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Conformations of the Zwitterionic Intermediates from Tetracyanoethylene and Alkyl Propenyl Ethers.

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Table 2. Rate constants of the reactions of ( 1 ) with enol ethers in alcohols
(to give dialkyl acetals) and in acetonitrile (to give 2+2-cycloadducts) at
25 c.
Enol ether
Solvent
102 k >
[I.rn~l~~s-']
3,4-Dihydro-ZH-pyran
Methanol
Ethanol
Acetonitrile
Ethanol
Acetonitrile
Ethanol
Acetonitrile
Methanol
Ethanol
Acetonitrile
0.38
0.21
0.70
0 93
50
0.79
41
33
21
63
Methyl c is-propenyl ether
Methyl trans-propenyl ether
Ethyl I-isobutenyl ether
As a result of kinetic control, tetracyanoethylene ( I ) and
enol ethers in alcohol afford the acetal as trapping product
and the 2 2-cycloadduct side by side. The relative rate constants of the reactions of the intermediate of type (2), i.r.
k A [Alcohol]/k~,,~i,., depend on the structure of the enol ether
and of the alcohol; for the cis-form (trans-form) of methyl
propenyl ether at 0°C the following values of the ratio acetal/
cyclobutane are found: in methanol 12.3 (2.3), in ethanol
5.3 (2.3), in 2-propanol 4.6 (1.4) and in tert-amyl alcohol 1.4
(0.59).
( 8 ) , respectively, with tetracyanoethylene (TCNE)"], leads
to the acetals ( 4 ) and ( 7 ) which contain two chiral centers
(C-1 and C-2). Surprisingly, the addition of ethanol occurs
stereospecifically. With ethanol at 0 ' C the cis-propenyl ether
( 1 ) gives 84% of the acetals ( 4 ) and ( 7 ) [ I J in the ratio
18: I together with 16% of the cis-cyclobutane ( 2 ) (Scheme
1).The reaction is complete in 15 min ;NMR analysis identifies
the products of the kinetically controlled reaction.
H
(1)
H
'.C-C,'
H3C'
OCH,
+
(2)
cis
f3)
(4)
Ib
J) 2
H
Received. October 25, 1973 [Z 946a I€]
German version: Angew. Chem. 86,47 (1974)
H,
,OCII3
(CN)
c=c( cN) z
[ I ] R. Huisqm and G. Steiner. J. Amer. Chem. Soc. 95, 5054 (1973). cf.
the experiments with ( 1 ) and cis-anethol- P. D. Bartlrtr, Quart. Rev. Chem.
Soc. 24, 473 (1970).
123 R. Hiiisgen and G. Strinrr, J. Amer. Chem SOC. 95, 5055 (1973).
[3] G. Srciner and R. Huisgtw, J. Amer. Chem. SOC.95, 5056 (1973).
[4] R. Hoisgen and G. Strinrr, Tetrahedron Lett. 1973, 3763.
Scheme I
[5] G. Strinpr and R. H i r i s g m , Tetrahedron Lett. 1973, 3769.
O n the other hand, the frans-propenyl ether (8) combines
with TCNE in ethanol to give preferentially the acetal (7)
[ ( 4 ) :( 7 ) = 1 :171; there is also formed 30% of the 2 + 2-cycloadducts, these being composed of 29% of the trans-cyclobutane ( 5 ) and = 1 YOof the cis-isomer (2) (Scheme I). The
methyl and ethyl groups of the enol ether and the alcohol
can be exchanged: now TCNE in methanol reacts with the
ethyl cis-propenyl ether (9) to give the acetals ( 4 ) and ( 7 )
in the proportion I :29, while the trans-enol ether (10) gives
rise to ( 4 ) and ( 7 ) in a 13: 1 ratio. Reaction of the cis-zwitter-
[6] F. K . Fkischmunn and H . Krlm, Tetrahedron Lett. 1973, 3773
[7] J . K . Williams, D. W Wilry, and 5. C. M c K ~ r s i c k ,J. Amer. Chem.
Soc. 84, 2210 (1962).
[8] K . Dimroth, C . Rrwhurdr, 7: Siepmann, and F . Bohlmann, Liebigs Ann.
Chem. 661, I (1963): C. Reichardt. Ldsungsmitteleffekte in dcr organischen
Chemie. Verlag Chemie, Weinheim 1969.
[9] Replacements by piperidine in p-halonitrobenzenes: H . Snhr, Chem. Ber.
97, 3277 (1964).
[ 101 Addition constants of secondary amines t o acetylenecarboxylic esters:
B. Cirse and R. H u i s g m , Tetrahedron Lett. IY67, 1889.
Table I . Reactions of alkyl cis- and trans-propenyl ethers with TCNE in alcohols and alcoholysis of the cyclobutanes.
~ _ _ _ _ _ _ ~ _ _ _ _ ~ _ - - ~-. - ~
~
Reaction
Acetals [",;I
14,
li,
Cyclobiitanes
/41:171
cis
I",,]
I1 C1ll.S
~
(i)
(8)
(9)
110)
(2)
( I / )
(12)
+ T C N E in ethanol
+ TCNE in ethanol
+ T C N E in methanol
+ T C N E in methanol
+ ethanol
+ methanol
+ methanol
Conformations of the Zwitterionic Intermediates from
Tetracyanoethylene and Alkyl Propenyl Ethers
By Rolf Huisgen, Reinhard Schug, and Gerd Ste.iner[*]
The addition of ethanol to the zwitterions ( 3 ) and (6) that
are formed from methyl cis- ( 1 ) and trans-propenyl ether
I'[
Prof. R. Huisgen, DipLChem. R. Schug, and Dr. G. Steiner
Institut fur Organische Chemie der Universitit
X Munchcn 2, Karlstr. 23 (Germany)
Angew. Chum. inrurnor. Edit. j Vol. 13 (1974) j No. I
79.6
4
3
58.6
98
I
98
4.4
66
86
4.4
2
99
2
18: 1
1.17
1.29
13: 1
49: I
I :99
49: I
=
16 ( 2 )
I(2)
lo(//)
< 1 (11)
< 1(5)
29 (5)
% I(l2)
37 ( 1 2 )
ions of type ( 3 ) with alcohols is 5-8 times faster than the
cyclization to (2). This preference for formation of acetals
rather than of cyclobutanes is less for the trans-zwitterions of
type ( 6 ) , the ratios being 2.3 and 1.7, respectively (Table 1).
If the zwitterions from the enol ethers (1) and (8) with
TCNE were formed in random conformations, the stereospecificity should be small and the reaction in alcohol would be
expected to afford the diastereoisomeric acetals ( 4 ) and (7)
in approximately identical ratios. From the high stereospecificity of the acetal formation it must be concluded that most
81
(9)
\
densationofaikyl iodidewith Grignard reagent in the presence
of dilithium tetrachlorocuprate as catalyst produced yields
which were hardly satisfactory (63% of decane) in view of
our actual aim of preparing optically active dipropylcycloprostarting from (R,R)-bis(hydroxymethy1)cyclopane (2)
propane ( I ) (X = OH).
/
c=c
H3C‘
’OC&
of the zwitterions arise in the ‘U’-conformations (3) and
(6), i.e., in the state where the Coulomb potential of the
charge centers is lowest; a donor-acceptor interaction between
the electrophilic and nucleophilic centers may contribute to
the stabilization of the spatial arrangement.
In acetonitrile the zwitterions (3) and (6) undergo cyclization
to (2) and ( 5 ) and cis-trans-isomerization by rotation in
the proportions f :5 and 1 :4, respectively, which correspond
to 16% and 20% of non-stereospecifically formed cyclobutanes”].
The stereospecificity of acetai formation in alcohol is higher.
Clearly a configuration-stabilizing interaction of the zwitterion
and alcohol occurs at an early stage. The concurring 2 f2-cycloaddition in alcohol is strikingly more stereospecific than
the one in acetonitrile. In alcohol the ring closure to the
cyclobutane has a chance only if the zwitterions are formed
in the conformations (3) and (6) that are ideal for cyclization.
Some “badly oriented” zwitterions, nevertheless, are formed
and are trapped by the alcohol as shown by comparison
with the 98-99% stereospecificity of acetal formation on
ethanolysis of the pure methoxycyclobutane (2) and on methanolysis of the ethoxycyclobutanes ( 1 1 ) and (12) (Table 1).
The comparatively slow ring opening of the cyclobutanes“]
produces almost wholly “ideal” zwitterions whose reaction
with alcohol is highly stereospecific.
Our attempts to improve the method were guided by the
following considerations:
I ) Copper(1) compounds13] prefer tetragonal coordination.
Cuprates of the type MCuXRC4.51 (M = MgX, Li, etc. ; X = halide, alkoxide, or organic group) acquire the stable tetracoordination by self-association or by association with another
organometallic species that may be present in excess. It is
important that the mixed complex[61 expected in our case,
i.e. (3), be allowed to form without interference.
2) The role of initiator is assumed by the monomeric cuprate
or the cation i 4 ) derived therefrom which, as an electron-deficient compound, can undergo carbene-like ((6)) insertion into
R‘MgBr; BrMgCuBrR‘
4
L
R-R’
[MgOTos]
(3)
a CX bond (e.g. to give ( 5 ) ) . Insertion should be facilitated
by leaving groups which, like p-toluenesulfonates ( 7 ) , are
capable of 1,4-interactions.
The configuration of the acetals ( 4 ) and (7) must be proved
by X-ray structure analysis. As shown in Scheme 1 and in
(13) we favor selective attack of the alcohol on the carboniumoxonium center “from above” rather than reaction via the
hydrogen bonded chelate ( 1 4 ) which should lead to compounds that are epimeric at C-1.
Received: October 25,1973 [Z 946b IEJ
German version: Angew. Chem. 86.48 (1974)
[ I ] For isolation and determination of structure of ( 4 ) and (7) see R.
Huisgen, R. Schug, and G. Sreiner, Angew. Chem. 86, 47 (1974); Angew.
Chem. internat. Edit. 13, 80 (1974).
[Z] R. H u i s g m and G. Srrinvr, J. Amcr. Chem. SOC. 95. M54, 5055 (3973).
Improved Carbon-Carbon Linking
by Controlled Copper Catalysis[**J
By Gerd Fouquet and Manfred Schlosser[*l
During a search for the simplest and most effective pathway
from I-octanol to decane most of the conventional C-C
linking reactions failed. Even a new variant“] involving con[*] Prof. Dr. M. Schlosser and Dr. G. Fouquet
Institut de Chimie Organique de I’UniversitG
Rue de la Barre 2, CH-100s Lausanne (Switzerland)
This work was supported by the Schweizerischer Nationalfonds (Project
No. 2.593.71). Thanks are also due to BASF, Ludwigshafen, for supplying
starting materials.
[**I
82
As shown in Table 1, a “catalyst breeding” step (mixing of
Grignard reagent and copper catalyst at -78°C followed
by gradual warming to room temperature, below which no
condensation occurs) and use of p-toluenesulfonates instead
of iodides or even bromides leads to a striking improvement.
The Grignard component can be varied at will. In contrast,
only primary p-toluenesulfonates react well, while secondary
and tertiary ones give poor yields of product -if any-presumably owing to steric hindrance of insertion. The smooth
reaction of (1) (X=OTos) to give (2) also confirms that
no carbocationic, -radical, or -anionic intermediates are
involved.
Surprisingly, it also proved possible to replace acetate
groups[’ occupying allylic and benzylic positions by the
hydrocarbon moiety of Grignard compounds, apparently
because acetates can likewise exhibit a chelating action. Thus
trans-crotyl acetate was converted regio- and stereoselectively
into trans-2-octene (8) (88 %).
Angew. Chem. intrmnat. Edit.
1 Vol. 13 (1974) i
No. I
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