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Re: CTRL "V" thread
Regulation of Structural Isomerism in Simple
Diels-Alder Adducts
Sir:
A few years ago, two research groups reported the
very pronounced catalytic effect of Lewis acids on
the Diels-Alder reaction.'S2 We now wish to report
that Lewis acids not only accelerate the Diels-Alder
reaction, but also regulate the structural isomerism of
the adducts.
It is well known that two structural isomeric adducts
are obtained from the reaction of an unsymmetrical
diene with an unsymmetrical dienophile. Nazarov,
et al.,3 have shown that when 1- or 2-alkyl-1,3-butadienes
are allowed to react with esters of a-alkylacrylic
acids, two structural adducts are formed in which the
predominant isomer is the one that has the greatest
separation between bulky g r o ~ p s . ~
(1) P. Yates and P. Eaton, J . Am. Ckcm Soc., 84, 4436 (1960)
(2) G. I. Fray and R. Robinson, ibid., 8S, 249 (1961).
(3) I. N. Nazarov. Yu. A. Titov, and A. I. Kuznetsova, Dokl. Akad. Nauk
SSSR, 104, 586 (1959), Chcm. Abrfr., 63, 11,268f (1959).
(4) H. E. Hennis, 1. &E. Ckcm., 18, 2570 (19631, has found in the Diels-
Alder reaction of isoprene with methyl acrylate that methyl 4-methyl-3-
cyclohexene-1-carboxylate accounts for 70% of the two isomeric adducts
formed and that the reaction is temperature independent.
3900 COMMUNICATIONS TO THE EDITOR Vol. 86
TABLE I
DIELS-ALDER REACTION BETWEEN ISOPRENE AND DIENOPHILE
RCPCtemp..
adducts
tioo '% of isomeric
Dienophile Catalyst Solvent OC. 111 IV
1 Methyl vinyl
2 Methyl vinyl
3 Acroleind None None 150 5V.' 41
4 Acrolein' SnCl4.5H*0 Benzene <25 96"* 4
a Carried out in a sealed tube for 15 hr. in conventional manner
according to H. L. Holmes in R. Adams, "Organic Reactions,"
Vol. IV. John Wiley and Sons, Inc., New York, N. Y., 1948,
p. 132. Direct analysis by gas chromatography, using an F. and
M. Model 609 instrument equipped with 220 ft. of R' (Ucon
Lubricant 50-HB-2000). capillary column, and &me ionization
detector. The physical properties of the ketones matched those
reported by A. A. Petrov and N. P. Sopov, Zh. Obshch. Khim.,
22, 591 (1952); Chem. Absfr.. 47, 2735 .(1953). Elemental
analyses (Calbraith Laboratories, Knoxville, Tenn.) for both
ketones were acceptable. Adduct mixture 2 spiked with mixture
1 showed only two peaks with the expected peak area
alteration. Carried out according to the method of Fray and
Robinson, ref. 2. d Reaction in sealed tube according to the
procedure of ref. a. p. 98. Indirect analysis, see discussion and
example below. I Melting point of 2,4-DNP derivative of
adduct mixture 161.3-166.5". Elemental analysis (Huffman
Microanalytical Laboratory, Wheatridge, Colo.) acceptable
with theory and with that of 2.4-DNP below. * Melting point
of 2.4-DNP derivative of adduct mixture, 176.4-177.5'. Elemental
analysis (Huffman Microanalytical Laboratory) acceptable.
ketone" None Toluene 120 71' 29
ketone" SnC1,.5HrO Benzene <25 93* 7
We have now found that Lewis acid catalysis of the
Diels-Alder reaction of simple, unsymmetrical reactants
markedly affects the structural isomeric distribution of
the resulting adduct^.^ Our studies have been confined
to the reaction of isoprene (I) and methyl vinyl
ketone or acrolein (11). The results obtained are
summarized in Table I.
!! 0 0
I1 II
f H 2 ,C-R CR & CH
CHa
H3C CHa 11 I11 IV
I R = H, CHs
The relative amounts of the two ketone adducts
were determined by gas chromatography (g.c.) and
the analysis of the thermal adducts checked closely
with the work of Henni~.~ The aldehyde adducts,
which we were unable to separate by g.c., were analyzed
indirectly by converkion to their mixed hydrazones
followed by reduction6 and disproportionation in a
single step over Pd/C to p- and m-xylenes.
N-NHa
(5) H. M. Walborsky. I. Barash, and T C. Davis, Tetrahedron, 19, 2333
(1963), have ohserved asymmrtric synthesis via Lewis acid catalyzed Diels-
Alder reactions at low temperatures The product after LiAIH, reduction
had the oppositc sign and configuration.
(6) This is an unusual Wolff-Kishner reduction. which does not appear
to he general: unpublished data of fi;. F. Lutz.
Although the disproportionation and reduction of
the isomeric hydrazones is not clean cut,l it is believed
that the ratio of the aromatics obtained is, at least
qualitatively, representative of the isomeric adducts
initially formed. Analyzing the ketone adducts by
this method and comparing the results with those
obtained by g.c. showed excellent agreement (90 us.
930/0) for adduct I11 from the Lewis acid catalyzed
reaction. The comparison for the thermal reaction
was less satisfactory (54 us. 71Y0).*
It is believed that the adduct distribution here is
regulated by coordination of the large Lewis acid with
the carbonyl oxygen to increase greatly the steric bulk
at the carbonyl group, causing the approaching isoprene
molecule to orient in such a fashion that the
pendant methyl group does not lie above the complexed
carbonyl. This intermediate configuration leads to
the formation of the 1,4-disubstituted adduct. This
correlates well with the work of Nazarov,j summarized
above, and with the accepted mechanism of adduct
formation by the approach in parallel planes of diene
and dienophile so that maximum accumulation of
double bonds is a~hieved.~
An example illustrative of the procedure used to synthesize
and analyze the aldehyde adducts follows.
Using the catalyst and conditionsI0 shown for expt. 4
in Table I, the aldehyde adduct of isdprene and acrolein
was synthesized in a 53.6% yield by the method
of Fray and Robinson.2 The hydrazone was prepared
by adding 3.7 g. (0.03 mole) of the mixed rnethylcyclohexenecarboxaldehydes
to a refluxing solution of
9.6 g. (0.3 mole) of hydrazine in 20 ml. of absolute
ethanol. The product was isolated by pouring the
reaction solution on ice, extracting rapidly with ether,
drying the ether extract, and stripping the solvent
under vacuum. The crude hydrazone was distilled
at 82-87O (0.08-0.06 mm.), 1 2 % ~ 1.5168, and gave an
acceptable elemental analysis (Table I, ref. f).12 To a
5-ml. round-bottomed flask equipped with magnetic
stirrer, condenser, and gas buret, was added 0.3607 g.
of methylcyclohexenecarboxaldehyde hydrazone, 0.723 1
g. of n-decane, and 0.0962 g. of 10~o Pd/C. This solution
was maintained at about 100° until gas evolution
ceased. G.c. analysisI3 of the reaction solution showed
the relative percentages of the xylenes formed were
96% p- and 4% m-xylene. The p-xylene was further
identified by spiking with the authentic compound and
by trapping the g.c. peak to obtain its ultraviolet
spectrum, which was identical with that in the A.P.I.
file. G.c. analysis further indicated that about 3070
of the reaction had proceeded with disproportionation
and reduction, with the remaining 70% going to nitro-
(7) In addition to the products of disproportionation and reduction that
are shown, other products were alsu obtained which were presumed to be
primarily nitrogen-containing compounds (C.E.. azines. etc.).
(8) We feel that our data on the aldehyde adducts are somewhat more
accurate than this becaux. they represent an average of several determinations.
(9, See K. B. Woodward and T. J. Katz, Tclrahcdron, 6, 70 (1958). for
discussion and lending references.
(10) Hydrated catalysts can he used so long as the temperature of reac
tion is low enough that the catalyst d l not hydrolyze to produce protons.
Proton formation leads to trimerization of the aldehyde adductsL1
( 1 1 ) E. P Lutz. irnpuhlished research.
(12) The hydrazone appeared to decompose in the presence of air and PP
parently oxidizes to the corresponding azine upon prolonged contact with
(13) G. c. anniyrrs was performed on an F. and M. Model 720 gaschromalograph,
equipped with ': m. of 5% Bentone-34-5% diisudecyl phthalate on
Chromosorh W. and programmed from 50 to 170O at 7.5'/miu.
a,r
Sept. 20, 1964 COMMUNICATIONS TO THE EDITOR 3901
gen coupled or reduced products (e.g., azines and
hydrazines).
DENVER RESEARCH CENTER
LITTLETON, COLORAW
MARATHON OIL COMPANY
EUGENE F. LUTZ
GEORGE M. BAILEY
RECEIVED APRIL 18, 1964
__________________
Shannon
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