Rolf Hulsgen Universitat Mllnchen Mllnchen, W. Germany translator Hans-Georg Gilde' ~ariettaCollege Marietta. Ohio 45750
II Rudolf Criegee (1902-1975) I
Those who knew Criegee were notably impressed by his modesty and matter-of-factness,but also by his breadth of interest. Criegee's Background . . Rudolf Criegee belongs to the scientists who in the last forty sears sianificantlv influenced the German contribution to brganicchemistr;. A short sketch of his works must necessarily he limited to a few specific areas. Rudolf Criegee was horn in 1902 in Diisseldorf and came from a family of lawyers. At age eighteen, he commenced his studies of chemistrv at the Universitv of Tiihineen without a specific goal. Afte; transferring to t6e ~ n i v e r s i i yof Greifswald. however. it became clear to him that oreanic chemistrv would he his choice. In 1925 Criegee receivid his doctorat from the University of Wiirzbura for work on acridiniuni salu under the direction of Otto ~ h r o t h (For . one to receive a doctorate a t age twenty-three today is a rarity). Criegee qualified himself for university teaching in 1930 and moved in 1932 to the University of Marhurg where Hans Meerwrin taught. Meerwein had-read the firit pul,lications of Crieyec and offwed the young scientist a position as Oberassist~ntIn 1937 Crieeee answered a call to rhe Technical Institute a t Karlsruhe a s ~ r o f e s s o of r Organic Chemistry, a position which in 1947 became a chair of organic chemistry. After the war, Criegee remained loyal to the rebuilt Karlsruhe Institute and declined calls to Kiel, Marhurg, and Bonn.
The curved arrows are used for electron bookkeeping and indicate that during the C-C cleavage, the Ph(1V) accepts two electrons and is reduced t o Ph(I1). The influence of structural changes of the diols-the designation glycol is derived from the simplest diol, the ethylene glycol HOCH2-CH2-OH-on the rate constants in LTA cleavage occupied Criegee even in the fifties. The reaction path via the cyclic lead ester I11 cannot he the only one since trans-diols such as IV and V, which for configurational reasons cannot form the cyclic ester, are cleaved by LTA although slower by a power of ten as th'e isomeric cis-diols.
Criegee's LTA-Selective Oxidizing Ageni Criegee's paper, the Habilitations work, which qualified him for universitv teachine introduced lead tetraacetate (LTA) ad a srlective oxidizing agent was considered a "Treifer." a real hit. and made Crieree's name immedintrlv known. ~ v k in u his fiist publications-criegee's precise, terse form of revortina was revealed. This oolished stvle conceals the lah&ous means and frequentl~ohscuresthe masterful experimentation required to overcome difficulties involved. The cleavage of glycols can he found in every organx texthook. As demonstrated with the crs.I,!!-diol I. Crieree suspected the site of the redox reaction in the via 11'form~dcyclic lead ester 111.
One also finds in the above mentioned Habilitations work. the acetoxylation reaction of alkenes in the so-called allylic position, that is, the pmitiou next to the double bond, another svecific reaction of LTA. shown here for the transformation o? cyclohexene VI-VII
In order to convert alkenes to cis-diols and thereby make the former accessible to LTA cleavage, Criegee searched for a new oxidant which he discovered in osmiumtetroxide with the cyclic osmic ester IX as an intermediate. This method, which
'
Permission to publish a translated version of this article, which was published originally in Chemie in unserer Zeit, [12,49 (1978)], was granted by Verlag Chemie and is hereby gratefullyacknowledged. The translator thanks Mrs. H. C. Gilde and Mrs.E. A. Marcum far their assistance in the preparation of the manuscript. The Photoreproduction was kindly supplied hy Mr. Phil Mazzara. Volume 56, Number 6, June 1979 1 369
Richard Willstatter classified scientists into two groups, those who patiently listen and learn from nature and those who give orders and force her to yield the answer.
has been used primarily in structuredeterminationof natural products, isshownat~wewithacenaphthalene VIll asan example. Autoxidation is defined as the flameless combination of a compound with oxygen a t room or moderate temperature. Alkenes under these conditions yield a mixture of oxidation products. In 1936 Criegee successfully isolated for the first time the 1:l primary product X from cyclohexene and oxygen under light-induced autoxidation followed by high-vacuum distillation purification. Since peroxides a r e notoriously unstable a n d frequently form explosive compounds, t h e greatest care is required i n their manipulation especially during distillations.
0
R-0-0'
R O - O H + FC During the forties i t was discovered that oxygen in the air (that is, triplet oxygen) gives a chain reaction with the organic substrate RH and that traces of certain metal ions are required for initiation. The CH2 group is activated by the neighboring double hond as in VI, as well as by the benzene ring as in XI. In 1944 Hock developed on the basis of the side-chain oxidation of isopropylhenzene (cumene) a phenol synthesis, which is used presently as an industrial process. H o-OH
H XVI
xv X
Criegee recognized that the oxygen attacked the allylic position and not the double bond. Three years before, H. Hock had obtained the crystalline hydroperoxide XI1 from tetralin and oxygen. Since the benzene nucleus of the 1,2,3,4-tetrahydronaphthalene XI did not appear to serve as a site for attack by oxygen, Hock suggested a t first to replace structure XI of tetralin with that of XI11 for only this structure could yield the cyclic peroxide XIV-so ingrained was the prejudice that oxygen must attack the double hond!
XI
--
RH
H
"(jO-OH
VI
R'C 0 2
R-O-0'+
t several
a3 H
XVII
Saturated hydrocarbons, likewise, may undergo autoxidation. Criegee isolated from cis-decalin XV the crystalline hydroperoxide XVI, which has a trans configuration with respect to ring juncture. The intermediate tertiary radical XVII has a planar configuration, that is, all C atoms bonded to the radical site are coplanar. With oxygen the energetic, more favorahle, trans orientation XVI is formed.
steps
IroPropyl
XVIII
alcohol
XI11
XIV
Only in the last decade was it recognized that certain alkenes, indeed, combine with singlet oxygen to form 1.2-dioxetanes, that is, the four-membered rings with two oxygen8 (like XIV). The normal oxygen, in contrast, has two unpaired electrons (triplet oxygen).
Acetone
The industrial synthesis of hydrogen peroxide and acetone by the gas-phase autoxidation of isopropyl alcohol via the hydroperoxide XVIII is one of the most important applications of this reaction scheme. Criegee's studies supplied the scientific basis for these processes.
dXIX
XX
While the a-hydroxy-hydroperoxide XVIII decomposes at 370 1 Journal of Chemical Education
A university education should be bound inextricably and obviously to a performance principle.
high temperature, the reaction can he reversed at low temperature and in solution. Using cyclohexanoue and hydrogen peroxide as an example, Criegee ascertained the route of formation of.the dimeric and trimeric ketoperoxide? XIX and XX. In the conversion of cyclohexanone to the trimeric peroxide XX-in contrast to nine-membered carbocycles, the uinemembered ring with six oxygen atoms in XX is formed easily-several intermediates (among others XIX) could be isolated. Many of these compounds explode upon heating; some explode even by gentle touch. Criegee's Scientific Nature Brilliant experimental technique and courage while working with the peroxides and ozonides permitted Criegee to surmount dangers from which others had shied away. His own pleasure in experimental work and his quality as an academic teacher attracted more than 170 co-workers, who under Criegee's direction did their M.S. and Ph.D. work. Many of these former co-workers now hold academic posts themselves. Criegee probably belongs among those few chemists, who besides supervising a large group of co-workers themselves experimented at the bench late into life, in fact, even more actively after achieving emeritus status. An experimental study published in "Chemische Berichte" as late as 1975 carries Criegee's name alone. Richard Willstdtter classified scientists into two groups, those who patiently listen and learn from nature and those who eive orders to nature and force her to vield the answer. ~ u d l l Criegee f probably was more of the first type. Results obtained through fine observation were connected in thought to previous experience, permitting generalization and opening new aspects. Specific oxidation effect was the main thought dominating the above discussed research. This work included the ozonides Criegee's Mechanlsm of Ozonolysls The elucidation of the ozonolysis of olefins revealed the hand of a master. Two years before his death Criegee had an article on "Die Ozonolyse" published in Chemie in unserer Zeit [VII, 75 (1973)l. Success in these mechanistic investigations depended upon the choice of evincive model systems and Criegee's originality in the choice of such models was most admirable. In 1904 C. Harries introduced the method of ozonolysis as a technique to cleave double bonds in hydrocarbon chains. In fact, it was by this method that the structure of natural rubber was determined.
posed by H. Staudinger. The question, however, remained, how these form with cleavage of the CC-double bond. Criegee's mechanism (1949) envisions a three-step reaction sequence. The primary addition of the ozone to the CCdouble hond is followed by a dissociation into a carbonyl compound and a carhonyloxide (called "Zwitterion" by Crieeee). These two s ~ e c i ethen s recombine hut in a reversed dirt,cr~onto form the isolable ozonide. All rhree reaction steps were later inter~reredas concerted 1.3-di~x)lar cscloadditions . . or the reverse, ?ycloreuersionen.
El .'-[a$] XXI
+
I
The first publication (1948) on this subject culminated in the proof of the existence of carhinoloxide hy trapping it with methanol. What open-mindedness was required to use methanol as a solvent in the reaction of the strong oxidizing agent 0 3 with 9-octalin XXI! The speed of reaction is determined not by the oxidation potential but rather by the energy requirement of the various transition states. The carbonyloxide XXIII, formed in the fragmentation of the primary ozonide, readily reacts as a "1,3-dipole" with oxygen to form a-methoxvhvdroneroxide XXII. . . The carkonyloxide and carbonyl groups in XXIII are both Dart of the same molecule. How does one establish that these functionalities completely separate during the cycloelimination of the monocyclic primary ozonide? T o answer this question, the ozonolysis of tetramethylethylene was carried out in the presence of formaldehyde. Since the carhonyloxide XXV reacts more readily with the added formaldehyde than with the acetone, it forms the ozonide of isobutylyene XXIV. Thus, the "smuggled-in" aldehyde then finds itself in the ozonide.
,+,
0
ir~.,:(JH leobutvlene Ozonide XXIV 55% Ozonide
Carbonvloxide
The ozonides can he degraded reductively to aldehydes and ketones or oxidatively to carboxylic acid. Studies by R. Pummerer (1938) and A. Rieche (1942) confirmed the 1,2,4-trioxolane formulation for ozonides as pro-
xxv
In 1960 Criegee and G. Schroder successfully isolated the primary ozonide from the reaction of trans-1,2-di-tertbutvl-ethvlene XXVI with ozone at -75OC. Reduction to the Sacemic i,2-diol XXVIII points to the preservation of the CC-single hond in XXVII. Volume56. Number 6, June 1979 I 371
.
Autooxidation.. is the flameless combination o f a compound and oxygen at room or moderate temperatures.
0,
lCHJ'C\C~H
\WCHShPentane
H/
-7 5%
XXVI
80
(c"JhCII,~IIIH
H LYCH,),
/ xxvrr
When a carhonyloxide is trapped during ozonolysis by means of an added aldehyde, as illustrated with 25 which reacts with formaldehyde to form XXIV. Then an '80-labeled aldehyde should yield an ozonide with the labeled oxygen in the ether link. Story and Murray carried out such an experiment in 1968 and found that the distribution of the heavy oxygen isotope was between the ether and peroxide bridge, with the latter even in slightly larger amount. This, as well as the findines of the steric event in the ozonide formation prompted the American group to propose an alternative mechanism to the Criegee mechanism of ozonolysis. The alternative mechanism, however, requires several steps which have no reality in terms of known reactions. Formulation of this mechanism is actually not necessary since the contradictions to the Criegee mechanism have been resolved. According to a recent report, the apparent 'sO-distrihution was the result of an unsuitable analytical method; the lsO-isotope appears exactly at the position proposed by Criegee-namely at the ether bridge of the ozonide. In 1975, his last year of life, Criegee published a comprehensive article on the "Mechanism of Ozonolysis." In the cool, factual discussion there is no hint of satisfaction or even triumph that the mechanism, conceived 25 years before, affirmatively survived all attacks. Characteristically, thereview ended with a series of questions indicating the course of future research. Just days before the article appeared in Angewandte Chemie in November 1975, Rudolf Criegee closed his eyes forever.
-
XXVIIl
XXlX
The "rearrangement" of XXVII into the ozonide XXXII does not occur until -60°C is reached, and it is here that the last CC-bond of the original alkene double bond is sacrificed. Through trapping with methanol and the formation of adduct XXIX, it was shown that cleavage once again yielded carbonyloxide XXX and trimethylacetaldehyde XXXI. Studies later also revealed that a t temperatures below -130°C even the primary ozonides of branched alkenes are stable. 1+) "(-1 o/ XXVlI
II
-60%
C
----r
,cH,d
I
+
\H
C
H'
XXX lCH,lC,,
,+"., o*,
\ctcH,I XXXl
_I
WCHds XXXll
The ozonide XXXII has, in contrast to its building units XXX and XXXI, two chiral centers (marked with asterisks). Whereas ozonolysis of the trans-alkene XXVI leads exclusively to the trans-ozonide XXXII, the ozonide of cis-l,2di-tert-hutylethene is made up of 70% cis-ozonide and 30% trans-ozonide as shown by gas-chromatographic separation. The Karlsruher laboratory as well as the research group of P. R. Story and R. W. Murray in the USA viewed this evidence as a contradiction of Criegee's mechanism of ozonolysis.
XXXIlla anti
XXXIIIb
XXXIV syn
XXXV
This contradiction, however, exists only as long as the carhonvl-oxide is written as a Zwitterion XXXIIIa with, more or less, free rotation around the CO single bond. On the other hand, the resonance contributing structure XXXIIIh with completed octets-four T-electroks in three parallel r-orbitals-has restricted rotation. As in the case of the related nitrone XXXV, the carbonyloxides should likewise appear in syn-anti isomeric forms XXXIII and XXXIV. I t is even possible, that the syn-anti isomeric carhonyloxides could he formed in different ratios during the concerted cycloreuersion of the cis-trans-isomeric primary ozonides. 372 / Journal of Chemical Education
Criegee's Sense of Responsibility to German Chemistry and Its Publications Those who knew Criegee were not only impressed by his modesty and matter-of-factness, hut also by his breadth of interest. In a discussion the facts themselves always held precedence over any undeflying personal motives. Clearly he presented his thouahts and meaningfully posed his questions. in straightforwardness, he was a model fo; his co-workers and friends. Criegee was a brilliant academic teacher. When lecturing about his own research interests, he tried very hard not to overwhelm his audience with slides. Didactically outstanding, he developed the problem and its solution. Furthermore, he oermitted his listeners to ex~eriencethe drama of question and answer interplay-for such is the situation in a gonl-oriented exocriment. Usinr: chalk and the t~lackhoard,he Droduced iliustrations, which, themselves, were a source of admiration as evidence of the lecturer's power of concentration and discipline. The author rememhers&ch a masterful expos4 given by Criegee at the time he received an honorary degree from the University of Munich which celehrated its 500th anniversary. Great concentration was reauired to accomolish his unusually heavy work load. ~ e s i d ehis s duties as director of the Institute and as supervisor of his doctoral students, Criegee was for 17 years the leading editor of Chemische Berichte. The careful consideration of the manuscripts not onlv lead to a tightening in policy hut also sought-to encourage an improvement in the quality by the authors, themselves. This in no small way contributed to the prestige of this old and renowned journal. In addition to completing the first reading on a quarter of the manuscripts received, Criegee was also responsible for the second reading of all manuscripts, including those of the other three co-editors. It was a deep sense of responsibility toward German chemistry and its publica-
Oxygen in the air gives a chain reaction with the organic substrate a n d . traces o f certain metal ions are required for initiation.
..
tions that prompted him to make such a large sacrifice of time. Until 1972 the oreanic chemistrv colloauium of the Karlsruher Institute heg& by the wish b f ~ r i e g e eevery , Saturday morning at $o'rl0ck.'l'his hroucht theccl-workera to the Institute & Saturdays, and the early start left sufficient time in the remaining morning hours for experimental work. Criegee was amazed that the great demands by our affluent society for free time also included the group of young ~. scientists and students. For Criepee a universitv education should be hound inextricably and obviously to a performance principle. That this principle at the end of the sixties became a favorite target of radical student protests-which hardly touched Karlsruhe-made him outright sad. Criegee's Interest in the Carbocyclics Among the pearls which we would like to display of this scientific work of a lifetime, there remains yet only the small carhocyclicsthat have dominated researcher's interest for the last 15 vears. since the days of R. Willstitter, the literature reports unsuccessful efforts to synthesize cyclobutadiene. Newer theoretical conceptions of this antiaromatic vinylogue of benzene attempt to make the failures understandable. "In fact, cvrlobuiadiene would have a negalise resonanrv energy m i I cannot envouraw to try to make it" t l l . .I. .-;. I)ewar, . anvonp . 1958). While Nenitzescu in Bukarest a t the end of the fifties was striving toward a synthesis of the basic structure, Criegee's experiments aimed toward tetramethylcyclobutadiene XXXVII. With G. Louis he successfullv in 1957 effected a C17 elimination ofthedichloride XXX\'I &th theaideof lithiui amnleam: in DIR(!P of XXX\'II the dimer. ortamethvl-cxntricyzooctadiene XXXVIII, was isolated.The formation of XXXVIII can he viewed as a Diels-Alder reaction of two molecules of XXXVII, where one functions as adiene and the other as a dienophile.
The photolysis of the ozonides XLI and XLII, obtained from Dewar-benzene derivatives, also offered a route to the cyclobutadienes XLIII and XLIV where once again the dimer appeared.
-
XLI
XLII
I
hv
I
XLIIl
XLIV
XLV
That irradiation of a mixture produced the mixed dimer XLV, prompted Criegee to make the following optimistic statement: "One can hardly explain these reactions without the assumption of tetrachlor- or tetramethylcyclobutadiene." XXXVI
+ NiCa
Boiling Benzene
70% XLVI
. . Ha
H"
XXXVIlI
Treatment of the diiodide XXXIX with mercury in the presence of maleic anhydride yields XL, a Diels-Alder adduct with XXXVII as the diene. "Do these experiments provide evidence for the existence of a free tetramethylcyclobutadiene intermediate? We would like to answer that question in the negative," so ended Criegee a critical discussion in 1962.
As early as 1959 Criegee and G. Schroder succeeded in the preparation of the nickel complex XLVI of the tetramethylcyclobutadiene, which resulted from the dehalogenation of XXXVI tetracarbonyl nickel. The violet crystals of XLVI are stable in air and soluble in water. Quantum mechanical considerations also permitted the fo~mationof such transition metal complexes of cvclobutadiene. A fruitfd developdent was thus started. T o describe today's extensive knowledge of cyclobutadiene would be possible only in hook form. The basic &ructure itself has been identified by American groups as well as by a student of Criegee, G. Maier. usine the so-called matrix-isolation techniaue. Suitable &solved in a solid solvent are photol&ed at liquid nitroeen or helium temperature: wherehv the cvclohutadiene and Fts derivatives cad be identified sp"ectroscopically. Cyclohutadiene with bulky groups, for example, with several tert-butyl groups, may even be isolated as such.
""
.. XLVII
XLVIII
In 1959 Criegee reported the thermal ring-opening of the cis-,trans-isomers of tetramethylcyclobutene to produce the Volume 56, Number 6,June 1979 / 373
stereoisomers of 3.4-dimethvl-2.4-hexadienes. The observed - . steric result, for example as in the conversion of the cis-isomer XLVII to cis.trans-2.4-hexadiene XLVIII. one now calls conrotation. driegee9skxperiments are earliexamples of the principle of the conservation of orbital symmetry, which was recognized in 1965 by R. B. Woodward and R. Hoffmann. Using polvcvclic compounds as models. Criegee was able to predict thk energetically more favorable conrotation as compared with the disrotation in electrocyclic ring-openings of
prism-derivative L rearranges a t a llO0C to a mixture of XLIX and LI with a half-life of tl/z = 2 hr; whereas XLIX finally isomerizes to hexamethylbenzene LI a t 150°C and t , / ~ = 2 hr. Criegee's Unusual Professional and Private Productivity Criegee's scientific harvest is recorded in 132 publications of which the writing is exemplary in precision and hrevity. Frequently the reader misses statements about how the author came upon the problem and about how he finally arrived a t its solution. Errors and negative exoeriments were deleted for the sake of hrrvity and did nor 1:ntrr the journals. 'I'he puhlicationsas"s~~ccrssstories"trilnsmir the impression of an author with u lifvtinle hlessed with good lurk. Thiisiruation reminds oneut'an anenlote told ahout Adolf von Baeyer. When a visitor expressed envy that fortune had blessed so much of Baeyer's work with success, Baeyer retorted dryly: "Herr Kollege, I experiment more than you." With Crieeee. .. . too. diligence. imagination. and brilliant experimentnrim guaranteed thc "luck." Crienre'i arh~e\,ementsrm-iwd the deserved recugnition. He wa'a member of the Academies of Science of ~ e a e l h e r g (1955) and Bavaria (1962), the New York Academy of Science (1966), and the Deutschen Akademie der Naturforscher Leopoldina (1968). In 1960 he received the Emil Fischer medal of the Society of German Chemists. The Universities of Giessen (1967) and Miinchen (1972) awarded him honorary doctorates. Rudolf Criegee's unusual productivity received support in a harmonious family life. His enthusiasm for research and experiment was transmitted to his co-workers, who will remember him with reverence. His creative imagination evoked the admiration of his colleagues and friends. One is inclined to remember Criegee, himself, by the words which he once dedicated to his teacher Otto Dimroth. He gave his students a great deal, hut the most important was his joy in~experimentation.He used praise and reproof sparingly, but when he praised or reprimanded (frequentlyas a result of a successful or unsuccessful experiment),both left a lasting effect. [He] had the gifts, which every chemist could wish for: sharp faculty of observation, a wide view along with the healthy common sense to separate the essential from the nonessential, and a never-tiringdiligence.And soshall we, who had the good fortune tn he his students, keep his memory. R. Criegee.
-
Ir
XLIX
CH , HC
CH, CH*
Li
That a scientist shortly before his retirement should begin a new research areaand follow through with great activity, is most unusual. Such activity Criegee brought to the Dewarhenzene problem. Upon irradiation of the hexamethylderivative XLIX, Criegee observed in 1967 the formation of hexamethylprismane L a n d hexamethylhenzene LI in a ratio of 1:3. W. Schafer a t the Chemische Werke Huls oerformed thc same experiment; whereupon hoth authurs juintly puhlished their rrs~llr.;.Roth rearrimsare aIIow~dhs the M'oodward-Hoffmann rules, if one starts with the so-cailed SI state; this excited state is reached following the absorption of a quantum of light. An intramolecular 2 2 cycloiddition is responsible for the formation of L: whereas the reaction of XLIX L involves a disrotatory electrocyclic ring-opening. A. Ladenburg in 1868 proposed a corresponding prismstructure for henzene, which was to account for the slight tendency of henzene to undergo addition reactions. The
-
374 1 Journal of Chemical Education
+
