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

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Geometrically Strained Carbenes: Interdependence Among Geometry, Spin Multiplicity, and Reactivity 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 ƒ¢ES-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.
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