close

Вход

Забыли?

вход по аккаунту

?

Formation and Detection of Vinyl Cations.

код для вставкиСкачать
C O N F E R E N C E REPORTS
Formation and Detection of Vinyl Cations
By C. A. Grub"]
Vinyl cations ( I ) , unknown and rejected a few years ago,
are now the subject of an increasing number of investigations. In these cations the formal positive charge resides on
the carbon atom of an olefinic double bond. With twopresumably linearly-attached ligands they belong to the
group of disubstituted carbenium ions.
with ordinary carbenium ions, such as the tendency to
isomerize[6] or to rearrange by way of ion pairs. Detailed
mechanistic studies are in progress in several laboratories.
Lecture at Leverkusen, April 19,1972 [VB 338 IE]
German version: Angew. Chem. 84,592 (1972)
[l] C. A . Grob and G . Cseh, Helv. Chim. Acta 47, 194 (1964).
[2] C. A. Grob and R. Spaar, Tetrahedron Lett. 1969,1439; Helv. Chim.
Acta 53, 2119 (1970).
[3] M . Hanack and '
I
Biissler, J. Amer. Chem. SOC.91, 2117 (1969);
S. A . Sherrod and R . G. Bergmann, ibid. 91, 2115 (1969).
[4] C. A. Grob and H . R. Pfaendler, Helv. Chim. Acta 54,2060 (1971).
[5] C. A. Grob and H . R . Pfaendler, Helv. Chim. Acta 53,2130 (1970).
[6] C. A . Grob and R. Nussbaumer, Helv. Chim. Acta 54, 2528 (1971).
(a), R = Aryl
(b), R = Alkenyl
(c), R = Cyclopropyl
Energy Coupling and Electron Transport
The objections which have frequently been raised against
the existence of vinyl cations are justifiable inasmuch as
simple vinyl halides ( 2 ) @=halogen) resist hydrolysis
and do not precipitate silver halide from aqueous silver
nitrate as is observed with alkyl halides. However, an
electron donating aryl substituent in the a-position, as in
( 2 a ) , can promote ionization under relatively mild conditions, as was first shown in 1964[']. Thus a-halo styrenes
( 2 a ) react in aqueous alcohol to form ketones (via the
enol), enol ethers, and acetylene derivatives by the S,I--E 1
mechanism. a-Alkenyl ( 2 b )['I and a-cyclopropyl substituents ( 2 c ) 131 likewise increase reactivity.
The reaction rates of a-aryl substituted vinyl bromides of
the type ( 2 a ) depend on substituents in the benzene ring
in a predictable manner. Since they are not influenced by
the hydrogen ion concentration of the medium a mechanism involving acid-catalyzed hydration to the intermediate (3)["'can be excluded.
1
2
3
4
(5)
R2C=C-CH-CRL
I
Br
R,C=C=CH-CR',
(41
It is noteworthy that the solvolysis rates of 2-bromodienes
(4/ are hardly influenced by the presence of one or two
methyl groups on C-I [ ( 4 ) , R=CH3]. However, two
methyl groups on C-4 [ ( 4 ) , R'=CH,] accelerate the reaction by more than five powers of ten, as expected on the
basis of the electron distribution in the mesomeric vinyl
cation ( 5 ) . However, charge delocalization can only occur
if C-I, C-2, and C-3 have a linear arrangement, as required
by the limiting allenic form. If this alignment is
prevented, as in the cyclic bromodiene (6), the negligible
solvolytic reactivity characteristic of vinyl halides ret u r n ~ [ ~Vinyl
].
cations have many features in common
[*] Prof. Dr. C. A. Grob
Institut fur Organische Chemie der Universitat
CH-4056 Basel, St.-Johanns-Ring 19 (Switzerland)
544
By Britton Chance"]
In recent years titrations of redox potentials of the redox
components of respiration chains (flavoproteins, ubiquinones, cytochromes) have led to a new classification of
the components of respiration chains. In this scheme four
groups of redox systems are differentiated, within each of
which the potential is approximately the same and fixed:
1. Flavoproteins and iron-sulfur proteins in the region
- 300 mV ;
2. Flavoproteins, iron-sulfur proteins, ubiquinones and
cytochrome b, in the region 0 mV;
3. Cytochrome c l r c and a, as well as copper- and ironsulfur proteins in the region + 240 mV ; and
4. Oxygen in the region
+ 840 mV.
Electrons are moved from one of these groups to another
by means of electron transport components of alternating
redox potential. They adjust their potentials to those of the
acceptor or donor groups in the redox systems of fixed
potential which function as redox buffers ;the energy loss is
minimal"]. The transport systems are called energy transducer components. The nature of the transducer components of the first and second groups is unclear, but cytochrome b, was recognized by redox potential titrations
and kinetically as the transducer component between the
second and the third group, and cytochrome a3, spectrophotometrically as the linking compound between the
third group and oxygen.
This classification has been supported by analysis of the
kinetic models by computer simulation and by optimization of the kinetic constants for suitable mechanisms. In
particular it was possible to differentiate cytochromes a
and a3, by measuring the kinetics of the reaction with oxygen at low temperatures (in presence of ethylene glycol
down to -30°C)[21. In these experiments reaction of the
respiratory system of intact mitochondria with oxygen was
initiated by using a laser flash to decompose a preparation
[*I Prof. Dr. B. Chance
Johnson Research Foundation,
University of Pennsylvania, Medical School
Philadelphia, Pennsylvania (USA)
Angew. Chem. internat. Edit. 1 Vol. I1 (1972) 1 NO. 6
Документ
Категория
Без категории
Просмотров
0
Размер файла
121 Кб
Теги
detection, formation, vinyl, cation
1/--страниц
Пожаловаться на содержимое документа