MELVINS. NEWMANAND LOUISL. ITr0o~,JR.
4300
Vol. SI
derivative IX of a heptanedione; rccrystallization froni nitrobenzene gave in .p. 203-203'. Anal. Calcd. for C I ~ H ~ O X ~ C, O , :46.70; H , 4.12; S, 22.95. Found: C,46.82; H,3.88; N,22.71. The ethanol extract gave two red derivatives, 111.p. 103170" and m.p. 60-70°, which could riot be purified. A portion of A , 0.9 g., was shaken overnight with 1.5 g. of potassium permanganate in 50 ml. of water. The colorless solution was filtered, acidified with hydrochloric acid and distilled to dryness under reduced pressure. The dry residue was extracted with hot ethyl acetate. The ethyl acetate was removed at reduced pressure, leaving a sirupy residue. A portion of the sirup gave a seinicarbazone. Recrystallization from hot water gave m.p. 169.5' (literature" 168169"). Anal. Calcd. for C1H13N303: C, 44.95; H, 6.95; N, 22.45. Found: C,44.68; H , 7.04; N,22.44. The remainder of the sirup was neutralized with sodium hydroxide and boiled for 1 hr. with alcoholic p-brornophenacyl bromide. Fractional recrystallization of the product gave the bis-ester of succinic acid, m.p. 206-208" (melting point undepressed by authentic specimen, 111.p. 208-200°), and the bis-ester of 2-methylglutaric acid, III . p . 96-99', Anal. Calcd. for CazH2006Br2: C, 48.00; H , 3.TO; Rr, 29.55. Found: C, 48.07; H , 3.97; Br, 29.93. Repeated crystallization from absolute ethyl alcohol failed to change the melting point. The bis-ester prepared froni authentic 2-methylglutaric acid12 had the same range of melting point. The melting point of a mixture of thc esters )vas 96-99", Oxidation of Tetrah--A solution of 331 g. (2.5 inolesj of redistilled tetralin in 383 g. (3.75 moles) of acetic a n hydride was oxidized a t 50-55' for 21 lir. arid then at 7073" for 17 hr. Fractional distillation of the ainber solution under reduced pressure gave 42 g. of a-tetraloiie, 2 , k i i n i t r o phenylhydrazone m.p. 256-257" ( l i t e r a t ~ r e l259-260°), ~ acetate ( 6 ) C. W, Smith, D. G. h-orton and S . A . Ballard, T H I S J O U R N A L S and 81 g. (15.770 yield) of 2-oxa-3,4-benzcyclol1eptyl ( S I ) as a n oily solid. Several crystallizations froin aqucous 73, 5270 (1951). ethanol gave colorless plates, m . p . 80-61". ( 7 ) K. H. Hall and B. K. Howc, J . Cht,i;:. SCJC.,2180 (1YZl). ( 8 ) G . N. Chelnokova and V. V . KorshaK, Sbovizik, Slufei O h l i ' k e i Anal. Calcd. for C12H1403: C, 69.99; H , 6.85. Found: Khim., 2, 1070 (1953); C. A , , 49, A297 (1955). c, 69.93; H,7.17. (81 . , R. T . Arnold. G. G . Smith and K . RI. Dodson, J . Oi,p. Chein , 15, Treatment of XI with acidified 2,4-diriitropheliyl~1~drd1256 (1950). zine gave the golden yellow derivative of o-hydroxyphen~1(10) (a) T h e following melting points are t o he found for t h e 3butyraldehyde, m.p. 143-144O (literature'l 149'). anhydride was placed in the ultraviolet ~ e s s e l . ~ Oxygen was bubbled continuously for 24 hr. through the solution a t 50". Fractional distillation under reduced pressure gave 2-oxa-3-cyclohexenyl acetate ( I I ) , 16.5 g. (4% yield), ~ dz6a1.107. b.p. 47-49' (3 mm.), n Q 01.4532, Anal. Calcd. for C7H1001: C, 59.14; H , 7.04. Found: C, 59.34; H , 6.90. The compound I1 has been made by other workers6 from vinyl acetate and acrolein; the physical constants given were: b.p. 77-78"(20 mm.), n z o D 1.4596, dzo41.1178. Treatment of 11 with acidified 2,4-dinitrophenylhydrazine gave the yellow derivative of glutaraldehyde, m.p. 189-90" (nitrobenzene) (literature 186-18707; 19Z06). 2-Cyclopentene-1-one was identified in the lower boiling fractions by its 2,4-dinitrophenylhydrazone, m.p. 166.5167.5' (literature* 166'). Oxidation of 3-Methylcyclohexene.-The 3-methylcpclohexene was prepared from 3-bromocyclohexene and methplmagnesium i ~ d i d e . ~ Oxidation of 192 g. (2 moles) in 408 g. ( 4 moles) of acetic anhydride was carried out a t 55-65' for 40 hr. Fractional distillation of this yellow solution under reduced pressure gave 65 g. of products and 185 g. of undistillable residue. Further fractional distillation of the 65 g. of products gave the ketonic product 111, b.p. 45-47' ( 2 mm.), fizoD 1.4646, and 19.75 g. (5.8y0yield) of the hemi-acetal ester mixture boiling between 58-66' (2 mm.) and having a range of nz$ of 1.4578 t o 1.4589. Of this material 6.2 g., b.p. 61-62 (2 mm.), %*OD 1.4583, comprised the product A. Anal. Calcd. for CgH1408: C, 63.53; H , 8.24. I'ountl: C, 63.72; H, 7.96. A 2,4-dinitrophenylhydrazone, 1n.p. 170-172', was obtained from 111. This was the derivative of either 3methyl- or 4-methylcyclohex-2-en-1-one . l o Identification of A.-Addition of acidified 2,4-dinitrophenylhydrazine solution gave an orange deposit, m.p. 175-185'. Extraction with hot ethyl alcohol left the yellow
methyl-derivative: 172.5-173', C . S . Marvel, THISJ O U R N A L , 60, 280 (1938); 173O, A. J. Birch, J . Che;iz. Soc., -130 ( 1 9 4 4 ) ; 173-174', A. J . Birch, J. C h e m S o c . , 593 (1946); 170-171', L l , Nousseron, Bull. sac. chim. France, 462 (1952); 178-179'. R I . S. r\-ewman, J . 01.g. C h e m . , 17, 577(1952); 177-17S0, G . F. U'oods, THISJOURKAI., 71,2028 (1949); 1751 7 7 O , hl. W.Cronyn, THISJ O U R N . ~ L , 75, 12-17 (1963). (11) The4-methylderivative is given a s 173-171', A. J. Birch, J . C h c m . Soc., 593 (1946); 172-173O, hl. Rlousseron, Blill. SOL. c h i l i : . Tvniice, 1210 (1951).
[CONTRIBUTIOS FROM
(11) G. T. Tatevosyan, el nl., B d ! . Avmeizinii branch A r d i l . S c i .
U S S R , 5-6, 37 (1944); C. A , , 40, 3398 (19413). (12) Kindly supplied by Prof. C . G. Overberger. (13) F. Rarnirez and .A. F. Kirby, Tms J O U R K A I . , 7 4 , 43:31 (19521, (14) A. Robertson and &'. .4. lx'aters, J . Chcn;. Soc., 1674 (1948).
WAYNE,Ti.J,
THE ~ ~ C P H E R S OCHEMlSTRY K
LABORATORY O F THEO H I O
STATE
UXIVERSITY]
Concerning the Mechanism of the Reaction of Phosphorus Pentachloride with Ketones BY MELVIN S.NEWXIXN AND LOCISL. WOOD,J R . KECEIYED I"EI3RTARY 19,195'3
A mechanism for the reaction of phosphorus pentachloride with ketones is presented, the tnsiri feature of wiiic!i iiiv,ilve\ the formation of a chlorocarboniuin ion. The known reactions of ketones with phosphorus pentachloride are explained b y the proposed mechanism.
Although phosphorus pentachloride has been used for a long time as a reagent which attacks the carbonyl group in aldehydes and ketones, there has been little discussion of possible mechanisms for these reactions. Because of an unexpected result in the reaction of 3,4-dimethyl-4-trichloromethyl-2,5-cyclohexadienone (I) with phosphorus pentachloridel we became interested in the inech( 1 ) hl. S Kewman and L. L. IX'ood, J r , J . O i p . C h e a z , 23, 1230 (1958).
anism of reaction of phosphorus pentachloride with carbonyl-containing compounds in general and with I and the analogous 4-methyl-4-trichloroniethyl2,5-~yclohexadienone~ (11) in particular. hluch work must be done before any mechanism call be considered to be cstxblishetl, but the scheme be!onr accounts so well for a number of f x t s that it s w i l l \ worthwhile to present at this time. ( 2 ) K \ o n Auwer, a n d IT l u l i c h e r BLi 65 21b7 flrl.22>
Aug. 20, 1959
REACTION OF PHOSPHORUS PETACHLORIDE WITH KETONES
4301
It is also possible that the ion formed in equation 2 may collapse directly to the chlorocarbonium ion V with loss of phosphorus oxychloride.
+
[
R-CCH3
[
R~CH,
[.;I
+ a-
4
[;c .H3]
(3)
OPC4 111
dPCl1
c1
-%
RC!!=CHa
4- Poc13 + HCl
(4a)
IV
OPCL I11
[
RFCH.1
--f
+
[RTCH.]
(3a)
+
V
Equation 4a.-The formation of a chloroolefin IV, a product often found in significant amount on the reaction of phosphorus pentachloride with ketones, may be the result of the collapse of 111, possibly by a cyclic mechanism involving a six-atom ring, e.g. c1 I
t -H+ b_
OPCL,]
R- C--,CH, 05
4- POCla
+ C1-
H'
+
IECCl=CH*
IV
(4b)
+
POC13
+ HCI
(4a)
v
Equation 4b.-The intermediate I11 may undergo ionic decomposition to yield the chlorocarbonium ion V which would be stabilized by resonance inRrCHs] C1- --f RCCltCH3 (5) volving the a-electrons of the chlorine atom.' v VI We believe that the chlorocarbonium ion V is the most significant species involved in the general Equation 1.-The structure of solid phosphorus case of the reaction of aldehydes and ketones with pentachloride has been established as PCL+ phosphorus pentachloride. This ion V may be PC16- by X-ray work,3 the PC14+ ion being tetra- formed by more than one route, but the reactions hedral and the PCle- ion being octahedral. In of aldehydes and ketones can readily be accounted the vapor state, phosphorus pentachloride is for by the assumption that V is the species involved monomeric and of a trigonal bipyramidal structure. in going to products. Since certain solutions of phosphorus pentachloride The chloroolefin I V may result from the loss of a are conducting3 it is reasonable to propose that in proton from V. The dissociation of I11 may inthe media usually involved in treating phosphorus volve steps and species similar to those involved pentachloride with ketones, the tetrachlorophos- in the case of the analogous chlorosulfites of alphonium ion, PC14+, is present and r e a ~ t i v e . ~cohols.8 The position of the equilibrium of equation 1 should Equation 5.-The final step in the formation of be sensitive to solvent and to t e m p e r a t ~ r e . ~ dichioride VI is the reaction oi the chlorocarbonium Equation 2,-This may be thought of as a nucleo- ion V with some species capable of providing a philic attack of the carbonyl oxygen on phosphorus chloride ion, such as the PCl6- ion. It is also or as an electrophilic attack of the phosphorus on possible that I11 may dissociate into an ion pair, the carbonyl oxygenS6 This step is probably an which then collapses to products, as8 equilibrium, the position of which may be markedly affected by solvent. Equation 3.-The reaction of the electron-de- I11 + R-C', OPCL- + RCClSCHa POC18 ficient ion formed in equation 2 with chloride ion T'I (or some species capable of yielding a chloride ion, such as the PCl6- ion) yields the intermediate (5a) I11 whose stability depends on the usual variables We do not believe that the product RCC12CH3 such as structure of R,solvent, temperature, etc. is formed by an sN2 type attack by chloride ion on I11 because of the fact that pinacolone (R = t(3) D. Clark, H. X.Powell and A. F. Wells, J . Chein. Soc., 642 butyl) reacts so readily with phosphorus penta(1942). I n this paper other references t o PCIS are given. (4) Se'e A. V. Kirsanov and V. P. Molosnova, Z h u r . Obshchei Khiin., chloride in the cold to form about equal amounts of 26, 30 (1958); C. A , , 62, 1276Ub (19581, for mention of addition of dichloride and chl~roolefin.~Such an s N 2 mechPCla+ ion t o t h e carbonyl group of a n ester. anism would involve the neopentyl type of steric ( 5 ) On heating PCIK a different equilibrium becomes important. hindrance in this case. namely, PCls a PCla + Clz ( l a ) . For this reason chlorinated side products are often obtained if t h e temperature of a reaction involving Discussion phosphorus pentachloride is not controlled, preferably in t h e &20° range. I n surveying the products formed by the re(6) Interestingly evidence for existence of t h e compounds PCla+ action of ketones of varying structure with phosAlClr- and PCla+FeCla- has been obtained. See Y . A. Fialkov and
[
+
[ r:I'
Y . B. Buryanov, Doklady Aka,!. .Vauk S.S.S.R.,92, 585 (1953); C. A , , 46, 5 7 0 F (1954); Zhuu. Obskcizei K h i m . , 26, 2391 (1955); C. A , , 60, 91971 (1956). It should be of interest t o see what effect t h e use of aluminum chloride and ferric chloride would have on reactions involving phosphorus pentachloride.
1
+
(7) See A. Streitwieser, Jr., Cheiiz. Rev., 66, 674 (19561, for a dis cussion of t h e effect of halogen atoms attached to a n electron-deficient carbon. (8) See ref. 7, p. 730 ff. (9) P. D. Bartlett and L. J. Rosen, THISJOURNAL, 6 4 , 541 (1942).
A ~ E L V I N S. NEWMAN AND LOUISL. WOOD,JR.
4302
Vol. 81
phorus pentachloride one can account for most The conversion of methyl cyclopropyl ketone of the products by application of the equations in 92y0 yield to 2,5-dichlor0-2-pentene~~ may be listed above and by taking into account special explained as features introduced by varying the nature of R. Specific examples are cited below. Simple aliphatic ketones such as 2 - b ~ t a n o n e ~ ~ and pinacoloneg react to give about equal amounts of dichloride and chloroolefin if the temperature is held low. The ready tendency of phosphorus + ClCH,CHzCH=CClCH3 c-CH,CH,CH=CClCH,j pentachloride to act as a chlorinating agent5 a t moderate temperatures (e.g., 70") is illustrated Similar rearrangements involving the cycloby the conversion of 2,2-dimethyl-3-pentanone into 2,2-dirnethyl-4-chlor0-3-pentanone~~ as compared propylmethyl cation are known.I7 The reaction of 4-methyl-4-trichloromethylto the ready conversions of pinacolone into dichloand chloro- 2,5-cyclohexadienone (VII) to yield 2-methyl-4ride (2,2-dimethyl-3,3-dichlorobutane) chlorobenzotrichloride2 (VIII) is explained as olefin (2-chloro-3,3-dimethyl-l-butene) a t 0-5". The fact that pinacolone yields unrearranged products may be interpreted as evidence either that the chlorocarbonium ion has a lesser tendency toward rearrangement than the corresponding hydrocarbon carbonium ion or that the inter0 c1 c1 mediate I11 collapses directly to dichloride and VIIa VI11 VI1 chloroolefin. That steric hindrance of reaction of ketones In this case the chlorocarbonium ion of general with phosphorus pentachloride may be an impor- formula V is pictured as in VIIa. The rearrangetant factor is indicated by the fact that 2-methyl- ment of the methyl group occurs when sufficient 5-isopropylacetophenone is chlorinated a t the positive charge is built up on a carbon adjacent to tertiary hydrogen of the isopropyl group to yield the quaternary carbon atom containing the 2 -methyl-5- (2-chloro - 2 -propyl) - acetophenone,12 methyl and trichloromethyl groups. whereas acetophenone readily yields the expected The rearrangement of the analogous 3,4-didichloride and chloroolefin.13 methyl - 4 - trichloromethyl - 2,5 - cyclohexadienone In accounting for the reaction products of simple (IX) to 3-(/3,/3,/3-trichloroethy1)-4-methylchloroketones the chlorocarbonium ion V is not necessary, benzene (XI) follows a different course1 because the as the collapse of 111, or of an intimate ion-pair, methyl in the 3-position of I X stabilizes the carcould readily lead to dichloride VI. However, in bonium ion IXa essentially as shown in IXa. more complicated cases the involvement of V There is no tendency for the 4-methyl group in seems indicated, although other possibilities are not I X a to migrate to the carbon containing the methyl ruled out. For the sake of simplicity in the follow- group since an aromatic ring would not result. ing examples, ions of type V will be used. Therefore the ion I X a loses a proton to yield the Crotonaldehyde yields a mixture of 1,l-dichloro- triene X which undergoes a 1: 3 rearrangement of 2-butene and 1,3-di~hloro-l-butene~~ (ca. 90%). the trichloromethyl group to yield XI.'* The chlorocarbonium ion (CH,CH-CH-CHCl) + reacts preferentially a t the 3-position. The formation of 2,5-dipheny1-3-chlorofuranin high yield (90%) from both cis- and trans-diben~oylethylenel~ a t moderate temperatures (25-40') is explained as C,H,COCH=CHCOC6H5
t PC1,'
c;' HC-C-H II IJ 3-PclAv-c C-C6H5
/ >4
HC-CCI I1 I1 c&c, ,CCbH5 + HC1
61 XI
c1
X
+
0
C A
(10) B. Charpentier, Bull. SOC. chim., [ 5 ] 1, 1407 (1934). (11) V. Vasselier, ibid., [ 4 ] 43, 563 (1928); t h e reaction mixture \\-as heated to 70'. (12) M . S. Malinovskii and N. bf. hledyantseva, Z h u r . Ohshcirei K h i f n . , 19, 324 (1919); C. A , , 43, 6593' (1949). Thereaction mixture v a s held a t low temperature first and then was warmed. '. Taslor, J. Cheni. SOC.,301 (1937). Ire believrd t h a t chloro(13) U ulefin and dichloride were formed "simultaneously" from some inter-
mediate. (14) L. J . hndrews, THISJ O U R N A L , 68, 2584 (1946). (15) I
