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Aryl-Substituted Dialkoxycarbonium Ions as Alkylating Agents.

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For instance, the salts (10)-(14) cannot be obtained by reaction of nucleophiles with trialkyloxonium salts [41.
The reaction is best carried out by using the unsubstituted dialkoxycarbonium salts that are readily accessible from orthoformic esters [31, and it is convenient to use these as the wellcrystalline hexachloroantimonates, e . g . ( I n ) . The substance
to be alkylated is added in slight excess to a solution of this
salt in CH2C12. The products crystallize immediately [e. g. (11)
and (IZ)] or overnight [ e . g . (7) and (S)] or can be precipitated by CC14.
Dialkoxycarbonium salts derived from higher carboxylic
esters react appreciably more slowly, e. g . (Ib), and often give
poorer yields and less pure products 151.
Received: June 6th, 1966
[ Z 259 IE]
German version: Angew. C h e m . 75,714 (1966)
[ I ] H. Meerwein, K. Bodenbenner, P. Borner, F. Kunert, and K.
Wimderlich, Liebigs Ann. Chem. 632, 38 (1960).
[2] S. Hiinig, Angew. Chem. 76, 400 (1964); Angcw. Chcm. internat. Edit. 3, 548 (1964).
[3] The alkylation of ethers was already observed by H . Meerwein, Angew. Chcm. 72, 927 (1960); cf. also [4], p. 341.
[4] H. Meerwein in Houben- IVeyl: Methoden der organischen
Chemie. Thieme Verlag, Stuttgart 1965, Vol. V1/3, p. 329.
[ 5 ] Aryl-substituted dialkoxycarbonium salts also have alkylating
properties: Angew. Chem. 78,714 (1966); Angew. Chem. internat.
Edit 5 , 676 (1966).
However, under the same conditions pyrocatechol gives no
detectable amount of its mono- or di-methyl ether with the
salt ( I c ) but instead adds to it; the product, which is at once
precipitated, yields o-hydroxyphenyl benzoate on addition of
water. If it is desired to add both O H groups of pyrocatechol
to ( I c ) , giving methyl o-phenylene orthobenzoate, then it is
necessary to treat pyrocatechol with trimethyl orthobenzoate
and a catalytic amount of a strong acid, such as 2-hydroxy5-sulfobenzoic acid, the reaction then occurring vin ( I c ) [31.
The salt ( I d ) is readily obtained crystalline from trimethyl
2,4,6-trimethylorthobenzoateand anhydrous HBF4 in ether
(93 % yield); for steric reasons the cation thereof no longer
gives addition reactions but merely methylates the pyrocatechol to guaiacol (83 % yield).
Compound ( I c ) is the first containing a reactive ambident
cation for which the normally favored kinetically controlled
addition is linked, owing to the high activation energy due to
steric hindrance, with a thermodynamically favored alkylation [41.
(c)
b
+
c
+
(Id
(b)
a: 2 . 5 2 p p m (S)
d : 6 . 9 5 p p m (M)
(b)
(c)
b
+
c
+
a: 4 . 2 3 p p m (S)
d : 7.48ppm (M)
Aryl-Substituted Dialkoxycarbonium Ions as
Alkylating Agents
By Prof. K. Dimroth and P. Heinrich
BF,@
Institut fur Organische Chemie, Universitat Marburg
(Germany)
Dialkoxycarbonium ions (1) can react with nucleophiles
(:Nu) either by addition, to give saturated 1,l-dialkoxy
compounds (Z), or by alkylation of the nucleophile, to give
compounds (3) and carboxylic esters (4) [ I 3 la].
a: 2.62ppm (S)
b: 6.39ppm (S)
c: 2 . 0 2 p p m (S)
d : 1 . 7 7 p p m (S)
a: 4.23ppm (S)
b: 6.71ppm (S)
c:}l.88ppm
d:
(S)
N M R spectra of (fc) and ( I d ) in liquid SO2 at -10 "C (TMS
as external standard) show equal chemical shifts for the OCH3
protons in ( I c ) and ( I d ) ; the polarization of the OCH3
(3)
( 4 ) 'OK'
R' : CH3, CzH,;
(a), R : H;
( b ) , R : CH3; It' : CH3, CzH,;
( c ) , R : CSH,;
groups should therefore also be about equal. Studies with
para-substituted cations analogous to ( I c ) are in progress.
R': CH3;
fd), R : 2,4,6-(CH3)3C~Hz;R' : CH3;
Xo : SbCk'
Xo : SbCl,'
xo : BFP
xo : B F , ~
Independently of KabussL21, who worked with ( l a ) and (Ib),
we have found that the aryl-substituted dimethoxycarbonium
salts ( I c ) (m.p. 88-90 'C[lal) and ( I d ) (m.p. 89-91 "C) are
powerful alkylating agents. For instance, on addition of 2,6dimethyl-4-pyrone to a solution of the salt ( f c ) or (Id) in
CzH4C12 at room temperature, 4-methoxy-2,6-dimethylpyrylium fluoroborate is at once precipitated in ca. 60%
yield.
676
Received: June 20th, 1966
[Z 271 IE]
German version: Angew. Chem. 78, 715 (1966)
[ l ] H . Meerwein P . Borner, 0 . Fuchs, H. J . Sosss, H. Schrodt,
and J . Spille, Chem. Ber. 89, 2060 (1956); H . Meerwsin in
Houben-Weyl: Methoden der organischen Chemie. 4th Ed.,
Thieme, Stuttgart 1965, Vol. VIi3, pp. 299, 329.
[la] H . Meerwein, K. Bdenbenrier, P. B x n e r , F. Kunert, and
K. Wimderlich, Liebigs Ann. Chem. 632, 38 (1960).
[2] S . Kabuss, Angew. Chem. 78, 714 (1966); Angew. Chern.
internat. Edit. 5, 675 (1966).
[3] K. Dimroth, P . Heinrich, and K . Schronm, Angew. Chcrn.
77, 863 (1965); Angew. Chem. internat. Edit. 4, 873 (1965).
[4] S. Hiinig, Angew. Chem. 76, 400 (1964); Angcw. Chem.
internat. Edit. 3, 548 (1964).
A n g e w . Chem. internat. Edit. { Vol. 5 (1966) I No. 7
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dialkoxycarbonium, alkylation, agenti, ions, substituted, aryl
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