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Vinamidines and Vinamidinium SaltsЧExamples of Stabilized Push-Pull Alkenes.

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[I91 Th. G. F.
Schooit
and G . Rieber, Angew. Makromol. Chem. 23, 43
( 1 972).
[20] D. Braioi, J. Polym. Sci. Symposium No. 50, 149 (1975).
[21] Ju. B. GrehenSfikou, V. I . Irtak, L. I . Kuzub, P . P . KuSf, and N. S.
Eiiikolopjan, Dokl. Akad. Nauk SSSR, Ser. Khim. 210, I124 (1973).
[22] L. Minnema and A. J . Staverman, J. Polym. Sci. 29, 281 (1958).
[23] M. Wesslau, Angew. Makromol. Chem. I, 56 (1967).
1241 K . DuSek, Collect. Czech. Chem. Commun. 34, 1891 (1969).
[2S] W E. Gihhs and J . M . Barton in G . E. H u m : Vinyl Polymerization.
Dekker, New York 1967. Part I . p. 59.
[26] D. Braurt and W Brendlein, Makromol. Chem. 167, 203 (1973).
[27] H. Rirrgsdorfand G . Greber, Makromol. Chem. 31, 50 (1959).
1281 D . Brauri and W Brertdlein, Makromol. Chem. 167, 21 7 (1973).
[29] S. Loshuek and 7: G . Fox, J. Am. Chem. SOC. 75. 3544 (1953).
[30] D. Kut; and A. V. Tobolsky, J. Polym. Sci. ,4-2, 1595 (1964).
[31] A . V Toholsky, D. Kotz, M . Takahashi, and R. Scha#l~auser, J. Polym.
Sci. A-2, 2749 ( I 964).
1321 H. Wesslau, Makromol. Chem. 93, 55 (1966).
[33] W Schloik, J . Appmrodt, A. Michael. and A. Thal, Ber. Dtsch. Chem.
Ges. 47, 473 (1914).
Vinamidines and Vinamidinium Salts-Examples
Pull Alkenes
[34] J . Jugur, M . Levy, M . Feld, and M . S m a r r , Trans. Faraday SOC.
58, 2168 (1962).
[35] J . Jagur, H . Monteiro, and M . Szwarc, Trans. Faraday SOC.59, 1353
(1963); cf. also D. J . Adams, J . Clfton, and M . Iguchi, Br. Polym.
J. 6, 265 (1974).
[36] D. Braun, M . H . Tio, and W Neumann, Makromol. Chem. 123, 29
(1 969).
[37] D. Braurt and G . Traser, Makromol. Chem. 175, 2275 (1974).
[38] D. Bruun and F.-J. Quesado Lucas, Makromol. Chem. 142, 313 (1971).
1391 W Kirhn and H . Majer, Makromol. Chem. 18/19, 239 (1956).
[40] W Kuhn and G . Balmer, J. Polym. Sci. 57, 311 (1962).
[41] D. Braurt and E. Walrer, Colloid Polym. Sci. 254, 396 (1976).
[42] P. Long;, F. G r e w and U. Rossi, Makromol. Chem. 129, 157 (1969).
[43] M. Hutrntanrt and M . Hiiller, Faserforsch. Textiltech. 26, 509 (1975).
[44] D. Braun and H . Boudeuska, Eur. Polym. _I.. i n PI’L‘~\.
[45] D. Brauii and U . Y Kim, Kolloid-2. Z. Polym. 2 / 6 / 2 / 7 , 321 (1967).
[46] H. Batzer, F . Lohse. and R . Schmid, Angew. Makromol. Chem. 29/30,
349 (1973); cf. also F. Lohse and R . Schmid, Chimia 28, 576 (1974).
[47] W Funke, Fatipec-Congr. 1974. 18.
[48] H. Burell, J. Paint Technol. 43, No. 559, 48 (1971).
of Stabilized Push-
By Douglas Lloyd and Hamish McNab[*]
The name vinamidines has been proposed for 1,5-diazapentadienes; these compounds can
be regarded as vinylogous amidines and at the same time as push-pull substituted (and thereby
stabilized) alkenes. Vinamidine and vinamidinium structural elements may also form part
of a ring system. Characteristic of this class of compounds are their reactivity toward electrophiles
(at C,) and nucleophiles (C,) and their regenerative character, i. e. their tendency to undergo
substitution instead of addition reactions. Reactions of vinamidinium salts with nucleophiles
are of special preparative value: they lead e.g. to pyrazoles, oxazoles, pyrimidines, diazepines,
quinolines, and quinolizines.
1. Introduction-‘Push-pull’
Properties
Alkenes and Their General
The ‘push-pull’ concept has in recent years received some
attention because of the way it has been used to provide
extra stability to otherwise unstable systems. Perhaps the most
obvious example has been its use in providing stable comy.~mriF,vhjd?. f~.wdl:c ymsvso B ~qdrJULfid2Pr?a
+ng.q,.
( I a ) [ ’ ] .This is stabilized by a push-pull effect between the
electron-donating amino groups and the electron-withdrawing
ester groups. Thus, in resonance terminology it is possible
to write a further contributing form which is dipolar ( 1 b).
Any push-pull alkene has an electron-donating group (Do)
attached to one end of a double-bond and an electron-with-
r] Dr. D. Lloyd and Dr. H. McNab
Department of Chemistry, Purdie Building
University of St. Andrews
St. Andrews, Fife (Scotland)
Aiiqew. Chmi. l i l t . Ed. Engl. J Vol. 15 ( 1 9 7 6 ) No. 8
drawing group (A) to the other end, and these groups interact
with each other as illustrated in ( 2 ) .
Whmdectmnrdmnr cu. dec.frrn,accerpm r g q c WLWrately attached to alkenes they give rise to, respectively, electron-rich and electron-poor double-bonds, and this results in
such alkenes having characteristic properties, e.g. the nucleophilic character of enamines, and the electrophilic character
of a,p-enones, as shown, for example, by their participation
in Michael reactions.
What happens when both electron-donor and electronacceptor bonds are attached to an alkene group? Depending
upon the reaction partner the molecules behave as nucleophiles
or as electrophiles. This is nicely exemplified by the reactions
of the push-pull cyclobutadiene ( 1 )I2].
Another property of push-pull alkenes is their enhanced
stability, due to delocalization of electrons from the donor
group through the double bond to the acceptor group. This
stability leads to such molecules having a tendency to undergo
substitution reactions rather than the addition reactions characteristic of most alkenes. For example, in the case of electrophilic attack on push-pull alkenes, loss of a proton from
459
Nnc
Hence they readily undergo both electrophilic and nucleophilic substitution. This picture of the structure and reactivity of tropolones is in accord with the results of
X-ray structure determinations, which show the C(1t C ( 2 )
bond to be effectively single and little involved in the conjugation.
OzE
NEtz
EtOZC
M e 0 13
EtZN,
)$“OzEt
...
CozEt
EtzN#
EtOzC
NEtz
EtOzC
NEtz
Acyclic
prod u cts
the positively charged intermediate ( 3 ) tends to take place
rather than addition of an anion, because the fundamental
stability of the push-pull alkene system is thereby regenerated.
H H
1
9
DoCH=CAE
H H
I
DO-CC-A
Ee
+
I
1
@
I
% DoCHX-CHAE
It was at one time thought that in the metal derivatives
there was complete cyclic delocalization of electrons involving
both the diketone moiety and the metal atomr4], but the
present interpretation regards the delocalization as being confined to the diketone part of the molecule[51.
P-Diketones ( 4 ) provide excellent examples of the reactivity
of such systems towards electrophiles and nucleophiles and
also of their meneidic character.
-BraR
R
Tropolones ( 5 ) are vinylogs of b-diketones and their stability and characteristic reactivity are ascribable to their being
push-pull alkened6?
460
0
Q
RN-C =C-C =NHR
c+
+F
Thus, to summarize, push-pull alkenes are characterized
by their reactivity towards both electrophiles and nucleophiles,
and by their enhanced stability, which makes them prone
to substitution rather than to addition reactions. The compounds thus show meneidic or regenerative characterl31, and
are also relatively easy compounds to handle. For these reasons
they are also suitable reagents for a variety of syntheses.
Probably the first push-pull alkenes to be investigated were
the P-diketones ( 4 ) and their metal derivatives.
R
2\ n G.
RN= C-C =C-NHR
DO-C-C-E
A
13)
B r B r g H
In the examples considered hitherto acarbonyl group always
acts as the electron-acceptor group. An imino group
can likewise take over this function. If the electron-donor
group is an amino group we then have the push-pull alkene
system (6).
This has been called a vinamidine system because it represents a vinylog of an amidine[’]. Even more interesting are
the vinamidinium salts ( 7 ) , because they can be symmetrical
systems and are therefore particularly stable: The illustrated
.q
0 0.
0
RNH=C-C =C-NHR
W l
I
C)
I
XQ
0
RNH-C =C-C =NHR
I
(7)
I
l b
X@
canonical forms are indeed identical and can be likened to
Kekule forms for benzene. Vinamidine and vinamidinium
salts may be open-chain or included in ring systems.
IIOI
Examples of vinamidinium salts are the compounds (8) and
( 9 ) , and the dihydrodiazepinium salts (10). Some of the
properties of such systems will now be considered.
2. Shape and Structure of Vinamidines and Vinamidiniurn Salts
The vinamidine system may take up one of three distinctive
shapes: all-cis (‘U’) (I I), all-trans (‘W’) (IZ), and cis-trans
(‘sickle’) (13).
Open-chain vinamidines having an NH-group exist predominantly in the U-forrnC8- “1, wherein they are stabilized
by intramolecular hydrogen bonding ( I 4 ) . The possibility
that these bases possess a non-classical cyclic delocalized system of rc-electrons involving the N-H
bonds[”] has been
ruled out[”]; they are indeed hydrogen-bonded vinamidines.
Atiguw. Chrm. I n r . Ed. Eirgil.
Vol. 15 l l Y 7 6 ) No. 8
In contrast, their salts take up the W-configuration. This
is shown quite unambiguously by the large vicinal coupling
constants in their 'H-NMR spectra[", 131.
In the case of cyclic vinamidines their configuration is forced
upon them by the shapes of the rings in which they are
incorporated. Thus dihydrodiazepines [cf. ( l o ) ] must have
a U-form and derivatives of 3-imino-I -cyclohexenylamine [cf.
( 9 ) ] must have a W-form, while the cyclohexene derivative
(15)[141 probably has a sickle-form.
While the commonest use of this reactivity has been to
provide suitable substituted vinamidinium salts, it can also
be used for more general purposes, as shown in the first
R
xIe2rd*.NMe2
XO
NHPh
io
cia
(IS)
Within the chains of each of these forms the electrons
are completely delocalized. This is clearly shown by the vicinal
coupling constants in the 'H-NMR spectra['5. 16J, and has
been confirmed in the case of dihydrodiazepinium salts by
X-ray structure determination[' 'I. There is an alternation of
electron density along the systems, the terminal nitrogen atoms
bearing the greatest density, while the a-and P-carbon atoms
are relatively electron-poor and electron-rich, respectively.
CalculaThis is clearly shown by their 3C-NMR spectra''
tions which have been made on the vinamidine system are
in accord with this data["].
3. Reactions of Vinamidines and Vinamidinium Salts
Because of the alternating electron density in vinamidines
and vinamidinium salts their a- and a-carbon atoms are,
respectively, electrophilic and nucleophilic. Furthermore,
attacks by nucleophiles and electrophiles, respectively, at these
positions provide intermediate o-complexes which can then
lose a suitable fragment to give substitution products. These
reactions proceed as shown in Scheme 1.
Scheme 2
example, where the reaction ultimately provided a method
for the preparation of tricarbonyl compounds.
3.2. Reactionswith Nucleophiles
The reactions of vinamidinium salts with nucleophiles have
almost all been carried out in order to prepare other types
of compounds; sometimes when nitrogen nucleophiles are used
the products are themselves examples of different types of
vinamidines. Examples are depicted in Scheme 3[' 6*23.241.
The latter type of reaction has been of particular utility.
Some examples of both types will now be given.
Me
3.1. Reactions with Electrophiles
NH 2
Picratem
Both cyclic and acyclic vinamidinium salts react with electrophiles to give P-substituted products. The apparent anomaly
that this involves electrophilic attack by one cation on another
cation is easily understood when it is realized that vinamidinium ions are examples of electron-rich cations, having six
electrons shared over five sites. Examples of such reactions
are shown in Scheme 2['9-22~.
Angew. Chem. Inr. Ed. Engl.
/ Vol. I5
(1976) No. 8
1241
H
Picrate@
(N?
...
N
H
Me
Picrate 0
Scheme 3
461
The first of these reactions provides a very convenient and
valuable method for the preparation of dihydrodiazepinium
salts; indeed the parent salt (16) is presently only obtainable
in satisfactory yield by such a
Reaction of vinamidinium salts with a m i d i n e ~ r261,~ ~guani,
dine[25-271or t h i o ~ r e a s [ ~leads
~ - ~to~ a] similar displacement
of the terminal amino groups to give cyclic products, which
in this case are not vinamidinium salts, but pyrimidines such
as ( I 7).
R1 R3
R'
(17)
alcohols of similar boiling point[l4I. This provides a very
useful method for the synthesis of 2,3- and 1,2,3-substituted
quinoline derivatives. Reaction presumably proceeds by the
following mechanism 41.
Quinolines have also been prepared from vinamidinium salts
by treatment of the latter with concentrated sulfuric
or with aluminum ~ h l o r i d e [ ~ ' ~ l .
A further example of nitrogen nucleophilic attack on a
vinamidinium salt which is followed by intramolecular cyclization is provided by the reaction of (18) with N,N-diphenylhydrazine to give a 1,2-benzodiazepine (21 )[281. Cyclization
could involve a process related to that involved in quinoline
formation.
R3
With suitably cc-substitutedvinamidinium salts, these substituent groups, instead of the terminal amino groups, may be
displaced by the attacking nucleophile. Thus a,"-dichlorovinamidinium salts ( 2 8 ) react with hydrazines to give bis(dimethylamino)pyrazoles[28~'I.
+ Phz N N H z
.+.NMe2
Me2N
-
Ph
' 0
a ( 8 N M e .
XO
xo
MeZN
Ph
The salts (18) react in similar fashion with hydroxylammonium chloride, benzamidine, and o-phenylenediamine to
give, respectively, bis(dimethy1amino)-oxazoles, -pyrimidines,
and -benzodiazepines[28!
P-Naphthylamine reacts with vinamidinium salts to give
an intermediate which subsequently undergoes intramoIecular
cyclization to 4-azaphenanthrene derivatives ( I 9)L3'1.
mNH2
...
'y
NMe,
1
- HCI
Q :
,
MezN
+
R
I
Carbon nucleophiles react with vinamidinium salts in an
analogous fashion to nitrogen nucleophiles. The carbon nucleophiles are provided by suitable C-H acids in the presence
of base; hence a variety of useful syntheses have been performed.
The carbanions derived from a-methyleneketones displace
one of the amino groups of vinamidinium salts to give S-aminopentadienones (22)C3'1, which can be further converted into
xo
NaOEt
Some vinamidinium salts are capable of undergoing intramolecular cyclization. For example N,N'-diphenylvinamidinium salts (20) undergo cyclization with formation of quinoline derivatives when they are heated in acetic acid o r in
4
xo
462
H
XO
H
pyridine
ClO,G
122)
vinylogs of the vinamidinium salts. O n reaction of the vinamidinium salt (23) with acetonedicarboxylic ester both amino
groups are displaced and a phenol is produced[32!
A variety of ring-systems has been prepared by the interaction between vinamidinium salts and the carbanions derived
from a number of substituted acetonitriles, RCH2CN. The
A I I ~ P MChem.
..
Inr. E d . E n g l . ; L'of. 1 5 ( 1 9 7 6 ) N o . 8
COzEt
A number of similar synthetic procedures involving other
types of C-H acids have also been reported. Thus pyrenes
(29) have been prepared by reactions of vinamidinium salts
with phenalene[21~30~35.36!
COzEt
R
A
following examples show how this method has been utilized
to obtain phenanthrenes (24)13'1, carbazoles (25)I3'1, benzofurans (26a )[' '1, benzothiophenes (26b )[' 'I, indoles (26c)r' 'I,
quinolizines (27)'331, and benzobiphenylenes (28)[341.
L.NaOMe,
2. A
R
YO
By reaction of a vinamidinium salt with the cyclopentadienide ion the interesting anion (30) was obtained[37! Indene
and fluorene analogs were also preparedc3'* 381.The cyclopentadienide ion has also been used for the preparation of 6(dimethylamino~inyl)fulvenes[~2!
vN
+
'
~ase_
M e 2 W - yo,:..,
-"Me2
R
PJ
XO
H
.;
H
R
2 =
Me
.Q..-.
Me2"- y N M e z
R
X@
F,C1
-
.. @.';
PhMeN-Y-NPhMe
Nao~c
XO
Me
dR
(27)
Yet another interesting cyclopentadienide derivative, (31),
was obtained by reaction of a vinamidinium salt with the
reactive methyl group of the 1,4-dimethyIpyridinium ion and
subsequent displacement of the second amino group by cyclopentadienide
MezN,@++NMez
R
xO
I. Base
+
O
M
e
2. A
CN R
r
CN
:... a,.--.
M e zN'(\'NMez
1. Base
R
Me
All of these reactions thus consist of initial displacement
of one amino group of the vinamidinium salt by the carbanion,
with subsequent loss of the second amino group in an intramolecular cyclization reaction.
A i ~ g r i r .Chw?i. l t i t .
E d . Engl. 1 Vol. I S ( 1 9 7 6 ) No. 8
R
(33)
463
Methyl groups attached to the seven-membered ring of
azulene can react as C-H acids; their reactions with vinamidinium salts have been utilized to prepare benzazulenes (32)
and cyclopentaheptalenes (33)[361.
As a final example the interesting synthesis of hexahelicene
using the iminiomethylvinamidinium salt (34) may be
Me R
*NMe2
Me2N
+
Me2NCH(OR1)2
-
Xa
CN
NaOMc
4. Preparation of Vinamidines and Vinamidinium Salts
NMe2
2
clop
(341
;:,:::::
160-170°C
c
12,
hu
au-
NC'
\
\
The two commonest methods for the preparation of vinamidines and their salts are the reactions of amines with a$alkynones or P-dicarbonyl compounds.
Thus one of the simplest and most commonly used vinamidinium salts is the 1,5-diazapentadienium derivative (37),
which has been prepared from pr~pargaldehyde[~~!
CH-C-CHO + Me2NH
--D
MezN-CH=CH-CHO
/
A different type of reaction, which presumably involves
the vinamidine behaving as a nitrogen nucleophile in addition
H
Ph
Ph
No cyclic analogs appear to have been made to date by
reaction of alkynones with diamines, but the dihydrodiazepine
(38) has been obtained by addition of ethylenediamine to
a 1,3-di~ne[~'!
Me
R2
H2
C02R'
.f$
Ph
Me
R2
R2
H
COzR'
H
'B3
H
-=-
Me
R2
C02R'
C02R'
C02R'
1I
Me&
R3NH2
By contrast the reactions between P-diketones and amines
have been used to prepare both
and
vinamidines, and their salts, e.g. (39) and (40).
C02R'
PhRH, Br',
(35) R2
C02R'
to undergoing nucleophilic attack, occurs with acetylenedicarboxylic esters, and results in the formation of pyridinedicarboxylic esters (35)140a1. A possible mechanism is outlined below.
3.3. Reactions Involving a-Methyl Groups of Vinarnidinium
Salts
u-Methyl groups of vinamidinium salts have potential carbanionic reactivity, like that of a- and y-methyl groups in
pyridinium ions; but the synthetic possibilities that this provides have not yet been explored. One example which has
been reported is the preparation of 4-(dimethylamino)pyridines
(36)[411.
If R also represents a methyl group, reaction only takes
place at one of the two methyl groups. As might be expected
similar reactivity is not shown by P-methyl groups.
464
0
Me Me
NHPh
ffe
"
Me
140)
A review of the preparation of dihydrodiazepines has already appeared in the
In the preparation of open-chain vinamidinium salts, e.g.
(39), the reaction is frequently carried out in two steps as
shown in the above
An alternative stepwise process which is often advantageous involves alkylation of the
intermediate oxoenamine (42 j , followed by replacement of
the alkoxy group by an amine[41,46al.Diamines provide
cyclic vinamidinium
Anyebb. Chern. Int. E d . Engl. J Vol. 15 ( 1 9 7 6 ) N o . 8
ble2NH_
0
Rw Me6y&Mez
R2
R2
R*R3
0
OH
R’ R3
I.Me2S04
(41~
NMe2
___+
2.H20
R2
142)
The mechanism of formation from the acids is not fully
understood; however, it is known to involve diformylation
coupled with decarboxylation. In the case of the reaction
from cyanoacetic acid an intermediate (48) is obtained[20!
An example of the formation of a vinamidinium salt by
Vilsmeier reaction of an alkene ( 4 9 ) has also been
e
X@
R’C
In place of dicarbonyl compounds ( 4 1 ) their monoace~ * ~also
~ ] be used. Thus the unsubtals[’ 1,481 or d i a c e t a l ~ [ may
stituted N,N’-arylvinamidinium salts ( 4 3 ) have been prepared
from the tetracetal of malondialdehyde[8z231.
Q””. Q
CICO-COCl
(491
*NMe
MezN
2
c1@
(50) and
P - A l k o ~ y - [and
~ ~ ]p-aminovinamidinium
( 5 1 ) , respectively, and (52)[601 have also been prepared by
Vilsmeier-type reactions.
DMF
M e zN-C H2C H( OE t ) 2
Me 2N+NM
_3
OE t
ma,
c10
P-Halovinamidinium salts ( 4 4 ) have been prepared from
mucochloric and mucobromic acids, which are masked Pdicarbonyl compounds[50].
144)
Z = C1 o r
Br
DMF
p-Chlorovinylaldehydes ( 4 5 ) , which may be regarded as
the acid chlorides of the enol forms of P-dicarbonyl compounds, can also serve as starting materials for the preparation
of vinamidinium salt^['^.^ 1.521 . S’mce the chloroacrylaldehydes are themselves readily prepared from E-methyleneketones[51s53]
this serves as another convenient route to the
salts. Vinamidines have also been obtained from P-chlorovinylketones[30a,53a1
--+.
VllSmClCr
R’CH~COR~
R‘-c,
redCllOn
FH0
CC1R2
-
RNH-CH 2C OzH
mCl3
a;)
Me2“
152)
u-Amino- and E ,a’-diaminovinamidinium salts (53) and
( 5 4 ) have been obtained by addition of an a,a-diaminoethylene to a formamidinium salt, and to an unsubstituted vinamidinium salt,
2R’R4NH
( M e 2 N ) 2 C = C H 2+ M e 2 N ~ ” N M e z X’
+
145)
R2
R3R4N+NR3R4
R’
clo
A third general method for the preparation of vinamidinium
salts utilizes the Vilsmeier formylation of aldehyde acetals
(46)[541, haloacetic acidsr21 , 3 2 , 5 5 , 561, cyanoacetic acidc2’],
phenylacetic acids (47)[21, 561 or malonic acids[”].
,.. Q..‘.
MeCH(OEt12
DMF’
-&
Me2N/\(-NMez
Me
DMF
pea,
M e z N y N M e z
Ph
C 1 ~
(46)
.. .@..--..
PhCHzCOzH
(47)
Anyew. Chmni. Int. Ed. Enyl.
1 Vol. I S ( 1 9 7 6 ) N o . 8
C 1 ~
a-Aminovinamidinium salts of type (53) have also been
prepared in high yield from the reaction between P,p-dichlorovinylketones and
The synthetically valuable dichlorovinamidinium salts ( 1 8),
mentioned previously in Section 3.2 in connection with their
use in the preparation of heterocycles, are obtained by reaction
465
A detailed analysis of vibrational spectra of (57) (R = R'= H)
of phosgene immonium salts with tertiary amines containing
two ci hydrogen atoms[28~29*62].
shows that the vinamidinium structure is indeed the
The dichloro compounds (18) have also been prepared
favored canonical
Nitration of the cation (57)
by addition of phosgene immonium salts to alkynylamine~[~~J. ( R = R = M e ) and of the free base ( 5 8 ) at the 3-position
show similar rates over a wide pH range@*], which
suggests that reaction proceeds via the cation and then by
[Me2N-CC12l0 Xo + R C H 2 C O N M e 2
a typical vinamidinium mechanism (see Scheme 1).
c1 c1
Vinamidinium systems may play a part in stabilizing nonbenzenoid systems. Two examples are the di- or triaminocyclopropenium salts (59)[691 and the diaminopentalene (61 )["I.
-
Monochlorovinamidinium salts have been prepared by addition of phosgene immonium salts to enaminesf2*1, and by
means of a Vilsmeier reaction from N,N-dimethylamides containing two ci hydrogen
Dibromovinamidinium salts
are obtained by the action of hydrogen bromide on malononitriIe[63al.
Two non-general methods which have provided vinamidine
derivatives merit mention. u-Aryl and u,a'-diaryl derivatives
are obtained when dithiolylium salts react with dimethylamine; the thiaaza salt ( 5 5 ) is postulated as intermediate[64!
,--,
R = H
or
Ar'
Dihydropyrimidines (56) are obtained by a "witches' brew
reaction" from a ketone, a ketoacetal and ammonia in the
presence of ammonium nitrate[65'241.
-NH,
MeCOCH2CH(OMe)2 + RR'CO
"O,R'
N
H Me
(56)
Finally the reader is referred back to Section 3.2 of this
article which deals with reactions of vinamidines with nucleophiles. Examples are given of the interconversion of different
vinamidines. Such interconversions are often most valuable
for the preparation of specific vinamidines.
5. Special Vinamidines and Vinamidinium Salts
In natural product chemistry the corrins and porphyrins,
which are of wide occurrence, contain vinamidine systems.
Like their simpler analogs they undergo electrophilic substitution at the fbposition[661.
The 4-aminopyridinium system (57) is an example of a
vinamidinium system which is electronically perturbed by
a bridging alkene group.
Z = NRZ o r
160)
H
159)
161)
162)
KMe2
&Mez
Compound (61) unlike simple pentalenes, is stable in air
for some hours. A helping factor here may be a contribution
from the vinamidinium-cyclopentadienide canonical form
(62)r701. The aminocyclopropenium salts are stable in air
and in particular are non-hygroscopic and very resistant to
attack by water[69]. The bridged vinamidinium form (60)
could contribute to stabilization in these compounds also.
It would be extremely interesting to know if the diamino
salts (59)-(60), Z = H, could undergo electrophilic substitution at the 2-position of the ring-unlike normal cyclopropenium salts but in keeping with vinamidinium character.
The ready protonation of the push-pull cyclobutadienes
such as ( I ), mentioned in Section 1, may also be assisted
by the stabilization conferred upon the resultant ions by their
bridged vinamidinium ion structure.
The vinamidinium system was also present in the first
Wheland intermediates (63), to be isolated and stable at
ambient temperatures[70a'.
@j
R z N:. y R R z
163)
NR z
Finally, mention should be made of the many vinylogs
of vinamidines and vinamidinium salts. For example the readily accessible dianils of glutacondialdehyde (64), prepared
by the action of amines on pyridinium
are important
synthetic intermediates, inter d i n , in the synthesis of azulene~[~'].
PhMeN-
.-...g,....,,,-.
'NPhMe
,rO
(64)
NaOH
PhMeN-0
L'-
Like the vinamidinium salts their vinylogs also undergo
electrophilic substitution, at the P-position[zol. Naturally
occurring examples of such vinylogs are
466
Anguw. Chmi. Int. Ed. Engl. J Vol. 15 ( 1 9 7 6 ) No. 8
Vinylogous vinamidinium salts are also ofconsiderable commercial importance, for the cyanines, which are used as photographic sensitizers, all contain this structural feature. There
is an enormous amount of literature available in this field;
a review on cyanines comments that they “have made color
photography and high-speed photography
6. Conclusion
There are many interesting aspects of the chemistry of vinamidines and vinamidinium salts. Some of the cyclic compounds of this type have been the subject of detailed
but for the remainder, although there has been much information published about them, especially concerning their use in
synthesis, there has been little in the way of systematic
comparative studies. Thus comparisons of the effects of
changing the amino groups have received little attention.
Similarly the reactions of substituent groups, and the effects
of such groups on the stability, structure and chemistry of the
system, remain relatively little explored. It would also be
:...o..-...
E tS-
SEt
(65)
of interest to compare the properties of other hetero-analogs,
e.g. compounds such as the dithiapentadienium ion ( 6 5 ) ,
whose spectrum has been
Received: February 20, 1976 [A 120 IE]
German version: Angew. Chem. 88.496 (1976)
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The Chemistry of Simple Phosphorus-Carbon Compounds
By Heinz H a r n i s c h [ * l
With the aid ofselected examples an overview is given of the development trends in phosphoruscarbon chemistry over the past few years. An attempt is made to demonstrate the relationships
between various parameters and properties such as constitution, basicity, substitution by
functional groups, reaction behavior etc. of the compounds. In the case of basis compounds
containing methylphosphorus groups the state of development of industrially interesting processes
is also outlined. In addition, the synthesis of a few bifunctional phosphorus-carbon compounds
which can be employed as comonomers in the production of polymers is described.
2. Properties and Reaction Behavior of Simple
1. Introduction
Phosphorus-Carbon Compounds
The unique properties of the phosphorus atom offer
the preparative chemist numerous reaction possibilities, the
theoretical chemist the study of numerous phenomena, and
the practically oriented researcher access to novel or extended
applications in areas such as corrosion resistance, flame
resistance, flotation, extraction, and complexation.
PH,
2.1. Influence of Basicity
The basicity of phosphorus-carbon compounds can be
varied according to type and number of organic groups bound
to the phosphorus. For example, in the series phosphane,
<
[PA,]@
4.31
Phosphorus-carbon chemistry has gone through a phase
of rapid development in the past few years. Research in this
sector was stimulated in particular by the introduction of
nuclear magnetic resonance spectroscopy, used for elucidating
constitution, and the improved accessibility of starting compounds containing simple alkyl or arylphosphorus groups.
In this review article an attempt will be made to outline
some of the more important developments and findings of
current interest in phosphorus-carbon chemistry.
[*] Dr. H. Harnisch
Hoechst AG, Werk Knapsack
D-5030 Hiirth-Knapsack (Germany)
468
4.04
3.95
3.88
3.82
Scheme 1. Basicity of the methylphosphanes. The figures below the cations
are their relative electron densities (calculated after Saiulersoii).
methylphosphane, dimethylphosphane, trimethylphosphane
the basicity of the respective compounds progressively
increases, which phenomenon is also evidenced by the decreasin relative electron density of the corresponding phosphonium
ions (Scheme 1).
Owing to the particularly large difference between the relative electron densities of PHf and CH3PH: the three methylphosphanes can be easily separated from phosphane by
treatment with moderately concentrated hydrochloric acid
and formation of methylphosphonium ions. By increasing
A n g n r Cltrm. 1111. Ed. Eiigl. J Vol.
15 (1976) No. 8
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