March 20, 1959
was recrystallized from petroleum ether to yield 0.80 g. of labeled ethyldimethylacetamide, m.p. 103-104" (reporteds1 m.p. 103-104°). Plates of varying thickness, spread over a uniform area, were made so that the specific activity could be evaluated for an infinitely thin plate. The amide gave 38.0 c./min./ mg. (4380 c./min./mmole) above background of TO c./ miii., extrapolated to zero thickness. T h e labeled ethyldimethylacetamide, 0.7 g., was rearranged t o the carbamate ester using the procedure described for the amide from Vc. I n this case the carbamate ester did not crystallize. It was hydrolyzed and decarboxylated as described above to give 0.25 g. (33%) of l-amylamine hydrochloride (VIb), m.p.229-231°, andO.TBg. (6372) of barium carbonate (yields based on the amide). Anal. Calcd. for CSHIJC1: C , 48.5;; H, 11.42; S , 11.33; C1, 28.68. Found (unlabeled duplicate sample): C , 48.12; H , 11.57; S,11.44; C1, 28.58.49 T h e picrate melted a t 180-182" (reported6* 182-183'). Anal. Calcd. for CllH16N40i: C, 41.77; H, 5.10; S , 17.72. Found (unlabeled duplicate sample): C, 41.76; H, 5.11; S , 17.81.40 Both the t-amylamine hydrochloride and the barium carbonate were radioactive. Plates of varying thickness were made from portions spread over a uniform area so t h a t the specific activity could be evaluated for an infinitely thin plate. The amine salt gave 23.5 c./min./mg. (2900 c./ min./mmole) and t h e carbonate gave 6.45 c./min./mg. (1270 c./min./mmole) above background of TO c./min., extrapolated t o zero thickness. The results were corrected for the 2.4Tc t-butylamine hydrochloride (vide znfm), which had all the activity of the ethyl t-butyl ketone, and the 2.4y0 barium carbonate, which had none of the activity of the ethyl t-butyl ketone, t o give 28T0 c./min./mmole for tamylamine hydrochloride and 1300 c./min./mmole for barium carbonate from the methyl t-amyl ketone. Liberation of the labeled t-amylamine from its hydrochloride and subjection t o vapor-phase chromatography48 (31) A. Haller and E. Bauer. A n n . -clicphthalate ( I ) "1kl;cyed"" 2.20 dl-Stilbenetlii~l 0.230 oi-er the niiriiial al)utitliiiiec o f 0,204 atom rixygeti-18. Defined in previiius paragrapli. c 0.98 atoni excess oxygen-18 per carboxylate group.
Solution of equations (1) and (2) for # leads to a value of 1.5' 24' for this angle. Solution of equation ( 3 ) for y leads to a value of 19' 3' for this angle, and consequently a dihedral angle (2y) of 38' 6'. The calculated carbonyl carbon-to-peroxygen--to peroxygen angle ( L AOE) is 111' 5 1 '. The extent to which the dihedral angle may differ from the value of 3s' 6' will be dependent on the balance between internal angle strains and any possible strain resulting from repulsion between the lone pair electron orbitals on the adjacent oxygeil atoms.
~)erositlewas used for both scrics of csperiiiicti ts.
(li) \V. C S r h u m h , C , S h,itt?rlivld diihing Corpc,r,tti,,n, N e w Ym!. . N e i i
L)ISTRII3rTIOX FROM KEACTIUN O F PHTIlALOVL
I'ERuXIDE-Ccil.bl~nyl-01"
AND
f/'(In,S-STILBESE
IS
CARBOS
TETRACHLORIDE
Coiisequetitly similar oxygen- 1 S excesses are expected for the two samples o f cyclic phthalate. ('The oxygen- 18 coiitent of cyclic phthalate might be expected to be slightly greater from the "delayed" reaction than from the direct reaction if the oxygen-18 exerts an isotope effect on the decomposition step of the peroxide.) T h e ratios of oxygen18 excess in diol to oxygen-18 excess in cyclic phthalate for the "direct" and the "delayed" reactions are 0.111 and 0.107, respectively. Calculation Concerning the Structure of Phthaloyl Peroxide.-~-In view of the established preferciice o f hytlrogeii 1)croxide for the couforinatioii
\'hal>le cixygen-oxyxen Clistanct. u l 1 . 1 4 .1 , I,, S. Silhert, I.. P. Witnaucr, r). Swern and C. Ricciuti, Abstracts uf t h e 134th Rleeting of the Americarl Chemical Society, r h i c a g o . Illinois, September. 1928. p. :32-S. (8) (a) Average value in large number of aryl compounds: (I>) salicylic acid, nicotinic acid. isatin: (c) methyl formate, methyl acetate; G. W. Wheland, "Resonance in Crganic Chemistry," John Wiley and Sons, I n c . , Kew York, S . >-,, 195.5, Appendix on Interatomic Distances and Bond Angles in Organic Molecules (9) T h e best value for t h e distance between two singly-bound carhon atoms, when hoth are In .i1,2 hyhrldized state appears t o be 1.17 A . ; X I . J. S. Dew-ar, Conferencr on Hyperconjugation, Indiana University. Bloomini't~in,I n r l , J u n e 2-1, I ! 4 S (10) See ref. ti, l i p . 32:3 :!2.7.
PHTHALOYL PEROXIDE-carbonyl-0l8
March W , 19.X
Discussion The data of Table 1 afford two principal results: (1) only a small amount of oxygen-18 originally located in a carbonyl group of phthaloyl peroxide becomes bonded to a carbon atoni of the olefin; ( 2 ) the amount of oxygen-18 ultimately attached to carbon of the olefin is essentially independent of the time that phthaloyl peroxide itself has been subjected to SOo in carbon tetrachloride. The main conclusion drawn from these results is t h a t equilibria between phthaloyl peroxide and any species in which a carbonyl oxygen atom becomes symmetrically located with respect to an oxygen atom of the oxygen-oxygen link are unimportant. Bearing of the Oxygen-18 Results on the Stability of Phthaloyl Peroxide.-One of the unique features of phthaloyl peroxide is its enhanced stability in carbon tetrachloride compared with benzoyl peroxide, a 37-fold difference in rate of decomposition. (For both cases the rate of decomposition in carbon tetrachloride is first order in peroxide and essentially free of contribution from induced decomposition processes.) This stability, a priorz, may be inherent in the cyclic cliacyl ring system, or may be the result of an equilibrium between phthaloyl peroxide and the diradical formed by homolytic fission of the oxygenoxygen bond.
a. 0
I11
6
a'"-
products (4)
coo
IV
Diradical I V represents a species with the option for a built-in cage reaction, and return to I11 might be expected to be faster than competing processes such as decarboxylation or attack on solvent. The equilibrium as written implies the equilibration of carbonyl oxygen and oxygen of the peroxidic link. I n terms of a labeled oxygen atom such equilibration can be achieved only by rotation of the carbonyl-carbon to ring-carbon bond. The observation t h a t the amount of oxygen-18 a t tached to carbon of olefin in the cyclic phthalate is independent of the length of time t h a t phthaloyl peroxide is subjected to bOo in carbon tetrachloride makes it clear t h a t conversion of I I I a and IIIb, by any mechanism. is unirrportant under the experimental conditions. 1 7 t h regard to the question
of diradical formation ( I I I a t o IVa), i t is clear from the calculations of the structure of phthaloyl peroxide t h a t thermal homolytic fission of the oxygeli-oxygen bond could only be achieved by
1505 A0 LG GG
I 37i = l47i : I 39A
:
o0
:
46a
OE
:
0 73J
AFcnlc LAGG LOAG LOAB LOAC LCAD
LCAB LODC LEOG
:
I 4
:
:
120" 112' 68"
:
Y
:
e
i
=
# d
i
G-
:
3 ~
0 Fig. 1,-hI(1dc1 for calculatioii of dihedral angle (2-, 1 in phthaloyl peruside.
considerable twisting of the carbonyl carbon-toring carbon bonds. The condition imposed by the oxygen-1S results is t h a t k1be much greater than k,. The exclusion of k3 of importance relative to kL1, even from the necessarily twisted configuration of IVa, places a major restriction on the lifetime of a diradical IV, and, in our opinion, effectively excludes this fragment from serious consideration as an intermediate in the reactions of phthaloyl peroxide. The basis for the greater stability of phthaloyl peroxide over the acyclic analogs well may lie in the fact t h a t oxygen-oxygen fission in an acyclic peroxide may be achieved by a simple stretching of the oxygen-oxygen bond whereas the related rupture in phthaloyl peroxide could be achieved only by twisting of the carbonyl carbon t o ring carbon b0nds.l' The oxygen-1S results also eliminate from consideration equilibrium between I11 and V, and obviously exclude V from consideration as the structure of phthaloyl peroxide. Structure V was initially rejected on the basis of the close correspondence in ultraviolet absorption spectra of phthaloyl peroxide and phthalic anhydride.12
4I11
* V
-0
Relationship of the Oxygen-18 Results to the Reaction of Phthaloyl Peroxide with 0lefins.The exclusion of thermal processes for the eyuilibration of carbonyl oxygen with oxygen of the oxygenoxygen link still leaves several possibilities for oxygen-18 incorporation into alkyl oxygen of the ester: (1) oxygen-IS shuffling in the process of formation of phthaloyl peroxide (from phthaloyl chloride arid hydrogen peroxide in ether in the presence of sodium carbonate) ; ):( isomerization of lactonic ortho-ester I1 to cyclic phthalate I ; (3) isomerization of cyclic phthalate-carbonyl-0 IX to cyclic phthalate-uLkyLO1x; (4) oxygen-1s shuf(11) For a detailed picture of the relation of structure a n d reactivity in a number of oxygen-oxygen cleavage reactions of peresters see P. D. Bartlett and R. R. H i a t t , THISJ O U R N A L , 80, 1398 (1958). (12) T h e elegant work of W. D. Emmons, THISJ O U R N A L , 79, 5739 (1957),o n the oxaziranes (structures containing nitrogen, oxygen and carbon in a three-membered ring) should substantially reduce a n y prejudice against the pussible occurrence of an oxygen-oxygen link in a three~memberedring.
fling in the process by which cyclic phthalate is formed from peroxide and olefin. ll-e see no plausible path for isotopic shuffling in the process of formation of peroxide and exclude the first possibility on this basis. Earlier work3 has shown that the lactonic ortho-ester I1 and cyclic phthalate I have high thermal stability and are izot interconverted, excluding the second possibility. T h e third possibility represents an interconversion that might take place by way of an ion-pair process ( a ) or a molecular S N i process (b). Although
the reaction of phthaloyl peroxide with carboncarbon unsaturation. Experimental Phthaloyl Chloride-018.-A mixture of 11 g . of phtlialoyl chloride (Eastman Kodak 01. white label, redistilled, b . p . 86-88" a t 0.1 r n m . ) and 100 nil. of water containing 1.1 :itom c; ox>-gen-18(Stuart Oxygen Co.) w a s heated a t rcflus in-ernight. From the cooled mixture 9.4 g. of phthalic acid was obt:iined, dec. 11. 207-208". This inaterial was converted to phtlialoyl chloride-0'8 by nie;ins of phosphorus pentachloride. Phthaloyl peroxide-coiboiz~l-0'" was prepared from thc above acid chloride by the procedure described previously. Reaction of Phthaloyl Peroxide-carbonyZ-018 with transStilbene.-This reaction was carried out under two sets of conditions. The oxygen-18 analyses are reported in Table
I.
s\y neither of these possibilities is excluded by the evidence a t hand, the resistance of simple esters t o this type of i n t e r c o n ~ e r s i o nand ' ~ the slowness of isomerization of tosylates under conditions t h a t are f a r more conducive to ionization" do not lend encouragement to the acceptance of alternatives a or b as the mechanism of oxygen transposition.';' On the other hand, isotopic shuffling in the olefin reaction itself is consistent with kinetic and product evidence.' This matter will be discussed in detail in a forthcoming publication in conjunction with additional information bearing on the mechanism of (13) 6. B. \Tiberg, T. 1%'. Shryne and R . R . Kintner. THISJ O U R X A L , 79, :3l(iO (19Z7). (14) D . B. Denney a n d B. Goldstein, ibid., 79, 4918 (1957). ( 1 5 ) Although processes of t h e t y p e a and b are consldered unlikely under t h e experimental conditions employed here, these processes m a y occur under other circumstances.
By Direct Reaction.--Peroxide :ind olefin were heated a t reflux in carbon tetrachloride f(ir 16 h r . anti worked u p 21s descrihed p r e v i o ~ s l y . ~The cyclic phthalate melted a t 206207' after recrvstallizatioxi from carbon tetmchloritlc (reported3 m.p. 206-21)i'0). .Alkaline hydrolysis of the cyclic plitliahte3 :tff(irdcd d1,2-diplien\.letlianediol o f 1n.p. 119.5-120" after recr1.stalliza-titinn from hexane. solutitm After Prior Heating of Phthaloyl Peroxide.-.\ o f 1.OO E. of uhthalovl Deroxide-cai.bonvl-Ol6 in 100 1111. of carbon tetraciiltiride k a i sealed in a flask under nitrogen and heated a t 80" for 96 lir. Iodometric analysis nf a 5ml. aliquot indicated oniy 137; destruction of peroxide in this time period. T:I the remainder o f the phthaloyl pcroxide solution TWS added :in equimol:ir amount of transstilbene, and the sr>lutiiin 'iras heated a t reflux for 18 h r . .-I &mi. piirtiiin W:IS evap(ir;tteti to dryness for spcctral :inalysis. The infrared ahsiirption spectrum o f this xtmplc in clilriroform was the s i m e a s t h a t iihtained Ixeviously from direct re:iction of the peroxide ant1 frans-stilbeiie.3 The cyclic phthalate was isolated a s before, 11i.p.%flii-207°. Alkaline h\-drolysis of the c\.clic phthalate :ifforded di1,2-dipIien!-lethanedii,l, 1n.11. i19.5-12U0 after recrystallizntion from liesnne.
Acknowledgment.---lye are indebted to Professor 11yron L. Bender for the oxygen-18 analyses and for his encouragement in this problem. CAMBRIDGE :39,~~IASSACIIUSETTS
[ C O S ? . R I U L T I O S F R U X T I E 1)EPAKT.\IEST UI: C H E A f I X R T OF THE P O L T T E C H S I C
ISSl Il'Ll'E
CIF ~ROOKLTS]
The Thermal Decomposition of Substituted !-Butyl N-Phenylperoxycarbamates' BY EUGENE L. O'BRIEN, F.I I A R S H X L L BERINGERAND KOBERTB.
IIESROBIAX4
RECEIVED J U L Y 30, 1958 ildditioii of t-butyl hydroperoxide to substituttd phenyl isocy:itiates gavc substituted t-but>.]S - p l i e r i y l p e r o s y c a r b ~ i i i i a t ~ ~ . Their ultraviolet and infrared absorption spectra and their rates of deconiposition in toluene over a 30' range of temperatures mere nieasured. First-order rate constants increased with t h e electron-releasing power of the substituents; a Hamrnctt plot with H. C . Brown's u+-values had a slope of -2.2. This ma)- explain the observation that pcroxycarbamates benring p-lriethyl or p-rnethosyl groups could not be isolated.
In a previous paper3 the first-order therinal decuiiipositions of t-butyl and cuniyl K-phenylperoxy11) P a r t of thib work wab suiiported by t h e Otlicc of S a \ al Research under Contract SIjunr-2630
