Preparation of tetraalkylformamidinium salts and related species as precursors to stable carbenes

Article abstract of DOI:10.1039/b104110j

The preparation of a range of tetraalkylformamidinium salts for use as precursors for the formation of stable diamino-and amino-carbenes was discussed. The preparation methods included nucleophilic addition to formamides followed by trapping with electrophiles such as triflic anhydride and various methods of formamide activation. Exchange reactions involving orthoesters, the transamination of amidinium salts and alkylation methods were other methods suitable for the synthesis.

Full text of DOI:10.1039/b104110j

View Article Online / Journal Homepage / Table of Contents for this issue
Preparation of tetraalkylformamidinium salts and related species
as precursors to stable carbenes
Roger W. Alder,*a Michael E. Blake,a Simone Bufali,a Craig P. Butts,*b A. Guy Orpen,a
Jan Schütza and Stuart J. Williamsa
1
a School of Chemistry, University of Bristol, Cantock’s Close, Bristol, UK BS8 1TS
b School of Chemistry, University of Exeter, Stocker Rd, Exeter, UK EX4 4QD
Received (in Cambridge, UK) 4th May 2001, Accepted 7th June 2001
First published as an Advance Article on the web 27th June 2001
Methods are described for the preparation of a range of N,N,N Ј,N Ј-tetraalkylformamidinium and N,N-dialkyl-
iminium ions 4–20 as anhydrous salts for use as precursors to diamino- and amino-carbenes. These methods include
a novel method involving nucleophilic addition to formamides followed by trapping with electrophiles such as triflic
anhydride, various methods of formamide activation, exchange reactions involving orthoesters and the
transamination of amidinium salts, and alkylation methods.
Introduction
We required a wide range of amidinium and iminium ions as
precursors to stable carbenes.1–7 We have been particularly
interested in the preparation of carbenes where the carbene
centre is not part of a ring such as 1–3 (we will use the term
non-cyclic for these cases, even if there are rings elsewhere in the
structure). Preparation of these carbenes is best achieved by
deprotonation of per-alkylated and often sterically hindered
formamidinium and iminium ions using non-nucleophilic
amide bases,8 and this therefore requires access to tetra-
alkylformamidinium and N,N-dialkyliminium ions as pure,
anhydrous salts which are at least sparingly soluble in ether or
hydrocarbon solvents.
While the literature on preparative methods for amidinium
and iminium salts is extensive,9 we found many published
methods to be ineffective, especially for the preparation of
highly hindered tetraalkylated examples. We also found that
literature methods of isolation were inadequate in many cases,
as many salts of this type have been previously prepared only in
situ and used without isolation. In this paper, we report reliable
methods for making amidinium and iminium salts 4–20.
by-products. The crude product in these reactions is typically
dark and intractable, and the isolation of the amidinium
salt is complicated by the mixture of counterions present
Results and discussion
The methods discussed here can be divided into four broad
Ϫ
(e.g. PO2Cl2 , as well as ClϪ), so that the salts form low melting
waxy solids. Thus, we reported4 the preparation of tetraiso-
categories: 1) formamide activation; 2) nucleophilic additions
to formamides; 3) exchange reactions of orthoesters or amidin-
ium salts; and 4) alkylation of amidines.
propylformamidinium salt 4 (Scheme 1), using phosphorus
1
Formamide activation
The use of Vilsmeier–Haack chemistry to prepare N,N Ј-di-
alkyl- or N,N Ј,N Ј-trialkyl-amidinium salts is well estab-
lished.9,10 However the preparation of N,N,N Ј,N Ј-tetraalkyl-
formamidinium salts by these methods is severely complicated
by difficulties in isolating the desired salt from ammonium salt
Scheme 1
1586
J. Chem. Soc., Perkin Trans. 1, 2001, 1586–1593
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b104110j

View Article Online
Table 1 Yields of formamide activation reactions
(Scheme 2), suggesting that the dication is also the active
electrophile in solution.‡ Other hindered salts, such as 8 and 9,
were prepared in a similar manner (Table 1).
Preparative method and yields (%)
Salt
Product mixtures from Tf2O reactions are usually colourless
crystalline solids as opposed to the dark intractable tar typically
formed in the corresponding POCl3 reactions. Careful recrystal-
lisation of the amidinium triflate away from the ammonium
salt side-products is now sometimes possible, for example,
in the case of salt 4, but this generally gives low isolated
yields, and precipitation of the hexafluorophosphate salt from
aqueous solutions, as described above, remains the most reliable
method of purification if the salt is sufficiently stable.
POCl3
Tf2O
Nucleophilic addition
4
66,a 75b
36a
62
38
—
—
—
85
16
—
—
—
52
—
—
—
37
5
6
7
77b
—
8
9
59a
—
10
a Aqueous NH4PF6 work-up. b Worked-up by aqueous NaBF4 wash.
2
Nucleophilic addition to formamides
As the removal of the ammonium salt by-products is the
fundamental obstacle to isolation of tetraalkylformamidinium
salts, methods of preparing the amidinium or iminium salts
without an organic salt impurity were investigated. A solution
of diisopropylformamide in diethyl ether§ was treated firstly
with phenyllithium and then with triflic anhydride at Ϫ78 ЊC
to trap the resultant lithium alkoxide 24 (Scheme 3). As
oxychloride, but in less than 25% yield after a difficult
work-up.† A number of other tetraalkylformamidinium salts,
5–9, have been successfully generated via the POCl3–formamide
activation method (Table 1), but in some cases, e.g. 8, it proved
impossible to isolate the salt in a satisfactory state of purity,
even when NMR spectra indicated that the amidinium cation
had been cleanly formed.
Many tetraalkylformamidinium salts are hydrolysed quite
rapidly in contact with water, although this is less of a problem
with hindered cations. A useful alternative for hindered salts is
to dissolve the product mixture in cold water and immediately
add an aqueous solution of ammonium hexafluorophosphate.
Amidinium hexafluorophosphate salts usually precipitate from
the aqueous mixture, while other species (e.g. ammonium ions)
remain in solution.11
As alternatives to POCl3, oxalyl and thionyl chloride were
found to be insufficiently electrophilic for our purposes,
however trifluoromethanesulfonic (triflic) anhydride has been
shown to be effective in related processes by ourselves7 and by
Sforza et al.12 Activation of a secondary formamide with triflic
anhydride forms a monotriflate species, such as 22 (Scheme 2),
Scheme 3
the reaction mixture warms to room temperature, triflate is
eliminated to give the extremely hygroscopic iminium salt 10
which precipitates from solution and was isolated in reasonable
yield (Table 1). Attempts to use bulky aryllithium reagents,
such as mesityllithium, were less successful—the products were
1
observed by H NMR, but in mixtures (ca. 25% impurities)
from which they could not be easily isolated. This novel
methodology has been successfully employed during the
preparation of the highly hindered C3-symmetric amine
(R,R,R)-tris(1-phenylethyl)amine.15
Reaction of lithium diisopropylamide (LDA) with benzalde-
hyde, with subsequent trapping by oxalyl chloride, also gave
phenyliminium salt 10 in the same manner. The corresponding
reaction to generate formamidinium salts by reaction of lithium
amides with formamides also proved to be a viable method
for the one case to which it was applied. The preparation of
formamidinium salt 6 by reaction of lithium piperidide and
diisopropylformamide with subsequent trapping by Tf2O
proceeds in reasonable yield (52%, see Table 1).
Scheme 2
analogous to the chloroiminium salt 21 in Scheme 1. It has
been shown however13,14 that the monotriflate species then
reacts with a second molecule of formamide to generate a
dication, such as 23, which precipitates from even relatively
polar solutions. We confirmed the identity of the precipitate
formed upon slow addition of Tf2O to a cooled dichloro-
methane solution of diisopropylformamide as the dication 23.
Reaction of this mixture with diisopropylamine gives reason-
able yields of the tetraisopropylformamidinium salt 4 but
also regenerates one equivalent of the original formamide
3
Exchange reactions of orthoesters or amidinium salts
Orthoester exchange. Saba et al. reported the synthesis of
cyclic amidinium salts by treatment of suitable diamines with
‡ Crossover studies showed that the monocation and dication are in
equilibrium in solution.
§ It is interesting to note that this reaction does not proceed in
tetrahydrofuran—even when rigorously dried. An intractable polymeric
solid precipitates upon addition of the trapping agent (Tf2O,
Cl(CO)2Cl, or SOCl2)—the nature of this substance is unclear, although
presumed to be polymerised solvent.
† In this earlier report an accurate elemental analysis of the salt 4ؒX was
never achieved due to the mixture of counterions.
J. Chem. Soc., Perkin Trans. 1, 2001, 1586–1593
1587

View Article Online
Table 2 Yields of exchange reactionsa
Preparative method and yields (%)
Transamination of amidinium salts. The elimination of
ammonia from amidinium acetate by exchange with
a
secondary amine, might also be expected to afford tetra-
alkylamidinium salts. As with orthoesters, the formation
of cyclic salts, such as 14, by this method was successful
(Table 2). However, the corresponding reaction with diiso-
propylamine or piperidine gave 1,1,5,5-tetraisopropyl-1,3,5-
triazapenta-1,3-dienium acetate 27 and 1-(piperidinium-1-yl-
idene)-3-(piperidin-1-yl)-2-azapropene acetate 28 respectively—
confirmed by NMR spectroscopy and X-ray crystallography
after conversion to the corresponding hexafluorophos-
phate salts. 1,1,5,5-Tetramethyl-1,3,5-triazapenta-1,3-dienium
chloride (“Gold’s reagent”), a dimethylamino analogue of
these compounds, has been previously reported18 (prepared
from cyanuric chloride and N,N-dimethylformamide) and was
shown to be a general β-dimethylaminomethylenating agent.19
The synthetic utility of 27 and 28 in this sense is currently under
examination.
To avoid this side reaction, amidinium acetate was replaced
by an anhydrous solution of N,N,N Ј,N Ј-tetramethylform-
amidinium chloride. Reaction with piperidine under reflux
gave the desired tetraalkylamidinium salt 17 in good yield.10
However, the corresponding reaction with diisopropylamine
did not form 4, but stopped at the unsymmetrical N,N-
diisopropyl-N Ј,N Ј-dimethylformamidinium salt 5.
Salt
Orthoester
exchange
Oxonium salt
exchange
Amidine
exchange
11
12
13
14
15
17
18
83b
56b
48b
60c
33b
—
59
—
—
98
—
62
88
—
—
—
66,d 65e
—
68e
—
—
ϩ
a Yields not optimised. b NH4 salts used as acid source. c Ethereal
HBF4 used as acid source. d Amidinium acetate used. e Tetramethylform-
amidinium chloride used.
an orthoester (as solvent) in the presence of an acid (an
example is shown in Scheme 4).16 Reaction of trimethyl ortho-
formate with an acid and N,N Ј-dialkylated diamines proceeded
Scheme 4
In general, the preparation of cyclic or unhindered non-cyclic
examples such as 11–18 by these exchange reactions works well,
but hindered non-cyclic tetraalkylamidinium salts cannot be
made by these methods.
in generally high yields in our hands (Table 2). We also found
that it was possible to prepare the 2-13C-labelled amidinium
salt 11 from 13C-labelled triethyl orthoformate using just one
equivalent of orthoester and ethanol as solvent. The reaction
rate was significantly decreased, but the subsequent isolation
was actually easier.
Application of this method to the synthesis of non-cyclic
tetraalkylformamidinium salts proved less useful. Reaction of
2 equivalents of a secondary monoamine with trimethyl ortho-
formate under the same conditions resulted in methylation of
half of the added amine and elimination of the corresponding
formamide. Attempts to prevent this by using triethyl ortho-
formate, where alkylation should be slower, also proved
unsuccessful.
4
Alkylation of amidines
Stepwise alkylation is an obvious route to tetraalkylamidinium
salts, but the final alkylation is limited by problems with elimin-
ation (except for methylations) and with low reactivity (methyl
triflate is therefore the reagent of choice). N,N Ј-Di-tert-butyl-
N,N Ј-dimethylamidinium triflate salt 19 was prepared by
dimethylating di-tert-butylamidinium acetate 29 using methyl
triflate as shown in Scheme 5 in 73% yield.20 It was a surprise to
Scheme 5
find that trace quantities of water were sufficient to cause
elimination of isobutene, resulting in the isolation of [(tert-
Cyclic orthoester 25,17 in which any SN2 alkylation reaction
would have to occur at a hindered neopentyl-like centre, should
avoid this side-reaction. In fact, we found it more convenient
to prepare and isolate the corresponding oxonium salt 26
immediately before reaction with the amine, although its
extreme sensitivity to hydrolysis makes careful handling essen-
tial. Reaction of secondary diamines with oxonium salt 26 is
an efficient method of generating cyclic salts such as 11 and 14
(Table 2). It has also proved effective in the synthesis of
relatively unhindered non-cyclic tetraalkylformamidinium ions
such as 16–18. Attempts to generate the sterically hindered
salt 4 by reaction of diisopropylamine with the oxonium ion 26
were unsuccessful, presumably due to the reluctance of a
second molecule of amine to attack the highly hindered inter-
mediate oxyiminium salt.
butylmethylamino)methylidene]dimethylammonium
triflate
30ؒOTf. The perimidinium salt 20 was also prepared by quater-
nisation of 1H-perimidine21 with methyl triflate in reasonable
yield (73%). We have previously reported the application of
alkylation to be useful in the synthesis of some iminium salts.7
Conclusions
We have developed a range of methods to prepare a variety of
tetraalkylformamidinium salts and related compounds for
application to the preparation of novel diaminocarbenes. For
cyclic or unhindered tetraalkylformamidinium salts, the alkyl-
ation and exchange/transamination methods allow straight-
forward preparation of high purity material with relatively little
1588
J. Chem. Soc., Perkin Trans. 1, 2001, 1586–1593

View Article Online
Table 3 Summary of single crystal X-ray diffraction data
Compound
4ؒOTf
6ؒPF6
12ؒPF6
17ؒBF4
23ؒOTf
28ؒPF6
Formula
Formula weight
Crystal system
Space group
a/Å
b/Å
c/Å
α/Њ
C14H29F3N2O3S
362.45
C12H25F6N2P
342.31
Orthorhombic
P212121
9.610(2)
12.226(3)
14.090(2)
90
90
90
1655.3(6)
4
0.220
C8H17F6N2P
286.21
Monoclinic
P21/c
8.1410
11.230(1)
14.131(2)
90
101.22(1)
90
1267.2(3)
4
C11H21BF4N2
268.11
Orthorhombic
Pbcn
18.086(3)
9.734(1)
7.642(2)
90
C16H30F6N2O7S2
540.54
Orthorhombic
Pbca
11.322(1)
17.900(2)
24.468(4)
90
C12H22F6N3P
353.30
Orthorhombic
Pca21
12.094(2)
16.099(1)
16.456(1)
90
Triclinic
¯
P1
8.996(2)
9.238(2)
11.992(2)
90.02(2)
91.36(2)
112.50(2)
920.4(3)
2
β/Њ
90
90
90
90
90
90
γ/Њ
V/Å3
1345.46(4)
4
0.116
4958.5(12)
8
0.297
3204.05(6)
8
0.232
Z
µ/mmϪ1
0.218
0.272
Temperature/K
Measured reflections
173
5877
293.2
1109
293.2
3051
173
7836
173
2689
173
19842
Independent reflections
Rint
R
4034
0.0175
0.0572
1109
Not calc.
0.0593
2212
0.0117
0.0509
1532
0.0255
0.0525
2194
0.0345
0.0472
6988
0.0402
0.0398
isolation required. For highly hindered salts such as 4, 8 and 9,
which are of particular interest in diaminocarbene chemistry,
the formamide activation and aqueous hexafluorophosphate
isolation techniques can be used. Nucleophilic addition/electro-
phile trapping offers a useful alternative, and merits further
development. It is applicable to the preparation of iminium
as well as these hindered formamidinium salts and removes the
necessity for complex isolation procedures other than recrystal-
lisation from anhydrous solvents.
Aqueous counter-ion conversion to hexafluorophosphate. A
reaction mixture (2–4 g) containing the desired product was
dissolved in ice-cold distilled water or dilute acid (ca. 25 cm3,
0.5 M) and added to a cooled aqueous solution (ca. 25 cm3)
of ammonium hexafluorophosphate (ca. 2 equivalents). The
resulting precipitate was filtered and washed first with cold
water (ca. 2 × 10 cm3) and then diethyl ether (ca. 10 cm3).
General procedures for preparing formamidinium and iminium
salts. 1. Formamide activation
1.1 Phosphorus oxychloride. Typically a solution of a N,N-
dialkylformamide (ca. 10 g) in anhydrous dichloromethane
(40 cm3) was added to a solution of phosphorus oxychloride
(1 equivalent) in dry dichloromethane (100 cm3; Ϫ78 ЊC). The
reaction mixture was allowed to warm to room temperature
and stirred under nitrogen for 2 h. The solution was then cooled
to 0 ЊC and a mixture of the appropriate secondary amine
(1 equivalent) and a tertiary amine base (typically triethylamine
or diisopropylethylamine, 1 equivalent) in dichloromethane
(50 cm3) was slowly added. The reaction mixture was again
allowed to warm slowly to room temperature and stirred for
2 h.
Experimental
Instrumentation
NMR spectra were recorded using standard pulse sequences on
JEOL JNM-LA300 or JEOL JNM-GX400 spectrometers.
Coupling constants are quoted in hertz (Hz). Mass spectra
(FAB and EI) were measured on a Fisons VG Analytical
Autospec spectrometer. Melting points were determined on
a Kofler hotstage electrothermal capillary apparatus and are
uncorrected. Elemental combustion analyses were measured
using a Perkin-Elmer 240C elemental analyser.
General notes
1.2 Triflic anhydride. Typically a solution of a N,N-dialkyl-
formamide (2.5 g) in dry dichloromethane (10 cm3), was
added to a solution of triflic anhydride (0.5 equivalents) in
anhydrous dichloromethane (30 cm3; Ϫ78 ЊC) under a nitrogen
atmosphere. After stirring at room temperature for 1 h, a
mixture of N,N-dialkylamine (0.5 equivalents) and diisopro-
pylethylamine (0.5 equivalents) in dichloromethane (10 cm3)
was then added dropwise to the suspension of the resultant
dication at 0 ЊC. The solution was allowed to warm to room
temperature and stirred for an hour.
All procedures were performed under an inert atmosphere
unless otherwise specified. All solvents and reagents were
rigorously dried by using standard techniques and stored
under an inert atmosphere. All glassware was oven-dried at
140 ЊC before use. Amidinium and iminium salts were generally
stored under anhydrous conditions in a vacuum desiccator
(over P2O5), a vacuum oven (80 ЊC, 20 mmHg) or a nitrogen
atmosphere glove box.
Crystal structure analyses. Many of the details of the
structure analyses of 4ؒOTf, 6ؒPF6, 12ؒPF6, 17ؒBF4, 23ؒOTf,
and 28ؒPF6 are presented in Table 3. Data collection was
carried out at low temperature on Bruker P4 or SMART dif-
fractometers. Data collection for 23ؒOTf was incomplete due to
crystal decomposition. In 28ؒPF6 the two independent formula
units are nearly, but not exactly, related by a centre of inversion.
Attempted refinement in the likely alternative space groups
(Pbca and Pbcm) was unsuccessful. The indeterminate value of
the absolute structure parameter (0.56(8)) may imply some
inversion twinning.
2. Nucleophilic addition to formamides
Typically a solution of a N,N-dialkylformamide (1 g) in diethyl
ether (10 cm3) at Ϫ78 ЊC was slowly treated with an alkyllithium
reagent (diluted in alkane solvent, 1 equivalent) or lithium
amide (1 equivalent, prepared from the appropriate secondary
amine and n-BuLi in diethyl ether). The mixture was stirred at
Ϫ78 ЊC for 30 minutes, followed by a further 30 minutes at
room temperature. The solution was then cooled to Ϫ78 ЊC
before an ethereal solution of triflic anhydride (ca. 1.5 equiv-
alents), or other trapping reagent e.g. oxalyl or thionyl chloride,
was added dropwise with vigorous stirring. The solution was
stirred for a further 10 minutes before being allowed to warm to
room temperature. During warming, a white precipitate
appeared (also significant gas evolution was observed if oxalyl
Full details of the structure determinations are available.
CCDC reference numbers 155803–155808. See http://
www.rsc.org/suppdata/p1/b1/b104110j/ for crystallographic
files in .cif or other electronic format.
J. Chem. Soc., Perkin Trans. 1, 2001, 1586–1593
1589

View Article Online
chloride was used). Once the reaction was complete, the solid
General notes. The resultant solid was recrystallised from hot
was isolated by filtration under an inert atmosphere.
acetone–chloroform as colourless needles (66%); mp 326–
326.5 ЊC (decomp.) (Found: C, 43.7; H, 7.9; N, 7.7. C13H29F6-
N2P requires C, 43.6; H, 8.2; N, 7.8%); δH(300 MHz; (CD3)2CO)
1.46 (24 H, d, J 7, 8 × CH3), 4.11–4.49 (4 H, br m, 4 × CH) and
7.66 (1 H, s, NCHN); δC(100 MHz; (CD3)2CO) 19.7–25.0 (br),
49.6–55.9 (br) and 152.3; m/z (FAB) 213 (Mϩ, 100%), 169 (10)
and 133 (6).
3. Exchange reactions
3.1 Exchange with trialkyl orthoformates.15 Typically a mix-
ture of an N,N Ј-dialkyldiamine (3.0 g), trimethyl or triethyl
orthoformate (15 cm3) and a suitable ammonium salt (ca. 1.5
equivalents) was heated under nitrogen at 120 ЊC, until no more
alcohol was distilled (2–4 h). The excess orthoformate was then
distilled from the reaction mixture under reduced pressure
before work-up.
Alternatively, reaction of a diamine with 1 equivalent of
orthoformate and a slight excess of ammonium salt in refluxing
ethanol for 12–18 hours afforded reaction products with com-
parable yields, but that were more easily isolated.
Diisopropylaminomethylidene(diisopropyl)ammonium tetra-
fluoroborate, 4ؒBF4. Formamidinium salt 4ؒBF4 was also pre-
pared by method 1.1 as described above. The reaction mixture
was washed with aqueous sodium hydroxide solution (2.0 M)
and saturated sodium tetrafluoroborate and extracted with
dichloromethane. The organic layer was dried over anhydrous
magnesium sulfate before filtration and removal of solvent in
vacuo. The product was dissolved in acetone and precipitated
with a mixture of ether and pentane (4 : 1). The resultant solid
was isolated by filtration and was purified by crystallisation
from acetone–diethyl ether to afford colourless crystals (75%);
mp 311–312 ЊC (Found: C, 52.1; H, 9.8; N, 9.1. C13H29BF4N2
requires C, 52.0; H, 9.7; N, 9.3%). Spectroscopic details were
in agreement with the corresponding hexafluorophosphate salt
above.
3.2 Oxonium ion reaction. A solution of a N,N-dialkylamine
(2 equivalents) or N,N Ј-dialkyldiamine (1 equivalent) in
anhydrous dichloromethane (5 cm3) was added dropwise to a
stirred suspension of the freshly prepared oxonium salt 26ؒBF4
(4 g, see below for preparation) in dry dichloromethane (45 cm3;
Ϫ78 ЊC) under nitrogen. The reaction was allowed to warm to
room temperature and stirred overnight. The resulting yellow–
orange solution was then poured into ether and the product
precipitated as an off-white powder, which was isolated by
filtration.
Diisopropylaminomethylidene(diisopropyl)ammonium
tri-
fluoromethanesulfonate, 4ؒOTf. Salt 4ؒOTf was prepared from
N,N-diisopropylformamide, triflic anhydride and diisopropyl-
amine using procedure 1.2. The pale yellow reaction mixture
was poured into diethyl ether. The colourless precipitate was
collected by filtration, then shaken with a mixture of distilled
water and ethyl acetate and the crude product was recovered as
a solid floating at the interface of the two solutions. Subsequent
recrystallisation from acetone–ethyl acetate gave the salt 4ؒOTf
as colourless needles (38%); mp 323–325 ЊC (decomp.) (Found:
C, 46.5; H, 8.0; N, 7.6. C14H29F3N2O3S requires C, 46.4; H,
8.1; N, 7.7%). Spectroscopic details were in agreement with the
corresponding hexafluorophosphate salt above. Crystals of
sufficient quality for single crystal X-ray diffraction studies
(Table 3) were obtained by diffusion of diethyl ether into an
acetone solution of 4ؒOTf.
5,5-Dimethyl-5,6-dihydro-4H-1,3-dioxinium
tetrafluoro-
borate, 26ؒBF4. The oxonium salt 26 was prepared using
a modification of literature procedures16,22 in rigorously dry
conditions and under a nitrogen atmosphere. A solution of
ethereal tetrafluoroboric acid (5.3 cm3, 38.4 mmol) in dry
dichloromethane (5 cm3) was added dropwise to a stirred solu-
tion of dioxane 2516 (5.53 g, 37.9 mmol) in dichloromethane (45
cm3; Ϫ78 ЊC). This was allowed to warm to room temperature
and was stirred for 30 min. Dry diethyl ether (10 cm3) was
added, the solution cooled to Ϫ78 ЊC and the resulting crystals
(which were almost transparent in solution) were filtered under
nitrogen. The precipitate was then recrystallised at least twice
from nitromethane (5 cm3) with dichloromethane (50 cm3)
and diethyl ether (10 cm3). Cooling to Ϫ78 ЊC ensured full
precipitation of the oxonium salt 26ؒBF4 as fine, highly hygro-
scopic, colourless flakes (7.38 g, 96%); δH(400 MHz; CD3NO2)
1.29 (6 H, s, 2 × CH3), 4.89 (4 H, s, 2 × CH2) and 9.16 (1 H, s,
OCHO); δC(100 MHz; CD3NO2) 21.0, 30.4, 83.5 and 176.6.
Dimethylaminomethylidene(diisopropyl)ammonium
hexa-
fluorophosphate, 5ؒPF6. Salt 5ؒPF6 was prepared from N,N-
dimethylformamide, phosphorus oxychloride, diisopropyl-
amine and diisopropylethylamine using procedure 1.1 and
hexafluorophosphate anion exchange as described in the
General notes. The formamidinium hexafluorophosphate 5ؒPF6
separated as a brown oil which was dissolved in ethyl acetate–
acetone and heated with activated charcoal. After filtration and
removal of the solvent in vacuo the resulting solid was recrystal-
lised from hot ethyl acetate to afford colourless flakes (36%);
mp 179–181 ЊC (Found: C, 35.5; H, 7.2; N, 9.2. C9H21F6N2P
requires C, 35.8; H, 7.0; N, 9.3%); δH(300 MHz; (CD3)2CO) 1.40
(12 H, d, J 7, 4 × CH3), 3.41 (6 H, br s, 2 × CH3), 3.81–4.36
(1 H, br m, CH), 4.41–5.01 (1 H, br s, CH) and 7.84 (1 H, s,
NCHN); δC(100 MHz; (CD3)2CO) 19.3–25.4 (br), 41.3, 47.2,
48.6–53.0 (br) and 155.2; m/z (FAB) 157 (Mϩ, 100%) and 113.
3.3 Amidine and formamidinium exchange. Typically, a
solution of an N,N Ј-dialkyldiamine (ca. 2–3 g) or N,N-
dialkylamine (ca. 2–3 g) and amidinium acetate or tetra-
methylformamidinium chloride (1 equivalent, see below for
purification) in anhydrous ethanol (2 cm3) was heated at
100 ЊC for 4 h under nitrogen with stirring. The gas flow from
the reaction was bubbled through dilute aqueous acid solution
to neutralise the ammonia/dimethylamine evolved.
Purification
of
N,N,NЈ,NЈ-tetramethylformamidinium
chloride. A quantity of the salt was dissolved in dichloro-
methane and anhydrous sodium sulfate added. The solution
was filtered and the filtrate treated with equal quantities of
acetone and THF to precipitate all salts. The resulting colour-
less solid was redissolved in dichloromethane and acetone and
any trace of insoluble material removed by filtration. THF was
added to the filtrate and the resulting precipitate collected by
filtration. The hygroscopic salt was stored under an inert
atmosphere as a colourless crystalline powder. Mp 146–147 ЊC
(lit.23 152–154 ЊC).
Dimethylaminomethylidene(diisopropyl)ammonium
tetra-
fluoroborate, 5ؒBF4. Salt 5ؒBF4 was prepared from the corre-
sponding hexafluorophosphate salt 5ؒPF6 by ion exchange
chromatography. The tetrafluoroborate salt 5ؒBF4 was
recrystallised by evaporation of acetone (ca. 5–10%) from hot
ethyl acetate to afford colourless needles; mp 97–98 ЊC (Found:
C, 44.45; H, 8.45; N, 11.4. C9H21BF4N2 requires C, 44.3; H, 8.7;
N, 11.5%). Spectroscopic details were in agreement with the
corresponding hexafluorophosphate salt above.
Diisopropylaminomethylidene(diisopropyl)ammonium hexa-
fluorophosphate, 4ؒPF6. Salt 4ؒPF6 was prepared from N,N-
diisopropylformamide, phosphorus oxychloride, diisopropyl-
amine and diisopropylethylamine using procedure 1.1 and
hexafluorophosphate anion exchange as described in the
Diisopropyl(piperidin-1-ylmethylidene)ammonium hexafluoro-
phosphate, 6ؒPF6. Salt 6ؒPF6 was prepared from N,N-
1590
J. Chem. Soc., Perkin Trans. 1, 2001, 1586–1593

View Article Online
diisopropylformamide, phosphorus oxychloride, piperidine and
diisopropylethylamine using procedure 1.1 and hexafluoro-
phosphate anion exchange as described in the General notes.
Salt 6ؒPF6 was recrystallised from hot acetone–chloroform to
afford colourless needles (62%); mp 209–210 ЊC; δH(400 MHz;
CDCl3) 1.38 (12 H, d, J 7, 4 × CH3), 1.76 (6 H, br m, 3 × CH2),
3.69 (4 H, m, 2 × CH2), 4.02 (2 H, br m, 2 × CH) and 8.15 (1 H,
s, NCHN); δC(100 MHz; CDCl3) 22.3 (br), 22.9, 25.9, 50.5, 54.5
and 152.8.
dried with anhydrous magnesium sulfate. Most of the solvent
was removed in vacuo and then the crude product was separated
as an orange oil by the addition of diethyl ether and n-pentane.
The oil was dissolved in water and filtered. The filtrate was
concentrated to afford 9ؒOTf as a colourless solid (16%); mp
165–170 ЊC. [α]2D0 Ϫ24.5 (Found: C, 51.9; H, 8.8; N, 6.6; S, 7.5.
C19H37F3N2O3S requires C, 53.0; H, 8.7; N, 6.5; S, 7.45%)
(NMR spectra were run at elevated temperatures to simplify
interpretation) δH(300 MHz; DMSO, 60 ЊC) 0.80–1.30 (5 H, br
m, 2 × CH2 and 1 × CH), 1.30–1.39 (21 H, 7 × CH3), 1.50–1.78
(6 H, br m, 3 × CH2), 3.53 (1 H, br dq, J 7, 1 × CH), 4.09 (3 H,
br septet, J 7, 3 × CH) and 7.48 (1 H, s, NCHN); δC(75.5 MHz;
DMSO, 60 ЊC) 20.33, 21.0 (br), 25.40, 25.47, 29.05, 30.09, 53.6
and 152.32; m/z (FAB) 281 (Mϩ, 100%).
Diisopropyl(piperidin-1-ylmethylidene)ammonium
salt
6ؒPF6 was also prepared from formylpiperidine, lithium diiso-
propylamide (prepared in diethyl ether from diisopropylamine
and n-BuLi) and triflic anhydride using procedure 2 and hexa-
fluorophosphate anion exchange as described in the General
notes. Physical and spectroscopic details were identical to the
sample prepared above (52%). Crystals of sufficient quality for
single crystal X-ray diffraction studies (Table 3) were obtained
by diffusion of diethyl ether into an acetone solution of 6ؒPF6.
(R)-(Ϫ)-{[(1-Cyclohexylethyl)isopropylamino]methylidene}-
diisopropylammonium tetrafluoroborate 9ؒBF4. Formamidinium
salt 9ؒBF4 was prepared from N-isopropyl-N-(R)-(Ϫ)-1-cyclo-
hexylethylformamide, phosphorus oxychloride and diiso-
propylamine by procedure 1.1 as described above. After the
reaction was complete, dichloromethane was added and the
reaction mixture was washed with sodium hydroxide solution
(2.0 M) and saturated sodium tetrafluoroborate solution. The
aqueous washings were extracted with dichloromethane and
the combined organic fractions were dried over anhydrous
magnesium sulfate and filtered. Most of the solvent was
removed in vacuo before the crude product was precipitated as
an orange oil by the addition of n-pentane. The product was
purified by column chromatography (10 : 1, CH2Cl2–MeOH),
to yield a tacky yellow oil. After removing traces of solvent,
repeated heating, cooling and removal of solid allowed
isolation. This procedure yielded the salt 9ؒBF4 as a pale yellow
solid (59%). Spectroscopic details were in agreement with those
of 9ؒOTf as prepared above.
(S)-(؉)-Diisopropyl(2-methoxymethylpyrrolidin-1-ylmethyl-
idene)ammonium tetrafluoroborate, 7ؒBF4. Formamidinium
salt 7ؒBF4 was prepared from (S)-(Ϫ)-1-formyl-2-(methoxy-
methyl)pyrrolidine, phosphorus oxychloride and diisopropyl-
amine using procedure 1.1. Dichloromethane was added to
the reaction mixture and the solution was re-cooled in ice and
washed with aqueous sodium hydroxide (2.0 M) and a saturated
aqueous solution of sodium tetrafluoroborate. The aqueous
washings were extracted with dichloromethane before being
dried over anhydrous magnesium sulfate, filtered and the
solvent removed in vacuo. The crude product, an orange oil,
was then purified by column chromatography (10 : 1, CH2Cl2–
MeOH). After removing traces of solvent, heating and cooling
of the resultant oil allowed crystallisation of salt 7ؒBF4 as a low
melting solid (77%); mp 50–51 ЊC; [α]2D0 ϩ108.7 (Found: C, 49.7;
H, 8.8; N, 9.3. C13H27BF4N2O requires C, 49.65; H, 8.7; N,
8.9%); δH(300 MHz; CDCl3) 1.36 (6 H, d, J 7, 2 × CH3), 1.38
(6 H, d, J 7, 2 × CH3), 1.65 (1 H, m, ring CH), 2.15 (3 H, m, ring
CH ϩ CH2), 3.37 (3 H, s, OCH3), 3.41–3.54 (2 H, m, CH2O),
3.82 (1 H, septet, J 7, CH), 3.90, 3.82–3.84 (2 H, m, CH2), 4.25
(1 H, m, CH), 4.62 (1 H, septet, J 7, CH) and 7.75 (1 H, s,
NCHN); δC(75.5 MHz; CDCl3) 24.4, 22.5, 20.6, 20.3, 25.7, 25.0,
49.0, 50.6, 58.6, 65.3, 74.1 and 151.4. m/z (EI) 227 (Mϩ, 55%).
Benzylidenediisopropylammonium chloride 10ؒCl. Iminium
salt 10ؒCl was prepared from N,N-diisopropylformamide,
phenyllithium and oxalyl chloride in diethyl ether using pro-
cedure 2 as described above. The easily hydrolysed solid product
was isolated by filtration under N2 and washed with Et2O
(37%); mp 225–230 ЊC (decomp.); δH(400 MHz; CD3OD) 1.58
(6 H, d, J 7, 2 × CH3), 1.64 (6 H, d, J 7, 2 × CH3), 4.63 (1 H,
septet, J 7, CH), 5.14 (1 H, septet, J 7, CH), 7.70–7.90 (5 H, m,
5 × ArH) and 9.18 (1 H, s, NCHN); δC(75 MHz; CD3OD) 20.6,
24.7, 56.9, 59.1, 129.0, 131.1, 133.0, 136.6 and 173.1.
N,NЈ-Dicyclohexyl-N,NЈ-diethylformamidinium
trifluoro-
methanesulfonate, 8ؒOTf. Salt 8ؒOTf was prepared from
N-cyclohexyl-N-ethylformamide, triflic anhydride and N-
cyclohexyl-N-ethylamine using procedure 1.2. The solution
was stirred at room temperature for 1 h before being poured
into diethyl ether giving a yellow oil. The oil was partitioned
between diethyl ether and water, with the ethereal fraction being
dried over anhydrous magnesium sulfate before filtration and
removal of the solvent in vacuo. The residue was recrystallised
from ethyl acetate and ether (85%) as fine colourless flakes;
mp 65–66 ЊC (Found: C, 52.6; H, 8.3; N, 6.8. C18H33F3N2O3S
requires C, 52.2; H, 8.0; N, 6.8%); δH(300 MHz; CDCl3) 1.26
(6 H, br t, J 7, 2 × CH3), 1.03–1.98 (20 H, br m, 10 × CH2), 3.48
(4 H, br q, J 7, 2 × CH2), 3.66 (2 H, br s, 2 × CH) and 7.94 (1 H,
br s, NCHN); δC(75 MHz; CDCl3) 15.6, 24.7, 25.2, 31.8, 41.0,
68.1, 122.9 (q, J 321) and 153.7; m/z (FAB) 265 (Mϩ, 100%), 236
(3), 213 (5), 182 (3) and 138 (6).
1,3-Diisopropyl-3,4,5,6-tetrahydropyrimidin-1-ium tetrafluoro-
borate, 11ؒBF4. Salt 11ؒBF4 was prepared from N,N Ј-diiso-
propylpropane-1,3-diamine, ammonium tetrafluoroborate and
triethyl orthoformate using procedure 3.1. The product mixture
was dissolved in acetonitrile and the insoluble ammonium
salt was removed by filtration. The acetonitrile was removed
under vacuum and the residue recrystallised by evaporation of
acetone (ca. 5–10%) from hot ethyl acetate to give fine colour-
less needles (83%); mp 144–145 ЊC (Found: C, 47.2; H, 8.4; N,
10.8. C10H21BF4N2 requires C, 46.9; H, 8.3; N, 10.9%); δH(400
MHz; (CD3)2CO) 1.34 (12 H, d, J 7, 4 × CH3), 2.14 (2 H, quin,
J 6, CH2), 3.51 (4 H, t, J 6, 2 × CH2), 4.01 (2 H, septet, J 7,
2 × CH) and 8.27 (1 H, s, NCHN); δC(100 MHz; (CD3)2CO)
19.8, 20.1, 39.7, 57.4 and 151.7; m/z (FAB) 169 (Mϩ, 100%) and
126 (3).¶
(R)-(Ϫ)-{[(1-Cyclohexylethyl)isopropylamino]methylidene}-
diisopropylammonium trifluoromethanesulfonate 9ؒOTf. Form-
amidinium salt 9ؒOTf was prepared from N-isopropyl-N-
(R)-(Ϫ)-1-cyclohexylethylformamide, triflic anhydride and
diisopropylamine by procedure 1.2 as described above. Once
the reaction was complete, dichloromethane was added and the
reaction mixture was washed with sodium hydroxide solution
(2.0 M) and distilled water. The aqueous layer was extracted
with dichloromethane and the combined organic washings
Formamidinium salt 11ؒBF4 was also prepared by reaction of
N,N Ј-diisopropylpropane-1,3-diamine with freshly prepared
oxonium salt 26ؒBF4 using procedure 3.2. The resulting precipi-
tate was recrystallised using the method above to give the
formamidinium salt 11ؒBF4 (59%) as fine colourless needles.
Spectroscopic details were in agreement with the product
isolated above.
¶ Single crystal X-ray structure previously reported.15
J. Chem. Soc., Perkin Trans. 1, 2001, 1586–1593
1591

View Article Online
1,3-Diethyl-3,4,5,6-tetrahydropyrimidin-1-ium
hexafluoro-
procedure 3.2. The resulting colourless precipitate was gathered
phosphate, 12ؒPF6. Salt 12ؒPF6 was prepared from N,N Ј-diethyl-
propane-1,3-diamine, ammonium tetrafluoroborate and triethyl
orthoformate using procedure 3.1 and hexafluorophosphate
anion exchange as described in the General notes. The desired
formamidinium hexafluorophosphate salt 12ؒPF6 was recrystal-
lised by evaporation of acetone (ca. 5–10%) from hot ethyl
acetate to give colourless prisms (56%); mp 87–88 ЊC (Found:
C, 33.8; H, 6.2; N, 9.8. C8H17F6N2P requires C, 33.6; H, 6.0; N,
9.8%); δH(300 MHz; (CD3)2CO) 1.30 (6 H, t, J 7, 2 × CH3), 2.18
(2 H, quin, J 6, CH2), 3.52 (4 H, t, J 6, 2 × CH2), 3.59 (4 H, q,
J 7, 2 × CH2) and 8.18 (1 H, s, NCHN); δC(75 MHz; (CD3)2CO)
13.4, 19.6, 43.1, 50.9 and 153.1; m/z (FAB) 141 (Mϩ, 100%)
and 112 (4). Crystals of sufficient quality for single crystal
X-ray diffraction studies (Table 3) were obtained by cooling
(Ϫ30 ЊC) an acetone–ethyl acetate solution of 12ؒPF6.
by filtration. δH(400 MHz; CDCl3) 1.36 (12 H, t, J 7, 4 × CH3),
3.56 (8 H, q, J 7, 4 × CH2) and 7.69 (1 H, s, NCHN). Sub-
sequent recrystallisation and filtration resulted in the colourless
solid turning to a viscous oil (1H NMR of the decomposition
product indicated that salt 16ؒBF4 was rapidly hydrolysed to the
corresponding diethylammonium salt and diethylformamide).
Piperidin-1-ylmethylidenepiperidinium
tetrafluoroborate,
17ؒBF4. Salt 17ؒBF4 was prepared from piperidine and freshly
prepared oxonium salt 26ؒBF4 using procedure 3.2. The pre-
cipitate was recrystallised from hot ethyl acetate, affording the
formamidinium salt 17ؒBF4 as fine colourless flakes (62%);
mp 199–200 ЊC (Found: C, 49.4; H, 8.0; N, 10.4. C11H21BF4N2
requires C, 49.3; H, 7.9; N, 10.45%); δH(300 MHz; CDCl3) 1.73
(12 H, br s, 6 × CH2), 3.62 (8 H, br s, 4 × CH2) and 7.67 (1 H, s,
NCHN); δC(100 MHz; (CDCl3) 23.0, 25.7, 49.1 (br), 54.6 (br)
and 153.7; m/z (FAB) 181 (Mϩ, 100%). Crystals of sufficient
quality for single crystal X-ray diffraction studies (Table 3) were
obtained by diffusion of ether into an acetone solution of
17ؒBF4.
1,3-Dimethyl-3,4,5,6-tetrahydropyrimidinium iodide, 13ؒI. Salt
13ؒI, previously reported by Alici et al.,24 was prepared in 48%
yield from N,N Ј-dimethylpropane-1,3-diamine, ammonium
iodide and triethyl orthoformate using procedure 3.1. δH(400
MHz; (CD3)2CO) 2.21 (2 H, quin, J 6, CH2), 3.33 (6 H, s,
2 × CH3), 3.50 (4 H, t, J 6, 2 × CH2) and 8.75 (1 H, s, NCHN);
δC(75 MHz; (CD3)2CO) 19.5, 42.1, 45.2 and 154.4; m/z (FAB)
353 (2MϩIϪ, 5%) and 113 (Mϩ, 100).
Piperidin-1-ylmethylidenepiperidinium hexafluorophosphate,
17ؒPF6. Salt 17ؒPF6 was prepared from piperidine and
N,N,N Ј,N Ј-tetramethylformamidinium chloride using pro-
cedure 3.2 and hexafluorophosphate anion exchange as
described in the General notes The resultant solid was recrystal-
lised by evaporation of acetone (ca. 5–10%) from hot ethyl
acetate to give colourless needles (68%). Spectroscopic details
were in agreement with those of the corresponding BF4 salt
above.
1,3-Di-tert-butyl-4,5-dihydro-3H-imidazol-1-ium tetrafluoro-
borate, 14ؒBF4. Salt 14ؒBF4 was prepared from N,N Ј-di-
tert-butylethylenediamine, ethereal tetrafluoroboric acid and
triethyl orthoformate using procedure 3.1. The reaction product
was recrystallised from hot ethyl acetate to give colourless flakes
(60%); mp 298–299 ЊC (lit.24 277 ЊC) (Found: C, 49.0; H, 8.5;
N, 10.2. C11H23BF4N2 requires C, 48.9; H, 8.6; N, 10.4%);
δH(300 MHz; (CD3)2CO) 1.46 (18 H, s, 6 × CH3), 4.16 (4 H, s,
2 × CH2) and 8.24 (1 H, s, NCHN); δC(75 MHz; (CD3)2CO)
27.9, 46.1, 57.4 and 153.7; m/z (FAB) 183 (Mϩ, 100%), 127 (9)
and 111 (15).
Formamidinium salt 14ؒBF4 was also prepared from N,N Ј-
di-tert-butylethylenediamine and freshly prepared oxonium salt
26ؒBF4 using procedure 3.3. The resulting colourless precipitate
was gathered by filtration (98%). Spectroscopic details were in
agreement with the product isolated above.
Pyrrolidin-1-ylmethylidenepyrrolidinium
hexafluorophos-
phate, 18ؒPF6. Salt 18ؒPF6 was prepared from pyrrolidine
and freshly prepared oxonium salt 26ؒBF4 in CH2Cl2, using
procedure 3.2 and hexafluorophosphate anion exchange as
described in the General notes. The resultant solid was
recrystallised by evaporation of acetone (ca. 5–10%) from hot
ethyl acetate to give colourless needles (88%); mp 210–211 ЊC
(Found: C, 36.2; H, 5.6; N, 9.2. C9H17F6N2P requires C, 36.25;
H, 5.75; N, 9.4%); δH(300 MHz; (CD3)2CO) 1.96 (4 H, quin, J 7,
2 × CH2), 2.09 (4 H, quin, J 7, 2 × CH2), 3.81 (4 H, br t, J 7,
2 × CH2), 4.06 (4 H, br t, J 7, 2 × CH2) and 8.22 (1 H, s,
NCHN); δC(75 MHz; (CD3)2CO) 24.7, 26.6, 49.1, 55.0 and
152.4; m/z (FAB) 153 (Mϩ, 100%) and 111 (7).
1,3-Di-tert-butyl-4,5-dihydro-3H-imidazol-1-ium hexafluoro-
phosphate, 14ؒPF6. Salt 14ؒPF6 was prepared from di-tert-
butylethylenediamine and amidinium acetate using procedure
3.3 and hexafluorophosphate anion exchange as described in
the General notes. The desired formamidinium salt 14ؒPF6
was recrystallised by evaporation of acetone (ca. 5–10%) from
a hot ethyl acetate solution to give colourless prisms (66%);
mp 317–318 ЊC (Found: C, 40.3; H, 7.1; N, 8.4. C11H23F6N2P
requires C, 40.25; H, 7.1; N, 8.5%). Spectroscopic details were
in agreement with the corresponding tetrafluoroborate salt
above.
tert-Butyl[(tert-butylmethylamino)methylidene]methyl-
ammonium trifluoromethanesulfonate, 19ؒOTf. Methyl trifluoro-
methanesulfonate was added dropwise to a solution of N,N Ј-
di-tert-butyl-N-methylformamidine (prepared by the method of
Nivard et al.20) in acetonitrile and allowed to stir overnight at
room temperature. The solvent was removed and the resulting
yellow solid was recrystallised from ethyl acetate giving colour-
less flakes 19ؒOTf (73%); mp 111–112 ЊC (Found: C, 43.2; H,
7.9; N, 8.4. C12H25F3N2O3S requires C, 43.1; H, 7.5; N, 8.4%);
δH(300 MHz; CDCl3) 1.48 (18 H, s, 6 × CH3), 3.31 (6 H, s,
2 × CH3) and 7.80 (1 H, s, NCHN); δC(75 MHz; CDCl3) 28.1,
34.5, 62.8, 120.7 (q, J 320) and 153.4; m/z (FAB) 185 (Mϩ,
100%) and 128 (5).
Formamidinium salt 19ؒPF6 was prepared from the corre-
sponding triflate salt, prepared above, using the anion exchange
method described in the General notes. The resultant solid was
recrystallised by evaporation of acetone (ca. 5–10%) from ethyl
acetate to give colourless prisms; mp 222–223 ЊC (Found:
C, 40.2; H, 7.8; N, 8.4. C11H25N2F6P requires C, 40.0; H, 7.6;
N, 8.5%); δH(300 MHz; (CD3)2CO) 1.53 (18 H, s, 6 × CH3), 3.43
(6 H, s, 2 × CH3) and 8.12 (1 H, s, NCHN); δC(75 MHz;
(CD3)2CO) 28.2, 35.2, 62.3 and 155.8; m/z (FAB) 185 (Mϩ,
100%).
1,3-Diisopropyl-4,5-dihydro-3H-imidazol-1-ium tetrafluoro-
borate, 15ؒBF4.15 Salt 15ؒBF4 was prepared from N,N Ј-di-
isopropylethylenediamine, ammonium tetrafluoroborate and
triethyl orthoformate using procedure 3.1. After filtration to
remove the insoluble ammonium salt, the reaction mixture was
recrystallised by evaporation of dichloromethane (∼5–10%)
from an ethyl acetate solution to give colourless flakes (33%);
mp 144.5–146.5 ЊC (lit.15 152–153 ЊC); δH(300 MHz; (CD3)2CO)
1.41 (12 H, d, J 7, 4 × CH3), 3.90 (4 H, s, 2 × CH2), 4.03 (2 H,
septet, J 7, 2 × CH) and 8.10 (1 H, s, NCHN).
Diethylaminomethylidene(diethyl)ammonium
borate, 16ؒBF4. Salt 16ؒBF4 was prepared from N,N-diethyl-
amine and freshly prepared oxonium salt 26ؒBF4 using
tetrafluoro-
1592
J. Chem. Soc., Perkin Trans. 1, 2001, 1586–1593

View Article Online
1,3-Dimethyl-1H-perimidin-1-ium trifluoromethanesulfonate,
20ؒOTf. 1H-Perimidine (0.50 g, 2.98 mmol) was prepared by
the literature method of Sachs,21 and treated with methyl triflate
(0.70 cm3, 6.19 mmol) and 2,6-di-tert-butylpyridine (0.7 cm3,
3.02 mmol) in dichloromethane (300 cm3). The mixture was
stirred overnight at room temperature. The solvent was
removed and the yellow solid dissolved in the minimum amount
of saturated bicarbonate solution, washed with diethyl ether
and extracted into dichloromethane. The solvent was removed
to give 20ؒOTf, which was recrystallised as yellow needles from
ethanol (73% yield); mp 275–276 ЊC (Found: C, 48.7; H, 3.7; N,
8.1. C14H13F3N2O3S requires C, 48.55; H, 3.8; N, 8.1%); δH(300
MHz; CDCl3) 3.72 (6 H, s, 2 × CH3), 6.84 (2 H, br dd, J 7 and 1,
2 × ArH), 7.49 (2 H, br dd, J 8 and 7, 2 × ArH), 7.58 (2 H, br
dd, J 8 and 1, 2 × ArH) and 9.28 (1 H, s, NCHN); δC(75 MHz;
CD3OD) 39.7, 109.1, 121.9 (q, J 318), 122.2, 125.5, 129.6,
134.2, 136.1 and 154.5; m/z (FAB) 197 (Mϩ, 100%), 182 (19)
and 133 (31).
(CD3)2CO) 1.66–1.84 (12 H, br m, 6 × CH2), 3.77 (4 H, br t, J 5,
2 × CH2), 3.96 (4 H, br t, J 5, 2 × CH2) and 8.49 (2 H, s,
2 × NCHN); δC(75 MHz; (CD3)2CO) 24.4, 26.2, 27.3, 46.5, 53.8
and 165.7; m/z (FAB) 208 (Mϩ, 100%) and 124 (4). Crystals of
sufficient quality for single crystal X-ray diffraction studies
(Table 3) were obtained by diffusion of diethyl ether into an
acetone solution of the title compound to Ϫ30 ЊC.
Acknowledgements
We thank the EPSRC for a grant (GR/K76160) and for a
studentship (M.E.B.). S.J.W. thanks the University of Bristol
for a postgraduate scholarship.
References
1 For recent reviews see: (a) D. Bourissou, O. Guerret, F. P. Gabbai
and G. Bertrand, Chem. Rev., 2000, 100, 39; (b) A. J. Arduengo, Acc.
Chem. Res., 1999, 32, 913; (c) A. J. Arduengo and R. Krafczyk,
Chem. Unserer Zeit, 1998, 32, 6.
1,1,5,5-Tetraisopropyl-1,5-diazonia-3-oxapenta-1,4-dienyl
bis(trifluoromethanesulfonate), 23ؒOTf. Dication 23ؒOTf was
prepared by the addition of triflic anhydride (100 µL, 0.594
mmol) to a solution of N,N-diisopropylformamide (68 µL,
0.593 mmol) in d3-nitromethane (0.867 g), at Ϫ78 ЊC in dry
conditions under nitrogen. The solution was then allowed to
warm to room temperature and the extremely hygroscopic solid
precipitate was isolated by filtration. δH(400 MHz; CD3NO2)
1.59 (12 H, d, J 7, 4 × CH3), 1.67 (12 H, d, J 7, 4 × CH3), 4.58
(2 H, septet, J 7, 2 × CH), 4.87 (2H, septet, J 7, 2 × CH) and
9.46 (2 H, br s, 2 × NCHO); δC(75 MHz; CD3NO2) 19.6, 22.7,
58.1, 59.3, 121.2 (q, J 318) and 162.8. Crystals of sufficient
quality for single crystal X-ray diffraction studies (Table 3) were
obtained by cooling a dichloromethane–nitromethane solution
of 23ؒOTf to Ϫ30 ЊC.
2 R. W. Alder, P. R. Allen and S. J. Williams, J. Chem. Soc., Chem.
Commun., 1995, 1267.
3 R. W. Alder, M. E. Blake and J. M. Oliva, J. Phys. Chem. A, 1999,
103, 11200.
4 R. W. Alder, P. R. Allen, M. Murray and A. G. Orpen, Angew.
Chem., Int. Ed. Engl., 1996, 35, 1121.
5 R. W. Alder and M. E. Blake, Chem. Commun., 1997, 1513.
6 R. W. Alder, M. E. Blake, C. Bortolotti, S. Bufali, C. P. Butts,
E. Linehan, J. Oliva, A. G. Orpen and M. Quayle, Chem. Commun.,
1999, 241.
7 R. W. Alder, C. P. Butts and A. G. Orpen, J. Am. Chem. Soc., 1998,
44, 11526.
8 R. W. Alder, M. E. Blake, C. P. Butts, A. G. Orpen and S. J. Williams,
in preparation.
9 (a) W. Kantlehner, Comprehensive Organic Synthesis, ed. B. M. Trost
and I. Fleming, Pergamon Press, Great Britain, 1991, Vol. 6;
(b) S Patai, The Chemistry of Amidines and Imidates, Wiley, New
York, 1975, Ch. 7.
10 (a) W. Kantlehner, Adv. Org. Chem., 1979, 9, 321; (b) G. Simchen,
Methoden der Organischen Chemie (Houben Weyl), 1985, E5/1, 1.
11 Our thanks to Otto Meth-Cohn for this extremely helpful
suggestion.
12 S. Sforza, A. Dossena, R. Corradini, E. Virgili and R. Marchelli,
Tetrahedron Lett., 1998, 39, 711.
13 P. J. Stang, G. Maas, D. L. Smith and J. A. McCloskey, J. Am. Chem.
Soc., 1981, 103, 4837.
14 T. Gramstad, S. Husebye and J. Saebo, Tetrahedron Lett., 1983, 24,
3919.
1,1,5,5-Tetraisopropyl-1,3,5-triazapenta-1,3-dienium
hexa-
fluorophosphate, 27ؒPF6. Salt 27ؒPF6 was prepared from diiso-
propylamine and amidinium acetate using procedure 3.3.
The solvent was removed and the product was isolated as the
hexafluorophosphate salt as described in the General notes.
The hexafluorophosphate salt 27ؒPF6 was recrystallised from
acetone–diethyl ether (2.25 g, 24%) to afford colourless prisms;
mp 173–174 ЊC (Found: C, 43.7; H, 8.1; N, 10.85. C14H30F6N3P
requires C, 43.6; H, 7.85; N, 10.9%); δH(300 MHz; (CD3)2CO)
1.41 (12 H, d, J 7, 4 × CH3), 1.42 (12 H, d, J 7, 4 × CH3), 4.11
(2 H, septet, J 7, 2 × CH), 4.83 (2 H, septet, J 7, 2 × CH) and
8.59 (2 H, s, 2 × NCHN); δC(75 MHz; (CD3)2CO) 19.9, 23.1,
50.8, 52.3 and 166.2; m/z (FAB) 240 (Mϩ, 100%), 225 (3), 196
(11), 182 (6), 154 (5) and 129 (14).
15 P. Wyatt, C. P. Butts, V. Patel and B. Voysey, J. Chem. Soc., Perkin
Trans. 1, 2000, 4222.
16 S. Saba, A. Brescia and M. Kaloustian, Tetrahedron Lett., 1991, 32,
5031.
17 S. Kabusz and W. Tritschler, Synthesis, 1971, 312.
18 H. Gold, Angew. Chem., 1960, 72, 956.
19 J. T. Gupton and S. A. Andrews, Org. Synth., 1986, 64, 85 and
references therein.
1-(Piperidinium-1-ylidene)-3-(piperidin-1-yl)-2-azapropene
hexafluorophosphate 28ؒPF6. Salt 28ؒPF6 was prepared from
piperidine and amidinium acetate using procedure 3.3 and
hexafluorophosphate anion exchange as described in the
General notes. The hexafluorophosphate salt was recrystallised
from dichloromethane–diethyl ether (63%) as thick colourless
needles; mp 106–107 ЊC (Found: C, 41.05; H, 6.2; N, 11.8.
C12H22F6N3P requires C, 40.8; H, 6.3; N, 11.9%); δH(300 MHz;
20 This is a modification of a preparation reported in S. C. Wicherink,
J. W. Scheeren and R. J. F. Nivard, Synthesis, 1977, 273.
21 F. Sachs, Liebigs Ann. Chem., 1909, 365, 66.
22 B. Chalova, E. S. Kurmaeva, E. A. Kantor, T. K. Kiladze and
D. L. Rakhmankulov, J. Org. Chem. USSR, 1982, 18, 1434.
23 R. H. Toeniskoetter and K. A. Killip, J. Am. Chem. Soc., 1964, 86,
690.
24 B. Alici, E. Cetinkaya and B. Cetinkaya, Heterocycles, 1997, 45,
29.
J. Chem. Soc., Perkin Trans. 1, 2001, 1586–1593
1593