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Light-Induced Formation of Acids from Cyclic Ketones.

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understandable in the light of recent investigations.
Nevertheless, the incorporation of hetero-atoms may be
of advantage in preparing siloxanes for special applications. The properties of the heterosiloxanes synthesized so far clearly show the wide range of possibilities.
Dr. H. S. Arnold, Dipl.-Chem. W . Findeiss, Dr. H. Hussek, Dr. J. A. Perez-Garcia, and Dr. F. Schindler. We
would like to thank Prof. Max Schmidt for his constant
and generous support.
Received: August 4th, 1964
The author’s own investigations reported in this article
were made possible only by the dedicated cooperation of
[A 416/201 IE]
German version: Angew. Chem. 77, 206 (1965)
Translated by Express Translation Service, London
Light-Induced Formation of Acids from Cyclic Ketones
Ultraviolet irradiation of aqueous solutions of cyclic ketones having a tetrahedral carbon
atom in at least one of the two positions adjacent to the light-absorbing carbonyl group
results in ring opening accompanied by the formation of a carboxylic acid. Cyclohexadienones which are blocked in the ortho-position are converted to hexadienoc acids. Cycloalkanones give saturated carboxylic acids and, in the presence of oxygen, also some unsaturated ones. The scope of the reactions is illustrated, and favorable conditions are
described. The structure of the products and the route by which they are presumably formed
are discussed.
I. Introduction
Much work has recently been carried out in the field
of organic photochemistry, since both efficient chromatographic methods capable of separating complex
mixtures of products, and serviceable light sources [*]
are now available.
Photochemical reactions are n o longer curiosities, even in the
syntheses of natural products. A few examples may serve t o
illustrate this:
Scheme I. Principle of the Barton reaction [l].
[*I The development of organic photochemistry has been
reviewed by A . Schonberg : Praparative Organische Photochemie.
Springer, Berlin 1958; P. de Mayo in: Advances in Organic
Chemistry. Interscience, New York 1960, Vol. 11, p. 361; P. de
Mayo and S.T. Reid, Quart. Reviews 15, 393 (1960); and by
various authors in Advances in Photochemistry. Interscience,
New York 1963, Vol. I. (The last-named book could not be
considered for the purpose of this manuscript).
111 D.H . R. Barton, J. M . Beaton, L. E. Geller, and M . M . Pechet,
J . Amer. chem. SOC.82,2640 (1960); 83,4076 (1961); A . L. NussAngew. Chem. internat. Edit. / Vol. 4{1965)
No. 3
baum, F. E. Carlon, E. P . Oliveto, E. R. Townley, P. Kabasakalian,
and D.H . R. Barton, ibid. 82, 2913 (1960); D. H. R. Barton and
J. M. Beaton, ibid. 83,750 (1961); 84,199 (1962); C. H. Robinson,
0. Gnoj, A . Mitchel, R. Wayne, E. R.Townley, P. Kabasakalian,
E. P. Oliveto, and D. H. R. Barton, ibid. 83, 1172 (1961); A. L.
Nussbaum, C. H. Robinson, E. P. Oliveto, and D. H . R. Barton,
ibid. 83, 2400 (1961); H . Reimann, A . S . Copomaggi, T.Strauss,
E. P. Oliveto, and D. H . R. Barton, ibid. 83, 4481 (1961); M .
Akhtar and D. H . R . Barton, ibid. 84, 1496 (1962); M. Akhtar,
M . Akhtar, D. H . R. Barton, J. M . Beaton, and A . G. Hortmann,
ibid. 85, 1512 (1963); P. Kabasakalian and E. R.TownIey, ibid.
84, 2711, 2123, 2124 (1962); P. Kabasakalian, E. R. Townley, and
M . D.Yudis, ibid. 84, 2116, 2118 (1962); M . Akhtar and M. M .
Pechet, ibid. 86, 265 (1964); Review: A . L. Nussbaum and C. H .
Robinson, Tetrahedron 17, 35 (1962).
21 1
T h e reaction discovered by Barton [ I ] (Scheme I) provides a
remarkably simple partial synthesis of aldosterone (2) from
corticosterone acetate ( I ) [2].
Corey has developed t w o elegant total syntheses of sesquiterpenes. T h e synthesis of DL-caryophyllene ( 5 ) begins with the
photochemical addition of isobutylene (3) t o cyclohexen-2one ( 4 ) 131.
and were able t o synthesize cantharidin (16) from dehydrocantharidin (15) using t h e s a m e reaction IS].
a* @$
' :
1 part
T h e synthesis of dihydrocostunolide (8) involves a light-induced rearrangement of t h e conjugated cyc!ohexadiene (6) t o
its valence isomer, the conjugated triene (7) [4].
According t o Scott 191, the unsensitized photo-oxidation of 7chloroanhydrotetracycline(17) leads t o formation of t h e 6hydroperoxy compound ( I 8 ) which can be catalytically hydrogenated t o tetracycline (19); this reaction sequence provides a partial synthesis of tetracycline.
Eschenmoser used a photochemical acyl rearrangement ( 9 ) +
(10) in his studies o n t h e synthesis of highly substituted corrin derivatives [5].
f 8)
i I81
T h e last step in Inhofen's total synthesis of vitamin D3 is the
photochemical trans-cis isomerization of 5,6-trans-vitamin
( I I ) to give the 5,6-cis-vitamin D3 (12) [6].
Finally, t h e light-induced selective autoxidation of purpurin
(20) t o t h e ketoaldehyde (21) plays a n important part in
Woodward's synthesis of chlorophyll a (22) [lo].
Schenck a n d Ziegler used t h e photosensitized autoxidation of
a-terpinene (13) f o r the partial synthesis of ascaridol (14) [7],
[ 2 ] D. H . R. Barton and J. M . Beufon, J. Amer. chem. SOC.82,
2641 (1960); 83, 4083 (1961).
[3] E. J. Corey, R. B. Mitruu, and €2. Uda, J. Amer. chem. SOC.
85, 362 (1963).
[41 E. J. Corey and A . G. Hortmann, J. Amer. chem. SOC. 85,
4033 (1963).
(51 A . Eschenmoser, Pure appl. Chem. 7, 297 (1963).
[6] H. H. Inhoffen, G . Quinkert, H . 4 . Hem, and H . Hirschfeld,
Chem. Ber. 90,2544 (1957); Review: H. H. Inhoffen and K . Irmscher, Fortschr. Chem. org. Naturstoffe 17, 70 (1959); K . Irmscher in H. LettrP, H. H. Inhoffen, and R. Tschesche: u b e r Sterine, Gallensauren und verwandte Naturstoffe. 2nd Edit., Thieme,
Stuttgart 1959, Vol. 11.
171 G . 0. Schenck and K . Ziegler, Naturwissenschaften 32, 157
(1944); Review: G . 0. Schenck, Angew. Chem. 64, 12 (1952).
[8] K. Ziegler, G. 0. Schenck, E. W. Krockow, A . Siebert, A . Wenr,
and H. Weber, Liebigs Ann. Chem. 551, I (1942); G . 0. Schenck
and R. Wirtz, Naturwissenschaften 40, 581 (1953); Review: G . 0.
Schenck and K . Ziegler in: Festschrift A. Stoll. Birkhauser, Basel
1957, p. 620.
[9] A. 1. Scott and C.T. Bedford, J . Amer. chem. SOC. 84, 2271
[lo] R . B. Woodward et al., J. Amer. chem. SOC.82, 3800 (1960);
Review: R. B. Woodward, Angew. Chem. 72, 651 (1960); Pure
appl. Chem. 2, 383 (1961).
Angew. Chem. internat. Edit.
Vol. 4 (1965) 1 No. 3
The following review deals with three photochemical
reactions which lead to similar results: cyclic ketones
are converted to open-chain carboxylic acids by the
action of ultraviolet light. Although the mechanisms of
the three reactions are not yet fully known, they certainly
differ from one another in detail. Nevertheless, a
review ought to be useful even at this stage. The following
discussion will be deductive, i. e. for the sake of clarity the
general principle will be explained first and after
a provisional interpretation has been given, the experimental facts will be described.
11. Photochemical Formation of Substituted
Hexadienoic Acids or Their Derivatives from
ortho-Blocked Cyclohexadienones
and (266) are present together. Their ratio depends
on the rate constants kl, k2, k3, and k4, i. e.
on the wavelength of the incident light [Ilc], as
well as on the difference between the free energies
of (25) and (26a) on the one hand, and of (26a) and
(266) on the other. Disturbance of the system
leading to a permanent chemical change, can occur
either by reaction of the unsaturated ketenes with a
nucleophile H-X containing a proton to give the acids
or their derivatives, or by irreversible removal of the
substituent R2 from (25). In the latter case, phenols are
formed, either by a dienone/phenol photo-rearrangement to (27), or by homolytic photo-dissociation followed by abstraction of hydrogen, e.g. from the solvent,
to give (28).
1. General
R 3
R 3
If, for example, a solution of (23) in ether saturated
with water is irradiated with the unfiltered light from a
high-pressure mercury lamp, the unsaturated carboxylic
acid (24) can be isolated after a short reaction-time.
This example is typical of the recently discovered photochemical ring-opening in cyclohexadienones having
blocked ortho-positions [l I], which has already proved
to be widely useful (see Table 1). The yield of about
50 % [l la] is certainly not the best possible, since the
ketene diacetate grouping in (24) is very unstable.
Scheme 2. Possible courses for the photochemical reaction of
cyclohexadienones having blocked orrho-positions.
1958, 197; J. chem. SOC.(London) 1960, 1.
Assuming that the values of kl, k2, and k4 are relatively
large, the extent to which the competing reactions take
place is determined by koI and koII, and by the sums
(k3 + k51)and (k3 + k511). Provided that the substituents
in (266) do not cause mutual steric hindrance, k3 is so
large that a weakly nucleophilic reactant H-X is
sufficient to make the rates of the reactions via (26b)
considerably greater than the rate of phenol formation.
If, on the other hand, mutual repulsion occurs between
R1 or R2 and R3, and between R3 and R4 in (26b), k3
decreases and the formation of the acid derivatives
(29) and (30) predominates over the formation of
phenols only if a strong nucleophile H-X ( e . g . a
primary amine) is used. With weaker nucleophiles H-X
( e . g . HzO), (k3 + k51) and (k3 + k511) are comparable
to kor and ko11, and in the extreme case only phenols
are formed. This situation always exists when a solution
of (25) is irradiated with ultraviolet light in the absence
of protonic nucleophiles.
[ l l a ] The yield determined by ultraviolet spectroscopy of the
irradiation product is 86 94.
[ I I b l The structure of the excited ketones and the mechanism of
their physical and chemical stabilization are not considered in
this paper.
[ I Ic] The photochemical investigations described here were
usually carried out with the unfiltered light from a high-pressure
mercury lamp. In glass vessels the reactions are qualitatively the
same, though somewhat slower, a s those in quartz vessels.
The ease with which the reaction proceeds depends on
the number and positions of the substituents in the
cyclohexadienone molecule, and on the nucleophilic
character of the protonic compound H-X which must
be added to the solution.
2. Reaction Mechanism
After the ketone molecule (25) [ l l b ] is excited by the
absorption of light, it is stabilized by cleavage of the
bond between the carbonyl group and the adjacent
tetrahedral carbon atom. This results in the formation
of a conjugated diene-ketene which can occur in two
geometrically isomeric configurations (26a) and (26b).
In the photostationary state the isomers (25), (26a),
[ I I ] D . H . R . Barton and G. Quinkert, Proc. chem. SOC.(London)
Angew. Chem. internot. Edit.
Vol. 4(l965)
No. 3
However, in the reactions leading to phenol formation
the substituent R2, which is to be eliminated, must have
certain characteristics. The dienone/phenol photorearrangement (25) + (27) requires R2 to be a
nucleophilic group. The term “dienone/phenol photorearrangement” refers only to the overall reaction; the
assumption that polar species are involved in the primary photochemical step [l l -131remains to be proved.
3. Scope of the Reaction
Table 1 contains a list of ortho-blocked cyclohexadienones, the photochemical behavior of which corresponds to Scheme 2. Apart from (41) (which will be
discussed in greater detail below), all ortho-blocked
cyclohexadienones investigated yield acid derivatives
of type (29) or (30) in the presence of a suitable
Table 1. Photochemical reactions of orlho-blocked cyclohexadienones accoi‘ding to Scheme 2.
[type (25)l
Irradiation product [type (29) or (3011 in the presence of
type (29)
RI = CH3, R2 = OAc,
R3 = R4 = H, X = OH
R1= OAc, R2 = CH,,
R3= R4= H, X = OH [a,bl
R ’ = C H 3, R2 = OAc,
R3 = R4 = H [I 11
type (29)
R’ = R2 = CH3,
R3 = R4= H, X = OH [d]
R1= R2= OAc
R3 = R4 = H 1141
type 1291
R1= RZ = OAc,
R3= R4= H, X = OH [el
type (29)
RI = R2 = OAc,
R3 = CH3, R4 = H, X == OH
[= (2411
R1= R3 = CH3
RZ= OAc, R4 = H [11]
oily mixture consisting mainly of
type (30)
R1= R3 = CH3, RZ = OAc,
R4 == H, X = OH
R1= OAc, R2 = R3 = CH3,
R4 = H, X = OH [c]
and up to 10 % of a phenol If]
type (30)
R’ R3 = CHI, R2 = OAc, R4 = H,
X = NH-CaHl,
R’ = OAc, R2 = R3 = CH3,
R4= H, X = N H - C ~ H ~Id]
oily mixture of roughly equal
parts of (29) and (30)
type (29)
R1= R4 = CH3, R2 = OAc,
R1= OAc. Rz = R4 = CH3,
R3 = H, X = OH
type (30)
R1= R4 CHI, Rz = OAc,
R1= OAc, R2 = R4 = CH3,
R3 = H, X = OH
type (29)
R1= R4 = CH3, R2 = OAc,
R3 = H, X = N H - G H I ~
also in the absence of a nucleophile mixture of various phenols.
including (27) with
R l s R3 = R4 = CH3,
R2 = OAc [a)
and (28) with
R1= R3= R4= CH, [fl
type (30)
R1= OAc, R2 = R3 = R4 = CH3,
R1= R3 = R4 = CH3, R2 = OAc,
X = NH-CsH11 [c)
RI = OAc, R2 = R4 = CH3,
R3 = H, X = NH-C aHii Id1
R1= R4 = CH3,
R2 = CHt-CH=CHz,
R! = R3 = R4 = CHI,
1121 H. E. Zimmermann and D . I. Schuster, J. Amer. chem. SOC.
84,4527 (1962); H. E. Zimmermann, Tetrahedron 1963, 393.
1131 W. G. Dauben, K. Koch, 0. L. Chapman, and S. L. Smith,
J . Amer. chem. SOC. 83, 1768 (1961); 0. L. Chapman and S. L.
Smith, J. org. Chemistry 27, 2291 (1962).
Angew. Chem. internat. Edit. / Voi.4 (1965) / No. 3
Table I . (Continued)
together with a mixture of
phenols, including
l-methyl-5,6,7,8-tetrahydro2-naphthol [gl
f41b) may yield 1 5 % of amide
mixture of dimers from (41a)
( 4 l a ) , R = OAc
(4Ib), R = CH3
[a1 Chemically proved.
[bl On irradiation in the absence of a nucleophile, (28) with R1 = CH3, R3 = R4 = H is formed together
with other phenols.
Icl Irradiation of (31) in the presence of aniline yields the crystalline anilide of type (28), with R1 = CH,,
= NH-C6H5 or R1 = OAc, R2 = CH3, R3 = R4 = H,X = NH-C6H5.
R2 = OAc, R3 = R4 = H, X
Id1 Verified by ultraviolet spectroscopy.
[el Deduced by analogy.
If1 Determined by paper chromatography.
Igl Mixture of phenols on irradiation in the absence of nucleophiles.
protonic nucleophile. The strongly nucleophilic cyclohexylamine gives the highest yields, and - with the
exception of (41) - always leads to the product formed
by ring cleavage.Water, being a weaker nucleophile(smal1
values of kgI and k511), does not form carboxylic acids if
there are two 1,3-interactions [cf. (26b)l between substituents R1 (or R2) and R3 and between R3 and R4.This is
the case with, for example, (37) and (38). In contrast to
(37), compound (38) can be converted to phenols. The
formation of a phenol in competition with the formation
of acid derivatives is noticeable even if only substituents
R1 (or R2) and R3 are present, as in (34); up to 10 % of
phenol are formed in this case [13a]. A number of
cyclohexadienones having an acetoxy group on the
tetrahedral ring-carbon atom were irradiated in the
absence of protonic compounds; they all reacted to
form phenols [see (31), (38), (39)].
T h e peculiar behavior of (41) can be explained by the fact
that one of the two C = C bonds of the cyclohexadienone system belongs to a fused aromatic ring. Consequently, ( 4 1 ~ )
preferentially undergoes dimerization in the presence of water, like an qP-unsaturated ketone. Even in the presence of
cyclohexylamine, no amide could be isolated from (41b) [IS];
if any was formed, the yield must have been less than S o / , .
That the fused ring must be aromatic in order t o produce the
behavior of (41) is shown by the irradiation of (39): in
aqueous ether, the main product is (40);this is accompanied
by a mixture of phenols, in which 1-methyl-5,6,7,8-tetrahydro2-naphthol has been identified. As expected, therefore, (39)
behaves like (34) [15].
The light-induced conversion of ortho-blocked cyclohexadienones to dienecarboxylic acids has also been used in natural-product chemistry. Thus (43a) has been obtained from
(42) which was derived from Y-santonin 1161, and photo[13a] The mother liquor was not investigated in the case of (23).
1141 G. Billek, J . Swoboda, and F. Wessely, Tetrahedron 1962,
(151 G. Quinkert, G. Buhr, and A. Strijewski, unpublished work.
1161 W. G. Dauben, D . A. Lightner, and W. K. Hayes, J. org.
Chemistry 27, 1897 (1962).
Angew. Chem. internat. Edit.1 Vol. 4(1965)
No. 3
santoninic acid (47) has been produced from the recently
discovered cyclohexadienone (46u) [17, IS], which is itself
formed from santonin (44), via lumisantonin (45), by-the
action of light 1191.
(43a): R = H
(43b): R = CH3
The racemization of optically active usnic acid [(48a) or (48b)l
is interpreted [20) as involving the ketene (49), which is formed by ring opening, as an intermediate. In keeping with this
I171 0.L. Chapman and L. F. Englert, J. Amer. chem. SOC.85,
3028 (1963).
[18] M . H. Fisch and J. H . Richards, J. Amer. chern. SOC.85,
3029 (1963).
1191 D . H. R . Barton, P. de Mayo, and M . Shafiq, J. chem. SOC.
(London) 1958, 3314; D. Arigoni, H . Bosshard, H. Bruderer, G.
Biichi, 0. Jeger, and L. J. Krebaum, Helv. chim. Acta 40, 1732
(1957); E. E.vanTumeIen, S. H. Levin, G . Brenner, J. Wolinsky,
and P . Aldrich, 3. Amer. chem. SOC.81, 1666 (1959).
I201 G. Stork, Chem. and Ind. 1955, 915.
interpretation, optically active usnic acid looses its optical
activity [21] a n d is converted into t h e racemate [I11 when
irradiated with ultraviolet light.
open-chain or cyclic ketones, discovered shortly after
the turn of the century by Ciamician and Silber 123-271.
For a long time these reactions have attracted little
attention and have only recently found application in
the assignment of constitution in terpene chemistry
[28,29]. Their mechanism has recently been largely
elucidated [30-331. Investigation of the light-induced
formation of carboxylic acids (and their derivatives)
from cyclic unconjugated ketones of different ring sizes
and different degrees of substitution in the presence of a
protonic nucleophile has led to the interpretation shown
in Scheme 3.
2. Reaction Mechanism
4. Structure of the Reaction Products
A priori it is difficult to predict which of the two
diene-carboxylic acids in Scheme 2 [(29) and/or (30)
or their derivatives] will be formed. It is conceivable
that the primary product is not the most stable one and
undergoes partial or complete isomerization [ 111, especially in the presence of an amine; however, it is more
likely that competing 1,2- and 1,4-additions onto (26b)
or even onto (26a) occur [21a]. The constitution of
(29) and the configuration of the internal double bond
of the conjugated diene portion follow from the characteristic ultraviolet absorption [l 11.
111. Photochemical Conversion of Cycloalkanones
to Open-Chain Saturated Carboxylic Acid
1. General
If a solution of Scc-cholestan-3(3-ol-6-one (50) in acetic
acid is irradiated in a quartz vessel under nitrogen with
the unfiltered light of a high-pressure mercury-lamp,
acid (51) can be
isolated in 54 % yield. This is a typical example [22] of
the “photochemical hydrolysis” of unconjugated [22a]
[21] S. Mackenzie, J. Amer. chem. SOC.77, 2214 (1955).
[21a] In the ultraviolet irradiation of verbenone [ J . J . Hnrst and
G . H. Whitham, J . chem. SOC.(London) 1960, 28641 in the presence of protonic nucleophiles, base-catalysed isomerization of
the primary product to that isolated was found to be unlikely.
[22] G. Quinkert, 8. Wegemund, F. Homburg, and G. Cimbollek,
Chem. Ber. 97, 958 (1964).
[22a] The term “unconjugated ketones” includes saturated
ketones, as well a s unsaturated ketones in which the unsaturated
function is not conjugated with the carbonyl group.
The ketone molecule (52) is excited by the absorption
of light and is stabilized by homolytic cleavage of
the bond between the carbonyl group and the C-atom
which most favors fission owing to the nature and the
number of its substituents. The alkyl-acyl diradical
(53), which is the primary product of the photochemical
reaction [33a], then preferentially undergoes reactions
in which both radicals disappear simultaneously.
Recombination leading to the reformation of (52) is
reflected in the quantum yield. If coupling of the un[23] G. Ciamician and P . Silber, Ber. dtsch. chem. Ges. 36, 1582
1241 G. Ciamician and P. Silber, Ber. dtsch. chem. Ges. 40,2415
[25] G. Ciamician and P. Silber, Ber. dtsch. chern. Ges. 41, 1071
[26] G. Ciamician and P. Silber, Ber. dtsch. chem. Ges. 41, 1928
[27] G. Ciamician and P. Silber, Ber. dtsch. chem. Ges. 43, 1340
(1 9 10).
1281 D. Arigoni, D . H . R. Barton, R. Bernasconi, C. Djerassi,
J. S . Mills, and R. E. Wolfl, Proc. chem. SOC.(London) 1959, 306;
J. chern. SOC.(London) 1960, 1900.
I291 J. Krepinsky, M . Romanik, V . Herout, and F. Sorm, Tetrahedron Letters 1960, No. 7, 9 ; 1962, No. 5, 169; H. Hikino,
Y. Hikino, Y. Takeshita, K . Meguro, and T. Takemoto, Chem.
pharmac. Bull. (Tokyo) 11, I207 (1963).
DO] G. Quinkert, B. Wegemund, and E. Blanke, Tetrahedron
Letters 1962, No. 6, 22.
I311 G. Quinkert, B. Wegemund, F. Homburg, and G. Cimbollek,
Chem. Ber. 97, 958 (1964).
[32] G. Quinkert, E. Blanke, and F. Homburg, Chem. Ber. 97,
1799 (1954).
I331 G. Quinkert, G. Buhr, and A . Moschel, Chem. Ber., in the
[33a] In contrast to the alkyl-acyl radical pairs formed in gasphase photolysis of open-chain ketones below 100 “ C [40],
occurrence of the alkyl-acyl diradical of cyclic ketones has not
been experimentally proved. Nevertheless, the light-induced
formation of carboxylic acid derivatives from cyclic ketones via
the alkyl-acyl diradical is discussed here; until formation of this
diradical will be experimentally confirmed o r refuted, this is
justified by analogy with open-chain ketones. Regarding the
problem of intermediate diradicais, see [41].
Angew. Chem. internat. Edit.
1 Vol. 4
(1965)I No. 3
paired electrons leads to the formation of an unsymmetrically substituted center, the epimeric configurations
(54) and (52) may arise. Since both ketones absorb in
approximately the same spectral region, the system
reaches a photostationary state via the diradical. The
position of this state depends on the wavelength
of the incident light and on the difference between the
rates of the competing recyclization reactions.
Scheme 3. Photochemical formation of carboxylic acid derivatives from
unconjusated ketones.
In the case of 17-ketosteroids, this photo-epimerization
was first observed by Butenandt [34]. It was used
preparatively in this field [35-381 and was later recognized to be reversible [39]. The photochemical
pseudo-equilibrium between (52) and (54) lies far on
the side of (54).
Intramolecular disproportionations of (53) (by migration of a hydrogen atom from the position adjacent
to the acyl radical to the alkyl radical-site) can lead
to the formation of the isomeric ketenes (57) or (57a),
or to a mixture of both. These react with water to give
the carboxylic acids (59), X = OH, and (59a), X = OH;
[34] A . Butenandt and A. Wolff;Ber. dtsch. chem. Ges. 72, 1121
(1939); A. Butenandt and L. Poschmann, ibid. 73, 893 (1940);
A . Butenandt, A . M‘olff; and P . Karlsen, ibid. 74, 1308 (1941);
A . Butenandt, W . Friedrich, and L. Poschmann, ibid. 75, 1931
(1942); A . Butenandt and L. Poschmann, ibid. 77, 392, 394 (1944).
[35] J . R . Billeter and K . Miescher, Helv. chim. Acta 34, 2053
1361 D . H . R . Barton, A . da Campos-Neves, and A. 1. Scott, J .
chem. SOC.(London) 1957, 2698.
1371 J . P. L. Bots, Recueil Trav. chirn. Pays-Bas 77, 1010 (1958).
1381 H . Zeugner, Ph. D. Thesis, Technische Hochschule Braunschweig, 1963.
[39] H . Wehrli and K . Schaffner, Helv. chim. Acta 45, 385 (1962).
Angew. Cliern. internat. Edit.
Vol. 4(1965)
1 No. 3
reaction with cyclohexylamine yields the amides (59),
x = N H - C ~ H I I ,and (Sga), X = N H - C ~ H I ~When
the ketone (52) is irradiated in the absence of protonic
nucleophiles, the intermediate formation of ketenes can
be detected by infrared spectroscopy [32].
A reaction which competes with the formation of
ketenes is the formation of the unsaturated aldehyde
(58), also by intramolecular disproportionation ; in this
case, a hydrogen atom migrates from the position
adjacent to the alkyl radical to the acyl radical-site.
Such ring cleavages of unconjugated cyclic ketones
leading to unsaturated aldehydes have been known
since the time of Ciamician and Silber [24-27,42,43];
other examples were described recently [31,44-491.
That the H-migrations leading to the formation of
(57), (57a), and (58) from (53) are intramolecular has
been shown with the aid of deuterated ketones, both in
the formation of ketenes [31] and of aldehydes [45].
Owing to the competition between both photo-isomerizations of the epimers (52) and (54), the yields of
(57), (57a), and (58) are mutually interdependent. The
cyclic transition states of the competing processes can
be represented by formulae (55), (55a), and (56). All
other factors being equal, the extent to which each
disproportionation occurs will be determined by the
differences in ring strain [33].
Intramolecular hydrogen shifts are important in the
chemistry of free radicals. They may remain unnoticed,
or cause branching in a growing polymer chain [SO]. A
six-membered cyclic transition state seems to be most
favorable [51]. This is so in the Hofmann-LofflerFreytag reaction [52] as well as in the photo-isomerizations described here and in the case of the Barton
reaction [1,52a]. In contrast to the formation of the
aldehyde (58), there is no uncertainty regarding the
source of the migrating H-atom in the formation of the
ketenes (57) and (57a). If the terminus of the hydrogen
shift becomes an unsymmetrical center [as in the case
of (57) and (57a)], its configuration generally cannot be
predicted. If the homolytic transfer of the H-atom in (53)
leads via (55) and (55a) to different reaction products
[401 E . W . R . Steacie: Atomic and Free Radical Reactions. 2nd
Edit., Reinhold, New York 1954, Vol. 1.
[41] G. Quinkert, Pure appl. Chem., in the press.
1421 G. Ciamician and P . Silber, Ber. dtsch. chern. Ges. 42, 1510
[43] G. Ciamicinn and P . Silber, Ber. dtsch. chem. Ges. 46, 3077
(191 3).
[44] M . S. Kharasch, J . Kuderna, and W . Nudenberg, J. org.
Chemistry 18, 1225 (1953).
[45) R. Srinivasan, J. Amer. chern. SOC.81, 1546 (1959).
[461 R . Srinivasan, 1. Amer. chem. SOC. 81, 2601 (1959).
[47] R. Srinivasan, J. Arner. chern. SOC.81, 2604 (1959).
I481 R . Srinivasan, J. Amer. chem. SOC.81, 5541 (1959).
[49] P. Bladon, W . McMeekin, and I. A . Williams, J. chem. SOC.
(London) 1963, 5727.
1501 C. Walling: Free Radicals in Solution. Wiley, New York
1957, p. 195.
[51] C. A . Grob and H . Kammiiller, Helv. chim. Acta 40, 2139
[52] E. J. Corey and W . R . Hertler, J. Amer. chern. SOC. 82,
1657 (1960).
[52a] Note added in proof: see also K . Heusler and J . Kalvoda,
Angew. Chem. 76, 518 (1964); Angew. Chem. internat. Edit. 3 ,
525 (1964).
(57) and (57a), the difference in the ring strains of (55)
and (55a) influences the stereoselectivity of this reaction
(see below and especially [33]).
3. Scope of the Reaction
Apart from acetone and methyl ethyl ketone (which
give acetic acid together with methane or ethane on
irradiation in aqueous solution [23,24]), only cyclic
ketones were studied. Table 2 shows a number of
The yields of the acidic products or their derivatives are
generally between 30 and 70 %.
In the case of the ketones (68) and (70) no preferred
direction for the %-cleavageis to be expected. While (69)
gives no nidication of the site of the ring cleavage, the
3-ketosteriod (70) yields two carboxylic acids (71) and
(72) by approximately equal cleavage of ring A betwzen
C-3 and C-2 and between C-3 and C-4. The other
ketones listed in Table 2 are substituted with alkyl
groups to varying degrees at the positions adjacent to
the carbonyl group. In each case, only the product
Table 2. Photochemical conversion of cycloalkanones to saturated carboxylic acids and their
derivatives according to Scheme 3.
Irradiation product
HOOC- ( C H Z ) ~ - C H 16
~ I)
1 oooH
examples which illustrate the scope of the reaction and
the relationship between the structures of the cycloalkanones irradiated and the resulting carboxylic acids.
[531 F. Homburg, Ph. D. Thesis, Technische Hochschule Braunschweig, 1964.
corresponding to ring cleavage on the side of the more
highly substituted a-C-atom was described. The 17-ketosteroids (73) and (75) have been particularly closley
studied with respect to the specificity of the ketene
formation [33]: in the presence of water, both (73) and
Angew. Chem. internot. Edit. / Vol. 4 [1965) 1 No. 3
(75) yield only the primary carboxylic acid (74); the
tertiary acid (76)is not formed [53a].
The photochemical conversion of unconjugated cycloalkanones into the carboxylic acids by addition of one mole of
H20 has also been used in natural-product chemistry [28].
The ultraviolet irradiation of P-amyrone (80) yields the dihydro derivative (81) of nyctanthic acid (82). The structure
of (82) was deduced from this reaction 1541. Likewise, the
y-lactone (83) obtained from hydroxydammarenol-I1 can be
converted to (84) which was correlated to a reaction product
of dammarenolic acid.
photochemically excited keto group, as in (85), then a
photofragmentation [e.g. to yield (86) and (86a)I or a
photocyclization [59], e. g. leading to (87), may take
place. These reactions are initiated by an intramolecular
hydrogen shift. Of course, the entire process may occur
in a single step [59a].
186) 1860)
Scheme 4. Photofragmentation and photocyclization of ketones.
If the structural requirements for intramolecular
hydrogen transfer to the oxygen atom of the photochemically excited keto group (Scheme 4) are not satisfied, other reactants (particularly the solvent) may
assume the role of H-donors. Depending on the nature
of the ketone and the coreactant, the products may be
alcohols [e.g. (88) and (90) from (89)I [31] or alcoholic
condensation products [e.g. in the addition of isopropanol to maleic acid, photosensitized by benzo(91) which reacts further to yield
Only very small quantities of an acid derivative are obtained
on irradiation of solutions of camphor (64) containing a protonic nucleophile. Comparison with homocamphor (66)
shows that ketene formation increases with the transition
from the cyclopentanone ring to the higher homologue. The
same is true for the homologues (73) and(77). Thecompound
with the cyclopentanone ring (73) gives a 28% yield of the
carboxylic acid (74), while (77) is converted into the corresponding carboxylic acids in a yield of almost 50% 1331.
Besides the formation of unsaturated aldehydes, which is related to the formation of ketenes, there are other competing
reactions which limit the synthetic usefulness of this method
of preparing acids or their derivatives. Such reactions may be
due to the structure of the ketone t o be irradiated: a+'phenyl- 155, 561 or vinyl-substituted 1571 ketones undergo rapid light-induced decarbonylation which prevails over the
formation of acids (or their derivatives).
The formation of acids (or their derivatives) can also
decrease as a result of photoreduction. If a hydrogen
atom occupies a favorable position with respect to the
[53a] The absence of (76) is consistent with the ketene formation
by homolytic hydrogen abstraction from the position adjacent to
the acyl radical, since a-cleavage of (73) and (75) between C-16
and C-17 yields an alkyl-acyl diradical with no hydrogen in the
position adjacent to the acyl radical.
1541 G. H . Whitham, J . chem. SOC.(London) 1960, 2016.
1551 G. Quinkert, K . Opitz, W. W . Wiersdorff, and J . Weinlich,
Tetrahedron Letters 1963, 1863.
1561 K. Mislow and A . J. Gordon, J. Amer. chem. SOC.85, 3521
[57] 0. L. Chapman, D. J . Pasto, G. W . Borden, and A . A . Griswold, J . Amer. chem. SOC.84, 1220 (1962); G . 0 . Schenck and
R . Steinmetz, Chem. Ber. 96, 520 (1963); D. L. Schuster, M . Axelrod, and J. Auerbach, Tetrahedron Letters 1963, 191I .
[SS] R . G . Norrish and M. E. S . Appleyard, J . chem. SOC.(London) 1934, 874; W . Davis j r . u. W . A. Noyes jr., J . Amer. chem.
SOC.69, 2153 (1947); J. N. Pitis jr., J. chem. Educat. 34, 112
(1957); R . Srinivasan, J. Amer. chem. SOC.81, 5061 (1959).
Angew. Chem. internat. Edit. / Vol. 4(1965)
/ No. 3
i 91)
[59] N. C . Yang and D. H. Yang, J. Amer. chem. SOC.80, 2913
[59a] In particular, see the photofragmentations and photocyclizations carried out with steroid ketones in the laboratories
of 0. Jeger and N. C. Yang [60-621.
1601 J. M . Erikson and D . L. Forbess in: Steroid Reactions.
Holden-Day, San Francisco 1963, p. 321.
[61] N. C. Yang, A. Morduchowitz, and D. H . Yang, J. Amer.
chem. SOC.85, 1017 (1963).
[62] I. Orban, K . Schaffner, and 0.Jeger, J. Amer. chem. SOC.85,
3033 (1963); K. Schaffner, D. Arigoni, and 0. Jeger, Experientia
16, 169 (1960).
[63] G. Ciamician and P. Silber, Ber. dtsch. chem. Ges. 44, 1280
(1911); G. 0. Schenck, Angew. Chem. 69, 579 (1957); A . Schonberg: Praparative Organische Photochemie. Springer, Berlin
1958, p. 109; P. de Maya in: Advances in Organic Chemistry.
Interscience, New York 1960, Vol. 11, p. 372; W. G . Moore, G . S.
Hammond, and R. P. Foss, J. Amer. chem. SOC. 83,2789 (1961);
G. S. Hammond, W. P. Baker, and W . M. Moore, J. Amer. chem.
SOC.83, 2795 (1961).
[64] G. 0. Schenck, G. Koltzenburg, and H . Grossmann, Angew.
Chem. 69, 177 (1957).
Unsaturated substituents, the p-orbitals of which
overlap with the p-orbital of the alkyl radical, favor
homolysis and help to determine the site of ring cleavage.
T h e solvent should have the lowest possible tendency t o donate
hydrogen a toms t o free radicals. This condition is best satisfied by water a nd benzene; however, these can scarcely b e considered a s solvents f o r the formation o f carboxylic acids, since
ketones usually a r e n o t sufficiently soluble in water, and t h e
water required for the reaction is not sufficiently solublein benzene. Acetonitrile, glacial acetic acid, andalcohols (particularly
tertiary alcohols) have been used, whereas aliphatic or cycloaliphatic ethers strongly favor the intermolecular photoreduction.
The configuration of the carboxylic acid (or derivative)
formed cannot be predicted when a center of
asymmetry is involved in the light-induced ring cleavage,
as in the case o f (73) and (77). Comparison of the
products obtained in these cases [(74) from (73), and
(78) + (79) from (77)] shows that the stereoselectivity
Finally, a group of unconjugated ketones should be
[67a] decreases as the size of the cycloalkanone ring
mentioned, from which the light-induced formation of
This can be plausibly explained by assuming
acids or their derivatives is impossible. In fenchone (93)
ketone gives less ring strain in the
and 16,16-dimethyl-3~-methoxy-A*-androsten-17-one
cyclic transition state for the formation of ketenes than
(94), neither C-atom adjacent to the light-absorbing
a five-membered ketone [higher yield of acid from (77)
carbonyl group carries a hydrogen atom; consequently,
these compounds cannot undergo intramolecular hydrogen shift to form a ketene. On irradiation in the
presence of water, (93) and (94) do not yield the carboxylic acids which would arise by incorporation of the
elements of water [32,65].
than from (73)], and that the difference between the
ring strains of the transition states (95) and (95a), which
are configurational isomers, is less than in the case of(55) and ( H a ) .
/ 94)
4. Structure of the Reaction Products
The structure of the carboxylic acids and their derivatives which result from the photochemical reaction of a
cycloalkanone [e.g. (52)]is determined by constitutionspecific cleavage of the ketone ring to yield the most
stable diradical [e.g. (53)], and consequently depends
on the nature of the substituents in the a- and a'positions. The rule that light-induced cleavage of cyclic
ketones takes place on the side of the more highly alkylsubstituted carbon atom was stated by Ciamician and
Silber [24,25,42] and has subsequently been used
repeatedly as a guide ( e .g. [28]) [65a].
[65] B. Wegemund, Diploma Thesis, Technischc Hochschule
Braunschweig, 1960.
[6SaJ The constitution-specific a-cleavage is not self-evident,
since this reaction starts with a photochemically excited molecule. In the gas-phase photolysis, e.g. of methyl ethyl ketone, acleavage occurs in both possible directions [66]. It seems likely
that two different excited species are responsible for the formation
of the different reaction products. If the photolysis is carried out
in the gas phase at low pressure, the electronically excited molecule reacts from the vibrational state reached in the excitation.
In solution, deactivation by a chemical reaction is preceded
by thermodynamic equilibration of the vibrational energy with the
surroundings; this is shown in the gas-phase photolysis of methyl
ethyl ketone by the decrease in specificity of the a-cleavage with
decreasing wavelength of the incident light 1661. Apart from the
chemical reaction, fluorescent emission also yields information as
to the nature of the excited species. Whereas the absorption
spectrum of a substance in solution normally shows that the
electronically excited molecule is also vibrationally excited, the
fluorescent emission spectrum of the same compound in solution
shows that the emitting molecule is already in the vibrational
ground state [67].
[661 See [401, p. 358.
[671 T. Forster: Fluoreszenz Organischer Verbindungen. Vandenhoeck und Rupprecht, Gottingen 1951, p. 133; A. Ehrenberg
and H . Theorell in M . Florkin: Comprehensive Biochemistry.
Elsevier, Amsterdam 1962, Vol. 3, p. 170.
IV. Photochemical Conversion of Cycloalkanones
to Open-Chain, Unconjugated, Unsaturated
Carboxylic Acids
1. General
If a solution of AWanosten-3-one (96) and cyclohexylamine in benzene [68a] is irradiated with the unfiltered
light of a high-pressure mercury lamp, while oxygen is
continuously passed through the solution, a 20 % yield
of 3,4-seco-A4(23)~~-lanostadien-3-oic
acid (97) can be
isolated after 24 hours.
This is a typical example of the recently discovered
autoxidative ring cleavage of cyclic unconjugated
ketones to unsaturated carboxylic acids containing one
[67a] The terms "constitution-specific'' and "stereoselective"
imply 8 connection between the constitution of the starting
material and that of the product, but none between their configurations. Thus, the formation of the carboxylic acid ( 7 4 ) with a
13cr-CH3 group from both ( 7 3 ) and (75) is constitution-specific
and stereoselective; see [33,68].
[68J H . E. Zimmerman, L. Singer, and B. S . Thyagarajan, J. Amer.
chem. SOC.81, 108 (1959).
[68a] Cyclohexylamine is added to convert the ketenes, which
are also formed in the presence of oxygen (see Section Ill), to
cyclohexylamides. This facilitates isolation of the acids formed
by autoxidation.
Angew. Cliem. internat. Edit.
1 Vol. 4
No. 3
Table 3. Photochemical conversion of cyclic ketones to unsaturated carboxylic acids according to Scheme 5 .
(621 (711
Acidic photo-oxidation product
(73) and (75) [69,70]
1104a): R = CHzOH
11046): R = CH3
oxygen atom more than the original ketone 169,701.
This reaction has been used in partial syntheses, especially of the 3-ketotriterpenes (see Table 3).
2. Reaction Mechanism
The products assumed or shown to be formed when
unconjugated cyclic ketones are irradiated with ultraviolet light in the absence of oxygen (see Section 111)
are not precursors of the unsaturated carboxylic acids
which are formed in the presence of oxygen. The hypothetical diradical, e.g. (53), which has been assigned a
key position as the product of the primary photochemical process is also a possible reaction partner for
the molecular oxygen. However, such a reaction is
ruled out by the structure of the acidic photo-oxidation
products [69,70] and by experiments aimed at testing
the unknown reactivity of diradicals towards 0 2 [72].
For structural reasons, the ketene photoisomer of (52),
e.g. (57), cannot be a n intermediate of the photo[69] G . Qrrinkert and H.-G. Heine, Tetrahedron Letters 1963,
[70] H . 4 . Heine, Ph. D. Thesis, Technische Hochschule Braunschweig, 1964.
[70aJ Photo-oxidations which proceed in the absence of sensitizers, and which d o not involve the reaction of radicals formed
by photodissociation with 0 2 , have so far received little attention
[see R . M . Hochstrasser and G . B. Porter, Quart. Reviews 14,
146 (1960)l.
[71] G. Quitikert and H . Oehlschluger, unpublished work.
[72] H. H . Rieckert, Diploma Thesis, Technische Hochschule
Braunschweig, 1963.
Angew. Chrm. internat. Edit.
Vof.4 (1965) / N o . 3
oxidation, and the other disproportionation product
(58) of the alkyl-acyl diradical (53) can also be ruled
out [69,70].
As a working hypothesis it is assumed that the cyclic
ketone (105) in the photoactivated state reacts with the
oxygen molecule [70a]. The postulated hydroxy-hydroperoxy diradical (106) undergoes intramolecular rearrangement to yield the unsaturated peroxyacid (107)
which then forms a carboxylic acid (108) in the usual
manner (see Scheme 5).
Scheme 5. Photo-oxidation of cyclic ketones to unsaturated carboxylic
Reaction of the photo-oxidation product (106) is
initiated by an intramolecular hydrogen-shift [cf. the
light-induced conversion of (85) to (86) and (86a)I ; of
course, this reaction may also proceed in a single step.
A six-membered cyclic transition state is required for
both processes. Thus the cyclic ketone must have at
least one H-atom in the (%position with respect to the
carbonyl group; furthermore, the orientation of this H-
22 1
atom must be such that the centers H-C@)-C(a)-C-0-0
in the hydroxy-hydroperoxy diradical can assume a
cyclic arrangement.
3. Scope of the Reaction
Table 3 shows a number of cases studied so far, in which
a cyclic ketone is converted to an unsaturated carboxylic
acid containing an additional oxygen atom. With the
exception of (62), all the ketones contain a CH3substituent a to the carbonyl group. In these examples
the new C=C double bond, which is formed together
with the carboxyl group, is always terminal.
Unsaturated carboxylic acids of this type have very
recently been found in nature: dammarenolic acid (109)
[28], nyctanthic acid (82) [28,54], roburic acid (102)
[73], and canaric acid (1046) [74].
Compounds (102) and (82) have been obtained in 20 %
yield by irradiation of a-amyrone (101) and P-amyrone
(80) in the presence of oxygen [69]. Similarly, betulone
(103) gave the doubly unsaturated hydroxycarboxylic
acid (104a) [7OJ which presumably occurs in nature, too.
I obtained my first impressions of phatochemistry from
the instructive “studies in the vitamin D series” carried out
together with Prof.H . H. Inhoffen. I then had the good
fortune to witness the development of the study of “photochemical fransformations” under Prof.D. H. R. Barton,
and subsequently embarked on the investigation of “lightinduced reactions” together with capable young colleagues
who find a challenge in unsolved problems and who are
n7entioned by name in the list of references. To all with
whom I could work together I am sincerely gratejul.
i 109)
Consequently, this new photo-oxidation can be used
as a step in the partial synthesis of 3,4-seco-triterpenecarboxylic acids with a A4(23)-doublebond from 3-ketotriterpenes [e.g. (96) -+ (97)J
Received: March 31st. 1964
[A 414/200 IEJ
German version: Angew. Chem. 77, 229 (1965)
Translated by Express Translation Service. London
[73] L. Mangoni and M. Belardini,Tetrahedron Letters 1963,921.
[74] R. M. Carman and D. E. Cowley, Tetrahedron Letters 1964,
Fused Salts and Their Use as Reaction Media [*]
Apart from being of interest jor physicochemical investigations, ionic liquids are a very
important supplement to the non-aqueous and water-like solvents. The present discussion
of the physical properties and current ideas on the structure of fused salts is followed by a
report on the solubilities of gases, salts and metals. Our knowledge of fused salt baths and
their use in electrochemical and electrometallurgicalprocesses has recentiy been considerably
expanded. Special attention is drawn to chemical reactions in fused electrolytes. Fused
salts can also act as catalysts, so that they may often be advantageous reaction media for
synthesis, if not the only media which can be used.
I. Introduction
Most operations in modern preparative chemistry are
carried out either in or with the participation of a liquid
phase, in which the mobility of the molecules is similar
to that in the vapor phase, but where the density of matter is almost the same as that of a solid. However, reactions are not confined to aqueous systems and to solutions in organic solvents, which could also be regarded
as ~6molecular
melts,9. Verv- *Drofitableuse is made. both
[*] Extended version of lectures delivered to the International
Congress for Pure and Applied Chemistry, London, July 15th,
1963, and in Bonn (Germany) on January 14th, 1964.
in the laboratory [ l ] and in industry, of reaction media
which range from condensed gases to melts of predominantly covalent metal halides, and which are characterized by a slight self-dissociation, similar to that of water.
Surprisingly, however, we rarely find papers describing
the preparative use of ionic liquids as solvents, although
the physicochemical properties of these liquids suggest
that they should also be useful in fields other than electrochemistry.
Fused electrolytes are generally completely dissociated
into ions, and are excellent solvents for salts, metals, and
[I] G. Jnnder, H . Spandau, and C. C. Addison: Chemie in nichtwafirigen Losungsmitteln. Vieweg, Braunschweig 1963.
Angew. Chem. internnt. Edit. / VoI. 4 (1965) No. 3
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