Geometrically Strained Carbenes: Interdependence among Geometry, Spin Multiplicity, and Reactivity

Yasutake Takahashi*, Athanassios Nicolaides*, and Hideo Tomioka*

ChemistryDepartment for Materials, Faculty of Engineering, Mie University,

Received July 5, 2001

Abstract : The singlet and triplet states of cyclobutenylidene 1 and benzocyclobutenylidene 2 have been stud- ied computationally (using ab initio and DFT methods) in order to assess the effect of angle strain on the S-T gap of vinyl- and phenylcarbenes. It is found that both carbenes have a singlet ground state. Cyclobutenylidene 1 has a more significant singlet-triplet gap (-25 kcal mol-1) than benzocyclobutenyli- dene 2 (-14.5 kcal mo1-1). The strong preference of 1 for a singlet ground state may be understood if it is viewed as bicyclobut-l-ene with a considerably puckered ring. Singlet benzocyclobutenylidene (12) is com- puted to have also a puckered cyclobutene ring albeit less pronounced. The lowest isomerization path avail- able for singlet cyclobutenylidene (11) is the formation of vinylacetylene, which is predicted to have a barrier of around 9 kcal moil. Our calculations suggest that singlet benzocyclobutenylidene (12) lies in a rather deep potential well and should be observable under suitable experimental conditions. However, under low-temperature Ar matrix conditions no evidence for the formation of carbene 2 was obtained from the photolysis of precursor 15. On the other hand, laser flash photolysis (LFP) of precursor 16 enables us to observe benzocyclobutenylidene at room temperature. The lifetime of carbene 12 is determined to be 0.6-0.7 ,us in cyclohexane. The mcxy value for carbene 12 is determined to be 0.43, revealing a strong electrophilic nature and ranking 12 second among the electrophilic carbenes listed in the carbene philicity spectrum.

1. Introduction ground multiplicity depends upon the relative energy of the singlet and triplet states. The four lowest states of a carbene

Carbenes are divalent carbon species with a wide variety of have electronic configurations described as ƒÐ1p1, ƒÐ2, or p2. chemical properties. Although there are many useful reac- In the ƒÐlpi configuration the electron spins may be paired, tions in which carbenes play an important role as intermedi- giving rise to a singlet, or parallel forming a triplet, while the ates, they often remain elusive without complete characteriza- σ2 and p2 configurations must be electron-paired singlets. tion1. A key parameter in understanding the overall reactivi- Thus, the triplet state has ƒÐlp1 (3B1) configuration, while ƒÐ2 ty of a carbene is the spin state of the two nonbonding elec- (1A1) is generally thought to be the lowest energy configura- trons and the singlet-triplet splitting (ƒ¢ES-T), both of which tion for the singlet (Figure 1). are closely related to its molecular structure. The origin of The small difference between the energies of the lowest sin- the interdependent relationship between the energy gap and glet state (So) and the first triplet state (T1) may easily be the molecular structure can be considered schematically (Fig- overturned by the effects of substituents on the carbene cen- ure 1). ter. The factors that influence the S-T spacing can be ana-

The carbene carbon is linked to two adjacent groups by lyzed in terms of electronic and geometric (steric) effects. covalent bonds and possesses two nonbonding electrons Electronic effects can be sub-divided to inductive and reso- which may have antiparallel spins (singlet state) or parallel nance effects. Electronegative substituents can stabilize induc- spins (triplet state). A linear carbene has two degenerate p tively the ƒÐ orbital, promoting a singlet ground state. Reso- orbitals, and Hund's first rule predicts a triplet ground state. nance effects are often more important and arise when sub-

If the carbene unit is bent, the two orbitals are energetically stituents have p or ƒÎ orbitals available for overlap with the 2p different. The orbital perpendicular to the plane defined by orbital. Both strong ir donors and Ir acceptors favor the sin- the three atoms is designated as "p", while the in-plane one glet compared to the triplet. The former by destabilizing the is called "ƒÐ". Bending causes the a orbital to acquire some s πorbital and the latter by stabilizing theσ. character and thereby become stabilized, while the p orbital The electronic effects have been investigated systematically remains largely unchanged. by experiments and theoretical calculations. Carbenes bear-

Practically, most carbenes are more or less bent and the ing a vinyl or aryl substituent generally display a triplet

ground state, while carbenes bearing an electron-donating heteroatom substituent, such as amino, alkoxy, or halogen,

display a singlet ground state. For example, parent methy-

lene2a-f and its phenyl derivative3a-h have triplet ground states,

which are more stable than the singlet states by 9 and 4 kcal

mol-1, respectively2f, 3f-h. On the other hand, dichlorocar-

bene2g, h and phenychlorocarbene3i have singlet ground states

with ĢES-T of -13.5 and -7 kcal moil, respectively.

Figure 1. Linear methylene with two degenerate p orbitals and bent Geometric effects also control the AES-T of a carbene. In methylene with a and p orbitals. this case the energy level of the a orbital of a carbene can be

1070 ( 34 ) J. Synth . Org . Chem . , Jpn . perturbed. A narrow bond angle at the carbene center is angle affects the geometrical characteristics and the S-T expected to increase the s character of the 6 orbital and splitting of cyclobutenylidene 1. Also we examined the ener- therefore stabilize the singlet relative to the triplet state. gy surface of the following intramolecular rearrangement It is interesting to note that geometric effects have not been pathways: the 1, 2 H- and 1, 2 C-shifts, and the ring-opening examined in a systematic way4. This is probably due to the to give vinylacetylene. general belief that geometrically strained carbenes are too Geometries. Calculations at the G2 level of theory predict reactive. However, nowadays sophisticated techniques like that cyclobutenylidene 1 has a singlet ground state with an time-resolved spectroscopy5 and matrix isolation spec- estimated AEs_T of -25 kcal moil. As shown in Figure 2, troscopy6 allow us to observe highly reactive intermediates. carbene 1 has a rather unusual geometry at its singlet state. In addition, computational chemistry methods have been use- The singlet adopts a puckered structure of low symmetry ful in this field of research. Indeed, high-level ab initio cal- (11-(C1)) with the C2-C3 bond length being shorter than the culations can predict singlet-triplet gaps with reasonable C1-C2 by 0.04 A. On the other hand, the triplet (31) is calcu- accuracy7 and, thus, it is possible to explore the effect of sub- lated to be planar (Cs) and displays an allylic feature with the stituents and geometrical constraints on the AES_T systemati- C1-C2 bond length being virtually equal to the C2-C3 bond cally and consistently. length. Partial optimization of singlet 1 under Cs symmetry Cycloalkenylidenes constitute interesting models for the leads to a saddle point (1-TS), which corresponds to the study of the interdependence among the geometry, spin mul- transition state for the ring flipping between the two enan- tiplicity, and reactivity for vinylcarbenes. An extreme exam- tiomeric structures of 11-(C1). The barrier for this process is ple is cyclopropenylidene. In fact, this intriguing molecule predicted to be 21.3 kcal moil. has been studied both experimentally and computationally8. It is believed to have a singlet ground state, unlike its acyclic analogue vinylcarbene, which has a triplet ground state9-11. This difference is partly due to the small angle of the divalent carbon in cyclopropenylidenesa. On the other hand, very lit- tle is known for the higher homologue cyclobutenylidene 112,13 and its benzo analogue, benzocyclobutenylidene 2, which remain elusive. Our interest in the influence of geometry on the S-T gap and reactivity of vinylcarbenes prompted us to start our stud- ies'''. First, cyclobutenylidene 1 was explored computationally Figure 2. Selected B3LYP/6-31G (d) optimum geometrical parame- and the results were compared with those of its higher homo- ters of carbene 1 (distances in A, angles in degrees). logues, cyclopentenylidene 3, cyclohexenylidene 4, and cyclo- heptenylidene 5. Some unusual features in the structure and In the case of the saddle point 1-TS, the C2-C3 bond is reactivity of 1 prompted us to pursue the benzo analogue, longer than the C,-C2 (by 0.12A) and its C2-C3-C4 angle is benzocyclobutenylidene 2 both theoretically and experimen- narrower than that of the triplet state (by 8°). It appears tally. then that 1-TS displays more of the geometrical characteris- tics expected for a simple structural drawing of cyclobutenyli- dene than the energy minimum structure of 11. The latter could also be described as the twisted alkene bicyclobut-l-ene due to the rather short C2-C3 bond (1.372 A) and C,-C3 1 2 3 4 5 diagonal distances. From this point of view, 1-TS can be thought of as a carbene serving as the transition state for the ring inversion of the strained bicycloalkene (structure II ). 2. Cyclobutenylidene This is reminiscent of homocub-9-ylidene18' 19. In that case, Background. When Shevlin et all2a. attempted to generate the carbene is in equilibrium with its isomer homocub-1(9)-ene cyclobutenylidene 1 by deoxygenation of cyclobutenone using (a strained alkene) via a reversible C-C bond insertion mech- atomic carbon, the final product was vinylacetylene 6. No anism. products attributable to a 1, 2-hydrogen shift or to a 1, 2-car- bon shift were detected, implying that carbene 1 does not rearrange to cyclobutadiene 7 or methylenecyclopropene 8. This is somewhat surprising, since singlet carbenes bearing a-hydrogens are known to rearrange preferentially by 1, 2-H shift15 or in certain cases of sterically constrained carbenes, by 1 2-C shift16,17 Whether 11 is better regarded as a strained olefin or as a carbene is debatable, and our studies have not addressed this issue. Nevertheless, the geometrical features of 11 strongly suggest that both resonance structures shown below should be important descriptors of its electronic state. 6 7 8 1, 2-Hydrogen and 1, 2-carbon migration. Generally, 1, 2-hydrogen shifts in alkyl carbenes occur rather easily15. We have examined how the angle strain at the carbenic However, a significantly larger barrier of 50.4 kcal moll is

Vol.59 No.11 2001 ( 35 ) 1071 predicted for 11. This large difference can be attributed to a) planarization of the ring for the hydrogen migration, which as discussed above requires approximately 21 kcal moil. Another factor that may be also contributing to the high 1, 2-H barrier for 11, is the formation of the antiaromatic cyclobutadiene. 1, 2 C-shifts are much less common than 1, 2-H shifts in singlet carbene reactions, but they have been reported in a few cases of conformationally rigid systems16. A recent example is cyclobutylidene where 1, 2-C migration is preferred over 1, 2-H migration17. Similarly in singler cyclobutenylidene (11), the 1, 2-C shift barrier (34.3 kcal moll) is computed to be lower in energy than the 1, 2-H barrier. However, in this particular case neither of these shifts can compete with the ring opening to butadienylidene as dis- cussed later on. Higher homologue of cyclobutenylidene 1. In contrast to the unusual features in the structure and reactivity of 1, cal- culations for its higher homologues, cyclopentenylidene 3, cyclohexenylidene 4, and cycloheptenylidene 5 provided more or less expected results. In Table 1 are summarized the heats of formation and the derived S-T gaps. For higher homo- logues, the break-even point seems to be the six-membered ring for which the singlet and triplet states are calculated to be nearly isoenergetic. The seven-membered cyclic vinylcar- b) bene 5 has a triplet ground state like the parent vinylmethy- lene.

Table 1. Best Estimates of heats of formation (AHf298,kcal mol-1) and derived S-T splittings (AEs_T,kcal moll) for small- and medium-size cycloalkenylidenes 1, 3, 4, and 5

Figure 3. a) Lowest energy rearrangement path of 11 leading to the formation of vinylacetylene. (G2 Energies (kcal moil) The carbenic angles of the singlet states of 3, 4, and 5 are relative to 11). b) QCISD (T) relative energies (kcal 95.1°, 103.1•‹, and 118.5°, respectively. The double bond mol-1) of butadienylidene conformers. lengths (C1-C2) in the singlet states (3: 1.363A, 4: 1.379A, and 5: 1.373A) are consistently shorter than the C2-C3 bond moll). The s-trans conformer easily isomerizes to viny- lengths by 0.03-0.09A while the triplet states have allylic-type lacetylene via a 1, 2 H-shift, with a barrier of 0.6 kcal moll, structure with similar C1-C2 and C2-C3 bond lengths. Like which is comparable to the barrier reported for the isomer- normal alkylcarbenes, cyclopentenylidene 3, 4, and 5 are cal- ization of the parent vinylidene to acetylene (1-1.5 kcal culated to have low barriers for 1, 2-H shifts (3: 7.5, 4: 9.7, mo-1)21. A direct path between 9 and 6 is via TS (9-6), and 5: 5.6 kcal moll) but significantly higher barriers for 1, which corresponds to a 1, 2 C-shift. However, this path is 2-C shifts (3: 55.6, 4: 38.7, and 5: 11.8 kcal moil). 5.7 kcal mo1-1 higher in energy than the isomerization path Formation of vinylacetylene 6 from singlet 1. The forma- via s-trans-9, and therefore not likely to be competitive at tion of vinylacetylene 6 is predicted to be the lowest energy low temperatures. path available for the intramolecular rearrangement of 1 Overall, it appears that the bottleneck in the 1 6 rear-

(Figure 3a), in agreement with the experimental observations rangement is the cleavage of the C3-C4 bond. The intermedi- of Shevlin et a112. This isomerization proceeds via rupture of ate butadienylidenes (9 and s-trans 9) lie in shallow potential the C3-C4 bond, which is the longest C-C bond of 11, to minima (of less than a couple of kcal moil in depth). Based form butadienylidene (9, Figure 3a). on this, one may conclude that the detection of butadienyli- Butadienylidene 9, by analogy to butadiene, has two mini- dene would be difficult. On the other hand, singlet ma: a planar s-trans structure (s-trans 9) and a twisted one cyclobutenylidene (11) appears to be in an energy well of

(C2 symmetry for butadiene, but C1 for 9)14a The planar around 9 kcal moV1 in depth. Presumably, under Shevlin's s-cis structure is a transition state connecting the two Cl con- conditions, the deoxygenation of cyclobutenone by atomic formers of 9, and the barrier for this enantiomerization is 0.4 carbon leaves behind a hot carbene that has enough energy kcal moll (Figure 3b). The rotational barrier of 9, towards to overcome this barrier. the more stable s-trans conformer, is slightly higher (1.7 kcal Generation of cyclobutenylidene 1 and its derivatives under

1072 ( 36 ) J. Synth . Org . Chem., Jpn . suitable conditions should make it experimentally accessible. Durr and co-workers23 reported that thermolysis and pho- Previously, halogen-metal exchange of hexachlorocy- tolysis of the sodium salt of benzocyclobutenone tosylhydra- clobutene in the presence of alkenes resulted in products zone afford the formal dimer of the carbene 2, and that nei- which are, at least formally, adducts between tetrachlorocy- ther benzocyclobutadiene nor its dimer were detected. clobutenylidene and alkenes20a,b. Similar adduct formation According to their semiempirical calculations carbene 2 was found in the photolysis of sodium salt of 2-fluoro-3- should have a triplet ground state. However, their trapping pheny1-2-cyclobutenone tosylhydrazone with 2-butenes2(k. experiments with alkenes were essentially stereospecific in However, no direct evidence has been provided for the inter- nature showing no evidence of stepwise addition. In order to vention of cyclobutenylidenes. to accommodate the computational and the experimental data, the idea of an equilibrium between singlet and triplet states was promoted23.

Although all of the earlier reactions gave products consis- 3. Benzocyclobutenylidene tent with those expected from carbene 2, it has not been

Another attractive model for exploring the interdepen- observed spectroscopically and its involvement in the afore- dence of carbene geometry, substitution and multiplicity is mentioned chemistry has not been unequivocally established. benzocyclobutenylidene 2. Again, the bond angle at the We therefore decided to investigate this interesting carbene by divalent carbon in 2 is significantly smaller than that of the attempting to observe it directly in an inert matrix at low parent phenylcarbene (90•‹vs. 1 35-1 55•‹)3, although in other temperature and studying it at definitive levels of theory. respects the electronic perturbation introduced by the small Thermochemistry and energetics of 2 and the related C8H6 ring is minimal. isomers. The G2 (MP2,SVP) relative energies for several

Several groups attempted to study benzocyclobutenylidene C8H6 isomers (2, 10-13), computed using B3LYP/6-3 1G(d)

2 or its simple derivatives by photolysis or thermolysis of var- geometries, are given in Scheme 1. According to our calcula- ious precursors22-24. Photolyses of sodium salts of benzocy- tions the ground state of benzocyclobutenylidene 2 is singlet clobutenone tosylhydrazones resulted in hydrocarbon prod- and its geometry is nonplanar. Singlet 2 undergoes confor- ucts which were assignable to the (formal) carbene dimers mational ring inversion through a planar transition state with and trimer22a, 24. a computed barrier of ca. 7 kcal mol-1. Triplet 2 is found to

be planar and to lie 8-15 kcal mol-1 higher in energy than the

singlet ground state. This situation is similar to the simple

analogue, cyclobutenylidene 1, which displays a pronounced

deviation from planarity in its singlet state12, 14a However, in

the case of 2, the presence of the rather rigid benzene ring

does not allow for much out-of-plane deformation. The

degree of stabilization afforded in the singlet state by virtue

of the geometric distortion is reflected in the planarization

barrier, which is calculated to be 7 kcal mol-1 for benzocy-

clobutenylidene 2 but 21 kcal mol-1 for cyclobutenylidene

114a. The singlet-triplet energy gap (AEs_T) of benzocy-

Photolysis of a similar sodium salt afforded an isomeric clobutenylidene 2 was estimated in two waysl4b. The value of

mixture of (formal) carbene dimers while thermolysis resulted 7.8 kcal mol-1 was determined directly from the BLYP rela- - in 9a, 10-dihydrobenz[a]azulene presumably via the initial tive energies, while the value of -14.5 kcal mol-1 was deter-

formation of a benzene adduct22b,c. mined using the experimental singlet-triplet gap of methylene

(ĢES-T = +9 kcal mol-1)2f and the G2 (MP2,SVP) energy of the isodesmic reaction 32 + 1CH2 12 + 3CH2.

The intramolecular rearrangement pathways of singlet ben-

zocyclobutenylidene (12) was examined computationally

(Scheme 1): (a) 1, 2-hydrogen shift (the most common rear- rangement path for carbenes bearing a-hydrogens)15, (b) 1,

2-carbon shift (this path can become dominant in sterically

strained carbenes)17'26, and (c) ring-opening of the fused ring

(forming vinylidene 10) followed by a C-C insertion leading to strained cycloalkyne 11. Path (c) may seem unusual, but it

is calculated to be the most likely path followed in the iso-

Vol.59 No.11 2001 ( 37 ) 1073 Scheme 1 a)

b) merization of cyclobutenylidene as described above12, 14a.

From the calculated enthalpic barriers for the three processes,

it is found that while path (c) is preferred its barrier of 21.7

kcal mo1-1 is still large. Therefore, it appears that benzocy-

clobutenylidene lies in a rather deep potential well and

should be observable under suitable experimental conditions.

Attempted generation of 2 in an Ar matrix. An obvious

photochemical precursor for 2 is diazobenzocyclobutene (14), but this species was found to be too unstable for isolation

and purification under normal preparative conditions. Thus,

the more robust N-aziridinylimine 15 was chosen as a pre-

cursor, which was prepared in a pure form and deposited

directly into the Ar matrix. Irradiation of 15 (ƒÉ•„ 300 nm)

gave rise to 14 and styrene. The structure of 14 was assigned Figure 4. Comparison of experimental (Ar, 10 K) and theoretical on the basis of the strong, characteristic C=N=N absorption IR spectra: a) Calculated IR spectrum (B3LYP/6-31G (d) at 2042 cm-1 and the good agreement of the experimental scaled by 0.961) for cycloalkyne 11. b) Experimental IR and computed IR spectra. spectrum of the photoproducts formed by irradiation of N-aziridinylimine 15. Bands due to photoproduct A are marked with S and those due to styrene with •~ . Scheme 2

Although, cycloalkyne 11 is the photoproduct obtained

upon irradiation of the diazo precursor 14, no evidence for

the formation of carbene 2 was obtained. If carbene 2 is

indeed formed as an intermediate during the photolysis of

diazo compound 14, it must either be formed "hot" having

Photolysis of 14 (using either A •„ 300 nm or A •„ 564 nm) sufficient vibrational energy to overcome the barrier to iso- gave rise to new peaks assigned to A (II, Figure 4a). When merization to alkyne 11 or it must be photosensitive under the photolysis was carried out in an Ar matrix doped with the irradiation conditions (ƒÉ•„534 nm).

0.3% oxygen, the same final photoproduct was observed. Laser flash photolytic studies. Previous results in support

Furthermore, annealing the matrix containing the products at of benzocyclobutenylidene 2 as an intermediate were provid-

30-35 K resulted in no appreciable changes in the spectrum. ed by photolysis of various precursors in solution at ambient

An ESR study performed under similar experimental condi- temperature. In this respect, application of time-resolved tions failed to detect the presence of any triplet species. The spectroscopy is a straightforward method and may enable us agreement between the experimental IR spectrum of A and to observe benzocyclobutenylidene 2 at room temperature. the computational spectrum for singlet or triplet benzocy- Thus, we undertook laser flash photolytic studies with oxadi- clobutenylidene is very poor. These pieces of evidence are azoline precursor 1627. inconsistent with a benzocyclobutenylidene intermediate (12 Laser flash photolysis of oxadiazoline 16 in acetonitrile or 32), suggesting that intermediate A is perhaps a rearrange- afforded transient absorption spectra as shown in Figure 5. ment product of 2. A broad and weak absorption centered around 450 nm

The IR spectra of several C8H6 isomers were computed appeared immediately after laser excitation and decayed with and compared to the experimental IR spectrum of A. Two the rate constant of k=2.1 X 106 s-1 (z=490 ns) while con- good matches were found for vinylidene 10 cycloalkyne 11. current growth of a new transient absorption at 360 nm was The main discriminating factor in their computed IR spectra observed. The 360 nm transient decayed with the rate con- is the presence of the bond absorption in the latter. In stant of k=2.4 X 103 s-1 (Ą= 420 its). After these transients the experimental spectrum there is a weak absorption at 2098 disappeared, a longer-lived species was observed as a weak -1 cm, which enables the assignment of photoproduct A as absorption at 510 nm, whose lifetime was 660 ,us.

7-methylenecyclohepta-3, 5-dien-l-yne (11) (Figure 4). The initially formed transient at 450 nm can be assigned as

Additional evidence for this assignment comes from 13C being due to carbene 2 in its singlet state based on the obser- labeled experiments, which gave the expected isotopic shift vation that the decay rate remained unchanged in oxygenated for the bond absorptionl4b. acetonitrile but dramatically increased in the presence of elec-

1074 ( 38 ) J. Synth. Org . Chem ., Jpn .

18 19 20 trans-21a, b

23 24a 24b 25

Figure 5. Transient absorption spectra obtained by laser flash pho- tolysis of oxadiazoline 16 in acetonitrile at 75, 200, and 500 ns dulation. Inset shows an absorbance decay moni- tored at 450 nm. tron-rich alkenes. The 450 nm transient was also quenched by alcohols. While the transient due to carbene 2 (450 nm) was quenched by added alkenes or alcohols, the growth rate of the 360 nm transient remained unchanged. However, the maximum intensity decreased with increasing concentration of added alkenes or alcohols. These results suggest that the

360 nm transient be derived from carbene 2. We found that the 360 nm transient is quenched by acetone (kad = 2.3 •~ 109 Figure 6. Transient absorption spectra obtained by laser flash pho- tolysis of oxadiazoline 16 in cyclohexane at 75, 200, and M-1s-1) while carbene 2 is not。Thus,the360-nmtransientis 500 ns dulation. Inset shows an absorbance decay moni- assigned to acetonitrile ylide 17 formed from carbene 2 and tored at 450 nm. the solvent, acetonitrile (Scheme 3). The 510 nm transient can be assigned as diazobenzocyclobutene 14 because this the remarkable growth of a new transient absorption at 380 species can be quenched by dipolarophiles such as fumaroni- nm. This spectral change can be ascribed to the formation of trile, while carbene 2 is not quenched. ylides 22a, b by the reaction between photogenerated carbene 2 and oxadiazoline 16 (Scheme 4) since the decay rate of car-

Scheme 3 bene 2 (450 nm) and the growth rate of ylides 22a, b are iden-

tical within experimental error and both rates increased with

increasing concentration of oxadiazoline 16. Plots of the

decay rate (450 nm) and the growth rate (380 nm) against the

concentration of oxadiazoline 16 were linear and the inter-

cepts of the plots (1.7 and 1.4 •~ 106 NC s-1) give the lifetime

of carbene 2, 0.59-0.71 is in cyclohexane. Since carbene 2

Consistent with the above results is that steady-state pho- reacts with cyclohexane, its inherent lifetime may be longer. tolysis of 16 in the presence of cyclohexene or methanol Steady-state photolysis experiments with 16 in cyclohexane resulted in the formation of adduct 18 or 19. Also consistent showed that the C-H insertion product 23 was a major pho- is that adduct 20 was obtained as one of the photoproducts from the steady-state photolysis of oxadiazoline 16 in an ace- Scheme 4 tonitrile-acetone mixture. Photoirradiation of 2 in the pres- ence of fumaronitrile afforded a mixture of trans adducts 21a, b in high yield. Figure 6 shows the transient absorption spectra obtained by LFP of oxadiazoline 16 in cyclohexane. Again, a tran- sient assignable to carbene 2 was observed 22a 22b around 450 nm as a short-lived absorption Table 2. Absolute rate constants and relative rates for addition of carbenes to alkenes with the lifetime of 340 ns. In addition, a longer-lived absorption band due to dia- zobenzocyclobutene 14 appeared at 510 nm. Plots of the observed decay rate constants (kobs)monitored at 450 nm against varying concentration of alkenes showed linear rela- tions and the slopes afforded the second order rate constants (kad) for the reaction of carbene 2 with alkenes as shown in Table 2. In neat cyclohexane, decay of the carbene absorption at 450 nm was concurrent with

Vol.59 No.11 2001 ( 39 ) 1075

toproduct when the concentration of 16 is 0.01 M but 23 was a highly puckered structure, which is virtually a resonance no longer a major product when the concentration of 16 is hybrid of a cyclobutenylidene form and a strained bicy- 0.1 M or higher. Instead, small amounts of azines and clobutene form. The lowest energy path available for the dimers were produced. Presumably, the resulting ylides 22 rearrangement of cyclobutenylidene 11 is calculated to be the may give azines and/or dimers in low yields. In the presence formation of vinylacetylene 6, which proceeds through a step- of alkenes in excess, however, alkene adducts are produced in wise mechanism involving the initial rupture of the C3-C4 high yields regardless of the concentration of 16. Carbene 2 bond followed by a hydrogen migration in butadienylidene 9. is found to react with cis- or trans-2-butene stereospecifical- Singlet benzocyclobutenylidene (12) is also computed to ly to give two cis-adducts 24a, b or trans-adduct 25, respec- have a puckered cyclobutene ring. Our calculations suggest tively. The observed stereospecificity is in accord with the that 12 lies in a rather deep potential well and should be prediction that carbene 2 has a singlet ground state with the observable under suitable experimental conditions. However, ΔEs-T of - 14.5 kcal mol-114b. photolysis of several precursors under low-temperature Ar Cyclopropanation is one of the most typical reactions of matrix conditions gave no evidence for the formation of car- singlet carbenes and their reactivity can be assessed based on bene 2. Instead, cycloalkyne 11 was observed by IR spec- the second order rate constants or relative rates. Moss and troscopy. Calculations find that 11 can be formed from 2 by co-workers developed the "carbene selectivity index", MCXY, a pathway which is conceptually similar to that followed by which describes differential reactivities of carbenes in cyclo- vinylcarbene to give vinylacetylene. propanation reactions by the common use of the term "car- On the other hand, LFP of precursor 16 in conjuction with bene philicity" 28. Figure 7 shows a "carbene philicity spec- time-resolved UV spectroscopy is capable of detecting benzo- trum" constructed by Moss, where carbenes are classified cyclobutenylidene 12 at room temperature in solution. Pre- into three broad regions; electrophiles, ambiphiles, and nude- liminary results find the lifetime of carbene 12 to range ophiles. The mcxy index is defined as the least-squares slope between 0.59-0.71 is and its Mcxy value to be 0.43. The of log (ki/k0)cxy vs. log (ki/k0)ccl2 by taking dichlorocarbene latter reveals the highly electrophilic nature of carbene 12, as a standard carbene. ranking it second among the electrophilic carbenes listed in the carbene philicity spectrum. Acknowledgment This work was supported by Grant-in Aid for Scientific Research from the Ministry of Education, Sci- ence, Sports and Culture of Japan. We thank Prof. Robert J. McMahon (University of Wisconsin) for helpful discussions.

References 1) For carbenes in general: (a) Kirmse, W Carbene Chemistry; 2nd ed.; Academic Press: New York, 1971. (b) Jones, M.; Figure 7. A "carbene philicity spectrum". Reconstructed based on Moss, R. A., Eds. Carbenes; R. E. Krieger: Malabar, FL, the data in ref 28a with the addition of the data for 2. 1983. (c) Wentrup, C. Reactive Molecules; John Wiley: New Nucleophile region is omitted for simplicity. York, 1985. 2) (a) Wasserman, E.; Kuck, V. J.; Hutton, R. S.; Yager, R. S. J. The Mcxy value for carbene 2 is preliminarily determined to Am. Chem. Soc. 1970, 92, 7491. (b) Wasserman, E.; Yager, R. S.; Kuck, V. J. Chem. Phys. Lett. 1970, 7, 409. (c) Wasserman, be 0.43. For comparison, the kobsvalues for the reaction of E.; Kuck, V. J.; Hutton, R. S.; Anderson, ; Yager, R. S. J. phenylchlorocarbene with the same alkenes are also shown in Chem. Phys. 1971, 54, 4120. (d) Bernheim, R. A.; Bernard, H. Table 2. Based on those data the Mcxy value for phenylchloro- W; Wang, P. S.; Wood, L. S., Skell, P. S. J. Chem. Phys. 1970, carbene is determined to be 0.84, which is in excellent agree- 53, 1280. (e) Bernheim, R. A.; Bernard, H. W.; Wang, P. S.; Wood, L. S., Skell, P. S. J. Chem. Phys. 1971, 54, 3223. (f) ment with the value of 0.83 determined previously based on McKellar, A. R. W; Bunker, P. R.; Sears, T. J.; Evenson, K. competition kinetics28a. It is noteworthy that the Mcxy value M.; Saykally, R. J.; Langhoff, S. R. J. Chem. Phys. 1983, 79, of carbene 2 is much smaller than that of phenylchlorocar- 5251. (g) Kim, S.-J.; Hamilton, T. P.; Schaefer, H. F., III J. bene, 0.84. Indeed, carbene 2 ranks second among the elec- Chem. Phys. 1991, 94, 2063. (h) Russo, N.; Sicilia, E.; Toscano, M. J. Chem. Phys. 1992, 95, 5031. trophilic carbenes31 listed in the carbene philicity spectrum 3) (a) Trozzolo, A. M.; Murray, R. W; Wasserman, E. J. Am. (Figure 7). Chem. Soc. 1962, 84, 4990. (b) Higuchi, J. J. Chem. Phys. The strongly electrophilic nature of carbene 2 explains why 1963, 39, 1339. d) Wasserman, E.; Trozzolo, A. M.; Yager, W A.; Murray, R. W. J. Chem. Phys. 1964, 40, 2408. (c) West, P. 2 reacts with acetonitrile or the oxadiazoline itself, whereas R.; Chapman, 0. L.; LeRoux, J.-P. J. Am. Chem. Soc. 1982, phenylchlorocarbene is reported to be unreactive to acetoni- 104, 1779. (d) McMahon, R. J.; Abelt, C. J.; Chapman, 0. L.; trile. The high electrophilicity of 2 seems to stem from its Johnson, J. W; Kreil, C. L.; LeRoux, J.-P.; Mooring, A. M.; lower HOMO (a) and LUMO (p). However, for a more West, P. R. J. Am. Chem. Soc. 1987, 109, 2456. (e) Haider, K. W; Platz, M. S.; Despres, A.; Migirdicyan, E. Chem. Phys. detailed and quantitative consideration additional data based Lett. 1989, 164, 443. (f) Matzinger, S.; Bally, T.; Patterson, E. on both experimental and theoretical studies are required. V.; McMahon, R. J. J. Am. Chem. Soc. 1996, 118, 1535. (g) Wong, M. W; Wentrup, C.; J. Org. Chem. 1996, 61, 7022. (h) 4. Concluding Remarks Schreiner, P. R.; Karney, W L.; Schleyer, P. v. R.; Borden, W T.; Hamilton, T. P.; Schaefer, H. F. J. Org. Chem. 1996, 61, Geometric constraints have significant effects on unsaturated 7030. carbenes. In contrast to the parent vinylcarbene and phenylcar- 4) (a) Shuster, G. B. Adv. Phys. Org. Chem. 1986, 22, 311. (b) bene, cyclobutenylidene and benzocyclobutenylidene are shown Mueller, P. H.; Rondan, N. H.; Houk, K. N.; Harrison, J. F.; Hooper, D.; Willen, B. H.; Liebman, J. F. J. Am. Chem. Soc. to have a singlet ground state with S-T gaps of -25 and 1981, 103, 5049. (c) Hoffmann, R.; Zeiss, G. D.; Van Dine, G. 14.5 kcal moil, respectively. Singlet cyclobutenylidene 11 has- W. J. Am. Chem. Soc. 1968, 90, 1485.

1076 ( 40 ) J. Synth. Org. Chem., Jpn. 5) See for example: Platz, M. S. Ed. Kinetics and Spectroscopy of Soc. 1997, 119, 3191. (c) Sulzbach, H. M.; Platz, M. S.; Carbenes and Biradicals; Plenum: New York, 1990. Schaefer, H. F.; Hadad, C. M. J. Am. Chem. Soc. 1997, 119, 6) For a recent review about carbenes in matrices, see Sander, W; 5682. Bucher, G.; Wierlacher, S. Chem. Rev. 1993, 93, 1583. 27) Takahashi, Y; Inaba, M.; Tomioka, H. unpublished results. 7) For some recent examples, see: (a) Karney, W. L.; Borden, W. 28) (a) Moss, R. A. Acc. Chem. Res., 1980, 13, 58. (b) Moss, R. T. J. Am. Chem. Soc. 1997, 119, 3547. (b) Nicolaides, A.; A. Ace. Chem, Res., 1989, 22, 15. Smith, D. M.; Jensen, F.; Radom, L. J. Am. Chem. Soc. 1997, 29) Chateauneuf, J. E.; Johnson, R. P.; Kirchhoff, M. M. .1. Am. 119, 8083. (c) Matzinger, S.; Bally, T.; Patterson, E. V.; Chem. Soc. 1990, 112, 3217. McMahon, R. J. J. Am. Chem. Soc. 1996, 118, 1535. (d) 30) (a) Moss, R. A.; Prez, L. A.; Turro, N. J.; Gould, I. R.; Wong, M. W; Wentrup, C. J. Org. Chem. 1996, 61, 7022. (e) Hacker, N. P. Tetrahedron Lett. 1983, 24, 685. (b) Gould, I. R.; Balkova, A.; Bartlett, R. J. J. Chem. Phys. 1995, 102, 7116. (f) Turro, N. J.; Butcher, Jr. J.: Doubleday, Jr. N. C.; Hacker, N. Cramer, C. J.; Dulles, F. J.; Falvey, D. E. J. Am. Chem. Soc. P.; Lehr, G. F.; Moss, R. A.; Cox, D. P.; Guo, W; Munjal, R. 1994, 116, 9787. C.; Perez, L. A.; Fedorynski, M. Tetrahedron Lett. 1985, 41, 8) (a) Lee, T. J.; Bunge, A. Schaefer, H. F. J. Am. Chem. Soc. 1587. 1985, 107, 137. (b) Maier, G.; Reisenauer, H. P.; Schwab, W; 31) Difluorovinylidene is found to be a highly reactive and Carsky, P.; Spirko, V.; Hess, B. A. J.; Schaad, L. J. J. Am. extremely electrophilic singlet carbene: (a) J. Breidung, H. Chem. Soc. 1987, 109, 5183. (c) Clauberg, H.; Minkek, D. W; Burger, C. Kotting, R. Kopitzky, W. Sander, M. Senzlober, W. Chen, P. J. Am. Chem. Soc. 1992, 114, 99. Thiel, H. Willner Angew. Chem. Int. Ed. Engl. 1997, 36, 1983. 9) (a) Roth, H. D.; Hutton, R. S. Tetrahedron 1985, 41, 1567. (b) (b) C. Kotting, W. Sander, J. Breidung, W. Thiel, M. Senzlober Hutton, R. S.; Manion, M. L.; Roth, H. D.; Wasserman, E. J. und Hans Burger J. Am. Chem. Soc. 1998, 120, 219. (c) C. Am. Chem. Soc. 1974, 96, 4680. Kutting, W Sander, M. Senzlober und H. Burger Chem. Eur. J. 10) (a) Davis, J. H.; Goddard III, W A.; Bergman, R. G. J. Am. 1998, 4, 1611. (d) C. Kotting, W. Sander, M. Senzlober und H. Chem. Soc. 1977, 99, 2427. (b) Honjou, N.; Pacansky, J.; Burger Chem. Eur. J. 1998, 4, 2360. (e) W. Sander, C. Kotting Yoshimine, M. J. Am. Chem. Soc. 1985, 107, 5332. (c) Chem. Eur. J. 1999, 4, 24. (f) W. Sander, C. Kotting und R. Yoshimine, M.; Pacansky, J.; Honjou, N. J. Am. Chem. Soc. Hubert J. Phys. Org. Chem. 2000, 561. 1989, 111, 2785. (d) Yoshimine, M.; Pacansky, J.; Honjou, N. J. Am. Chem. Soc. 1989, 111, 4198. 11) Poutsma, J. C.; Nash, J. J.; Paulino, J. A.; Squires, R. R. J. Am. Chem. Soc. 1997, 119, 4686. 12) Dyer, S. F.; Kammula, S.; Shevlin, P. B. J. Am. Chem. Soc. PROFILE - 1977, 99, 8104. 13) Kollmar, H.; Carrion, F.; Dewar, M. J. S.; Bingham, R. C. J. Yasutake Takahashi is Associate Profes- Am. Chem. Soc. 1981, 103, 5292. sor at Mie University. He was born in 14) (a) Nicolaides, A.; Matsushita, T.; Tomioka, H. J. Org. Chem. Iwate in 1955. He received his educa- 1999, 64, 3299. (b) Nicolaides, A.; Matsushita, T.; Yonezawa, tion from Iwate University (B. Eng.) and K.; Sawai, S.; Tomioka, H.; Stracener, S. S.; Hodges, J. A.; Tohoku University (M. Sc. and D. Sc.). McMahon, R. J. J. Am. Chem. Soc. 2001, 123, 2870. After several years of postdoctoral 15) See, for example: (a) Liu, M. T. H. Acc. Chem. Res. 1994, 27, work, he jointed the faculty at Tohoku 287. (b) Evanseck, J. D.; Houk, K. N. J. Am. Chem. Soc. University as Assistant Professor in 1990, 112, 9148. (c) Evanseck, J. D.; Houk, K. N. J. Phys. 1987. He moved to Mie University in Chem. 1990, 94, 5518. 1996. His research interests are in the 16) (a) Shevlin, P. b.; McKee, M. L. J. Am. Chem. Soc. 1989, 111, area of organic photochemistry. 519. (b) Wiberg, K. B.; Burgmaier, G. J.; Warner, P. J. Am. Chem. Soc. 1971, 93, 246. 17) For a review see: Backes, J.; Brinker, U. H. Carbene (oide), Athanassios Nicolaides is Assistant Pro- Carbine; Regitz, M., Ed.; Houben-Weyl, Thieme: Stuttgart, fessor of Chemistry at the University of 1989: Vol. E19b; p 511. Cyprus. He was born in Greece in 1962. 18) (a) Eaton, P. E.; Hoffmann, K.-L. J. Am. Chem. Soc. 1987, He received his education from Univer- 109, 5285. (b) Eaton, P. E.; White, A. J. Org. Chem. 1990, 55, sity of Athens, Greece (B. Sc.) and Uni- 1321. (c) Eaton, P. E.; Appell, R.B. J. Am. Chem. Soc. 1990, versity of Washington, USA (Ph.D.). 112, 4055. After postdoctoral work at the Aus- 19) (a) Chen, N.; Jones, M., Jr. J. Phys. Org. Chem. 1988, 1, 305. tralian National University and Mie (b) Chen, N.; Jones, M., Jr. Terahedron Lett. 1989, 30, 6969. University, he jointed the faculty at Uni- (c) White, W R.; Platz, M.S.; Chen, N.; Jones, M., Jr. J. Am. versity of Cyprus in 2000. His research Chem. Soc. 1990, 112, 7794. (d) Chen, N.; Jones, M., Jr.; interests are in the area of computation- White, W. R.; Platz, M.S. J. Am. Chem. Soc. 1991, 113, 4981. al organic chemistry. 20) (a) Semmelhack, M. F.; DeFranco, R. J. Tetrahedron Lett. 1971, 1061. (b) Semmelhack, M. F.; DeFranco, R. J. J. Am. Chem. Soc. 1972, 94, 8838. (c) Yue, V. T.; Courson, C. J.; Brinkman, M. R.; Gasper, P. P. J. Org. Chem. 1978, 43, 4873. Hideo Tomioka is Professor of Chem- 21) (a) Gallo, M. M.; Hamilton, T. P.; Schaefer, H. F. J. Am. istry at Mie University. He was born in Chem. Soc. 1990, 112, 8714. (b) Ervin, K. M.; Ho, J.; Ise in 1941 and received his B. Sc. at Lineberger, W C. J. Chem. Phys. 1989, 91, 5974. Nagoya Institute of Technology. He 22) (a) Blomquist, A. T.; Heins, C. F. J. Org. Chem. 1969, 34, obtained his Ph. D. from Nagoya Uni- 2906. (b) O'Leary, M. A.; Wege, D. Tetrahedron Lett. 1978, versity in 1969. He joined the faculty at 2811. (c) O'Leary, M. A.; Richardson, G. W; Wege, D. Tetra- Mie University in 1971 as Associate hedron 1981, 37, 813. Professor and promoted to Professor in 23) Durr, H.; Nickels, H.; Pacala, L. A.; Jones, Jr., M. J. Org. 1985. He received the Award of the Chem. 1980, 45, 973. Japanese Photochemistry Association in 24) Frimer, A. A.; Weiss, J.; Rosental, Z. J. Org. Chem. 1994, 59, 1991 and the Divisional Award of the 2516. Japan Chemical Society in 1998. Cur- 25) (a) Wiberg, K. B.; Burgmaier, G. J.; Warner, P. J. Am. Chem. rent research interests include spec- Soc. 1971, 93, 246. (b) Shevlin, P. B.; McKee, M. L. J. Am. troscopy and chemistry of reactive inter- Chem. Soc. 1989, 111, 519. mediates, and design and synthesis of 26) (a) Friedman, L; Shechter, H. J. Am. Chem. Soc. 1960, 82, stable triplet carbenes. 1002. (b) Pezacki, J. P.; Pole, D. L.; Warkentin, J.; Chen, T.; Ford, F.; Toscano, J. P.; Fell, J.; Platz, M. S. J. Am. Chem.

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