminoallenes predict that
these push–push-substituted allenes,which they described as
“carbodicarbenes” (compounds featuring
a
divalent
carbon(0) center[12–13] with two N-heterocyclic carbene
(NHC) ligands),are highly flexible.
On the basis of these results,it seemed likely that a push–
push substitution pattern could enable the synthesis of allenes
confined within relatively small ring systems. Herein we
report the synthesis and X-ray crystallographic character-
ization of the first lithium-salt adduct and salt-free derivative
of five-membered ring allenes consisting only of second-row
elements. A preliminary account of the ligand properties of
these species is also given on the basis of infrared analysis of a
rhodium(I) dicarbonyl chloride complex.
The readily available,thermally and air-stable 35, -diami-
nopyrazolium salt 2a[14] was a logical precursor to a five-
membered ring allene with four p-donor amino groups
(Scheme 1). When 2a was treated with n-butyllithium at
À788C, 1H NMR spectroscopy of the reaction mixture
revealed clean deprotonation,as shown by the disappearance
of the signal at d = 5.4 ppm due to the hydrogen atom on the
pyrazolium ring. The 13C NMR spectrum showed two distinct
resonances at d = 114 and 176 ppm,which are in the range of
those observed for the central and terminal CCC carbon
atoms of the acyclic bent allene A8 (d = 110.2 and 144.8 ppm,
respectively). An X-ray diffraction study[15] of the compound
obtained from the deprotonation of 2a revealed that it was
not the desired cyclic allene,but its lithium tetrafluoroborate
adduct 3a (orange crystals,26% yield; Figure 1). It is known
that some highly basic compounds,such as bis(diisopropyl-
amino)cyclopropenylidene[16] and phosphorus ylides,[17] are
formed as the lithium salt adduct whenever a lithium base is
used. If the salt-free compound is desired,then alternative
sodium or potassium bases must be used. A number of such
was incarcerated in a molecular container by Warmuth and
Marvel.[4] The kinetically protected 1,2-cyclooctadiene A3
(calculated C-C-C angle: 1588),[5] the trisilicon-[6] and diphos-
phorus-containing[7] hexacycles A4–A6 (crystallographically
[*] V. Lavallo, Dr. C. A. Dyker, B. Donnadieu, Prof. G. Bertrand
UCR-CNRS Joint Research Chemistry Laboratory (UMI 2957)
Department of Chemistry, University of California
Riverside, CA 92521-0403 (USA)
Fax: (+1)951-827-2725
E-mail: guy.bertrand@ucr.edu
Homepage: http://research.chem.ucr.edu/groups/bertrand/
guybertrandwebpage/
[**] We are grateful to the NSF (CHE 0518675) for financial support of
this research, the NSERC for a Postdoctoral Fellowship (C.A.D.),
andthe ACS for a Graduate Fellowship (V.L.).
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In the hope of favoring the formation of a free five-
membered-ring allene,we modified the pyrazolium scaffold
to make the central carbon atom less basic. We chose to
replace the exocyclic amino groups of 2a by weaker-p-donor
and more-electronegative aryloxy groups. Thus,the treatment
of 1 (X = I) with excess 2,6-dimethylphenol and triethylamine
in chloroform afforded the new pyrazolium salt 2b in 45%
yield. The reaction of 2b with KHMDS was then monitored
by NMR spectroscopy,which indicated clean and quantitative
deprotonation and the formation of a new species, 3b. Single-
crystal X-ray diffraction demonstrated that 3b is a salt-free
cyclic allene with an extremely small C-C-C angle of 97.58
(Figure 2),[15] almost 208 smaller than that predicted for the
Scheme 1. Synthesis of the allene–lithium adduct 3a andthe salt-free
allene 3b.
Figure 2. Molecular structure of 3b in the solidstate (hydrogen atoms
are omittedfor clarity; ellipsoids are drawn at 50% probability).
Selectedbondlengths [ꢀ] andangles [ 8]: C1–C2 1.370(4), C1–C3
1.386(3), O1–C2 1.358(3), O2–C3 1.347(3), N1–C2 1.388(3), N2–C3
1.385(3), N1–N2 1.401(3); C2-C1-C3 97.5(2), C1-C2-N1 117.5(2), N2-
C3-C1 116.9(2), C2-N1-N2 103.8(2), C3-N2-N1 104.1(2), C2-O1-C16
120.0(2), C3-O2-C22 117.1(2), C2-N1-C4 125.0(2), N2-N1-C4 118.3(2),
C3-N2-C10 128.7(2), N1-N2-C10 118.2(2).
all-carbon analogue A1! Furthermore,in contrast with the
perpendicular arrangement of the substituents in classical
allenes,the 69 8 twist angle observed for the acyclic push–
push-substituted allene A8,and even the trans arrangement of
the exocyclic allene substituents calculated for A1,the two
nitrogen and two oxygen centers of 3b are coplanar with the
Figure 1. Solid-state structure of one of the enantiomers of 3a·thf
(hydrogen atoms and a noncoordinated THF molecule are omitted for
clarity; ellipsoids are drawn at 50% probability). Selected bond lengths
[ꢀ] andangles [ 8]: C1–C2 1.403(2), C1–C3 1.398(2), C2–N1 1.411(2),
N1–N2 1.444(2), C3–N2 1.408(2), C2–N3 1.349(2), C1–Li 2.106(3),
C3–N4 1.353(2); C2-C1-C3 100.8(1), C2-C1-Li 129.5(1), C3-C1-Li
129.2(2), C1-C3-N4 128.3(2), C1-C3-N2 114.7(1), N2-C3-N4 117.0 (1),
C2-N1-N2 104.2(1), C2-N1-C10 122.0(1), N2-N1-C10 109.6(1), N1-N2-
C4 111.9(1), N1-N2-C3 104.9(1), C4-N2-C3 121.0(1), C1-C2-N3
127.3(2), C1-C2-N1 114.8(1), N1-C2-N3 117.9(1), C2-N3-C16 125.5(1),
C2-N3-C19 120.2(1), C19-N3-C16 111.6(1), C3-N4-C23 123.7(1), C3-
N4-C20 120.7(1), C20-N4-C23 111.6(1).
À
À
C2 C1 C3 fragment (maximum deviation: 0.0478 Š).
À
Finally,the C C bond distances (C1–C2: 1.370 Š; C1–C3:
1.386 Š) are significantly longer than the standard allene
bond length (1.31 Š).[18] These peculiar features of 3b are all
due to the polarization of the allenic p bonds towards the
central carbon atom C1. Clearly,the oxygen centers act as
2
p donors,as they are sp
hybridized (C-O-C: 120.0 and
117.18),they have rather short bonds to the allenic linkage
(O1–C2: 1.358 Š; O2–C3 = 1.347 Š),and they are arranged
to enable lone-pair conjugation with the allene p system (C-
O-C-C torsional angles: 2.65 and 6.298). Surprisingly,
although p donation from the nitrogen atoms is evidenced
bases (NaHMDS (HMDS = hexamethyldisilazide),NaNH
,
2
KHMDS,KH,KO tBu,KN iPr2) were screened for the
deprotonation of 2a; however,no clean deprotonation was
observed.
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Chemie
À
by the short C N bond lengths (C2–N1: 1.388 Š; C3–N2:
Experimental Section
1.385 Š), 3b exists in the solid state as a racemic mixture as a
result of the pyramidalization of the nitrogen atoms with a
trans arrangement of the Ph groups (sum of the angles at N1:
347.18,N2: 351.0 8).[19]
All experiments were carried out under dry argon by using standard
Schlenk or dry-box techniques. Solvents were dried by standard
methods and distilled under argon.
Synthesis of 3a: nBuLi (2.5m,0.9 mL,2.24 mmol) was added to a
suspension of 2a[14] (1.0 g,2.24 mmol) in THF (50 mL) at À788. The
mixture was stirred for 15 min at this temperature and then removed
from the cold bath. When the temperature of the reaction mixture
had reached room temperature,the solvent was concentrated to 50%
of the original volume,and diethyl ether (20 mL) was added. The
solution was maintained at À208C overnight,and then orange crystals
of 3a (0.308 g,26.2%) suitable for X-ray diffraction were collected.
M.p.: 130–1328C (decomp.); 1H NMR (300 MHz,C 6D6): d = 1.26–
1.30 (br m,8H),1.43–1.48 (br m,4H),3.28–3.33(br m,8H),3.60–3.65
The ligand properties of the cyclic allene 3b were assessed
by infrared analysis of the corresponding rhodium(I) dicar-
bonyl chloride complex 4 (Figure 3),which was prepared
(br m,4H),6.95–7.05 ppm (br m,10H); 13C NMR (75 MHz,C D6):
6
d = 26.0 (CH2),26.1 (CH 2),51.2 (NCH 2),68.3 (OCH 2),114.4 (C CC),
127.9 (CHAr),128.2 (CH Ar),129.3 (CH Ar),144.7 (NC Ar),176.3 ppm
(NCN).
Synthesis of 2b: Triethylamine (7.5 mL,54 mmol) was added in a
steady stream with a syringe to a stirred mixture of 1 (5.0 g,12 mmol)
and 2,6-dimethylphenol (4.1 g, 34 mmol) in chloroform (50 mL). The
resulting mixture was stirred overnight,then diethyl ether (250 mL)
was added to precipitate the product and ammonium salts. The
precipitated material was filtered,washed with diethyl ether (40 mL),
and then washed with water (150 mL) with vigorous stirring for
20 min. The solid material was then filtered,washed with water (2 ”
80 mL) then diethyl ether (4 ” 60 mL),recrystallized from methanol,
and dried under vacuum to give 2b (3.2 g,45%) as a colorless solid.
M.p.: 306–3078C; 1H NMR (300 MHz,CDCl 3): d = 2.32 (s,12H),4.75
(s,1H),7.05–7.07 (m,6H),7.45–7.48 (m,6H),8.06–8.10 ppm (m,
4H); 13C NMR (75 MHz,CDCl ): d = 17.2 (CH3),75.5 (CH),127.7
3
(CHAr),129.5 (NCO),129.8 (CH Ar),129.9 (CH Ar),130.1 ( CCH3),
Figure 3. Molecular structure of 4 in the solidstate (hydrogen atoms
are omittedfor clarity; ellipsoids are drawn at 50% probability).
Selectedbondlengths [ꢀ] andangles [ 8]: C1a–C3a 1.384(3), C3a–N2a
1.374(3), N2a–N1a 1.398(2), N1a–C2a 1.361(3), C2a–C1a 1.400(2),
C1a–Rh1a 2.096(2); C3a-C1a-C2a 100.6(2), C3a-C1a-Rh1a 132.6(1),
C2a-C1a-Rh1a 125.9(2), C1a-Rh1a-C33a 91.1(1), C33a-Rh1a-C32a
92.6(1), C32a-Rh1a-Cl1a 87.5(1), C1a-Rh1a-Cl1a 88.8(1).
130.2 (CHAr),131.9 (CH Ar),150.2 (NC Ar),157.8 ppm (OC Ar).
Synthesis of 3b: Diethyl ether (3 mL) was added to a stirred
mixture of 2b (0.639 g,1.09 mmol) and KHMDS (0.217 g,1.09 mmol)
at À788C. The cold bath was then removed,and the mixture was
stirred for a further 50 min and then filtered. The solid collected was
washed with diethyl ether (2 ” 3 mL). Volatiles were removed from
the solid under vacuum,hexanes (6 mL) were added,and the mixture
was stirred vigorously for 15 min. The mixture was then cooled to
À788C,and the hexanes were removed by filtration at this temper-
ature. This process was repeated,and the product was dried under
vacuum to give 3b (0.236 g,47%) as a pale-yellow solid. Single
crystals suitable for X-ray diffraction were grown from a solution in
hexanes at À208C. M.p.: 958C (decomp. begins); 1H NMR (300 MHz,
C6D6): d = 2.18 (s,12H),6.76–6.79 (m,8H),6.91–6.96 (m,4H),7.34–
readily by the addition of [{Rh(CO)2Cl}2] (0.5 equiv) to 3b.
Interestingly,the average
nCO values for complex
4
(2018 cmÀ1) are very similar to those observed for the
corresponding complex with the acyclic bent allene A8
(2014 cmÀ1). All bent allenes appear to be stronger donor
ligands than phosphines,five-membered NHCs (2036–
2060 cmÀ1),and even strongly basic bis(diisopropylamino)-
carbene (2020 cmÀ1).[20]
7.38 ppm (m,4H); 13C NMR (75 MHz,C D6,): d = 17.2 (CH3),115.5
6
(CCC),125.0 (CH Ar),125.9 (CH Ar),127.5 (CH Ar),129.3 (CH Ar),129.7
(CHAr),130.8 ( CCH3),137.4 (NC Ar),152.9 (OC Ar),174.8 ppm (NCO).
Synthesis of 4: Benzene (2 mL) was added to a mixture of 3b
(0.111 g,0.24 mmol) and [{Rh(CO) 2Cl}2] (0.046 g,0.12 mmol) in a
Schlenk flask. The reaction mixture was stirred for 30 min,then
concentrated to dryness. The resulting solid was washed with hexanes
(2 ” 3 mL) and dried under vacuum to afford 4 (0.150 g,97%). Yellow
crystals suitable for X-ray diffraction were grown by the slow
The results reported herein demonstrate that extremely
bent allenes can be prepared by using a push–push substitu-
tion pattern and confining the CCC skeleton to a five-
membered ring. The remarkable stability of allene 3b,which
is stable for weeks at room temperature (the crystals
decompose at 958C) suggests that a variety of cyclic push–
push-substituted allenes could be isolated. Moreover,the
facile electronic and steric tuning of these systems should give
rise to a diverse family of new ligands for transition metals. Of
special interest is the resonance
evaporation in air of a solution in acetone. M.p.: 234–2368C
À1
(decomp.); IR (CH2Cl2): n˜ = 2059,1978 cm
;
1H NMR (300 MHz,
CDCl3): d = 2.20 (br s,12H),7.03–7.06 (br m,6H),7.34–7.42 ppm
(br m,10H); 13C NMR (75 MHz,CDCl 3): d = 17.3 (CH3),96.3 (d,
1JC,Rh = 34.6 Hz,CRh),126.3 (CH Ar),126.4 (CH Ar),129.3 (br,CH Ar),
129.5 (CHAr),129.8 (CH Ar),130.3 (br, CCH3),133.9 (NCO),150.9
(NCAr),166.8 (OC Ar),183.7 (d, 1JC,Rh = 76.9 Hz,RhCO),185.3 ppm (d,
1JC,Rh = 56.6 Hz,RhCO).
structure 3b’,which shows that bent
allenes are potentially four-electron
donors. We are currently investigat-
ing this possibility.
Received: March 11,2008
Published online: June 12,2008
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