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Skeletal rearrangements induced by chemical ionization—I
SKELETAL
REARRANGEMENTS INDUCED 10NIzATI0N-I ANALOGY
TO SOLVOLYSISt
G. SUDHAKAR &DDY, Nuional C&mid
lnstitut f8r &pnischc
BY CHEMICAL
M. VAIRAhlANI and K. G. D,s* Poo~411008. ludi~
Laboratory.
Cbeah
dcr
Univanftit K&n. West Gamany
CWhon Collcp ol Tcdmo&8y. Potdun. New York
13676, USA
(RaxkdinUK3Apd1979) ALaweIn amlras4 to the OrlId expeuatio in chemical iotlhcioo (CI) amm spectmmCQ, the cl qxctm of I &es of aryl4-nitro-A’-cyclohercaer show very low intensity protoorted mokculu ions
rhikamrj~~rhrrtdtbeioncurrentirarrialbyrc~ -gemcot long known in the dvolyais
of cyclohcxeoyl
otions.Tlleintuvmtionofr carkoium ionr un a-1 for tbc
obscmcd CI bchviour.
The analytical
value of chemical ionization mass rpcctrometry’ derives from the usual observation of simple spectra dominated by the intensity of the protonatcd mokcular ion (M+ 1)‘. This expcctatioa is dramatically breached in the CI rpectrr of a l&es of aryl rubetitutcd nitrocydo&un?d reported herein. Our observations arc exemplidcd by tbc isobutane CI spectrum of 3,5&ripbenyl4nitrocyclohexeae which cxhibitx an Fi+ l)+ ion carrying only 0.4% of the total ionization above m/z 90 (%&,,) while the base peak in the spectrum which carries 30%& is a rearrangement ion cornpored of two of the phenyl groups and a mcthine carbon [cJI,)KW. Skektal rearrangement proaaus which are cornmon in electron ionization mass spectra’ arc of less frequent occurrence in chemical ionixation masa spectra. In the literature there arc only a limited number of skeletal reamngementx reported under CI conditions.“’ One reason for these dif?ercntial obmvationr is the driving force pushing the odd electron ions, formed on EI, to fragment so as to separate the radical from the charge. Thus prior rearrangement within the molecular cation radical will be revealed when it falls apart. This driving force is abrent in the a>mmonly encountered protonated and hena even ekctron ions encountered in CI sources. Although the w+l)+ ion may be extensively rearranged this will remain hidden in the absence of fragmentation. Of great importma is tbc oppommity thcae rcvcdcd rearrangements give to elucidating the overlap between carbcnium ion chemistry in solution and in mass spectrometers.’ In the former situation the details of molecular rcorganixation t NU Gzuununimtio
No. 2434
can be monitored via NMR spectrometry whereas in the mass spectrometer we are constrained to product anaIysis. In the case herein we shall see that the interesting extruxion of a single carbon bearing hydrogen and two aryl groups from the protonated triarylnitrocycIohexencs can be tenonably connected to the long known contraction of 6to S-membered ring carbcnium ions.’ Moreover we have prcviourly studied the EI khaviour of thin class of aryl nitrocyclohexencs’ and the present study actx to complete this picture for tbe bchaviour of these materials in mass rpcctrometcn subject lo difIerential iotition mcchanLME3. The compounds under discussion arc exhibited in !5cbcmc 1. -AL compaurdr. The di- and tri~~o/~ rubrtituted nitrocydohexenfa were prepared by well des&bed procedurc8.~’ Ibe a0 and e&o ddpctr formlxf were carefully separated oo dim gel by chromatogmphic Iedlniqm. structum and SIerooch~ were dglld ffOlllSpXddrt&AUtllCNMRSpCCU8WC~a V&an T-60 instrument. !bnningofdwSom*’ CIspfxtmofaUtheampound8 were recorded 00 hnigao Mua Spectrometer 32~.dll~UlCthAlXIDdkbUtaDCU~t@8C%,tbc oo
presWcdwhktlwuulailltaiucdrr1torr.Tbetunpof ioairation chamkr was betwceo 14V to 214’. REsULTaANDDKWWsIo?4 Tabk I cxhibftx the peraotagc of total ionization above m/t 90 for the outstanding ion8 in the CI spectra of the compounds listed in Scheme 1. A heuristic scheme, which must await further evidcace for confirmation, (from low temperature
2697
2698
G. S~D+WLNI &DDY ei OL
(A) (1) (2
(0)
RI * RS=phonyl~V2=H )
RI = R1 = phrnyli
R2 =H
IeOm@r
A
I8om.r
9
(3)
RI = Rp = Rl=phrnyl
Iromor
A
(4)
RI = Rz = RS = phony1
lsomrr
B
(5)
RI=
Ra=ph’onyl,
R2=-CIH,-CttI(~)
lromrr
A
(6)
RI=
Ra=phrnyl
R2=
lromrr
B
(7)
RI=
RS=phrnylj
lromrr
A
j
-Cell,-CHJP)
R25-CIH4-Cl(P)
(8)
RI 8 R, x phony1
;
R2 a -CIH4-
(9)
RI = R,=
phony1
f
R2 =-CIH4-F
(10)
RI = RS = phony1
i
R2 = -CIH4
-F
lromrr
B
(PI
lromrr
A
(PI
Iaomrr
B
lromrr
A
lromrr
B
(p)
(II)
RI
phrnyl;
R2 = -CeH4
-Br(P)
(12)
RI o Ra = phrnyl;
RP t -Calf4
-Er
=
Rs=
Cl
(P)
(13)
RI m Ra = phonyl;
R2 = 2-
Fury1
lromrr
A
(14)
RI = RI
R2 - 2 - Fury1
Iromor
B
(15)
RI = Ra = phony1
(IS)
RI = R3=
= phonyl;
phrnyl;
;
R2 = 2 - Thirnyl
lromrr
A
R2=CIH4-CN
lromrr
A
scbcmc
NMR and solvolytic studies) would reasonably involve the initial protonatioa at the nitro group’ so as to give loss of HNC&. The loss of HNOl to lead to the carbcnium ion might also be aided by the adjacent rhenyl group” or by solvolytically precedent&’ participation of the homoallylic double bond. Thus the observation of a low intensity (M+ H)’ ion is reasonable and not of exceptional note. The loss of the phenyl or aryl groups plus one hydrogen and the protonated dicne product of the retro-Dieb-Alder reaction can reasonably be ascribed to alternative protonation by the ionized reagent gax on the phenyl or aryl groups or the ring double bond. Protonation at various sites in multifunctional molecules can be expected.” The key and striking observation in these molecules is the abundant (Table I) extrusion of the charged fragmcntx [CHR2RJ, [CHR,RJ’ and [CHR,RJ. It is di5cult to understand how the substituted cyclohcxene structure could yield such ions. It is ccrtainly aafc to conclude that an extensive rcmngemtnt has occurred. As outlined below, a reasonable solution to this apparent conundrum may be found in the literature of cydohexenyl carbenium ions. Dcno and Houxer first reported on the basis of NMR data the opening of bicyclic alcohols to cyclohexenyl ations which then undergo ring contraction to cyclopcntcnyl ions.” Sorenson ef (11.” have reported that cydopropyl ally1 cations rexrrangc to cyclohcxenyl cations which then rearrange further to cyclopcntenyl cationx. These workers have also shown that this carbocation system undergoes a series of degenerative rearrangements whkb scrambk the ring positions”. Olah et al.“‘” in their
(PI
1.
studies of carbocations in super acid systems have observed that many substituted cyclohexenyl cations undergo ring contraction to form the substituted cyclopcntcnyl cations. Among these, generation of methyl cydopcntenyl cation from cyclohcx3-cnol” in super acid provides a key analog for the present system. In related work when 2-phenyl cyclohexylamine and 2.3 dimethyld-phenylcyclohexylamine were treated with nitrous acid the corresponding ring contracted alcohols have been obtained in good yields.” Further research in this area has led to details of the scrambling mechanisms available in these ions.” The rezarch outlined above suggests the possibility of ring contraction in the w+ H- HNOJ ion produced in the Cl of compounds 1-16 (!Scbcme 1). As part of the work reported in solution and dixc~S4 above the cyclohcxenyl cation with positive charge at tbc &3-poxition has been dimmed Scheme 2 outlinea some of tbcpoxaibkdire&rtxthixionwouldtakebaxcd on * xt.u&_*lLl~lbl~ The interconnections exhibited in Scheme 2 constitute an extremely complex kinetic s)atcm which has been studied in pxrt by many workers and in which the individual rates and equilibria are strongly dependent on the substitution pattern of the cyclohexenyl cation precursor. These studies have for the most part been conducted in strong acid media at low temperatures, e.g. C-50”. Although temperature has a strong effect on these intcrconvenions, the ring contraction i8 observed at the highest temperatures reported.” There is no readon to doubt that these rcarrangancnts outlined
Sktkti
rearraugeme~~induced by chcmiat ionixation-I
Known Intcrmedirtcs From Cycbbcre~yl
in S&me 2 would be easily accessible to the cyclohexenyl cation generated by ~BS of HNOa from compounds 146 (Scheme 1) in the Cl source of a mass spectrometer. we are now offered an explanation for the expulsion rearrangement loss of [CHRR~ (Table I). Ring contraction from the initially produced cyclobexenyl cation could cast one carbon out of, although for the moment still attached to, the cycle. I&fore addressing the question as to how this now distinguiuhed carbon may be ejected from the newly formed cyclopenteayt cation let us dii the c&traction step further. The initial precursor A (Scheme 2) can only push out either C,(la) or C;(p).
ref. 19 8nd refsteaccr therein.
Chtionr’
might very well explain the observed fact (Table I) that the expelled group in all cases overwhelmingly contains R1. Neverthek-ss either via intermediate C (Scheme 2) and/or by double bond pupation to D (Scheme 2) C-3 and C-6 may oaarpy the expelled position. IIris longer route places R, and R3 on the C to be ejected. The ion [CHR,RJ is a&o encountered although with lower intensity. The CI re~ultb are not contradicted by expectations based on the known NMR intermediates (Scheme 2). In the NMR studies d&cussed above, it was found that the ring contracted ion, e.g., E (Scheme 2) rapidly transfem hydride from C-3 to C-4 so as
Itis
scheme 3* ‘See
2699
2700
Scheme 4
Tin proposal in Scheme 4 is supported by: analogy to known solution carbocation chemistry; the expulsion of diilmetbinyl cations predominantly containing R1 aryi gulps of widely varying structure; a strong stereochemical dependence. The lab tcr two observations would be difficult to understand if more random and extensive rearrangcmen& were occurring prior to fragmentation. l’be present study indi~rtes that the substituted cyclohexenyl cation undergoes the same type of rearrangements urxkr solvolytic conditions and in the cl XNlrce.
to place the charge within the ring, A large body of evidence on intramolecular Ftiodel-CT* alkylntions and acylations’* shows that the process depicted in Scheme 3 is rapid. (3rbocations located so as to close the S-membered ring are favourable. This is precisely the situation in the mokcuks d&zussed here (Scheme 1; Table B and moreover such an intervention hypothesis could reasonably lead to the CI observed ~~~e~nyl cation (Table f). This hypothesis leads to an overall mechanism which is outlined for compound 3 (Scheme 1) in Scheme 4.
Tabk I’ Partial tptftrum expmsed as percent of total ionizationsbovc m/z 90 for compounds 1-16
Retro-Dieb w
[(M+W-@@QCmp.
Reagent
No.
GM*
I
I
2 3
:
4 : 7 8 9 10 11 12 13 14 15 16
I I : x z fl: I I :
(Rflr FI+Hr
1.3 12 <.S <.S i .S < .s < .5 C.5 C.1 <.l C.l C.1 3.0 7.5 :::
f(M+H)--(HNodr
K=WJ Diene Rotoaat&t and/or
an&or
fR,wr
ww
52
2.0
-
z
1.4 *
-
2 30 18 1s :: :d 8.9 15 13 7.3
:;
ion
::; .6 c.5 c.5 1.7
fmR,r
-
z 2.7 1.2 3.2 1.4 4.2 3.0
*DC
266.2 14 27 5.4 8.0 4.4 4.6 2.2 i.4 l:o
1.2
.39
3.5 2.3 3.9 2.0 4.2 3.S 4.7 5.2 13 4.5 44 8’5 2:o
2.4 2.7 .76 .75 4.2 1.6 3.0 1.9 4.6 1.2 44 3’1 :66
%I ;; 10.5 19 8.4 15 14 12 6.7 11.4 9.2 21 5.3
‘ Refer to Scheme 1, for the swum comrpoading to the [email protected]. ben and the uyf poop dh$pwcd and R,. bIhi80butuxandMbmethuse ‘Comprising 1.4 substituted (R, ti R,l butadianc d In tbeu compound R, - & - R, - phcnyl(R). Heacs it is denoted a [(M+ H)-_(HNO~l’+
DfR,RS 5.4 3.4 -
1.9 .78 4.8 1.8 4.5 3.0 3.0 1.5 1.2 <.S 1,s 1.3 by R,, R,
Skekul munogemeots
Acknowkdgcment--The
luthux thaok Dr. K. Levseo
induced by chemical ion&tio~I
of
the Iostihl1 flir Pllysikauscbc aleolk. Uoivenitat Booo, w8.t Gnoaoy for CA data and Dr. B. schubch,
Universitlt BkJfeld. Wcrt Germany for MIKE dam to the of the authors (GSR) thanks CSIR, New Delhi. India, for the award of a junior rcuarcb feuowIi’ip.
aappoe the El obwvathn:
rcvkr is:A_ L. Buriingamc. C. H. L. S&a&ktoo, 1. Howe and 0. S. Chizhov. Anafyr C&m. SO, 346 (1978) and paw 35>356 in parthhr. See abo: F. H. Feld. Acauna Qum Ra. 1, 42 (1968) and subaequeot papen in that wriu; F. H. Field, MIP hemaliOMfRcokWofScLnu,Pfly¶hJCkRit@y&iUOnc (Edited by A. MeoIl) Vol. 5; CL 5, Butterworths, Loodon (1972). A beautiful rcaot example of thh uralyhml value is: A. M. Scpukhre. P. Varencc, B. C. Du. S. D. Gro and H. E. Audier. Nowrau J. Cl&& 2, 405 (1978). ‘P. Bran and C. Djcraai, Angry. Glum. Inr Ed 6, 477 (1967). For geoeral discus&o. rcfs aml cumpka w: R. A. W. Johnatone, h&u Spuuuwrry fa wan&c C7wmim. Cambridge Univ. F’ras, Caahidgt (1972); D. H. Wii and I. Howe, PriKipltt 4 Gr8anic Mass Spchumerry. McGraw-Hill, New York, (1972). F. W. Mchfferty, rll&rFwfarionof Man specw (2nd Edidoo) Benjamin. Reading Ma.. (1978); K. G. Du. Ogank Mou Specrrcmwrry.IEH and Oxford, New Delhi (1976). Rurrangemcat reactions in EI and Cl mass rpcc(rometry UC valuable probes of stercolomcrism. !kc: M. M. Gnxo, Mars Specaometry of Nahuol Fnhcrr Pergamon Pras, New York (1978). ‘A few outsmding exampks arc: F. H. Field and M. S. B. Mumoo, 1. Am. CJwm. Sot m, 4272 (1967); F. H. Fkld, ibid 9.,5649 (1968); Ibid. 89.5328 (1967). J. G. Lkhr and A. McCloskev. 0*1. Mou Swccnmr 9. 491 (1974); P. Loogevialk,-6. +. A. Miinc and ti. M. Faka. 1. Am. C&m. Sot 9S, 6666 (1973). A lcaot exccknt review is: W. Richter and H. !khwarz, Angcw. Owm. Int. Ed. 17, 424 (1978). ‘H. C. Dcno and J. J. Houser. 1. Am. Owm. !hc. w 1741 (1964). ‘A rtaot
2701
‘K.G.DuandK.P.Madhuaudumn,Og.MassSpecwon. ll, 135 (1976). ‘J. S. Bii, C. Jain and N. Anand, Ind. 1. C&m. 9, 388 (1971). ‘K. P. M&a’usudhMM, Ph.D. Therh. Kcrala Univcnity, Kcrak, India (1974). ‘In afktitktn, the El 8pcctra wrc recorded for all materiala in !&&c-me1. Tbc results support our carikr pablishcd oburvrtins.’ we have in the aunt of Ihi& recorded Ibc normal metastabk ions at 70 eV and dctcrmincd the MIKE qccrn for tbe FI_NO]’ and (M-HNOJ’ ioru from ampwnd 3 in Scbfsnc 1. These data and diwusioo mry be found in: G. Sudhaku Rmddy. Ph.D. Thesis, in preparatioo, National Chcmkal bboratory, Powa, India (1979). Thir work will form the buia of a future publication. PD. P. Mudnwo. %. hi(uI $ecrmm. ll, 762 (1976). ‘9. H. Shapiro and T. F. Jenkins, %. hfasa Qecaum 2, 771 (1969); W. J. Rkbter and W. Vetter. Ibid 2,781 (1969); G. W. A. Milne. T. Axenrod, H. M. F&s, 1. Am. C7wm. Sot. n 5170 11970). “G. A. Okb. G. L&g uni Y. i<. MO. IMd 94, 3544 (1972). ‘?ke ref.3 and especially W. Richter and H. !khwur. “N. C. Dew and J. J. Houser. 1. Am. C&m. Sot_ $6, 1741 (1964). set also: N. C. Dcno and R. R. Ltrtoahky, lb&d. 90,408s (1968). ‘q. S. Soremoo and K. Rajahwui. Ibid X4, 4222 (1971); T. S. Sorncaon and K. Ranganayakulu. IMd 9& 6539 (1970); T. S. Sorensoo. Ibid 09, 3794 (1967). ‘?. S. Sorewo and K. Raiahwari. CM. 1. Chcm. 56. 2939 (1972); T. S. Soren&o. R. Coot, R. P. H&tin; and P. Kamukr. Ibid. S2 3320 11974). “G. A. O&b and & Li&I. Am.‘ Cl&, Sot w, 6434 (1972). “D. V. Nighdngalc and M. Makothal, Ibid. 72, 4823 (19%); D. V. Niatingak. J. D. Kerr. J. A. G&&x and M. Makothai, 1. &g. C7wm. 17, 1017 (1952).“G. Sevbold. P. VoseL M. Saunders sod K. B. Wibcre J. Am. bum Sa -% 2045 (1973); D. Fucasiu, &d. 100, 1015 (1978). ‘pL.RorrandcBuday.FweI-cmfrrandRe&&d Rcoaiar (Edited by G. A. m) Vol. II; Ch. XXII. II’IenckDcr (1964); s. Sethnr, ad. Vol. m, a’. XXXV.
REARRANGEMENTS INDUCED 10NIzATI0N-I ANALOGY
TO SOLVOLYSISt
G. SUDHAKAR &DDY, Nuional C&mid
lnstitut f8r &pnischc
BY CHEMICAL
M. VAIRAhlANI and K. G. D,s* Poo~411008. ludi~
Laboratory.
Cbeah
dcr
Univanftit K&n. West Gamany
CWhon Collcp ol Tcdmo&8y. Potdun. New York
13676, USA
(RaxkdinUK3Apd1979) ALaweIn amlras4 to the OrlId expeuatio in chemical iotlhcioo (CI) amm spectmmCQ, the cl qxctm of I &es of aryl4-nitro-A’-cyclohercaer show very low intensity protoorted mokculu ions
rhikamrj~~rhrrtdtbeioncurrentirarrialbyrc~ -gemcot long known in the dvolyais
of cyclohcxeoyl
otions.Tlleintuvmtionofr carkoium ionr un a-1 for tbc
obscmcd CI bchviour.
The analytical
value of chemical ionization mass rpcctrometry’ derives from the usual observation of simple spectra dominated by the intensity of the protonatcd mokcular ion (M+ 1)‘. This expcctatioa is dramatically breached in the CI rpectrr of a l&es of aryl rubetitutcd nitrocydo&un?d reported herein. Our observations arc exemplidcd by tbc isobutane CI spectrum of 3,5&ripbenyl4nitrocyclohexeae which cxhibitx an Fi+ l)+ ion carrying only 0.4% of the total ionization above m/z 90 (%&,,) while the base peak in the spectrum which carries 30%& is a rearrangement ion cornpored of two of the phenyl groups and a mcthine carbon [cJI,)KW. Skektal rearrangement proaaus which are cornmon in electron ionization mass spectra’ arc of less frequent occurrence in chemical ionixation masa spectra. In the literature there arc only a limited number of skeletal reamngementx reported under CI conditions.“’ One reason for these dif?ercntial obmvationr is the driving force pushing the odd electron ions, formed on EI, to fragment so as to separate the radical from the charge. Thus prior rearrangement within the molecular cation radical will be revealed when it falls apart. This driving force is abrent in the a>mmonly encountered protonated and hena even ekctron ions encountered in CI sources. Although the w+l)+ ion may be extensively rearranged this will remain hidden in the absence of fragmentation. Of great importma is tbc oppommity thcae rcvcdcd rearrangements give to elucidating the overlap between carbcnium ion chemistry in solution and in mass spectrometers.’ In the former situation the details of molecular rcorganixation t NU Gzuununimtio
No. 2434
can be monitored via NMR spectrometry whereas in the mass spectrometer we are constrained to product anaIysis. In the case herein we shall see that the interesting extruxion of a single carbon bearing hydrogen and two aryl groups from the protonated triarylnitrocycIohexencs can be tenonably connected to the long known contraction of 6to S-membered ring carbcnium ions.’ Moreover we have prcviourly studied the EI khaviour of thin class of aryl nitrocyclohexencs’ and the present study actx to complete this picture for tbe bchaviour of these materials in mass rpcctrometcn subject lo difIerential iotition mcchanLME3. The compounds under discussion arc exhibited in !5cbcmc 1. -AL compaurdr. The di- and tri~~o/~ rubrtituted nitrocydohexenfa were prepared by well des&bed procedurc8.~’ Ibe a0 and e&o ddpctr formlxf were carefully separated oo dim gel by chromatogmphic Iedlniqm. structum and SIerooch~ were dglld ffOlllSpXddrt&AUtllCNMRSpCCU8WC~a V&an T-60 instrument. !bnningofdwSom*’ CIspfxtmofaUtheampound8 were recorded 00 hnigao Mua Spectrometer 32~.dll~UlCthAlXIDdkbUtaDCU~t@8C%,tbc oo
presWcdwhktlwuulailltaiucdrr1torr.Tbetunpof ioairation chamkr was betwceo 14V to 214’. REsULTaANDDKWWsIo?4 Tabk I cxhibftx the peraotagc of total ionization above m/t 90 for the outstanding ion8 in the CI spectra of the compounds listed in Scheme 1. A heuristic scheme, which must await further evidcace for confirmation, (from low temperature
2697
2698
G. S~D+WLNI &DDY ei OL
(A) (1) (2
(0)
RI * RS=phonyl~V2=H )
RI = R1 = phrnyli
R2 =H
IeOm@r
A
I8om.r
9
(3)
RI = Rp = Rl=phrnyl
Iromor
A
(4)
RI = Rz = RS = phony1
lsomrr
B
(5)
RI=
Ra=ph’onyl,
R2=-CIH,-CttI(~)
lromrr
A
(6)
RI=
Ra=phrnyl
R2=
lromrr
B
(7)
RI=
RS=phrnylj
lromrr
A
j
-Cell,-CHJP)
R25-CIH4-Cl(P)
(8)
RI 8 R, x phony1
;
R2 a -CIH4-
(9)
RI = R,=
phony1
f
R2 =-CIH4-F
(10)
RI = RS = phony1
i
R2 = -CIH4
-F
lromrr
B
(PI
lromrr
A
(PI
Iaomrr
B
lromrr
A
lromrr
B
(p)
(II)
RI
phrnyl;
R2 = -CeH4
-Br(P)
(12)
RI o Ra = phrnyl;
RP t -Calf4
-Er
=
Rs=
Cl
(P)
(13)
RI m Ra = phonyl;
R2 = 2-
Fury1
lromrr
A
(14)
RI = RI
R2 - 2 - Fury1
Iromor
B
(15)
RI = Ra = phony1
(IS)
RI = R3=
= phonyl;
phrnyl;
;
R2 = 2 - Thirnyl
lromrr
A
R2=CIH4-CN
lromrr
A
scbcmc
NMR and solvolytic studies) would reasonably involve the initial protonatioa at the nitro group’ so as to give loss of HNC&. The loss of HNOl to lead to the carbcnium ion might also be aided by the adjacent rhenyl group” or by solvolytically precedent&’ participation of the homoallylic double bond. Thus the observation of a low intensity (M+ H)’ ion is reasonable and not of exceptional note. The loss of the phenyl or aryl groups plus one hydrogen and the protonated dicne product of the retro-Dieb-Alder reaction can reasonably be ascribed to alternative protonation by the ionized reagent gax on the phenyl or aryl groups or the ring double bond. Protonation at various sites in multifunctional molecules can be expected.” The key and striking observation in these molecules is the abundant (Table I) extrusion of the charged fragmcntx [CHR2RJ, [CHR,RJ’ and [CHR,RJ. It is di5cult to understand how the substituted cyclohcxene structure could yield such ions. It is ccrtainly aafc to conclude that an extensive rcmngemtnt has occurred. As outlined below, a reasonable solution to this apparent conundrum may be found in the literature of cydohexenyl carbenium ions. Dcno and Houxer first reported on the basis of NMR data the opening of bicyclic alcohols to cyclohexenyl ations which then undergo ring contraction to cyclopcntcnyl ions.” Sorenson ef (11.” have reported that cydopropyl ally1 cations rexrrangc to cyclohcxenyl cations which then rearrange further to cyclopcntenyl cationx. These workers have also shown that this carbocation system undergoes a series of degenerative rearrangements whkb scrambk the ring positions”. Olah et al.“‘” in their
(PI
1.
studies of carbocations in super acid systems have observed that many substituted cyclohexenyl cations undergo ring contraction to form the substituted cyclopcntcnyl cations. Among these, generation of methyl cydopcntenyl cation from cyclohcx3-cnol” in super acid provides a key analog for the present system. In related work when 2-phenyl cyclohexylamine and 2.3 dimethyld-phenylcyclohexylamine were treated with nitrous acid the corresponding ring contracted alcohols have been obtained in good yields.” Further research in this area has led to details of the scrambling mechanisms available in these ions.” The rezarch outlined above suggests the possibility of ring contraction in the w+ H- HNOJ ion produced in the Cl of compounds 1-16 (!Scbcme 1). As part of the work reported in solution and dixc~S4 above the cyclohcxenyl cation with positive charge at tbc &3-poxition has been dimmed Scheme 2 outlinea some of tbcpoxaibkdire&rtxthixionwouldtakebaxcd on * xt.u&_*lLl~lbl~ The interconnections exhibited in Scheme 2 constitute an extremely complex kinetic s)atcm which has been studied in pxrt by many workers and in which the individual rates and equilibria are strongly dependent on the substitution pattern of the cyclohexenyl cation precursor. These studies have for the most part been conducted in strong acid media at low temperatures, e.g. C-50”. Although temperature has a strong effect on these intcrconvenions, the ring contraction i8 observed at the highest temperatures reported.” There is no readon to doubt that these rcarrangancnts outlined
Sktkti
rearraugeme~~induced by chcmiat ionixation-I
Known Intcrmedirtcs From Cycbbcre~yl
in S&me 2 would be easily accessible to the cyclohexenyl cation generated by ~BS of HNOa from compounds 146 (Scheme 1) in the Cl source of a mass spectrometer. we are now offered an explanation for the expulsion rearrangement loss of [CHRR~ (Table I). Ring contraction from the initially produced cyclobexenyl cation could cast one carbon out of, although for the moment still attached to, the cycle. I&fore addressing the question as to how this now distinguiuhed carbon may be ejected from the newly formed cyclopenteayt cation let us dii the c&traction step further. The initial precursor A (Scheme 2) can only push out either C,(la) or C;(p).
ref. 19 8nd refsteaccr therein.
Chtionr’
might very well explain the observed fact (Table I) that the expelled group in all cases overwhelmingly contains R1. Neverthek-ss either via intermediate C (Scheme 2) and/or by double bond pupation to D (Scheme 2) C-3 and C-6 may oaarpy the expelled position. IIris longer route places R, and R3 on the C to be ejected. The ion [CHR,RJ is a&o encountered although with lower intensity. The CI re~ultb are not contradicted by expectations based on the known NMR intermediates (Scheme 2). In the NMR studies d&cussed above, it was found that the ring contracted ion, e.g., E (Scheme 2) rapidly transfem hydride from C-3 to C-4 so as
Itis
scheme 3* ‘See
2699
2700
Scheme 4
Tin proposal in Scheme 4 is supported by: analogy to known solution carbocation chemistry; the expulsion of diilmetbinyl cations predominantly containing R1 aryi gulps of widely varying structure; a strong stereochemical dependence. The lab tcr two observations would be difficult to understand if more random and extensive rearrangcmen& were occurring prior to fragmentation. l’be present study indi~rtes that the substituted cyclohexenyl cation undergoes the same type of rearrangements urxkr solvolytic conditions and in the cl XNlrce.
to place the charge within the ring, A large body of evidence on intramolecular Ftiodel-CT* alkylntions and acylations’* shows that the process depicted in Scheme 3 is rapid. (3rbocations located so as to close the S-membered ring are favourable. This is precisely the situation in the mokcuks d&zussed here (Scheme 1; Table B and moreover such an intervention hypothesis could reasonably lead to the CI observed ~~~e~nyl cation (Table f). This hypothesis leads to an overall mechanism which is outlined for compound 3 (Scheme 1) in Scheme 4.
Tabk I’ Partial tptftrum expmsed as percent of total ionizationsbovc m/z 90 for compounds 1-16
Retro-Dieb w
[(M+W-@@QCmp.
Reagent
No.
GM*
I
I
2 3
:
4 : 7 8 9 10 11 12 13 14 15 16
I I : x z fl: I I :
(Rflr FI+Hr
1.3 12 <.S <.S i .S < .s < .5 C.5 C.1 <.l C.l C.1 3.0 7.5 :::
f(M+H)--(HNodr
K=WJ Diene Rotoaat&t and/or
an&or
fR,wr
ww
52
2.0
-
z
1.4 *
-
2 30 18 1s :: :d 8.9 15 13 7.3
:;
ion
::; .6 c.5 c.5 1.7
fmR,r
-
z 2.7 1.2 3.2 1.4 4.2 3.0
*DC
266.2 14 27 5.4 8.0 4.4 4.6 2.2 i.4 l:o
1.2
.39
3.5 2.3 3.9 2.0 4.2 3.S 4.7 5.2 13 4.5 44 8’5 2:o
2.4 2.7 .76 .75 4.2 1.6 3.0 1.9 4.6 1.2 44 3’1 :66
%I ;; 10.5 19 8.4 15 14 12 6.7 11.4 9.2 21 5.3
‘ Refer to Scheme 1, for the swum comrpoading to the [email protected]. ben and the uyf poop dh$pwcd and R,. bIhi80butuxandMbmethuse ‘Comprising 1.4 substituted (R, ti R,l butadianc d In tbeu compound R, - & - R, - phcnyl(R). Heacs it is denoted a [(M+ H)-_(HNO~l’+
DfR,RS 5.4 3.4 -
1.9 .78 4.8 1.8 4.5 3.0 3.0 1.5 1.2 <.S 1,s 1.3 by R,, R,
Skekul munogemeots
Acknowkdgcment--The
luthux thaok Dr. K. Levseo
induced by chemical ion&tio~I
of
the Iostihl1 flir Pllysikauscbc aleolk. Uoivenitat Booo, w8.t Gnoaoy for CA data and Dr. B. schubch,
Universitlt BkJfeld. Wcrt Germany for MIKE dam to the of the authors (GSR) thanks CSIR, New Delhi. India, for the award of a junior rcuarcb feuowIi’ip.
aappoe the El obwvathn:
rcvkr is:A_ L. Buriingamc. C. H. L. S&a&ktoo, 1. Howe and 0. S. Chizhov. Anafyr C&m. SO, 346 (1978) and paw 35>356 in parthhr. See abo: F. H. Feld. Acauna Qum Ra. 1, 42 (1968) and subaequeot papen in that wriu; F. H. Field, MIP hemaliOMfRcokWofScLnu,Pfly¶hJCkRit@y&iUOnc (Edited by A. MeoIl) Vol. 5; CL 5, Butterworths, Loodon (1972). A beautiful rcaot example of thh uralyhml value is: A. M. Scpukhre. P. Varencc, B. C. Du. S. D. Gro and H. E. Audier. Nowrau J. Cl&& 2, 405 (1978). ‘P. Bran and C. Djcraai, Angry. Glum. Inr Ed 6, 477 (1967). For geoeral discus&o. rcfs aml cumpka w: R. A. W. Johnatone, h&u Spuuuwrry fa wan&c C7wmim. Cambridge Univ. F’ras, Caahidgt (1972); D. H. Wii and I. Howe, PriKipltt 4 Gr8anic Mass Spchumerry. McGraw-Hill, New York, (1972). F. W. Mchfferty, rll&rFwfarionof Man specw (2nd Edidoo) Benjamin. Reading Ma.. (1978); K. G. Du. Ogank Mou Specrrcmwrry.IEH and Oxford, New Delhi (1976). Rurrangemcat reactions in EI and Cl mass rpcc(rometry UC valuable probes of stercolomcrism. !kc: M. M. Gnxo, Mars Specaometry of Nahuol Fnhcrr Pergamon Pras, New York (1978). ‘A few outsmding exampks arc: F. H. Field and M. S. B. Mumoo, 1. Am. CJwm. Sot m, 4272 (1967); F. H. Fkld, ibid 9.,5649 (1968); Ibid. 89.5328 (1967). J. G. Lkhr and A. McCloskev. 0*1. Mou Swccnmr 9. 491 (1974); P. Loogevialk,-6. +. A. Miinc and ti. M. Faka. 1. Am. C&m. Sot 9S, 6666 (1973). A lcaot exccknt review is: W. Richter and H. !khwarz, Angcw. Owm. Int. Ed. 17, 424 (1978). ‘H. C. Dcno and J. J. Houser. 1. Am. Owm. !hc. w 1741 (1964). ‘A rtaot
2701
‘K.G.DuandK.P.Madhuaudumn,Og.MassSpecwon. ll, 135 (1976). ‘J. S. Bii, C. Jain and N. Anand, Ind. 1. C&m. 9, 388 (1971). ‘K. P. M&a’usudhMM, Ph.D. Therh. Kcrala Univcnity, Kcrak, India (1974). ‘In afktitktn, the El 8pcctra wrc recorded for all materiala in !&&c-me1. Tbc results support our carikr pablishcd oburvrtins.’ we have in the aunt of Ihi& recorded Ibc normal metastabk ions at 70 eV and dctcrmincd the MIKE qccrn for tbe FI_NO]’ and (M-HNOJ’ ioru from ampwnd 3 in Scbfsnc 1. These data and diwusioo mry be found in: G. Sudhaku Rmddy. Ph.D. Thesis, in preparatioo, National Chcmkal bboratory, Powa, India (1979). Thir work will form the buia of a future publication. PD. P. Mudnwo. %. hi(uI $ecrmm. ll, 762 (1976). ‘9. H. Shapiro and T. F. Jenkins, %. hfasa Qecaum 2, 771 (1969); W. J. Rkbter and W. Vetter. Ibid 2,781 (1969); G. W. A. Milne. T. Axenrod, H. M. F&s, 1. Am. C7wm. Sot. n 5170 11970). “G. A. Okb. G. L&g uni Y. i<. MO. IMd 94, 3544 (1972). ‘?ke ref.3 and especially W. Richter and H. !khwur. “N. C. Dew and J. J. Houser. 1. Am. C&m. Sot_ $6, 1741 (1964). set also: N. C. Dcno and R. R. Ltrtoahky, lb&d. 90,408s (1968). ‘q. S. Soremoo and K. Rajahwui. Ibid X4, 4222 (1971); T. S. Sorncaon and K. Ranganayakulu. IMd 9& 6539 (1970); T. S. Sorensoo. Ibid 09, 3794 (1967). ‘?. S. Sorewo and K. Raiahwari. CM. 1. Chcm. 56. 2939 (1972); T. S. Soren&o. R. Coot, R. P. H&tin; and P. Kamukr. Ibid. S2 3320 11974). “G. A. O&b and & Li&I. Am.‘ Cl&, Sot w, 6434 (1972). “D. V. Nighdngalc and M. Makothal, Ibid. 72, 4823 (19%); D. V. Niatingak. J. D. Kerr. J. A. G&&x and M. Makothai, 1. &g. C7wm. 17, 1017 (1952).“G. Sevbold. P. VoseL M. Saunders sod K. B. Wibcre J. Am. bum Sa -% 2045 (1973); D. Fucasiu, &d. 100, 1015 (1978). ‘pL.RorrandcBuday.FweI-cmfrrandRe&&d Rcoaiar (Edited by G. A. m) Vol. II; Ch. XXII. II’IenckDcr (1964); s. Sethnr, ad. Vol. m, a’. XXXV.