The 1,2-diphenylethyl cation via carbene fragmentation

Article abstract of DOI:10.1016/S0040-4039(01)01225-4

2,2-Diphenylethoxychlorocarbene fragments with k_frag=2.1×10⁶ s^-1 in MeCN, largely with 1,2-phenyl migration and the formation of rearranged products. Related chemistry is observed for the 2,2-diphenylethyldiazonium cation.

Full text of DOI:10.1016/S0040-4039(01)01225-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 6045–6048
The 1,2-diphenylethyl cation via carbene fragmentation
Robert A. Moss* and Yan Ma
Department of Chemistry, Rutgers, The State University of New Jersey, New Brunswick, NJ 08903, USA
Received 5 June 2001; accepted 5 July 2001
Abstract—2,2-Diphenylethoxychlorocarbene fragments with kfrag=2.1×106 s−1 in MeCN, largely with 1,2-phenyl migration and
the formation of rearranged products. Related chemistry is observed for the 2,2-diphenylethyldiazonium cation. © 2001 Elsevier
Science Ltd. All rights reserved.
+
N
The fragmentation of alkoxyhalocarbenes, (2) in Eq.
Ph2CHCH2O
Cl
NH2
(1), generates alkyl cation–halide anion ion pairs (3).1
The carbenes, in turn are readily available by the
photolysis (or thermolysis) of alkoxyhalodiazirines (1).
With this methodology, we could revisit the chemistry
of several pivotal cations of physical organic chemistry,
including the cyclopropylmethyl2 and cyclobutyl
cations,3 and the exo- and endo-2-norbornyl cations.4
We have found that product distributions from ion
pairs 3 resemble those obtained from analogous deami-
native processes (R+N2X−), and that the kinetics of
carbene fragmentation, Eq. (1), can be measured by
laser flash photolysis.1,5 From these perspectives, we
report here on the fragmentation of 2,2-diphenyleth-
oxychlorocarbene, which features a 1,2-phenyl shift, a
common motif in carbocation chemistry.
Ph2CHCH2OCNH2, X-
N
4
5
Photolysis of 5 in MeCN (u >320 nm, A360=1.0, 25°C)
or in CD2Cl2 afforded the products shown in Eq. (2),
which we attribute to 2,2-diphenylethoxychlorocarbene,
6. Thus, 2,2-diphenylethyl chloride (7), 1,1-
diphenylethylene (8), cis- and trans-stilbene (9), a-
chlorobibenzyl (10), and 2,2-diphenylethyl formate (11)
were identified by GC–MS and NMR, as well as GC
and NMR spiking experiments with authentic materi-
als.8 Products 7–10 reflect prior fragmentation of car-
bene 6, whereas formate 11 stems from the capture of 6
by adventitious water.5a
N
RO
..
kfrag
hν
ROCX
[ R+ OC X- ]
Products
..
-N2
hν
MeCN
X
N
Ph2CHCH2OCCl
Ph2CHCH2Cl + Ph2C=CH2
5
2
3
1
6
7
8
O
(1)
+
PhCH=CHPh + PhCHClCH2Ph + Ph2CHCH2OCH
10 11
2,2-Diphenylethanol was converted to isouronium salt
4 by the reaction of 10 mmol of the alcohol with 5
mmol each of cyanamide and triflic acid in 20 ml of dry
THF (25°C, 24 h).6 Oxidation7 of 4 with 12% aqueous
NaOCl afforded diazirine 5, which was extracted into
pentane as formed, and subsequently purified by
column chromatography on silica gel, eluted with cold
1:3 CH2Cl2/pentane. Diazirine 5 exhibited umax 352 nm
in this solvent and umax 360 nm in MeCN.
9
(2)
The product distributions were sensitive to the GC
injector temperature: chloride 10 afforded stilbene 9 by
elimination. Therefore, we determined product distribu-
1
tions by H NMR integrations of crude product mix-
tures in CDCl3 or CD2Cl2. Table 1 records the results
for photolytic (25°C) and thermal (35°C) decomposi-
tions of diazirine 5, as well as for the HCl-initiated
decomposition of 2,2-diphenyldiazoethane (12).
Keywords: carbenes; diazirines; kinetics.
* Corresponding author. Tel.: 732-445-2606; fax: 732-445-5312;
e-mail: moss@rutchem.rutgers.edu
Ph2CHCH=N2
12
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01225-4

6046
R. A. Moss, Y. Ma / Tetrahedron Letters 42 (2001) 6045–6048
Table 1. Product distributions from diazirine 5 and diazoalkane 12a
Run
Subst.
Cond.
% 7
% 8
% tr-9
% cis-9
% 10
% 11
Rearr.b
1c,d
2c,e
3f
5
5
5
5
12
12
hw, MeCN
hw, CD2Cl2
D, CD2Cl2
hw, CD2Cl2
HCl, MeCN
HCl, MeCN
1.2
2.2
0.9
17.3
12.6
9.2
1.9
1.2
0.6
3.3
3.8
8.9
34.7
38.9
43.2
24.1
56.8
48.3
8.5
5.5
2.2
4.8
–
38.0
38.7
41.9
36.2
26.9
32.7
4.0
3.7
–
14.5
–
6
24
58
3.2
5.1
4.5
4c,g
5c
6c,g
0.8
–
a NMR integration is relative to PhCH2CH(Ph)Cl of product 10. Results are averages of two experiments.
b The extent of rearrangement is taken as (9+10)/(7+8).
c At 25°C.
d 11.7% of an unknown product was also present.
e 9.8% of an unknown product was present.
f At 35°C for 12 h; 11.2% of unknown was present.
g tetra-n-Butylammonium chloride (2.52 M) was present.
When 2,2-dipenylethoxychlorocarbene (6) is generated
photochemically (runs 1 and 2) or thermally (run 3)
..
(4)
-
+
Ph2CHCH2Cl + CO + Cl
Cl- Ph2CHCH2OCCl
from diazirine 5, carbene fragmentation very largely
occurs with 1,2-Ph migration, affording products 9 and
10 which derive from the 1,2-diphenylethyl cation; cf.
Eq. (3). The rearranged cation, as part of ion pair 13,
can give either the stilbenes (9) by proton loss (proba-
bly to Cl−), or a-chlorobibenzyl (10) by cation/anion
collapse.
To further the analogy1 between carbene fragmentation
and dediazoniation reactions,12 we examined the HCl-
catalysed decomposition of diazoalkane 1213 in MeCN
saturated with gaseous HCl (3.5 M HCl). Reaction of
HCl with 12 generates 2,2-diphenylethyldiazonium
chloride (Ph2CHCH2N2 Cl−, 14); decomposition of 14
+
Ph
then affords 7, 8, trans-9, and 10; cf. Table 1, run 5.
..
+
PhCHCH2OCCl
[ PhCHCH2Ph OC Cl- ]
c/t-9 + 10
Comparison of runs 5 and 1 shows that the diazonium
ion decomposition produces more unrearranged chlo-
ride (7), more trans-stilbene (9), and less rearranged
chloride (10) than does the carbene fragmentation. The
overall extent of rearrangement (defined above) is only
ꢀ5 in run 5, but reaches 26 in run 1. Chloride ion
attack at the a-carbon of the diazonium chloride ion
pair 14 (competitive with 1,2-phenyl migration) must be
responsible for the enhanced yield of 7; Eq. (5), paths a
versus b. Other decompositions of primary alkyldiazo-
nium ions, for example the acetolysis of the chiral
butanediazonium-a-d cation, occur bimolecularly with
inversion.14
6
13
(3)
We imagine that the fragmentation of 6“13 is con-
certed with phenyl migration, as depicted in Eq. (3),
and in harmony with the fragmentations of neopentyl-
oxychlorocarbene and 1-(adamantyl)methoxychloro-
carbene which also mainly afford rearranged products
(derived from the t-amyl or homoadamantyl cations,
respectively).5a,9,10
Very little product of runs 1–3 can be attributed to the
unrearranged primary 2,2-diphenyl-1-ethyl cation
(Ph2CHCH2 ). Indeed, products 7 and 8 might arise
from chloride or HCl attack directly on carbene 6, or
loss of H+ concerted with the fragmentation. Avoidance
of primary cation formation is also suggested by the
relatively high rate constant observed for the fragmen-
tation of 6.
+
Ph
b
HCl
MeCN
+
-
PhCHCH2N2 Cl
7 + 9 + 10
12
(5)
-
a
Cl
14
Curiously, the formation of 7 from 14 is not subject to
enhancement by added chloride (Table 1, run 6),
whereas the opposite holds for carbene fragmentation
(runs 2 and 4). This probably reflects the substantial
excess of HCl already present in the MeCN solvent for
the decomposition of 12 and 14.
When the carbene is generated in the presence of 2.5 M
added chloride, we observe a significant increase in the
yield of unrearranged primary chloride 7 (Table 1, run
4). The extent of rearrangment [expressed as (9+10)/(7+
8)] decreases from 24 (run 2) to 3.2 (run 4); cf. Table 1.
We attribute this to fragmenting SN2 attack of Cl− on
the carbene, Eq. (4), competitive with the phenyl migra-
tion fragmentation process of Eq. (3).11 The ‘bimolecu-
lar’ fragmentation mechanism has been shown to
govern the fragmentations of n-BuOCCl and i-
BuOCCl.5b In these cases, as in the present example, the
intermediacy of a primary alkyl cation is avoided.
The product-based evidence suggests that both carbene
6 and diazonium ion 14 decompose by composits of
bimolecular attacks at their a-carbon atoms and 1,2-
phenyl migrations that are (weakly) concerted with CO
or N2 expulsion. In related solvolytic processes, such as

R. A. Moss, Y. Ma / Tetrahedron Letters 42 (2001) 6045–6048
6047
the 75–100°C acetolysis of 2,2-diphenylethyl tosylate,15
phenyl participation leads to solvolytic rate enhance-
ment and to the rearranged product, trans-stilbene.16
The phenyl participation which accompanies expulsion
of the tosylate leaving group is sufficient to exclude
competing mechanisms that would afford unrearranged
products. In the decompositions of 6 or 14, however,
the activation energies for the loss of CO or N2 are so
low that displacements at the a-carbon are able to
compete with phenyl migration.
unlikely. Similar chemistry is found for the 2,2-
diphenylethyldiazonium chloride ion pair (14). The low
activation energies of the carbene fragmentation (or
dediazoniation) reactions make possible competitive
displacement reactions that afford at least some unrear-
ranged products, in contrast to the solvolysis of 2,2-
diphenylethyl tosylate (DH*=27.1 kcal mol−1)15a where
phenyl partcipation strongly dominates, and only rear-
ranged products form.15
Rate constants for the fragmentations of 6 in MeCN or
dichloroethane (DCE) were determined by laser flash
photolysis (LFP)17 using the pyridine visualisation
method.18 LFP at 351 nm and 25°C of diazirine 5 in
MeCN (A360=1.0) in the presence of pyridine afforded
a UV absorption due to the formation of a carbene–
pyridine N-ylide (monitored at 436 nm in MeCN and
428 nm in DCE). A correlation of the apparent rate
constants for ylide formation, kobs (3.0–8.7×106 s−1)
versus [pyridine] (0.41–3.3 M) was linear (eight points,
r=0.997) with a slope of 2.1×106 M−1 s−1, which repre-
sents the rate constant for ylide formation. The Y-inter-
cept was 2.2×106 s−1, which is the sum of the rate
constants for all processes that destroy the carbene
when [pyridine]=0. We take 2.2×106 s−1 as a good
estimate of kfrag, the rate constant for the fragmentation
of Ph2CHCH2OCCl.
Acknowledgements
We thank Drs. Fengmei Zheng and Lauren Johnson for
helpful discussions, and the National Science Founda-
tion for financial support.
References
1. Moss, R. A. Acc. Chem. Res. 1999, 32, 969.
2. Moss, R. A.; Ho, G. J.; Wilk, B. K. Tetrahedron Lett.
1989, 30, 2473.
3. Moss, R. A.; Zheng, F.; Johnson, L. A.; Sauers, R. R. J.
Phys. Org. Chem. 2001, 14, 400.
4. Moss, R. A.; Zheng, F.; Sauers, R. R.; Toscano, J. P. J.
Am. Chem. Soc., in press.
A repetition of these experiments led to kfrag=1.9×106
s−1, so that the average kfrag=(2.05 0.15)×106 s−1. Simi-
larly, kfrag was determined in DCE; two sets of experi-
ments gave kfrag=(4.6 0.4)×105 s−1. The fragmentation
appears to be ꢀ4.4 times slower in the less polar
solvent, DCE.19
5. (a) Moss, R. A.; Ge, C.-S.; Maksimovic, L. J. Am. Chem.
Soc. 1996, 118, 9792; (b) Moss, R. A.; Johnson, L. A.;
Merrer, D. C.; Lee, Jr., G. E. J. Am. Chem. Soc. 1999,
121, 5940; (c) Moss, R. A.; Johnson, L. A.; Yan, S.;
Toscano, J. P.; Showalter, B. M. J. Am. Chem. Soc. 2000,
122, 11256.
6. Moss, R. A.; Kaczmarczyk, G.; Johnson, L. A. Synth.
Commun. 2000, 30, 3233. Salt 4 was fully characterised.
7. Graham, W. H. J. Am. Chem. Soc. 1965, 87, 4396.
8. (a) Chloride 7 was prepared from 2,2-diphenylethanol,
thionyl chloride, and pyridine in refluxing THF: Hamlin,
K. E.; Weston, A. W.; Fischer, F. E.; Michaels, Jr., R. J.
J. Am. Chem. Soc. 1949, 71, 2734; (b) Chloride 10 was
prepared from benzyl chloride and n-butyllithium (−100
to −80°C, THF): Hoeg, D. F.; Lusk, D. I. J. Organomet.
Chem. 1966, 5, 1; (c) Formate 11 was made by formic
acid esterification of 1,1-diphenylethanol (BF3·2MeOH,
85–125°C). A satisfactory NMR spectrum and elemental
analysis were obtained.
A surprisingly good Arrhenius correlation was obtained
between ln kfrag and 1/T for the fragmentation of 6 in
MeCN studied between 231 and 294 K (seven points,
r=0.97). We obtained Ea=1.5 kcal mol−1, ln A=18.2
s−1, and DS*=−24 e.u. (298 K). The remarkably low Ea
underscores the concerted nature of the 6“13 fragmen-
tation, Eq. (3); it is hard to believe that a primary
+
cation (Ph2CHCH2 ) could be formed at such a low
energetic cost, even by fragmentation of 6.20 Negative
activation entropies, which may reflect solvent restric-
tion during these polar fragmentations, have been pre-
viously noted. For example, the fragmentation of
1-adamantylmethoxychlorocarbene, largely to the
homoadamantyl cation, had Ea=3.6 kcal mol−1, DS*=
−18 e.u. (298 K).5c The more negative DS* attending
the fragmentation of 6 may signal additional rotational
restrictions connected with phenyl participation.
9. (a) 1,2-Hydride shift concerted with fragmentation of 6
would give Ph2C+Me. No Ph2CClMe corresponding to
this cation was observed (l 2.30 in CDCl3),9b although
some 8 could have been formed by proton loss from the
cation; (b) Strazzolini, P.; Giumanini, A. G.; Verardo, G.
Tetrahedron 1994, 50, 217.
In summary, 2,2-diphenylethoxychlorocarbene frag-
ments with kfrag=2.1×106 s−1 (Ea=1.5 kcal mol−1,
DS*=−24 e.u.) in MeCN, largely with 1,2-phenyl
migration to ion pair 13, from which rearranged prod-
ucts (stilbene, a-chlorobibenzyl) form. Small quantities
of unrearranged fragmentation products (2,2-
diphenylethyl chloride, 1,1-diphenylethene) also form,
probably directly from the carbene; fragmentation to
the unrearranged 2,2-diphenyl-1-ethyl cation is
10. The 1,2-Me shift that converts chiral Me3CCHDOCBr to
the t-amyl cation occurs with inversion and is likely
concerted with fragmentation: Sanderson, W. A.;
Mosher, H. S. J. Am. Chem. Soc. 1966, 88, 4185.
11. The increase of 7 comes largely at the expense of trans-
stilbene, which decreases from 38.9% in run 2 to 24.1% in
run 4. The enhanced yield of formate 11 in run 4 proba-
bly reflects adventitious water introduced with the chlo-
ride salt.

6048
R. A. Moss, Y. Ma / Tetrahedron Letters 42 (2001) 6045–6048
12. For a contemporary review of aliphatic dediazoniation,
see: Zollinger, H. Diazo Chemistry II; Weinheim: VCH,
1995; Chapter 7.
Soc. 1957, 79, 2888.
15. (a) Winstein, S.; Morse, B. K.; Grunwald, E.; Schreiber,
K. C.; Corse, J. J. Am. Chem. Soc. 1952, 74, 1113; (b)
Burr, J. G. J. Am. Chem. Soc. 1953, 75, 5008; (c) Saun-
ders, Jr., W. H.; Glaser, R. J. Am. Chem. Soc. 1960, 82,
3586; (d) Fujio, M.; Maeda, Y.; Goto, M.; Saeki, Y.;
Mishima, M.; Tsuno, Y. Bull. Chem. Soc. Jpn. 1993, 66,
3015.
16. With substituted 2,2-diarylethyl tosylates, added acetate
ion leads to (rearranged) 1,2-diarylethyl acetates and
stilbenes.15d
13. 2,2-Diphenyldiazoethane was prepared by the method of
Hellerman, L.; Cohn, M. L.; Hoen, R. E. J. Am. Chem.
Soc. 1928, 50, 1716 with some modifications. 2,2-
Diphenylethylamine was converted to the urethan with
ethyl chloroformate; the urethan was nitrosated with
NaNO2 in HOAc–Ac2O at 0°C; and the N-nitro-
sourethan was treated with NaOEt/EtOH (−2 to −10°C, 3
h), affording diazoalkane 12 (65%). Red, crystalline 12,
was freed of ethanol under vacuum at −10°C, and exhib-
17. See Ref. 5b for a description of our LFP system.
18. Jackson, J. E.; Soundararajan, N.; Platz, M. S.; Liu, M.
T. H. J. Am. Chem. Soc. 1988, 110, 5595.
1
ited wN2=2059 cm−1; H NMR (l, DMSO-d6) 7.20–7.35
(m, 10H, 2Ph), 4.95 (d, J=7 Hz, 1H, CH), 4.02 (d, J=7
Hz, 1H, CH).
19. mMeCN=35.6; mDCE=10.7.
14. (a) Brosch, D.; Kirmse, W. J. Org. Chem. 1991, 56, 907;
(b) Streitwieser, Jr., A.; Schaeffer, W. D. J. Am. Chem.
20. kfrag is also ‘high,’ consistent with concerted fragmenta-
tion, e.g. kfrag for PhCH2OCCl is 0.7–1.3×106 s−1
.
.