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Metal Complexes of Bredt Olefins Synthesis and Structure of Bicyclo[4.2.1]non-1(8)-enebis(triphenylphosphane)platinum(0)

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[4] L. K. Hansen, A. Horduik. L. J. Saethre, J. Chem. SOC.Chern. Commun.
1972, 222.
[S] A . Horduik, Acta Chem. Scand. 25, 1583 (1971).
161 D. L. Whenuer, Inorg. Chem. 9. 2250 (1970); M . A. Bennett. R. N. Johnron,
G. B Robertson, I . B. Tomkins, P. 0. W i m p , J. Am. Chem. SOC.98, 3514
(1976): J. Dehaud, J. Fischer, M. Pfefler, A. Mitschler, M. Zinius, Inorg.
Chem. IS, 2675 (1976): P. M. Maitlis: The Organic Chemistry of Palladium.
Vol. I. Academic Press, New York 1971, p. 38 and references cited therein.
[7) G. W . Parshall, ACC.Chem. Res. 3, 139 (1970); S . Trofimenko, Inorg. Chem.
12, 1215 (1973); H:P. Abicht, K. Issleib, Z . Chem. 17, l(1977); M.I. Bruce,
Angew. Chem. 89, 75 (1977); Angew. Chem. lnt. Ed. Engl. 16, 73 (1977).
[El H. Alper, J. Organomet. Chem. 61, C 62 (1973).
191 The mixture was boiled under reflux for 2 h and then stirred for 20 h at
20°C.
[lo] We thank Ursuln Wilcrok for experimental assistance.
-@?\a /
P1
U
Metal Complexes of Bredt Olefins: Synthesis and
Structure of Bicyclo[4.2.l]non-l(8)-enebis(triphenylphosphane)platinum(o)[**I
By Erich Starnrn, Konrad B. Becker, Peter Engel, Otto Ermer,
and Reinhart Keese[']
Bredt olefins['l derived from trans-cyclohexene or trans-cycloheptene'21 have been detected as labile intermediate^'^],
yet they have never been studied in detail. Because of their
unusual structural and chemical properties we decided to
check whether Bredt olefins can be reversibly stabilized with
transition-metal
We have investigated the
reaction of ethylenebis(triphenylphosphane)platinum(o) (1)
with bicyclo[4.2.l]non-l-(8)-ene (2), a Bredt olefin of the
trans-cyclooctene series stable at room temperature, to give
the complex (3) (see Procedure).
The structure of (3) was determined by X-ray crystallographylsl (Fig. 1). The bridgehead double bond is bound on the
ex0 side to the bis(tripheny1phosphane)platinum.The platinum atom is almost planar coordinated; the angle between
the P1 -Pt-P2
and C1-Pt-C8
planes is 6" (cf. [6,4c1). The
most important bond lengths are: Pt-Pl*= 2.30,
Pt -P2=2.26, Pt--Cl=2.16 and Pt -C8=2.11 A. oThe
length of the bridgehead double bond C1- C8 (1.41 A) is
comparable with that observed in other complexed olefins of
this type (e.g. (1): 1.43 A)[6a1.
The conformation of bicyclo[4.2.l]non-l(8)-ene in the
crystalline platinum complex (3) corresponds to the energetically preferred conformation['] of the free olefin (2) as ob-
~
[*I Prof. Dr. R. Keese [+I, Dr. E. Stamm
lnstitut fur organische Chemie der Universitat
Freiestrasse 3, CH-3012 Bern (Switzerland)
Priv.-Doz. Dr. P. Engel
Mineralogisch-petrographischeslnslitut der Universitat
CH-3012 Bern (Switzerland)
Priv.-Doz. Dr. K. B. Becker
lnstitut fur organische Chemie der Universitat
CH-4056 Basel (Switzerland)
Dr. 0. Ermer
Abteilung fur Chemie der Universitat
D-4630 Bochum (Germany)
I '1
Fig. 1. a) Structure of bicyclo[4.2.1]non-l(8)-enebis(triphenylphosphane)platinum(o) (3) (phenyl groups not shown): b) torsion angles ["I in bicyclo[4.2.l]non1(8)-ene (2). Upper values: measured in complex (3); lower values: calculated 171
for the free olefin (2).
tained by force field calculations. In Figure 1 the doublebond geometry observed in the crystalline complex (3) is
compared with that calculated for the free olefin. As can be
expected from the back-bonding influence of platinum(0) on
the olefinic double bond, the torsional angle
C2-C1=C8--C7
in the complex (3) (124") is smaller than
the value calculated for the free olefin (2) (136.5"). The increased pyramidal character at the olefinic bridgehead, i. e.
the angle between the C1, C2, C8 and C1, C8, C9 planes
("out-of-plane bending" parameter, see ['I) is also in accord
with this observation: The value found for this angle in (3)
(39") is greater than the value (33.3") calculated for the corresponding angle in (2). X-ray structure analysis is not accurate enough to provide more detailed information.
The complex (3) is stable at room temperature; surprisingly however, at higher temperatures in solution a mixture of
four isomeric bicyclo[4.2.l]nonenes is formed[*I.Isomer-free
bicyclo[4.2.l]non-l(8)-eneis obtained from (3) in toluene by
addition of carbon disulfide or tetracyanoethylene (TCNE).
Further experiments have revealed that the stability of complexes of the general formula [(C6H5)3P]2Pt--Lin solution
increases along the series L=H2C=CH2 < (C6H5),P < O2
< (2) < CS2 2 TCNE.
Procedure
To a solution of ( I ) (0.56 g, 0.75 mmol) in toluene (30 ml)
is added (2) (0.095 g, 0.75 mmol) at -60"C[9]. After 1 hour
the mixture is allowed to warm to 0°C and then concentrated in uacuo. The residue is dissolved in 1 :1 ethyl acetate/
dichloromethane (ca. 3 ml) and chromatographed at - 10 "C
on 60 g aluminum oxide (Camag, neutral, activity I). Olefins
are eluted with hexane, and the complex (3) with hexane/
ethyl acetate (1 :1). The main fraction is concentrated in uaCUO at 0 "C; the residue crystallizes spontaneously from ether
at 0 "C. A total of 0.328 g (52%)of (3) is obtained as pale yellow crystals["'.
Author to whom correspondence should be addressed.
[**I Delivered in part in a lecture at the Conference of the Schweizerische
Chemische Gesellschaft, Geneva, October 9, 1976.-This work was supported by
the Schweizerischer Nationalfonds zur Forderung der wissenschaftlichen Forschung (Projects 2.103-0.74, 2.560-0.76 and 2.627-0.76).
Angeu. Chem. I n t . Ed. Engl. 18 (197Y) No. 9
Received June 8, 1979 (2278 IEl
German version Angew Chem Y I, 746 (1 979)
CAS Registry numbers
( I ) , 12120-15-9 (2), 23057-35-4, (3), 71302-37-9
0 Verlag Chemie, GmbH, 6Y40 Weinheim.
1Y7Y
0570-0833/79/0909-0685
$ 02.50/0
685
~~~
[I] For the definition of "Bredt olefin" and "bridgehead olefin" cf. [3], p. 568
and 528, respectively, footnote [***I.
[21 a) J. Bredt, Ann. Acad. Sci. Fenn. 29, 2, 15 (1927); b) J. R. Wiseman, W. A .
Pletcher, J. Am. Chem. SOC.92, 956 (1970).
I31 R. Keese, Angew. Chem. 87, 568 (1975); Angew. Chem. Int. Ed. Engl. 14,
528 (1975).
141 a) Inter a h , (2 +4J-cycloadd1tions are also suitable for reversible stabilization; however, furan adducts of I-norbornene are stable up to 260 "C: R.
Keese, E. P. Krebs, Angew. Chem. 83, 254 (1971); 84, 540 (1972); Angew.
Chem. Int. Ed. Engl. to, 262 (1971); 11,518 (1972); E. P. Krebs, Dissertation
ETH No. 4994, Zurich 1972; b) for similar examples cf. M. E. Jason. J. A.
McGinnety, K. B. Wiberg, J. Am. Chem. SOC.96, 6531 (1974); c) M.E. Juson, J. A. McGinnery, Inorg. Chem. 14, 3025 (1975); d) Fe(C0)4 complex of
hicyclo[4.3.1Jdeca-1(9),7-diene:M . u. Biiren, H. J. Hansen, Helv. Chim.
Acta 60, 2717 (1977).
151 /3), triclinic, space group PI: d,= 1.46, d , = 1.53 g/cm': Z = 2 ; a= 11.34(1),
b = 11.90(1), c=16.14(1)
a=79.4(1), p=70.4(1), y=67.2(1)". R=9.5%
for 6280 reflections (isotropic refinement). Standard deviation for bond
lengths 0.1
for bond angles 5'. The unit cell contains two independent
molecules of (3); the structural parameters of one molecule are unreliable
(possibly due to disorder) and are not considered here.
161 a) P. T. Cheng, S.C. Nyburg, Can. J. Chem. 50, 912 (1972); b) J. J. de Boer,
D. Bright, J. Chem. SOC.Dalton Trans. 1975, 662.
[7] 0.Ermer, Z. Naturforsch. B 32, 837 (1977).
(81 E. Slamm, K. B. Becker. P. Engel, R. Keese, Helv. Chim. Acta, in press.
191 K. B. Becker, Helv. Chim. Acta 60, 82 (1977).
[lo] M.p. = 137-141 "C (dec.); Rr (aluminum oxide, ethyl acetate/hexane,
1: 1)=0.8; 'H-NMR (D,-toluene, 80 MHz, -2O"C, TMS): S=0.95-3.4
(ca. 14H), 7.12 (s, 18H), 7.68 (12H), contains a small amount ofether; "CNMR (25.2 MHz, - 2 0 ° C TMS internal): 6=25.9, 28.9, 36.4 (ZC), 39.1,
41.2, 47.2, 63.6, 70.4, signals of the ChHs groups overlapping with &toluene signals; "P-NMR (40.5 MHz, - 2 0 ° C (CH,O),P external): S = 107.7
[Pa; J(P, Pt) = 3503 Hz, J(P, Pi,) = 54.4 Hz], 108.4 [Pb; J(Pb Pt)= 3292
Hz]; MS:m/e=262, 183, 154, 122, 93, 80 (loo%),79, 78, and ( l o x amplified) 841 (M+), 719, 457.
?
T pt-0-C-NH-CH<
/
/
T pt-OH
A
(3)
-
\
=
OOH
Toc-Amino acid
9
T pt-0-e -NH( C H2)4-C H-C OOH
I
NH-Z
14) = Z - L y s ( T o c )
Z-NH-C H-COOTpt
A
A,
( 5 ) = Z-Amino acid T p t e s t e r
A,
Solubilizing Acid-Labile Peptide Protecting Groups
By Hermann Anzinger, Manfred Mutter, and Ernst Bayer"'
A central problem in the synthesis of polypeptides is the
sparing solubility of larger peptides". 21. Particularly valuable
as solubilizing protecting groupsI2 'I are, e. g., polyethylene
glycol residues, which increase the solubility both in organic
as well as in aqueous media'3.51.
For general application as amino acid protecting groups
these solubilizing residues must be so modified that they can
be cleaved off under the normal conditions of peptide synthesis. We describe here the first 0-hydroxyalkylation of a
primary alcohol to a tertiary alcohol [(2)] using triethyleneglycol monomethyl ether (1) as example. Acid-labile solubilizing protecting groups of the tert-butylcarboxyl and tert-butyloxycarbonyl type thus become accessible.
We synthesized (2) by addition of (1) to methyl vinyl ketone and subsequent Grignard reaction with CH3MgIr6'.Direct reaction of (1) with e p o x i d e ~ ~was
~ . ~unsatisfactory.
'
C H3
I
CH,-(OC HzCH,),-OCHzCH2-C-OH
I
C H3
(2) = Tpt-OH
CH3-( OCHzCH,),-OH
(1)
The introduction of the Tpt -0-CO
group ("Toc"
group)['' as acid-labile a-amino protecting group as in (3) or
€-amino protecting group of lysine as in (4) was achieved in
high yields via the mixed carbonates[*'. The reaction of N protected amino acids with (2) leading to the acid-labile Tpt
esters (5) was accompanied by considerable racemization.
Investigations on model peptides showed that the Toc and
Tpt groups are just about as acid-labile as the Boc and t-Bu
groups, respectively. The new protecting group has already
proven useful, especially in conformational studies['].
['I
['I.
Dipl.-Chem. H. Anzinger, Prof. Dr. E. Bayer
Institut fur Organische Chemie der Universitat
Auf der Morgenstelle 18, D-7400 Tubingen (Germany)
Author to whom correspondence should be addressed
[*] Prof. Dr. M. Mutter
686
0 Verlag Chemie, GmbH, 6940 Weinheim. 1979
Procedure
Synthesis of (2): (1) (8.2 g, 50 mmol) is treated with
NaOCH3 (81 mg, 1.5 mmol) and, after warming to 35"C,
methyl vinyl ketone (3.5 g, 50 mmol) is added dropwise to
the stirred mixture such that the temperature remains constant. Stirring is continued for a further 2 h at 35 "C and the
mixture neutralized with glacial acetic acid. The ketone is
purified by column chromatography (silica gel); yield 4.8 g
(41%) as colorless oil (characterization: 'H-NMR).-A solution of the ketone (4.7 g, 20 mmol) in ether (40 ml) is treated
with 40 mmol CH3MgI. The resulting precipitate is filtered
o f fand hydrolyzed with ice/NH4C1. The aqueous solution is
washed with ether and then evaporated to dryness. The residue is digested with CH2C12/acetone(1/1) and filtered; the
filtrate is dried over KzC03, concentrated by evaporation,
and purified by column chromatography (silica gel). 2.6 g
(52%) of (2) is obtained as a colorless oil (characterization:
'H-NMR).
Synthesis of (3), R=CH3: L-alanine (360 mg, 4.05 mmol)
is dissolved in a 40% alcoholic solution of Triton B. After removal of solvent by evaporation the residue is stripped of
water by azeotropic distillation with 2 x 10 ml dimethylformamide. The residue is taken up in 12 ml dimethylformamide, treated with 1.5 g Tpt-phenyl carbonate (4.05 mmol) and
stirred for 3 h at 50°C. The mixture is added to ice-water
and washed three times with ether. The temperature is kept
at 0 "C and the pH is adjusted to 2-3 with citric acid. The
mixture is then saturated with NaC1, exhaustively extracted
with CH2CI2,and the combined extracts washed with NaCl
solution, dried and evaporated down. The oil thus obtained
is thin-layer chromatographically pure; yield 1.09 g (74%).
Racemization test: < 1% D-component (characterization: 'HNMR, elemental analysis).
Synthesis of (S), R = H: 636 mg (3.04 mmol) of 2-glycine,
760 mg (3.04 mmol) of (2), and 37 mg of 4-dimethylaminopyridine are dissolved in 5 ml of CH2C12. The solution is
cooled to 0°C and 3 mmol of dicyclohexylcarbodiimide
(DCC) is added dropwise with stirring. After continued stirring for 0.5 h at 0 "C and 4 h at 25 "C, the mixture is filtered,
the filtrate diluted with CH2C12,washed with dilute acetic
acid (saturated with NaCI) and 5% NaHC03 solution (saturated with NaCI), dried, and evaporated down. Subsequent
column chromatography (silica gel) affords 360 mg (27%) of
(5), R = H, as a colorless oil (characterization: 'H-NMR, elemental analysis).
Received April 9, 1979 [Z 279 IE]
German version Angew Chem 91, 747 (1979)
[ I ] E Wunsch, Angew. Chem. X3, 773 (1971); Angew. Chem. Int. Ed. Engl. /0,
786 (1971).
[21 A . Hubbuch, W Danko, H. Zahn in M. Goodman, J. Meienhofer: Peptides.
Proceedings of the 5th American Peptide Symp. Wiley, New York 1977, p.
540.
0570-0833/79/0909-0686
$ 02.50/0
Angew'. Chem. Inr. Ed. Engl. 18 (1979) No. 9
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triphenylphosphate, platinum, structure, synthesis, bicycle, metali, olefin, non, complexes, enebis, bred
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