The fragmentation of alkoxychlorocarbenes in hydrocarbon solvents and in low temperature argon matrixes

Article abstract of DOI:10.1021/jp0217076

Alkoxychlorocarbenes (ROCCl, R = benzyl, cyclohexyl, and 1-octyl) were generated from the corresponding diazirines in acetonitrile, dichloroethane, benzene, cyclohexane, and pentane solutions at 25°C, and the fragmentations of these carbenes were studied. A prominent product was phenacyl chloride (PhCH₂CoCl), a formal rearrangement product of the carbene. In polar solvents, the two fragmentation transition states for CH₃OCCl and C₂H₅OCCl were stabilized to the point where cis-trans isomerization, loss of CO, and concerted loss of CO + Cl all have similar activation energies. In less polar solvents, such as dichloroethane, heterolytic cleavage was probably not competitive.

Full text of DOI:10.1021/jp0217076

12280
J. Phys. Chem. A 2002, 106, 12280-12291
The Fragmentation of Alkoxychlorocarbenes in Hydrocarbon Solvents and in Low
Temperature Argon Matrixes
Robert A. Moss,* Yan Ma, Fengmei Zheng, and Ronald R. Sauers*
Department of Chemistry and Chemical Biology, Rutgers, The State UniVersity of New Jersey,
New Brunswick, New Jersey 08903
Thomas Bally* and Alexander Maltsev
Institute for Physical Chemistry, UniVersity of Fribourg, Perolles, CH-1700 Fribourg, Switzerland
John P. Toscano and Brett M. Showalter
Department of Chemistry, Johns Hopkins UniVersity, Baltimore, Maryland 21218
ReceiVed: July 23, 2002; In Final Form: October 15, 2002
Alkoxychlorocarbenes (ROCCl, R ) benzyl, cyclohexyl, and 1-octyl) were generated from the corresponding
diazirines in acetonitrile, dichloroethane, benzene, cyclohexane, and pentane solutions at 25 °C, and the
fragmentations of these carbenes were examined. Formation of RCl (and alkenes when R ) cyclohexyl or
1-octyl) occurred efficiently in all of the solvents. The rate constant for the fragmentation of PhCH2OCCl
(determined by laser flash photolysis) was ∼105 s-1, and relatively independent of solvent. Photolysis of
PhCH2OC(N2)Cl in Ar matrixes led to the carbene, PhCH2OCCl, as well as its primary fragmentation products,
the benzyl and COCl radicals. A prominent product was also phenacyl chloride, PhCH2COCl, a formal
rearrangement product of the carbene. Computational studies of the rearrangements and fragmentations of
MeOCCl, EtOCCl, and PhCH2OCCl at the DFT and coupled cluster levels afforded transition states and
energetics. These studies allowed us to identify several mechanistic pathways, including concerted and
homolytic processes that are predicted to prevail in nonpolar solvents and matrixes as opposed to heterolytic
(ionic) processes in polar solvents. The existence of cis and trans forms of R-OC-Cl, and their interconversion,
are complicating factors that were considered computationally.
In polar solvents such as acetonitrile, methanol, or 1,2-
dichloroethane (DCE), benzyloxychlorocarbene (1) fragmented
to benzyl chloride, carbon monoxide, and solvent trapping
products (e.g., benzyl methyl ether in methanol); cf., eq 1.1 It
a mechanistic formulation of the fragmentation via ion pair 2.1,3
The absolute rate constant of the fragmentation, measured by
laser flash photolysis (LFP) using either UV4,5 or time-resolved
infrared (TRIR) monitoring,5 gave kfrag ∼ 4.5 × 105 s-1 (MeCN)
and ∼ 1.8 × 105 s-1 (DCE).5
In polar solvents, ionic fragmentation is fast enough such that
trapping of the carbene by MeOH is not competitive; no
carbene-methanol products are observed.1,3,6
However, we would not expect ionic fragmentation in
nonpolar solvents. Indeed, in 0.9 M MeOH in pentane (as
opposed to MeOH in MeCN), up to 40% of 1 is trapped by the
methanol affording benzyl alcohol, presumably by the sequence
in eq 2, where traces of water and/or HCl hydrolyze the initially
formed chloroacetal (3).1,7 The yield of benzyl chloride is not
affected on going from acetonitrile to methanol-pentane as a
solvent, and 18% of benzyl methyl ether is also formed under
these conditions.1
The substantial trapping of carbene 1 in methanol/pentane
raises the question of how alkoxychlorocarbenes fragment in
hydrocarbon solvents because one would not expect ionic
fragmentations to proceed in such media. To address this
is known that both cis and trans forms of MeOCCl and PhOCCl
are generated by diazirine photolysis in cryogenic matrixes,8
and for purposes of interpretation and discussion it seems most
reasonable to assume that this is also the case for the photolytic
generation of PhCH2OCCl in ambient temperature solution as
well. (In Ar matrixes, both cis and trans isomers of 1 are formed,
see below.)
Stereochemical retention during the fragmentation of chiral
R-deuterio-1,2 as well as the persistence of 43% of benzyl
chloride even when the reaction occurred in methanol,1 led to
10.1021/jp0217076 CCC: $22.00 © 2002 American Chemical Society
Published on Web 11/28/2002

Fragmentation of Alkoxychlorocarbenes
J. Phys. Chem. A, Vol. 106, No. 51, 2002 12281
Figure 1. B3LYP/6-31G* transition state for the gas-phase fragmenta-
tion of cis-1: r1 ) 2.200; r2 ) 3.349; r3 ) 1.173; r4 ) 2.396 (all
distances in Å); R ) 118.5°; â ) 120.3°.
question, we have to consider that carbene 1 can exist or be
generated in two conformations (configurations), cis-1 and
trans-1 whose fragmentation behavior may be quite different.8
of 511), in polar solvents (MeCN, DCE). Here, we demonstrate
for the first time the persistence of efficient fragmentation in
hydrocarbon solvents. Moreover, we extend studies of carbene
1 to Ar matrixes at 12 K, where we encounter reactive
intermediates not readily observed in solution at ambient
temperature. At the same time we extend the previous compu-
tational investigations on the fragmentations of oxychlorocar-
benes10 to provide a coherent framework for the interpretation
of the new experimental findings that are presented in this work.
Whereas in sufficiently polar solvents trans-1 can cleave to
form an ion pair (separated by a molecule of CO), cis-1 may
fragment via a tight ion pair9 or, in the limit, by a concerted
process where the incipient benzyl cation and chloride anion
recombine to form benzyl chloride as CO is expelled. A
corresponding transition state (Figure 1) was located in a
previous computational study where the calculated activation
energy for the fragmentation of cis-1 in a vacuum was found
to be only 6.7 kcal/mol,10 so that this fragmentation should
certainly occur at ambient temperature in pentane. Interestingly,
the activation energy dropped to 1.45 kcal/mol in acetonitrile,10
which indicates that the transition state is much more polar than
the reactant carbene.
Experimental Results and Discussion
Reactions and Products. Carbenes 1, 4, and 5 were
generated by photolysis or thermolysis of 3-alkoxy-3-chlorodi-
azirines 6, which were prepared by hypochlorite oxidation
(Graham reaction)12 of isouronium salts 7. The latter were
In the transition state of Figure 1, r2 is 3.35; a strong
benzyl-C (δ+) and chloride (δ-) interaction clearly exists. The
fragmentation is advanced at the cationic center, where the C-O
separation (r1) is 2.20 Å and bond angles R and â approach
120°.
synthesized from the appropriate alcohol, cyanamide, and strong
acid (HX) by previously described methods.1,13 Except for
diazirine 6c, these carbene precursors have been previously
prepared and were characterized by their IR, UV, and NMR
spectra.1,14 All diazirines were purified before use by short-
path chromatography over silica gel, with pentane elution. When
experiments required alternative solvents, pentane was removed
under reduced pressure and replaced by the solvent of choice.
Photolysis (λ > 320 nm) or thermolysis of diazirines 6 were
carried out at 25 °C. Products were analyzed by GC and GC-
MS and confirmed by spiking comparisons with authentic
materials.
Benzyloxychlorocarbene (1). The fragmentation of carbene
1 in MeCN led to benzyl chloride and N-benzylacetamide, a
“Ritter” product formed by the attack of the benzyl cation on
MeCN, followed by hydrolysis with adventitious water.1,4 In
DCE, only benzyl chloride formed from the fragmentation of
1.5 These results, together with product distributions for the
fragmentations of 1 in cyclohexane, benzene, and pentane,
appear in Table 1.
On the other hand, the attraction between the nascent chloride
anion and the benzyl cation is strongly attenuated by the
intervening CO molecule in the transition state for the frag-
mentation of trans-1 (see eq 3). However, according to SCI-
PCM calculations,10 the fragmentation of 1 to and PhCH2+, CO,
and Cl- is slightly exothermic in acetonitrile (and, presumably
also in methanol), so that the development of solvated ions from
trans-1 is conceivable under these conditions and may indeed
account for the formation of PhCH2OMe in MeOH.1
Due to lack of solvation for the developing ions, heterolytic
fragmentation of trans-1 in a vacuum (and, presumably, pentane)
is energetically prohibitive (∆Ecalc > 100 kcal/mol10), but
homolytic fragmentation (∆Ecalc ∼ 8.6 kcal/mol) to give the
benzyl radical and COCl (which itself loses CO quite easily)
may become competitive with other processes in nonpolar
solvents.
In the present report we examine the fragmentations of several
representative alkoxychlorocarbenes in hydrocarbon solvents.
We consider benzyloxychlorocarbene (1), cyclohexyloxychloro-
carbene (4), and n-octyloxychlorocarbene (5) in MeCN, DCE,
benzene, cyclohexane, or pentane.
From Table 1, it is clear that quantitative or near quantitative
fragmentation to benzyl chloride and CO is the principal fate
of carbene 1 in MeCN, DCE, benzene, or pentane solutions at
ambient temperature. Only in cyclohexane is there substantial
side product formation. The cyclohexyl chloride and cyclohex-
Prior product and kinetics studies have appeared for the
fragmentations of 14,5 and n-butoxychlorocarbene (an analogue

12282 J. Phys. Chem. A, Vol. 106, No. 51, 2002
Moss et al.
TABLE 1: Product Distributions (%) from Benzyloxy-
chlorocarbene (1)a
solvent
methodb PhCH2Cl PhCH2NHCOMe reference
MeCN
DCE
benzene
cyclohexane
pentane
pentane
hν
hν
hν
hν
hν
∆
63
100
100
80d
94e
98f
37
4
5
c
c
c
c
a All reactions at 25 °C; the absorbance of diazirine 6a was 1.0 at
λmax.
b Decomposition method for the diazirine. c This work. d 9% of
cyclohexyl chloride and 11% of cyclohexanol also formed. e 3% of
benzaldehyde and 2% of benzyl formate also formed. f 2% of benzal-
dehyde formed.
SCHEME 1: Photolysis of Diazirine 6a in an Argon
Matrix at 12 K
Figure 2. Difference UV-vis spectra obtained in an Ar matrix at 12
K: (a) after irradiation of diazirine 6a at 365 nm; (b) after subsequent
bleaching of the radical pair (PhCH2‚‚‚COCl) at >375 nm; (c) after
subsequent bleaching of carbene 1 at 340 nm; (d) high-resolution (0.1
nm) scan of the first band of benzyl radical, based on spectrum (a).
anol may result from reaction pathways involving cyclohexyl
and benzyl radicals. LFP1,15,16 and matrix isolation experiments
(see below) do indeed indicate minor benzyl radical formation
(<10%) in the fragmentation of 1. Note, however, that the only
benzyl-containing product formed from carbene 1 in cyclohex-
ane is the expected fragmentation product, PhCH2Cl, whereas
toluene (the H-abstraction product expected from benzyl radical)
is not observed.
In contrast, the most conspicuous product observed after 365
nm photolysis of diazirine 6a embedded in an Ar matrix at 12
K is the benzyl radical with its signpost optical bands at 400-
450 and 280-320 nm (see traces a and d in Figure 2). However,
a featureless absorption extending from 260 to 360 nm indicates
the presence of another species (carbene 1) that can be bleached
by >340 nm photolysis with the formation of more benzyl
radical (Figure 2c). Finally, >375 nm irradiation leads to loss
of the benzyl radical, which is accompanied by a partial recovery
of the above 260-360 nm UV band. These results are shown
in Scheme 1.
Figure 3. Difference IR spectra obtained in an Ar matrix at 12 K (a)
after 365 nm irradiation of diazirine 6a; (b) subsequent bleaching of
the PhCH2‚‚‚COCl radical pair at >375 nm; (c) bleaching of carbene
1 at 340 nm. Inverted triangles denote IR bands of phenacyl chloride,
filled squares those of benzyl chloride, asterisks are for IR bands of
benzyl radical; (d) spectrum of a 75:25 mixture of trans- (open circles)
and cis-1 (filled circles) calculated by B3LYP/6-31G*.
calculated spectrum of a 75:25 mixture of the anti- and syn-
conformers, Figure 3d).
More detailed information is gained from the IR spectra
shown in Figure 3. The difference spectrum (a) for the 365 nm
photolysis of 6a very clearly shows the rise of the prominent
bands of CO (2143 cm-1), COCl (1870 cm-1), and phenacyl
chloride 8 (1800-1820 cm-1). On subsequent bleaching at
>375 nm (difference spectrum b), the COCl peaks and several
bands that we attribute to the benzyl radical17 decrease, whereas
those of CO and phenacyl chloride increase (the latter very
weakly). In addition, a group of bands attributed to benzyl
chloride (by comparison with an authentic sample) also gain in
intensity. On subsequent 340 nm irradiation (difference spectrum
c, which corresponds to the bleaching of the broad UV band)
the COCl and benzyl radical bands are partially recovered. Most
of the IR peaks that decrease in this experiment can be attributed
to the two conformers of benzyloxychlorocarbene 1 (cf., the
By internal calibration experiments involving known quanti-
ties of the stable products (CO, benzyl chloride, phenacyl
chloride), we are able to quantitate the yields of all the species
obtained in the various photolyses using integrated IR band
intensities (Table 2). From this it becomes evident that, despite
their prominence in the UV/vis spectra (Figure 2), the radical
cleavage products, benzyl and COCl, are minor products of the
bleaching of diazirine 6a. However, these products gain more
prominence in the photolysis of carbene 1.
The occurrence of phenacyl chloride 8 as a major product
constitutes the chief difference between the room temperature
solution experiments and those in Ar matrixes at 12 K. LFP-
TRIR experiments suggest the possible minor involvement of

Fragmentation of Alkoxychlorocarbenes
J. Phys. Chem. A, Vol. 106, No. 51, 2002 12283
TABLE 2: Results of Photolyses in Ar Matrixes at 12 K
TABLE 3: Product Distributions (%) from Cyclohexyloxy-
chlorocarbene (4)a
irradiation
wavelength (nm) percenta
products
solvent
methodb
c-C6H11Cl
cylcohexene
reference
reactant
MeCN
DCE
benzene
pentane
pentane
hν
hν
hν
hν
∆
62
70
53
53
43
32c
30
14
14
e
e
e
diazirine 6a
365
46 ( 6
27 ( 5
20 ( 8
7 ( 2
85 ( 3
15 ( 3
53 ( 5
33 ( 5
14 ( 2
59
PhCH2-COCl
PhCH2-Cl + CO
PhCH2-OC¨ Cl
36d
42f
57
PhCH2• + COCl•
PhCH2-Cl + CO
PhCH2-COCl
•
PhCH2 ‚‚‚COCl•
>375
a All reactions at 25 °C; the absorbance of diazirine 6b was 1.0 at
PhCH2OC¨ Cl
340
PhCH2-COCl
λmax.
b Decomposition method for the diazirine. c 6% of N-cyclohexyl-
PhCH2• + COCl•
PhCH2-Cl + CO
PhCH2-COCl
acetamide also formed. d 11% of cyclohexyl dichloromethyl ether
formed. e This work. f 5% of an unidentified product was also present.
diazirine 6a
(overall)
41
PhCH2-Cl + CO
TABLE 4: Product Distributions (%) from n-Octyloxy-
chlorocarbene (5)a
a Uncertainties arise mainly from the accuracy with which partial
pressures of CO, PhCH2Cl, and PhCH2-COCl could be gauged in the
calibration experiments. In the case of the carbene, overlapping bands
caused additional difficulties. The quoted uncertainties are conservative
estimates.
n-Oct
n-Oct
solvent methodb 1-octene n-Oct-Cl 2-Oct-Cl formate OCHCl2
MeCN
hν
hν
hν
∆
37
30
32
39
25
19
41
55
10
5
5
13
2
12
15
29
benzenec
pentaned
pentane
8 during the LFP of 6a in cyclohexane. The formation of 8
could be rationalized as a recombination product of the radical
pair [9‚‚‚COCl] that is formed by homolytic scission of 1.
Interestingly, only 15% of 8 is formed in the photochemical
bleaching of this radical pair; the main process is Cl abstraction
by 9 from COCl, a reaction that is exothermic by 57 kcal/mol.
Hence, any radicals that might be formed by homolytic cleavage
of 1 in solution are likely to decay by this route. Alternatively,
8 could arise by direct 1,2 migration of benzyl from oxygen to
the carbene center, a mechanism that is perhaps operative in
the bleaching of 1, which leads to over 50% of 8. This
mechanistic possibility will be examined by computational
means below.
The general absence of phenacyl chloride (or products derived
therefrom) in the solution LFP experiments is probably due to
the fact that ∆H298 for the dissociation of COCl is only about
8 kcal/mol.18a Hence, in contrast to the situation in a matrix at
12 K, the lifetime of COCl at room temperature may be too
short to undergo efficient recombination with the benzyl radical
to form 8. However, the Cl radicals that remain after dissociation
of COCl can easily recombine with 9 to form benzyl chloride
or abstract hydrogen atoms from the solvent to form HCl, which
may explain some of the other products that are isolated in the
solution runs in cyclohexane (see Table 1).
6
a At 25 °C; the absorbance of diazirine 6c was 1.0 at λmax
.
b Decomposition method for the diazirine. c 15% of another component,
possibly an azine, was present. d 9% of an unknown component was
present.
provide some insight as to the mechanism of these reactions in
nonpolar solvents. We do not have an explanation for the
apparent dependence of the cyclohexyl chloride/cyclohexene
distribution in pentane on the method of carbene generation
(Table 3), but both products arise by carbene fragmentation.
As will be shown below, isolated ethoxychlorocarbene is
predicted to undergo concerted elimination of CO and HCl with
∆Hq ) ∼20 kcal/mol and fragmentation with ∆H q ) ∼26 kcal/
mol.
n-Octyloxychlorocarbene (5). Primary alkyloxychlorocarbenes
(RCH2OCCl, R ) n-C4H9 or i-C4H9) fragment in MeCN by
bimolecular processes that involve SN2 attack of Cl- at the
R-methylene carbon atom of the carbene.11 We examined
carbene 5 to determine if a MeCN to hydrocarbon solvent
change impeded carbene fragmentation. The products obtained
from 5, as generated by the photolysis of diazirine 8c in MeCN,
are shown in eq 4; product distributions appear in Table 4.
Thus, the Ar matrix results suggest that benzyl chloride may
form from 1 in pentane via different pathways, i.e., a concerted
one from cis-1, or a sequential one from trans-1. Which pathway
dominates will depend on the relative activation energies for
the dissociation of trans-1 to 9 and COCl and for the
rotamerization to cis-1 (which will promptly decay to benzyl
chloride by “concerted” elimination of CO.
Cyclohexyloxychlorocarbene (4). Fragmentation of carbene
4 in MeCN or DCE mainly produces cyclohexyl chloride and
cyclohexene, which can be rationalized by cation-anion
recombination or “HCl” elimination by â-carbon-to-chloride
proton transfer within an ion pair.19 Product distributions appear
in Table 3 for the fragmentations of carbene 4 in MeCN, DCE,
benzene, and pentane.
Apart from the formation of 11% of cyclohexyldichloro-
methyl ether (the HCl-trapping product of carbene 4) in
benzene,20 carbene fragmentation is the dominant fate of this
sec-alkoxychlorocarbene in the hydrocarbon solvents benzene
and pentane. Because ionic fragmentation is thermochemically
impossible in these solvents (see computational part), concerted9
or radical processes must account for the formation of the
observed fragmentation products. The computational studies will
1-Octene (10), n-octyl chloride (11), and 2-chlorooctane (12)
are fragmentation products of carbene 5, whereas octyl formate
(13) and n-octyl dichloromethyl ether (14) represent carbene
trapping by water or HCl, respectively. The products are
analogous to those observed from n-butoxychlorocarbene in
MeCN,11 although we did not detect carbene dimer or azine
(see below). In MeCN, fragmentation of 5 accounts for ∼72%
of the total product. Fragmentation (10 + 11 +12) decreases
to 53% in benzene, where about 15% of azine appears to form
(on the basis of the MS fragmentation pattern of an additional
product). In pentane, fragmentation accounts for 78-100% of
the products, depending on the method of carbene generation.
Clearly, fragmentation is not inhibited by a solvent change from
polar MeCN to nonpolar pentane, even though ionic fragmenta-
tion should not occur in pentane.

12284 J. Phys. Chem. A, Vol. 106, No. 51, 2002
Moss et al.
TABLE 5: Rate Constants (kfrag, s-1) for Fragmentations of
PhCH2OCCl (1)a
methodb
MeCNc
DCEc
benzened
pentaned
UV-pyridine
3.6 × 105
6.2 × 104
5.0 × 105
e
TRIR
4.4 × 105
2.9 × 105
e
1.8 × 105
a At 25 °C. Errors in kfrag are 10-15%. b Monitoring method for the
LFP experiments; see text. c Data from ref 5. d This work. e Not
determined.
Kinetics. We determined rate constants for the fragmentations
of carbenes 1 and 5 in various solvents. For carbene 1, the rate
constants were obtained by LFP, using both UV and TRIR
monitoring methods.5 LFP-UV studies were previously reported
for carbene 1 in MeCN and DCE solvents;4,5 the most recent
values of kfrag are (3.6 ( 0.45) × 105 s-1 (MeCN) and (6.2 (
0.2) × 104 s-1 (DCE).5 We repeated these measurements in
Figure 4. Correlation of kobs (s-1) for the formation of the pyridinium
ylide from carbene 5 and pyridine as a function of added Bu4NCl (M)
in 5.77 M pyridine-MeCN solutions at 25 °C. The slope of the
4,5
benzene, using the pyridine ylide method21 to derive kfrag
.
Thus, LFP of diazirine 6a (A350 ) 1.0) with a xenon fluoride
excimer laser22 at 351 nm and 25 °C in benzene containing
added pyridine produced an absorption at 412 nm due to ylide
15.23
correlation (k2) is 1.2 × 107 M-1 s-1
.
differ by a factor of 4.7 for DCE. This may be due to specific
solvent effects of the pyridine probe, although the dielectric
constants of pyridine and DCE are very similar. Suffice it to
say that all the kfrag values are near 105 s-1; the fragmentations
are fast and relatively independent of solvent, consistent with a
very low activation energy for fragmentation, even in pentane.
We also measured kfrag for n-octyloxychlorocarbene (5) in
MeCN and benzene. The rate constant for the analogous
fragmentation of n-butyloxychlorocarbene in MeCN was previ-
ously determined as 1.75 × 106 s-1 (Ea ) 3.1 kcal/mol).11
However, kfrag varied with added chloride, affording k2 ) 8.2
× 106 M-1 s-1, consistent with a “bimolecular” fragmentation
induced by SN2 attack of Cl- at the carbene’s R-carbon that
avoids a “unimolecular” fragmentation via a primary alkyl
cation; cf., eq 5.11
A correlation of the apparent rate constants for ylide formation
(kobs, (1.2-2.5) × 106 s-1) vs pyridine concentration (2.47-
7.42 M) was linear (7 points, r ) 0.992) with a slope of 2.7 ×
105 M-1 s-1, which can be taken as the second-order rate
constant for ylide formation, ky.21,24 The Y-intercept at [pyr] )
0 was 5.5 × 105 s-1, which represents the sum of the rate
constants for all processes that destroy carbene 1 in the absence
of pyridine. As indicated in Table 1, PhCH2OCCl quantitatively
fragments to benzyl chloride in benzene, so that 5.5 × 105 s-1
is a good estimate of kfrag. Repetition of this determination led
to kfrag ) 4.5 × 105 s-1, giving an average value of kfrag ) (5.0
( 0.5) × 105 s-1 25
.
Rate constants for the fragmentation of
carbene 1 appear in Table 5.
For comparison with previous determinations, we also
measured kfrag for 1 in DCE using the UV-pyridine monitoring
method. We obtained kfrag ) (6.2 ( 0.3) × 104 s-1. The kfrag
measurement is in excellent agreement with the earlier value
[(6.2 ( 0.2) × 104 s-1].5 In DCE, fragmentation of carbene 1
to benzyl chloride is quantitative; cf., Table 1.
We were unable to determine kfrag for 1 in pentane (or
cyclohexane) by the LFP-UV-pyridine method. The UV
absorption of ylide 15 was very poor in these solvents and could
not be exponentially fitted at lower [pyr]. Similar difficulties
were encountered with carbene 4 in both benzene and pentane.
Accordingly, we used the LFP-TRIR spectroscopy26 to measure
With the UV-pyridine method, LFP studies of the fragmen-
tation of 5 in MeCN gave kfrag ) (1.6 ( 0.03) × 106 s-1 (and
ky ) (1.9 ( 0.06) × 105 M-1 s-1), in excellent agreement with
the kinetic data for the fragmentation of n-BuOCCl in MeCN.11
Measurement of kfrag for carbene 5 in benzene gave (5.1 ( 0.3)
× 105 s-1 and ky ) (2.5 ( 0.04) × 105 M-1 s-1
.
The bimolecular fragmentation mechanism of eq 5 was
verified for the decomposition of carbene 5 in MeCN. LFP of
diazirine 6c in MeCN solutions containing a constant concentra-
tion of pyridine (5.77 M) and varying quantities of tetra-n-
butylammonium chloride (0.17-0.76 M) gave a linear (r )
0.995) correlation of kobs for the formation of ylide vs [C1-],
affording k2 ) 1.2 × 107 M-1 s-1; cf., Figure 4.27 This value is
in good agreement with the analogous rate constant for the
chloride-induced fragmentation of n-BuOCCl in MeCN (8.2 ×
106 M-1 s-1).11
k
frag for carbene 1 in pentane. We monitored the time-dependent
formation of CO at 2120 cm-1 by TRIR during the fragmenta-
tion of carbene 1 in pentane. Analysis of the growth of the CO
over 9 µs after the laser flash gave k ) 1.84 × 105 s-1; cf.,
Table 5.
The TRIR-determined rate constants in Table 5 manifest small
differences that may reflect the decreasing solvent polarity from
MeCN to DCE to pentane. The UV-pyridine results, however,
are not in a parallel sequence. To avoid overinterpretation of
the data, we will not analyze small differences between kfrag
values determined by different monitoring methods. The UV-
pyridine and TRIR rate constants agree very well for ACN but
The foregoing experiments support the contention that neither
the product distribution nor the fragmentation rate constants are
greatly altered by solvent variation between MeCN, DCE,
benzene, and pentane. This is surprising in view of the ∼5.2
kcal/mol computed reduction of activation energy for the
fragmentation of PhCH2OCCl in MeCN vs vacuum (and,
presumably pentane); see above.10

Fragmentation of Alkoxychlorocarbenes
J. Phys. Chem. A, Vol. 106, No. 51, 2002 12285
TABLE 6: Geometry Parameters,a Relative Energies (kcal/mol), and Partial Charges of CH3OCCl Isomers and Transition
States in the Gas Phase and in Simulated CH3CN (SCI-PCM)
trans-carbene
cis-carbene
fragmentation TSb
cis-trans-isom TS
rearrangement TSc
gas phase
CH3CN
gas phase
CH3CN
gas phase
CH3CN
gas phase
CH3CN
gas phase
CH3CN
H3C-O
1.453 Å
1.296 Å
1.809 Å
3.882 Å
114.9°
105.3°
180.0°
(0)g
(0)
(0)
29.28
-0.087
-0.202
+0.289
1.466 Å
1.280 Å
1.848 Å
3.924 Å
115.9°
105.0°
180.0°
(0)h
1.468 Å
1.278 Å
1.872 Å
2.941 Å
129.1°
111.1°
-0.02°
+0.93
1.478 Å
1.264 Å
1.904 Å
2.981 Å
130.2°
111.3°
-0.01°
+0.58
2.185 Å
1.176 Å
2.337 Å
2.795 Å
122.8°
99.8°
2.181 Å
1.160 Å
2.624 Å
2.968 Å
129.6°
95.9°
1.446 Å
1.289 Å
1.898 Å
3.513 Å
125.6°
107.1°
95.7°
+18.11
+20.89
+20.88
28.47
1.470 Å
1.225 Å
2.119 Å
3.875 Å
146.8°
106.6°
105.6
1.796 Å
1.246 Å
1.946 Å
3.326 Å
74.8°
111.8°
114.1°
+40.46
+43.90
+43.14
27.36
1.918 Å
1.200 Å
2.213 Å
3.490 Å
77.1°
110.3°
105.0°
O-C
C-Cl
H3C‚‚‚Cl
H3C-O-C
O-C-Cl
H3C-O-C-Cl
B3LYP/6-31G*
CCSD(T)/DZd
CCSD(T)/TZe
ZPE (B3LYP)
δ(Cl)f
0.0°
0.22°
+30.60
+37.26
+38.17
+42.26
26.07
-0.394
-0.034
+0.428
+15.90
+36.00
+1.06
+1.66
29.22
-0.133
-0.173
+0.306
29.12
29.12
25.87
28.10
26.79
-0.151
-0.241
+0.392
-0.191
-0.160
+0.351
-0.656
+0.025
+0.631
-0.165
-0.106
+0.271
-0.425
+0.094
+0.331
-0.246
-0.046
+0.292
-0.532
+0.019
+0.513
δ(OC)f
δ(CH3)f
a For structures see Figure 6. b TS for loss of CO and concomitant formation of CH3Cl. c TS for concerted CH3OCCl f CH3COCl rearrangement.
d Single-point calculation with Dunning’s cc-pVDZ basis set at the B3LYP geometry. Total energy: -612.40737 h. e Single-point calculation with
Dunning’s cc-pVTZ basis set at the B3LYP geometry. Total energy: -612.63992 h. f CHelpG charges for the indicated fragments. g Total energy:
-613.34568 h. h Total energy: -613.351974 h.
Computational Results and Discussion
Previous Studies. Several recent computational studies deal
with the thermochemistry and the kinetics of the fragmentation
of oxychlorocarbenes.10,19,28 However, these studies focused on
alkoxychlorocarbene fragmentations in polar solvents where
fragmentation is presumed to lead to ion pairs. The same cannot
happen in nonpolar solvents such as pentane or cyclohexane
(benzene may stabilize ions through π donor/acceptor interac-
tions in certain cases). In view of the present experimental
findings we set out to examine in more detail processes that
may contribute to the decay of alkoxychlorocarbenes RCH2-
OCCl under conditions where solvation is ineffective, and to
examine the changes that are then brought about by introducing
dielectric stabilization of the reacting species.
The previous studies have demonstrated that the activation
barriers for the loss of CO from cis-carbenes (via transition states
such as the one depicted in Figure 1) depend strongly on the
nature of R and the presence or absence of a polar solvent,
whereas the rotational barriers for interconversion of cis- and
trans-carbenes are rather independent of these factors.
Methoxychlorocarbene. We started our investigation with
methoxychlorocarbene, 16, the smallest member of the series,
because it is amenable to very high-level calculations, which
allow us to assess the performance of the more economical
B3LYP/6-31G* method that will be used subsequently. The
results of this study are given in Table 6 and Figure 5 which
illustrate the following scenario: the cis-and trans-conformers
of 16 are predicted to interconvert prior to any decomposition.
Figure 5. Energies of CH3OCCl and products of rearrangement and
fragmentation by different methods relative to the trans-carbene.
The lowest energy dissociation pathway is straight homolytic
cleavage of either conformer to •CH3 + •COCl for which 18-
22 kcal/mol are required at 0 K (zero-point energy differences
calculated by B3LYP included). Interestingly the B3YLP/6-
31G* energy difference for this process is closer to that obtained
at the CCSD(T) level with the triple-ú than at the same level
with a double-ú basis set.
CH3Cl whose gas-phase 0 K activation energy varies between
ca. 34 (B3LYP) and 39 kcal/mol (CCSD(T)/cc-pVTZ at the
B3LYP geometry ZPE differences included). It seems that
B3LYP/6-31G* underestimates the activation energy for this
process, a feature that we will have to keep in mind when we
interpret the results of these calculations for larger molecules.
On going to acetonitrile, the barrier for this process is lowered
by ca. 6.7 kcal/mol at the B3LYP level, which is easily
understandable in view of the high degree of charge separation
between the CH3 and the COCl moieties that prevails at the
transition state (cf., Table 6). We were surprised to note by how
much the predictions of the different methods varied with regard
to the energy liberated in the formation of MeCl + CO, and
•
Experimentally, the dissociation enthalpy of COCl is about
•
8 kcal/mol18 so this energy must be added to arrive at CH3+
•
•
CO + Cl.29 Due to the high ionization energy of CH3,
heterolytic dissociation of 16 is of course prohibitively expensive
in the gas phase, and although it is predicted to drop to 44.4
kcal/mol in MeCN, this process is not competitive with
homolytic cleavage, even in this highly polar solvent.
If more energy becomes available, the next process that comes
to the fore is the concerted elimination of CO and formation of

12286 J. Phys. Chem. A, Vol. 106, No. 51, 2002
Moss et al.
Figure 6. Structures of CH3OCCl and corresponding species.
also how much this result depends on the basis set in the
CCSD(T) calculations (cf., Table 6). Conversely, solvation by
MeCN does not affect the exothermicity of this process very
much.
We were also interested in the possibility of a concerted
process leading from 16 to the much more stable isomeric acetyl
chloride (a process that amounts to a 1,2-O-C methyl shift).
Despite the strong exothermicity of this process, the corre-
sponding transition state was found to lie 18-24 kcal/mol above
the dissociation limit of 16 on a 0 K enthalpy scale. Again, we
note that the B3LYP results are closer to those obtained at the
CCSD(T) level with the triple-ú than with the double-ú basis
set. However, all methods agree in predicting that the formation
of alkanoyl chlorides from alkoxychlorocarbenes requires dis-
sociation and recombination, even in polar solvents where the
transition state for the concerted rearrangement is strongly
stabilized due to the strong charge separation that also prevails
in this geometry (cf., Figure 6)
Figure 7. Energies of CH3CH2OCCl and products of rearrangement
and fragmentation by different methods relative to the trans-carbene.
7. The energy difference between the cis and trans conformers
does not differ much from the values found for CH3OCCl,
whereas the presence of the additional methylene group lowers
the 0 K barrier for their interconversion by about 1.3 kcal/mol.
In contrast, the activation barrier for concerted elimination of
CO and formation of R-Cl decreases by 4-8 kcal/mol,
although it is still higher than the barrier for cis f trans
isomerization.
In the case of MeOCCl and its products, all structures were
fully reoptimized with B3LYP in MeCN, using the SCI-PCM
method, for which gradients are available. As can be seen from
Table 6, the geometries of most equilibrium structures did not
change very much on solvation, whereas those of the transition
states are more strongly affected. The main change is invariably
a lengthening of the C-Cl bond, which attains nearly 0.3 Å in
the (very polar) fragmentation transition state. The reason for
this is that the partial negative charge on Cl increases from 0.25
to 0.53 on going from the gas phase to MeCN, which results in
the observed 7 kcal/mol differential stabilization of the transition
state. A similar increase in the polarity of the transition state
for cis-trans isomerization (charge on Cl goes from -0.165
to -0.425) leads to a similar effect, and a significant stabilization
of the transition state by solvation. Notable also is the finding
that the nonbonded H3C‚‚‚Cl distance decreases significantly
on going from the cis-carbene to the transition state for loss of
CO in the gas phase, whereas in MeCN it remains almost the
same. Thus, in solution, the tendency for the Cl atom to bond
to the carbon center is attenuated.
Most importantly, in our search for the transition state for
the fragmentation process, we found another saddle point that
was shown by IRC calculations to be connected adiabatically
to ethene, CO, and HCl on the product side, and to a different
cis-conformer of 17 on the reactant side (see Figure 8 for
pictorial representations of the relevant geometries). The activa-
tion energy for this concerted elimination of CO and HCl was
found to be 6-7 kcal/mol lower than that for fragmentation to
CO + C2H5Cl so the two processes are likely to be competitive,
in accord with the experimental findings for the two alkoxy-
chlorocarbenes 4 and 5, given the differing substitution patterns.
The energy for homolytic dissociation to R• + •COCl is also
slightly lowered by the additional methyl group in 17, so that
this process is again predicted to be competitive with the
concerted eliminations described above. Different methods of
calculations differ in the ordering of the energies for the three
transition states: B3LYP/6-31G* predicts elimination of CO
+ HCl to be favored by 3.6 kcal/mol over homolytic cleavage
and by 5.8 kcal/mol over fragmentation to CO + C2H5Cl,
whereas CCSD(T) calculations favor homolytic cleavage by 3
kcal/mol over elimination of CO + HCl. The same may also
be true for the 1-octyl and cyclohexyl sytems investigated
experimentally, so that HCl might also arise by abstraction of
a H atom â to the alkyl radical center by a Cl radical in nonpolar
solvents, although we find no product evidence for extensive
radical chain processes.
Ethoxychlorocarbene. The finding that olefins are prominent
products after decomposition of diazirines 6b and 6c led us to
investigate possible pathways that would account for their
formation. As a model system we chose ethoxychlorocarbene,
17 (CH3CH2OCCl), whose potential surface is depicted sche-
matically in Figure 7, on the basis of the data presented in Table

Fragmentation of Alkoxychlorocarbenes
J. Phys. Chem. A, Vol. 106, No. 51, 2002 12287
TABLE 7: Geometry Parameters,a Energies (kcal/mol), and Partial Charges of C2H5OCCl Isomers and Transition States
trans-carbene
cis-carbene
1.487 Å
fragmentation TSb cis-trans-isom TS elimination TSc rearrangement TSd
CH2-O
1.469 Å
1.293 Å
1.815 Å
3.902 Å
114.89°
-112.32°
105.38°
179.20°
(0)f
2.278 Å
1.169 Å
2.459 Å
3.003 Å
1.467 Å
1.284 Å
1.912 Å
3.543 Å
126.11°
1.976 Å
1.182 Å
2.392 Å
3.694 Å
138.19°
1.884 Å
1.230 Å
2.018 Å
3.337 Å
O-C
1.274 Å
1.882 Å
2.968 Å
129.84°
180.00°
111.10°
0.00°
C-Cl
CH2‚‚‚Cl
H2C-O-C
CH3-CH2-O-C
O-C-Cl
124.82°
77.06°
-93.00°
101.31°
168.96°
107.11°
18.58°
112.51°
-96.45°
111.33°
CH2-O-C-Cl
B3LYP/6-31G*
-2.95°
30.00
96.11°
17.91
10.36°
22.63
-105.61°
38.53
1.02
B3LYP/6-31G*/SCI-PCM (0)g
0.71
1.20
47.08
19.75
34.37
43.67
-0.497/-0.633
-0.046/-0.059
+0.413/+0.577
16.41
20.94
46.34
-0.186/-0.232
-0.079/-0.074
+0.314/+0.434
15.57
27.93
43.94
-0.458/-0.593
-0.024/-0.027
+0.331/+0.456
33.89
42.59
44.97
-0.332/-0.415
-0.048/-0.056
+0.244/+0.315
CCSD(T)/cc-pVDZe
ZPE (B3LYP)
δ(Cl)i
(0)h
47.24
-0.098/-0.177 -0.156/-0.183
-0.214/ -0.176 -0.186/-0.197
+0.376/+0.401 +0.472/+0.507
δ(OC)i
δ(CH2)i
a For structures see Figure 6. b Transition state for loss of CO and concomitant formation of EtCl. c Transition state for simultaneous loss of CO
+ HCl, formation of ethylene. d Transition state for concerted Et-OCCl f Et-COCl rearrangement. e Single point at B3LYP/6-31G* geometry.
f Total energy: -652.666125 h. g Total energy: -652.672248 h. h Total energy: -651.611325 h. i CHelpG charge on the fragments (in vacuo/in
acetonitrile).
Figure 8. Structures of CH3CH2OCCl and corresponding species.
Due to homoconjugative stabilization of the incipient car-
bonium ion, the added methyl group assists the process of
heterolytic cleavage to give R+, CO, and Cl- 30 even more than
the above homolytic cleavage, although the energy for formation
of ions remains prohibitively high in the gas phase. However,
in acetonitrile, the energy of this assemblage of ions relative to
the trans-carbene falls to 22 kcal/mol, so that heterolytic
cleavage may indeed become competitive with homolytic
fragmentation in acetonitrile even for primary alkoxychloro-
carbenes. However, because the barriers for the concerted
processes described above are also lowered significantly by
solvation (see hashed bars in Figure 7), these processes are still
predicted to dominate, even in MeCN, at least in 17.
Hence the trans-carbene would rather isomerize to the cis-
carbene than to undergo fragmentation and the cis-carbene
prefers concerted loss of CO + HCl over elimination of CO
alone, both in the gas phase and in polar solvents. Thus our
calculations predict a change of mechanism on going from
nonpolar solvents where the two conformers of the alkoxy-
chlorocarbene interconvert prior to any fragmentation to polar
solvents where these processes are at least competitive. As in
the case of 16, formation of propanoyl chloride (EtCOCl)
requires homolytic cleavage and recombination of the fragments,
because the transition state for concerted rearrangement lies
significantly higher than the dissociation limit.
Benzyloxychlorocarbene. Finally, we repeated the above
calculations for PhCH2OCCl (1), which stands at the focus of
the present experimental investigation, and for which the greatest
amount of experimental data are available. This study revealed
some interesting new features that are highlighted in Figures 9
(on the basis of the data in Table 8) and 10. Unsurprisingly,
the benzyl substituent leads to a substantial stabilization of the
products of both homolytic and heterolytic ROCCl fragmenta-
tion. In fact, the latter process is even predicted to be slightly
exothermic in acetonitrile (in DCjE it is calculated to be
endothermic by 6.8 kcal/mol), thus supporting the idea that the
formation of ions is likely an important process in polar
solvents,3 although the pyridine ylide carbene-trapping experi-
ments indicate that heterolytic cleavage cannot be activationless,
even in acetonitrile.
The activation barrier for cis f trans isomerizaton of the
carbene remains essentially unaffected by the benzyl substituent.
In contrast, the barrier for loss of CO decreases to ca. 7 (B3LYP)
or 15 kcal/mol (CCSD(T)), respectively, whereas single point
SCI-PCM calculations in MeCN at the B3LYP gas-phase
geometry of the transition state even place this 2.4 kcal/mol
below the starting cis-carbene.31 This implies that the transition
state for loss of CO lies much earlier on the reaction coordinate
in the presence of MeCN; i.e., the gas-phase geometry of that
transition state represents a point well beyond the transition state

12288 J. Phys. Chem. A, Vol. 106, No. 51, 2002
Moss et al.
TABLE 8: Important Geometry Parameters,a Energies (kcal/mol), and Partial Charges of PhCH2OCCl, 1, Isomers and
Transition States for Fragmentation and Isomerization
trans-carbene (C1)
cis-carbene (Cs)
isomerization TSc (C1)
fragmentation TSb (C1)
H2C-O
1.481 Å
1.292 Å
1.815 Å
3.911 Å
114.52°
110.20°
105.43°
179.27°
-109.23°
(0)g
(0)
(0)
(0)
80.52
1.510 Å
1.270 Å
1.890 Å
2.975 Å
129.33°
108.11°
111.40°
0.00°
180.0°
0.78
-0.13
2.26
1.35
80.21
-0.168/-0.259
-0.175/-0.131
+0.431/+0.416
1.486 Å
1.281 Å
1.923 Å
3.558 Å
125.03°
109.05°
107.15°
96.46°
167.66°
17.96
16.57
22.53
21.46
2.204 Å
1.173 Å
2.397 Å
3.350 Å
127.03°
107.05°
109.83°
0.58°
94.64°
9.34
-0.20
18.89
17.39
78.35
-0.510/-0.668
-0.048/-0.084
+0.198/+0.282
O-C
C-Cl
H2C...Cl
H2C-O-C
C5H5-C-CH2-O
O-C-Cl
H2C-O-C-Cl
C-CH2-O-C
B3LYP/6-31G*
B3LYP/SCI-PCMd
MP2/6-31G* e
CCSD(T)/cc-pVDZf
ZPE
79.51
δ(Cl)h
-0.110/-0.139
-0.180/-0.181
+0.317/+0.340
-0.200/-0.244
-0.103/-0.105
+0.329/+0.378
δ(OC)h
δ(CH2)h
a For structures see Figure 10. b TS for loss of CO and concomitant formation of PhCH2Cl. c TS for cis f trans isomerization. d Single point
calculation at gas-phase B3LYP geometry, Total energy for trans-1: -844.406137 h. e At MP2/6-31G* optimized geometries, total energy for
trans-1: -842.589978 h. f Single point calculation at B3LYP geometry, total energy for trans-1: -842.809607 h. g Total energy: -844.399623 h.
h CHelpG charges for the fragments (in vacuo/in CH3CN).
subtle changes on going from ground to transition states cannot
be modeled accurately by either of the procedures at our
disposition.
In any event, all methods agree in predicting that the trans-
carbene would rather undergo cleavage to benzyl + COCl
(homolytic in nonpolar, heterolytic in polar solvents30) than to
isomerize to the cis-carbene, whereas the latter conformer is
depleted by loss of CO in a process that will have a very small
activation barrier in polar solvents.
In a next step we investigated the fate of cis-1 after it has
crossed the transition state for loss of CO. In the cases of the
methyl and ethyl derivatives, discussed above, IRC calculations
had indicated that the decay leads directly to the corresponding
alkyl chlorides. In the case of 1, the situation turned out to be
more complicated in that the IRC procedure (which amounts
to an infinitely damped decay along the bottom of a potential
valley leading from a transition state to a product) led to a
structure, slightly below the transition state, where the Cl atom
has moved into the plane of the benzyl moiety, bonded to two
hydrogens, as indicated in Figure 10. At this point the CO had
withdrawn so far from the PhCH2‚‚‚Cl complex that gradients
became too small for the IRC procedure to continue.
Removal of the CO and reoptimization of the remaining
complex led to a stationary point that turned out to be a transition
state in the gas phase that decays spontaneously to benzyl
Figure 9. Energies of PhCH2OCCl and products of rearrangement and
fragmentation by different methods relative to the trans-carbene.
chloride upon the slightest twisting of the benzylic CH2 group
or displacement of the Cl atom out of the plane. This complex
shows an unusually high degree of charge separation (Chelpg
charge of -0.67 on Cl), which indicates a strong ion-pair nature,
even in the gas phase. However, in the isolated species, this
feature turned out to be artifactual and due to the imposed
constraint of a closed-shell wave function. When this constraint
was removed, the energy dropped by 1.5 kcal/mol and the
species adopted partial triplet character ( S2 increased to a value
of 0.37) and the charge on Cl fell to -0.50. Concomitantly, a
negative spin of 0.33 appeared on the Cl atom, balanced by a
positive spin of the same magnitude in the π system of the
benzyl moiety. These results indicate that the above complex
has a high biradical character and that the lowest triplet and
closed-shell singlet surfaces are in very close proximity.32
A polar solvent can of course again change the situation.
Indeed, reoptimization of the above complex in acetonitrile by
in MeCN, which may explain why it lies below cis-1 in that
solvent. Clearly, the geometry of this very polar (and in MeCN
even more polar) transition state will be quite strongly affected
by the presence of the solvent. Because we were unable to locate
the transition state in acetonitrile, we cannot provide a prediction
for the activation barrier for the elimination of CO in this
solvent. However, we were able to confirm that the starting cis-
carbene still represents a minimum on the B3LYP/SCI-PCM
potential energy surface, so that this species is expected to have
a finite lifetime even in polar solvents.
We have no explanation for the substantial discrepancy of
the predictions made by the B3LYP and the CCSD(T) methods
for the activation energies of this process. Apparently, the
presence of the benzyl group’s π system increases the contri-
bution of dynamic electron correlation to the point where its

Fragmentation of Alkoxychlorocarbenes
J. Phys. Chem. A, Vol. 106, No. 51, 2002 12289
Figure 10. Structures of PhCH2OCCl and resulting species.
B3LYP/SCI-PCM led to a potential energy minimum where the
charge on Cl had increased to 0.90. We have not been able to
find a transition state for the decay of this species to benzyl
chloride, but our calculations seem to indicate that a tight
benzyl-chloride ion pair may indeed have a fleeting existence
in polar solvents (note that this species lies about 8 kcal/mol
below the separated ions in acetonitrile, ca. 4 kcal/mol in
dichloroethane), in contrast to nonpolar solvents where the
corresponding biradicaloid does not represent a minimum on
the potential surface.
give alkyl chlorides requires ca. 6 kcal/mol more activation
energy. At room temperature the COCl radicals formed by
homolytic cleavage will rapidly dissociate to CO + Cl• (∆H )
8 kcal/mol, ∆G close to zero)18 and recombination of the Cl
with the alkyl radical will also lead to alkyl chlorides. Thus,
the main products expected from fragmentation of alkoxy-
chlorocarbenes in nonpolar solvents are alkyl chlorides and
alkenes, in accord with experiment.
In polar solvents the two fragmentation transition states for
CH3OCCl and C2H5OCCl are stablized to the point where cis-
trans isomerization, loss of CO, and concerted loss of CO + Cl
all have similar activation energies. Our calculations yield no
indication that ion pairs intervene in these two fragmentation
processes; i.e., loss of CO leads directly to alkyl chlorides. In
acetonitrile, heterolytic cleavage becomes about 4 kcal/mol more
favorable than homolytic cleavage but still requires 5-8 kcal/
mol more energy than the above concerted processes (if
heterolysis is an activated reaction, it is even more strongly
disfavored compared to the other processes). In less polar
solvents, such as dichloroethane, heterolytic cleavage is probably
not competitive. Note that the COCl anion dissociates spontane-
ously to CO + Cl-, which can recombine with the alkyl cation
to yield alkyl chloride. Thus, the calculations do not lead us to
expect major changes in the product composition from frag-
mentations of these alkoxychlorocarbenes on going from polar
to nonpolar solvents.
(b) In the case of benzyloxychlorocarbene, the situation is
different in various respects. First, due to the resonance energy
of the incipient benzyl moiety, the transition states for all
fragmentations fall significantly below that for cis-trans-
isomerization, so that the latter process is unlikely to be of
importance in this case. Whereas the trans-carbene will undergo
simple cleavage (homolytic in nonpolar, heterolytic in polar
solvent), the cis-carbene will lose CO in a process whose exact
mechanism was found to be difficult to assess, mainly because
the predictions by different methods are at variance. According
to spin-restricted B3LYP calculations, loss of CO requires only
7.5 kcal/mol of activation and leads via a PhCH2+‚‚‚Cl-
complex with a high degree of charge separation, which
represents a transition state on the potential energy surface, to
benzyl chloride. Unrestricted calculations show, however, that
a diradicaloid state with a much smaller charge separation lies
ca. 1 kcal/mol below the ion pair whose nature is therefore
spurious.
All efforts to locate the transition state for concerted rear-
ragement of 1 to 8 failed, because in the presence of the phenyl
substituent the degree of negative charge that accumulated on
the COCl moiety near this transition state was high enough to
provoke spontaneous loss of Cl from that fragment.30 (In the
gas phase collapse to benzyl chloride + CO ensued.) However,
the fact that the activation barrier for concerted formation of
RCOCl barely changed on going from CH3OCCl to C2H5OCCl
indicates that its energy relative to 1 is not very sensitive to
substitution, from which we conclude that, if it exists, it must
also lie well above the dissociation limit in the case of 1. Hence,
formation of 8 requires dissociation, and because heterolytic
cleavage of 1 leads directly to CO + Cl-,30 8 will have no
chance of forming in polar solvents. Even in nonpolar solvents,
the COCl radical probably does not have a sufficient lifetime
at room temperature to recombine with the benzyl radical to
form 8 before it falls apart to CO + Cl, which explains the
absence of this species from the TRIR experiments. In contrast,
COCl persists in Ar matrixes and can reorient and recombine
with the benzyl radical, in part after photoactivation of the latter.
As in the previous cases, we were dismayed to note the large
discrepancies between the exothermicities predicted for the
fragmentation to RCl + CO by the B3LYP and the CCSD(T)
methods. Although we do not think that these discrepancies
affect our conclusions, it might be worthwhile to investigate
their causes.
Conclusions
The calculations described above lead us to the following
conclusions:
(a) Alkoxychlorocarbenes in the gas phase or in nonpolar
solvents undergo rapid cis-trans rotamerization at room tem-
perature (Ea ) 16-18 kcal/mol) before any fragmentation can
intervene. Homolytic cleavage of the trans-carbene and con-
certed loss of CO + HCl from the cis-carbene are predicted to
be competitive (Ea ) 20-25 kcal/mol), whereas loss of CO to
In acetonitrile, the above ion pair turns into a minimum on
the potential surface, which indicates that an ion pair of finite
lifetime may also be involved in the decay of cis-benzyloxy-

12290 J. Phys. Chem. A, Vol. 106, No. 51, 2002
Moss et al.
chlorocarbene in sufficiently polar solvents. In the absence of
suitable trapping agents this ion pair, which lies about 12 kcal/
mol below individually solvated PhCH2+ + Cl-, will, however,
invariably decay to PhCH2Cl. Thus, product analysis cannot
yield any mechanistic information in this case.
Finally, we mention that preliminary experiments with
cyclopropylmethoxychlorocarbene and cyclobutoxychlorocar-
bene indicate that the fragmentations and alkyl group rearrange-
ments that afford mixtures of cyclopropylmethyl chloride,
cyclobutyl chloride, and allylcarbinyl chloride in acetonitrile
and dichloroethane28a,33 persist in pentane. Given the unlikely
intervention of ion pairs in the latter solvent, deep-seated
concerted rearrangements must be coupled with the fragmenta-
tions of the carbenes. Further experimental and theoretical
studies are planned.
were identified by GC and GC-MS comparisons to commercial
or synthesized authentic samples. Product distributions are
detailed in Tables 1, 3, and 4.
LFP experiments were performed with the apparatus de-
scribed in ref 11.22 Results are described above and in Table 5.
Products from diazirine 6c. 1-Octene (10) and 1-chlorooctane
(11) were identified by comparisons to commercial samples.
2-chlorooctane (12) was prepared by the method of Hudson.34
Thionyl chloride (13.7 mL, 189 mmol) was added dropwise
with strirring to a mixture of 2-octanol (10 mL, 63 mmol) and
pyridine (5.1 mL, 63 mmol). Overnight reflux afforded a brown
liquid from which SOCl2 was stripped at the water pump. The
residue was dissolved in 100 mL of ether, extracted with 2 ×
100 mL of brine, 100 mL of saturated aqueous Na2CO3, and
again with 100 mL of brine. The ethereal phase was dried over
Na2SO4 and stripped, affording 7.0 g (47 mmol, 75%) of
1
2-chlorooctane. H NMR35 (δ, CDCl3): 4.0 (m, 1 H, CHCl);
Experimental Section
1.67 (m, 2 H, CH2CHCl); 1.48 (d, J ) 3 Hz, 3 Hz, 3 H, CH3-
CHCl); 1.26 (m, 8 H, CH3(CH2)4); 0.87 (m, 3 H, CH3CH2).
The sample also contained 1- and 2-octene.
Solvents. Acetonitrile, benzene, and pyridine (Fisher, Certi-
fied ACS) were refluxed over CaH2, distilled, and stored over
5A molecular sieves. Dichloroethane, cyclohexane (Aldrich,
Certified ACS), and pentane (Fisher, HPLC grade) were stored
over 5A molecular sieves and then used directly.
Diazirines. The preparations of diazirines 6a1,4 and 6b14 have
been described. Here we will detail the preparation of 3-n-
octyloxy-3-chlorodiazirine (6c), and its precursor isouronium
salt (7c), as illustrative of all cases.
O-n-Octyloxyisouronium Trifluoromethanesulfonate (7c, X )
OTf). This material was prepared by the general method of ref
13. 1-Octanol (20 mL, 127 mmol), cyanamide (1.07 g, 25
mmol), and trifluoromethanesulfonic acid (2.25 mL, 25 mmol)
were stirred magnetically for 24 h at ambient temperature in a
100 mL round-bottom flask protected by a drying tube. Pentane
(200 mL) was added, the mixture was refrigerated, and 6.0 g
(73%) of 7c, was harvested by filtration, recrystallized from
CH2Cl2 and dried in vacuo; mp 79-80 °C. 1H NMR (δ, DMSO-
d6): 8.34-8.40 (br s, 4 H, 2NH2), 4.22 (t, J ) 6 Hz, 2 H,
OCH2), 1.66 (m, 2 H, OCH2 CH2), 1.23-1.27 (m, 10 H, (CH2)5),
0.87 (crude t, 3 H, CH3).
1-Octyl formate (13) was prepared36 from 3.5 mL (22 mmol)
of 1-octanol, 8.0 mL (220 mmol) of formic acid, and 2.0 mL
of boron trifluoride-dimethanol complex. The mixture was
heated for 30 min at 85 °C, and then at 125 °C (reflux) for 1 h.
Distillation at 45 °C then removed the BF3 complex. The residue
was dissolved in 100 mL of ether, washed with 2 × 100 mL of
brine, saturated aqueous Na2CO3 until no CO2 evolution was
observed, and again with 100 mL of brine. The ether phase
was dried over Na2SO4, and then stripped to give 2.6 g (16.4
1
mmol, 75%) of 1-octyl formate. H NMR37 (δ, CDCl3): 7.91
(s, 1 H, OCH); 4.00 (t, J ) 6 Hz, 2 H, OCH2); 1.49 (m, 2 H,
OCH2CH2); 1.12 (m, 10 H, (CH2)5); 0.72 (t, J ) 6 Hz, 3 H,
CH3).
Matrix Isolation and Spectroscopy. Diazirine 6a was mixed
with a 1000-fold excess of argon, and the mixture was deposited
on a CsI crystal held at ca. 20 K inside an APD Cryogenics
HC-2 closed-cycle cryostat. After about 5 mmol of Ar was
deposited, the inlet valve was closed and the matrix was cooled
to ca. 8 K. Irradiations at the wavelengths indicated in the text
were effected with a 1 kW Hg-Xe lamp through the appropriate
cutoff or interference filters. UV/vis spectra were recorded on
a PE Lambda 900 instrument, and IR spectra were observed on
a Bomem DA3 interferometer.
Anal. Calcd. For C10H21F3N2O4S: C, 37.23; H, 6.57; N, 8.70.
Found: C, 37.01; H, 6.44; N, 8.60.
3-n-Octyloxy-3-chlorodiazirine (6c). The method of Graham
was used.12 Lithium chloride (4.2 g), DMSO (90 mL), isouro-
nium salt 7c (1.0 g, 3.1 mmol), and 40 mL of pentane were
combined, stirred magnetically, and maintained at <20 °C (ice
bath), while 150 mL of 12% commercial aqueous NaOCl (“pool
chlorine”), saturated with NaCl, was added slowly over 30 min.
The mixture was separated; the pentane phase was washed twice
with ice water and then dried over CaCl2 at 0 °C for 2 h. The
pentane solution of diazirine 6c was passed through a short silica
gel column. Pentane could then be removed by rotary evapora-
tion and replaced by another solvent of choice (e.g., MeCN,
benzene). About 30 mL of diazirine solution was thus prepared.
UV: λmax 348, 366 nm (pentane); 362 nm (MeCN); 350, 370
Theoretical Methods
The geometries of all species were fully optimized by the
B3LYP density functional method,38 using the 6-31G* basis
set. All stationary points were characterized by second derivative
calculations. In the case of MeOCCl, and at selected geometries
of PhCH2OCCl, stationary points were re-optimized in a
dielectric continuum whose permittivity corresponds to that of
acetonitrile, using the SCI-PCM method.39 In the other cases,
the same method was used in single-point calculations at the
geometries optimized in vacuo to assess the influence of
solvation.
Single-point calculations were also carried out by the coupled-
cluster method, which accounts for the effect of single and
double excitations and includes a perturbative estimate of triple
excitations (CCSD(T)), using Dunning’s correlation-consistent
polarized valence double-ú (cc-pVDZ) and triple-ú basis sets
(cc-pVTZ, the latter only in the case of MeOCCl).40,41
Atomic charges were calculated according to the ChelpG
method.42 All calculations were done with the implementation
of the above methods in the Gaussian9443 (solvation studies)
1
nm (benzene). H NMR (δ, C6D6): 3.48 (t, J ) 6 Hz, 2H,
OCH2); 1.22-1.00 (m, 12 H, (CH2)6); 0.84 (t, J ) 7.5 Hz, 3H,
CH3).
Photolysis of Diazirines. Solutions of diazirines 6 (3 mL in
solvent of choice, A ∼ 1.0 at λmax) were photolyzed in quartz
cuvettes for 1 h at 25 °C with a focused 200 W Oriel UV lamp,
λ > 320 nm (uranium glass filter). UV spectroscopy indicated
the absence of diazirine after photolysis. The photolysis products
were analyzed by capillary GC and GC-MS using a 30 m ×
0.32 mm × 0.25 µm (film) HP-5 (5% PhMe siloxane) column
at 30 °C (2 min), programmed to 260 °C at 30 deg/min. Products

Fragmentation of Alkoxychlorocarbenes
J. Phys. Chem. A, Vol. 106, No. 51, 2002 12291
and Gaussian9844 suites of programs. Cartesian coordinates and
total energies (including thermal corrections) of all stationary
points are available in ASCII form from the Supporting
Information.
(19) Cf.: Moss, R. A.; Zheng, F.; Sauers, R. R.; Toscano, J. P. J. Am.
Chem. Soc. 2001, 123, 8109.
(20) The dichloride product increased to 22% when carbene 4 was
generated in benzene that contained 3.5 M added HCl. The dichloride was
not formed in benzene that contained 4.0 M pyridine (which scavenges
HCl4).
(21) Jackson, J. E.; Soundararajan, N.; Platz, M. S.; Liu, M. T. H. J.
Am. Chem. Soc. 1988, 110, 5595.
Acknowledgment. The authors at Rutgers and at Johns
Hopkins are grateful to the National Science Foundation for
financial support and to the center for Computational Neuro-
science of Rutgers University (Newark) for computational
support. J.P.T. acknowledges NSF Faculty Early Career De-
velopment and Camille Dreyfus Teacher-Schlolar Awards. The
work in Fribourg was supported by the Swiss National Science
Foundation, project No. 2000-061560.00.
(22) For a description of our LFP system, see ref 11. The 1000 W Xe
monitoring lamp described there has now been replaced with a Photophysics
LS.1, 150 W pulsed Xe lamp light source.
(23) In MeCN, ylide 21 absorbs at 460 nm.4
(24) Ambiphilic carbenes (such as 1) react “slowly” with pyridine; ky
) 9 × 105 for MeOCCl in MeCN: Ge, C.-S.; Jang, E. G.; Jefferson, E. A.;
Liu, W.; Moss, R. A.; Wlostowska, J.; Xue, S. Chem. Commun. 1994, 1479.
(25) The average value of ky ) (2.9 ( 0.2) × 105 M-1 s-1
.
(26) (a) Iwata, K.; Hamaguchi, H. Appl. Spectrosc. 1990, 44, 1431. (b)
Yuzawa, T.; Kato, C.; George, M. W.; Hamaguchi, H. Appl. Spectrosc.
1994, 48, 684. (c) Toscano, J. P. AdV. Photochem. 2001, 26, 41.
(27) At [Cl-] ) 0, the Y-intercept of Figure 4 should equal kobs for the
formation of ylide at 5.77 M pyridine. Agreement is observed; the intercept
Supporting Information Available: Geometries for all
structures optimized in this work and further information about
partial atomic charges. This material is available free of charge
via the Internet at http://pubs.acs.org.
is 2.5 × 106 s-1, whereas the experimental kobs ) 2.7 × 106 s-1
.
(28) (a) Moss, R. A.; Zheng, F.; Johnson, L. A.; Sauers, R. R. J. Phys.
Org. Chem. 2001, 14, 400. (b) Moss, R. A.; Zheng, F.; Fede; J.-M.; Ma;
Y.; Sauers, R. R.; Toscano, J. P.; Showalter, B. M. J. Am. Chem. Soc. 2002,
124, 5258.
References and Notes
(29) The different quantum chemical methods employed in this work
differ strongly in their prediction of D0 of the COCl radical (B3LYP/6-
31G*, +11.45; CCSD(T)/cc-pVDZ, -0.45; CCSD(T)/cc-pVTZ, +4.1 kcal/
mol), so that we prefer to use experimental data here.
(1) Moss, R. A.; Wilk, B. K.; Hadel, L. M. Tetrahedron Lett. 1987,
28, 1969.
(2) Moss, R. A.; Kim, H.-R. Tetrahedron Lett. 1990, 31, 4715.
(3) Moss, R. A. Acc. Chem. Res. 1999, 32, 969.
(4) Moss, R. A.; Ge, C.-S.; Maksimovic, L. J. Am. Chem. Soc. 1996,
118, 9792.
(30) COCl- is not bound and undergoes activationless cleavage to CO
+ Cl- even in the gas phase.
(31) Note that according to IRC calculations, fragmentation occurs from
a slightly different conformer of the cis-carbene which lies a fraction of a
kcal/mol above the lowest energy structure (cf., Figure 10).
(32) Reoptimization of the complex with UB3LYP (Guess)mix.,
always) led to a slightly different geometry, which did, however, still
represent a saddle point on the potential surface.
(5) Moss, R. A.; Johnson, L. A.; Yan, S.; Toscano, J. P.; Showalter,
B. M. J. Am. Chem. Soc. 2000, 122, 11256.
(6) For ambiphilic carbenes such as 1, methanolic trapping can be rather
slow, with k ∼ 104-106 s-1
: Du, X.-M.; Fan, H.; Goodman, J. L.;
Kesselmayer, M. A.; Krogh-Jespersen, K.; LaVilla, J. A.; Moss, R. A.; Shen,
S.; Sheridan, R. S. J. Am. Chem. Soc. 1990, 112, 1920.
(33) Moss, R. A.; Ho, G. J.; Wilk, B. K. Tetrahedron Lett. 1989, 30,
2473.
(7) Smith, N. P.; Stevens, I. D. R. J. Chem. Soc., Perkin Trans. 2 1979,
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Bull. Akad. Sci. USSR, DiV. Chem. Sci. 1983, 10, 2090.
(8) Alkoxy- and aryloxychlorocarbenes exist in cis or trans forms,
where partial double bond character restricts rotation about the O-C
(carbene) bond: Kesselmayer, M. A.; Sheridan, R. S. J. Am. Chem. Soc.
1986, 108, 99 and 844.
(36) Dymicky, M. Org. Prep. Proced. Int. 1982, 14, 177.
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100, 16098. Self-consistent reaction field methods model solvation effects
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in the SCF computations.
(9) The question how to define an intimate ion pair arises here. We
quote from a classic formulation by S. Winstein and G. C. Robinson of
1958: “some covalent character may be visualized for the cation-anion
attraction in an intimate ion pair. ... Because of the character of intimate
ion pairs, there is no sharp distinction between such an ion pair and a
covalently bound intermediate in a so-called cyclic rearrangement. These
are not qualitatively distinct, but form extremes in a graded series. Thus,
there is no sharp distinction between formation of an intimate ion pair
followed by internal return and a cyclic rearrangement, and marginal cases
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(14) Diazirine 6b (and isouronium salt 7b) are described in: Johnson,
L. A. Ph.D. Dissertation, Rutgers University, New Brunswick, NJ, 2001.
For diazirine 6c, see the Experimental Section.
(15) Benzyl radical was observed spectroscopically upon LFP of 6a in
MeCN, DCE, or pentane solutions (λ 303, 315 nm16). However, photolysis
of 6a in BrCCl3 or cumene gave largely fragmentation product PhCH2Cl,
rather than the radical products PhCH2Br (10%) or toluene (2%), suggesting
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(16) McAskill, N. A.; Sangster, D. F. Aust. J. Chem. 1977, 30, 2107.
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is less than 1 kcal‚mol-1! In solution, where the translational degrees of
freedom of the two fragments are not fully realized, ∆S is a bit smaller,
but ∆G is still so low that the lifetime of COCl is expected to be vanishingly
small at room temperature.
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