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Preparation of 2 4 6-Tri-tert-butylpyrylium Tetrafluoroborate 2 4 6-Tri-tert-butylpyridine and 2 4 6-Tri-tert-butylphosphorin.

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2-en-1-a1 ( l a ) 121, 4-(dimethylamino)but-3-en-2-one (lb) 131,
and methyl 3-(dimethylamino)acrylate (Ic) [41 leads almost
quantitatively t o the hydrobromides (2). which react with
triethylamine t o give the bromine derivatives (3) in yields of
more than 90 % [51. The acetylenes (4) are obtained from (3)
in yields of about 70 % by elimination of HBr with potassium
rert-butoxide [61. Compound (4a), a pale yellow oil, is stable
for long periods only below -5OoC, whereas (4b) and (4c)
can be kept for several hours at room temperature.
( c H ~ ) ~ N - c - c-C -H
(11
-,
+HR
(CH3)2N,
5?
C
, =C H -C - H
R
(21
Received: January 15, 1968
[ Z 713a IE]
German version: Angew. Chem. 80, 443 (1968)
Publication delayed at the authors' request
[*I Prof. Dr. K. Hafner and Dr. M. Neuenschwander
Institut fur Organische Chemie der Technischen Hochschule
61 Darmstadt, Schlossgartenstr. 2 (Germany)
[**I M. N. is grateful to the Schweizerische Siiftung fur Stipendien auf dern Gebiete der Chemie for a grant. We thank Fraulein
M . Ruppert for her skilful experimental assistance.
[l] K. Hafner and J . Schneider, Liebigs Ann. Chem. 624, 37
(1959); K. Hafner and G. Schneider, ibid. 672, 194 (1964); K. Hafner and M . Kreuder, Angew. Chem. 73,657 (1961); K . Hafner, R.
Fleischer, and K. Fritz, Angew. Chem. 77, 42 (1965); Angew.
Chem. internat. Edit. 4, 69 (1965).
[Z] F. Wille and F. Knorre, Chem. Ber. 85, 841 (1952); German
Pat. 944852 (1956), Chem. Abstr. 52, 16300b (1958). We are
grateful to Badische Anilin- und Sodafabrik AG., Ludwigshafen,
for the supply of compound ( l a ) .
[3] E. Benary, Chem. Ber. 63, 1573 (1930).
[4] Propiolic acid after V. WOK Chem. Ber. 86, 735 (1953);
methyl ester by refluxing for two days with 10% conc. H2SO4
in anhydrous methanol; (tc) by addition of dimethylamine to
methyl propiolate in tetrahydrofuran at 20 "C.
[5] Correct analytical data were obtained for all the compounds
described.
[6] Overall yield for all the steps. The acetylenes were purified by
distillation in a bulb tube at 10-4 torr and oven temperatures of
20°C (4a), 40°C (4b), and 30°C (4c). The yields decrease
rapidly at higher temperatures.
A New Rearrangement of Substituted
3-AminopropenalsI* *I
By M . Neuenschwander and K. Hafner [*I
3-(Dimethylamino)prop-2-yn-l-al (1) reacts with an equimolar quantity of HCl in anhydrous tetrahydrofuran at 0 O C
t o give the reactive aldehyde (2),R = CI [I], which rearranges
during isolation t o form isomer-free
trans-3-chloro-N, Ndimethylacrylamide (3a), R = CI (yield 80 %) [31. The strong
band in the UV spectrum of ( 3 ~ )R. = CI, occurs at 216 nm
(n-hexane), a position characteristic of amides of similar
structure, and the AB system of the vinyl protons in the
N M R spectrum occurs at T = 2.77 and 3.32 (J = 1 3 Hz141);
hydrogenation of (3a), R = CI, with Pd black (2OOC in
benzene) yields N,N-dimethylpropionamide. The aldehyde
(2). R = OAc, cannot be isolated in the analogous reaction
of ( 1 ) with glacial acetic acid; the product obtained (80 %
yield) is isomer-free 121 cis-3-acetoxy-N,N-dimethylacrylamide (3b), R = OAc [ N M R in CDC13: T = 2.58/D (J = 7.5
Hz)/l H; T = 4.43/D/1 H ; T = 6.90 and 6.94/2S/6H; T =
7.75/S/3H], which rearranges when heated t o 6OoC to give
the thermodynamically more stable frans isomer (3a),
R = OAc W M R in CDCl3: T = 1.73/D ( J = 1 2 H z ) / l H ;
T = 3.73/D/1 H ; T = 6.93/S/6H; T = 7.79/S/3 HI. Compound
(3b), R = OAc, can be converted into 3-acetoxy-N,N-dimethylpropionamide by hydrogenation(Pd/Hz. 2OoC in benzene).
Compound ( I ) reacts highly stereoselectively 161 with a n equimolar quantity of diethylamine or with a n excess of methanol
or ethanolrsl t o give the aldehydes ( 2 ) , R = N(CzH&,
OCH3, and OC;H5 respectively, which can be isolated since
their tendency toward rearrangement is less pronounced than
that of the adducts of ( I ) with HCl or acetic acid, and
decreases in the order OCH3 > OCzH5 > N(CzH5)z.
460
Whereas only traces of 3-(diethylarnino)-3-(dimethylamino)prop-2-en-1-a1 (2). R = N(CzH5)z. rearrange at 60 t o 80 "C,
3-methoxy-3-(dimethylamino)prop-2-en-l-al
(2),R = OCH3,
bellow oil, yield 90 %. N M R in CDCl3: T = 0.72/D (J =
8Hz)/l H ; T = 5.46/D/l H; T = 6.16/S/3H; T = 7.06/S/6H] is
converted when heated at 60 "C in CHCI3 into the isomeric
methyl truns-3-(dimethylamino)acrylate (41, R = OCH3
157 % yield, m.p. 46-47OC, Amax in CHzC12: 278 nm, log
E = 4.28, N M R in CDCI3: T = 2.57/D (J = 13 Hz)/l H; T =
5.50/D/1 H ; T = 6.35/S/3 H; T = 7.09/5/6H], the constitution
of which was verified by comparison with (4), R = OCH3,
prepared from methyl propiolate and dimethylamine.
The new rearrangement of 3-aminopropenal derivatives (2)
evidently proceeds with high stereoselectivity; a four-membered ring intermediate or a corresponding transition state is
probably involved. The configuration of the aldehyde (2),
which is important for the elucidation of the mechanism 171,
cannot be established from the above results. We are now
proceeding with the further study of this reaction.
Received: January 15. 1968
[Z 713b IE]
German version: Angew. Chem. 80, 444 (1968)
Publication delayed at the authors' request
-.
~~
[*] Dr. M. Neuenschwander and Prof. Dr. K. Hafner
Institut fur Organische Chemie der Technischen Hochschuie
61 Darmstadt, Schlossgartenstr. 2 (Germany)
[**I M . N, is grateful to the Schweizerische Stiftung fur Sripendien auf dem Gebiete der Chemie for a grant. We thank Fraulein
M . Ruppert for her skilful experimental assistance.
[l] The aldehyde (2), R = C1, can be obtained in the impure
state by careful working-up [UV in CHzC12: Amax = 297 nm;
NMR in CDC13:T = 0.45/D/1 H and T = 4.80/D ( J = 7 Hz)/l H;
T = 6.81/S/6 HI.
[2] Purity checked by NMR spectra.
[3] Similarly, the addition of HCl to 3-(N-methylanilino)prop2-yn-1-a1 (NMR in CDCI3: T = 0.73/S/1 H; T approx. 2.75/M/5 H;
T = 6.62/S/3H; IR in CC4: v c = c = 2170 cm-1) leads by
rearrangement to trans-3-chloro-N-methylacrylanilide[NMR in
CDC13: T = 2.3-2.9/M/6H, including T = 2.64/D/(J = 13 Hz);
T = 3.78/D (J = 13 Hz)/l H; T = 6.63/S/3 HI.
141 For coupling constants of similar compounds, cf. E. Winterfeldt and H. Preuss, Chem. Ber. 99, 450 (1966).
[5] Since the reaction rate in dilute equimolar solutions is too
slow, the reactions was carried out in the corresponding alcohol.
[6] According to the NMR spectra, the adducts (Z), R =
N(CzH&, OCH3, OCzH5, are formed with a stereoselectivity of
about 90 %.
[7] For the steric course of additions to C E C bonds, cf. R. Huisgen, B. Giese, and H . Huber, Tetrahedron Letters 1967, 1883
(further references cited); E. Winterfetdt, Angew. Chem. 79, 389
(1967) (further references cited); Angew. Chem. internat. Edit. 6 ,
423 (1967).
Preparation of 2,4,6-Tri-tevt-butylpyrylium
Tetrafluoroborate, 2,4,6-Tri-tert-butylpyridine,
and 2,4,6-Tri-tert-butylphosphorin
By K. Dirnroth and W. Mach[*]
Pyrylium salts substituted by tert-butyl groups in the 2,4,6positions have not previously been known. The synthesis of
such a salt, which we describe here, made possible the prepaAngew. Chem.'internat. Edit. / Vol. 7 (1968) / No. 6
ration of 2,4,6-tri-tert-butyl-pyridine
and 2,4,6-tri-tert-butylphosphorin, the first phosphorin derivative containing only
aliphatic substituents.
We obtained 2,4,6-tri-tert-butylpyrylium tetrafluoroborate
(61 in a total yield of 42% by the route shown in the following
scheme. The methoxy ketone (3) is converted mainly into
the unsaturated ketone (4)when boiled in a basic medium,
whereas it is converted by acids into a cyclic carboxonium
salt [ I ] .
+
t
t
30% sodamide suspension in benzene (0.15 mole of NaNH2).
Compound (5), m.p. 53OC. is formed in 85 % yield; this
is boiled with an equimolar amount of triphenylmethyl
tetrafluoroborate in glacial acetic acid, then treated with
ether, whereupon the salt ( 6 ) crystallizes in 75 % yield. It is
recrystallized from ethylene dichloride/ether. M.p. 21 6 "C.
N M R : 6 = -7.82 ppm (2 H), 6 = -1.46 ppm (9 H). 6 =
-1.53 pprn (18 H).
Independently of us, W. Rundel (Tubingen) has prepared
2,4,6-tri-tert-butylpyrylium
perchlorate by a different
route [41.
[Z 727a I€]
Received: January 2. 1968. Revised: April 22, 1968
German version: Angew. Chem. 80, 489 11968)
+
O
'
[*I Prof. Dr. K. Dimroth and Dip1.-Chem.
W. Mach
Institut fur Organische Chemie der Universitat
355 Marburg, Bahnhofstr. 7 (Germany)
[l] K . Dimroth and W. Mach, Angew. Chem. 80, 490 (1968);
Angew. Chem. Internat. Edit. 7, 462 (1968).
[2] G. Markel, Angew. Chem. 78, 907 (1966); Angew. Chem.
internat. Edit. 5, 846 (1966); K . Dimroth, N. Gref, W. Stade, and
F. W. Sleuber, Angew. Chem. 79, 125 (1967); Angew. Chem.
internat. Edit. 6,711 (1967).
[3] A . T. Balaban and C. D. Nenitzescu, Chem. Bet-. 93,599 (1960).
141 W. Rundel, private communication.
+
Stable Cyclic Carboxonium Salts
2,4,6-Tri-tert-butylpyridine(7). m.p. 69 OC, is formed quantitatively by the action of alcoholic ammonia on the pyrylium
salt (6). N M R : singlet 6 = -7.02 ppm (2 H), 6 = -1.34 ppm
(18 H), and 6 = -1.30 ppm (9 H). The base (7) can be isolated and purified by recrystallization from ethanol/water.
Tri-tert-butylphosphorin (8) [21 is formed in 37 % yield from
(6) and P(CH20H)3; it has m.p. 88OC, and in the N M R
shows 6 = -7.77 ppm (2 H), JP-CH= 6 Hz, doublet 6 =
-1.46 ppm (18 H), JP-CH = 2 Hz, singlet 6 = -1.37 (9 H)
(8) is isolated and purified by recrystallization from methanol/water.
With HzOz in ethanolr31, compound (8) affords a 30% yield
of tert-butyl 3,5-di-tert-butyl-2-furylketone (9), which can
also be obtained from (6) by use of hydroxylammonium
chloride.
Experimental:
Compounds (2) (86 g, 1 mole) and ( I ) (100 g. 1 moIe) are
boiled for 4 h in 10% NaOCH3 solution (540 g, 1 mole of
NaOCH3), whereupon (3) and ( 4 ) are formed in approximately equal amounts in a total yield of about 67 %. To 18.2g
of this mixture and 10 g of ( I ) (0.1 mole) is added 20 g of a
Angew. Chem. internat. Edit.
Vol. 7 (1968)
1 No. 6
By K . Dimroth and W. Mach[*]
Whereas carbonium salts having two ethereal oxygen atoms
in the or-position are known to exist as stable carboxonium
salts [I], the corresponding cyclic carboxonium salts with only
one ethereal oxygen in the or-position have hitherto only been
detected kinetically or by N M R measurements at low temperature 121.
We have now succeeded in isolating the carboxonium salt
(6). whose stability can be ascribed to the formation of a
heterocyclic five-membered ring. There are three ways in
which this salt can be synthesized: a) by cleavage of methoxidefrom5-methoxy-2,2,6,6-tetramethyl-3-heptanone
( I ) [31;
b) by addition of a proton to 2,2,6,6-tetramethyl-3-hepten-5one (2) [31; and c) by addition of a proton to 2-tert-butyl4,5,5-trimethyl-4,5-dihydrofuran
(3).
In methods a) and b) the formation of (6) takes place on
boiling the reactants with a 5 0 % solution of water-free
HBF4 in 1,2-dichloroethane; the carbonium ion ( 4 ) is
probably formed first, and migration of a methyl group leads
to the formation of ( 5 ) and subsequently to ( 6 ) . Compound
( 6 ) can be isolated by precipitation with ether (yield 75 %).
In method c) the cyclic carboxonium salt ( 6 ) is formed
immediately, in quantitative yield, o n addition of ethereal
HBF4 in the cold.
46 1
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preparation, butylpyrylium, tetrafluoroborat, tri, butylphosphorin, butylpyridine, tert
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