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On the efficiency of intramolecular trapping of carbenes and biradicals by a proximate methyl group.
TetmhedronLettersNO. 24, pp 2217- 2220,1973. Per@mcn kess.
ONTW
Printed in Great Britrin.
F,FFIcIB?CY OF INTRAMOLEcULARTRAPPINGOF CARBRES ANDBIRADICALS BY A PKOXDJAT~ZETHYL GROUP.
#I.D.Crow and &zhael N. Paddon-Row ChemistryDepartment,School of General Studies,AustralianNational kivsreity, Canberra,A.C.T. 2600 (Receivedin UK 10 April 1973; locepted for pbliwtia
The formationof fulvsnallene,(I), from the gas We
19 April 1973)
1,2 poses 8n pJrrolysis of indssole
interestingmechanisticdichotomy. Thus, dependingupon the seat of migrationof the hydro@n 293 atom from position 1, a "biradioal"pathway (3 or a "oarbene"pathway (u mw be taken (scHEM?&x = II). Incorporationof a me-1
group in the 3-positionof N-deuterioindaEole led
to the intramoleoulartrappingof the tao intewediates (2) and (1) giving the labelledstyrenes.
SCHQ4El
R = H, CR3; CD3.
2217
2218
No. 24
Assay of the deuteriumcontentin the ring and in the exooyolicdouble bond gave the result that the *'biradical*8 pathvey ocourredto the extent of 82.5s betvem 700' end 760°, 0.05 mm. However the correctnessof our conclusionsdepends on the validityof the tacit assumptionthat little or no interconversion betmen the biradioal(2) and the carbene (2) occurs (pathwey (2) of the SCHWA) - in other words, that the methyl group acts ae a very efficienttrap for carbenesend biraticals. Although there is Bornequalitativeevidencein the literatureto support this a9.3umption4'5 we decided to obtain quantitativeevidenceconcerningthis point. The dry sodium salt of the tosylhydrazonederivative6of aoetophanone(4) (85$ triwas led directly deuteriatedat the aethyl gwup'l) was heated at 95’ and the 1-phenyldiazoethane into the furnace8at 630' or 720' at 0.05 mm. Under these conditionsthe carbene intermediate
(1;
11- CD3) is generated. ltirecttrappingof this carbene by the methyl group would give
styrene labelledwith deuteriumsolely at the olefinic sites (2). Law
to the biradical &;
a = CDj) end subsequenttrappingof this intermediategeneratesstyrene vith deuteriumincorporated into the ortho positionof the aromaticring (6). Assay of the deuteriumcontent of the
ring may be achieved through oonvers1on of the styrene to benzoic acid.9 As we have shown previously'that essentiallyall of the deuteriumlabel in the ring is locatedat the ortho positionit was only neceaaary then to estimatethe total deuteriumcontentof the etyrene and the benzoic acid by low voltagemaea spectrometry. The results are presentedin the Table. For the biradioalmnerator we cnose the D-labelledmethylhcmophthalic enbydride(1) since Spengler and r3.mhave shown that, 011gas phase pyrolysis, homophthalicanhydrideis convertedmto
2219
Ro. 24
Table $llinatyrene
$l)inbenaoic aoid"
$b&xd$al
&.&ar=
compound
Temp.
(2)
630'
85.4
1.8
2.1
97.9
(2)
720~
84
4.4
5.2
94.8
(1)
7m"
26.7
2.2
91.8
8.2
12 fulvenallen*." Pyrolysisof this compound at 720°, 0.05 mm gave the expeotedetyrene and the resulta are given in the Table. Je define the effioienoyof the methyl group in trappingthe biradical &) and the oarbene (1) in terms of the ratios kd(k3 + k5) end kd(k4 + k6) respectively(see SGHEZdE). conditioneprevail
:Jemake the ~s~pti~t~teq~~b~~
in
the furpace whioh implies that,
at constant temperatureand pressure,the energiesof the biradioal(2) and the oarbene (& are independentof the chemicalnature of their generators. Thm the relevantrate constants remain unalteredfor the generators(4) and (z) and the kinetic saheme for the direct generation of the biradioal (a mey be representedby:
k5 (1) ----c(2)
-
(If
-
Biradiaalproduct
k6 Carbene product
and a similar scheme ma~rbe written for the oarbene generator. From these two schemes,and using the steady state approximation,tao equationsmay be derived: BiradicalGenerator
$ biradicaluroduot $ carbene product
Carbene Generator
$ Carbene product $ Biradicalproduot
from which the trappingefficienciesmay be readily obtained. .lefind that, at 720' (0.05 mm), the methyl group traps the biradical@) with 915 effidency end that the carbene (2) ie trapped with 96 efficiency. ?Jeconclude,therefore,that the metbyl group does indeed trap oarbenea and b&radicalsefficiently. RLnally, the possibilitythat a significantproportionof the ring label was derived from soramblingin etyrene from the double bond into the aromaticnucleus was al-ted
by
No. 24
2220
subjectinga-D-styreneto the same pyrolysisconditions. At 720°, only 0.6s scramblinginto the aromaticring aae observedwith no overall loss of deuterium. However at 8CC", an overall loss of 13$ deuteriumresultedwith eoramblinginto the aromatic ring occurringto the extent of 155. Acknowled@aente. :k are gratefulto the AustralianResearch Grents Committeefor financial are indebtedto kr. R. Brig@ for the aa support. !'ie
spectrometrywork.
Referencesand Footnotes 1.
-I.D.Crow, A.R. Lea end b1.N.Paddon-Row,TetrahedronLetters, 2235 (1972).
2.
X.D. Crow end ti.N.Paddon-Row,Aust. J. Chem., 1973, in press.
3.
.r'.D. Crow end li.N.Paddon-Row,TetrahedronLetters, 3207 (1972).
4.
G.L. Closs, L.N. Kaplan end V.I. Bendall,J. her. Chem. Soc., a,
5.
d.J. Baron, II.Jones, Jr. end P. Gasper,i&i., 2, ibid., s,
3376 (1967).
4739 (1970); P. Hedaya and ki.P.Kent,
3283 (1971); G.G. Vander Stouw, A.R. Kraska and H. Shechter,ibid., a,
1655
(1972). 6.
G.Li.Kaufman,J.A. Smith, G.G. Vander Ytouw end H. Sheohter,J. her. Chem. Soo., a, 935 (1965).
7.
Preparedby refluxingaoetophenonewith D2C and K2C03.
8.
Wi.D.Crow and H.K. Sally, Aust. J. Chein., u,
9*
Ry oxidationof the etyrene with H2SO&in04.
10.
Correctedfor natural abundance.
11.
R.J. Spanglerand J.H. Kim, TetrahedronLetters, 1249 (1972).
12.
The dimethyl
2119 (1966).
ester of homophthalioacid was methylated(NaNR2/liq.NR3/i.ieI) and
subsequentlydeuteriated(NaWe/l,ZeOd) and hydrolyzedto the diacid. Conversionto (1) was effeotedby dioyclohexylcarbodiiaide.
ONTW
Printed in Great Britrin.
F,FFIcIB?CY OF INTRAMOLEcULARTRAPPINGOF CARBRES ANDBIRADICALS BY A PKOXDJAT~ZETHYL GROUP.
#I.D.Crow and &zhael N. Paddon-Row ChemistryDepartment,School of General Studies,AustralianNational kivsreity, Canberra,A.C.T. 2600 (Receivedin UK 10 April 1973; locepted for pbliwtia
The formationof fulvsnallene,(I), from the gas We
19 April 1973)
1,2 poses 8n pJrrolysis of indssole
interestingmechanisticdichotomy. Thus, dependingupon the seat of migrationof the hydro@n 293 atom from position 1, a "biradioal"pathway (3 or a "oarbene"pathway (u mw be taken (scHEM?&x = II). Incorporationof a me-1
group in the 3-positionof N-deuterioindaEole led
to the intramoleoulartrappingof the tao intewediates (2) and (1) giving the labelledstyrenes.
SCHQ4El
R = H, CR3; CD3.
2217
2218
No. 24
Assay of the deuteriumcontentin the ring and in the exooyolicdouble bond gave the result that the *'biradical*8 pathvey ocourredto the extent of 82.5s betvem 700' end 760°, 0.05 mm. However the correctnessof our conclusionsdepends on the validityof the tacit assumptionthat little or no interconversion betmen the biradioal(2) and the carbene (2) occurs (pathwey (2) of the SCHWA) - in other words, that the methyl group acts ae a very efficienttrap for carbenesend biraticals. Although there is Bornequalitativeevidencein the literatureto support this a9.3umption4'5 we decided to obtain quantitativeevidenceconcerningthis point. The dry sodium salt of the tosylhydrazonederivative6of aoetophanone(4) (85$ triwas led directly deuteriatedat the aethyl gwup'l) was heated at 95’ and the 1-phenyldiazoethane into the furnace8at 630' or 720' at 0.05 mm. Under these conditionsthe carbene intermediate
(1;
11- CD3) is generated. ltirecttrappingof this carbene by the methyl group would give
styrene labelledwith deuteriumsolely at the olefinic sites (2). Law
to the biradical &;
a = CDj) end subsequenttrappingof this intermediategeneratesstyrene vith deuteriumincorporated into the ortho positionof the aromaticring (6). Assay of the deuteriumcontent of the
ring may be achieved through oonvers1on of the styrene to benzoic acid.9 As we have shown previously'that essentiallyall of the deuteriumlabel in the ring is locatedat the ortho positionit was only neceaaary then to estimatethe total deuteriumcontentof the etyrene and the benzoic acid by low voltagemaea spectrometry. The results are presentedin the Table. For the biradioalmnerator we cnose the D-labelledmethylhcmophthalic enbydride(1) since Spengler and r3.mhave shown that, 011gas phase pyrolysis, homophthalicanhydrideis convertedmto
2219
Ro. 24
Table $llinatyrene
$l)inbenaoic aoid"
$b&xd$al
&.&ar=
compound
Temp.
(2)
630'
85.4
1.8
2.1
97.9
(2)
720~
84
4.4
5.2
94.8
(1)
7m"
26.7
2.2
91.8
8.2
12 fulvenallen*." Pyrolysisof this compound at 720°, 0.05 mm gave the expeotedetyrene and the resulta are given in the Table. Je define the effioienoyof the methyl group in trappingthe biradical &) and the oarbene (1) in terms of the ratios kd(k3 + k5) end kd(k4 + k6) respectively(see SGHEZdE). conditioneprevail
:Jemake the ~s~pti~t~teq~~b~~
in
the furpace whioh implies that,
at constant temperatureand pressure,the energiesof the biradioal(2) and the oarbene (& are independentof the chemicalnature of their generators. Thm the relevantrate constants remain unalteredfor the generators(4) and (z) and the kinetic saheme for the direct generation of the biradioal (a mey be representedby:
k5 (1) ----c(2)
-
(If
-
Biradiaalproduct
k6 Carbene product
and a similar scheme ma~rbe written for the oarbene generator. From these two schemes,and using the steady state approximation,tao equationsmay be derived: BiradicalGenerator
$ biradicaluroduot $ carbene product
Carbene Generator
$ Carbene product $ Biradicalproduot
from which the trappingefficienciesmay be readily obtained. .lefind that, at 720' (0.05 mm), the methyl group traps the biradical@) with 915 effidency end that the carbene (2) ie trapped with 96 efficiency. ?Jeconclude,therefore,that the metbyl group does indeed trap oarbenea and b&radicalsefficiently. RLnally, the possibilitythat a significantproportionof the ring label was derived from soramblingin etyrene from the double bond into the aromaticnucleus was al-ted
by
No. 24
2220
subjectinga-D-styreneto the same pyrolysisconditions. At 720°, only 0.6s scramblinginto the aromaticring aae observedwith no overall loss of deuterium. However at 8CC", an overall loss of 13$ deuteriumresultedwith eoramblinginto the aromatic ring occurringto the extent of 155. Acknowled@aente. :k are gratefulto the AustralianResearch Grents Committeefor financial are indebtedto kr. R. Brig@ for the aa support. !'ie
spectrometrywork.
Referencesand Footnotes 1.
-I.D.Crow, A.R. Lea end b1.N.Paddon-Row,TetrahedronLetters, 2235 (1972).
2.
X.D. Crow end ti.N.Paddon-Row,Aust. J. Chem., 1973, in press.
3.
.r'.D. Crow end li.N.Paddon-Row,TetrahedronLetters, 3207 (1972).
4.
G.L. Closs, L.N. Kaplan end V.I. Bendall,J. her. Chem. Soc., a,
5.
d.J. Baron, II.Jones, Jr. end P. Gasper,i&i., 2, ibid., s,
3376 (1967).
4739 (1970); P. Hedaya and ki.P.Kent,
3283 (1971); G.G. Vander Stouw, A.R. Kraska and H. Shechter,ibid., a,
1655
(1972). 6.
G.Li.Kaufman,J.A. Smith, G.G. Vander Ytouw end H. Sheohter,J. her. Chem. Soo., a, 935 (1965).
7.
Preparedby refluxingaoetophenonewith D2C and K2C03.
8.
Wi.D.Crow and H.K. Sally, Aust. J. Chein., u,
9*
Ry oxidationof the etyrene with H2SO&in04.
10.
Correctedfor natural abundance.
11.
R.J. Spanglerand J.H. Kim, TetrahedronLetters, 1249 (1972).
12.
The dimethyl
2119 (1966).
ester of homophthalioacid was methylated(NaNR2/liq.NR3/i.ieI) and
subsequentlydeuteriated(NaWe/l,ZeOd) and hydrolyzedto the diacid. Conversionto (1) was effeotedby dioyclohexylcarbodiiaide.