Thermal valence isomerization of 2,3-diborata-1,4-diphosphoniabuta-1,3- dienes to bicyclo[1.1.0]butanes and cyclobutane-1,3-diyls

Article abstract of DOI:10.1002/anie.200701578

(Chemical Equation Presented) Mind your Ps and Bs: Valence isomerizations of PBBP 1,3-butadienes are investigated and compared to those of PCCP and all-carbon systems. The 2,3-diborata-1,4-diphosphoniabuta-1,3-dienes thermally isomerize to bicyclo[1.1.0]butanes or cyclobutane-1,3-diyls. Under photolytic activation, the bicyclo[1.1.0]butanes rearrange into cyclobutenes.

Full text of DOI:10.1002/anie.200701578

Angewandte
Chemie
DOI: 10.1002/anie.200701578
Valence Isomerization
Thermal Valence Isomerization of 2,3-Diborata-1,4-diphosphoniabuta-
1,3-dienes to Bicyclo[1.1.0]butanes and Cyclobutane-1,3-diyls**
Jean-Baptiste Bourg, Amor Rodriguez, David Scheschkewitz, Heinz Gornitzka,
Didier Bourissou,* and Guy Bertrand*
In his Nobel lecture entitled “Bridges between Inorganic and
Organic Chemistry”, Hoffmann concluded: “The isolobal
analogy is a model. It is the duty of our scientific craft to push
it to its extremes, and being only a model it is certain to fail
somewhere”.[1] Comparison of the relative stabilities and the
interconversions of the different valence isomers of butadiene
(C4H6) with those of its isolobal analogues provides a good
demonstration of Hoffmannꢀs conclusions. In carbon chemis-
try, s-trans-buta-1,3-diene (1C) is the most stable valence
isomer, cyclobutene (2C) and bicyclo[1.1.0]butane (3C) being
14.8 and 31.1 kcalmolꢀ1 higher in energy, respectively
(Scheme 1, left).[2] The singlet cyclobutane-1,3-diyl (4C) was
interconversion may be either similar or very different.[6] For
example, it has been shown that 1,4-diphosphabutadienes 1PC
[7]
can undergo a ring closure to form 2PC
,
but only thermally
(Scheme 1, right). Moreover, diradicals 4PC are not only a
minimum on the potential energy surface, but can even be
isolated, provided they have the right set of substituents.[8]
Photolysis of 4PC affords the bicyclic derivatives 3PC, from
[8c]
which subsequent thermolysis gives butadienes 1PC
.
Most of the possible interconversions between isomers 1–
4 have been observed for systems with various hetero-
atoms.[5–8] However, there is only one example of a photo-
chemical isomerization of a butadiene (1Ca) into a bicyclo-
[1.1.0]butane (3Ca; Scheme 2, top)[9,10] and no definitive
evidence for this process under thermal conditions.[11] This
Scheme 1. Known valence isomerizations of singlet C4H6 (left) and of
P2C2R4 derivatives (right).
predicted only as a transition state for the inversion of 3C.[3,4]
The interconversion of these different valence isomers 1C–4C
has been studied extensively, both experimentally and
theoretically.[5] It has been found that the thermolyses of
Scheme 2. Photochemical isomerizations of butadiene 1Ca (top), and
two possible mechanisms for the formation of bicyclo[1.1.0]butanes
3BP and/or butane-1,3-diyls 4BP (bottom).
[5a–c]
[5d,e]
both 2C
and 3C
lead to butadiene 1C, the photolysis of
which gives back 2C.[5f–h] When some or all of the carbon
atoms of the backbone are replaced by heavier elements, the
order of stability of the valence isomers and their modes of
situation prompted us to study in detail the mechanism
of formation of 1,3-diborata-2,4-diphosphoniabicyclo-
[1.1.0]butanes 3BP and 1,3-diborata-2,4-diphosphoniacyclobu-
tane-1,3-diyls 4BP.[12,13] We postulated the involvement of
transient 2,3-diborata-1,4-diphosphoniabuta-1,3-dienes 1BP
(Scheme 2, bottom). However, a second pathway via a
cationic three-membered heterocycle 6BP could also explain
the formation of 3BP and 4BP.
Herein, we report the synthesis and single-crystal X-ray
diffraction study of a derivative of type 1BP, and its thermal
valence isomerization into the corresponding bicyclic deriv-
ative 3BP. Strong evidence that butadienes 1BP can also
isomerize thermally into diradicals 4BP is presented. More-
over, it is shown that, under photolytic activation,
heterobicyclo[1.1.0]butanes 3BP undergo a ring-opening reac-
tion accompanied by a 1,2-shift, a process similar to that
observed for the all-carbon analogue 3Ca.[9]
[*] Dr. D. Scheschkewitz, Dr. H. Gornitzka, Dr. D. Bourissou
Laboratoire HØtØrochimie Fondamentale et AppliquØe du CNRS
(UMR 5069)
UniversitØ Paul Sabatier
118, route de Narbonne, 31062 Toulouse Cedex 09 (France)
Fax: (+33)561-558-207
E-mail: dbouriss@chimie.ups-tlse.fr
Dr. J.-B. Bourg, Dr. A. Rodriguez, Dr. D. Scheschkewitz,
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: gbertran@mail.ucr.edu
[**] We are grateful to the NSF (CHE 0518675) and Rhodia for financial
support of this work.
To probe the putative formation of a three-membered
BBP heterocycle of type 6BP, we first treated 1,2-di-tert-butyl-
1,2-dichlorodiborane with one equivalent of lithium diphe-
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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nylphosphide at ꢀ608C (Scheme 3). However, only the
nylphosphide was added to 1,2-diduryl-1,2-dichlorodiborane
at ꢀ788C, and 1-chloro-2-phosphinodiborane 7BPc was iso-
lated as yellow crystals in 62% yield (Scheme 4). An X-ray
diffraction analysis[15] showed that the phosphorus center has
bicyclic compound 3BPa was formed,[12b] and an equimolar
amount of the starting diborane remained. In the hope of
preventing the addition of the second phosphide group, the
Scheme 4. Synthesis of diborane 7BPc and butadiene 1BPc, and valence
isomerization of 1BPc into bicyclo[1.1.0]butane 3BPc.
Scheme 3. Synthesis of bicyclo[1.1.0]butane 3BPa, and syntheses of
butane-1,3-diyl 4BPa and bicyclo[1.1.0]butane 3BPb via transient buta-
a slightly pyramidalized environment (sum of the angles =
343.18) and that the B1–P1 distance (1.845(2) Š) is halfway
between those of a single and a double bond (Figure 1, left).[16]
This geometry indicates that the lone pair of the phosphorus
atom interacts to some extent with the vacant orbital of the
adjacent boron atom. The phosphorus lone pair is therefore
not available to attack the second boron center to induce ring
closure into a three-membered heterocycle of type 6BP.
Monitoring the reaction of one equivalent of lithium
diphenylphosphide with 7BPc at ꢀ608C, or of two equivalents
with the dichlorodiborane at ꢀ608C, we observed the
formation of a new species. Importantly, only one signal was
observed both in the 31P NMR (d = 12 ppm) and the 11B NMR
(d = 76 ppm) spectra, suggesting a symmetrical structure.
These signals disappeared in a few minutes at ꢀ108C and
were replaced by 31P NMR (d = ꢀ51 ppm) and 11B NMR (d =
ꢀ13 ppm) signals in the range expected for the bicyclic
derivative 3BPc. After work up, 3BPc was obtained as yellow
crystals in 66% yield and was characterized by X-ray
diffraction (Figure 1, right).[15] These results prompted us to
further investigate the structure of the intermediate detected
by NMR spectroscopy, and we were fortunate to obtain single
crystals by recrystallization at ꢀ408C. The X-ray diffraction
dienes of type 1BP
.
bulkier diisopropylphosphide was used. A clean reaction
occurred, and derivative 7BPa was isolated as a yellow oil in
93% yield. The nonsymmetrical and acyclic structure of 7BPa
was apparent from the presence of two signals in the
11B NMR spectrum (d = 93, 63 ppm) and from the rather
low-field position of the signal in the 31P NMR spectrum (d =
50 ppm).[14] Addition of one equivalent of lithium diisopro-
pylphosphide to 7BPa gives rise to the known diradical 4BP
a
(68% yield).[12a] Monitoring the reaction by 31P NMR spec-
troscopy did not allow the observation of any intermediate.
Hypothesizing that the presence of a less basic phosphine
fragment might slow down the valence isomerization of the
putative butadiene intermediate of type 1BP, we added lithium
diphenylphosphide to 7BPa. This experiment also did not allow
the detection any intermediate and led to the unsymmetri-
cally substituted bicyclic compound 3BPb (69% yield).
In the hope of crystallizing a phosphinodiborane of type
7BP, we replaced the tert-butyl groups by duryl groups (Dur=
2,3,5,6-tetramethylphenyl). One equivalent of lithium diphe-
Figure 1. Molecular structure of 7BPc in the solid state (left; hydrogen atoms are omitted for clarity). Selected bond lengths [Š] and angles [8]: P1–
B1 1.845(2), B1–B2 1.687(3); B1-P1-C4 115.78(9), B1-P1-C3 119.83(10), C4-P1-C3 107.48(9), P1-B1-B2 119.85(15), P1-B1-C1 121.21(14), B2-B1-C1
118.65(16), B1-B2-Cl 119.86(15), B1-B2-C2 121.57(17), Cl-B2-C2 118.50(15). Molecular structure of 1BPc in the solid state (center; hydrogen atoms
are omitted for clarity). Selected bond lengths [Š] and angles [8]: P1–B1 1.8346(16), B1–B1’ 1.712 (3); B1-P1-C4 121.73(6), B1-P1-C3 121.69(7),
C4-P1-C3 107.15(7), P1-B1-B1’ 117.03(12), P1-B1-C1 116.08(9), B1’-B1-C1 126.44(15). Molecular structure of 3BPc in the solid state (right;
hydrogen atoms and noncoordinated atoms of the phenyl groups on the phosphorus atoms are omitted for clarity). Selected bond lengths [Š]:
B1–B2 1.971(2), B1–P1 1.8874(15), B1–P2 1.8858(15), B2–P1 1.8819(16), B2–P2 1.8870(16).
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Chemie
analysis[15] showed that the intermediate corresponds to the
planar butadiene 1BPc with an s-trans conformation (Figure 1,
center).[17,18] The B1–P1 bond length (1.8346(16) Š) and the
environment around the phosphorus centers (sum of the
angles: 350.18) are similar to those observed for 7BPc.
The thermal rearrangement of 1BPc!3BPc (Scheme 4), is
opposite to that observed for the parent hydrocarbon (3C!
1C) and the PCPC system (3PC!1PC; Scheme 1). However, the
rearrangement is analogous to that reported for sterically
hindered carbon skeletons (1Ca!3Ca; Scheme 2, top), in
which case it was photochemically induced. To extend these
comparisons, we investigated the photochemical behavior of
analogue of 5Ca. A similar rearrangement was observed upon
irradiation of 3BPc, but the resulting four-membered ring 5BP
c
could only be characterized at low temperature. It quickly
rearranges into the acyclic product 8BPc, which features an
allylic 2-p-electron, rather than a butadiene 4-p-electron
structure. Indeed, as shown by X-ray diffraction,[15] the
internal phosphorus atom P1 has a planar environment
(sum of the angles = 3598), while the environment of the
terminal phosphorus atom P2 is strongly pyramidalized (sum
of the angles = 3178; Figure 3). The instability of 5BPc relative
to 5BPb probably results from the weaker basicity of the
diphenylphosphine fragment relative to the diisopropylphos-
phine fragment.
3BPb and 3BPc.
Upon irradiation at l = 254 nm, 3BPb was quantitatively
converted into 5BPb after 2 h at room temperature in toluene
solution (Scheme 5). The cleavage of the B–B bond and the
Figure 3. Molecular structure of 8BPc in the solid state (hydrogen
atoms are omitted for clarity). Selected bond lengths [Š] and angles [8]:
P1–B1 1.863(7), P1–B2 1.866(7), P2–B2 1.911(8); B1-P1-B2 124.5(3),
B1-P1-C3 118.6(3), B2-P1-C3 116.8(3), B2-P2-C5 106.7(3), B2-P2-C6
105.8(3), C5-P2-C6 104.5(3), P1-B1-C4 124.2(5), P1-B1-C1 115.7(5),
C4-B1-C1 120.0(6), P1-B2-P2 116.1(4), P1-B2-C2 120.3(5), P2-B2-C2
122.2(5).
Scheme 5. Photochemical isomerizations of bicyclo[1.1.0]butanes 3BP
and 3BPc.
b
1,2-shift of a phenyl group from phosphorus to boron were
indicated by the spectroscopic data and confirmed by an
X-ray diffraction analysis (Figure 2).[15] Notably, 5BPb is
obtained as a single regioisomer; the migration selectively
involves a phenyl ring rather than an isopropyl group. The
four-membered ring of 5BPb is perfectly planar. The short P2–
B1 bond length (1.795(8) Š) and the planar environment at
P2 (sum of the angles = 3608) are indicative of a double bond.
These geometric parameters suggest that 5BPb is a hetero
These results as a whole highlight the analogies and
dichotomies in the interconversion between the valence
isomers of various butadienes and heterobutadienes. Surpris-
ingly, the closest analogue of the inorganic PBPB system is an
all-carbon butadiene bearing bulky substituents. These are
the only known systems in which an isolated butadiene
valence isomerizes into a bicyclo[1.1.0]butane, although in the
all-carbon case, the rearrangement is photochemically
induced. Both bicyclo[1.1.0]butane systems rearrange under
photolytic conditions into a cyclobutene isomer. Note also
that, unlike for most known stable diradicals,[19,20] but as for
cyclobutane-1,3-diyls, the ring closure of BPBP diradicals 4BP
into their bicyclic isomers 3BP is thermally allowed. Accord-
ingly, the substituents at the phosphorus and boron atoms
strongly influence the ground-state structure of these com-
pounds. Thus, the diradical 4BPa or the bicyclic compound
3
BPb are formed depending on the phosphide used
(Scheme 3).[12b] This result suggests that the BPBP diradical
BPa results from the thermal rearrangement of the corre-
4
sponding transient butadiene 1BP, which is a type of valence
isomerization that has no precedent.
Figure 2. Molecular structure of 5BPb in the solid state (hydrogen
atoms omitted for clarity). Selected bond lengths [Š] and angles [8]:
P1–B1 1.795(8), P1–B2 2.014(7), P2–B1 1.949(8), P2–B2 2.053(7);
B1-P1-B2 99.8(3), B1-P1-C1 128.4(3), B2-P1-C1 131.8(3), P1-B1-P2 87.2
(4), P1-B1-C2 136.1(5), P2-B1-C2 136.7(5), B1-P2-B2 93.6(3), P1-B2-P2
87.2(4).
Experimental Section
7BPc: A solution of Ph2PLi (0.53 g, 2.8 mmol) in THF (10 mL) was
added to
a solution of 1,2-diduryl-1,2-dichlorodiborane (1.0 g,
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Communications
2.8 mmol) in toluene (10 mL) at ꢀ788C. The volatiles were removed
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under vacuum at 08C, the residue was dissolved in hexane, and the
salts were removed by filtration. Yellow crystals of 7BPc were obtained
from a concentrated hexane solution at ꢀ308C (0.88 g, 62% yield).
M.p. 127–1288C (dec.); 31P{1H} NMR (121.5 MHz, CDCl3): d =
17.2 ppm; 11B{1H} NMR (96.3 MHz, CDCl3): d = 81, 56 ppm.
1BPc: A solution of Ph2PLi (0.11 g, 0.56 mmol) in [D8]THF
(0.8 mL) was added to 1,2-diduryl-1,2-dichlorodiborane (0.10 g,
0.28 mmol) at ꢀ788C. The reaction mixture was stirred for 10 min
at ꢀ788C. Multinuclear NMR spectroscopy indicated the quantitative
formation of 1BPc. Red crystals suitable for a single-crystal X-ray
diffraction study were obtained from a concentrated THF solution at
ꢀ408C. 31P{1H} NMR (121.5 MHz, [D8]THF): d = 12 ppm;
11B{1H} NMR (96.3 MHz, [D8]THF): d = 76 ppm.
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3BPc: When a [D8]THF solution of 1BPc, prepared as described
above, was warmed to ꢀ108C, the color changed from red to yellow.
The volatiles were removed under reduced pressure, and toluene
(30 mL) was added. After filtration and reduction of the volume,
cooling at ꢀ308C gave 3BPc as yellow crystals (0.12 g, 66% yield).
31P{1H} NMR (121.5 MHz, [D8]THF): d = ꢀ51 ppm; 11B{1H} NMR
(96.3 MHz, [D8]THF): d = ꢀ13 ppm.
Photolysis of 3BPb: A solution of 3BPb (0.10 g, 0.22 mmol) in
[D8]toluene (0.8 mL) was irradiated at l = 254 nm for 2 h at room
temperature. 31P NMR spectroscopy indicated the quantitative con-
version of 3BPb into 5BPb. Colorless crystals of 5BPb were obtained
from [D8]toluene solution at ꢀ308C. M.p. 124–1278C; 31P{1H} NMR
(121.5 MHz, C6D6): d = 65 (d, JPP = 301 Hz), 5 ppm (d, JPP = 301 Hz);
11B{1H} NMR (96.3 MHz, C6D6): d = 56, 7 ppm.
Photolysis of 3BPc: When the irradiation at l = 254 nm of a
solution of 3BPc (0.10 g, 0.20 mmol) in [D8]toluene at ꢀ608C was
monitored by 31P NMR spectroscopy (121.5 MHz), signals corre-
sponding to 5BPc were observed at d = 80 (d, JPP = 324 Hz) and
ꢀ12 ppm (d, JPP = 324 Hz). Upon warming to ꢀ508C, 5BPc quantita-
tively isomerized into 8BPc, which was obtained as colorless crystals.
31P{1H} NMR (121.5 MHz, C7D8): d = 31 (d, JPP = 48 Hz), ꢀ3 ppm (d,
2JPP = 48 Hz); 11B{1H} NMR (96.3 MHz, C7D8): d = 80 ppm.
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Received: April 10, 2007
Published online: June 25, 2007
Keywords: boron · butadienes · heterocycles · phosphorus ·
.
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