On the electronic character of localized singlet 2,2-dimethoxycyclopentane-1,3-diyl diradicals: substituent effects on the lifetime

Photodenitrogenation of the diazenes 4 affords exclusively the housanes 5 through intramolecular cyclization of the spectrally detected and characterized singlet diradicals 3. The lifetime of singlet diradical 3, determined by transient absorption measurements, depends on the Y and Z substituents at the para position of the phenyl ring and has the following order: Y, Z = OMe, OMe > OMe, CN > CN, CN > OMe, H > Cl, Cl approximately CN, H approximately Me, Me > H, H. This unprecedented substituent effect reveals stabilization of the singlet 2,2-dimethoxycyclopentane-1,3-diyl diradicals 3 through radical, zwitterionic, pi-bonding, and hyperconjugative structures.     

Structural and dynamic studies on amido-bridged rhodium and iridium complexes

Treatment of [[M(mu-Cl)(diolefin)](2)] with the lithium salts of primary and secondary amines (LiNRR') in diethyl ether affords the complexes [[M(mu-NRR')(diolefin)](2)] (M=Rh, Ir; diolefin=1,5-cyclooctadiene (cod), tetrafluorobenzobarrelene (tfb); R'=H, R=tBu, Ph, 4-MeC(6)H(4); R=R'=Ph, 4-MeC(6)H(4)). Mixed-bridged chloro/amido complexes are intermediates in these syntheses, two of which, [[Rh(cod)](2)(mu-NHR)(mu-Cl)] (R=tBu, 4-MeC(6)H(4)), have been isolated. Replacement of the diolefin ligands by carbon monoxide or tert-butyl isocyanide in selected compounds takes place with retention of the binuclear structure to give the corresponding complexes [[M(mu-4-HNC(6)H(4)Me)(CO)(2)](2)], [[Rh(mu-4-HNC(6)H(4)Me)(CNtBu)(2)](2)] (12), and [[Rh(mu-NPh(2))(CNtBu)(2)](2)] (13). Single-crystal X-ray diffraction analyses of the complexes [[Rh(mu-NRR')(cod)](2)] (R'=H, R=4-MeC(6)H(4) (3); R=R'=4-MeC(6)H(4) (5)), 12, and 13 have shown that the conformation of the "RhN(2)Rh" four-membered metallacycle is planar in 5 and folded in 3, 12, and 13. The complexes with primary amides, 3 and 12, were found to exist as the syn,endo stereoisomers. The fluxionality of the complexes with secondary amides is due to rotation of the aromatic substituents about the N-C(ipso) bond and, in the case of 13, to the inversion of the "RhN(2)Rh" metallacycle as well. The complexes [[M(mu-NHR)(cod)](2)] (R=Ph, 4-MeC(6)H(4)) were found to exist as isomeric mixtures in solution, the syn/anti ratio being 2:3 for the rhodium derivatives and 1:1 for their iridium counterparts. Again, the motion detected was due to rotation of the aromatic substituents, and could be frozen only in the case of the syn isomers. The complex [[Rh(mu-NHtBu)(cod)](2)] with aliphatic amido ligands was found to be the anti folded isomer and proved to be nonfluxional. The most common conformation of the "RhN(2)Rh" metallacycle in these compounds is folded, and the preferred configuration varies from syn for the less encumbered compounds to anti on increasing the bulkiness of the bridging and ancillary ligands.     

Computational study on the origin of the stereoselectivity for the photochemical denitrogenation of diazabicycloheptene

The origin of the inversion stereoselectivity of housane formation via photochemical nitrogen extrusion of diazabicycloheptene (DBH) has been investigated using reaction path computations and multireference second-order perturbation theory within a CASPT2//CASSCF scheme. We show that the primary photoproduct of the reaction is an exo-axial conformer of the diazenyl diradical ((1) DZ) which displays a cyclopenta-1,3-diyl moiety with a Cs-like structure. (1) DZ is selectively generated via decay at a linear-axial conical intersection, and it is located in a shallow region of the ground state potential energy surface that provides access to five different reaction pathways. Reaction path analysis (including probing with classical trajectories) indicates that production of inverted housane can only occur via impulsive population of an axial-to-equatorial pathway, and it is thus inconsistent with thermal equilibration of the primary (1) DZ conformer. Similarly, according to the same analysis, the decrease of inversion stereoselectivity and even the retention (stereochemical memory effect) observed for suitably substituted DBHs are explained by dynamics effects where the axial-to-equatorial impulsive motion is restrained by the inertia and/or steric hindrance of the substituents. These results shade light on the poorly understood mechanisms that allow a photochemical reaction, in which a large amount of energy is deposited in the reactant by photon absorption, to show a high degree of stereoselectivity.     

Stereochemical memory versus Curtin-Hammett behavior in the rearrangement of 1,3-cyclopentanediyl radical cations derived from housanes through structural effects on conformational control

The electron-transfer-catalyzed rearrangement of the housanes 1 affords regioselectively the two cyclopentenes 2 and 3 by 1,2-migration of a group at the methano bridge. Appropriate ring annelation in the intermediary cyclopentane-1,3-diyl radical cation 1(*+) changes the stereochemical course of the rearrangement from complete stereoselectivity (stereochemical memory) for the structurally simple housane 1b to partial loss of stereoselectivity through competing conformational interconversion for the tricyclic housane 1c. Additional cyclohexane annelation, as in the tetracyclic housane 1a, results in complete loss of stereocontrol through Curtin-Hammett behavior, as substantiated by the viscosity dependence on the product ratio of the rearrangement. Whereas in the radical cations 1b(*+) and 1c(*+) the 1,2-shifts (k(2) and k(3)) are faster than the conformational anti <==> syn change (k(1), k(-1)), the reverse applies for the radical cation 1a(*+). Such structural manipulation of conformational effects in radical cation rearrangements has hitherto not been documented.     

Viscosity dependence of the denitrogenation quantum yield in azoalkane photolysis: experimental evidence for reversible formation of the diazenyl diradical

[reaction: see text] Experimental evidence is reported for the reversible formation of the singlet diazenyl diradical ((1)DZ), photolytically generated from the structurally elaborate DBH-type azoalkane. Reversiblity of the (1)DZ formation manifests itself through the decrease of the photodenitrogenation quantum yield over a ca. 40-fold viscosity variation (from 0.5 to 19.3 cP). This viscosity behavior is interpreted in terms of frictional effects on the competitive reaction modes of the diazenyl diradical.     

Secondary deuterium kinetic isotope effect for aquation, solvolysis, and isomerization reactions of trans-[Co(en)2(OSMe2)N3]2+, and the resolution of a mechanistic anomaly

The two closely spaced NH signals in the (1)H NMR spectrum of trans-[Co(en)(2)(OSMe(2))(N(3))](2+) have been reassigned using 2D NMR and other techniques. Thus, the unusual syn to anti (to Co-N(3)) NH rearrangement on base catalyzed substitution of the selectively deuterated complex in ND(3)(l) has been reinterpreted as "normal", with inversion of the effective deprotonation site accompanying the act of substitution. The re-examination of this system required a repeat study of the secondary isotope effect for the acid hydrolysis reaction, previously used to assign syn and anti amine sites, and this has been extended to other solvents (Me(2)SO, MeCN). The relative NH proton exchange rates are also reconsidered. A systematic rate reduction for Me(2)SO substitution is observed for deuterium incorporation into the cis-NH centers, irrespective of whether these are syn or anti, and the effect is much greater in Me(2)SO than in water. The results are interpreted in terms of zero point energy effects and coupled vibrations.     

Highly enantioselective michael addition of silyl nitronates to alpha,beta-unsaturated aldehydes catalyzed by designer chiral ammonium bifluorides: efficient access to optically active gamma-nitro aldehydes and their enol silyl ethers

Highly enantioselective Michael addition of silyl nitronates to alpha,beta-unsaturated aldehydes has been accomplished by the utilization of designer N-spiro C2-symmetric chiral quaternary ammonium bifluoride 1 as an efficient catalyst, providing direct access to both optically active gamma-nitro aldehydes, a very useful precursor to various complex organic molecules including aminocarbonyls, and their enol silyl ethers, a Mukaiyama donor of potential synthetic utility for further selective transformations. For instance, the reaction of trimethylsilyl nitronate 2 (R1 = Me) with trans-cinnamaldehyde (R2 = Ph, R3 = H) in toluene in the presence of (R,R)-1 (2 mol %) proceeded smoothly at -78 degrees C to give the desired enol silyl ether 3 (R1 = Me, R2 = Ph, R3 = H) in 90% isolated yield (anti/syn = 83:17) with 97% ee (anti isomer), and simple treatment of 3 thus obtained with 1 N HCl in THF at 0 degrees C afforded the corresponding gamma-nitro aldehyde 4 quantitatively without loss of diastereo- and enantioselectivity.     

Temperature and viscosity dependence in the stereoselective formation of the inverted housane for the photochemical nitrogen loss from the deuterium-stereolabeled parent diazabicyclo[2.2.1]hept-2-ene.

For the liquid-phase photolytic denitrogenation of the stereolabeled DBH derivative exo-d2-diazabicyclo[2.2.1]heptene (exo-d2-1), the k(inv)/k(ret) ratio of the inverted [2(inv)] and retained [2(ret)] housanes (bicyclo[2.1.0]pentanes) depends on the viscosity of the medium. For this purpose, the viscosity was varied by changing the solvent (various alcohols and diols, n-hexane, and acetonitrile) at constant temperature and by changing the temperature (-50 to +100 degrees C) in one single solvent, namely n-butanol. This viscosity effect is consistent with a stepwise denitrogenation mechanism in the liquid-phase photolysis of DBH, which proceeds through an unsymmetrical, nitrogen-containing transient, namely the singlet diazenyl diradical. The simple free-volume model adequately accounts for the observed viscosity behavior of the k(inv)/k(ret) ratio in terms of frictional effects. The temperature dependence discloses a small but measurable difference in the internal activation energies for the inversion and retention processes of the proposed diazenyl diradical.     

Olefin substitution in (silox)3M(olefin) (silox = (t)Bu3SiO; M = Nb, Ta): the role of density of states in second vs third row transition metal reactivity

The substitution chemistry of olefin complexes (silox)3M(ole) (silox = (t)Bu3SiO; M = Nb (1-ole), Ta (2-ole); ole = C2H4 (as 13C2H4 or C2D4), C2H3Me, C2H3Et, cis-2-C4H8, iso-C4H8, C2H3Ph, cC5H8, cC6H10, cC7H10 (norbornene)) was investigated. For 1-ole, substitution was dissociative (deltaG(double dagger)(diss)), and in combination with calculated olefin binding free energies (deltaG(o)(bind)), activation free energies for olefin association (deltaG(double dagger)(assoc)) to (silox)3Nb (1) were estimated. For 2-ole, substitution was not observed prior to rearrangement to alkylidenes. Instead, activation free energies for olefin association to (silox)3Ta (2) were measured, and when combined with deltaG(o)(bind) (calcd), estimates of olefin dissociation rates from 2-ole were obtained. Despite stronger binding energies for 1-ole vs 2-ole, the dissociation of olefins from 1-ole is much faster than that from 2-ole. The association of olefins to 1 is also much faster than that to 2. Linear free energy relationships (with respect to deltaG(o)(bind)) characterize olefin dissociation from 1-ole, but not olefin dissociation from 2-ole, and olefin association to 2, but not olefin association to 1. Calculated transition states for olefin dissociation from (HO)3M(C2H4) (M = Nb, 1'-C2H4; Ta, 2'-C2H4) are asymmetric and have orbitals consistent with either singlet or triplet states. The rearrangement of (silox)3Nb(trans-Vy,Ph-cPr) (1-VyPhcPr) to (silox)3Nb=CHCH=CHCH2CH2Ph (3) is consistent with a diradical intermediate akin to the transition state for substitution. The disparity between Nb and Ta in olefin substitution chemistry is rationalized on the basis of a greater density of states (DOS) for the products (i.e., (silox)3M + ole) where M = Nb, leading to intersystem crossing events that facilitate dissociation. At the crux of the DOS difference is the greater 5dz2/6s mixing for Ta vs the 4dz2/5s mixing of Nb. This rationalization is generalized to explain the nominally swifter reactivities of 4d vs 5d elements.     

Conformational studies by dynamic NMR. 90. Structure and stereodynamics of the rotamers of di- and tri-alpha-naphthylphenyl derivatives

The crystal structure of 1,3,5-tris(4-methylnaphth-1-yl)benzene, 1, shows one naphthyl substituent in an anti relationship to the other two. On the other hand, low temperature (-70 degrees C) (1)H NMR spectra in solution show the presence of a second rotational conformer (rotamer) having all the three naphthyl substituents in a syn relationship. The interconversion barrier between the anti (77%) and syn (23%) rotamers of 1 was determined by line shape simulation of the temperature-dependent NMR spectra (Delta G(++) = 12.1 kcal mol(-1)). In the analogous disubstituted meta and paraderivatives, that is, 1,3- and 1,4-bis(4-methylnaphth-1-yl)benzene (2 and 3, respectively), the presence of both the anti and syn rotamers was also detected by low-temperature NMR spectroscopy. In the latter compounds, the proportions of the anti and syn forms are nearly equal, and the corresponding anti to syn interconversion barriers were found to be lower (11.4 and 11.1(5) kcal mol(-1), respectively) than those of the trisubstituted derivative 1.     

Off-center oxygen-arene interactions in solution: a quantitative study

[reaction: see text] Triptycenes with C1-MeO/RCOO (R = H, Me, Et, i-Pr, CF3) and C9-XC6H4CH2 (X = Me, H, F, CN, CF3) have been prepared to determine lone pair-arene interactions in the off-center configuration. The ratios of the syn and anti conformers were determined by low-temperature NMR spectroscopy. The syn conformer allows the attached arene and the MeO/ester to interact with each other while the anti conformer does not. The free energies of interaction have been derived from the syn/anti ratios. Compound 7 in the ester series with X = H and R = CF3 is the only compound that shows a slightly repulsive interaction (0.08 kcal/mol). Compound 2e in the MeO series with X = CF3 exhibits an attractive interaction (-0.47 +/- 0.05 kcal mol). All other compounds show smaller attractive interactions.     

Development of a new Lewis acid-catalyzed [3,3]-sigmatropic rearrangement: the allenoate-Claisen rearrangement

A new Lewis acid-catalyzed Claisen rearrangement has been developed that allows the stereoselective construction of beta-amino-alpha,beta,epsilon,zeta-unsaturated-gamma,delta-disubstituted esters from simple allylic amines and allenoate esters. This reaction, which is contingent upon the use of Lewis acid, can be conducted with a range of metal salts (Yb(OTf)3, AlCl3, Sn(OTf)2, Cu(OTf)2, MgBr2.Et2O, FeCl3, Zn(OTf)2) with catalyst loadings as low as 5 mol %. This catalytic process provides access to a diverse range of beta-amino-alpha,beta,epsilon,zeta-unsaturated-gamma,delta-disubstituted esters in high yield and with excellent levels of diastereoselectivity for a series of allyl pyrrolidines (R1 = H, Me, i-Pr, Ph, NR2 = pyrrolidine, piperidine, NMe2; >/=81% yield, >/=94:6 syn:anti) and allenoate esters (R2 = H, Me, i-Pr, Ph, allyl, NPht, Cl; >/=75% yield, >/=91:9 syn:anti). The capacity of this new Claisen rearrangement to provide catalytic access to elusive structural motifs has also been demonstrated in the stereospecific formation of quaternary carbon bearing frameworks arising from geranyl- and neryl pyrrolidine (>/=93% yield, >98:2 dr).     

Designer chiral quaternary ammonium bifluorides as an efficient catalyst for asymmetric nitroaldol reaction of silyl nitronates with aromatic aldehydes

Designer chiral quaternary ammonium bifluoride 1 has been prepared, and both its catalytic and its chiral efficiency have been clearly demonstrated by achieving the first catalytic asymmetric nitroaldol reaction of silyl nitronate with aldehydes. For instance, the reaction of trimethylsilyl nitronate 2 (R(1) = Me) with benzaldehyde (R(2) = Ph) in THF in the presence of (S,S)-1 (2 mol %) proceeded smoothly at -78 degrees C, giving the corresponding nitroaldol adduct 3 (R(1) = Me, R(2) = Ph) in 92% isolated yield (anti/syn = 92:8) with 95% ee (anti isomer). The method was found to be successfully applicable to other aromatic aldehydes and silyl nitronates, and a high level of anti selectivity and enantiomeric excess was constantly observed. This finding should lead to the further development of fluoride ion-catalyzed asymmetric carbon-carbon bond-forming reactions.     

Silicon chelation in aqueous and nonaqueous media: the furanoidic diol approach

Anhydroerythritol (AnEryt) shares some of its ligand properties with furanosides and furanoses. Its bonding to silicon centers of coordination number four, five, and six was studied by X-ray and NMR methods, and compared to silicon bonding of related compounds. Diphenyl(cycloalkylenedioxy)silanes show various degrees of oligomerization depending on the diol component involved. For example, Ph(2)Si(cis-ChxdH(-2)) (1) and Ph(2)Si[(R,R)-trans-ChxdH(-2))] (2; Chxd = cyclohexanediol) are dimeric, Ph(2)Si(AnErytH(-2)) (3) is monomeric, and Ph(2)Si(L-AnThreH(-2)) (4; AnThre = anhydrothreitol) is trimeric both in the solid state and in solution. Ph(2)Si(cis-CptdH(-2)) (5) (Cptd = cyclopentanediol) is monomeric in solution but dimerizes on crystallization. Si(AnErytH(-2))(2) (6) and Si(cis-CptdH(-2))(2) (7) are monomeric spiro compounds in solution but are pentacoordinate dimers in the crystalline state. Pentacoordinate silicate ions are found in A[Si(OH)(AnErytH(-2))(2)] (A = Na, 8 a; Rb, 8 b; Cs, 8 c). Related compounds are formed by substitution of the hydroxo by a phenyl ligand. K[SiPh(AnErytH(-2))(2)]1/2 MeOH (9) is a prototypical example as it shows the two most significant isomers in one crystal structure: the syn/anti and the anti/anti form (syn and anti define the oxolane ring orientation close to, or apart from, the monodentate ligand, respectively). syn/anti Isomerism generally rules the appearance of the NMR spectra of pentacoordinate silicates of furanos(id)e ligands. NMR spectroscopic data are presented for various pentacoordinate bis(diolato)silicates of adenosine, cytidine, methyl-beta-D-ribofuranoside, and ribose. In even more basic solutions, hexacoordinate silicates are enriched. Preliminary X-ray analyses are presented for Cs(2)[Si(CydH(-2))(3)] 21.5 H(2)O (10) and Cs(2)[Si(cis-InsH(-3))] cis-Ins8 H(2)O (11) (Cyd = cytidine, Ins = inositol).     

PtII-mediated 1,3-dipolar cycloaddition of oxazoline N-oxides to nitriles as a key step to dihydrooxazolo-1,2,4-oxadiazoles

A novel type of heterocycle, viz., 2,3a-disubstituted 5,6-dihydro-3aH-[1,3]oxazolo[3,2-b][1,2,4]oxadiazoles, was generated by an intermolecular PtII-mediated 1,3-dipolar cycloaddition (1,3-DCA) between the oxazoline N-oxide C(Me)2CH2OC(R)=N+(O-) (R = Me, Et) and coordinated nitriles in the complexes trans/cis-[PtCl2(R'CN)2] [R' = Me, Et, CH2Ph, Ph, N(C5H10)]. The reaction is unknown for free RCN and oxazoline N-oxides, but under PtII-mediated conditions, it proceeds smoothly (CH2Cl2, 20-25 degrees C, 18-20 h) and gives pure complexes [PtCl2{N=C(R')ONC(R)OCH2CMe2}2] [R/R' = Me/Me, 1; Me/Et, 2; Me/CH2Ph, 3; Me/Ph, 4; Me/N(C5H10), 5; Et/Me, 6; Et/Et, 7; Et/CH2Ph, 8; Et/Ph, 9; Et/N(C5H10), 10] in 42-84% yields after column chromatography. Compounds 1-10 were characterized by elemental analyses (C, H, N), FAB+-MS, IR, and 1H and 13C{1H} NMR spectroscopies, and X-ray diffraction (for 1, 2, 5, and 9). With the exception of benzonitrile complexes, 1,3-DCA of oxazoline N-oxides to the PtII-ligated nitriles occurred diastereoselectively and afforded mixtures of enantiomers. Depending on the substituents on nitriles, asymmetric atoms in both of the formed heterocyclic ligands have the same (SS/RR) or different (SR/RS) configurations. The heterocyclic ligands were liberated from 1-4 and 6-9 by treatment with excess ethane-1,2-diamine (en) in CH2Cl2 for 1 day at 20-25 degrees C (for R' = Me, Et, CH2Ph) and at 50 degrees C (for R' = Ph) to achieve the free organic species and the well-known [Pt(en)2](Cl)2; the products were separated, and 2,3a-disubstituted 5,6-dihydro-3aH-[1,3]oxazolo[3,2-b][1,2,4]oxadiazoles (11-18) were characterized by ESI+-MS and 1H and 13C{1H} NMR spectroscopies.     

Reactions of Diazo Compounds with Alkenes Catalysed by [RuCl(cod)(Cp)]: Effect of the Substituents in the Formation of Cyclopropanation or Metathesis Products

The reaction of diazo compounds with alkenes catalysed by complex [RuCl(cod)(Cp)] (cod=1,5‐cyclooctadiene, Cp=cyclopentadienyl) has been studied. The catalytic cycle involves in the first step the decomposition of the diazo derivative to afford the reactive [RuCl(Cp){?C(R¹)R²}] intermediate and a mechanism is proposed for this step based on a kinetic study of the simple coupling reaction of ethyl diazoacetate. The evolution of the Ru–carbene intermediate in the presence of alkenes depends on the nature of the substituents at both the diazo N₂?C(R¹)R² (R¹, R²=Ph, H; Ph, CO₂Me; Ph, Ph; C(R¹)R²=fluorene) and the olefin substrates R³(H)C?C(H)R⁴ (R³, R⁴=CO₂Et, CO₂Et; Ph, Ph; Ph, Me; Ph, H; Me, Br; Me, CN; Ph, CN; H, CN; CN, CN). A remarkable reactivity of the complex was recorded, especially towards unstable aryldiazo compounds and electron‐poor olefins. The results obtained indicate that either cyclopropanation or metathesis products can be formed: the first products are favoured by the presence of a cyano substituent at the double bond and the second ones by a phenyl.     

Deuterium-labeling studies establishing stereochemistry at the oxypalladation step in Wacker-type oxidative cyclization of an o-allylphenol

The stereochemical pathway (syn or anti) of the oxypalladation step was studied in the palladium(II)-catalyzed Wacker-type cyclization of stereospecifically deuterated 6-(2-hydroxyphenyl)-3-deuteriocyclohexenes, cis-3-d-1 and trans-3-d-1, giving dihydrobenzofuran derivatives. The stereochemistry was determined to be syn in the reaction catalyzed by a dicationic palladium(II) catalyst generated from Pd(MeCN)4(BF4)2 and (S,S)-ip-boxax in the presence of benzoquinone in methanol, while it is mainly anti in the reaction catalyzed by PdCl2(MeCN)2 in the presence of a chloride ion.     

Stereospecificity and stereoselectivity in electrophilic substitution reactions of non-alpha-heterosubstituted organolithiums and stannanes: a rotationally restricted amide as an internal stereochemical marker.

The complete stereochemical course of a tin-lithium exchange/electrophilic quench sequence has been unambiguously determined by stereochemical characterization (using X-ray crystallography or NOE studies) at every step. Pairs of diastereoisomeric stannanes of known stereochemistry bearing atropisomeric amide substituents undergo tin-lithium exchange with alkyllithiums to give diastereoisomeric benzylic organolithiums whose stereochemistry can be assigned by NMR. For one atropisomer of the stannanes, the tin-lithium exchange is fully stereospecific and proceeds with retention of stereochemistry. The other atropisomer undergoes nonstereospecific tin-lithium exchange: the first reported example of a lack of stereospecificity in electrophilic substitution of tin for lithium. One of the diastereoisomeric atropisomeric organolithiums produced by the tin-lithium exchange is deuterated and alkylated with retention but stannylated with inversion of stereochemistry. The other is alkylated nonstereospecifically but stannylated with retention.     

Conformational Studies by Dynamic NMR. 57.(1) Stereodynamics of Syn-Anti Interconversion of Disubstituted Acyl Durenes

The orthogonal syn and anti isomers, originated by the restricted rotation about the Ar-C(O)Bu(t) single bonds in 1,4-bis(2,2-dimethylpropanoyl)durene (2e), have been separated by preparative thin layer chromatography. In solution they reach an equilibrium where the syn-anti ratio depends upon the polarity of the solvent. This allowed us to assign the anti structure, which has a null dipole moment, to the least retained isomer. The free energy of activation (DeltaG) for the interconversion was found to be 22.5 kcal mol(-)(1), a value high enough for identifying these species as configurational isomers. When less hindered derivatives, also having two RCO (R = Pr(i), Et, Me) substituents in the positions 1,4 of the durene moiety, were examined, the syn and anti forms could be detected only at low temperature by means of NMR spectroscopy. The corresponding interconversion barriers (DeltaG = 13.4, 11.7, 10.9 kcal mol(-)(1), respectively) are, in fact, much lower than for R = Bu(t), indicating that in these cases we are dealing with conformational rather than with configurational isomers.     

The almost bottleable triplet carbene: 2,6-dibromo-4-tert-butyl-2',6'-bis(trifluoromethyl)-4'-isopropyldiphenylcarbene.

Computations on 2,6-dibromo-4-tert-butyl-2',6'-bis(trifluoromethyl)-4'-isopropyldiphenylcarbene (1) using ab initio and density functional theory methods underscore the unusual stability of the triplet over the singlet state. At the B3LYP/6-311G(d,p) level, the triplet state had a slightly bent central C-C-C bond angle of 167 degrees, whereas this angle in the singlet was 134 degrees. The B3LYP singlet-triplet splitting (12.2 kcal/mol) was larger than that of the parent molecule (5.8 kcal/mol), diphenylcarbene (2), which also has a triplet ground state. The energy of a suitable isodesmic reaction showed the triplet and singlet states of (1) to be destabilized, by 6.3 and 12.5 kcal/mol, respectively, due to the combined effects of the CF3, Br, and alkyl substituents. The linear-coplanar form of (3)(1), which might facilitate dimerization or electrophilic attack at the more exposed diradical center, was prohibitively (35.9 kcal/mol) higher in energy. Our results confirm Tomioka's conclusion that the triplet diarylcarbene, ortho-substituted with bulky CF3 and Br substituents, is persistent due to steric protection of the diradical center. Dimerization and other possible reaction pathways are inhibited, not only by the bulky ortho substituents but also by the para alkyl groups. The increase in stability of the triplet ((3)(1)) state relative to the singlet ((1)(1)) state does not influence the reactivity directly.