Photochemical nucleophile-olefin combination, aromatic substitution (photo-NOCAS) reaction: Methanol, β-myrcene, and 1,4-dicyanobenzene. Intramolecular cyclization of an ene-diene radical cation

Article abstract of DOI:10.1139/v98-156

The photochemical nucleophile-olefin combination, aromatic substitution (photo-NOCAS) reaction of methanol, 7-methyl-3-methylene-1,6-octadiene (β- myrcene, 1), and 1,4-dicyanobenzene yields five 1:1:1-adducts: cis-2(4- cyanophenyl)-4-(1-methoxy-1-methylethyl)-1-methylenecyclohexane (15), trans- 2-(4-cyanophenyl)-4-(1-methoxy-1-methylethyl)-1-methylenecyclohexane (16), 1- (4-cyanophenylmethyl)-4-(1-methoxy-1-methylethyl)cyclohexene (17), 4[4- methoxy-3,3-dimethylcyclohex-(E)-1-ylidenyl]methylbenzonitrile (18), and 4- (1-vinyl-4-trans-methoxy,3,3-dimethylcyclohexyl)benzonitrile (19). All of these adducts are cyclic; variation in the product ratio as a function of methanol concentration indicates cyclization is occurring, 1,6-endo, with both the initially formed radical cation and with the intermediate β- alkoxyalkyl radicals. Evidence based upon comparison of the ionization and oxidation potential of β-myrcene with model alkenes and with conjugated dienes indicates the initial electron transfer involves the trisubstituted mono alkene moiety; the diene moiety, mono-substituted at a nodal position, has a higher oxidation potential. High-level ab initio molecular orbital calculations (MP2/6-31G(*)//HF/6-31G(}) provide useful information regarding the nature (relative energies and charge and spin distribution) of the intermediate radical cations, which supports the proposed reaction mechanism.

Full text of DOI:10.1139/v98-156

1238
Photochemical nucleophile–olefin combination,
aromatic substitution (photo-NOCAS) reaction:
methanol, ␤-myrcene, and 1,4-dicyanobenzene.
Intramolecular cyclization of an ene–diene
radical cation1
Donald R. Arnold and Kimberly A. McManus
Abstract: The photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction of
methanol, 7-methyl-3-methylene-1,6-octadiene (β-myrcene, 1), and 1,4-dicyanobenzene yields five 1:1:1 adducts: cis-2-
(4-cyanophenyl)-4-(1-methoxy-1-methylethyl)-1-methylenecyclohexane (15), trans-2-(4-cyanophenyl)-4-(1-methoxy-1-
methylethyl)-1-methylenecyclohexane (16), 1-(4-cyanophenylmethyl)-4-(1-methoxy-1-methylethyl)cyclohexene (17), 4-
[4-methoxy-3,3-dimethylcyclohex-(E)-1-ylidenyl]methylbenzonitrile (18), and 4-(1-vinyl-4-trans-methoxy-3,3-
dimethylcyclohexyl)benzonitrile (19). All of these adducts are cyclic; variation in the product ratio as a function of
methanol concentration indicates cyclization is occurring, 1,6-endo, with both the initially formed radical cation and
with the intermediate β-alkoxyalkyl radicals. Evidence based upon comparison of the ionization and oxidation potential
of β-myrcene with model alkenes and with conjugated dienes indicates the initial electron transfer involves the
trisubstituted mono alkene moiety; the diene moiety, mono-substituted at a nodal position, has a higher oxidation
potential. High-level ab initio molecular orbital calculations (MP2͞6-31G*͞͞HF͞6-31G*) provide useful information
regarding the nature (relative energies and charge and spin distribution) of the intermediate radical cations, which
supports the proposed reaction mechanism.
Key words: photoinduced electron transfer, radicals, radical cations, β-myrcene, cyclization.
Résumé : La réaction photochimique de combinaison nucléophile–oléfine avec substitution aromatique (photo-NOCAS)
entre le méthanol, le 7-méthyl-3-méthylène-octa-1,6-diène (β-myrcène, 1) et le 1,4-dicyanobenzène fournit cinq adduits
1:1:1: cis-2-(4-cyanophényl)-4-(1-méthoxy-1-méthyléthyl)-1-méthylènecyclohexane (15), trans-2-(4-cyanophényl)-4-(1-
méthoxy-1-méthyléthyl)-1-méthylènecyclohexane (16), 1-(4-cyanophénylméthyl)-4-(1-méthoxy-1-
méthyléthyl)cyclohexène (17), 4-[4-méthoxy-3,3-diméthylcyclohex-(E)-1-ylidényl]méthylbenzonitrile (18) et le 4-(1-
vinyl-4-trans-méthoxy-3,3-diméthylcyclohexyl)benzonitrile (19). Tous ces produits sont cycliques; les variations dans
les rapports de produits en fonction de la concentration de méthanol indiquent que la cyclisation se produit, 1,6-endo,
tant avec le cation radical formé initialement qu’avec les radicaux β-alkoxyalkyles intermédiaires. Des données
obtenues sur la base d’une comparaison des potentiels d’ionisation et d’oxydation du β-myrcène avec ceux d’alcènes et
de diènes conjugués indiquent que le transfert initial d’électron implique la portion trisubstituée du monoalcène; le
potentiel d’oxydation de la portion diénique monosubstituée en position nodale est supérieur. Des calculs ab initio
d’orbitales moléculaires à un niveau élevé (MP2͞6-31G*͞͞HF͞6-31G*) fournissent des informations utiles relatives à
la nature (énergies relatives et distributions de charge et de spin) des cations radicaux intermédiaires qui supportent le
mécanisme réactionnel proposé.
Mots clés : transfert d’électron photoinduit, radicaux, cations radicaux, β-myrcène, cyclisation.
[Traduit par la Rédaction] Arnold and McManus 1248
The definition of the scope and limitations of the photo-
chemical nucleophile–olefin combination, aromatic substitu-
Received April 27, 1998.
D.R. Arnold2 and K.A. McManus. Department of
Chemistry, Dalhousie University, Halifax, NS B3H 4J3,
Canada.
tion (photo-NOCAS) reaction has received considerable
attention (1–3). This reaction has synthetic utility; yields of
the nucleophile, olefin, aromatic (1:1:1) adducts can be high.
The reaction is general for mono alkenes, and dienes, both
conjugated and nonconjugated, and the reaction can incorpo-
rate a variety of nucleophiles; e.g., methanol, fluoride anion,
cyanide anion, acetonitrile. Our understanding of the mecha-
nism of this reaction has progressed to the point where the
1See refs. 1 and 2 for preceding parts of this series.
2Author to whom correspondence may be addressed.
Telephone: (902) 494-3714. Fax: (902) 494-1310. E-mail:
Donald.Arnold@dal.ca
Can. J. Chem. 76: 1238–1248 (1998)
© 1998 NRC Canada

Arnold and McManus
1239
results from a new reaction can now provide useful funda-
mental information regarding the reactivity of the intermedi-
ates, i.e., radical cation, radical anion, and radical. A main
objective of this project was to determine the probability and
regiochemistry of intramolecular cyclization of the interme-
diate(s) involved during the reaction of a nonconjugated
alkene, 2-methyl-2-butene (3), yields both regioisomeric
1:1:1 adducts (4, 5) in 70% yield (reaction [1]) (2a). The
mechanism proposed for formation of the major (6:1) iso-
mer, 3-(4-cyanophenyl)-2-methoxy-3-methylbutane (4), in-
volves rapid equilibration of the initially formed distonic
radical cation intermediates, followed by irreversible
deprotonation, with preferential formation of the more stable
β-alkoxyalkyl radical (2a, b).
alkene
– conjugated diene, 7-methyl-3-methylene-1,6-
octadiene (β-myrcene, 1).
Relevant examples of this reaction are shown in reactions
[1]–[4] (2). The irradiation of an acetonitrile–methanol solu-
tion of 1,4-dicyanobenzene (2) and the unsymmetric mono
Similar irradiation of an acetonitrile–methanol solution of
1,4-dicyanobenzene and 2-methyl-1,3-butadiene (6) gave
four isomeric 1:1:1 adducts (reaction [2]) (2c). The major
[1]
[2]
[3]
[4]
© 1998 NRC Canada

1240
Can. J. Chem. Vol. 76, 1998
Table 1. The dependence of the product ratio
[(18 + 19)͞(16 + 17)] from reaction [5] on
the concentration of methanol.
Fig. 1. Variation in the product ratio [(18 + 19)͞(16 + 17)] from
reaction [5] as a function of the concentration of methanol.
[CH3OH] (M)
Product ratio
0.25
0.50
1.00
1.50
2.00
0.64 (0.03)a
0.64 (0.04)
0.89 (0.02)
1.01 (0.03)
1.02 (0.01)
aPopulation standard deviation.
(8:1) adducts (7, 8, 9) result from combination of methanol
with the diene radical cation at C-1, which again leads ulti-
mately to the more stable allylic radical (2b).
The irradiation of an acetonitrile–methanol solution of
1,4-dicyanobenzene and 2,5-dimethyl-1,5-hexadiene (11)
provides an example of cyclization of the intermediate radi-
cal cation (reaction [3]) (2d). The ratio of 1:1:1 adducts is
dependent upon the methanol concentration; lowering the
concentration of methanol favours radical cation cyclization,
increasing the yield of 13. Cyclization, 1,6-endo, endo, of
the initially formed alkene radical cation yields the more
heavily substituted, more stable, 1,4-distonic radical cation
intermediate, leading to the formation of 13.
(1) was present in excess) and were calculated from the total
weight of the combined products and the ratio of isomers de-
termined by integrated peak (GC–fid) area. The ratio of ad-
ducts was not dependent upon the extent of conversion;
these ratios remained constant during the irradiation.
A particularly interesting example of the photo-NOCAS
reaction involves irradiation of an acetonitrile–methanol so-
lution of 1,4-dicyanobenzene and 6,6-dimethyl-2-
methylenebicyclo[3.1.1]heptane (β-pinene) (14) (reaction
[4]) (2e). In this case, the initially formed alkene radical cat-
ion cleaves an allylic cyclobutyl bond to give the distonic
radical cation. The structures of the three 1:1:1 adducts (15,
16, 17) are consistent with reaction of this intermediate; no
other 1:1:1 adducts were obtained. The adducts maintain op-
tical activity, hence, the distonic radical cation does not
cleave; cleavage of this distonic radical cation would yield
the achiral radical cation of β-myrcene (1+·).
Three of the adducts, cis-2-(4-cyanophenyl)-4-(1-meth-
oxy-1-methylethyl)-1-methylenecyclohexane (15), trans-2-
(4-cyanophenyl)-4-(1-methoxy-1-methylethyl)-1-methylene-
cyclohexane (16), and 1-(4-cyanophenylmethyl)-4-(1-meth-
oxy-1-methylethyl)cyclohexene (17), were racemic mixtures
of the enantiomeric adducts previously characterized from
the reaction involving β-pinene (14) (reaction [4]) (2e). The
major adducts were the new compounds: 4-[4-methoxy-3,3-
dimethylcyclohex-(E)-1-ylidenyl]methylbenzonitrile
and 4-(1-vinyl-4-trans-methoxy-3,3-dimethylcyclohexyl)-
benzonitrile (19).
(18)
The ratio of products [(18 + 19)͞(16 + 17)] was depend-
ent upon the initial concentration of methanol. (The yield of
15 was too low for an accurate measurement.) Irradiations
were carried out in Pyrex tubes (5 mm) under conditions
similar to those used for the preparative scale reaction de-
scribed above. The concentration of methanol was varied
(0.25–2.0 M). The concentrations of the other reactants were
the same as in the preparative scale reaction. The ratio [(18 +
19)͞(16 + 17)] of products was determined from the corre-
sponding integrated peak (GC–fid) areas (Table 1 and
Fig. 1). An increase in methanol concentration caused an in-
crease in the yield of 18 and 19 relative to 16 and 17. At the
lower range of methanol concentration (≤ 0.5 M) the ratio of
An acetonitrile–methanol (3:1) solution of 1,4-
dicyanobenzene (2), biphenyl, serving as a codonor, and β-
myrcene (1) was irradiated with a medium-pressure mercury
vapour lamp through Pyrex. Progress of the reaction was
monitored by capillary column gas chromatography with a
flame ionization detector (GC–fid). The reaction produced
five 1:1:1 (methanol, 1, 2) photo-NOCAS adducts (reaction
[5]). The yields were based upon complete conversion of
1,4-dicyanobenzene as the limiting reagent (β-myrcene
[5]
© 1998 NRC Canada

Arnold and McManus
1241
Fig. 2. The X-ray crystal structure of 4-[4-methoxy-3,3-dimethylcyclohex-(E)-1-ylidenyl]methylbenzoic acid (18a).3
products remained constant. The ratios (18͞19) and (16͞17)
The assigned overall structure and stereochemistry of 18
was substantiated by X-ray crystallography. The nitrile
group in compound 18 was converted to a carboxylic acid;
4-[4-methoxy-3,3-dimethylcyclohex-(E)-1-ylidenyl]methyl-
were independent of methanol concentration.
1
Structural assignments
benzoic acid (18a). The H nmr spectrum of the nitrile (18)
Products 15, 16, and 17 were shown to have identical gas
chromatographic retention times and mass spectra as the
previously characterized adducts (2e).
and this carboxylic acid (18a) are almost identical; the X-ray
analysis of 18a (Fig. 2) provides firm evidence for the struc-
ture of the initial product (18).3
Compound 18 was identified as 4-[4-methoxy-3,3-
dimethylcyclohex-(E)-1-ylidenyl]-methylbenzonitrile. The
typical AA′XX′ pattern in the aromatic region of the H nmr
Compound 19 was identified as 4-(1-vinyl-4-trans-
methoxy-3,3-dimethylcyclohexyl)benzonitrile. The typical
AA′XX′ pattern in the aromatic region of the H nmr spec-
1
1
spectrum is indicative of the 4-cyanophenyl group. The pres-
ence of the trisubstituted alkene is supported by a triplet
(5.20 ppm), representing a vinyl proton with typical cou-
pling (7.3 Hz) to a geminal methylene, and a doublet
(3.43 ppm), representing this geminal methylene, aryl-
substituted. The doublet of doublets at 2.93 ppm represents
the axial proton of the methoxy-substituted methine (3J4ax–5ax
= 8.5 Hz, J4ax–5eq = 3.7 Hz). The H nmr also exhibits two
singlets at 0.84 and 0.94 ppm indicative of two nonequiva-
lent methyl groups.
trum is indicative of the 4-cyanophenyl group. The presence
of the terminal alkene is supported by two doublets and a
doublet of doublets (4.87, 5.03, and 5.86 ppm) in the vinyl
proton region. The distorted four-line multiplet at 2.81 ppm
represents the axial proton of the methoxy-substituted
methine. Individual coupling constants were not obtained;
however, based on the width of the multiplet at half-height
(13 Hz), this proton is axial. Typical equatorial–axial and
equatorial–equatorial couplings range from 0 to 3 Hz; if this
proton were equatorial, the separation of the outer lines of a
3
1
1
The 13C nmr spectrum of 18 exhibits
a
doublet
(85.91 ppm), which indicates a methine group substituted by
the methoxy group. There is lower field triplet
four-line multiplet would be, at most, 6 Hz. The H nmr
spectrum also exhibits two singlets at 0.72 and 0.96 ppm in-
dicative of two nonequivalent methyl groups.
a
(47.93 ppm), which is consistent with an aryl-substituted
methylene group (carbons adjacent to the 4-cyanophenyl
group tend to occur between 37–55 ppm (1, 2)). The
13C nmr spectrum also exhibits a doublet (120.12 ppm) for
the vinyl methine and a singlet (138.77 ppm) for the quater-
nary vinyl carbon, further supporting the existence of the
trisubstituted alkene.
A nuclear Overhauser enhancement (NOE) experiment
proved the aryl group was trans to the methoxy group on the
cyclohexyl ring. When the methyl protons at 0.72 ppm were
irradiated, a NOE was observed for the proton of the
methoxy-substituted methine, the methoxy–methyl protons,
and the methyl protons (0.96 ppm). When the methyl pro-
tons at 0.96 ppm were irradiated, a NOE was observed for
3 X-ray crystallographic data for 18a may be purchased from: The Depository of Unpublished Data, Document Delivery, CISTI, National Re-
search Council of Canada, Ottawa, Canada, K1A 0S2. Details of data collection, structural analysis and refinement, and tables of inter-
atomic distances and bond angles have also been deposited with the Cambridge Crystallographic Data Centre and can be obtained on
request from: The Director, Cambridge Crystallographic Data Centre, University Chemical Laboratory, 12 Union Road, Cambridge,
CB2 1EZ, U.K.
© 1998 NRC Canada

1242
Can. J. Chem. Vol. 76, 1998
the methoxy–methyl protons, the vinyl protons, and the
methyl protons (0.72 ppm). Based on the evidence that the
proton of the methoxy-substituted methine is axial, vide su-
pra, the methoxy group is equatorial, the aryl group is equa-
torial, i.e., the trans configuration.
exothermic, –21.6 kcal mol–1 for 1+· to 1a+· and –11.3 kcal
mol–1 for 1+· to 1b+·, consistent with the lack of reversibility.
At higher methanol concentrations the product ratio [(18 +
19)͞(16 + 17)] increases; for the preparative runs (6 M
methanol) this ratio becomes 1.7. Under these conditions, re-
action of the radical cation (1+·) with methanol competes
with cyclization; the measured bimolecular rate constant for
reaction of diene radical cations with methanol (>109 M–1 s–1)
is consistent (3a). Now the product ratio is controlled by the
addition of methanol to the radical cation 1+· at the alterna-
tive sites, C7 or C6, to yield the β-alkoxyalkyl radicals, 1c·
or 1d· (Scheme 2). As expected, based upon the product ra-
tio from reaction (1), addition of methanol to the alkene rad-
ical cation at C6, with subsequent formation of the more
stable tertiary radical (1d·), is favoured, relative to addition
at C7, to give the secondary radical (1c·). Both of these β-
alkoxyalkyl radicals then cyclize 1,6-endo on the way to the
observed products.
Adduct 18 results from cyclization of the radical cation
(Scheme 1) or radical (Scheme 2) at C10 of the diene moi-
ety. The E-configuration of this adduct (18) clearly indicates
this diene had the s-trans conformation. The result of an ex-
tensive stochastic search for conformers of β-myrcene (1),
using the molecular mechanics program (MM3-94 (6)), is
consistent with reaction of this conformation (Table 4). Con-
former 3 has close approach between C7 and C10, required
for cyclization to 1b+·, and the s-trans conformation of the
diene. Conformers 6 and 8 have close approach between C1
and C6 required for cyclization to 1a+·. None of the low-
energy conformers have the s-cis conformation and close ap-
proach of C7-C10 or C1-C6. The fully optimized (ab initio)
structures of β-myrcene (1) and of the β-myrcene radical cat-
ion (1+·) are similar (Table 4).
Clearly, none of the products from reaction 5 involve ini-
tial addition of the nucleophile, methanol, to the conjugated
diene moiety. This was, at first, a surprise; however, there is
convincing evidence that formation of the trialkyl-
substituted mono-ene radical cation (C6, C7) is energetically
favoured (1b). The oxidation potentials and the ionization
potentials of β-myrcene (1) and of the model compounds 2-
methyl-2-butene (3) and 2-methyl-1,3-butadiene (6) are
summarized in Table 5. The radical cation of the trialkyl-
substituted mono-ene moiety (C5, C6, C7, C8, C9) is more
stable than that at the conjugated diene moiety (C1, C2, C3,
C10, C4). The molecular orbital calculations support this
conclusion; the charge density of radical cation, 1+·, is
largely on C6, C7, C8, and C9, while the spin density is on
C6 and C7 (Table 3).
The 13C nmr spectrum of compound 19 exhibits a doublet
(85.58 ppm) indicative of a methine group substituted by the
methoxy group. There is a lower field singlet (44.31 ppm),
which is consistent with an aryl-substituted quaternary car-
bon. This spectrum also exhibits a triplet (112.52 ppm) for
the vinyl methylene and a doublet (146.73 ppm) for the vi-
nyl methine, thus further supporting the existence of the ter-
minal mono-substituted alkene.
Calculations
Ab initio molecular orbital calculations were carried out,
using the GAUSSIAN 94 package of programs (4), to obtain
fully optimized structures (HF͞6-31G*) of β-myrcene
(1) and the radical cations 1+·, 1a+·, and 1b+·. The results of
these calculations, optimized structures, and spin and charge
density distribution obtained from Mulliken population anal-
ysis, are listed in Tables 2 and 3. The total energies, cor-
rected for spin contamination, were obtained from single
point calculations with Møller–Plesset perturbation theory
(MP2) using the HF͞6-31G* optimized structures (5). The
results of an extensive stochastic search, using the molecular
mechanics program (MM3-94 (6)), identified 24 conformers
of β-myrcene (1); the more stable conformers are described
in Table 4.
A mechanism for the formation of 15, 16, and 17 involves
cyclization of the initially formed radical cation of myrcene
(1+·) to the distonic radical cation (1a+·) (Scheme 1). This
distonic radical cation is identical to that formed upon cleav-
age of the β-pinene radical cation (14+·). An alternative
mode of cyclization of 1+· yields the distonic radical cation
1b+·, which leads to formation of 18 and 19. The
regiochemistry of these adducts requires reaction of these
distonic radical cations — tertiary or secondary alkyl cation
with methanol and the allylic radical with the radical anion
of 1,4-dicyanobenzene (2–·). At lower methanol concentra-
tions (≤0.5 M) cyclization of the radical cation dominates,
the product ratio [(18 + 19)͞(16 + 17)] is relatively small
(0.64), 1,6-endo, exo cyclization of 1+· to 1a+· is favoured.
The results from the molecular orbital calculations are
consistent with these observations (Table 3). The distonic
radical cation 1a+· has most of the positive charge density on
the tertiary carbocation (C7, C8, and C9), while the spin
density is delocalized over the allylic radical (C2 and C10).
Similarly, the distonic radical cation 1b+· has greatest posi-
tive charge on C6, and the spin density is delocalized over
the allylic radical (C1 and C3). It seems likely that the alter-
native modes of cyclization of the initially formed β-
myrcene radical cation, 1+·, are dependent upon
conformational factors. The rate of the intermolecular reac-
tion of a diene radical cation with the precursor diene is
rapid, 108–109 M–1 s–1 (3a). Nevertheless, the molecular or-
bital calculations indicate that the major mode of cyclization
of 1+· yields the more stable distonic radical cation 1a+·. The
calculations also indicate that cyclization is significantly
Photoinduced electron transfer, from 7-methyl-3-
methylene-1,6-octadiene
(β-myrcene,
1)
to
1,4-
dicyanobenzene, in acetonitrile–methanol, leads to formation
of five 1:1:1 (photo-NOCAS) adducts (15–19). All of these
adducts are cyclic; both the initially formed radical cation
and the intermediate β-alkoxyalkyl radicals cyclize. The pre-
ferred modes of cyclization are for the radical cation 1+·,
1,6-endo, exo and 1,6-endo, endo, and for the radicals, 1c·
and 1d·, 1,6-endo. The observed products, the measured
oxidation and ionization potentials, and the molecular orbital
© 1998 NRC Canada

Arnold and McManus
1243
Table 2. Optimized (HF͞6-31G*) structures for ␤-myrcene (1), ␤-myrcene radical cation (1+C), ␤-
myrcene cyclized, distonic, radical cations (1a+C and 1b+C).
Bond length (Å)
Bond angle (deg)
β-Myrcene (1)
Dihedral angle (deg)
C1—C2
C2—C3
C3—C10
C3—C4
C4—C5
C5—C6
C6—C7
C7—C8
C7—C9
1.323
1.480
1.326
1.515
1.534
1.507
1.325
1.511
1.510
C1-C2-C3
C2-C3-C4
C2-C3-C10
C3-C4-C5
C4-C5-C6
C5-C6-C7
C6-C7-C8
C6-C7-C9
126.8
117.5
118.6
116.4
111.4
128.9
120.8
125.4
C4-C3-C2-C1
C5-C4-C3-C2
C6-C5-C4-C3
C7-C6-C5-C4
C8-C7-C6-C5
C9-C7-C6-C5
C10-C3-C2-C1
–0.1
–180.0
177.1
114.0
–180.0
0.1
179.8
β-Myrcene radical cation (1+·)
C1—C2
C2—C3
C3—C10
C3—C4
C4—C5
C5—C6
C6—C7
C7—C8
C7—C9
1.322
1.479
1.325
1.515
1.547
1.485
1.414
1.488
1.492
C1-C2-C3
126.8
116.5
119.7
114.4
110.0
123.8
127.0
120.1
121.5
118.4
C4-C3-C2-C1
–0.7
–178.1
2.1
–177.6
–115.3
–177.6
1.8
C2-C3-C4
C2-C3-C10
C3-C4-C5
C4-C5-C6
C4-C3-C10
C5-C6-C7
C6-C7-C8
C6-C7-C9
C8-C7-C9
C5-C4-C3-C2
C5-C4-C3-C10
C6-C5-C4-C3
C7-C6-C5-C4
C8-C7-C6-C5
C9-C7-C6-C5
C10-C3-C2-C1
179.1
β-Myrcene cyclized, distonic, radical cation (1a+·)
C1—C2
C1—C6
C2—C3
C3—C4
C3—C10
C4—C5
C5—C6
C6—C7
C7—C8
C7—C9
1.507
1.577
1.389
1.519
1.398
1.529
1.534
1.469
1.478
1.477
C1-C2-C3
C1-C6-C5
C1-C6-C7
C2-C1-C6
C2-C3-C4
C2-C3-C10
C3-C4-C5
C4-C5-C6
C4-C3-C10
C5-C6-C7
C6-C7-C8
C6-C7-C9
C8-C7-C9
125.3
109.8
103.7
110.0
120.1
120.9
112.7
109.1
118.9
117.4
121.6
119.9
118.5
C1-C6-C5-C4
C2-C1-C6-C5
C2-C1-C6-C7
C2-C3-C4-C5
C3-C2-C1-C6
C3-C4-C5-C6
C4-C3-C2-C1
C7-C6-C5-C4
C8-C7-C6-C1
C8-C7-C6-C5
C9-C7-C6-C1
C9-C7-C6-C5
C10-C3-C2-C1
C10-C3-C4-C5
66.0
–46.9
–173.2
19.8
15.9
–51.2
–2.2
–175.9
94.7
–26.6
–85.5
153.2
179.0
–161.4
β-Myrcene cyclized, distonic, radical cation (1b+·)
C1—C2
C2—C3
C3—C4
C3—C10
C4—C5
C6—C7
C7—C8
C7—C9
C7—C10
1.386
1.400
1.513
1.515
1.533
1.447
1.594
1.536
1.540
C1-C2-C3
C2-C3-C10
C3-C4-C5
C3-C10-C7
C4-C3-C10
C6-C7-C10
C8-C7-C10
C9-C7-C10
126.7
121.5
113.1
115.5
119.4
113.2
111.2
112.4
C1-C2-C3-C10
C2-C3-C10-C7
C3-C10-C7-C6
C4-C3-C10-C7
C5-C4-C3-C10
C8-C7-C10-C6
C9-C7-C10-C6
–1.2
163.6
39.0
–17.7
–22.0
–108.1
129.8
calculations all indicate the trisubstituted mono alkene moi-
ety is the site of the initially formed radical cation 1+·, not
the conjugated diene. Cyclization of this radical cation is
significantly exothermic and is irreversible. The observed
products and the molecular orbital calculations indicate that
on the cyclized, distonic radical cation intermediates, 1a+·
and 1b+·, the positive charge density is largely on the tertiary
and secondary alkyl moiety, and the spin density is
delocalized on the allylic radical. The addition of methanol
to the initially formed radical cation follows the established
pattern for alcohol addition to mono alkenes; addition occurs
preferentially to ultimately yield the more heavily substi-
tuted, more stable, β-alkoxyalkyl radical. Cyclization of
these radicals, 1c· and 1d·, occur 1,6-endo.
© 1998 NRC Canada

1244
Can. J. Chem. Vol. 76, 1998
Table 3. Charge and spin density distribution on the radical cations 1+C, 1a+C, and 1b+C.a
Charge
Spin
Atom
density
density
β-Myrcene radical cation, 1+· (E = –388.919 12)
1
2
3
4
5
6
7
8
–0.026
0.065
0.102
0.043
0.058
0.253
0.187
0.183
0.187
–0.051
–0.000
0.002
0.005
0.030
–0.068
0.610
0.451
–0.015
–0.014
–0.000
9
10
β-Myrcene cyclized, distonic, radical cation, 1a+· (E = –388.953 51)
1
2
3
4
5
6
7
8
0.060
0.029
0.073
0.066
0.059
–0.050
0.323
0.211
0.211
0.017
–0.068
0.914
–0.788
0.049
–0.008
0.010
–0.001
0.000
0.000
0.892
9
10
β-Myrcene cyclized, distonic, radical cation, 1b+· (E = –388.937 18)
1
2
3
4
5
6
7
8
0.023
0.040
0.016
0.092
0.110
0.476
–0.198
0.180
0.151
0.110
0.854
–0.787
1.027
–0.072
0.012
0.001
0.011
–0.002
–0.000
–0.073
9
10
aSpin and charge densities include hydrogens summed into heavy atoms.
3392A integrator was interfaced with the GC–fid to obtain
peak areas. An HP 5890 gas chromatograph with a 5%
phenyl methyl silicone fused silica WCOT column (25 m ×
0.20 mm, 0.33 µm film thickness) interfaced with an HP
5970 mass selective detector (GC–MS) was also used for
product analyses. Mass spectra are reported as m͞z (relative
intensity). Separation of product mixtures was generally car-
ried out using preparative medium-pressure liquid chroma-
tography (mplc) (1a). The mplc consists of a 2.5 cm × 1 m
column packed with thin-layer chromatography (TLC) grade
silica gel (without binder) (Rose Scientific Ltd., cat. 81632)
at a pressure of 19 psi using helium (1 psi = 6.9 kPa). Con-
nected to the mplc was a uv spectrophotometer – fraction
collector collecting ca. 10 mL fractions.
General information
1
The H and 13C nmr spectra were obtained from a Bruker
AC 250 F spectrometer. Spectra were recorded in parts per
million and frequencies are relative to tetramethylsilane. In-
frared spectra (ir) were recorded on a Nicolet 205 FTIR and
are reported in wave numbers (cm–1). Elemental analyses
were performed by Canadian Microanalytical Service Ltd.,
B.C. Exact mass determinations were obtained using a CEC
21-110 mass spectrometer at Dalhousie University, Halifax,
NS. Melting points were determined using a Cybron Corpo-
ration Thermolyne apparatus with a digital thermocouple
and are corrected. Product yields and progress of the reac-
tions were determined using a Hewlett–Packard (HP) 5890
gas chromatograph with a DB-1701 fused silica WCOT col-
umn (30 m × 0.25 mm, 0.25 µm film thickness) and a cali-
brated flame ionization detector (GC–fid) and are based
upon 1,4-dicyanobenzene (2) as the limiting reagent. An HP
Materials
Acetonitrile (Fisher ACS grade) was distilled twice, first
from sodium hydride and then from phosphorus pentoxide.
It was then passed through a column of basic alumina,
© 1998 NRC Canada

Arnold and McManus
Table 4. ␤-Myrcene (1) conformers.
1245
Energy diff.
(kJ͞mol)
C1—C6
(Å)
C7—C10
(Å)
C1—C7
(Å)
C6—C10
(Å)
C1-C2-C3-C10
(°)
Conformer
Stochastic search, molecular mechanics (MM3, 94 (6))
1
2
3
4
5
6
7
8
9
0.000
0.371
3.687
3.932
4.014
4.417
4.432
4.990
6.716
4.74
4.73
4.66
6.00
6.01
3.41
4.62
3.44
4.49
5.81
5.34
3.63
5.77
5.31
5.25
4.19
4.31
4.34
5.10
6.01
4.96
6.65
7.12
3.83
5.91
3.72
4.36
4.66
4.67
3.38
4.65
4.64
4.01
3.37
4.06
4.08
177.44
176.85
177.45
2.54
2.73
176.30
178.30
177.47
3.34
Ab initio molecular orbital theory (UHF͞6-31G*) (4)
5.34 5.43
6.28
4.44
179.8
179.1
β-Myrcene radical cation (1+·), ab initio molecular orbital theory (UHF͞6-31G*) (4)
5.30
5.42
6.30
4.38
(1 Torr = 133.3 Pa). Myrcene (1) (technical grade) was
obtained from the Aldrich Chemical Co. and purified by
vacuum distillation. Biphenyl (Eastman Kodak Co.) was
recrystallized from methanol.
Scheme 1. Cyclization of the initially formed β-myrcene radical
cation, to give the distonic radical cation, followed by reaction
of the distonic radical cation with nucleophile (methanol) and
with the aromatic radical anion.
Irradiation
The irradiation was carried out on an acetonitrile–metha-
nol (3:1) solution consisting of myrcene (1), 1,4-
dicyanobenzene (2), and biphenyl. Solutions were irradiated
in either 2 cm i.d. Pyrex tubes or 5 mm Pyrex nmr tubes,
which were degassed by nitrogen ebullition. These samples
were irradiated at 10°C using a CGE 1 MW medium-
pressure mercury vapour lamp contained in a water-cooled
quartz immersion well.
Cyclic voltammetric measurements
Cyclic voltammetry was used to obtain the oxidation po-
tential of myrcene (1). The apparatus has been described (8).
The working electrode was a platinum sphere (1 mm diame-
ter), and the counter electrode was a platinum wire. The ref-
erence electrode was a saturated calomel electrode (sce),
which was connected to the solution (TEAP 0.1 M,
acetonitrile) through a Luggin capillary. The alkene concen-
tration was ca. 0.005 M. Since the anodic wave was irrevers-
ible, the half-wave potential was taken as 0.028 V before the
anodic peak potential (9).
Formation of the methanol, β-myrcene (1), 1,4-
dicyanobenzene (2) photo-NOCAS adducts 15, 16, 17,
18, and 19: reaction [5]
A solution of β-myrcene (1) (10.0 mL, 8.0 g, 0.059 mol),
1,4-dicyanobenzene (2) (3.7 g, 0.029 mol), and biphenyl
(2.3 g, 0.015 mol) in acetonitrile–methanol (3:1) (240 mL)
was degassed by nitrogen ebullition and irradiated for 9 days
using a 1 kW lamp at 10°C. The solvent was removed to
yield a crude photolysate. The crude photolysate was
chromatographed (mplc) using a linear solvent gradient
(hexanes – 10% diethyl ether, 90% hexanes). Isolation of the
adducts was achieved via further chromatography using
mplc and a linear solvent gradient (hexanes – 1% diethyl
ether, 99% hexanes). The five 1:1:1 adducts were eluted in
refluxed over calcium hydride for 24 h (under a nitrogen at-
mosphere), fractionally distilled (under nitrogen), and stored
over molecular sieves (3 Å) (7). Methanol was distilled and
then stored over molecular sieves (4 Å). 1,4-
Dicyanobenzene (2) (Aldrich) was purified by treatment
with Norite in methylene chloride, followed by recrystal-
lization from 95% ethanol. Tetraethylammonium perchlorate
(TEAP) (Aldrich) was recrystallized three times from water
and then dried in a vacuum oven for 15 h, 70°C, 0.25 Torr
© 1998 NRC Canada

1246
Can. J. Chem. Vol. 76, 1998
Scheme 2. Reaction of the initially formed β-myrcene radical cation with nucleophile (methanol), to give the radical, followed by
cyclization of the radical and reaction with the aromatic radical anion.
substituted carbon (4-H)), 3.36 (s, 3H, OCH3), 4.87 (d, 1H,
Table 5. Calculated and experimental oxidation potentials and
ionization potentials of ␤-myrcene (1) and model systems.
3J1′–2′trans = 17.7 Hz, vinyl H (2′trans-H)), 5.03 (d, 1H, 3J1′–2′cis
3
= 10.7 Hz, vinyl H (2′cis-H)), 5.86 (dd, 1H, J1′–2′trans
=
IP v (calcd.)a
8.68d (8.93)
IP a (calcd.)b
(8.27)
EOX (V)c
3
1/2
17.7 Hz, J1′–2′cis = 10.7 Hz, vinyl H (1′-H)), 7.45 (d, 2H,
2-Methyl-2-
butene (3)
2-Methyl-1,3-
butadiene (6)
β-Myrcene (1)
2.03e
2.25f
2.12
3J2′–3′ = 8.5 Hz, J5′–6′ = 8.5 Hz, H’s adjacent to alkyl-
3
3
substituted aryl carbon (2′-H, 6′-H)), 7.58 (d, 2H, J2′–3′
=
8.89d (8.67)
(8.45)
3
8.5 Hz, J5′–6′ = 8.5 Hz, H’s adjacent to cyano-substituted
aryl carbon (3′-H, 5′-H)). The NOE experiment: the methyl
protons at 0.72 and 0.96 ppm were irradiated. When the
methyl protons at 0.72 ppm were irradiated, an NOE was
observed for the proton of the methoxy-substituted methine,
the methoxy–methyl protons, and the methyl protons
(0.96 ppm). When the methyl protons at 0.96 ppm were irra-
diated, an NOE was observed for the methoxy–methyl pro-
tons, the vinyl protons, and the methyl protons (0.72 ppm).
13C nmr (62.90 MHz, CDCl3), δ: 22.12 (t), 24.03 (q), 29.44
(q), 30.17 (t), 36.17 (s), 44.31 (s), 47.42 (t), 57.37 (q,
OCH3), 85.58 (d, CH, methoxy-substituted), 109.50 (s, qua-
ternary aryl carbon, cyano-substituted), 112.52 (t, alkene
CH2), 119.06 (s, CN), 127.27 (d, aromatic CH adjacent to
alkyl-substituted aryl carbon), 131.98 (d, aromatic CH adja-
cent to cyano-substituted carbon), 146.73 (d, alkene CH),
153.38 (s, quaternary aryl carbon, alkyl-substituted); MS,
m/z: 55(14), 58(22), 71(100), 115(15), 116(19), 121(17),
127(16), 140(18), 154(22), 155(13), 166(20), 181(13),
194(52), 195(13), 237(17). Exact mass calcd. for C18H23NO:
269.1780; found: 269.1773.
8.68g (8.65)
8.92h
8.36 (8.24)
a
Experimental vertical ionization potential; calculated vertical ionization
potential in parentheses, taken from the eigenvalue of the HOMO.
b
Experimental adiabatic ionization potential; calculated adiabatic
ionization potential in parentheses, taken as the difference between the
calculated total energy of the minimized neutral molecule and the
minimized radical cation.
c
Measured (CV) oxidation potential (vs. sce).
Reference 10.
Reference 2a.
Reference 2c.
HOMO.
The second band.
d
e
f
g
h
the order 19, 18, 16, 17, and 15. Adducts 15–17 were found
to be identical, mass spectra and GC retention times, with
photo-NOCAS adducts previously characterized from β-
pinene (14) (2e), cis-2-(4-cyanophenyl)-4-(1-methoxy-1-
methylethyl)-1-methylenecyclohexane (15, 1%), trans-2-(4-
cyanophenyl)-4-(1-methoxy-1-methylethyl)-1-methylenecy-
clohexane (16, 3%), 1-(4-cyanophenylmethyl)-4-(1-meth-
oxy-1-methylethyl)cyclohexene (17, 4%).
4-[4-Methoxy-3,3-dimethylcyclohex-(E)-1-ylidenyl]-
methylbenzonitrile (18)
The yield of 18 was 7%; infrared (Nicolet 205) ν: 2969
(s), 2950 (s), 2870 (m), 2822 (m), 2228 (s), 1607 (m), 1504
(m), 1452 (m), 1184 (m), 1101 (s), 823 (m); 1H nmr
(250.13 MHz, CDCl3), δTMS: 0.84 (s, 3H, CH3), 0.94 (s, 3H,
4-(1-Vinyl-4-trans-methoxy-3,3-dimethylcyclohexyl]-
benzonitrile (19)
The yield of 19 was 5%; infrared (Nicolet 205) ν: 3082
(w), 2930 (s), 2875 (m), 2822 (m), 2228 (s), 1632 (w), 1605
(m), 1504 (m), 1461 (m), 1408 (w), 1388 (w), 1363 (m),
1186 (m), 1101 (s), 1040 (w), 1019 (w), 997 (w), 915 (m),
3
3
CH3), 1.38–1.55 (m, 1H, J4ax–5ax = 8.5 Hz, J5ax–6eq
=
5.5 Hz, axial H of methylene group (5ax-H)), 1.77–2.08 (m,
1
2
2
3
835 (m); H nmr (250.13 MHz, CDCl3), δTMS: 0.72 (s, 3H,
4H, J6ax–6eq = 13.7 Hz, J2ax–2eq = 13.4 Hz, J5eq–6eq =
3
CH3), 0.96 (s, 3H, CH3), 1.71–1.97 (m, 5H, cyclohexyl H’s),
2.20 (m, 1H, cyclohexyl H), 2.81 (m, 1H, H of methoxy-
5.5 Hz, J4ax–5eq = 3.7 Hz, H of methylene groups on ring
(2ax-, 2eq-, 5eq-, 6ax-H)), 2.46 (dt, 1H, J6ax–6eq = 13.7 Hz,
2
© 1998 NRC Canada

Arnold and McManus
1247
3
3J5ax–6eq = 5.5 Hz, J5eq–6eq = 5.5 Hz, equatorial H of methy-
77(35), 79(27), 91(36), 93(36), 107(31), 121(78), 128(37),
129(50), 135(30), 141(41), 211(30), 213(50), 256(100). X-
ray data: see footnote 3.
lene group on ring (6eq-H)), 2.93 (dd, 1H, 3J4ax–5ax = 8.5 Hz,
3J4ax–5eq = 3.7 Hz, CH group, methoxy-substituted (4ax-H)),
3
3.37 (s, 3H, OCH3), 3.43 (d, 2H, J7–38 = 7.3 Hz, CH2 group,
aryl-substituted (8-H)), 5.20 (t, 1H, J7–8 = 7.3 Hz, vinyl H
3
3
(7-H)), 7.27 (d, 2H, J2′–3′ = 8.5 Hz, J5′–6′ = 8.5 Hz, H’s ad-
jacent to alkyl-substituted aryl carbon (2′-H, 6′-H)), 7.56 (d,
This work was supported by grants from the Natural Sci-
ences and Engineering Research Council of Canada. We
thank Dr. D.L. Hooper, Atlantic Region Magnetic Reso-
nance Centre at Dalhousie University, for help in the prepa-
ration of the manuscript, Dr. T.S. Cameron, Dalhousie
University, for the X-ray crystal structure of 18a, and Dr.
T.B. Grindley and Dr. M.S.W. Chan, Dalhousie University,
for assistance with the molecular mechanics calculations.
We also wish to thank Dr. D. Klapstein, St. Francis Xavier
University, for the photoelectron spectra of 1.
3
3
2H, J2′–3′ = 8.5 Hz, J5′–6′ = 8.5 Hz, H’s adjacent to cyano-
substituted aryl carbon (3′-H, 5′-H)); 13C nmr (62.90 MHz,
CDCl3), δ: 21.61 (q), 25.00 (t), 25.78 (t), 27.52 (q), 33.87
(t), 37.17 (s), 47.93 (t, CH2, aryl-substituted), 57.52 (q,
OCH3), 85.91 (d, CH, methoxy-substituted), 109.59 (s, qua-
ternary aryl carbon, cyano-substituted), 119.18 (s, CN),
120.12 (d, vinyl CH), 129.08 (d, aromatic CH adjacent to
alkyl-substituted aryl carbon), 132.22 (d, aromatic CH adja-
cent to cyano-substituted carbon), 138.77 (s), 147.53 (s);
MS, m/z: 55(27), 67(26), 71(25), 77(22), 79(23), 93(34),
116(55), 121(83), 154(44), 167(23), 180(22), 194(64),
195(24), 237(100). Anal. calcd. for C18H23NO: C 80.26, H
8.61, N 5.20; found: C 79.90, H 8.78, N 4.91.
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Preparation of 4-[4-methoxy-3,3-dimethylcyclohex-(E)-1-
ylidenyl]methylbenzoic acid (18a)
4-[4-Methoxy-3,3-dimethylcyclohex-(E)-1-ylidenyl]meth-
ylbenzonitrile (18) (0.041 g, 0.15 mmol) and 4 mL of a 40%
sodium hydroxide solution was refluxed for 2 days. This
mixture was cooled and extracted with dichloromethane.
The aqueous layer was acidified with hydrochloric acid
(conc.) and extracted twice with benzene. Evaporation of the
solvent left a white solid (0.026 g, 0.090 mmol), which was
recrystallized from hexanes.
4-[4-Methoxy-3,3-dimethylcyclohex-(E)-1-ylidenyl]-
methylbenzoic acid (18a)
The yield of 18a was 60%: the melting point was 112–
113°C; infrared (Nicolet 205) ν: 2967 (s), 2938 (s), 2900
(m), 2871 (m), 2821 (m), 2674 (w), 2544 (w), 1682 (s),
1608 (m), 1572 (w), 1425 (s), 1318 (m), 1290 (s), 1182 (m),
1
1101 (s), 951 (w); H nmr (250.13 MHz, CDCl3), δTM3S: 0.86
(s, 3H, CH3), 0.95 (s, 3H, CH3), 1.39–1.56 (m, 1H, J4ax–5ax
3
= 8.6 Hz, J5ax–6eq = 5.5 Hz, axial H of methylene group
2
2
(5ax-H)), 1.80–2.06 (m, 4H, J6ax–6eq = 13.7 Hz, J2ax–2eq
=
13.6 Hz, 3J5eq–6eq = 5.5 Hz, J4ax–5eq = 3.7 Hz, H of methy-
3
lene groups on ring (2ax-, 2eq-, 5eq-, 6ax-H)), 2.49 (m, 1H,
3
3
2J6ax–6eq = 13.7 Hz, J5ax–6eq = 5.5 Hz, J5eq–6eq = 5.5 Hz,
equatorial H of methylene group on ring (6eq-H)), 2.94 (dd,
3
3
1H, J4ax–5ax = 8.6 Hz, J4ax–5eq = 3.7 Hz, CH group,
methoxy-substituted (4ax-H)), 3.38 (s, 3H, OCH3), 3.44 (d,
3
2H, J7–38 = 7.3 Hz, CH2 group, aryl-substituted (8-H)), 5.24
3
(t, 1H, J7–8 = 7.3 Hz, vinyl H (7-H)), 7.27 (d, 2H, J2′–3′
=
8.1 Hz, 3J5′–6′ = 8.1 Hz, H’s adjacent to alkyl-substituted aryl
3
3
carbon (2′-H, 6′-H)), 8.02 (d, 2H, J2′–3′ = 8.1 Hz, J5′–6′
=
8.1 Hz, H’s adjacent to cyano-substituted aryl carbon (3′-H,
5′-H)); 13C nmr (62.90 MHz, CDCl3), δ: 21.52 (q), 25.06 (t),
25.83 (t), 27.58 (q), 33.90 (t), 37.19 (s), 48.04 (t, CH2, aryl-
substituted), 57.53 (q, OCH3), 86.14 (d, CH, methoxy-
substituted), 120.79 (d, vinyl CH), 126.90 (s, quaternary aryl
carbon, COOH-substituted), 128.46 (d, aromatic CH adja-
cent to alkyl-substituted aryl carbon), 130.41 (d, aromatic
CH adjacent to COOH-substituted carbon), 138.13 (s),
148.43 (s), 172.13 (s, CϭO of COOH); MS, m/z: 55(26),
© 1998 NRC Canada

1248
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6175 (1986).
8. D.D.M. Wayner and D.R. Arnold. Can. J. Chem. 64, 100
(1986).
© 1998 NRC Canada