Ultrafast UV-Vis and IR studies of p-biphenylyl acetyl and carbomethoxy carbenes

Published on Web 07/26/2008

Ultrafast UV-Vis and IR Studies of p-Biphenylyl Acetyl and

Carbomethoxy Carbenes

†

‡

,†

Jin Wang, Gotard Burdzinski, Jacek Kubicki, and Matthew S. Platz*

Department of Chemistry, The Ohio State UniVersity, 100 West 18th AVenue, Columbus, Ohio,

4

3210, and the Quantum Electronics Laboratory, Faculty of Physics, Adam Mickiewicz

UniVersity, 85 Umultowska, Poznan 61-614, Poland

Received May 2, 2008; E-mail: platz.1@osu.edu

Abstract: The photochemistry of a p-biphenylyl diazo ester (BpCN

2 2 3

CO CH ) and diazo ketone

(

BpCN COCH ) were studied by ultrafast time-resolved UV-vis and IR spectroscopies. The excited states

2

3

of these diazo compounds were detected and found to decay with lifetimes of less than 300 fs. The diazo

ester produces singlet carbene with greater quantum efﬁciency than the ketone analogue due to competing

Wolff rearrangement (WR) in the excited state of the diazo ketone. Carbene BpCCO CH has a singlet-triplet

2 3

gap that is close to zero in cyclohexane, but the triplet is the ground state. The two spin states are in rapid

equilibrium in this solvent relative to reaction with cyclohexane. There is (for a carbene) a slow rate of

singlet to triplet intersystem crossing (isc) in this solvent because the orthogonal singlet must rotate to a

higher energy orientation prior to isc. In acetonitrile and in dichloromethane BpCCO

2 3

CH has a singlet

ground state. Ketocarbene BpCCOCH has a singlet ground state in cyclohexane, in dichloromethane,

3

and in acetonitrile and decays by WR to form a ketene detected by ultrafast IR spectroscopy in these

solvents. Ketocarbenes have more stable singlet states, relative to carbene esters, because of the superior

conjugation of the ﬁlled hybrid orbital of the carbene with the π system of the carbonyl group, the same

factor that makes methyl ketones more acidic than the analogous esters. The rate of WR of BpCCOCH

3

is faster in cyclohexane than in dichloromethane and acetonitrile because of intimate solute-solvent

interactions between the empty p orbital of the carbene and nonbonding electron pairs of heteroatoms of

the solvent. These interactions stabilize the carbene and retard the rate of WR.

1

. Introduction

The conversion of R-diazo ketones into ketenes was discov-

Scheme 1. s-Z and s-E Conformers for R-Diazo Carbonyl

Compounds

1

ered in 1902. This reaction, now known as Wolff rearrangement

(

WR), has been extensively applied in many diverse ﬁelds, such₂

as synthetic chemistry, photolithography, and in drug delivery.

Due to the importance of the WR, it has been extensively studied

both theoretically and experimentally.

R-Diazo carbonyl compounds usually have a planar conﬁg-

uration of the OdC-CdN2 group. Rotation around the C-C

bond will generate two conformers, s-Z and s-E as shown in

Scheme 1. The C-C bond possesses partial double-bond

character in the resonance structure of the s-Z conformer

Scheme 2. Proposed Reaction Pathways for p-Biphenylyl Diazo

Ketone and Ester

(

Scheme 1). An attractive interaction between the positively

charged nitrogen and the negatively charged oxygen atoms also

2

contributes to the stabilization of the s-Z conformer.

Two mechanisms have been proposed for WR in R-diazo

2

carbonyl compounds (Scheme 2). One pathway is a concerted

mechanism in which the migrating group moves as the dini-

trogen is extruded. The other possibility is a stepwise mecha-

nism, in which the diazo compound loses nitrogen to form the

carbene and, subsequently, the carbene undergoes WR to form

a ketene and/or undergoes bimolecular reactions. In 1966,

3

Kaplan and Meloy proposed that ketene formation is favored

upon decomposition of the s-Z conformer of R-carbonyl

†

The Ohio State University.

Adam Mickiewicz University.

‡

(

1) Wolff, L. Justus Liebigs Ann. Chem. 1902, 325, 129–195.

(

2) Kirmse, W. Eur. J. Org. Chem. 2002, 2193–2256.

(3) Kaplan, F.; Meloy, G. K. J. Am. Chem. Soc. 1966, 88, 950–956.

J. AM. CHEM. SOC. 2008, 130, 11195–11209 9 11195

1

0.1021/ja803096p CCC: $40.75  2008 American Chemical Society

A R T I C L E S

Wang et al.

-

1

Table 1. B3LYP/6-311+G(d,p)//B3LYP/6-31G(d) Calculations of Gibbs Free Energy Differences (kcal·mol ) for syn and anti Conformers of

a

BpCN2CO2CH3 and BpCN2COCH3 in the Gas Phase, Cyclohexane, Dichloromethane, and Acetonitrile

∆

G

298K

syn %

anti %

∆G298K

syn %

anti %

b

gas phase

cyclohexane

-1.81

-1.58

-1.14

-1.03

4

6

13

15

96

94

87

85

-1.15

-0.83

-0.27

-0.09

13

20

39

46

87

80

61

54

c

dichloromethane

acetonitrile

c

a

b

c

Bp ) p-biphenylyl. The zero-point energy (ZPE) for the gas-phase calculations is corrected by a factor of 0.9806. The solution-phase

calculations used PCM models, and the geometries are from the gas-phase B3LYP/6-31(d) calculations.

compounds. In contrast, it was proposed that s-E conformers

preferentially decompose to form trappable carbenes. This rule

has been supported by numerous experiments.

suppressed. They proposed a concerted mechanism in which

ketene formation proceeds from the excited state of the diazo

precursor without intervention of a carbene. This is an example

of a “rearrangement in the excited state” (RIES) mechanism.

Another possibility, which was not considered by the authors,

is that ketene formation may involve a stepwise mechanism and

arise from very short-lived “hot” carbenes (Scheme 2). Upon

photolysis of the diazo ketone, it can decompose to form

carbenes with excess vibrational energy. These “hot” ketocar-

benes can rapidly overcome the barrier for WR to form ketenes

before they thermalize or react with solvents.

2

The structures of carbonyl-substituted carbenes in their singlet

and triplet states have been thoroughly studied by modern

4

theoretical methods. Triplet acetylcarbene and carbomethoxy-

carbene are lower in energy than the most stable singlet state.

Simple triplet carbonyl carbenes are predicted to have the

carbonyl group reside in the same plane as that deﬁned by the

carbene carbon and the two atoms to which it is directly bound.

The story is quite different in simple singlet carbonyl carbenes

in which the carbonyl group is nearly orthogonal to the plane

of the carbene carbon and its two directly bonded atomic

substituents. Singlet carbenes adopt this conformation to allow

the ﬁlled hybrid orbital of the carbene to interact with the π*

system of the carbonyl group. This provides the same type of

stabilization present in enolate ions.

Assuming that the conformational differences between the

7

phenyl diazo ketone and ester are slight, Tomioka’s studies

3

illustrate that the Kaplan and Meloy rules are difﬁcult to apply

to photochemical reactions. In a photochemical experiment both

syn and anti conformers are excited, but theory has not yet

predicted whether or not excited syn or anti conformers

decompose more or less rapidly than they undergo intercon-

The Wolff rearrangement of carbonyl carbenes has also

4

3

received considerable study. The potential energy surface of

version. One cannot expect the Kaplan and Meloy rules to apply

acetyl carbene is rather ﬂat. Acetyl carbene rapidly interconverts

with the isomeric oxirene and rearranges to ketene over a small

if indeed conformational interconversion is rapid on excited-

state surfaces.

(

2.7-6.3 kcal/mol) barrier. In the case of carbomethoxycarbene

the barrier to Wolff rearrangement was predicted to be between

.4 and 7.2 kcal/mol, or similar to that of acetylcarbene. Halogen

In this paper, we will discuss the application of state of the

art ultrafast time-resolved laser spectroscopy with UV-vis and

IR detection methods to investigate the photochemistry of a

biphenylyl diazo ketone and a biphenylyl diazo ester. We will

try to distinguish the RIES mechanism from the “hot carbene

mechanism”. In addition, WR of ketocarbenes will be found to

proceed within 1 ns in ﬂuid solution. Some ketocarbene lifetimes

2

substituents on the carbene center stabilize the singlet relative

to the triplet state. In these carbenes the singlet is predicted to

be the ground state, and the barrier to Wolff rearrangement of

chlorocarbomethoxy carbene increases to 13.9 kcal/mol.

Diazo esters and ketones are also known to experience

different photochemistry. Wang et al. studied 2-NpCN2CO2-

CH3 using nanosecond time-resolved infrared spectroscopy and

found that the formation of ketene solely arises from the carbene,

have been indirectly measured by the pyridine ylide method

2

5

8–10

with the aid of various assumptions.

In this study, we are

able to directly monitor the carbene decay and measure the rate

of WR in various solvents.

6

indicating a stepwise mechanism. The Tomioka group inves-

tigated the photochemistry of PhCN2CO2CH3 and found that

photolysis of this diazo compound in neat alcohols does not

afford any WR-derived product. This observation again indicates

that the concerted mechanism of WR is not operative upon

photolysis of the parent diazo ester, PhCN2CO2CH3.

Tomioka et al. studied the photochemistry of the related

diazo ketone, PhCN2COCH3, and found that even in methanol,

an excellent carbene trap, the WR-derived product cannot be

2. Ultrafast Spectroscopic Results

p-Biphenylyl diazo compounds are studied instead of the

parent phenyl compounds because the diazo excited states and

carbenes absorb at longer wavelengths than the phenyl analogues

and as a result are more easily studied in the polycyclic aromatic

systems. First, the ground-state geometries of BpCN2CO2CH3

and BpCN2COCH3 were calculated in the gas phase and in

several solvents. The results are collected in Table 1 and predict

that anti rotomers of the ester and ketone will predominate in

7

(

4) Scott, A. P.; Platz, M. S.; Radom, L. J. Am. Chem. Soc. 2001, 123,

6

069–6076.

5) Wang, Y.; Yuzawa, T.; Hamaguchi, H.-o.; Toscano, J. P. J. Am. Chem.

Soc. 1999, 121, 2875–2882.

(8) Wang, J.-L.; Toscano, J. P.; Platz, M. S.; Nikolaev, V.; Popik, V.

J. Am. Chem. Soc. 1995, 117, 5477–5483.

6) Fujiwara, Y.; Tanimoto, Y.; Itoh, M.; Hirai, K.; Tomioka, H. J. Am.

Chem. Soc. 1987, 109, 1942–1946.

(9) Toscano, J. P.; Platz, M. S.; Nikolaev, V. J. Am. Chem. Soc. 1995,

117, 4712–4713.

7) Tomioka, H.; Okuno, H.; Kondo, S.; Izawa, Y. J. Am. Chem. Soc.

(10) Toscano, J. P.; Platz, M. S.; Nikolaev, V.; Popic, V. J. Am. Chem.

Soc. 1994, 116, 8146–8151.

1

980, 102, 7123–7125.

1

1196 J. AM. CHEM. SOC. 9 VOL. 130, NO. 33, 2008

Ultrafast Spectroscopy of Carbenes

A R T I C L E S

1

the latter to singlet carbene ester BpCCO2CH3. Time-dependent

density functional theory (TD-DFT) calculations predict that

1

BpCCO2CH3 absorbs at 368 nm (f ) 0.4569, Table S1), in

excellent agreement with the experimental results. The absorp-

tion band of BpCCO2CH3 sharpens with a time constant of

1

5

3

.5 ps and decays biexponentially with time constants of 20 (

ps and a few nanoseconds (Figure S1a). The fast components

(

5.5 and 20 ps) are assigned to vibrational cooling of the hot

1

2,19–23

carbene.

We speculate that the growth of the

1

BpCCO2CH3 signal is due to different oscillator strengths of

2

4

the vibrationally hot and the thermalized carbene. The slow

component is assigned to the intrinsic lifetime for the relaxed

singlet carbene, but its lifetime cannot be measured accurately

with our ultrafast spectrometer. Thus, our best estimate of τ of

the relaxed carbene is ∼2 ns in cyclohexane.

Nanosecond (ns) laser ﬂash photolysis (LFP) (λex ) 308 nm)

studies were performed to determine the ground-state multiplic-

ity of BpCCO2CH3 in argon-saturated cyclohexane, and the

spectra are shown in Figure S2. A transient absorption band

absorbing between 320 and 400 nm is observed 10 ns after the

laser pulse and decays with a time constant of 49 ns (Figure

S3a). A new transient, centered at 335 nm, can be observed 1

µs after the laser pulse, which decays biexponentially with time

constants of 20 and 90 µs (Figure S3b). The initially observed

transient on the nanosecond time scale (red spectrum of Figure

3

S2) is assigned to the triplet state of the carbene ( BpCCO2CH3),

and the transient observed at longer delay times (blue spectrum

of Figure S2) is assigned to the radical HR, formed by hydrogen

atom abstraction by the triplet carbene from the solvent. TD-

3

DFT calculations predict that BpCCO2CH3 and radical HR

absorb at 353 nm (f ) 0.5330, Table S2) and 358 nm (f )

0

.4736, Table S3), respectively, in good agreement with the

experimental results.

Figure 1. Transient spectra produced by ultrafast photolysis of

BpCN2CO2CH3 in cyclohexane. The spectra were generated by ultrafast

LFP (λex ) 308 nm) with time windows of (a) 0.10-1.00 ps, (b) 1-20 ps,

and (c) 20-3000 ps.

It is interesting to note that there is a shoulder accompanying

the triplet carbene transient absorption band, from 370 to 430

nm (red spectrum of Figure S2). This shoulder (spectral feature)

decays with the same rate as the triplet carbene. We posit that

this shoulder feature of the spectrum can be assigned to the

singlet carbene because it is similar to the singlet carbene

absorption obtained by ultrafast photolysis (Figure 1). Since the

absorption coefﬁcients of singlet and triplet BpCCO2CH3 are

all solvents under study. Thus, differences in the photochem-

istries of the diazo ester and ketone will not be due to differences

in their ground-state geometries. We note that NMR analysis

of both diazo compounds is consistent with rapid interconver-

sion; thus, the barriers connecting the rotomers are small and

are likely in the range of 13s16 kcal/mol observed with similar

2

compounds.

2

.1. Weakly Coordinating Solvents. Ultrafast photolysis (λex

(

15) Wang, J.; Burdzinski, G.; Kubicki, J.; Gustafson, T. L.; Platz, M. S.

)

308 nm) of BpCN2CO2CH3 in cyclohexane produces the

J. Am. Chem. Soc. 2008, 130, 5418–5419.

transient spectra shown in Figure 1. A broadly absorbing

transient with absorption maximum at 475 nm is formed within

the laser pulse and decays in 300 fs. As this transient absorption

decays, a new transient is observed with λmax ) 375 nm.

Following our previous studies,

to an excited state of the diazo precursor BpCN2CO2CH3* and

(16) Wang, J.; Kubicki, J.; Gustafson, T. L.; Platz, M. S. J. Am. Chem.

Soc. 2008, 130, 2304–2313.

(

17) Wang, J.; Kubicki, J.; Peng, H.; Platz, M. S. J. Am. Chem. Soc. 2008,

30, 6604–6609.

1

(

18) Wang, J.; Zhang, Y.; Kubicki, J.; Platz, M. S. Photochem. Photobiol.

Sci. 2008, 7, 552–557.

11–18

the former band is attributed

1

(19) Laermer, F.; Elsaesser, T.; Kaiser, W. Chem. Phys. Lett. 1989, 156,

3

81–386.

(20) Miyasaka, H.; Hagihara, M.; Okada, T.; Mataga, N. Chem. Phys. Lett.

(

11) Wang, J.; Burdzinski, G.; Gustafson, T. L.; Platz, M. S. J. Org. Chem.

1992, 188, 259–264.

2

006, 71, 6221–6228.

(21) Schwarzer, D.; Troe, J.; Votsmeier, M.; Zerezke, M. J. Chem. Phys.

1996, 105, 3121–3131.

12) Wang, J.; Burdzinski, G.; Gustafson, T. L.; Platz, M. S. J. Am. Chem.

Soc. 2007, 129, 2597–2606.

(22) Elsaesser, T.; Kaiser, W. Annu. ReV. Phys. Chem. 1991, 42, 83–107.

(23) Burdzinski, G.; Hackett, J. C.; Wang, J.; Gustafson, T. L.; Hadad,

C. M.; Platz, M. S. J. Am. Chem. Soc. 2006, 128, 13402–13411.

(24) Mohammed, O. F.; Vauthey, E. J. Phys. Chem. A 2008, 112, 3823–

3830.

13) Wang, J.; Kubicki, J.; Hilinski, E. F.; Mecklenburg, S. L.; Gustafson,

T. L.; Platz, M. S. J. Am. Chem. Soc. 2007, 129, 13683–13690.

14) Burdzinski, G.; Wang, J.; Gustafson, T. L.; Platz, M. S. J. Am. Chem.

Soc. 2008, 130, 3746–3747.

J. AM. CHEM. SOC. 9 VOL. 130, NO. 33, 2008 11197

A R T I C L E S

Wang et al.

Scheme 3. Product Studies of Photolysis (308 nm) of

BpCN2CO2CH3 in Cyclohexane

by GC-MS, indicating that the unimolecular decay of

1

BpCCO2CH3 in cyclohexane (vide supra) is mainly controlled

by isc to form a spin equilibrium mixture rather than WR.

The analogous photolysis of BpCN2CO2CH3 was performed

in a 1:1 mixture of cyclohexane and cyclohexane-d12. Analyzing

the cyclohexane C-H/C-D insertion product ratio, we deduce

a kinetic isotope effect of 2.5, which is consistent with the

25,26

previouslyreportedvaluesofsingletcarbeneinsertionreactions.

1

Since BpCCO2CH3 decays with the same rate in cyclohexane

and in cyclohexane-d12, the C-H insertion product must be

formed from the carbene singlet-triplet equilibrium mixture.

The absence of ketene-derived product indicates that the spin

equilibrated carbene does not undergo WR in cyclohexane even

on a time scale of tens of nanoseconds.

not known, we cannot deduce the yields of the singlet and triplet

1

The absorption of BpCCO2CH3 at 3 ns is ∼35% of its

carbenes and the singlet-triplet (S-T) gap. However, ∆GST

-1

maximum absorption determined 20 ps after the laser pulse

must be within (1 kcal ·mol at 298 K in cyclohexane to allow

1

(

Figure 1c). Since the decay of BpCCO2CH3 is mainly

us to simultaneously detect both spin isomers. Note that at 298

-1

controlled by isc, it can be deduced that in the S-T equilibrium

K, a 1 kcal ·mol of the carbene S-T gap corresponds to a

spin equilibrium mixture containing 16% of one spin isomer

and 84% of the other. We will later provide evidence that the

triplet is the ground state of the carbene in cyclohexane and

that the slow intersystem crossing (isc) rate is due to the

difference in singlet and triplet geometries.

3

mixture there is no less than 65% of BpCCO2CH3 and no more

1

than 35% of BpCCO2CH3 present, which corresponds to an

3

1

equilibrium constant Keq ) [ BpCCO2CH3]eq/[ BpCCO2CH3]eq

1

3

g 1.86, where [ BpCCO2CH3]eq and [ BpCCO2CH3]eq are the

singlet and triplet carbene concentrations at equilibrium, and

-1

the S-T energy gap ∆GST g 0.4 kcal ·mol . Combining our

When the cyclohexane solution was saturated with oxygen,

dramatically different results were obtained (Figure S4). Again,

-

1

previous analysis that ∆GST is within (1 kcal ·mol , we

-

1

3

conclude that ∆GST is within 0.4-1 kcal·mol

and that

the triplet carbene BpCCO2CH3 absorption spectrum was

3

BpCCO2CH3 is the ground state in cyclohexane.

observed in the region between 320 and 400 nm, 10 ns after

the laser pulse, but in the presence of oxygen the triplet lifetime

is shortened to 25 ns (Figure S5). The long component (340

ns) of the kinetic trace probed at 335 nm is assigned to radical

HR. However, 1 µs after the laser pulse, a new band centered

at 425 nm was generated in the oxygen-saturated solution, which

is strikingly different from the results obtained in argon-saturated

1

The lifetime of BpCCO2CH3 shortens to 1.2 ( 0.1 ns in

cyclohexene (Figure S6). In neat alkene, the carbene ester reacts

with solvent in competition with unimolecular decay by isc,

presumably to form cyclopropanes.

Ultrafast photolysis of diazoketone BpCN2COCH3 produces

the transient spectra shown in Figure 2. The excited state,

1

BpCN2COCH3*, is the ﬁrst species observed in the visible

cyclohexane solution. The lifetime of this species is greater than

6

region, and it decays within the instrument response time (300

2

00 µs. Following Tomioka’s studies, the carrier of transient

fs) to form singlet ketocarbene with λmax ) 365 nm. TD-DFT

absorption centered at 425 nm is assigned to the carbene

carbonyl oxide CO.

1

calculations predict that BpCCOCH3 absorbs at 348 nm (f )

0

.5041, Table S4), in good agreement with the experimental

results. The transient absorption in the region of 420 and 550

nm observed at longer delay times (Figure 2b) is due to the

re-excitation of the photoproduct ketene (cf., the Experimental

Section). Vibrational cooling (VC) of the carbene is observed

and has a time constant of 11 ( 3 ps (Figure S7). The relaxed

ketocarbene decays an order of magnitude more rapidly in

cyclohexane (τ ) 180 ( 20 ps, Figure S7) than the analogous

carbene ester (Figure S1). The lifetime of the relaxed ketocar-

Very similar ultrafast results were obtained in cyclohexane-

d12. The singlet carbene has the same lifetime in cyclohexane-

d12 (Figure S1b) as in cyclohexane. Clearly, the lifetime of

BpCCO2CH3 in cyclohexane is not controlled by reaction with

solvent and must be controlled instead by a unimolecular

process, either WR or isc.

To better appreciate the sign and magnitude of ∆GST in

cyclohexane, BpCN2CO2CH3 (∼1 mg/mL) was photolyzed (λex

1

bene BpCCOCH3 is the same in cyclohexane, cyclohexane-

1

d12 (Figure S7b), and in cyclohexene (Figure S8). This

1

demonstrates that the lifetime of BpCOCH3 is controlled by a

very rapid unimolecular processes, even in the more reactive

alkene solvent.

Nanosecond LFP studies were also performed for BpCN2-

COCH3 in argon-saturated cyclohexane. We could not observe

)

308 nm) in cyclohexane. After the photolysis was complete,

3

any spectral signatures of the triplet ketocarbene BpCCOCH3.

ethanol was added to quench formation of the ketene reaction

products. The reaction mixture was analyzed by NMR and gas

chromatography-mass spectrometry (GC-MS) (cf., the Ex-

perimental Section for details of product studies), and the results

of this product study are shown in Scheme 3. The major product

is the cyclohexane C-H insertion product (84%). The triplet

carbene double hydrogen abstraction product can also be

observed (7%) along with bicyclohexane (due to the combina-

tion of two cyclohexyl radicals). Interestingly, the WR product

does not form to an appreciable extent and cannot be detected

It is possible that a transient is hidden in the bleaching region

of BpCN2COCH3 that is centered at 300 nm. The transient

spectra observed centered at 300 nm does not decay on a time

window of 500 µs. We tentatively assign this transient to the

ketene, formed by WR of the carbene. This assignment is also

(

25) Barcus, R. L.; Hadel, L. M.; Johnston, L. J.; Platz, M. S.; Savino,

T. G.; Scaiano, J. C. J. Am. Chem. Soc. 1986, 108, 3928–3937.

26) Platz, M.; Admasu, A. S.; Kwiatkowski, S.; Crocker, P. J.; Imai, N.;

Watt, D. S. Bioconjugate Chem. 1991, 2, 337–341.

1

1198 J. AM. CHEM. SOC. 9 VOL. 130, NO. 33, 2008

Ultrafast Spectroscopy of Carbenes

A R T I C L E S

1

BpCCO2CH3 proceeds, a rising absorption centered at 340 nm

with a similar time constant (τ ) 120 ns, right panel of Figure

S10) is observed. The 340 nm absorbing species overlaps

1

severely with the spectrum of BpCCO2CH3 (Figure S11); thus,

1

the decay time constant of BpCCO2CH3 appears to differ

somewhat from the rise time constant of the 340 nm absorption

band because of the spectral interference. The lifetime of the

3

40 nm transient is 115 µs (Figure S12), and again, there is no

oxygen effect on the lifetime. Following previous studies, this

1

3

40 nm transient absorption is assigned to the BpCCO2-

CH3-acetonitrile ylide ACN-Y. TD-DFT predicts that the

acetonitrile ylide absorbs at 367 nm (f ) 0.4569, Table S6), in

good agreement with the experimental results. Furthermore, on

1

the basis of the lifetime of BpCCO2CH3 (180 ns), the absence

of an oxygen effect on the lifetime, and the lack of a spectral

3

1

signature for BpCCO2CH3, we conclude that BpCCO2CH3 is

the ground state of the carbene in acetonitrile.

The lifetime of the ketocarbene is 700 ( 30 ps in acetonitrile

Figure S13a). This is much shorter than that of the carbene

Figure 2. Transient spectra produced by ultrafast photolysis of

BpCN2COCH3 in cyclohexane. The spectra were generated by ultrafast LFP

(

λex ) 308 nm) with time windows of (a) 0.10-1.00 ps and (b) 2-500 ps.

ester but is much longer than the lifetime of the ketocarbene in

cyclohexane. The ketocarbene lifetime in acetonitrile-d3 (Figure

S13b) is the same as in acetonitrile indicating that the decay of

BpCCOCH3 in this solvent is unimolecular and most likely

due to WR as a nitrile ylide is not observed.

in accord with TD-DFT calculations for the ketene (λmax ) 302

nm, f ) 0.6321, Table S5).

1

A comparison of the two ﬁgures in Figure 3 reveals that the

optical yield of ketocarbene is consistently less than half of that

of the carbene ester, a trend which is found in other solvents as

well (Table 3). These experiments were performed on the same

day, using the same optical alignment of the spectrometer and

utilized solutions of equal diazo absorbance at the wavelength

of excitation (308 nm). This is reminiscent of our ultrafast

Nanosecond LFP (308 nm) of BpCN2COCH3 in acetonitrile

produces results similar to those obtained in cyclohexane.

3

Neither BpCCOCH3 nor the acetonitrile ylide is observed. The

only transient observed is the ketene, formed by WR, which

does not decay over a 500 µs time window.

Ultrafast photolysis of BpCN2COCH3 (λex ) 270 nm) with

1

2

studies of 1,2 hydrogen shift in the diazo excited states.

Following our previous study and a large body of literature,

IR detection was also performed in acetonitrile (Figure 6). In

1

2

-1

the region of 2060-2140 cm , the diazo bleaching band (2076

-1

we propose that the difference of the optical yields of the two

initially formed carbonyl carbenes is due to the fact that WR

proceeds efﬁciently in the diazo excited state of BpCN2COCH3

but is not a major deactivation pathway for the excited state of

BpCN2CO2CH3. A more detailed discussion will be given in

section 3.1.

cm ) is observed immediately after the laser pulse (300 fs).

-1

The ketene K band (2100 cm ) is also observed immediately

1

4,15

(τ < 0.4 ps) after the laser pulse.

The band narrows and

blue- shifts over 30 ps and then grows further over 700 ps

(Figure S14). The ketene is formed initially hot, and its band

blue-shifts and narrows 50 ps after the laser pulse. It is possible

that the ketene is formed by rearrangement of the hot singlet

2

.2. Coordinating Solvents. 2.2.1. Acetonitrile. The photo-

1

#

chemistries of BpCN2COCH3 and BpCN2CO2CH3 were also

studied in acetonitrile (ACN). The results are given in Figures

4

solvation dynamics are observed along with transients formed

by re-excitation of persistent photoproducts in the region of 480

nm for BpCN2CO2CH3, and 440 and 500 nm for BpCN2COCH3

carbene BpCCOCH3 . Alternatively, the hot ketene can be

formed by WR in the diazo excited state without intervention

of the carbene. We are leaning toward the latter explanation

for reasons detailed in section 3.1.

and 5. Excited diazo compounds, singlet carbenes, and VC/

2.2.2. Alcohols. Ultrafast time-resolved studies were also

performed in methanol. Interestingly, the transient absorption

1

(

cf., the Experimental Section for more details). The lifetime

spectra of BpCCOCH3 and BpCCO2CH3 shift to longer

wavelengths between 1-25 ps after the laser ﬂash. This is not

due to VC which typically involves a blue shift. This pattern

1

of singlet carbene ester BpCCO2CH3 (λmax ) 400 nm) exceeds

3

methods. With the use of ns time-resolved laser ﬂash photolysis

methods, the lifetime of BpCCO2CH3 is determined as 180 ns

ns in acetonitrile and cannot be determined by ultrafast

1

was ﬁrst reported with BpCCF3 and was interpreted as the

1

16

dynamics of solvation of the singlet carbene (Scheme 4).

(

left panel of Figure S10), and there is no oxygen effect observed

As these singlet carbenes have zwitterionic, closed-shell

electronic structures, one expects that the empty p orbital of

the carbene will interact with nonbonding electrons on oxygen

and that the hydroxyl proton will interact with the ﬁlled, in-

plane hybrid orbital of the ester carbene. The lifetimes of the

on the lifetime, which proves that the observed transient is not

a triplet carbene. This observation also demonstrates that the

carrier of the transient absorption is not in rapid equilibrium

with an accessible oxygen-sensitive species. As the decay of

J. AM. CHEM. SOC. 9 VOL. 130, NO. 33, 2008 11199

A R T I C L E S

Wang et al.

Figure 3. Transient spectra produced by ultrafast photolysis of (a) BpCN2CO2CH3 and (b) BpCN2COCH3 in cyclohexane. The spectra were generated by

ultrafast LFP (λex ) 308 nm) with a time window of 0.10-1.00 ps.

1

Table 2. Lifetimes and Spectral λmax Shifts of Carbenes BpCCO2CH3 and BpCCOCH3 in Various Solvents

1

BpCCO CH

2

3

BpCCOCH

3

a

τcbn/ns

λ

initial/nm

λ

ﬁnal/nm

τcbn/ps

λinitial/nm

λ

ﬁnal/nm

b

cyclohexane

cyclohexane-d12

cyclohexene

acetonitrile

acetonitrile-d3

methanol

∼2

375

385

400

375

385

400

180 ( 20

200 ( 30

170 ( 10

700 ( 30

680 ( 30

120 ( 10

130 ( 10

440 ( 40

280 ( 20

310 ( 30

230 ( 20

770 ( 40

365

375

380

390

420

370

375

380

390

365

375

380

395

435

375

385

390

1.2 ( 0.1

180 ( 20

-

3

(185 ( 7) × 10

(426 ( 6) × 10

390

385

375

385

390

400

410

415

395

385

410

400

methanol-OD

c

-3

TFE

(20 ( 2) × 10

d

THF

∼2

diisopropyl ether

∼5

2

-propanol

∼5

dichloromethane

790 ( 20

a

1

b

Most of the lifetimes of Bp-C-CO2CH3 are on a nanosecond time scale, except those in methanol, methanol-OD, and TFE. These lifetimes are

1

too long to be accurately measured on our ultrafast spectrometer. But by comparing the kinetic traces, the decay of BpCCO2CH3 has a similar rate in

cyclohexane and in cyclohexane-d12. TFE ) 2,2,2-triﬂuoroethanol. THF ) tetrahydrofuran.

c

d

carbene ester in methanol and methanol-OD are 185 ( 7 and

26 ( 6 ps, respectively. There is no time-resolved spectroscopic

evidence that reaction of the carbene ester with methanol leads

to a signiﬁcant yield of a cation formed by the abstraction of a

proton from the solvent. Thus, the kinetic isotope effect (KIE) of

seems likely and also explains the lack of a KIE on the lifetime

of the keto carbene with methanol.

4

A different story is told in R,R,R-triﬂuoroethanol (TFE). In

the acidic solvent, the decays of both BpCCOCH3 and

BpCCO2CH3 are accompanied by the growth of transient

absorptions at 490 nm (Figures S15 and S16). These transient

2.3 indicates that the carbene ester primarily reacts with methanol

+

by a concerted insertion of the carbene into an OH bond.

absorptions are attributed to cations BpCHCO2CH3 and

+

1

The ketocarbene lifetime is 120 ( 10 ps in methanol and

BpCHCOCH3 . The lifetime of BpCCO2CH3 is 20 ( 2 ps in

+

1

30 ( 10 ps in methanol-OD. The absence of a KIE, and the

TFE, which is obtained by ﬁtting the growth of BpCHCO2CH3

(Figure S17). The decay of BpCCO2CH3 in TFE is controlled,

1

lack of spectroscopic evidence of cation formation, persuades

1

us to posit that if BpCCOCH3 reacts with neat methanol, it

at least in part, by the cation formation route. The lifetime of

cation BpCHCO2CH3⁺in TFE is too long to be accurately

measured on our spectrometer, and our best estimate is ∼2 ns.

proceeds by initial formation of an ylide (CH3OH-Y) which

2

7

subsequently forms an ether by a proton shuttle mechanism.

1

The putative CH3OH-Y was not detected by spectroscopic

methods.

The lifetime of ketocarbene BpCCOCH3 is 440 ( 40 ps in

TFE (Figure S18), which is longer than that recorded in

methanol (120 ( 10 ps). In methanol, the decay of ketocarbene

1

BpCCOCH3 is controlled by ylide formation and, more likely,

WR. Because TFE is less nucleophilic than methanol the ylide

formation reaction pathway will be even less likely than in TFE.

1

The decay of BpCCOCH3 is controlled, at least in part, by

1

cation formation in TFE. Since the fate of BpCCOCH3 in TFE

is controlled by a different mechanism from that operative in

1

methanol, it is not surprising that BpCCOCH3 has a different

lifetime in TFE than in methanol.

We cannot rule out the possibility that WR of the ketocarbene

is faster than reaction with solvent. Given the product study

data of Tomioka et al. with PhCCOCH3, this interpretation

7

(

27) Bethell, D.; Newall, A. R.; Whittaker, D. J. Chem. Soc., Perkin Trans.

2

1971, 23–31.

1

1200 J. AM. CHEM. SOC. 9 VOL. 130, NO. 33, 2008

Ultrafast Spectroscopy of Carbenes

A R T I C L E S

Figure 4. Transient spectra produced by ultrafast photolysis of

BpCN2CO2CH3 in acetonitrile. The spectra were generated by ultrafast LFP

Figure 5. Transient spectra produced by ultrafast photolysis of

BpCN2COCH3 in acetonitrile. The spectra were generated by ultrafast LFP

(λex ) 308 nm) with time windows of (a) 0.05-1.00 ps, (b) 1-30 ps, and

(c) 30-2000 ps.

(

λex ) 308 nm) with time windows of (a) 0.05-1.00 ps, (b) 3-200 ps, and

c) 200-3000 ps.

The lifetimes of the ketocarbene and the carbene ester in

methanol are more than an order of magnitude longer than those

of singlet diphenylcarbene and p-biphenylylcarbene in the same

solvent. In the latter two carbenes, the proton transfer process

is signiﬁcant. The electron-withdrawing groups acetyl, ester, and

of a related, analogous, recently reported system, CF3 (τ ) 31

16

ps), destabilize carbocations and depress the rate of the proton

transfer reaction of these types of carbenes with methanol and

thus extend the lifetimes of carbenes substituted with electron-

withdrawing groups, in methanol. Indeed, the fates of

1

BpCCO2CH3 and BpCCOCH3 are mainly controlled by ylide

formation and/or O-H insertion mechanisms.

In 2-propanol, a more sterically hindered alcohol, both

1

BpCCO2CH3 and BpCCOCH3 display red shifts between

1

2

(

-20 ps after the laser pulse. The lifetime of BpCCOCH3 in

Figure 6. Transient IR spectra produced by ultrafast photolysis of

BpCN2COCH3 in acetonitrile. The spectra were generated by ultrafast LFP

(λ_e_x) 270 nm) with a time window of 1.3-1850 ps.

-propanol is 230 ( 20 ps, longer than its lifetime in methanol

1

120 ps). The lifetime of BpCCO2CH3 in 2-propanol is too

long to be measured accurately, and the best estimate of its

lifetime is ∼5 ns.

initially observed transient spectra of the carbene shifts to longer

2

.2.3. Ethers.

Ultrafast

time-resolved

studies

of

wavelengths between 1-20 ps after the laser pulse. Once again,

1

8

BpCN2COCH3 and BpCN2CO2CH3 were also performed in

tetrahydrofuran (THF). As shown in Figures S19 and S20, the

this is attributed to the dynamics of carbene solvation. The

1

lifetimes of BpCCO2CH3 and BpCCOCH3 are ∼2 (Figure

J. AM. CHEM. SOC. 9 VOL. 130, NO. 33, 2008 11201

A R T I C L E S

Wang et al.

Figure 7. Transient spectra produced by ultrafast photolysis of

BpCN2CO2CH3 in methanol. The spectra were generated by ultrafast LFP

Figure 8. Transient spectra produced by ultrafast photolysis of

BpCN2COCH3 in methanol. The spectra were generated by ultrafast LFP

(

λex ) 308 nm) with time windows of (a) 0.10-1.00 ps, (b) 1-25 ps, and

(

λex ) 308 nm) with time windows of (a) 0.10-1.00 ps, (b) 2-15 ps, and

(

c) 25-1000 ps.

(

c) 15-500 ps.

S21) and 0.28 ns (Figure S22), respectively, in THF. As the

1

transient absorption of BpCCO2CH3 decays, a transient absorp-

nm (X ) CH3, f ) 0.7848), in good agreement with the

experimental results.

tion centered at 360 nm is formed with an isosbestic point at

3

70 nm (Figure S19b), which is attributed to a THF-carbene

The experiments were repeated in diisopropyl ether which is

more sterically encumbered than is THF. In this solvent, the

1

ylide THF-Y (X ) OCH3). In the case of BpCCOCH3, as the

singlet carbene decays, the spectral maximum shifts to 365 nm

1

initial absorptions of BpCCO2CH3 and BpCCOCH3 were

observed at 385 and 375 nm, respectively. Although transient

absorption bands continue to grow 2-5 ps after the laser ﬂash,

unlike THF, the values of λmax, remain unchanged. The lifetime

(

Figure S20b), which is also assigned to THF-Y (X ) CH3)

ylide.

1

of BpCCOCH3 in diisopropyl ether is 310 ( 30 ps. The lifetime

1

of BpCCO2CH3 is too long to be resolved accurately in THF

on our ultrafast spectrometer, and the best estimate of its lifetime

is ∼5 ns.

2

.2.4. Halogenated Solvents. Ultrafast time-resolved studies

of the two R-carbonyl diazo compounds were also performed

in dichloromethane. The results in dichloromethane are similar

to those obtained in acetonitrile. The data are summarized in

Table 2, and the transient spectra are given in Figures S25 and

S26, respectively. Both carbene spectra experience a growth at

early times after the laser pulse (Figures S25a and S26a). These

growths are assigned to the solvation of the carbenes following

The lifetimes of these ylides were determined by ns time-

resolved LFP methods. The lifetime of the ester carbene ylide

X ) OCH3) is 120 µs in THF (Figure S23). For the ketocarbene

(

ylide (X ) CH3), the lifetime is too long to be determined

accurately by our ns spectrometer, and the best estimate of its

lifetime is ∼900 µs (Figure S24).

16

TD-DFT methods predict that the THF ylides will have

absorption maxima at 342 (X ) OCH3, f ) 0.5172) and 346

our previous studies. The lifetimes of the two singlet carbenes

are longer in dichloromethane than in cyclohexane (Table 2).

1

1202 J. AM. CHEM. SOC. 9 VOL. 130, NO. 33, 2008

Ultrafast Spectroscopy of Carbenes

A R T I C L E S

Figure 9. Kinetic traces of BpCN2CO2CH3 in (a) methanol and (b) methanol-OD; excited by ultrafast LFP at 308 nm and probed at 410 nm.

Figure 10. Kinetic traces of BpCN2COCH3 in (a) methanol and (b) methanol-OD; excited by ultrafast LFP at 308 nm and probed at 400 nm.

Scheme 4. Solvation Dynamics of BpCCO2CH3

(Figure S29). The rise time of the 350 nm transient absorption

cannot be determined due to its severe overlap with the spectra

of BpCCO2CH3. The decay of the 350 nm transient absorption

1

is biexponential with two time constants of 23 and 154 µs

(

Figure S30a), and the lifetimes of these species are shortened

in the presence of oxygen (Figure S30b). These features suggest

that the 350 nm band is a radical species. In dichloromethane-

1

d2, the lifetime of BpCCO2CH3 is 660 ns (Figure S28b), which

shows an inverse kinetic isotope effect KIE ) 0.84. If a

1

Table 3. Optical Yields of Singlet Carbenes BpCCO2CH3 and

1

BpCCOCH3 in Cyclohexane, Acetonitrile, and Methanol Produced

by Ultrafast LFP (308 nm) of BpCN2CO2CH3 and BpCN2COCH3

a

hydrogen abstraction product (HR) was formed by

1

BpCCO2CH3 in dichloromethane, it would have a normal KIE.

1

BpCCO CH

2

3

BpCCOCH

3

The inverse KIE in dichloromethane strongly suggests the

formation of radical ClR by chlorine abstraction of

cyclohexane

acetonitrile

methanol

0.057

0.065

0.077

0.022

0.030

0.040

1

28

BpCCO2CH3. This is consistent with previous ﬁndings of

2

9

30

Roth and subsequently Jones et al. that, in chlorinated

solvents, singlet carbenes can abstract chlorine atoms and that

the triplet carbene prefers to abstract hydrogen atoms. This

a

Optical yields are the highest intensities of the singlet carbene

bands, measured 1 ps after the laser pulse.

The lifetimes of singlet ketocarbene BpCCOCH3 (λmax ) 390

nm) is 770 ( 40 ps (Figure S27) and that of the carbene ester

BpCCO2CH3 (λmax ) 400 nm) are too long to measure by

ultrafast spectroscopy and are in excess of several nanoseconds

chlorinated ester radical is also observed in CCl (Figure S31).

There is no evidence for radical formation in acetonitrile, where

the formed transient absorbs at 340 nm (Figure S11) and its

lifetime is not affected by oxygen. This transient is assigned to

an ylide.

4

(

Figure S26b).

With the use of ns laser ﬂash photolysis methods, the lifetime

of the carbene ester was found to be 790 ns in dichloromethane

Figure S28a) and was not affected by oxygen. As the decay of

(

28) Wiberg, K. B. Physical Organic Chemistry; Wiley: New York, 1964.

29) Roth, H. D. J. Am. Chem. Soc. 1971, 93, 4935–4936.

30) Jones, M. B.; Jackson, J. E.; Soundararajan, N.; Platz, M. S. J. Am.

Chem. Soc. 1988, 110, 5597.

(

1

BpCCO2CH3 proceeds, a band centered at 350 nm, which was

1

buried under the absorption of BpCCO2CH3, can be discerned

J. AM. CHEM. SOC. 9 VOL. 130, NO. 33, 2008 11203

A R T I C L E S

Wang et al.

Due to the similarity of the electronic structures of HR and

ClR, they are expected to have similar absorption spectra. In

cyclohexane, radical HR absorbs at 335 nm (Figure S2), which

as expected resembles the absorption spectra of ClR. Further-

more, TD-DFT calculations predict that ClR has an absorption

at 365 nm (f ) 0.4057, Table S9), in good agreement with the

experimental data. On the basis of the inverse KIE, the absence

of an oxygen-sensitive species in acetonitrile, the oxygen

sensitivity of the 350 nm transient absorption, and TD-DFT

calculations in the gas phase, it is safe to assign the 350 nm

band to the radical ClR. We can also conclude that

1

BpCCO2CH3 has a singlet ground state in dichloromethane.

Figure 11. Decrease in diazo CdNdN IR absorbance of solutions of

BpCN2CO2CH3 and BpCN2COCH3 in acetonitrile as a function of exposure

to 308 nm light from a XeCl excimer laser.

3

. Discussion

.1. Wolff Rearrangements in a Diazo Excited State. To-

3

mioka and co-workers reported product studies for the corre-

agreement with experiment. The same calculations predict that

^t^h^e^c^a^r^b^eⁿ^e^e^s^t^e^r^wⁱ^l^l^h^a^v^e^λmax ⁾³⁶⁸ⁿ^m⁽^e^x^p^e^rⁱ^m^eⁿ^t^a^l^λmax

sponding phenyl analogues of the p-biphenyl diazo compounds

6,7

of this work. They found that photolysis of phenyl diazo ester

PhCN2CO2CH3 does not produce an appreciable amount of WR-

)

375) and f ) 0.4569. Thus, TD-DFT calculations predict that

6

there should not be a large difference in molar absorptivity. It

is, therefore, clear that differences in molar absorptivity do not

explain the optical yield data.

derived product in alcohols. In contrast, photolysis of phenyl

diazo ketone PhCN2COCH3 forms ∼50% of WR-derived

7

product in methanol. These results imply that the excited state

of the diazo ketone undergoes WR in concert with nitrogen

extrusion ∼50% of the time, relative to other chemical processes,

but that the excited state of the diazo ester does not.

The data can, in principle, be explained by differences in the

quantum yield of disappearance of BpCN2COCH3 and

BpCN2CO2CH3 if the former diazo compound has more efﬁcient

internal conversion than the diazo ester. To test this possibility,

solutions of diazo ketone and ester in acetonitrile were prepared

with identical, initial, optical densities of 1.0. Each solution was

then exposed to the same number of pulses of 308 nm light

from an excimer laser. As shown in Figure 11, the two diazo

compounds decompose with the same quantum yield. Thus,

differences in internal conversion efﬁciencies of the two diazo

compounds do not explain the data.

5

,31

Wang, Toscano, and co-workers

studied the photochem-

istry of 2-NpCN2CO2CH3 using ns time-resolved infrared

spectroscopy (ns-TRIR) and observed that the growth of the

ketene is monoexponential in Freon-113 and proceeds with the

same rate as the decay of the carbene S-T equilibrium mixture.

They concluded that there is no ketene formation directly from

the diazo ester singlet excited state in this solvent.

1

4

As shown in Figure S14, there is an instantaneous growth

of the ketene band (τ < 0.4 ps) upon photolysis of the p-biphenyl

diazo ketone. The band narrows and blue-shifts over 30 ps and

then grows over 700 ps, determined by ultrafast time-resolved

infrared spectroscopy. In principle, the fast growth of the ketene

band may be due to the formation of the ketene from “hot”

To explain the data and following our previously reported

12

studies of 1,2 hydrogen shifts in diazo excited states, we posit

1

that BpCN2COCH3* undergoes WR in competition with

carbene formation to a far greater extent than the diazo ester

excited state (Scheme 1). This conclusion is consistent with a

large body of literature and the instantaneous (τ < 0.4 ps)

1

#

BpCCOCH3 , a process which is much faster than ketene

1

2

formation from thermalized BpCCOCH3 but we expect will

require ∼10 ps. Alternatively, the ketene can be formed directly

in the singlet excited state of the diazo precursor without

intervention of a carbene and the 50 ps time constant is then

caused by VC of the “hot” ketene. This latter mechanism is

most consistent with the observation of instantaneous formation

of ketenes.

formation of the ketene.

1

Why is there so much more WR in BpCN COCH * than in

2

3

1

BpCN CO CH *? One can posit, following Kaplan and

2

3

Meloy, that the key difference resides in the syn-anti confor-

mational equilibrium constant is more favorable to WR in the

diazo ketone. Indeed inspection of Table 1 reveals that theory

predicts that there is more syn diazo ketone than diazo ester at

equilibrium, in the gas phase, and in the solvents of this work.

The calculated differences in percent syn conformation of diazo

ketone and ester at equilibrium (the conformation predicted by

A comparison of parts a and b of Figure 3 reveals that the

optical yield of ketocarbene is consistently less than half of that

of the carbene ester in all the solvents employed in this study

3

(

Table 3). One can simply argue that the molar absorptivity of

Kaplan and Meloy to be disposed to concerted WR) are

the carbene ester is twice that of the ketocarbene. To test this

signiﬁcant in acetonitrile but not in cyclohexane. As the ratio

of the carbene ester to keto carbene yield are the same in the

two solvents the conformation of the diazo ground state cannot

be the only determining factor. This is not unreasonable as

nothing is currently known of the rate of syn-anti intercon-

version on diazo carbonyl excited-state surfaces, and it may be

competitive with nitrogen extrusion (<300 fs).

1

possibility, TD-DFT calculations were performed on BpC-

1

COCH3 and BpCCO2CH3. These calculations predict that

1

BpCCOCH3 will have λmax ) 348 nm (f ) 0.5041), in good

(

31) Wang, Y.; Hadad, C. M.; Toscano, J. P. J. Am. Chem. Soc. 2002,

24, 1761–1767.

1

1204 J. AM. CHEM. SOC. 9 VOL. 130, NO. 33, 2008

Ultrafast Spectroscopy of Carbenes

A R T I C L E S

1

We speculate that a key factor is the loss of ester resonance

assumed. If NpCCO2CH3 has an isc rate similar to that of

BpCCO2CH3 (∼2 ns), then the spectral evolution will not

display signiﬁcant changes on the time scale of 100-1000 ps,

1

energy that accompanies the WR of BpCN2CO2CH3. The ester

-1

resonance energy of methyl acetate is 8.5 kcal ·mol in the

32,33

34

37

gas phase

and is about half of that value in solution. Thus,

the longest time delay used in the study of HKLPP.

the WR of a diazo ester excited state will be less exothermic

and slower than that of a diazo ketone excited state. The same

line of reasoning predicts that the WR of a singlet ketocarbene

will proceed faster than that of an analogous carbene ester (vide

infra).

HKLPP assigned the 370 nm transient absorption band to an

equilibrium mixture of singlet and triplet NpCCO CH (Figure 1

2

3

of ref 37) because the ratio of the maximum absorbance of the

long-wavelength band to the short-wavelength band decreases

on changing the solvent from acetonitrile to Freon-113. This is

the expected trend if the absorption in the long-wavelength band

3

.2. Carbene Intersystem Crossing and Wolff Rearrange-

1

ment. Different carbene spin states have different reactivities.

One must understand carbene ground-state multiplicity to fully

appreciate the chemistry observed. Phenylcarbomethoxycarbene

is primarily due to NpCCO CH and if the equilibrium constant

2

3

for S-T spin equilibration increases in the less polar solvent.

For this argument to be correct, it must be based on the

assumption that the ratio of the oscillator strength of the 370

6

PhCCO2CH3 was ﬁrst studied by Fujiwara et al. They

1

concluded that PhCCO2CH3 has a triplet ground state in a

cryogenic matrix, using electron spin resonance (ESR) spec-

and 420 nm bands of NpCCO CH is the same in acetonitrile

2

3

and in Freon-113. However, oscillator strength can vary with

solvent and it is not certain that the absorbance ratio of 370 nm

6

troscopy at 77 K. The groups of Toscano and Hadad studied

1

the S-T energy gaps of a series of para-substituted phenyl

over 420 nm is the same for NpCCO CH in solvents with

2

3

carbomethoxy carbenes PhCCO2CH3 using ns-TRIR and quan-

2 3

different polarities. Given that singlet and triplet NpCCO CH

35

36

tum mechanical calculations. They discovered that an electron-

donating para-substituted PhCCO2CH3 favors a singlet carbene

ground state, whereas PhCCO2CH3 with an electron-withdraw-

ing group at the para-position tends to have a triplet carbene

ground state. Their experimental studies of S-T gaps as a

function of solvent are consistent with their computational

have similar UV spectra (spectra b and c in Figure S33) and

1

that NpCCO CH is reported to be the ground state in

2

3

5

1

acetonitrile, we feel it is likely that only NpCCO CH was

2

3

observed in acetonitrile instead of the S-T equilibrium mixture

3

7

proposed by Hess et al.

Finally, Wang et al. deduced that the S-T gap for

3

5

-1

5

studies.

-Naphthylcarbomethoxy carbene (NpCCO2CH3) has been

studied extensively in cryogenic matrixes and in solution by

NpCCO CH is 0.2 ( 0.1 kcal ·mol in Freon-113. Ultrafast

2

3

39,40

2

2 3

isc of NpCCO CH is inconsistent with the Eisenthal’s rule

that the smaller S-T gap, the slower the rate of isc.

The S-T splitting (∆G ) of BpCCO CH was calculated as

3

6

5

37

37,38

the groups of Bally, Toscano, Kohler, and Platz.

It has

ST

2

3

been reported that NpCCO2CH3 has a singlet ground state in

a function of solvent using DFT methods (gas phase, cyclo-

hexane, acetonitrile, and dichloromethane). The results are given

in Table 4. The S-T gaps of BpCCO2CH3 are predicted to be

3

1

polar solvents but a triplet ground state in nonpolar solvents.

-1

2

.19, 0.49, -2.72, and -1.94 kcal·mol in the gas phase,

cyclohexane, acetonitrile, and dichloromethane, respectively,

where a positive sign denotes a triplet ground state and a

negative sign indicates that the singlet is the ground state. On

the basis of the ns LFP studies, it is clear that BpCCO2CH3 has

a singlet ground state in acetonitrile and dichloromethane and

a triplet ground state in cyclohexane, all of which is qualitatiVely

consistent with the DFT calculations. As discussed previously,

combining the ultrafast spectroscopic and ns LFP data for

BpCCO2CH3 in cyclohexane, we estimate that ∆GST is within

The ultrafast dynamics of NpCCO2CH3 in solution were ﬁrst

studied by Hess, Kohler, Likhotvorik, Peon, and Platz (HKLPP)

37

using femtosecond time-resolved spectroscopy. It was reported

that “all spectral evolution ceases after approximately 50 ps”,

after pulsed laser photolysis, and it was concluded that 2-naph-

thyl-carbomethoxycarbene undergoes very rapid isc (in 10 ps)

in acetonitrile and in Freon-113 (CF2ClCFCl2). This is at least

-1

3

0

.4-1 kcal ·mol and that BpCCO2CH3 is the ground state

in cyclohexane. The calculated S-T gap in cyclohexane is 0.49

1

order of magnitude faster than the isc rate found for other

-1

kcal·mol , which is in excellent agreement with the experi-

aryl carbenes. The authors attributed the rapid rate of isc to a

coupling of the motion of ester group to the isc process.

The conclusions of HKLPP are not consistent with our

mental results.

The calculations consistently predict more negative S-T

energy gaps for the ketocarbene relative to the carbene ester.

The singlet ketocarbene is consistently stabilized relative to the

corresponding triplet and relative to the carbene ester (Table

1

observation of a rather “slow” rate of isc of BpCCO2CH3. We

speculate that the spectrum observed by Hess et al. 50 ps after

the laser pulse in acetonitrile and in Freon-113 are both purely

4

). In cyclohexane, for example, the S-T splitting of the carbene

1

that of the singlet carbene NpCCO2CH3 rather than that of the

-1

ester is predicted to be +0.49 kcal ·mol and that of the keto

carbene is -0.46 kcal ·mol . Thus, if the carbene ester has a

S-T carbene equilibrium mixture (Figure S32) as previously

-1

singlet ground state in a particular solvent, it seems likely that

the ketocarbene will also have a singlet ground state in that

solvent as well.

(

32) Blom, C. E.; Guenthard, H. H. Chem. Phys. Lett. 1981, 84, 267–271.

33) Wiberg, K. B.; Laidig, K. E. J. Am. Chem. Soc. 1987, 109, 5935–

5

943.

1

(

34) Grindley, T. B. Tetrahedron Lett. 1982, 23, 1757–1760.

35) Geise, C. M.; Wang, Y.; Mykhaylova, O.; Frink, B. T.; Toscano, J. P.;

Hadad, C. M. J. Org. Chem. 2002, 67, 3079–3088.

The lifetime of BpCCO2CH3 is ∼2 ns in cyclohexane. On

the basis of product studies, there is very little WR-derived

(

36) Zhu, Z.; Bally, T.; Stracener, L. L.; McMahon, R. J. J. Am. Chem.

Soc. 1999, 121, 2863–2874.

(39) Eisenthal, K. B.; Turro, N. J.; Aikawa, M.; Butcher, J. A., Jr.; DuPuy,

C.; Hefferon, G.; Hetherington, W.; Korenowski, G. M.; McAuliffe,

M. J. J. Am. Chem. Soc. 1980, 102, 6563–6565.

37) Hess, G. C.; Kohler, B.; Likhotvorik, I.; Peon, J.; Platz, M. S. J. Am.

Chem. Soc. 2000, 122, 8087–8088.

38) Wang, J.-L.; Likhotvorik, I.; Platz, M. S. J. Am. Chem. Soc. 1999,

(40) Sitzmann, E. V.; Langan, J.; Eisenthal, K. B. J. Am. Chem. Soc. 1984,

106, 1868–1869.

1

21, 2883–2890.

J. AM. CHEM. SOC. 9 VOL. 130, NO. 33, 2008 11205

A R T I C L E S

Wang et al.

-1

Table 4. B3LYP/6-311+G(d,p)//B3LYP/6-31G(d) Calculations of Carbene Singlet-Triplet Gaps (kcal ·mol ) for BpCCO2CH3 and

a

BpCCOCH3 in the Gas Phase, Cyclohexane, Acetonitrile, and Dichloromethane

BpCCO CH

2

3

BpCCOCH

3

∆

H

ST(0K)

∆HST(298K)

∆GST(298K)

∆HST(0K)

∆HST(298K)

∆GST(298K)

b

gas phase

1.95

2.23

3.93

3.36

2.07

0.11

-3.05

-2.29

2.19

0.49

-2.72

-1.94

-2.43

1.57

3.16

-2.75

-0.70

-4.08

-3.28

-1.26

-0.46

-3.88

-3.06

c

cyclohexane

acetonitrile

dichloromethane

c

2.64

a

In the gas phase the energies of the fully optimized singlet and triplet carbenes are compared. The energies of the carbenes at the optimized

gas-phase geometries were then recalculated in the indicated solvents. The condensed phase single-point energies of the singlet and triplet carbene are

b

given. The zero-point energy (ZPE) for the gas-phase calculations is corrected by a factor of 0.9806. The proposed energy corrections by Woodcock et

c

al. (ref 44) were not applied on our results. The solution-phase calculations used PCM models, and the geometries are from the gas-phase B3LYP/

6

-31(d) calculations. For the triplet carbenes, the anti conformation is more stable than the syn conformation and is used for the S-T gap calculations.

1

coordinate. At each relaxed geometry of PhCCO2CH3 along

the reaction coordinate, the energies of the triplet spin isomers

were calculated to search for the surface crossing between the

singlet and triplet carbene PES. It was found that when

RO-C-C-C is 40°, the two PESs cross. The surface crossing point

-1

has an energy that is 6.2 kcal ·mol greater than that of relaxed

1

PhCCO2CH3.

3

A related, relaxed PES scan was computed for PhCCO2CH3

1

with a single-point energy calculation of PhCCO2CH3 at each

relaxed geometry of the triplet carbene. It was found that the

relaxed triplet and restrained singlet PES do not cross. The

smallest energy difference between these two surfaces is at

1

RO-C-C-C is 70°, and the restrained-to-be-planar PhCCO2CH3

-1

energy is 6.7 kcal ·mol higher than the relaxed orthogonal

1

PhCCO2CH3.

The relaxed singlet carbene PES crosses that of the relaxed

Figure 12. Potential energy surfaces (PES) for singlet and triplet

-

1

triplet carbene PES at an energy that is 2 kcal ·mol greater

PhCCO2CH3 in the gas phase. Relaxed PhCCO2CH3 PES is in red; rigid

1

3

1

PhCCO2CH3 PES is in yellow; relaxed PhCCO2CH3 PES is in blue; rigid

than that of the relaxed PhCCO2CH3. We note that a barrier to

PhCCO2CH3 PES is in green. See text for more details.

isc has been reported on isc of NpCCO2CH3 in a cryogenic

3

6

matrix. Thus, based on these calculations, we conclude that

if isc proceeds when the singlet and triplet carbene are

isoenergetic then a barrier to isc can be estimated to vary

product formed for photolysis of BpCN2CO2CH3 in cyclohexane

and the main product is that of solvent insertion (Scheme 3).

1

In addition, the lifetime of BpCCO2CH3 is the same in

-1

between ∼2 kcal · mol (if isc proceeds where the surfaces

cyclohexane and in cyclohexane-d12. We conclude that the decay

-1

cross) to 6.7 kcal ·mol (if the singlet carbene must rotate to

1

of BpCCO2CH3 in cyclohexane is mainly controlled by isc.

the triplet’s preferred geometry to allow isc) for the isc of

4

1

13

Previous studies on diphenylcarbene, ﬂuorenylidene, p-

1

PhCCO2CH3 to its triplet. The ultimate reaction product (C-H

11

12

biphenylylcarbene, and p-biphenylylmethylcarbene show that

insertion adduct, Scheme 3) must be formed from the equilib-

rium mixture of singlet and triplet carbene, in a process which

takes longer than 2 ns.

the isc time constants in cyclohexane are no longer than 200

1

ps. The isc of BpCCO2CH3 (∼2 ns) is around 1 order of

magnitude slower than the previously studied carbenes.

We posit that this effect is mainly due to the signiﬁcant

geometry difference between singlet and triplet BpCCO2CH3.

5

Since Wang et al. observed ketene formation for

2

-NpCCO2CH3 in acetonitrile and Freon-113, it is reasonable

to assume that WR also contributes to the decay of

BpCCO2CH3 in acetonitrile and dichloromethane where the

singlet state is the ground state of the carbene. Due to other

competing reactions, such as ylide formation in acetonitrile and

chlorine abstraction in dichloromethane, we cannot directly

deduce the WR rates in these two solvents.

1

The plane of the carbonyl group of BpCCO2CH3 is predicted

1

to be perpendicular to the plane of the phenyl ring. In

3

BpCCO2CH3, the carbonyl group is coplanar with the phenyl

ring, which imposes a dramatic structural change requirement

upon the isc of BpCCO2CH3 and thus decelerates the isc rate.

A potential energy surface (PES) scan was performed for

PhCCO2CH3 at the B3LYP/6-311+G(d,p)//B3LYP/6-31G(d)

level of theory to estimate where the singlet and triplet surfaces

cross (Figure 12). A phenyl counterpart was studied instead of

BpCCO2CH3 because they have similar S-T gaps in the gas

phase and we assume that the substitution of biphenyl with

phenyl will not alter the S-T isc barrier dramatically but will

facilitate the calculations. A relaxed PES scan in the gas phase

DFT calculations predict that the S-T gap for BpCCOCH3

-1

is -1.26, -0.46, -3.88, and -3.06 kcal ·mol in the gas phase,

cyclohexane, acetonitrile, and dichloromethane, respectively

(

Table 4), where the negative sign indicates a singlet ground

state. The calculations predict that the S-T gap of the keto

carbene is always more negative (more in favor of the singlet)

than the carbene ester. The experimental evidence indicates that

the carbene ester has an S-T splitting of approximately zero

in cyclohexane (with a slight preference for the triplet) and a

singlet ground state in acetonitrile and dichloromethane. Thus,

assuming that the error in the DFT calculations are the same

for BpCCO2CH3 and BpCCOCH3, we conclude that BpC-

1

was performed for PhCCO2CH3 with the dihedral angle of the

carbonyl group and the phenyl ring (RO-C-C-C) as the reaction

(

41) Peon, J.; Polshakov, D.; Kohler, B. J. Am. Chem. Soc. 2002, 124,

428–6438.

6

1

1206 J. AM. CHEM. SOC. 9 VOL. 130, NO. 33, 2008

Ultrafast Spectroscopy of Carbenes

A R T I C L E S

Scheme 5. Electronic Stabilization of Carbenes by Carbonyl

However, in the halogenated solvent dichloromethane, the

WR rate of the ketocarbene is comparable to that of the same

carbene in acetonitrile. On the basis of the ET(30) values,

dichloromethane is more polar than cyclohexane but less polar

than acetonitrile. The comparable WR rate in acetonitrile and

dichloromethane suggests that the rate of WR in solution is not

simply controlled by solvent polarity. We have reported a

halogenated solvent effect on the isc rates of certain carbenes

Groups

1

7

with triplet ground states. This was attributed to coordination

of the carbene with the halogenated solvent. We propose that

COCH3 has a singlet ground state in cyclohexane, acetonitrile,

and dichloromethane.

1

the same type of coordination of BpCCOCH3 with acetonitrile

The preference of a singlet ground state for the ketocarbene

as compared with the ester carbene is due to an electronic effect.

The singlet carbonyl carbene geometry prefers that the carbonyl

group and the plane of the phenyl ring are orthogonal (Scheme

and dichloromethane (solvent nonbonding electron pairs with

the empty p orbital of the singlet carbene) stabilizes the

ketocarbene and depresses the rate of WR in these solvents

relative to cyclohexane.

5

). A singlet carbene is much like a zwitterion. The orthogonal

conformation avoids unfavorable interactions between the

carbonyl group and the empty p orbital/positive charge. On the

other hand, in the orthogonal conformation, the carbonyl group

can stabilize the ﬁlled orbital/negative charge through conjuga-

tion. In the case of the ester carbene, stabilization of the ﬁlled

orbital by the carbonyl group diminishes the resonance of the

ester moiety. For the ketocarbene, the carbonyl group can better

stabilize the carbene ﬁlled orbital than the carbene ester. The

difference between the ketocarbene and ester carbene results in

the ketocarbene always favoring a singlet carbene ground state

relative to the ester carbene. The electronic effect that makes a

methyl ketone more acidic than the analogous ester is the same

electronic effect that favors a singlet ground state of a carbene

ketone relative to a carbene ester.

4

. Experimental Section

4

.1. Calculations. DFT and TD-DFT calculations were per-

formed using the Gaussian 03 suite of programs at The Ohio

42

Supercomputer Center. For the gas-phase calculations, geometries

were optimized at the B3LYP/6-31G(d) level of theory with single-

point energies obtained at the B3LYP/6-311+G(d,p)//B3LYP/6-

3

1G(d) level of theory. Vibrational frequency analyses at the

B3LYP/6-31G(d) level were utilized to verify that stationary points

obtained corresponded to energy minima. All the transition states

have one and only one imaginary frequency. The zero-point energy

(

ZPE) was corrected with a factor of 0.9806. The solution-phase

calculations used PCM models and were performed at B3LYP/6-

11+G(d,p)(PCM)//B3LYP/6-31G(d)(gas phase) level of theory

refer to the footnotes of tables). The electronic spectra were

3

(

computed using time-dependent density functional theory with

Gaussian 03 at the B3LYP/6-311+G(d,p) level and using the

B3LYP/6-31G(d) geometry; for these calculations, 20 allowed

electronic transitions were calculated. Carbonyl carbenes and their

derivatives have s-Z and s-E conformations. Both conformers were

calculated for each species, and the most stable conformers were

shown and used to calculate the electronic spectra.

6

Fujiwara et al. reported that triplet-sensitized photolysis of

PhCN2COCH3 in methanol did not form appreciable yields of

3

characteristic products of reaction of PhCCOCH3, such as

double hydrogen abstraction product. This observation is

consistent with a singlet ground state for PhCCOCH3 in this

solvent. Nanosecond laser ﬂash photolysis of BpCN2COCH3

also did not provide evidence for the formation of the triplet

carbene (Figure S9). On the basis of the ultrafast results, the

4

.2. Ultrafast Spectroscopy. Ultrafast UV-vis broad-band

absorption measurements were performed using the home-built

spectrometer described previously. Samples were prepared in 50

mL of solvent with absorption 1.0 at the excitation wavelength with

2

3

1

decay of BpCCOCH3 is a unimolecular process, either WR or

isc, or both. Since DFT calculations predict singlet ground states

for BpCCOCH3, the rate of an uphill isc process will be slow.

1

.0 mm optical length. For the bleaching control experiments (cf.,

section 4.3), the bleached solutions for BpCN CO CH and

BpCN COCH in acetonitrile were prepared with the same con-

2

3

1

Thus, we conclude that BpCCOCH3 mainly decays by WR in

2

3

cyclohexane, acetonitrile, and dichloromethane. WR must

proceed faster in a nonpolar solvent cyclohexane (τ ) 180 ps)

than in a polar solvent acetonitrile (τ ) 700 ps). This may be

due to the fact that the singlet carbene is more polar than the

transition state of WR. Assuming an Arrhenius pre-exponential

centration as in their corresponding ultrafast experiments, bleached

using a 308 nm excimer laser for 10 min, and ultrafast spectroscopic

experiments were then performed immediately to avoid decomposi-

tion of unstable photoproducts.

The ultrafast infrared laser system (see Scheme S1 of the

Supporting Information) consists of a short-pulse titanium-sapphire

oscillator(Coherent,Mira)followedbyahigh-energytitanium-sapphire

regenerative ampliﬁer (Coherent, Positive Light, Legend HE USP).

The beam is split into two beams to pump two OPAs (OPerA

Coherent). The OPA with an SFG module generates a UV pump

pulse (tunable from 240 to 310 nm), while the second OPA has a

DFG module producing IR pulses (tunable from 2 to 10 µm). A

Ge beamsplitter splits the IR beam into reference and probe beams,

and both are focused into the sample cell (Harrick Scientiﬁc), but

only the probe beam overlaps with the pump beam in the sample.

After passing through the sample the probe and reference beams

were spectrally dispersed in a grating spectrometer (Triax 320) and

independently imaged on a liquid nitrogen cooled HgCdTe detector

1

3

factor of WR is 10 , one can deduce the activation barrier of

1

-1

WR for BpCCOCH3 is 4.4 and 5.2 kcal ·mol in cyclohexane

4

and acetonitrile, respectively. The group of Radom calculated

the WR barriers for parent formyl carbene to be 2.4-7.2

-1

kcal·mol in the gas phase. We performed B3LYP/6-31(d)

-1

calculations and obtained WR barriers of 4.4 and 8.9 kcal ·mol

for the p-biphenyl keto carbene and p-biphenyl carbene ester,

respectively, in the gas phase (Table 5). Our calculations reveal

that the barriers to WR of the arylketone are similar to that

calculated for formylcarbene and are in excellent agreement with

1

our experimental values for the barrier to WR of BpCCOCH3.

The greater calculated barrier to WR of the carbene ester is

attributed to the loss of ester resonance energy in the WR of

(

2 × 32 elements) with 17 nm resolution. Every second UV pump

pulse is blocked by a synchronized chopper to eliminate long-term

1

BpCCO2CH3. Our calculations also explain why the lifetime

of the keto carbene is shorter than that of the analogous carbene

ester in acetonitrile, THF, and dichloromethane.

(

42) Frisch, M. J.; et al. Gaussian 03, revision C.02; Gaussian, Inc.:

Wallingford, CT, 2004.

J. AM. CHEM. SOC. 9 VOL. 130, NO. 33, 2008 11207

A R T I C L E S

Wang et al.

-1

Table 5. B3LYP/6-31G(d) Calculations of Relative Energies of the Transition State and the Product of Wolff Rearrangement (kcal ·mol ) for

1

a

BpCCO2CH3 and BpCCOCH3 in the Gas Phase

BpCCO CH

2

3

BpCCOCH

3

∆

H

ST(0K)

∆HST(298K)

∆GST(298K)

∆HST(0K)

∆HST(298K)

∆GST(298K)

WR T.S.

ketene

8.92

-29.08

8.79

-29.14

8.88

-28.74

3.67

-51.22

3.27

-51.54

4.40

-50.51

a

The zero-point energy (ZPE) for the gas-phase calculations is corrected by a factor of 0.9806. The energies are relative to its corresponding carbene

singlet state.

1

drift effects. Solution concentrations were adjusted to an absorption

of 0.7 in a 1 mm cell at the excitation wavelength. The pump pulse

energy was about 4 µJ at the sample position. The entire set of

pump-probe delay positions (cycle) is repeated at least three times,

to observe data reproducibility from cycle to cycle. To avoid

rotational diffusion effects, the angle between polarizations of the

pump beam and the probe beam was set to the magic angle (54.7°).

Kinetic traces are analyzed by ﬁtting to a sum of exponential terms.

Convolution with a Gaussian response function is included in the

global ﬁtting procedure. The instrument response was approximately

3

is formed (0.15 g, yield 60%). H NMR (500 MHz, CDCl ): δ

7.62-7.64 (m, 2H), 7.59-7.61 (m, 2H), 7.55-7.56 (m, 2H),

1

3

7.43-7.46 (m, 2H), 7.33-7.36 (m, 1H), 3.89 (s, 3H). C NMR

(125 MHz, CDCl ): δ 165.6, 140.3, 138.7, 128.8, 127.6, 127.4,

3

127.1, 126.9, 124.3, 124.3, 52.1.

4.7. p-Biphenylacetone. A 50 mL ﬂask is charged with 2.12 g

of biphenylacetic acid (10.0 mmol), 3.6 mL of thionyl chloride (50

mmol), and 20 mL of dichloromethane. The solution is reﬂuxed

overnight, and the solvent and excess thionyl chloride are removed

under vacuum to afford biphenylacetic chloride. Biphenylacetic

chloride is dissolved in 20 mL of dichloromethane and 1.07 g of

N,O-dimethylhydroxylamine hydrochloride (11.0 mmol). The solu-

tion is cooled in an ice bath, and 4.5 mL of DBU is added dropwise.

The reaction is monitored by GC-MS. The reaction mixture is

washed by saturated ammonium chloride (10 mL × 2), then water

3

00 fs (fwhm). All the experiments were performed at room

temperature.

4.3. Re-excitation of Persistent Photoproducts. An enigmatic

2 2 3

band is observed at 480 nm in the study BpCN CO CH in

acetonitrile. The lifetime of this transient absorption is hundreds

or thousands of picoseconds. A sample of the diazo compound in

acetonitrile was bleached with pulses of 308 nm excimer laser light,

and then the bleached solution was immediately studied by ultrafast

spectroscopic techniques. Pulsed laser photolysis of the bleached

solution does produce the 480 nm transient (Figure S34); thus, the

carrier of this species does not originate from the primary

photochemistry of the diazo compound. The bleached solution does

not produce a transient absorption at 400 nm, indicating that the

(

10 mL), and dried with sodium sulfate. The solvent is removed

under vacuum to afford BpCH

biphenylyl. BpCH CON(OCH )CH

anhydrous THF, and 5 mL of 3 M CH

2

CON(OCH

is dissolved in 30 mL of

MgCl solution is added

3 3

)CH , where Bp )

2

3

dropwise at 0 °C over 1 h. The reaction mixture is stirred for 4 h

and monitored by GC-MS. Upon completion of the reaction, it is

quenched by saturated ammonium chloride solution and washed

by brine. The solvent is removed under vacuum. The crude material

is puriﬁed by column chromatography with a mixture of pentane

4

00 nm transient absorption, which is assigned to the carbene, is

not due to re-excitation of reaction products.

1

and ether (10:1). White solid is afforded (1.26 g, yield 60%). H

Similarly, re-excitation was also performed in the case of

NMR (500 MHz, CDCl

H), 7.33-7.36 (m, 1H), 7.27-7.29 (m, 2H), 3.74 (s, 2H), 2.20

s, 3H).

.8. 1-(Biphenyl-4-yl)-1-diazopropan-2-one (BpCN

p-Biphenylacetone (0.210 g, 1.0 mmol) and tosyl azide (0.197 g,

.0 mmol) are dissolved in 10 mL of dichloromethane. DBU (0.3

3

): δ 7.56-7.59 (m, 4H), 7.42-7.45 (m,

BpCN

40 nm upon photolysis of bleached solution. In cyclohexane, a

transient absorption centered at 500 nm was visible. Ultrafast

spectroscopic studies of bleached BpCN COCH in acetonitrile

2 3

COCH . In acetonitrile, there was a shoulder observed at

2

(

4

2 3

COCH ).

2

3

produces two transients, centered at 440 and 500 nm (Figure S35),

suggesting the 440 and 500 nm transient absorptions are due to

re-excitation. This photolysis of the bleached solution does not

produce a transient absorbing at 370 nm, assuring the transient

observed at 370 nm is due to the primary photochemistry of the

diazo compound.

1

mL, 2.0 mmol) is added to the solution dropwise at 0 °C, and the

mixture is stirred overnight. The reaction is quenched by saturated

ammonium chloride solution, washed by brine, and dried by sodium

sulfate. The solvent is removed under vacuum. The crude material

is puriﬁed by column chromatography with a mixture of pentane

and dichloromethane (1:1). Orange crystal is afforded (0.14 g, yield

0%). H NMR (500 MHz, CDCl

.57-7.61 (m, 4H), 7.43-7.47 (m, 2H), 7.34-7.38 (m, 1H), 2.41

s, 3H). C NMR (125 MHz, CDCl

4.4. Synthesis and Product Studies. All materials and solvents

were purchased from Aldrich. The solvents for ultrafast studies were

spectrophotometric grade from Aldrich and used as received.

1

6

7

(

1

3

): δ 7.64-7.66 (m, 2H),

4

.5. Methyl p-Biphenylacetate. A 200 mL ﬂask is charged with

1

3

): δ 140.2, 139.8, 128.9, 127.7,

2

.12 g of biphenylacetic acid (10.0 mmol) and 0.95 mL of dimethyl

27.5, 126.9, 124.3, 27.0.

sulfate (10.0 mmol). The ﬂask is cooled in an ice bath, and 1.8 mL

of DBU (12.0 mmol) is added dropwise. The solution is stirred for

1

4

.9. H NMR and GC-MS Analysis of Photolysis Mixtures

for BpCN CO CH (Scheme 3). Photolysis of BpCN CO CH was

performed in cyclohexane using a 10 Hz excimer laser (XeCl, 308

4

h, and TLC is used to monitor the completion of the reaction.

2

3

2

3

Methanol is removed under vacuum. The crude material is run

through a short column using dichloromethane as eluent. The

nm, 17 ns, ∼0.5 J/pulse) source for 5 min. In a quartz cuvette, 3.1

solvent is removed under vacuum, and a white, solid product is

mg of BpCN

2

CO

2

CH

3

was dissolved in 2.48 g of cyclohexane (∼3.9

1

afforded (2.26 g, yield 100%). H NMR (500 MHz, CDCl

7

3

): δ

mM). The sample solution was purged with argon for 5 min before

photolysis. During photolysis, the sample solution was shaken and

argon was bubbled through the solution. The completion of the

photolysis was monitored by UV-vis spectroscopy. After pho-

tolysis, 0.5 mL of ethanol was added immediately to quench

formation of any ketene. The solvent was removed under vacuum,

.55-7.59 (m, 4 H), 7.42-7.45 (m, 2H), 7.32-7.37 (m, 3H), 3.72

s, 3H), 3.68 (s, 2H).

.6. Methyl 2-(biphenyl-4-yl)-2-diazoacetate (BpCN

). Methyl biphenylacetate (0.226 g, 1.0 mmol) and tosyl azide

0.197 g, 1.0 mmol) are dissolved in 10 mL of dichloromethane.

DBU (0.3 mL, 2.0 mmol) is added to the solution dropwise at 0

C, and the mixture is stirred overnight. The reaction is quenched

(

4

2 2

CO -

CH

3

(

and the product mixture was dissolved in CD

2

Cl

was used instead of CDCl

3

impurity has a signal at 7.26 ppm, which may

2

and analyzed by

1

°

H NMR (400 MHz). Note that CD

because CHCl

Cl

2 2

3

by saturated ammonium chloride solution, washed by brine, and

dried by sodium sulfate. The solvent is removed under vacuum.

The crude material is puriﬁed by column chromatography with a

mixture of pentane and dichloromethane (1:1). An orange crystal

overlap with the signals of the biphenyl moiety and complicate the

integration. We assumed that the products derived from

BpCN CO CH will have an intact biphenyl ring system. The

2 2 3

1

1208 J. AM. CHEM. SOC. 9 VOL. 130, NO. 33, 2008

Ultrafast Spectroscopy of Carbenes

A R T I C L E S

aromatic hydrogens (δ 7.2 to ∼8.0) were integrated and deﬁned as

the total yield (100%). The NMR peaks in the mixture were

identiﬁed either by comparison with authentic samples or the

literature NMR data of the same compound or their monophenyl

counterparts in the literature. Generally speaking, changing from

phenyl to biphenyl should only change the chemical shifts of their

nonconjugated substituents downﬁeld by less than 0.1 ppm, such

in these solvents and have no sensitivity to the presence of

oxygen. In acetonitrile, the singlet ester carbene lifetime is

mainly controlled by ACN-Y ylide formation. In dichlo-

1

romethane, BpCCO2CH3 abstracts a chlorine atom to form a

3

radical pair. In cyclohexane, BpCCO2CH3 is the ground state

1

and the lifetime of BpCCO2CH3 is controlled by isc (τ ∼ 2

ns) based on the spectroscopic data and product studies. The

slow isc rate is due to the need for the singlet carbene to rotate

to a high-energy conformation to allow relaxation. In methanol,

TFE, and THF, reactions with solvent control the fate of

as toluene (CH

acetophenone (CH

.61). On the basis of this rule and the NMR literature data of the

3

, δ 2.34) and p-phenyltoluene (CH

3

, δ 2.39),

, δ 2.59), and p-phenylacetophenone (CH

3

, δ

2

phenyl counterparts, the chemical shifts of the biphenyl photoprod-

ucts can be reasonably estimated.

1

^B^p^C^C^O²^C^H³+and form O-H insertion product, cation

The photolysis mixture was also analyzed by GC-MS. The WR

product was not observed either by NMR or by GC-MS analysis.

Two major peaks were observed by GC-MS, with retention times

BpCHCO2CH3 and THF-Y ylide, respectively.

1

The optical yields of the ketocarbene BpCCOCH3 are much

1

lower than that of the carbene ester BpCCO2CH3, from which

(

RT) of 8.45 and 12.25 min. The mass spectrum of the component

with RT of 8.45 min has a molecular ion peak at m/z 226. This is

we conclude that Wolff rearrangement in the excited state of

the diazoketone is a more signiﬁcant process than the analogous

process in the excited state of the diazo ester. This is attributed

to the loss of ester resonance in the Wolff rearrangement of the

excited diazo ester which makes it a less exothermic and slower

process.

assigned to methyl p-biphenylacetate, which is formed by double

3

hydrogen abstraction of BpCCO

2

CH

3

from cyclohexane. This

assignment is also conﬁrmed by comparing the MS fragmentation

4

3

pattern of its phenyl counterpart in the literature. The MS of the

component with RT of 12.25 min has a base peak at m/z 226, which

is also the peak with the highest mass observed. The fragmentation

pattern of this component is different from that of the RT 8.45

min component. The component with RT of 12.25 min is assigned

to the carbene-cyclohexane adduct. The m/z 226 peak in MS is

formed by a McLafferty rearrangement.

On the basis of DFT calculations and experimental results,

1

we conclude that BpCCOCH3 has a singlet ground state in

cyclohexane, acetonitrile, and dichloromethane. The lifetime of

1

BpCCOCH3 is shorter than that of BpCCO2CH3 in all the

Some minor components were observed by GC-MS. The

component with a RT of 5.30 min (m/zM+· ) 166) is assigned to

bicyclohexyl, and the MS spectrum is consistent with that reported

solvents employed in this study except TFE. The lifetime of

1

BpCCOCH3 is controlled by Wolff rearrangement in cyclo-

4

3

hexane, acetonitrile, and dichloromethane. The rate of Wolff

rearrangement of the ketocarbene is faster in cyclohexane than

in acetonitrile and dichloromethane. Solvent coordination

stabilizes the carbene and depresses the rate of Wolff rearrange-

in the literature.

The analogous photolysis was performed for BpCN

2

2 3

CO CH in

a 1:1 mixture of cyclohexane-cyclohexane-d12, and the reaction

mixture was analyzed by GC-MS. There are three major compo-

nents observed with RTs of 8.45, 12.15, and 12.23 min. The

component with RT of 8.45 min is assigned to methyl p-

biphenylacetate based on our previous analysis. The components

with RTs of 12.15 and 12.23 min were assigned to the adducts of

1

ment. The lifetime of BpCCOCH3 is the same in methanol and

in methanol-OD, indicating the deactivation of this singlet

carbene is mainly controlled by the formation of MeOH-Y

ylide, which is followed by an ultrafast proton shuttle to form

an ether. In THF, the THF-Y ylide formation is also observed

BpCCO

2

CH

3

-cyclohexane-d12 (base peak m/z ) 228) and

2

3

-cyclohexane (base peak m/z ) 226), respectively.

1

for BpCCOCH3.

The area ratio between the peaks for BpCCO

and BpCCO CH -cyclohexane-d12 is 2.5.

2 3

CH -cyclohexane

2

3

Acknowledgment. Support of the CCBD and of this project

by the National Science Foundation is gratefully acknowledged

along with the support of the Ohio Supercomputer Center. J.W.

gratefully acknowledges an Ohio State University Presidential

Fellowship. G.B. thanks Foundation for Polish Science and MF

EOG for a “Homing” Grant in the year 2008. The authors thank

Professor C. M. Hadad with guidance on the calculations performed.

5

. Conclusions

Photolysis of BpCN2CO2CH3 produces the excited state of

this compound (λmax ) 480 nm) within the instrument response.

The excited state decomposes to form singlet p-biphenyl

carbomethoxy carbene (BpCCO2CH3). This carbene has a singlet

ground state at ambient temperature in acetonitrile and in

dichloromethane as their lifetimes are hundreds of nanoseconds

Supporting Information Available: Spectra and kinetics of

ultrafast studies and TD-DFT calculations. Complete ref 42. This

material is available free of charge via the Internet at http://

pubs.acs.org.

(

43) National Institute of Advanced Industrial Science and Technology.

SDBS Web: http://riodb01.ibase.aist.go.jp/sdbs/.

44) Woodcock, H. L.; Moran, D.; Brooks, B. R.; Schleyer, P. v. R.;

Schaefer, H. F. J. Am. Chem. Soc. 2007, 129, 3763–3770.

JA803096P

J. AM. CHEM. SOC. 9 VOL. 130, NO. 33, 2008 11209

Article Doi

Ultrafast UV-Vis and IR studies of p-biphenylyl acetyl and carbomethoxy carbenes

DOI: 10.1021/ja803096p

Source and publish data:

Authors:

Article abstract of DOI:10.1021/ja803096p

Full text of DOI:10.1021/ja803096p

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