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Enols from Norrish Type II Cleavage of Carbonyl Compounds in Solution.

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electron pair
for closo compounds (n =number of skeletal atoms) 16 skeletal electrons are required for occupation
of the bonding molecular orbitals in the 7-atom MnzCzBzS
cluster; these are provided by the components C (2 x 3), B
(2 x 2), S (4) and MnCO, (2 x 1).
3,4-Diethyl-2,5-dimeth yl-l,2,5-thiadiborolenebis(tricarbonylmanganese) (2)
A mixture of Mn,(CO),, (2.67 mmol) and ( 1 ) (2.67 mmol)
in 40ml mesitylene is heated under reflux for 5 h. The small
amount of red [(l).Mn(C0)4]2 which crystallizes out on
cooling is separated off. After removal of mesitylene by distillation the residue is recrystallized twice from pentane (- 20°C):
orange-red crystals, m. p. 183-1 84"C, dec. > 230°C, yield 21 %.
Received: February 27, 1976 [Z 446b IE]
revised: March 22, 1976
German version: Angew. Chem. 88, 451 (1976)
CAS Registry numbers:
( l ) , 54677-51-9; ( 2 ) , 59318-06-8; Mn2(CO)lo, 10170-69-1; "B, 14798-13-1
[l] Diborylethylene Compounds as Ligands in Metal Complexes, Part 4.
This work was supported by the Deutsche Forschungsgemeinschaft and
3: W Siebert, R . Full, C .
the Fonds der Chemischen Industrk-Part
Kriiger, and K-H. B a y , Z. Naturforsch. 31 b, 203 (1976).
[2] a) K . Kinberger and W Siebert, unpublished: b) W Siebert, G . Augustin,
R . Full, C. Kriiger, and E-H. Tsay, Angew. Chem. 87,286 (1975); Angew.
Chem. Int. Ed. Engl. 14, 262 (1975); c) note added in proof (30. 6.
76): confirmed by X-ray structure analysis: C . Kriiger and J . Sekutowsky,
unpublished.
[3] G . E. Herberich, Lecture at Marburg, January 19, 1976: G . E. Herberich,
J . Hengesbach, U . Kolle, G. Huttner, and A . Frank, Angew. Chem. 88,
450 (1976); Angew. Chem. Int. Ed. Engl. 15, 433 (1976).
[4] H . Werner, Nachr. Chem. Tech. 22, 3 (1974); W 7: Robinson and R.
N . Grimes, Inorg. Chem. 14, 3056 (1975); literature cited therein.
[S] K . Wade, Chem. Brit. 1 1 , 177 (1975); literature cited therein.
Enols from Norrish Type I1 Cleavage of Carbonyl Compounds in Solution
By Andreas Henne and Hanns Fischerrl
We recently reported that 0.01-0.1 M stationary concentrations of acetone enol are observed during UV irradiation
of acetone/2-propanol mixtures at - 70"C, the enol being
formed by disproportionation [eq. (a)] of l-hydroxy-l-methylethyl radicals. It did not decay appreciably within 5000 s['].
(CI-I,),CO
+
(CH,),CHOH
h"
-2
(CH3),COH
-
X
R
Y
Z
further irradiation, and does not decrease by more than 10 %
within 2 h after cessation of irradiation. After briefly warming
up the solution to 20°C a second NMR spectrum is recorded
at - 70°C. Evidently an equivalent concentration of ( 4 a )
has been formed from ( 2 a ) at +2O"C. Photolysis of (1 a )
in CDjOD (-70°C) and CD3CN/TMS (9: 1, -40°C) produces similar enol concentrations. In the irradiation of (1 b)(1 d ) at T r -4O"C, polar solvents favor['] the formation of
the enols ( 2 b ) - ( 2 d ) in high concentrations (Table 1). Warming these enol solutions to room temperature gives the corresponding tautomers ( 4 b)-(4d).
The enol-keto tautomerization ( 2 a ) + ( 4 a ) was followed
between - 14 and - 35 "C by integration of the NMR signal
of the enol at 6=3.97ppm (H'). In TMS, CD30H, and
CD,OD this reaction obeys a first order rate law. In TMS
the exponential lifetimes of ( 2 a ) amount to 5300 (-35"C),
1600 (-26"C), 560 (-17"C), and 380 s (-14°C). For T 2
- 17 "C these values are smaller than those of di- and trialkylsubstituted enols in solution at 1&37"C[71. We believe that
the observed lifetimes are determined by acid- or base catalysis
arising from solvent impurities or by-products of UV irradiation.
+
Table 1. Enol concentrations [(2)], of ( 2 a ) - ( 2 d ) and reactant conversion
[(I)l0- [ ( I ) ] , (both in mol/l, 15 %) after irradiation time t at temperature
?: starting from ( I a ) - ( ] d ) in 1 M solutions.
(20)
(2b)
(2c)
(2d)
- 70
- 40
- 40
- 44
50
30
55
30
0.83
0.75
0.72
0.30
0.70
0.20
0.29
0.20
[a]
[b]
[b]
[b]
[a] TMS solvent.
[b] CD3CN/TMS (9: I) solvent.
OH
H,C=C
+
'c H3
(a)
(CH3)zCHOH
In this paper, we show that the enols ( 2 a ) - ( 2 d ) can
be obtained in even higher concentrations from Norrish Type
I1 photoreactions['] [eq. (b)] of the carbonyl compounds
( 1 a)-(1 d ) in appropriate solvents at T< -40°C. Only the
keto tautomers have previously been reported as products
from (1) in solution[3!
After 50 minutes' irradiation (filtered light from a Philips
SP 1000W lampr5],280 5 .; < 350 nm) of 1 M 5-methyl-2-hexanone (1 a ) in TMS at - 70°C in the sample tube of a VARIAN
HA-100 'H-NMR spectrometer, an NMR spectrum is
observed which apart from unconverted reactant (1 a ) (0.13 M),
reveals the presence of isobutene ( 3 a ) (0.80 M), acetone ( 4 a )
(O.O7M), and acetone enol ( 2 a ) ( 0 . 7 0 ~ ;NMRr6]: CH3,
6=1.75; Ha, 6=3.97; Hb, 6=3.71; OH, 6=8.40ppm). The
concentration of ( 2 a ) remains approximately constant on
[*] Dr. A. Heme and Prof. Dr. H. Fischer
Physikalisch-Chemisches Institut der Universitat
Ramistrasse 76, CH-8001 Zurich (Switzerland)
Angew. Chem. I n r . Ed. Engl. J Vol. 15 (1976) N o . 7
In addition to providing a direct proof for the mechanism
of the Norrish Type I1 reactions of (1 a)-(I d ) in solution,
this observation of enols ( 2 a ) - ( 2 d ) in high concentrations
may form the basis for the study of enol-keto tautomerizations.
Received: March 25, 1976 [Z 449 IE]
German version: Angew. Chem. 88, 445 (1976)
CAS Registry numbers:
( l a ) , 110-12-3; ( I b ) , 110-62-3; ( I c ) , 4436-99-1; ( I d ) , 1009-14-9;
( 2 a ) , 29456-04-0; ( 2 b ) , 557-75-5; ( 2 ~ ) 59138-75-9;
,
( 2 d ) , 4383-15-7;
( 4 a ) . 67-64-1; ( 4 b ) , 75-07-0; ( 4 c ) . 590-50-1; ( 4 d ) , 98-86-2
[I] A. Henne and H . Fischer, Helv. Chim. Acta 58, 1598 (1975); A . Henne,
Ph. D. Thesis, Universitat Zurich, 1975.
[2] P. J . Wagner, Acc. Chem. Res. 4, 168 (1971).
[3] IR detection of ( 2 a ) in the gas phase: G . R. McMillan, J . G . Caluerr,
and J . N . Pirrs, Jr., J. Am. Chem. SOC.86, 3602 (1964).
[4] We thank Dr. M . Schneider, Universitat Hohenheim, Germany, for a
sample of dineopentyl ketone (1 c).
[S] B. Blank, A . Henne, and H . Fischer, Helv. Chim. Acta 57, 920 (1974).
161 For ClDNP of ( 2 a ) see also: G. P. Laroff and H . Fischer, Helv. Chim.
Acta 56, 2011 (1973); B. Blank, A . Henne, G . P. LaroK and H . Fischer,
Pure Appl. Chem. 41, 475 (1975).
[7] H . M . R. Hofmann and E. A . Schmidt, Angew. Chem. 85, 227 (1973);
Angew. Chem. Int. Ed. Engl. 12,239 (1973); E. A . Schmidt and H . M . R .
Hofmann, J. Am. Chem. Soc. 94,7832 (1972).
435
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solutions, carbonyl, cleavage, compounds, enol, norris, typed
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