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Direct Determination of Rate Constants for the Addition of Carbenium Ions to Alkenes.

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[7] a ) Satiafdctory elemental analyses were obtained for D - 3 to 0-6 [Route
(a)] and I ) - / L - ~ [Route (b)]; b) due to difficulties in purification of such
compounds and uncertainties in the polarometric determinations of their
enantiomer ratios, the specific rotations of the triols (viscous oils) must be
taken as approximate values; cf. discussion by V. Schurig, Kontakte
(Darmstadt) 1985 ( I ) 54.
[S] G. B. Payne, J . Org. Chem. 27 (1962) 3819.
[9] A related approach based on divinylglycol was devised by Prof. Dr. R . R .
Schmidt (see U. Kufner, R. R. Schmidt, Angew. Chern. 98 (1986) 90, Angew. Chem. Int. Ed. Engl. 25 (1986) 89. We thank Prof. Schmidt for informing us about their results prior to publication and for discussions.
Synthesis of Deoxyhexoses from DivinylglycolsThe Synthesis of D- and L-Chalcose
By UIrike Kufner and Richard R. Schmidt*
Deoxyhexoses and especially 2,6- and 4,6-dideoxyhexoses are components of numerous natural products.['] We
have developed a variable concept for the synthesis of enantiomerically pure dideoxy sugars starting from racemic
or meso-di~inylglycols,[~.~~
which requires only a few steps
and is exemplified by the synthesis of D- and L-chalcose
(D-6 and L-6)(Scheme l).[4.51
DL-1: R
R = BzI
The threo-dipropenylglycol DL-I,which is obtained by
reductive dimerization of crotonaldehvde["l and simDle
separation of the diastereomers,[" reacts with benzyl bromide to give the monobenzyl derivative D L - 2 . Sharpless oxidation['] leads to kinetic resolution of the racemate with
formation of the enantiomerically pure epoxides D-3 and
L-3,respectively. In order to obtain the other isomer in
each case, the unreacted starting materials, enriched in L-2
and D - 2 , respectively, are allowed to react further, thereby
affording correspondingly higher yields (ca. 70%) of L-3
and D - 3 . The epoxide ring in L-3 is opened regioselectively with Red-Al (sodium hydridobis(2-methoxyeth0xy)aluminate) to give the trio1 derivative L-4; analogous opening of hydroxyalkyl-substituted epoxides has already been observed.[*]Ozonolysis of L-4 affords the 2 - 0 benzyl-protected 4,6-dideoxy-~-xylo-hexose
L-5 in only
four steps. Conversion into the benzyl glycoside, 3-0-methylation, and hydrogenolytic debenzylation give L-chalcose ~ - in 6good~ yield.
~ Similarly,
the 4,6-dideoxyhexose
D - 5 and D-chalcose ~ - 6are
' ~obtained
from D - 3 .
Received: October 7, 1985 [Z 1487 IE]
German version: Angew. Chem 98 (1986) 90
[ I ] S. Hanessian, Adu. Carbohydr. Chem. 21 (1966) 143; N. R. Williams, J. D.
Wander in W. Pigman, D. Horton (Eds.): The Carbohydrates. Chrmi\try
and Biochemistry. Vol I B . Academic Press, New York 1980, p. 761 ff.
[2] In addition, 4,6-dideoxy-lyxo-hexoseswere prepared from ervthro-dipropenylglycol and the 2.6-dideoxyhexose D-digitoxose from ervthro-divinylglycol. U. Kiifner, planned Dissertation, Universitat Konstanz.
131 The pheromone exo-brevicomin was synthesized from threo-divinyiglycol: U. Kiifner, R. R. Schmidt, Synthesis 1985. 1060
[4] Chalcose syntheses starting from carbohydrates. a) N. K. Kochetkov, A.
J. Usov, Tetrahedron Lett. 1963. 519: b) S. McNally, W. G. Overend,
Chem. lnd. (London) 1964. 2021: c) A. B. Foster, M. Stacey, J . M. Webber, J. H. Westwood, J . Chem. Soc 1965, 2319: d ) B. T. Lawton. D. J.
Ward, W. A. Szarek, J. K. N. Jones, Can. J . Chem. 4 7 (1969) 2899: e) K.
Kefurt, 2. Kefurtovi, J . Jary, Collect. Czech. Chem. Commun. 38 ( 1972)
2627; f ) H. Redlich, W. Roy, Carbohvdr. Re.,. 68 (1979) 275.
[5] De novo chalcose synthesis: a) R. M. Srivastava, R. K. Brown, Cun J
Chem. 48 (1970) 830: b) K. Torsell, M. P. Tyagi, Actu Chem. Scund. 831
(1977) 7: c) A. Banaszek. A. Zamojski, Rocz. Chem. 45 (1971) 391: d ) S.
Danishefsky, J . F. Kerwin, J . Org. Chem. 47 (1982) 1597.
161 W. G. Young, L. Levanas, 2 . Jesaitis. J . Am. Chem. Soc. 58 (1936) 2274.
[7] T. Katsuki, K. B. Sharpless, J . Am. Chem. Soc. 102 (1980) 5974: V. S.
Martin, S. S. Woodard, T. Katsuki, Y. Ydmada. M. Ikeda, K. B. Sharpless, ibid. 103 (1981) 6237.
[S] J. M. Finan, Y. Kishi, Tetrahedron Lett. 23 (1982) 27 19: S. M. Viti. ihid. 23
(1982) 4541.
[9] L-6: [a]?;-78 (c=0.66, H 2 0 after 3-24 h) [4b]; m.p.=89-90"C [4b]: 11-6.
[air,+77 (c=0.6, HzO after 3-18 h); m.p.=90.5-9IoC.
' M HOe
Direct Determination of Rate Constants for the
Addition of Carbenium Ions to Alkenes**
By Reinhard Schneider, Ute Grabis, and Herbert M a y *
L- 6
Scheme 1. Synthesis of D- and L-chalcose, 0-6 and L-6. a) PhCH,Br, dimethylformamide (DMF), BaO/Ba(OH)> (46%), b) Ti(OiPr),, tBuOOH, CH2C12,
( +)-diethy1 tartrate (35%), c) Ti(OiPr),, tBuOOH, CHZC12,(- )-diethy1 tartrate (35%), d) Red-Al, tetrahydrofuran (86%), e) 1. o,, MeOH/CH2C12; 2.
MelS (81%), 0 PhCHZOH, HCI (70%), g) Mel, NaH, D M F (83%)),h) Pd/C,
H2, ethyl acetate (89%).
The rates of the addition of carbenium ions to alkenes
are important for the understanding of carbocationic polymerizations and with regard to reactivity-selectivity relationships. Probably because such reactions are difficult to
control, kinetic investigations have only been carried out
under special conditions, e.g., with radiolytically generated
carbenium ions."] Based on our previous work on the selective formation of 1 : 1 products from carbenium ions
Prof. Dr. R. R. Schmidt, DiplLChem. U. Kiifner
Fakultat fur Chemie der Universitat
Postfach 5560, D-7750 Konstanz (FRG)
De Novo Synthesis of Carbohydrates and Related Natural Products,
Part 20. This work was supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen 1ndustrie.-Part 19: 131.
Angew. Chem. Int. Ed. Engl. 25 11986) No. 1
Prof. Dr. H. Mayr, Dipl.-Chem. R. Schneider, U. Grabis
lnstitut fur Chemie der Medizinischen Universitat
Ratzeburger Allee 160, D-2400 Liibeck I (FRG)
I"] This work was supported by the Deutsche Forschungsgemeinschaft and
the Fonds der Chemischen Industrie. We thank Prof. Dr. 0.Nu.yken.
Mainz, and R . Bederke. Lubeck, for their help in constructing the apparatus.
0 VCH Verlagsgesellschafr mbH. 0-6940 Weinheim. 1986
0570-0833/86/0101-0089 $ 02.50/0
Table I . Rate constants (CH:CI?, -70°C) and activation parameters for the
addition of diarylcarbenium tetrachloroborates 1 to 2-methyl- I-pentene 2.
la OCH,
l b OCH,
lc OCH,
kZ [a]
AG *
[kJ/mol] [J mol - ' K - '1 ( - 70°C)
2 . 9 ~lo-'
1.7 x l o - '
2.4x 10'
- 122
[a] Reproducibility for la-Id better than
and alkenes, we have now developed a method for the determination of the corresponding reaction rates.
The diarylcarbenium tetrachloroborates la -le can
quantitatively be generated from the corresponding diarylmethyl chlorides and boron trichloride in dichloromethane
at -20 to -90°C. When 2-methyl-I-pentene 2 is added to
these solutions, the addition products 3a-3e, which are
practically unionized under these conditions, are formed
( > 90% yield). The course of these reactions can easily be
followed conductometrically and photometrically (fiber
optics) since the electrical conductivity and the absorption
of visible light decrease to less than 3% of the initial values.
The reactions were found to be first order with respect
to both alkene and carbenium ion over the entire concentration range examined ( 2 : 1 . 1 x lO-'to 8.6 x lo-' mol/L;
Id: 4.1 x
to 4.1 x lo-' mol/L). One to two equivalents of BCI, are sufficient to achieve complete ionization
of la-ld, whereas complete ionization of the di-p-tolyl
compound l e requires approximately 500 equivalents of
BC13 in the concentration range studied. The reaction rate
of Id (2. I x
mol/L) is not influenced by variation of
the BC13 concentration from l o - ' to 3.6 x lo-, mol/L.
The addition of 1 to 2 initially yields the carbenium ion
4, and it is conceivable that 4 either reacts with BCI? to
give 3 and BC13 or undergoes retroaddition to give 1 and
2 . The latter possibility, i.e., the reversible formation of 4,
was excluded since the addition of lo-' mol/L of benzyltriethylammonium tetrachloroborate to a 10 -'M solution
of Id-BCI? did not affect the reaction rate. The same
conclusion can be derived from the observation that
the systems
la-BCIz (k2=2.9 x lo-' L mol-' s-I),
la-BBr3ClQ (2.7 x lo-' L mol-' S K I ) ,
and la-SnCl?
(2.5 x lo-' L mol-' s - ' ) show similar reactivity toward 2,
although the rate of the reaction 4 - 3 is altered when the
anion is varied.13] The observation that the reaction rate of
Id is not affected when the BCI? concentration is increased 100 fold indicates that either specific ion pairs d o
not exist or that ion pairs and free ions d o not differ in
their reactivity toward 2.
The decreasing electron-releasing ability of R ' and R' in
going from l a to l e is accompanied by an increase in
reactivity of 5 orders of magnitude (Table 1). In accordance with the solvolysis rates of methoxy-substituted diphenylmethyl derivative^,'^' we do not find a linear correla90
0 VCH Verlagsgesellsch~f,mhH. 0-69411 Weinheirn, 1986
- 120
55. I
47. I
for le, better than i- 10%).
tion between lgk2 and o+.A uniform reaction mechanism
and the absence of ion-pair effects is also indicated by the
linear correlation between Ink2/T and l / T in the temperature ranges investigated (mostly -30 to -70°C). These
plots allow the determination of the activation parameters
AH' and AS'. Especially in the additions of the less-stabilized carbenium ions, the entropy term TAS' is the
main contribution to the free energy of activation. Whereas
the activation entropy is almost unaffected by the variation
of R' and R2, A H c decreases considerably with decreasing
stabilization of the carbenium ions la-le . If AS' of the
corresponding addition of the diphenylmethyl cation 1
(R', R 2 = H ) is assumed to be of the same order of magnitude, AH' is estimated to be zero or negative.
In accord with these conclusions, a negative activation
enthalpy ( - 25 kJ/mol) was reported for the carbocationic
polymerization of p-methoxy~tyrene,[~]
in which attacking
and resulting carbenium ion correspond to the same structural type. The small (positive or negative) activation enthalpies and large negative activation entropies for the additions of reactive carbenium ions to alkenes are in accordance with previously reported data for the corresponding
reactions of carbenes and radicals.[61
Received: August 21, 1985:
revised: September 16, 1985 [Z 1437 IE]
German version: Angew. Chem. 98 (1986) 94
[ I ] a) Y . Wang, L. M. Dorfman, Macromolecules 13 (1980) 63; b) 0. Brede, J.
Bus, W. Helmstreit, R. Mehnerr, Radiat. Phy.7. Chenz. 19 (1982) I , and
references cited therein.
[2] a) H. Mayr, Angew. Chem. 93 (1981) 202; Angew. Chern. Inf. Ed. Engl. 20
(1981) 184; b) H. Mayr, W. Striepe, J. Org. Chem. 48 (1983) 1159: c) H.
Mayr, R. Schneider, Makromol. Chem.. Rapid Cornrnun. 5 (1984) 43.
131 G. Heublein, S. Spange, P. Hallpap, Makromol. Chem. 180 (1979) 1935.
141 S. Nishida, J Org. Chem. 32 (1967) 2692, 2695, 2697.
IS] R. Cotrel, G. Sauvet, J. P. Vairon, P. Sigwalt, Macromolecu!es 9 (1976)
[6] a) Carbenes: K. N. Houk, N. G. Rondan, J. Mareda, Tetrahedron 41
(1985) 1555; b) radicals: J. M. Tedder, J. C . Walton, Ada. Phys. Org.
Chem. 16 (1978) 5 I .
Amino Acid Derivatives from
N-(Ary1methylene)dehydroalanine Methyl Esters**
By Giinter Wulff* and Helmut Bohnke
N-Pyridoxylidenedehydroalanine 1 has been postulated
to be a reactive intermediate in the pyridoxal-phosphatecatalyzed biosynthesis of some amino acids ;I2' for example,
Prof Dr. G. Wulff, Dr. H. Bohnke
lnstitut fur Organische Chemie I 1 der Universitat
Universitatsstrasse I , D-4000 Dusseldorf (FRG)
[**I Activated Dehydroamino Acids, Pan 2. This work was supported by the
Fonds der Chemischen Industrie and the Deutsche Forschungsgemeinshaft.-Pan I : [I].
11870-11833/86/11101-11#90$ 02.50/0
Angew. Chern. Int. Ed. Engl. 28 11986) No. I
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constantin, carbenium, direct, additional, rate, determination, ions, alkenes
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