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a-Umpolung of Ketones via Enol Radical Cations A Mechanistic Study.

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61
171
Fig. 1. Structure of 6 in the crystal (hydrogen atoms omitted). Selected interatomic distances [A] and bond angles ["]: N...NJ 3.724(2), Na ..Na' 3.229(1);
Na'-Na-Na' 101.3(1),N-Na-Nk 163.6(1),N-Na-NJ93.5(1), N*-Na-NJ 102.9(1),
Na-N-Na' 163 6(1), Na-N-Na' 78.2(1), Na'-N-Na' 85.4(1).
ed tetramethyl pyrrole, but not as long as those in the fragment (x-C,Me,N)Fe.[*. The Na,N, quadrilaterals formed
by the double chains (interatomic distances and bond angles
shown in Fig. 1) are planar within the limits of experimental
error and form angles of 0.8" with each other, resulting in a
layer structure resembling a distorted rope ladder.
Unlike the analogous lithium compound, 6 is readily soluble in T H F and is present in solution as a dimer. In the 13C
NMR spectrum, the signals of the ring carbon atoms
(6 = 124.84 and 111.77) are only slightly shifted compared
with those of tetramethylpyrrole (6 = 119.96 and 113.33);
this finding indicates that the 7t bonding is disrupted upon
dissolution with formation of structure type 5 (L = THF).
The unique structural features of 2,3,4,5-tetramethyl-l -sodiopyrrole confirm that the azacyclopentadienyl ligand displays versatile coordination properties, which were until
now only presumed. In view of the rich structural chemistry
of cyclopentadienyl compounds of the main-group elements," O1 the coordination chemistry of this heteroarene
should offer promising applications.
Experimental Procedure
6 : NaH (0.57 g, 23.7 mmol) and 2,3,4,5,-tetramethylpyrrole (2.93 g,
23.7 mmol) were allowed to react in 100 mL of refluxing T H F for 18 h. The
reaction mixture was then filtered, the solvent removed, and the residue extracted with 30 mL of diethyl ether. Yield after recrystallization of the residue from
THF/diethyl ether: 1.28 g (43%) of colorless crystals. "CNMR (75.43 MHz,
[DJTHF,TMSint.): b = 124.84(C2,5),111.77(C3,4), 13.70(C2.5-CH3), 10.43
(C3,4-CH,). MS (70eV): m / r 145 ( M " , 1 Yo),122 ( M 8 - Na], l0Ooh).Molecular mass- found 265 (vapor pressure osmometry in T H F at 45 "C), calcd 145.2
(monomeric formula unit).
Received. April 9, 1990 12 3903 IE]
GemIan version. Angew. Chem. 102 (1990) 1179
CAS Registry number:
2,3,4.5-tetramethyl-l-sodiopyrrole,
128191-83-3
[I] a) R. Hacker, E. Kaufmann, P. von R. Schleyer, W. Mahdi, H. Dietrich,
Chem. Ber. 120 (1987) 1533-1538; b) K. Gregory, M. Bremer, P. von R.
Schleyer, P. A. A. Klusener, L. Brandsma. Angew. Chem. 101 (1989)
1261-1264; Angew. Chem. Int. Ed. Engl. 28 (1989) 1224-1226.
[2] Structure type 2 has been identified for transition metals in the chemistry
of azaferroceneIl1 a], I,l'-diazaferrocene[ll b], and azacymantrene[l2].
The linking of two molybdenum centers to form a structure like 3 has not
been confirmed by structure analysis[l3].
1144
$3 VCH
Verlagsgesellschuft mbH, 0-6940 Wemherm, 1990
[3] 6(150K):MonoclinicP2,/n,a = 11.090(4),b = 4.994(1),c= 15.705(5)A,
B = 106.60(2)", V = 8 3 3 . 5 5 A 3 , Z = 4 , @ d = 1.157gcm~',MoK,radiation, scan range 4" 5 28 I 5 4 " , 1834 unique reflections, 1548 observed
( I 2 1.96a(I)), R = 0.034, R, = 0.043, w = [cZ(Fo)f (0.01 . FO)*]-'.Further details of the crystal structure investigatlon may be obtained from the
Fachinformationszentrum Karlsruhe, Gesellschaft fur wissenschaftlichtechnische Information mbH, D-7514 Eggenstein-Leopoldshafen 2
(FRG), on quoting the depository number CSD-54660, the names of the
authors, and the journal citation.
(41 Cf., e.g., R. Griinig, J. L. Atwood, J. Organomet. Chem. 137(1977) 101111; D. J. Brauer, H. Burger, W. Geschwandtner, G. R. Liewald, C.
Kriiger, ibid. 248 (1983) 1-15,
[S] Cf., e.g., R. D. Rogers, J. L. Atwood, M. D. Rausch, D. W. Macomber,
W. P. Heist, J. Organomel. Chem. 238 (1982) 79-85; T. Aoyagi, H. M. M.
Shearer, K. Wade, G. Whitehead, ibid. 175 (1979) 21 -31.
161 The linking of three 0 s centers by the ligand C,H3NZe was recently
demonstrated: M. W. Day, K. I. Hardcastle, A. J. Deeming, A. J. Arce, Y.
De Sanctis, Orgunometallics 9 (1990) 6-12.
171 Cf. K. Jonas, D. J. Brauer, C. Kriiger, P. J. Roberts, Y.-H. Tsay, J Am.
Chem. SOC.98 (1976) 74-81.
[8] N. Kuhn, E:M. Horn, R. Boese, N. Augart, Angew. Chem. 100 (1988)
1433-1435; Anger%-.Chem. in!. Ed. EngL 27(1988) 1368-1369.
[9] A similar finding results from comparison of the ring bond lengths in the
fragments C,H,Na and C,H,Fe, cf [S] as well as C. Kriiger, B. L. Barnett,
D. Brauer in E. A. Koerner von Gustorf, F.W. Grewels, I. Fischler (Eds.):
The Organic Chemistrv oflron, Vol. I , Academic Press. New York 1978,
pp. 1-112.
[lo] Cf. review: P. Jutzi, Adv. Orgummet. Chem. 26 (1986) 217-295; P. Pure
Appl. Chem. 61 (1989) 1731-1736.
111) a) N. Kuhn, M Schulten, E. Zauder, N. Augart, R. Boese, Chem. Ber. 122
(1989) 1891-1896; b) N. Kuhn, E.-M. Horn, R. Boese, D. Blaser, ibid. 122
(1989) 2275-2277.
1121 W A. Herrmann, I. Schweitzer, P. S. Skell, M. L. Ziegler, K. Weidenhammer, B. Nuber, Chem. Ber. 112 (1979) 2423-2435; V. G. Andrianov, Y. T.
Struchkov, N. I. Pyshnograeva, V. N. Setkina, D. N. Kursanov, J.
Orgunomel. Chem. 206 (1981) 177-184, and references cited therein.
[I31 W. Dell, M. L. Ziegler, 2. Nafurforsch. B 37 (1982) 7- 12; cf. [6].
a-Umpolung of Ketones via Enol Radical
Cations: A Mechanistic Study **
By Michael Schmittel,* Ahmed Abufarag, Olaf Luche,
and Michael Levis
Numerous MO calculations ['I and mass-spectrometric
studies''] have confirmed that simple enol radical cations are
much more stable than the tautomeric keto ions in the gas
phase, in distinct contrast to the order of stability of the
neutral speciesc3](Fig. I).
Reversal of the thermodynamics of the keto/enol equilibrium by one-electron oxidation is also found in solution, as
exemplified by studies on stable en01s.[~~
For example, the
radical cations of 0,P-dimesityl-substituted enols are up to
25 kcal mol-' more stable than those of the tautomeric ketones in acetonitrile.
This finding opens up interesting possibilities for new synthetic procedures. Starting from the ketone, the small
amount of enol present in equilibrium can be selectively oxidized by suitable choice of oxidizing agent and the resulting
radical cation can be trapped by nucleophiles. If enolization
is sufficiently fast (acid or base catalysis), the carbonyl com['I
[**I
Dr. M. Schmittel. A. Abufarag, 0. Luche, Dip].-Chem. M. Levis
Institut fiir Organische Chemie und Biochemie der Universitat,
Albertstrasse 21, 7800 Freiburg (FRG)
En01 Radical Cations in Solution, Part 2. This work was supported by the
Deutsche Forschungsgemeinschaft, the Land Baden-Wurttemberg (FrNW 31), and the Fonds der Chemischen Industrie (Liebig-Stipendium for
M . S.). We thank Prof. C . Riichardt for supporting our work and D . Moral
for her dedicated assistance. Part 1: [4].
0570-0833/90/1010-11443 3.50+ .25/0
Angew. Chem. In[. Ed. Engl. 29 (1990) No. 10
lpE
f
0'0
OH@
I1I kcal mol-'
o!
A
1 1 4 kcal mol-'
A.
Fig. 1 . Relative energies of acetone and the tautomeric propen-2-01 as neutral
molecules and as radical cations in the gas phase [2].
pound could react quantitatively via enol radical cation intermediates.
To investigate this possibility, we first studied the reaction
of 1-(p-methoxyphenyl)propan-2-one (l),which has an enol
% 1-(p-methoxypheny1)propencontent of only about
2-01,'~' with tris(p-bromopheny1)ammoniumyl hexachloroantimonate ( 2 @ ) . The direct one-electron oxidation of 1
(E, = 1.67 VI6') by 2*@(2: E,,, = 1.06 VI7]) is so strongly
endergonic that, even for rapid subsequent reactions of l*@,
reactions should occur only slowly. On the other hand, the
oxidation potential of the enol of 1 should be less than 1 VJsl
ensuring its rapid oxidation by T@.Reaction of 1 with
200 mol YOof 2-@in acetonitrile/methanol (9: 1 ) for 14 min
gave 3[91 (56 %) [Eq. (a)]. In addition, p-anisaldehyde
( <9 "/a) and 1 -(p-methoxypheny1)propane-1,2-dione ( < 5 %)
were formed.
CH30cu
The products obtained are consistent with the assumed
course of the reaction. The acid released during the reaction
catalyzes the enolization of 1 and the enol is selectively oxidized and then trapped by methanol (Scheme 1, path a). Also
plausible would be the route involved in oxidative functionalization of alkylarenes["] (path b). This reaction is known
to proceed effectively despite strongly endergonic electrontransfer steps.
Control experiments allow a distinction to be made between the two reaction hypotheses. According to path b, a
strongly endergonic electron-transfer step is followed by deprotonation of the keto radical cation. The rate of the overall
reaction should therefore increase, or at least remain constant, upon addition of a base. In fact, however, the reaction
of 1 with T@ proceeds much slower in the presence of
200 mol % of 2,6-di-tert-butylpyridine: after 5 h, only 8% of
3, together with 58% of unreacted 1, is
This
finding rules out path b but is consistent with path a, since
the acid-catalyzed enolization should be suppressed by addition of base.['31
Additional arguments against path b are provided by
the reaction of 3-(p-methoxyphenyl)butan-2-one(5, E, =
1.69 V) with ammoniumyl salts. The direct oxidation of 5
can be suppressed or enhanced relative to acid-catalyzed
enolization by the use of ammoniumyl salts having various
oxidizing strengths (Table 1) [Eq. (b)].
200 mol-% 2'".
9,
or 10'"
5
cH
(b)
200 mol-% 2'" or 4
\
CH,CN/CH,OH 9:l
CH30
CH30Q0
+
'
+
CH,O
1
CH,O'
3
I
I
6
8
7
@-CH,-C,H,),N'@
(0,p-Br,-C,H,)N'@
9'0
Comparable yields of 3 (49%) are obtained when
200 mol YO of tris(o-phenanthroline)iron(rrI) tris(hexaflu0rophosphate) (4) is used. This finding supports the assumption of one-electron oxidation steps in the reaction mechanism, since 4 (El,2 = 1.09 V) is known to be an outer-sphere
oxidizing agent according to the work of Kochi et al.["]
>
CH,CN/CH,OH 9:l
100
Table 1. Products of the reaction of 5 with ammoniumyl salts (200 mol%) in
acetonitrile/methano1(9: 1) at room temperature. The yields and reaction times
are average values from at least three independent experiments.
Ammoniumyl salt
E,,, (SCE)[14] 6 ["/.I
[VJ
9 " [a]
Z a [a]
0.76
1.06
1.50
[a]
78
30
<1
["/.I
7 [%I
8
<1
10
7
<1
6
6
5 [%]
I
(121
5
29
41
33 h
153 min
2s s
[a] As hexachloroantimonate
cH30u
*
/
cH30u
+CH,OH
3
+CH OH
-H"-ee
The product analysis clearly shows that the use of strong
ammoniumyl salts results in a large decrease in the amount
of 6 and an increase in the formation of cleavage products
Apparently, 5'@ is not deprotonated
such as 7 and 8.''.
under the reaction conditions, as required according to path
b, but instead undergoes C-C bond cleavage. On the other
hand, the use of a weakly oxidizing ammoniumyl salt leads
to high yields of 6 ; in this case, the direct oxidation of 5 can
no longer compete with the acid-catalyzed enolization and
subsequent oxidation of the enol.
0,
Scheme 1.
Angen. Ch<>m.In!. Ed. Engl. 29 (1990) No. I0
CH3
0 VCH
VerlagsgeseNschaji mbH, 0-6940 Weinheim. 1990
0570-0833/90/1010-1145 $3.50+ .25/0
1145
The use of alcohols as nucleophiles necessitates the consideration of further reaction pathways in addition to those
discussed above, since, besides ketones and tautomeric enols,
small equilibrium concentrations of enol ethers and acetals
may also be present.['61 However, acetals and enol ethers
need not be intermediates in the umpolung reaction, as
shown by treatment of 2-(p-rnethoxyphenyl)cyclohexanone
(11, E, = 1.63 V) with 200 mol Yoof 9 @ in pure acetonitrile:
12 is formed in 74% yield [Eq. (c)].
11
Isomerization versus Decarboxylation of Protonated
Oxetanone: Comparison between Experimental
Results and Theoretical Calculations
12
The investigations show that ketones such as 1 and 5,
which exhibit a low tendency toward enolization, can be
methoxylated in the ct position via their enol radical cations.
It is still unclear how the enol radical cations react further to
give the products." 7i Elucidation of the mechanistic details
and scope of application of this novel umpolung reaction of
ketones, especially in comparison with known methods,llsi is
in progress.
Received: May 11, 1990 [Z 3950 IE]
German version: Angeiv. Chem. 102 (1990) 1174
CAS Registry numbers:
1, 122-84-9; r e ,24964-91-8; 3, 21165-40-2; 4, 28277-57-8; 5 , 7074-12-6; 6.
104741-73-3;?, 77525-91-8; 9a,65644-87-3; loe, 58047-17-9; 11,37087-68-6,
12, 128973-49-9.
[I] N. Heinrich, F. Louage, C. Lifshitz, H. Schwarz, J. Am. Chem. SOC.110
(1988) 8183, N. Heinrich, W. Koch, G Frenking, H. Schwarz, hid. 108
(1986) 593; W. J. Bouma, J. K. MacLeod, L. Radom. ibid. I01 (1979) 5540.
[2] J. L. Holmes, F. P. Lossing, J. Am. Chem. SOC.102 (1980) 1591; ibid. 104
(1982) 2648.
[3] J. Toullec, Adv. Phys. Org Chem. 18 (1982) 1; F. Turecek, L. Brabec, J.
Korvola, J. Am. Chem. SOC.If0 (1988) 7984.
[4] M. Schmittel, U. Baumann, Angew. Chem. 102(1990) 571; Angew,. Chem.
Int. Ed. Engl. 29 (1990) 541.
[5] J. R. Keeffe, A. J. Kresge, Y. Yin, J. Am. Chem. SOC.110 (1988) 8201.
[6] The oxidation potentials given here refer to the saturated calomel electrode
(SCE); they were measured in acetonitrile (100 mVs-'): 2-3 mM 1 or 5,
0 1 M tetra-n-butylammonium hexafluorophosphate, ferrocene as reference.
[7] W. Schmidt, E. Steckhan, Chem. Ber. 113 (1980) 577.
[8] The oxidation potentials of p,~-dimesityl-substitutedenols [4] are about
1 V below those of the tautomeric ketones. Assuming identical differences
in oxidation potential for 1 in enol and keto form, E, (enol) should be
much less than 1 V (SCE).
[9] A 30 mM solution of the ketone in acetonitriie/methanol (9: 1) was stirred
with 20 mol YOof 2.e at room temperature until the blue color of the
ammoniumyl salt vanished. The products were characterized by spectroscopy and by comparison with authentic substances.
[lo] C. L. Wong, J. K. Kochi, J. Am. Chem. SOC.101 (1979) 5593.
[ll] E. Baciocchi, A. Daila Cort, L. Eberson, L. Mandolini, C. Rol, J. Org.
Chem. 51 (1986) 4544; C. J. Schlesener, J. K. Kochi, ibid. 49 (1984) 3142;
L. Eberson, J. Am. Chem. SOC.105 (1983) 3192, E. Baciocchi, C. Rol, G. V.
Sebastiani, B. Serena, Garz. Chim. Nal. 113 (1983) 853.
[12] As a measure of the reaction rate, the time required for color change from
blue (color of 2 a ) to brown was determined.
[I31 Addition of 200 mol% of trifluoromethanesulfonic acid to the reaction
mixture gave 3 in 78% yield after 10 min.
[14] E. Steckhan, Top. Curr. Chem. 142 (1987) 1.
1151 The low conversion of5 upon oxldation with l o @is due to rapid reaction
of l o e with the solvent mixture. Furthermore, competition experiments
showed that, when l o e was used as oxidizing agent, 6 did not react faster
than 5.
1146
0 VCH
Verlagsgeseikchufl mhH. D-6940 Weinheim. 1990
[16] M. Gaudry, A. Marquet, Tetrahedron 26 (1970) 5617.
[17] Here, too, several reaction hypotheses are plausible (e.g., attack of methanol on the en01 radical cation or deprotonation of the enol radical cation).
Some of these reactions have already been described for the electrochemical oxidation of enol acetates to a-acetoxy ketones or of enol ethers to
a-alkoxy acetals: see S. Torii: Electroorganic Synfhese.r-Mezhods and Apphcations, Part 1 . Oxidations, Kodanshd, Tokio 1985, pp. 230, 234.
[18] R. M. Moriarty, 0. Prakash, M. P. Duncan, R. K. Vaid, H. A. Musallam,
J. Org. Chem. 52(1987) 150; R. M. Moriarty, 0.Prakash, Acc. Chem. Res.
19 (1986) 244.
By Gregorio Asensio,* Miguel A . Miranda,*
I
Sabater,
Julia Perez-Prieto, Maria .
and An tonio Simhn-Fuen tes
Decarboxylation of 2-oxetanones is a general and useful
method for the stereospecific synthesis of olefins, proceeding
with complete stereoretention in neutral organic
Conversely, upon addition of an acid catalyst, cis-3,4-disubstituted 2-oxetanones undergo carbon dioxide elimination
with almost total inversion of configuration and a marked
increase in the reaction rate."] Recent theoretical calculations13] on the decarboxylation of 2-oxetanone protonated
at the carbonyl oxygen atom (1H@)as a model for the acidcatalyzed thermolysis appear to support the intermediacy of
a carbenium ion (2H0), which is assumed to undergo fragmentation affording ethylene and protonated carbon di-
od
0
"OY
0
e
2
1
'
-co\
/Go,
H,C=CH,
1H@
I
"'
2H
@
3H
oxide. Hence, we decided, as a part of our work on the
thermolysis of protonated carbonyl compounds,[41to study
the behavior of the parent 2-oxetanone (1) in concentrated
sulfuric acid in order to determine the role of cationic
species in the decarboxylation and to check the feasibility
of the suggested C3-C4 fragmentation of 2H@to ethylene
and protonated carbon dioxide.
The protonated lactone 1H@['I was studied over a temperature range of 30 to 150 "C, during a period of up to several
days, by keeping the corresponding sulfuric acid solutions in
thermostated baths and recording periodically the 'H NMR
[*] Prof. Dr. G. Asensio, Prof. Dr. M. A. Miranda, Dr. J. Perez-Pneto,
M. J. Sabater, Dr. A. Simon-Fuentes
[**I
Departamento de Quimica Organica, Facultad de Farmacia
Avda. Blasco IbaBez, 13, E-46010 Valencia (Spain)
We thank the Direccion General de Investigacion Cientifica Y Tecnica
(DGICYT. Grant No. PB87-0989) for financial support.
0S70-0833/90/1010-1146 S 3 S0+.2S/O
Angew. Chem. I n ! . Ed. Engl. 29 (1990) No. 10
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