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Chair and Boat Stable Conformers of an Eight-Membered Ring System.

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We now report the first quantitative results of
FD-MS obtained by exploiting cationization.
Field desorption mass spectrometry and similar mass spectrometric methods give spectra for numerous organic compounds which consist predominantly of the ion [MI$ or
[M +Cat] +.These methods are particularly suitable for quantitative determinations because:
structurally significant fragments are not required,
interference due to impurities in the sample is reduced, and
the molecular ion or the cationized molecule carries almost
the entire ion current, thus leading to maximum possible
sensitivity.
A drawback of the FD-MS method is that it gives lower
ion currents showing greater fluctuations than electron impact
and chemical ionization. For this reason
the sensitivity was increased by cationization with alkali metal
cations,
the precision improved by an isotopically labeled internal
standard, and
the value determined for [M + Cat] was averaged over a
large number of repetitive scans.
Figure 1 shows a section of five scans recorded in the
quantitative determination of a non-derivatized monosaccharide, glucose, on a microgram scale.
The extent of labeling of the internal standard [1-13C]-glucose was determined by comparison of the attachment-FD
spectra A and B (Table 1). Taking account of the intensity
of the peak at m/e 202 of unlabeled glucose, it was calculated
that C-I of the ['3C]-glucose is 86.7% l3C. Use of monoisotopic [23Na] for cationization greatly simplifies quantitative
evaluation of the spectra. The quantitative data obtained are
supported by determination of the intensity distribution of
the [M + H]' group by the normal field desorption procedure
(spectrum D, Table 1).
The spectra were evaluated by making the simplifying
assumption that only [M+Na]+ and [M+H]+ ions are
formed. For spectrum C([M Na]+ group) a molar ratio
+
+
+
m / e 203
mle
-
I
Fig. 1. Five scans from a series of measurements comprising 105 consecutive
F D scans over the [M Na] + group of a mixture of glucose and [I -"C]-glucose (86.7'~; "C) in the presence of [Na]'. According to the amount of
sample weighed out the molar ratio was 1:1.17. Cationization was ensured
by dissolution of glucose and NaI in the approximate molar ratio 1 : l in
water. The solution was applied to a high-temperature activated F D emitter
with a microliter syringe. The emitter heating current was about 10 mA
while the spectrum was being recorded. The spectra were recorded at low
resolution (plateau peaks) by variation of the accelerating potential with
a Varian MAT 731 mass spectrometer (see also Table 1, spectrum C).
+
of 1:1.184 (error 11.1%) was obtained, and for spectrum
1.7%).
D ([M+H]+ group) a molar ratio of 1:1.191 (error
+
Angew. Chem.
lilt.
Ed. Engl. 16 (1977) No. 3
Table 1. Relative intensities of the [M + Na] + group (spectra A to C) and
the [ M + H ] + group (spectrum D) in the F D mass spectrum of glucose
in the presence and absence. respectively. of "a]+. I n order to record the
sodium attachment spectra the sample was dissolved together with Nal in
the approximate molar ratio of 1 : 1 in H 2 0 ; for recording the normal F D
mass spectrum pure H z O was used as solvent. The amounts of sample
applied were in the microgram range.
mle
202
203
204
205
Relative intensity [ ""1
Spectrum A
Spectrum B
Spectrum C
mle
23
3.4
180
100
17.5
100
8.9
I00
96.1
I81
182
63
7.1
183
Relative intensity [ %]
Spectrum D
10.5
100
96.5
9.8
Spectrum A: [ M t N a ] ' group of glucose +NaI, 60 consecutive scans, 5
mA emitter heating current.
Spectrum B: [M+Na]+ group of [l-"C]-glucose (86.7% "C)+Nal. 60
consecutive scans. 10 to 15 mA emitter current.
Spectrum C: [M+Na]+ group of a mixture of glucose and [1-13C]-glucose
(molarratio 1:1.17J+NaI. 105consecutivescans. l 0 t o 15 mAemitter heating
current, mean error of the individual measurements of relative intensity
of mje 204: +7.2%: mean error of average value: +0.7'%, (see also Fig.
1).
Spectrum D : [ M + H ] + group of a mixture of glucose and [l-'3C]-glucose
(molar ratio l:l.l7), 210 consecutive scans, 18 to 20 mA emitter heating
current, mean error of the individual measurements of the relative intensity
of nije 182: +6.8%; mean error of the average value: i 0 . 4 7 % .
In the quantitative determination of glucose by field desorption mass spectrometry the mean error of the individual measurement is about 7 % . On evaluation of 100 scans the mean
error of the average value is +0.7"/,, and this value decreases
accordingly as more scans are recorded. No isotope effects
were observed in these series of measurements. Our investigations show that field desorption maskspectrometry in conjunction with the isotopic dilution method possesses good sensitivity and precision for quantitative analysis and can be regarded
as a promising method for biochemical and medical analysis
(e.g. pharmakokinetic studies).
Received: December 16, 1976 [Z 632 IE]
German version: Angew. Chem. 89, 180 ( I 977)
CAS Registry number:
Glucose. 50-99-7
[ I ] H. D. Beckey, H.-R. Schulren, Angew. Chem. 87. 425 (1975): Angew.
Chem. lnt. Ed. Engl. 14, 403 (1975); H.-R. Schulten, f. W Riillyen,
Angew. Chem. 87, 544 (1975): Angew. Chem. Int. Ed. Engl. 14. 561
( 1 975).
[2] H.-R. Schulten, H . D. Beckey, Org. Mass Spectrom. 6. 885 (1971); 7.
861 (1973).
131 f. W Riillgen, H.-R. Schulten, Org. Mass. Spectrom. 10, 660 (1975);
H:R. Schulten, f. W Riillgen, ibid. 10. 649 (1975); cf. H. J . Veirh, Angew.
Chem. 88, 762 (1976); Angew. Chem. Int. Ed. Engl. IS, 695 (1976).
[4] S. Pfefer, H . D. Beckey, H.-R. Schulten, Z. Anal. Chem. 284, 193 (1977),
and references cited therein.
Chair and Boat: Stable Conformers of an Eight-Membered Ring System[**]
By Rolf W Saalfrank[*'
Eight-membered ring systems of type ( 1 ) can in principle
assume a rigid chair-like conformation or a flexible boat-like
[*] Priv.-Doz. Dr. R. W. Saalfrank
lnstitut fur Organische Chemie der Universitat
Erlangen-Nurnberg (Germany)
[**I Based on a lecture delivered at the V. Internat~onalConference of
Organic Chemistry, Gdansk (Poland), September 1974.
185
conformation[''. a-Disalicylide [(I ), X = Z = 0 ;Y = W =CO]
exists exclusively in the boat conformationrz1. In the case
of
5,6,11,I 2-tetrahydrodibenzo[a,e]cyclooctene
[(I ),
X =Z = Y =W = CH2], however, both the chair and the boat
conformations are present in almost equal proportions in
solution[3J, whereas in the crystalline state only the chair
conformation is knownr4]. In the following communication
the synthesis and isolation of the chair and boat conformers
of an eight-membered ring system is reported for the first
time.
6.p
Q Y - Z
Y-
X-W-Q
-z
X-W
(1)
Wittig reaction of the (2,2-diethoxyvinylidene)triphenylphosphorane ( 2 ) with 9,lO-phenanthrenequinone(3) leads,
probably via [4+4] cycloadditionr6I of the IP-dipoIe ( 5 )
and the "0-quinonoid allene (4)[51, to the cyclooctane derivative (6), whose chair and boat conformers ( 6 u ) and (66)
can be separated by fractional crystallization.
Heating of the chair boat conformer in 1,2-dichlorobenzene
leads to an equilibrium mixture. The position of the equilibrium (chair: boat = 1 :2)is practically independent of temperature between 30 and 90°C.
All conformers of the boat family are chiral. Their racemization ensues by flipping of the eight-membered rings. As shown
by the multiplet of the diastereotopic methylene protons, the
racemization at room temperature is slow with respect to
the 'H-NMR scale. With increasing temperature it becomes
more rapid, and at about 100°C the methylene protons are
magnetically equivalent.
The temperature dependence of the 'H-NMR spectra of
the chair conformer ( 6 a ) is surprising: Above 120°C its CH3
triplets begin to broaden, at about 160°C they coalesce, and
at 180°C a new triplet of double intensity appears exactly
in the middle of the two original triplets[']. This would indicate
that free rotation about the exocyclic C=C double bond
is possible in the chair conformer under these conditions[*].
Evidence for the isolated substances being conformers is
also provided by the fact that the differences between corresponding I3C-NMR signals do not exceed 1.5 ppm (see Table
1).
I
RO
h O R
OR
L
(4)
Identification of the conformers follows from the temperature dependent 'H-NMR and I3C-NMR spectra (Table 1).
On the basis of their molecular symmetry [Ci for ( 6 a ) ,
Cz for (6b)l one would expect the 'H-NMR spectra of both
conformers at room temperature to show two triplets for
the methyl groups together with a multiplet, since the methylene protons in both molecules are diastereotopic. A molecular
model shows that in the chair conformers ( 6 a ) two ethoxy
groups are more remote from the phenanthrene residues than
the other two. Hence two CH3 groups are more strongly
shielded and, as a consequence, their signals appear at higher
field in the 'H-NMR spectrum. In the flexible boat conformer
( 6 b ) , on the other hand, no ethoxy group is outstandingly
oriented. Hence the two CH3 triplets differ only slightly in
their chemical shift.
Experimentul:
A hot benzene solution of 8.3 g (40 mmol) 9,lO-phenanthrenequinone is added with vigorous stirring under nitrogen
to a benzene solution of (2,2-diethoxyvinylidene)triphenylphosphorane (2),prepared from 18.0g (40mmol) (2,2-diethoxyviny1)triphenylphosphonium tetrafluor~borate[~].
The mixture is allowed to stand overnight; the solvent is then removed
by evaporation and the residue treated with methanol. After
2 to 3 hours the precipitate (yellow crystals) is filtered off
and dried (orange-red crystals). Recrystallization from methylene chloride/methanol (1: 10) affords the boat conformer (6 b )
in the form of shiny-red platelets. The mother liquor is allowed
to stand for 2 to 3 days at room temperature. The precipitated
yellow chair conformer ( 6 a ) is recrystallized as above (yellow
Table I. Physical properties of the conformers ( 6 a ) and ( 6 b j
Yield
[x]
M.p.
rC]
IR (KBr)
[cm-'1
la1
U V ( C 2 H 5 0 H ) 'H-NMR
(60 MHz, CDCla, TMS intern)
(log E )
T
j.max
[nm]
I3C-NMR [el
(25.15 MHz, CDCI,, TMS intern)
6 [PPml
(6a)
9.8
190 [b]
1621; 1592 415;437
(ie 4.58)
9.70 (6H, t); 8.72 (6H, t); 7.1 bis 5.8
(8H, m) [c]: 2.9 bis 0.8(l6H, m)
-154.38: -132.33; -130.72; -129.99; -121.43;
-116.16; -61.19: -59.13: -15.17: -13.92
16b)
52
245
dec.
1613; 1585 450
(4.56)
8.77 (6H, t); 8.60 (6H, t); 6.04 (4H, q),
5.92 (4H, q) [d]; 3.7 bis 1.3 (16H, m)
-155.75; -132.81; -129.75: -128.69: -121.38:
-115.25; -61.64; -59.34; -15.59: -15.26
[a]
[h]
[c]
[d]
[el
Referred to amount of (2,2-d1ethoxyvinyl)triphenylphosphonium
tetrafluoroborate used.
Transformation point of the yellow chair-conformer into the orange-red boat-conformer.
Diasrereotopic methylene protons.
In 1,2-dichlorobenrenethe diastereotopic methylene protons in this conformer also show the expected splitting pattern.
In each case ten of the total of twenty I3C-NMR signals.
186
Angew. Chem. I n t . Ed. Engl. 16 (1977) No. 3
stat
needles on rapid crystallization, yellow platelets on slow crystallization).
dvn
exp
Received: December 22, 1976 [Z 642 IE]
revised: January 4, 1977
German version: Angew. Chem. 89, 184 (1977)
CAS Registry numbers:
(2), 21882-77-9; (31, 84-11-7; ( 4 ) , 61541-07-8; ( 6 ) , 61544-08-9
[l] W D. Ollis, J . F . Stoddart, I . 0 . Sutherland, Tetrahedron 30, 1903 (1974).
[2] P. G. Edgerley, L. E. Sutton, J. Chem. SOC.2951, 1069.
[3] R . Crossley, A . P . Downing, M . NdgrMi, A . Braga de Oliveira, W D.
Ollis, I . 0. Sutherland, J. Chem. SOC.2973, 205.
[4] W Baker, R. Banks, D. R . Lyon, F. G. Mann, J. Chem. SOC.2945, 27.
[5] R. W Saaljrank, Angew. Chem. 86, 162 (1974); Angew. Chem. Int. Ed.
Engl. 23, 143 (1974); Tetrahedron Lett. 2975, 4405.
[6] R . Gompper, Angew. Chem. 81, 348 (1969); Angew. Chem. Int. Ed. Engl.
8 , 312 (1969).
[7] The transformation is reversible.
[8] I am indebted to Priv. Dot. F. Dickerr for stimulating discussions; cf.
L . M . Jackmann in L. M . Jackmann, F. A . Cotton: Dynamic Nuclear
Magnetic Resonace Spectroscopy. Academic Press, New York 1975, p.
203.
[9] H. J . Bestmann, R. W Saaljrank, J . P . Snyder, Chem. Ber. 106,2601 (1973).
Observed and Calculated Electron-Density Distribution
in the Bonds of a Rectangular CyclobutadieneI”]
By Hewnann Irngartinger, Hans-Lothar Hase, Karl- Wilhelm
Schwlte, and Armin Schweig“’
According to the results of photoelectron spectroscopy[’“]
and X-ray structure analysis[1b]the cyclobutadiene derivative
(1 )[“I has a rectangular four-membered ring structure and
a singlet ground state. In order to study the distribution
of the bonding electrons for this structure and this state we
have determined the difference electron
for (I)
from the experimental X-ray data (4615 reflections up to
sin ell. = 0.663 A- ; Mo-K, radiation) and we have theoretically calculated the static (“do~ble-[[~l”
ab initid’] quality)
and dynamic (by a newly developed methodI6l)difference electron densities for cyclobutadiene[’]. Figure 1 shows three sections through each of the resulting densities (stat =static, dyn=dynamic, exp =experimental density). Section A is in the
plane of the four-membered ring, B at right angles through
the center of the double bonds, and C is analogous but through
the center of the single bonds).
As shown by the diagram, the density maxima (and minima)
of the dynamic densities are considerably reduced (by about
70% for the C=C bond, and by about 60% for the C-C
bond) relative to the maxima of the static densities. Therefore
__--.__
, ,--. -....
......._
I
I
I
I...
.
.
Fig. I. Calculated static (stat),dynamic (dyn),and experimental (exp)difference
electron densities in the four-membered ring of the cyclobutadiene system
in three planes: A, section within the plane of the four-membered ring;
B, section perpendicular to the plane of the four-membered ring through
thecenters of the double bonds; and C, section perpendicular to the four-membered ring through the centers of the single bonds. In all cases the contours
indicate differences of 0.04 e/A3. The zero contour is dotted, the positive
(negative) values being shown as solid (dashed) lines. The calculated densities
were obtained for cyclobutadiene and the measured ones for (I). In the
B sections the line of view is parallel to the C=C bonds in the direction
of a C-C bond and vice versa in the C sections.
the calculated dynamic densities are now in general agreement
with the experimentally determined densities. It is particularly
remarkable that both theory and experiment give density
maxima for the four-membered ring compound which lie significantly off the lines joining the C atoms. Thus bent bonds
are to be assumed for the four-membered ring of (1 ), and
also for cyclobutadiene,such as are known for three-membered
rings[’] and have recently also been found for cyclob~tane[~~.
In contrast, the density maxima of the exocyclic C-C bonds
of (I) and C-H bonds of cyclobutadiene lie in the center
of the bond axes. The sections perpendicular to the bonds
clearly show the elongation of the electron distribution in
the 7c bonds, whereas the density distribution in the CT bonds
is axially symmetric. More precise quantitative statements
will require low temperature measurements and extension
of the basis set in the calculations[10).
Received: December 3, 1976;
abridged: December 20, 1977 [Z 636 IE]
German version: Angew. Chem. 89, 194 (1977)
CAS Registry number:
( I ) , 40219-42-9
[*I
Doz. Dr. H. Irngartinger
Institut fur Organische Chemie der Universitat
Im Neuenheimer Feld 270, D-6900 Heidelberg (Germany)
Prof. Dr. A. Schweig, Dr. H.-L. Hase, Dip1.-Chem. K.-W. Schulte
Fachbereich Physikalische Chemie der Universitat
Auf den Lahnbergen, D-3550Marburg (Germany)
[**I This work was supported by the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie. The studies performed at
Heidelberg are part of the series “Electron Density Distribution in
Chemical Bonds”, Part 2.-Part 1: H. Irngartinger, H . 4 . Jiiger, Angew.
Chem. 88, 615 (1976); Angew. Chem. Int. Ed. Engl. 15, 562 (1976).
The work performed at Marburg was part of the investigationsperformed
within the framework of DFG-Sonderforschungsbereich 127 (“Crystal
Structure and Chemical Bonding”), Project H.
Angew. Chem. Int. Ed. Engl. 16 ( 3 9 7 7 ) N o . 3
[l] a) G . Lauer, C. Muller, K.-W Schulte, A . Schweig, A . Krebs, Angew.
Chem. 86, 597 (1974); Angew. Chem. Int. Ed. Engl. 13, 544 (1974);
G . Lauer, C. Muller, K.-W Schulte, A . Schweig, G . Maier, A . Alzbrreca,
&id. 87, 194 (1975) and 14, 172 (1975); resp. b) H. Irngartinger, H.
Rodewald, ibid. 86, 783 (1974) and 13, 740 (1974) resp.; c) H. Kimling,
A. Krebs, ibid. 84, 952 (1972) and 11,932 (1972), resp.
[2] The difference electron density of a molecule is defined as the difference
between the total molecular electron density and the superposed atomic
densities of the atoms of which the model considered is composed.
[3] The experimental difference densities have been averaged over the two
independent molecules (space group P i; Z = 2 ; molecular symmetry
C, (0) and over chemically equivalent regions in accord with Dlh
symmetry.
187
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