Israel Fernández

Structural Evidence for Antiaromaticity in Free Boroles

Unsaturated boron-containing heterocycles such as borirenes (I), boroles (II), and borepins (III) have attracted fundamental interest owing to their electronic structure. The interaction of the empty pz orbital at boron in these systems with the unsaturated carbon backbone might result in a stabilization (I and III) or destabilization (II) of the entire p system, depending on the number of available p electrons. In particular, boroles (II) are of interest because of their close relationship to the cyclopentadienyl cation (IV), which

Dyotropic Reactions: Mechanisms and Synthetic Applications†

In 1972, M. T. Reetz defined dyotropic (from the greek dyo, meaning two) rearrangements as a new class of pericyclic valence isomerizations in which two σ-bonds simultaneously migrate intramolecularly.1,2 Reactions in which the two migrating groups interchange their relative positions were designated as type I (Scheme 1A) whereas those of type II involve migration to new bonding sites without positional interchange (Scheme 1B,C). The uncatalyzed and concerted nature of these processes was also proposed.3,4 Nowadays, dyotropic transformations are standard tools for the construction of organic and organometallic molecules. Frequently, this unique rearrangement is the single entry to an efficient preparation of target molecules. Moreover, the discovery of new dyotropic reactions involving transition metals and excited states had widened the mechanistic scope of these processes to new reaction pathways well away from the original definition of uncatalyzed and concerted processes. This review comprehensively reports the dyotropic reactions studied since the seventies, accounts for the different reaction mechanisms for these rearrangements, and discusses their synthetic applications in organic and organometallic chemistry.

Rate-Determining Factors in Nucleophilic Aromatic Substitution Reactions

Quantum chemical calculations (OPBE/6-311++G(d,p)) have been performed to uncover the electronic factors that govern reactivity in the prototypical S(N)Ar reaction. It was found that intrinsic nucleophilicity--expressed as the critical energy (the energy required for forming the Meisenheimer structure Ph(X)(2)(-)) in the identity substitution reaction X(-) + PhX --> X(-) + PhX (Ph = phenyl)--shows the following approximate trend: NH(2)(-) approximately OH(-) approximately F(-) >> PH(2)(-) approximately SH(-) approximately Cl(-) > AsH(2)(-) approximately SeH(-) approximately Br(-). The periodic trends are discussed in terms of molecular properties (proton affinity of X(-) expressing Lewis basicity of the nucleophile and C(1s) orbital energy expressing Lewis acidity of the substrate) based on a dative bonding model. Furthermore, the stepwise progress of the reactions and the critical structures are analyzed applying energy decomposition analysis. Increased stability, and thereby increased intrinsic nucleophilicity, correlates with decreasing aromatic character of the Meisenheimer structure. This apparent contradiction is explained in consistency with the other observations using the same model.

Direct estimate of conjugation and aromaticity in cyclic compounds with the EDA method

The nature of the interatomic interactions in cyclic conjugated molecules has been investigated with the Energy Decomposition Analysis (EDA). The focus of this work is on the strength of the pi bonding and pi conjugation in carbocyclic and heterocyclic molecules. The calculated deltaE(pi) values suggest that the EDA may be used directly for estimating the strength of pi conjugation in aromatic, homoaromatic, homoantiaromatic and antiaromatic compounds. The theoretical data show a pattern which agrees with the 4n + 2 rule. The extra aromatic stabilization energy has been obtained by comparing cyclic conjugation with the strength of pi conjugation in acyclic reference compounds. The deltaE(pi) values indicate that benzene is significantly stabilized by aromatic stabilization, but the total pi bonding contribution to the carbon-carbon bonding becomes even more stabilizing when the geometry is distorted toward a D(3h) form which has three long and three short C-C bonds. The D(6h) equilibrium structure is enforced by the sigma orbital interactions and by the quasiclassical electrostatic attraction. The VRE and ASE values suggest that pyridine is more strongly stabilized by aromatic conjugation than benzene. The EDA data of the six-membered cyclic 6pi species C5H5E (E = CH, N-Bi) and (HB = NH)3 predicts strength of aromatic stabilization in the order N > CH > P > As > Bi >> borazine. The ASE values of the five-membered heterocyclic systems C4H4E (E = NH, O, S) are smaller than for the benzene analogues showing the order NH approximately S > O. The ASE value for Cp is very small which probably comes from the choice of the reference system. The VRE results indicate that Cp is strongly stabilized by cyclic conjugation. The ASE values for the cyclic singlet carbenes indicate weak aromatic stabilization in the 2pi system cyclopropenylidene which is much stronger in the 6pi compound cycloheptatrienylidene while the 4pi molecule cylopentadienylidene is clearly antiaromatic. The triplet states of all three cyclic carbenes are antiaromatic. The strength of the pi conjugation in the homoconjugated cyclic compounds suggests weak aromatic stabilization in the 6pi compounds 1,3-cyclobutene and 1,3-cyclopentadiene but weak antiaromatic destabilization in the 2pi compound cyclopropene and in the 8pi molecules 1,3-cyclohexadiene and 1,3,5-cycloheptatriene. The 4n + 2 rule thus holds also for homoconjugated systems. A large negative value for the ASE is calculated for 1,3-cyclobutadiene.

Exocyclic Delocalization at the Expense of Aromaticity in 3,5-bis(π-Donor) Substituted Pyrazolium Ions and Corresponding Cyclic Bent Allenes†

Small ring allenes are usually highly strained and highly reactive species, and for a long time considered only as transient intermediates. The recent isolation of a five membered heterocyclic allene 1f has raised questions and debate regarding the factors responsible for its stability. Since 1f has been derived by deprotonation of a pyrazolium ion 2f, it has been suggested that the stability of 1f comes from its aromatic character. Here we report computational evidence, including HOMA and NICS aromaticity indices, that allenes derived from 3,5-bis(pi-donor) substituted pyrazolium salts are weakly aromatic to nonaromatic, and that even their pyrazolium ion precursors have dramatically reduced aromaticity. Exocyclic delocalization, involving the pi-donor substituents, occurs at the expense of aromaticity and increases with the strength of the donor. Experimental support for these conclusions is found in the crystallographically determined structure of 3,5-bis(dimethlamino)pyrazolium ion 2g, which exhibits highly pyramidalized endocyclic nitrogen centers but planarized exocyclic ones, and from the facile C4 protonation to give a stable pyrazole-1,2-diium salt 3g, which has also been crystallographically characterized.

Why Do Cycloaddition Reactions Involving C60 Prefer [6,6] over [5,6] Bonds?

The origin of the experimentally known preference for [6,6] over [5,6] bonds in cycloaddition reactions involving C60 has been computationally explored. To this end, the Diels-Alder reaction between cyclopentadiene and C60 has been analysed by means of the recently introduced activation strain model of reactivity in combination with the energy decomposition analysis method. Other issues, such as the aromaticity of the corresponding transition states, have also been considered. These results indicate that the major factor controlling the observed regioselectivity is the more stabilising interaction between the deformed reactants in the [6,6] reaction pathway along the entire reaction coordinate.

Alder-Ene Reaction: Aromaticity and Activation Strain Analysis

We have computationally explored the trend in reactivity of the Alder‐ene reactions between propene and a series of seven enophiles using density functional theory at M06‐2X/def2‐TZVPP. The reaction barrier decreases along the enophiles in the order H2CCH2 > HCCH > H2CNH > H2CCH(COOCH3) > H2CO > H2CPH > H2CS. Thus, barriers drop in particular, if third‐period atoms become involved in the double bond of the enophile. Activation‐strain analyses show that this trend in reactivity correlates with the activation strain associated with deforming reactants from their equilibrium structure to the geometry they adopt in the transition state. We discuss the origin of this trend and its relationship with the extent of synchronicity between H transfer from ene to enophile and the formation of the new CC bond.

Twelve One‐Electron Ligands Coordinating One Metal Center: Structure and Bonding of [Mo(ZnCH3)9(ZnCp*)3]

The highest coordination number for metal complexes of monodentate ligands has been nine since the days of Alfred Werner. [1] The term “complex” refers to a molecule [MLm] that features a central metal atom M that bonds to ligator atoms E of ligands L by donor–acceptor interactions to yield a core structure MEn. [2] Metal atoms can also become captured inside an electron-precise cage En. These compounds obey the Wade–Mingos rules and are referred to as endohedral clusters M@En, typically with n> 9. Examples for the latter are the recently synthesized [M@Pb10] 2 and [M@Pb12] 2 (M=Ni, Pd, Pt). Herein we describe the synthesis of an unprecedented molecule containing a MoZn12 core, which offers a novel linkage between coordination compounds and cluster molecules (Figure 1). At first glance, the icosahedral structure of the title molecule [MoZn12Me9Cp*3] (1; Me=CH3, Cp*=C5Me5) is reminiscent of the endohedral clusters described above. The actual bonding situation is, however, intriguingly different. Quantum chemical analysis revealed a unique situation best described as a perfectly sd-hybridized molybdenum atom that engages in six Mo Zn two-electron-three-center bonds. There are six high-lying valence molecular orbitals (MOs) occupied by 12 electrons that can clearly be identified as Mo Zn bonding. Another six electrons are delocalized over the Zn cage, evoking only weak Zn Zn interactions (Figures 2 and 3). Before discussing these aspects in detail, we briefly report the synthesis and the analytical and structural properties of 1. The title compound [MoZn12Me9Cp*3] (1) was reproducibly obtained in 82% yield by the treatment of [Mo(GaCp*)6] (2) with 14 equivalents of ZnMe2 in toluene at 110 8C over a period of 2 h. Two mixed Ga–Zn compounds [MoZn4Ga4Me4Cp*4] (3) and [MoZn8Ga2Me6Cp*4] (4) are intermediates of this reaction and were isolated in nearly quantitative yields using 4 and 8 equivalents of ZnMe2 (Scheme 1).

Aromaticity and Activation Strain Analysis of [3 + 2] Cycloaddition Reactions between Group 14 Heteroallenes and Triple Bonds

We have computationally explored the trend in reactivity of [3 + 2] cycloaddition reactions between H(2)E=C=PH and HC≡CH as the terminal position in the phosphaallene is varied along E = C, Si, Ge, Sn, Pb. The reaction barrier drops significantly from E = C (nearly 50 kcal/mol) to E = Si-Pb (ca. 20 kcal/mol). Activation strain analyses tie this trend to a reduction in activation strain in the heavier phosphaallene analogues which, in contrast to the parent compound H(2)C=C=PH, do already possess the bent geometry required in the TS.

Borylene-Based Direct Functionalization of Organic Substrates: Synthesis, Characterization, and Photophysical Properties of Novel π-Conjugated Borirenes

Room temperature photolysis of aminoborylene complexes, [(CO)(5)M=B=N(SiMe(3))(2)] (1: M = Cr, 2: Mo) in the presence of a series of alkynes and diynes, 1,2-bis(4-methoxyphenyl)ethyne, 1,2-bis(4-(trifluoromethyl)phenyl)ethyne, 1,4-diphenylbuta-1,3-diyne, 1,4-bis(4-methoxyphenyl)buta-1,3-diyne, 1,4-bis(trimethylsilylethynyl)benzene and 2,5-bis(4-N,N-dimethylaminophenylethynyl)thiophene led to the isolation of novel mono and bis-bis-(trimethylsilyl)aminoborirenes in high yields, that is [(RC=CR)(mu-BN(SiMe(3))(2)], (3: R = C(6)H(4)-4-OMe and 4: R = C(6)H(4)-4-CF(3)); [{(mu-BN(SiMe(3))(2)(RC=C-)}(2)], (5: R = C(6)H(5) and 6: R = C(6)H(4)-4-OMe); [1,4-bis-{(mu-BN(SiMe(3))(2)(SiMe(3)C=C)}benzene], 7 and [2,5-bis-{(mu-BN(SiMe(3))(2) ((C(6)H(4)NMe(2))C=C)}-thiophene], 8. All borirenes were isolated as light yellow, air and moisture sensitive solids. The new borirenes have been characterized in solution by (1)H, (11)B, (13)C NMR spectroscopy and elemental analysis and the structural types were unequivocally established by crystallographic analysis of compounds 6 and 7. DFT calculations were performed to evaluate the extent of pi-conjugation between the electrons of the carbon backbone and the empty p(z) orbital of the boron atom, and TD-DFT calculations were carried out to examine the nature of the electronic transitions.