Reactive E=C(p-p)π - Systems 53 [1]: Reactivity studies on perfluoro-2-arsapropene: [2+2]-Cycloaddition reactions and quantum chemical calculations

Article abstract of DOI:10.1002/hc.20118

The extremely labile perfluoro-2-arsapropene F₃CAs=CF ₂ (1) has been generated by an improved pyrolysis process of Me ₃SnAs(CF₃)₂ and found to be stabilized by the presence of hexamethyldisiloxane and tert-butylphosphaethyne, thus allowing (i) reactivity studies with alkyne derivatives like tBu≡CP, (iPr) ₂NC≡P, MeC≡CN(iPr)₂, HC≡COEt and (ii) a full NMR investigation of 1 (¹⁹F, ¹³C). Due to the instability of 1 and some of the products, the [2 + 2]-cycloaddition reactions gave the expected arsaphospha- and arsa-cyclobutene derivatives, respectively, in moderate to good yields, but in some cases contaminated with side and/or decomposition products. Unequivocal characterization of the novel compounds was accomplished by spectroscopic investigations (¹H, ¹³C, ¹⁹F, ³¹PNMR, IR, MS) supported by comparison with the data of the more stable phosphorus analogues. An interesting isomerization was observed for the 2-dialkylamino-4,4-difluoro-1-trifluoromethyl-1-arsa-3-phospha- 2-cyclobutenes yielding the more stable 3-dialkylamino-2,4-difluoro-1- trifluoromethyl-1-arsa-2-phospha-3-cyclobutenes. Quantum chemical calculations [B3LYP/6-311+ G(d,p)] of HAs=CH₂, F₃CAs=CF₂, and F₃CP=CF₂ were carried out to compare the length of the As=C double bond with the literature data and to elucidate substituent effects on its electronic structure.

Full text of DOI:10.1002/hc.20118

Heteroatom Chemistry
Volume 16, Number 5, 2005
Reactive E C(p-p)π-Systems 53 [1]:
Reactivity Studies on Perfluoro-2-arsapropene:
[2+2]-Cycloaddition Reactions and Quantum
Chemical Calculations
Thomas Albers,1 Joseph Grobe,1 Duc Le Van,1 Andreas H. Maulitz,2
and Ernst-Ulrich Wu¨rthwein2
1Institut fu¨ r Anorganische und Analytische Chemie der Westfa¨lischen Wilhelms-Universita¨t
Mu¨ nster, D-48149 Mu¨ nster, Germany
2Organisch-Chemisches Institut der Westfa¨lischen Wilhelms-Universita¨t Mu¨ nster, D-48149
Mu¨ nster, Germany
Received 29 November 2004; revised 8 February 2005
compounds was accomplished by spectroscopic in-
ABSTRACT: The extremely labile perfluoro-2-
vestigations (1H, 13C, 19F, 31P NMR, IR, MS) supported
arsapropene F3CAs CF2 (1) has been generated by
by comparison with the data of the more stable
an improved pyrolysis process of Me3SnAs(CF3)2
phosphorus analogues. An interesting isomerization
and found to be stabilized by the presence of hexa-
was observed for the 2-dialkylamino-4,4-difluoro-
methyldisiloxane and tert-butylphosphaethyne, thus
1-trifluoromethyl-1-arsa-3-phospha-2-cyclobutenes
allowing (i) reactivity studies with alkyne derivatives
yielding the more stable 3-dialkylamino-2,4-difluoro-
like tBuC P, (iPr)2NC P, MeC CN(iPr)2, HC COEt
1-trifluoromethyl-1-arsa-2-phospha-3-cyclobutenes.
and (ii) a full NMR investigation of 1 (19F, 13C). Due
Quantum chemical calculations [B3LYP/6-311+
to the instability of 1 and some of the products, the
G(d,p)] of HAs CH2, F3CAs CF2, and F3CP CF2
[2 + 2]-cycloaddition reactions gave the expected
were carried out to compare the length of the As C
arsaphospha- and arsa-cyclobutene derivatives, re-
double bond with the literature data and to eluci-
spectively, in moderate to good yields, but in some
date substituent effects on its electronic structure.
cases contaminated with side and/or decomposition
ꢀ
C
2005 Wiley Periodicals, Inc. Heteroatom Chem
products. Unequivocal characterization of the novel
16:406–419, 2005; Published online in Wiley InterScience
(www.interscience.wiley.com). DOI 10.1002/hc.20118
Dedicated to Professor Alfred Schmidpeter on the occasion of
his 75th birthday.
The authors regret the much too early death of Dr. Duc Le Van,
the most skilled senior scientist in the team of J. G., on September
22nd 2004 aged 53 years.
INTRODUCTION
Correspondence to: Joseph Grobe; e-mail: grobe@uni-muenster.
In our extensive investigations on fluorinated
heteroalkenes, the extremely labile perfluoro-2-
de.
Contract grant sponsor: Fonds der Chemischen Industrie.
arsapropene F3CAs CF2 (1) was of particular in-
terest: (i) for comparison with the properties of
the considerably more stable phosphorus analogue:
(ii) for the study of its reactivity. Therefore, great
Contract grant sponsor: Deutsche Forschungsgemeinschaft
(Graduiertenkolleg “Hochreaktive Mehrfachbindungssysteme”).
Contract grant sponsor: Ministerium fu¨ r Wissenschaft und
Forschung Nordrhein-Westfalen.
c
ꢀ
2005 Wiley Periodicals, Inc.
406

Reactive E C(p-p)ꢀ-Systems 53 [1]: Reactivity Studies on Perfluoro-2-arsapropene 407
efforts were made to improve the preparation of
the pyrolysis zone. Under the conditions given above,
it was adjusted to 380◦C for reasonable to good yields
of the monomer, which was condensed at −196◦C
together with a suitable solvent or the reactant in a
small round bottom flask with a side-on fused NMR
tube. As a suitable solvent dichloromethane or its
perdeutero analogue was used. This procedure al-
lows the condensation of arsaalkene and solvent or
reactant layer by layer. The reactions are usually car-
ried out by quick melting and mixing of the com-
ponents at −78◦C. The reaction mixture then can be
easily transferred into the connected NMR tube.
By these measures, the yield of 1 could be in-
creased to 50%, enough for reactivity studies with
various partners. Since the [2 + 2]-self-addition of 1
is a preferred following reaction, di- and oligomer-
ization can only be avoided by use of highly reactive
partners. As a rule, several experiments at different
temperatures were necessary to obtain reasonable
yields of the products. The apparatus used for this
preparation is shown in Fig. 1.
the highly reactive As C double bond system for
which in an earlier investigation [2] the subsequent
reaction to dimers and oligomers had been ob-
served at temperatures as low as –80◦C. On the
other hand, a series of [4 + 2]-cycloaddition prod-
ucts of F3CAs CF2 could be prepared by quench-
ing the hetero-olefin immediately after its genera-
tion from the trimethyltin precursor Me3SnAs(CF3)2
[3]. A simple route to dialkylamino derivatives of
the type F3CAs C(F)NR2 was found in the elimina-
tion/addition reaction of (F3C)2AsH with secondary
amines HNR2, corresponding to the analogous be-
havior of (F3C)2PH [4,5]. In the present paper, we
report on an improved method for the generation
of F3CAs CF2, its stabilization with hexamethyldis-
iloxane, and the successful use of its solutions at low
temperatures for [2 + 2]-cycloaddition reactions.
RESULTS AND DISCUSSION
Improvement of the Generation of 1
[2+2]-Cycloaddition Reactions of
Perfluoro-2-arsapropene 1 with Phosphaalkynes
and Phosphaalkenes
The measures applied for this goal include increase
of the monomer yield and prevention of dimer and
oligomer formation. Both have been possible by ad-
justing the vapor pressure of Me3SnAs(CF3)2 in the
Compound 1 was expected to be particularly suitable
for addition reactions. This was already indicated by
the successful [4 + 2]-cycloaddition reactions of the
in situ generated 1 with 1,3-dienes [3].
According to the Woodward–Hoffmann rules
the [2 + 2]-cycloaddition of olefins is symmetry- al-
lowed only photochemically [7]. However, the pho-
tochemical activation of hetero-alkenes and -alkynes
is known to lead to decomposition in many cases.
Such reactions, therefore, have to be activated by
–3
pyrolysis apparatus close to 10 mbar by use of an
efficient vacuum pump and a Teflon valve for in-
troducing the organotin precursor. For a productive
thermal decomposition of Me3SnAs(CF3)2, the py-
rolysis tube is filled with glass wool to guarantee a
longer contact time. The thermolysis is supported
by the formation of the well-known polyadduct
(Me3SnF)n [6], deposited in the glass tube and on the
glass wool. A critical parameter is the temperature in
FIGURE 1 Pyrolysis apparatus for the improved preparation of perfluoro-2-arsapropene 1.

408 Albers et al.
substituent effects. Due to the fluorine substituents,
1 as its phosphorus analogue is an electron-poor het-
eroalkene with low-lying frontier orbitals and thus
expected to undergo cycloaddition reactions with
electron-rich alkenes and alkynes [8]. Because het-
eroatoms induce a symmetry reduction of the fron-
tier orbitals and lead to significantly different orbital
coefficients, cycloaddition reactions of such alkenes
often follow a nonconcerted pathway with a two-step
mechanism in which neither a photochemical nor a
thermal activation is necessary.
So far, only a few examples of [2 + 2]-cycloaddi-
tions of arsaalkenes are known in the literature.
Besides the cyclodimerization of 1 and the more
shielded derivatives RAs C(OSiMe3)tBu (R = Me,
Et) under UV irradiation [9], reactions of fer-
rioarsaalkenes like Cp*(CO)2FeAs C(R)NMe2 (R =
H, Ph) with fumaric acid dimethylester and fu-
marodinitrile, respectively, have been described
[10,11].
IR, and mass spectroscopic investigations.
(1)
The 19F NMR spectrum of 3 exhibits signals for
the CF3 and CF2 groups, both showing characteris-
tic patterns due to (i) the chirality of the As center
and (ii) F,F and P,F couplings. The F nuclei of the
2
CF2 group form an AB spin-system with a JFF(AB)
coupling of 261.5 Hz. The resonance at δF −99.6 is
assigned to the atom FB in cis-position to the CF3
4
group, because it shows the larger JFF constant of
4
10.2 Hz. The signal δF −86.1, JFF = 4.0 Hz is then
associated with the FA nucleus in trans-position to
the CF3 group. Both resonances exhibit a doublet of
quartet structure due to coupling with the 31P- and
the F-atoms of the CF3-substituent. The signal of the
CF3 group is detected as a multiplet at δF −49.8 con-
taining the above given 4 JFF couplings and a doublet
splitting 4 JPF = 4.0 Hz.
For the preparation of novel arsacyclobutene
derivatives by [2 + 2]-cycloaddition reactions of 1,
besides activated alkenes or alkynes, heteroana-
logues like phosphaalkenes or phosphaalkynes were
of particular interest. Due to their electronic struc-
ture, they are highly reactive and introduce the 31P
NMR probe for easy detection and characterization
of the resulting products.
1
The H NMR spectrum shows a doublet reso-
nance for the three equivalent methyl groups of the
tBu-substituent caused by a 4 JPH coupling of 1.7 Hz.
The signal of the 1H decoupled 31P nucleus exhibits a
ddq pattern associated with the JPF interactions dis-
cussed above. The 13C NMR spectrum exhibits the ex-
pected five signals at typical δC values with character-
istic coupling constants (see Experimental section).
The observed NMR data are in accord with
those of the analogous 1,3-diphospha-2-cyclobutene
derivative [12] for which, of course, due to the sec-
ond P-atom, an additional doublet splitting for all
other NMR active nuclei results.
The mass spectrum of 3, recorded on a GC/MS
instrument, shows the molecular ion peak [M+] with
a relative abundance of 5% at m/z = 294 and the
typical fragment peaks of [CF+3 ] (68%) and [tBu+]
(18%). On first sight unexpected is the basis peak
at m/z = 119 which corresponds to the fragment
[C4H9C PF+] and suggests a thermally induced re-
arrangement of 3 before fragmentation.
Reaction of 1 with Tert-butylphosphaethyne 2.
A surprising result was obtained in an attempt
to carry out this reaction. Neither during the
co-condensation of 1, obtained by the improved
pyrolysis, with 2, nor in the course of thawing and
warming the reaction mixture from −78◦C to room
temperature was a reaction observed. Even the rapid
di- and oligomerization of 1 was obviously hindered.
The NMR control detected the co-existence of the
two partners. Keeping the mixture below 0◦C, no
reaction occurred after several weeks. Only at room
temperature did the expected reaction start, but
with such a low rate that it came to its end only after
7 days. Additional investigations revealed that the
reaction rate was dependent on the content of hexa-
methyldisiloxane in the sample of 2. This is formed
as a by-product of the synthesis of 2 and cannot be
completely removed by vacuum condensation from
mmol-scale experiments. If the sample of 2 contains
only traces of hexamethyldisiloxane, the reaction
with 1 already begins at 0◦C, takes 4 days to finish,
and gives the expected [2+2]-cycloaddition product
2-tert-butyl-4,4-difluoro-1-trifluoromethyl-1-arsa-3-
phospha-2-cyclobutene 3 according to Eq. (1). 3 was
completely characterized by NMR (1H, 13C, 19F, 31P),
The gas phase IR spectrum of 3 was recorded af-
ter GC separation and is in agreement with the given
structure (data in Experimental section).
Reaction of 1 with Diisopropylaminophospha-
ethyne 4. This reaction was carried out as described
for 2. However, in this case cycloaddition already
occurs during the co-condensation of the partners

Reactive E C(p-p)ꢀ-Systems 53 [1]: Reactivity Studies on Perfluoro-2-arsapropene 409
at −196◦C, indicated by a color change of the re-
7a–d as main products as shown in Eq. (3).
action mixture to brown-orange. After thawing and
warming first at −78◦C and then at room temper-
ature, the reaction was almost complete, as ob-
served by NMR control measurements of the mix-
ture showing only small amounts of the reactants.
After keeping the mixture at room temperature for
1 h, the product formation according to Eq. (2) was
complete.
(3)
The hydrogen fluoride formed was probably
quenched to a smaller extent by addition to 1 yield-
ing the small amounts of (CF3)2AsH, detected in the
mixture, to a larger extent by reaction with 7a–d giv-
ing insoluble ammonium salts not showing up in the
NMR spectra.
(2)
Characterization of 5 and 7a–d
The cycloaddition product 2-diisopropylamino-
4,4-difluoro-1-trifluoromethyl-1-arsa-3-phospha-2-
cyclobutene 5 is regioselectively formed in a fast
reaction compared with the slow process for 2. This
can be explained by the allylic-type N C P system
producing a more electron-rich alkyne.
The novel derivatives were identified and character-
ized by spectroscopic investigations, in particular by
NMR and mass spectroscopy. The NMR results are
discussed here together, since characteristic differ-
ences are expected only in the 1H NMR spectra.
The 19F NMR spectra generally show three sig-
nals with the intensity ratio of 3 : 1 : 1, which can be
assigned to the CF3 group and the two diastereotopic
fluorine nuclei FA and FB of the AB spin-system in
the CF2 unit. The chemical shifts of the compounds
7a–d are found between δF − 73.3 and −74.2 for the
FA and between δF − 84.2 and 84.7 for the FB atoms
Reaction of 1 with Phosphaalkenes of the Type
HP C(F)NR2 (6). These reactions are based on the
former observation that compounds of type 6 usu-
ally behave as equivalents of the corresponding phos-
phaalkynes R2N C P which are formed in situ by
HF elimination, but are not accessible in free form
[12]. It was of interest whether dialkylaminophos-
phaalkynes with less bulky substituents at nitrogen
react differently with 1.
If 1 and the aminophosphaalkenes 6a–d are co-
condensed at −196◦C in analogy to the above pro-
cedures with phosphaalkynes, an immediate reac-
tion is observed. After thawing the reaction mixture
changes color to an intensive red-brown, indicating
a less selective reaction with formation of a greater
number of by-products. Two possible pathways have
to be considered:
2
with JFF(AB) constants between 239.0 and 246.3 Hz.
The stereochemical assignment of FA and FB corre-
sponds to that of the tert-butyl derivative 3. Due to
the different electronic structures of 5 and 7a–d, the
signals of the CF2 units show a low-field shift of 12
to 15 ppm. Both signals exhibit a ddq pattern with
2 JPF couplings between 82.8 and 89.6 Hz and JFF
4
constants of about 4 and 10 Hz, respectively. The 19
F
resonances of the CF3 groups appear as dd patterns
between δF − 50.3 and −50.7.
In the 1H NMR spectra of the N-alkyl-substi-
tuents, the mesomeric interaction of the nitrogen
lone pair with the ꢀ-system of the P C-bond leads
to a rotational barrier of the amino group around
the C N bond and, therefore, to a doubling of the
expected signals.
(i) HF elimination from 6a–d followed by the
[2+2]-cycloaddition,
(ii) [2 + 2]-cycloaddition of 1 to 6a–d followed by
The phosphorus nuclei in 7a–d exhibit chemical
shifts between 203.1 and 209.0 ppm with signal pat-
terns ddq due to couplings with the diastereotopic
F-atoms of the CF2 group and the equivalent
F-nuclei of the CF3 substituent. The resonances of
the P-atoms are, in comparison with the tert-butyl
HF elimination.
As in the case of 4, the reactions yield the cor-
responding 2-dialkyamino-4,4-difluoro-1-trifluoro-
methyl-1-arsa-3-phospha-2-cyclobutene derivatives

410 Albers et al.
analogue, shifted to high field by about 190 ppm,
due to the contribution of the zwitterionic formula
B to the ground state of the molecules (Scheme 1).
5 and 7a–d could not be isolated in pure form
because they undergo a subsequent reaction. With
the exception of the 13C NMR data which could not
1
be assigned unequivocally, the H, 19F, and 31P pa-
rameters allow the clear characterization of the new
arsa/phosphacyclobutenes. Mass spectra of 7a and
7b were recorded after gas chromatographic sepa-
ration. They show the molecular ions [M+] with in-
tensities of 8 and 6%, respectively, in relation to the
basis peaks [M+ CF3]. Typical fragments result from
splitting off F- and RF-substituents (see Experimen-
tal section).
SCHEME 2
more stable than purely organic bicyclobutanes,
because the heteroatoms P and As allow smaller
bond angles and thus reduce the energy difference
Isomerization of 5 and 7a–d
–1
between 5 and 9 to a value below 8 kcal mol as
When storing the arsaphosphacyclobutenes 5 and
7a–d at room temperature, a slow decrease of the
19F and 31P signals together with the appearance of
new characteristic patterns is observed. This is ob-
viously due to a thermodynamically determined re-
arrangement which will be demonstrated here for
5, because the analogues resulting from 7a–d con-
tain side- and decomposition-products making the
characterization more difficult. The NMR spectra of
the isomer 8 obtained from the intramolecular rear-
rangement of 5 exhibit new 19F and 31P resonances
supporting the rearrangement of Scheme 2.
This conforms to the analogous isomerization
of the related 2-dialkylamino-4,4-difluoro-3-trifluo-
romethyl-1,3-diphospha-1-cyclobutenes (alternative
name: 2-dialkylamino-4,4-difluoro-3-trifluoro-
methyl-1,3-diphosphetes) [12]. The isomerization
of 5 to 3-diisopropylamino-2,4-difluoro-1-trifluoro-
methyl-1-arsa-2-phospha-3-cyclobutene 8 is pre-
sumably initiated by a 1,2-fluorine migration from
the CF2 group to the neighboring P-atom. Similar
1,2- or 1,3-fluorine migrations leading to more
stable systems have been observed for acyclic phos-
phaalkenes [13]. The primarily formed intermediate
very probably is the bicyclic compound 9, which
undergoes a quick isomerization first to the 1-arsa-4-
phospha-1,3-diene 10 and then to 8 (Scheme 2). The
bicyclic derivative 9 is expected to be considerably
calculated for the above-cited diphospha analogues
[12]. The last step of the rearrangement presumably
is an electrocyclic reaction (valence isomerization)
to yield 8.
The spectroscopic results show that the isomer-
ization gives 8 as the main product (90%). In ad-
dition, two by-products of 5% each are observed
indicating the formation of the conformational
isomer of 8, 4-diisopropylamino-2,3-difluoro-1-
trifluoromethyl-1-arsa-2-phospha-3-cyclobutene 11,
very probably via the alternative heterobutadiene
derivative 10b (Scheme 3). Attempts to grow single
crystals of 8, suitable for X-ray diffraction, failed be-
cause of the impurities and its limited stability.
8 was completely characterized by NMR, MS,
and IR spectroscopic investigations. The 19F NMR
spectrum contains three signals with an intensity
ratio of 3:1:1 for the CF3 group and the CF and
PF ring units. Due to extensive F/F and P/F cou-
plings, the signals appear as ddd and ddq-patterns
at δF − 47.3 (AsCF3), −98.1 (CF), and −175.9 (PF).
The 31P-resonance at δP 115.3 appears as doublet of
1
3
pseudo-quintets, due to JPF = 990.7 Hz and JPF
=
17.1 Hz for both CF/P-couplings (CF3/P and CF/P).
The H-NMR spectrum at room temperature indi-
1
cates two equivalent isopropyl groups with a dou-
blet and a septet signal for the CH3 and CH protons,
respectively. This is in contrast to the doubling of
signals for 5 caused by hindered rotation of the R2N
group around the C N bond with double bond char-
acter. The mass spectrum of 8 supports the NMR
spectroscopic results: The molecular ion [M+] was
detected with an intensity of 17% relative to the basis
peak [M+ CF3]. Typical fragment ions are produced
by splitting off F, CF3, and propene C3H6. The IR spec-
trum also is in good agreement with the structure
SCHEME 1

Reactive E C(p-p)ꢀ-Systems 53 [1]: Reactivity Studies on Perfluoro-2-arsapropene 411
SCHEME 3
of 8, unequivocally demonstrated by the strong νC
the partners together with dichloromethane as sol-
vent. The 1H NMR spectrum of the product shows the
typical triplet and quartet signals for the ethyl group
with δH 2.45 and 2.90 ppm, respectively, together with
a singlet at 5.62 ppm for the olefinic proton, indicat-
ing structure 15a as the most probable one, because
for the isomer 15b coupling of the CH-unit with the
fluorine nuclei of the neighboring CF2 group should
lead to a dd pattern of the signal. This assignment
is supported by the cycloaddition direction observed
for the reaction of electron-poor phosphalkene com-
plexes with 14 [16]. Therefore, the analogous ring
structure can also be deduced for 13a.
Compounds 13a and 15a have been identified by
spectroscopic measurements, and the data of equiv-
alent moieties correspond to those of the heteroatom
derivatives 5 and 7a–d. For 15a the mass spectrum
could be recorded after GC separation proving the
molecular mass by detection of the molecular ion
[M+] = 264 (7%) and the structure by typical frag-
ment ions (see Experimental section).
=
C
–1
band at 1593 cm
.
[2 + 2]-Cycloaddition Reactions of 1 with
1-Diisopropylaminopropyne 12 and
Ethoxyethyne 14
The successful cycloaddition reactions of 1 with
phosphaalkynes led to the investigation of its re-
activity with electron-rich alkynes like di-isopro-
pylaminopropyne. The NMR control of the reaction
mixture, after thawing the co-condensate of reac-
tants and solvent from −196 to −78◦C, indicated a
fast and complete reaction within 15 min. The 19F
NMR spectrum is in accord with the formation of
the expected [2+2]-cycloaddition product 13. Since
this product, in contrast to 5, does not contain the
31P probe the conclusive identification of the mole-
cular structure of the main isomer was very difficult.
To determine the most probable of the two isomeric
structures 13a and 13b (Scheme 4), the analogous
reaction of 1 with ethoxyethyne 14 was studied. This
partner is expected to produce one or both of the iso-
meric structures 15a and 15b (Scheme 5), for which
Isolation and Spectroscopic
Characterization of 1
1
the pattern of the H resonance of the CH ring-unit
should allow to distinguish between the isomers.
Ethoxyethyne 14 undergoes a fast and complete
reaction with 1 after thawing the co-condensate of
In the attempted reaction of 1 with 2, a surpris-
ing stabilizing effect was observed depending on the
concentration of hexamethyldisiloxane (bp 100◦C)
SCHEME 4
SCHEME 5

412 Albers et al.
which could not be completely separated from 2 (bp
62◦C) by trap-to-trap vacuum condensation.
The As C double bond of HAs CH2 has been
calculated before, using the Hartree–Fock method
HF/3-21G(∗) [18] giving a bond length of 177.0 pm.
This method is not appropriate for atoms and sub-
stituents like arsenic and fluorine introducing a great
number of electrons. The basis set 6-311+G(d,p) was
employed in combination with the hybrid density
functional B3LYP [19]. The DFT calculations were
carried out applying the program GAUSSIAN 03 [20]
in order to establish the molecular structures, the
natural charges, and the NAO Wiberg bond orders
[21].
Therefore, investigations aimed at using the ob-
served stabilizing effect for a thorough spectroscopic
study were undertaken. Many of these experiments
led to the polymerization of 1, but in some cases
the [D2]dichloromethane solution of 1 was stable
enough to carry out NMR measurements at room
temperature. Di- and polymerization was detected
only after 30 min by formation of a cloudy suspen-
sion. The complete NMR spectroscopic investigation
was possible in a CD2Cl2 solution containing 1 and
(Me3Si)2O in equimolar amounts together with 2,
because numerous unsuccessful experiments indi-
cated a stabilizing effect also by 2. To avoid the self-
addition and/or the polymerization of 1, the NMR
measurements were carried out at −10◦C in a very
dilute solution.
Structure and Charge Distribution of HAs CH2.
The geometry and charge distribution of arsaethene
are presented in Fig. 2 and Table 1. The calculated
As C bond length amounts to 178.6 pm which cor-
responds to a real ꢁ + (p–p)ꢀ double bond with a
bond order of 1.95. It is unaffected by inductive,
mesomeric and steric substituent properties. The
charges on As ( + 0.44e) and C (–0.77e) are consis-
tent with the expected polarity of the double bond.
The 19F NMR spectrum contains three signals
4
due to the CF3 group (δF −37.6, JFF(AB) = 10.3/17.2)
and the nonequivalent nuclei FA and FB of the CF2
group (Cs symmetry). The signal of FA in trans-
position to CF3 with δF 24.8 has the expected dq-
4
pattern (2 JFF(AB) = 65.5 Hz; JFF(A) = 10.3 Hz). The
Structure and Charge Distribution of 1. 1 is of
particular interest in this study because the double
bond character will considerably differ from that
of HAs CH2 with consequences on both structure
and reactivity. The quantum chemical calculation
was performed for the staggered as well as for the
eclipsed conformation. The results are given in Fig. 3
and Table 2.
better-shielded FB in cis position to CF3 appears at
2
δF − 7.2 with dq couplings of JFF(AB) = 65.5 Hz and
4 JFF(B) = 17.2 Hz. Deviations of these data from those
obtained earlier [2] (see Experimental section) can
be attributed to the differing experimental condi-
tions, but also indicate interactions of 1 with com-
ponents of the solutions.
Because of the very low concentration of 1, long-
time measurements were necessary for a reasonable
13C NMR spectrum including the coupling pattern
with the fluorine nuclei. The resulting ddq (CF2) and
qdd (CF3) signals with δF 217.0 (1 JFC = 406.0 Hz) and
δF 130.0 ppm (1 JFC = 330.0 Hz) are in accord with
The As C double bond (182.1 pm) in the more
stable staggered conformation (a) is found to be
2.5 pm longer than that of HAs CH2, and conse-
quently the bond order is reduced to 1.66. On first
sight, this result is surprising, but can be explained
by ꢀ-donation from the substituents F(1) and F(2)
into the ꢀ* As C(1)-orbital, thus reducing the dou-
ble bond character. This interpretation gains support
1
those of F3CP CF2 (CF2: δF 204.1 (FA), JFC = 398.5
Hz, δF 198.7 (FB); 1 JFC = 407.6 Hz, and CF3: δF 129.7;
1 JFC = 309.0 Hz) studied earlier [17].
Quantum Chemical Calculations of 1,
Arsaethene and Perfluoro-2-phosphapropene
Quantum chemical calculations have been carried
out to explain the experimentally observed reactiv-
ity of 1 on the basis of its molecular and electronic
structure. For this purpose, we have studied the
perfluoro derivative 1 in comparison with the pa-
rent arsaethene and the corresponding perfluoro-2-
phosphapropene. Furthermore, the calculated geo-
metrical data are compared with X-ray structural
results for a series of known arsaalkene derivatives
to elucidate the influence of substituents on bond
lengths and angles.
FIGURE 2 Calculated molecular structure of arsaethene,
HAs CH2 [B3LYP/6-311+G(d,p)].

Reactive E C(p-p)ꢀ-Systems 53 [1]: Reactivity Studies on Perfluoro-2-arsapropene 413
TABLE 1 Calculated Data of Arsaethene HAs CH2 (Total Energy, Bond Length, Bond Order, Bond Angle, Atomic Charge) by
B3LYP/6-311+G(d,p) Total energy: E tot = −2275.76106 au
Bond Length (pm)
Bond Order
Bond Angle ( ◦)
Atomic Charge (e)
As C/178.6
1.95
0.94
0.94
0.97
H(3) As C/ 96.0
As C H(1)/124.4
As C H(2)119.3
H(1) C H(2)/116.3
As: +0.44
C: −0.77
C H(1)/108.3
C H(2)/108.5
As H(3)/153.4
H(1): +0.20
H(2): +0.20
H(3): −0.08
from the shortening of the involved C(1) F(1) and
C(1) F(2) bonds with bond orders of 0.95 vs. val-
ues of 0.88–0.89 for the C F bonds of the CF3 group.
The As C bond is stabilized by this interaction, and
the tendency to di- and oligomerization is reduced
in comparison with the hydrogen compound.
The C(1) As C(2) angle of 94.8◦ indicates a
strong deviation from sp2 hybridization and almost
pure p-orbital contributions of As in the As C ꢁ-
bonds. The lone pair of electrons on As consequently
has more than 80% s-character, in accord with the
small C(1) As C(2) bond angle. Steric interactions,
causing a widening of this angle, are missing.
In contrast to HAs CH2 with As(δ + ) C(δ–) po-
larity, the staggered conformation of 1 with charges
of +0.43e (As) and +0.47e (C1) contains an almost
nonpolar or even a slightly inversely polarized As C
bond.
(ii) a larger (3.6◦) C(1) As C(2) angle for the
eclipsed configuration due to repulsive interac-
tion between F(1) and F(3),
(iii) smaller structural differences for the other bond
lengths and angles which also can be rational-
ized on the basis of the discussed interactions.
Structure and Charge Distribution of Perfluoro-
2-phosphapropene, F3CP CF2. Since for the anal-
ogous phosphorus compound the gas phase struc-
ture has been determined by electron diffraction
[22], it was of interest to calculate also its data
for comparison both with the arsenic analogue and
with the experimental results. Figure 4 and Ta-
ble 3 present the results of the quantum chemical
calculation.
In general, the calculated structures of the re-
lated phospha- and arsaalkenes show only slight dif-
ferences in the molecular data, with the exception of
those caused by the different radii and electronega-
tivities of P and As. Thus the bond order of the P C
double bond is slightly higher, whereas the bond an-
gles and bond orders of the remaining bonds show
only small deviations. Due to the higher electronega-
tivity of phosphorus, the polarity of the P C double
bond with charges of +0.40e for P and +0.47e for
C(1) is a bit more strongly inverse than for As C.
To find out the thermodynamically preferred
configuration, the eclipsed structure (b) of 1 was
also calculated. It corresponds to the transition state
of the internal rotation. The molecular structure is
given in Fig. 3, and the bond parameters and the
natural charges are given in Table 2.
A comparison of the data in Table 2 indicates
(i) a clear preference for the staggered conforma-
–1
tion by 1.29 kcal mol
,
FIGURE 3 Calculated molecular structures of perfluoro-2-arsapropene 1: (a) staggered and (b) eclipsed conformation
[B3LYP/6-311+G(d,p)].

414 Albers et al.
TABLE 2 Calculated Data of Both the Staggered and Eclipsed Conformation of 1 by B3LYP/6-311+G(d,p): total energy:
E tot = −2811.43465 au (staggered); E tot = −2811.43259 au (eclipsed)
Length (pm)
Angle ( ◦)
Bond Order
Staggered
Atomic
Bond
Staggered
Eclipsed
Group
Staggered
Eclipsed
charge (ea)
As C(1)
182.1
131.2
132.0
203.8
134.8
134.8
135.4
182.0
131.0
132.4
204.3
134.1
135.4
135.4
1.67
0.94
0.94
0.86
0.89
0.89
0.88
C(1) As C(2)
F(1) C(1) F(2)
F(1) C(1) As
F(2) C(1) As
As C(2) F(3)
As C(2) F(4)
As C(2) F(5)
F(3) C(4) F(4)
F(4) C(4) F(5)
F(5) C(4) F(3)
94.8
109.5
129.2
121.2
114.0
114.0
106.8
107.4
107.2
107.2
98.4
109.2
131.5
119.3
117.7
108.6
108.6
107.4
106.7
107.4
As: +0.43
C(1): +0.47
F(1): –0.31
F(2): –0.32
C(2): +0.80
F(3): –0.35
F(4): –0,35
F(5): –0.36
C(1) F(1)
C(1) F(2)
As C(2)
C(2) F(3)
C(2) F(4)
C(2) F(5)
aData for the more stable staggered configuration.
The calculated barrier for internal rotation amounts
–1
culated data and study the influence of a variety of
substituents at the double bond on the bond order
and the electronic structure of the molecules. Most
of the presently known data are collected in Table 4.
The As C bond lengths vary between 180.7 and
199.0 pm and are generally widened in comparison
with that of HAs CH2 (178.7 pm), but in most cases
are considerably shorter than typical As C single
bonds with values around 196.0 pm. As already men-
tioned before, the main reason for the bond enlarge-
ment is the interaction with lone pairs of electrons
of substituents at the double bond. The bonds X As
involved, in general, experience a shortening. Atoms
of important influence at the doubly bonded C(1) are
the second row elements N > O > F. The mesomeric
effect on the structure can be enhanced by steric ef-
fects of bulky substituents, so that As C bonds even
exceed in length typical single bonds. This is demon-
strated by several examples in Table 4. A special ef-
fect results, if both donating and accepting groups
are present at the double bond system, as demon-
strated for the push/pull system with NEt2 on C(1)
and (CF3)2P on P in the phosphaalkene derivative
(CF3)2PP C(F)NEt2 [24] with a P C length of 176.0
pm as compared to 174.4 pm for CF3P C(F)NMe2
[23]. Especially, long E C bonds are observed if the
heteroatom is bound to two metal carbonyl frag-
ments and the C-atom is substituted with two NMe2
donor groups [34].
to 1.58 kcal mol
.
The agreement between the calculated and the
experimental data of F3CP CF2 is also quite good,
except for the angle C(1)–P–C(2): calc. 96.8◦; exp.
108.8◦. Because in the structures of CF3- and (CF3)2P-
substituted aminophosphaalkenes, determined by
X-ray diffraction, bond angles between 95◦ and 97◦
have been detected, the angle of 108.8◦ for the perflu-
orophosphaalkene may be in error and should pos-
sibly be re-determined. [23,24].
Comparison of Calculated As C Data with X-
Ray Structural Results for a Series of Arsaalkene
Derivatives. As C bond lengths from X-ray struc-
tural work are still quite rare in the literature. There-
fore, it is of interest to compare them with the cal-
CONCLUSION
In spite of the experimental problems arising from
the lability of perfluoro-2-arsapropene 1, numerous
interesting results could be obtained in low tempera-
ture reactions and by using the accidentally detected
stabilization effect of both hexamethyldisiloxane
FIGURE 4 Calculated molecular structure of perfluoro-2-
phosphapropene [B3LYP/6-311+G(d,p)].

Reactive E C(p-p)ꢀ-Systems 53 [1]: Reactivity Studies on Perfluoro-2-arsapropene 415
TABLE 3 Calculated and Experimental Data of the Staggered Conformation of Perfluoro-2-phosphapropene by B3LYP/6-
311+G(d,p); total energy: Etot = −916.93371 au
Length (pm)
Angle ( ◦)
Bond Order
Staggered
Atomic
Bond
Staggered
Experimental
Group
Staggered
Experimental
Charge (ea)
P C(1)
169.5
131.2
132.0
191.1
134.8
134.8
135.3
169.0
131.3
131.3
190.1
134.2
134.2
134.2
1.71
0.94
0.94
0.89
0.89
0.89
0.88
C(1) P C(2)
F(1) C(1) F(2)
F(1) C(1) P
F(2) C(1) P
P C(2) F(3)
P C(2) F(4)
P C(2) F(5)
F(3) C(2) F(4)
96.8
109.5
129.6
120.9
114.0
114.0
106.8
107.3
108.8
P: +0.40
C(1): +0.47
F(1): –0.30
F(2): –0.31
C(2): +0.81
F(3): –0.35
F(4): –0,35
F(5): –0.36
C(1) F(1)
C(1) F(2)
P C(2)
C(2) F(3)
C(2) F(4)
C(2) F(5)
125.3
122.9
114.3
114.7
106.7
106.3
aData for the staggered configuration.
and 2. Thus, [2 + 2]-cycloaddition reactions with the
phosphaalkynes (2), (4), and the phosphaalkenes
6a–d as synthetic equivalents of the corresponding
phosphaalkynes have been successfully investigated
yielding the novel arsaphosphacyclobutene deriva-
tives 3, 5, and 7a–d. The dialkylamino compounds
5 and 7a–d undergo an interesting intramolecular
rearrangement, leading to the more stable 1-arsa-
2-phospha-3-cyclobutene isomers, in the case of 5
closely studied by the characterization of the iso-
mer 8. As examples of electron-rich alkynes, 12 and
14 have been used as partners for 1 and are found
to be highly reactive giving the expected [2 + 2]-
cycloaddition products 13 and 15, respectively. Fi-
nally, quantum chemical calculations on the het-
eroalkenes HAs CH2, F3CAs CF2, and F3CP CF2
were carried out to rationalize the high reactivity of
1 and to compare the calculated As C bond lengths
with X-ray structural data. This allowed us to es-
timate the influence of various substituents at the
As C bond on its length and electronic structure.
EXPERIMENTAL
General
Since most of the compounds used as starting
materials and obtained products are volatile and
toxic chemicals, a standard vacuum-line in com-
bination with special devices and additional ap-
paratus was employed for handling as a precau-
tion. Solvents were purified, dried, and degassed.
TABLE 4 Experimental (X-ray) and Calculated (B3LYP/6-311+G∗) As C Bond Lengths of Arsaalkenes R1As CR2R3
R1As CR2R3
As C Bond Length (pm)
Experimental Calculated
R1
R2
R3
H
H
H
–
178.7
–
–
Mes∗
2,7-di-tert-butylfluorenylidene
( N(C6H12 )C(N )SiIs2
F
180.7 [25]
181.6 [26]
–
i Pr3Si
CF3
)
2
F
182.1
Cp(CO)2Fe
i Pr3Si
CF3
Me3SiO
tBu
182.1 [27]
182.7 [28]
–
–
–
N(c C6H11 )C(Nc C6H11)SiIs2
F
F
NH2
NEt2
NMe2
OH
185.8
187.7
CF3
186.7 [4]
187.6 [8]
190.0 [29]
192.1 [28]
183.0 [30]
184.9 [31]
178.9 [32]
191.4 [33]
197.5 [34]
199.0 [34]
180.0 [35]
Cp∗(CO)2Fe
MesAs(O)
NMe2
Mes
C(NMes)SiIs2
–
–
–
–
–
–
–
–
–
–
NHMes
arsatrimethynecyanine cation
Cp∗(CO)2Fe
Ph
Br
NMe2
NMe2
NMe2
SSiMe3
NMe2
SiMe3
NMe2
NMe2
NMe2
SSiMe3
Mes∗
Tpꢁ(CO)2W C
Cp∗(CO)2Fe[Cr(CO)5]
Cp∗(CO)2Fe[Fe(CO)4]
Cp∗(CO)2Fe[Cr(CO)5]

416 Albers et al.
Starting compounds: bis(trifluoromethyl)arsano-
trimethylstannane [36], 2 [37], 4 [38], and 6a–d [38]
have been prepared according to the literature. 12
and 14 are commercially available. NMR: Bruker
AC 200 (1H, 200.13 MHz, standard: TMS; 13C, 50.32
MHz, standard: TMS; 19F, 188.31 MHz, standard:
CCl3F; 31P, 81.02 MHz, standard: 85% H3PO4). In
Isolation of 1 for NMR Measurements
Following the prescription given above with hexa-
methyldisiloxane as stabilizing solvent condensed in
layers with 1, the layer system is carefully thawed,
then quickly mixed, cooled again with liquid nitro-
gen to a status still liquid, but at lower temperature
than −80◦C, and finally transferred into the NMR
tube. Quickly frozen again at −196◦C, the NMR tube
is sealed under vacuum. After thawing at room tem-
perature, 1 is only stable enough to record NMR
spectra for ca. 5–10 min. The formation of dimers
1
some cases, the H- and 13C-signals of the solvents
CH2Cl2 and CD2Cl2 were used as reference to deter-
mine the NMR data. GC/MS: Finnigan MAT ITD 800
spectrometer combined with a Dani gas chromato-
graph 8521a; column: Cp-Sil-8, 50 m, 0.32 mm, 025
ꢂm; IE = 70 eV. GC/IR: Bruker FTIR-spectrometer
IFS 48 combined with a Shimadzu GC 8A gas chro-
matograph; solvent: CH2Cl2.
shows up first in the 19F spectrum. To measure the 13
C
NMR spectrum, low concentrations of 1 in a mixture
of hexamethyldisiloxane and 2 were used, allowing
at −10◦C 30 min for recording. The 13C NMR spec-
trum did not allow us to measure more than the 1 JFC
coupling constants.
General Procedure for the Preparation of 1
4
19F NMR: δF − 37.6 (dd, JFF(A) = 10.3 Hz,
A special pyrolysis apparatus (Fig. 1) has been in-
stalled for the improved generation of 1. As a rule,
the calibrated tube with a Teflon valve is charged
with 5 mmol of Me3SnAs(CF3)2 and then connected
to the inlet of the furnace. The tube with the re-
actant and/or a suitable solvent is fixed at the end
of the apparatus. Then the whole system is thor-
oughly evacuated, the trap and the 25 mL reaction
vessel behind the furnace are cooled at −78◦C and
−196◦C, respectively, taking care of a constant level
of liquid nitrogen in the second Dewar vessel. Af-
ter heating the pyrolysis tube to 380◦C, the tap to
the vacuum-line is closed and the Teflon valve of the
Me3SnAs(CF3)2 reservoir is opened just enough to
guarantee a very low vapor density in the pyrolysis
tube. The reaction is carried out in 20% portions of
the precursor by closing the Teflon valve and con-
densing 1 completely in the reaction vessel. Next,
either the reactant and/or the solvent is transferred
from the respective tube into the vessel by vacuum
condensation. This procedure is repeated four times
until the 5 mmol amount of the precursor is used
up. In the case of a reactant the co-condensation
is finished with a layer of CD2Cl2 as suitable sol-
vent. The reaction is then carried out by thawing
the layered mixture at −78◦C and stirring the re-
sulting liquid for 15 to 60 min. The product mix-
ture or solution is then transferred under continued
cooling of the vessel into the NMR tube by turning
part of the apparatus, thus allowing the liquid to run
down. The NMR tube is sealed off after cooling to
−196◦C and used for a first control of the reaction.
The following working-up procedure depends on the
reaction partners involved and will be described fur-
ther down for the individual [2 + 2]-reactions. Typi-
cal yield of the generation of 1: ca. 0.5 g (2.5 mmol,
50%).
2
4 JFF(B) = 17.2 Hz; CF3); 24.8 (dq, JFF(AB) = 65.5 Hz,
2
4 JFF(A) = 10.3 Hz, FA); −7.2 (dq, JFF(AB) = 65.5 Hz,
1
4 JFF(B) = 17.2 Hz, FB). 13C{ H} NMR: δC 217.0
1
(ddq, JFC = 406.0/353.0 Hz, CF2); 130.0 (qdd,
1 JFC = 330.0 Hz, CF3). Comparison with 19F NMR
data obtained earlier in [D6]-dimethylether at
−110◦C on a 90 MHz Varian spectrometer [2]: δF −
4
4
41.2 (dd, JFF(A) = 8.5 Hz, JFF(B) = 17.0 Hz; CF3); 6.0
(dq, 2 JFF(AB) = 60.0 Hz, 4 JFF(A) = 8.5 Hz, FA); −11.4 (dq,
4 JFF(B) = 17.2 Hz, FB). The observed deviations are due
to differences in concentrations, temperatures, and
solvents and obviously indicate interactions of 1 with
the oxygen-containing compounds of the solutions.
[2 + 2]-Cycloaddtion Reactions of 1 with
Phosphaalkynes, Phosphaalkenes,
and Organic Alkynes
2-Tert-butyl-4,4-difluoro-1-trifluoromethyl-1-arsa-
3-phospha-2-cyclobutene 3. The general procedure
was carried out with 1 (ca. 2.5 mmol) and 2
(150 mg, 1.5 mmol) for preparing the layered
system. Surprisingly, no reaction was observed
during thawing and stirring for 15 min at −78◦C.
The reactant mixture was transferred into the NMR
tube which after sealing under vacuum was slowly
warmed to 0◦C and stored at this temperature
in the dark for several days. The end of reaction
was detected by recording 19F NMR spectra. In
a series of experiments, we found out that the
reaction time depends on the concentration of both
hexamethyldisiloxane and 2. Reaction times be-
tween four and seven days have been observed.
At −20◦C no reaction occurred allowing to store
the mixture of reactants, if necessary, for a longer
period. The product, 2-tert-butyl-4,4-difluoro-1-
trifluoromethyl-1-arsa-3-phospha-2-cyclobutene 3,

Reactive E C(p-p)ꢀ-Systems 53 [1]: Reactivity Studies on Perfluoro-2-arsapropene 417
was characterized by spectroscopic investigations.
be clearly characterized by NMR and in the case of
7a, b also by GC/MS measurements.
GC/IR: ν/cm–1(rel. intensity, assignment): 2970 (42,
νCH), 1653 (27), 1560 (20), 1506 (20, δCH), 1474 (18,
1
4
7a: H NMR: δ 2.95 (d, JPH = 3.0 Hz, (CH3)A),
4
δCH), 1458 (19, δCH), 1265 (42, νC P), 1155 (77, νCF),
3.08 (m, (CH3)B); 19F NMR: δ − 50.4 (dd, JFF
=
=
=
4
2
1142 (79, νCF), 1115 (94, νCF), 1086 (100, νCF), 814
12.3 Hz, JFF = 3.9 Hz, CF3); −73.7 (ddq, JFF(AB)
2 4
1
(51). 1H NMR: δ 1.25 (d, 4 JPH = 1.7 Hz, CH3);13 C{ H}
242.0 Hz, JPF = 87.2 Hz, JFF = 3.9 Hz, FA), −86.4
(ddq, 2 JFF(AB) = 242.0 Hz, 2 JPF = 88.5 Hz, 4 JFF = 9.5 Hz,
NMR: δ 30.2 (s, CH3), 42.3 (s, CCH3), 103.4 (2dd,
2
1
2
1 JFC = 314.8 Hz, JPC = 30.5 Hz, 130.1, CF2), 130.1
FB); 31P{ H} NMR: δ 207.5 (dd, JPF = 88.0 Hz,
C P CF2). GC/MS (70 eV), m/z (%): 281 (8) [M+],
212 (100) [M+ CF3], 162 (79) [Me2NCPAs+], 119 (9)
[Me2NC2HPF+], 87 (19) [Me2NCP+], 69 (25) [CF+3 ].
1
3
3
(qdd, JFC = 348.0 Hz, JF(B)C = 14.6 Hz, JF(A)C = 4.5
Hz, CF3), 217.0 (qm, JPF = 57.0 Hz, C P); 19F
1
4
4
NMR: δ − 49.8 (ddd, JFF = 10.4 Hz, JFF = 4.0 Hz,
4 JPF = 4.0 Hz, CF3), −86.1 (ddq, JFF(AB) = 261.9 Hz,
7b: H NMR: δ 1.22 (t, JHH = 7.1 Hz, (CH3)A),
2
1
3
2 JPF = 70.0 Hz, JFF = 4.4 Hz, F(A)), −99.6 (ddq,
1.38 (t, JHH = 7.1 Hz, (CH3)B), 3.31 (q, JHH = 7.1
4
3
3
2 JFF(AB) = 261.8 Hz, JPF = 72.4 Hz, JFF = 10.2 Hz,
Hz, (CH2)A), 3.64 (q, JHH = 7.1 Hz, (CH2)B); 19F
2
4
3
F(B)); 31P{ H} NMR: δ 392.2 (ddq, JPF = 70.0 Hz,
NMR: δ − 50.3 (dd, 4 JFF = 11.0 Hz, CF3); −73.6 (ddq,
1
2
2 JPF = 72.4 Hz, , JPF = 4.0 Hz, C P CF2); GC/MS
2 JFF(AB) = 242, 1 Hz, JPF = 87.2 Hz, JFF = 3.9 Hz,
4
2
4
(70 eV,), m/z (%): 294 (5) [M+], 175 (22) [C4H9CPAs+],
119 (100) [C4H9C PF+], 85 (29) [C4H6P+], 69 (68)
[CF+3 ], 57 (18) [C4H+9 ].
FA), −84.3 (ddq, JFF(AB) = 242.0 Hz, JPF = 88.5 Hz,
1 2
2
2
4 JFF = 9.5 Hz, FB); 31P{ H} NMR: δ 207.6 (dd, JPF
=
88.1 Hz, C P CF2). GC/MS (70 eV,), m/z (%): 309 (6)
[M+], 290 (6) [M+ F], 24+0 (100) [M+ CF3], 190 (72)
[Et2NCPAs+], 69 (25) [CF3 ].
2-Diisopropylamino-4,4-difluoro-1-arsa-3-phos-
pha-2-cyclobutene 5. The analogously performed
reaction of 1 with 4 started already during the co-
condensation of the reactants at −196◦C, indicated
by a color change to brown-orange. After thawing
of the mixture first at −78◦C, then at room temper-
ature, the reaction was complete within 1 h, giving
the expected [2 + 2]-cycloaddition product 5, which
was characterized spectroscopically. 1H-NMR: δ
1
3
7c: H NMR: δ 0.90 (t, JHH = 7.0 Hz, (CH3)A),
3
3
0.98 (t, JHH = 7.0 Hz, (CH3)B), 1.60 (qt, JHH = 7.0
3
Hz, (CH2)A), 1.77 (qt, JHH = 7.0 Hz, (CH2)B), 3.25
3
3
ꢁ
(t, JHH = 7.0 Hz, (CH2)A ), 3.58 (t, JHH = 7.0 Hz,
(CH2)B ); 19F NMR: δ −50.6 (dd, 4 JFF = 10.1 Hz, CF3);
ꢁ
−74.2 (ddq, 2 JFF(AB) = 242.3 Hz, 2 JPF = 83.6 Hz, 4 JFF
=
=
2
2
4.0 Hz, FA), −84.2 (ddq, JFF(AB) = 242.1 Hz, JPF
89.6 Hz, JFF = 10.1 Hz, FB); 31P{ H} NMR: δ 209.0
4
1
3
1.20 (d, JHH = 6.0 Hz, (CH3)A), 1.24 (d,3 JHH = 6.4
(dd, 2 JPF = 88.0 Hz, C P CF2).
1
Hz, (CH3)B), 3.41 (sept, (CH)A), 3.88 (sept, (CH)B);
7d: H NMR: δ 1.65 (m, 2H, CH2), 1.65 (m, 4H,
4
4
19F NMR: δ − 50.7 (ddd, JFF = 10.0 Hz, JFF = 4.2
2CH2), 3.40 (m, 4H, 2CH2); 19F NMR: δ − 50.6 (dd,
4
2
2
Hz, JPF = 3.3 Hz, CF3), −73.4 (ddq, JFF(AB) = 244.3
4 JFF(AB) = 3.8 Hz, CF3); −73.3 (ddq, JFF(AB) = 242.0
Hz, 2 JPF = 82.8 Hz, 4 JFF = 4.2 Hz, F(A)), −84.7 (ddq,
Hz, JPF = 86.6 Hz, JFF = 4.5 Hz, FA), −84.8 (ddq,
2 JFF(AB) = 241.9 Hz, 2 JPF = 89.1 Hz, 4 JFF = 9.5 Hz, FB);
2
4
2 JFF(AB) = 244.2 Hz, JPF = 87.0 Hz, JFF = 10.0 Hz,
2
4
FB); 31P { H}-NMR: δ 203.1 (ddq, JPF = 83.7 Hz,
31P{ H} NMR: δ 206.2 (dd, 2 JPF = 89.4 Hz, C P CF2).
1
2
1
4 JPF = 3.0 Hz, C P CF2).
3-Diisopropylamino-2,4-difluoro-1-trifluoro-
methyl-1-arsa-2-phospha-3-cyclobutene 8. At room
temperature 5 was found to undergo a thermody-
namically determined isomerization with formation
of 8 within 3 days. The resulting product mixture
was transferred into a Schlenk vessel, and the
volatile components pumped off at 10–3 mbar. 8 was
characterized by spectroscopic investigations.
2-Dialkylamino-4,4-difluoro-1-trifluoromethyl-
1-arsa-3-phospha-2-cyclobutenes (7a–d). Dialkyl-
amino derivatives of type 5 can be prepared also by
reactions of 1 with the phosphaalkenes HP C(F)NR2
(R = Me, Et, Pr; NR2 = Pip) (6a–d), which function
as equivalents of the corresponding phosphaalkynes
R2NC P. The reactions were carried out with 1.5
mmol amounts as described for 4 and proceed
similarly. After a reaction period of 15 min at
−78◦C, the resulting mixtures were kept at room
temperature for 1 h and then transferred into NMR
tubes for characterization. The spectra showed the
signals of the expected arsaphosphacyclobutenes
7a–d together with some by-products, mainly due
to side-reactions with the eliminated hydrogen
fluoride. These impurities could not be removed
from the main components which, however, could
8: GC/IR: ν/cm–1(rel. intensity, assignment):
2980 (27, νCH), 2947 (14, νCH), 2830 (9, νCH), 1593 (55,
νC C), 1560 (9, ?), 1410 (9, δCH), 1375 (18, δCH), 1259
=
(20, δCH), 1198 (13, νCF), 1123 (100, νCF), 1032 (12,
1
3
νCF). H NMR: δ 1.20 (d, JHH = 6.5 Hz, CMe3), 3.8
(sept, JHH = 6.5 Hz, CH); 19F NMR: δ − 47.3 (ddd,
3
3 JPF = 17.4 Hz, JFF = 1.8 Hz, JFF = 1.8 Hz, CF3);
4
4
−98.1 (ddq, 3 JPF = 16.1 Hz, 4 JFF = 1.8 Hz, CF), −175.9
(ddq, JPF = 990.4 Hz, JFF = 1.9 Hz, PF): 31P{ H}
1
4
1
1
3
NMR: δ 115.3 (ddq, JPF = 990.7 Hz, JPF = 17.1 Hz,

418 Albers et al.
C P CF2). GC/MS (70 eV,), m/z (%): 337 (17) [M+],
318 (5) [M+ F], 268 (100) [M+ CF3], 226 (41)
[M+ CF3 C3H6], 184 (33) [226–C3H6], 69 (25) [CF+3 ].
[11] Weber, L.; Kleinebekel, S.; Pumpenmeier, L.;
Stammler, H.-G.; Neumann, B. Organometallics
2002, 21, 1998–2005.
[12] Grobe, J.; Le Van, D.; Broschk, B.; Hegemann, M.;
Lu¨th, B.; Becker, G.; Bo¨hringer, M.; Wu¨rthwein, E.-U.
J Organomet Chem 1997, 529, 177–187.
[13] Grobe, J.; Le Van, D.; Nientiedt, J. New J Chem 1989,
13, 363–367.
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1992, 125, 567–570.
[15] Grobe, J.; Le Van, D. Chem Ber 1990, 123, 1047–1049.
[16] Marinetti, A.; Mathey, F. J Chem Soc, Chem Commun
1990, 153–154.
[17] Grobe, J.; Le Van, D. unpublished.
[18] Dobbs, K. D.; Boggs, J. E.; Cowley, A. H. Chem Phys
Lett 1987, 141, 372–375.
Reactions of 1 with 12 and 14. The electron-rich
alkyne derivatives 12 and 14 (1.5 mmol in 40% hex-
ane solution) were also reacted with 1 according to
the layer-by-layer process. Immediately after thaw-
ing the solution, a complete reaction was observed
by NMR control. To isolate the products the volatile
–1
components were pumped off at 10 mbar, leaving
behind the arsacyclobutenes 13 and 15, respectively.
They were found to decompose already at room tem-
perature and, therefore, were stored at −196◦C.
[19] Becke, A. D. J Chem Phys 1993, 98, 1372–1137, 5648–
5565.
[20] Gaussian 03, Revision C.01; Frisch, M. J.; Trucks,
G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.;
Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar,
S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.;
Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji,
H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.;
Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.;
Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo,
J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin,
A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.;
Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador,
P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman,
J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.;
Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.;
Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.;
Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara,
A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A.; Gaussian,
Inc: Wallingford CT, 2004 and earlier versions of the
program.
[21] Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem Rev
1988, 88, 899–926.
[22] Steger, B.; Oberhammer, H.; Grobe, J.; Le Van, D.
Inorg Chem 1986, 25, 3177–3181.
[23] Grobe, J.; Le Van, D.; Nientiedt, J.; Krebs, B.;
Dartmann, M. Chem Ber 1988, 121, 655–664.
[24] Grobe, J.; Le Van, D.; Winnemo¨ller, J.; Krebs, B.; La¨ge,
M. Z Naturforsch, B: Chem Sci 1996, 51, 778–784.
[25] Decken, A.; Carmalt, C. J.; Clyburne, J. A. C.; Cowley,
A. H. Inorg Chem 1997, 36, 3741–374.
3-Diisopropylamino-4,4-difluoro-1-trifluorometh-
yl-2-methyl-1-arsa-2-cyclobutene. 13a 1H NMR: δ
3
1.71 (d, JHH = 6.5 Hz, CMe2), 2.48 (s, CH3), 3.61
3
(sept, JHH = 6.5 Hz, CH); 19F NMR: δ − 49.0 (dd,
4
4 JFF = 11.1 Hz, JFF = 3.6 Hz, CF3); −83.2 (d,
4
2 FFF(AB) = 214.4 Hz, JFF = 3.6 Hz, FA), −91.0 (dq,
2 JFF(AB) = 214.3 Hz, 4 JFF = 10.9 Hz, FB).
3-Ethoxy-4,4-difluoro-1-trifluoromethyl-1-arsa-2-
1
3
cyclobutene 15a H NMR: δ 2.45 (t, JHH = 7.1 Hz,
3
CH3), 2.90 (q, JHH = 7.1 Hz, CH2), 5.62 (s, CH);
19F NMR: δ −49.4 (d, JFF = 8.6 Hz, CF3) −81.0
4
2
4
(d, JFF(AB) = 210.7 Hz, JFF = 4.6 Hz, FA), −89.4 (dq,
2 JFF(AB) = 210.5 Hz, 4 JFF = 9.8 Hz, FB); GC/MS (70 eV),
m/z (%): 264 (7) [M+], +245 (13) [M+ F], 236 (7) [M+
C2H4], 144 (1) [AsCF3 ], 119 (6) [F2CC(OEt)C+], 91
(100) [F2C(OH)C+], 69 (6) [CF3+].
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