Diradical mechanisms for the cycloaddition reactions of 1,3-butadiene,benzene, thiophene,ethylene, and acetylene on a Si(111)-7x7 surface

The cycloaddition chemistry of several representative unsaturated hydrocarbons (1,3-butadiene, benzene, ethylene, and acetylene) and a heterocyclic aromatic (thiophene) on a Si(111)-7x7 surface has been explored by means of density functional cluster model calculations. It is shown that (i) 1,3-butadiene, benzene, and thiophene can undergo both [4+2]-like and [2+2]-like cycloadditions onto a rest atom-adatom pair, with the former process being favored over the latter both thermodynamically and kinetically; (ii) ethylene and acetylene undergo [2+2] cycloaddition-like chemisorptions onto a rest atom-adatom pair; and (iii) all of these reactions adopt diradical mechanisms. This is in contrast to the [4+2] cycloaddition-like chemisorptions of conjugated dienes on a Si(100) surface and to the prototype [4+2] cycloadditions in organic chemistry, which were believed to adopt concerted reaction pathways. Of particular interest is the [4+2]-like cycloaddition of s-trans-1,3-butadiene, whose stereochemistry is retained during its chemisorption on the Si(111) surface.     

Surface chemistry of five-membered aromatic ring molecules containing two different heteroatoms on Si(111)-7 x 7

The surface chemistry of three representative aromatic molecules containing two different heteroatoms isoxazole, oxazole, and thiazole on Si(111)-7 x 7 was studied. These molecules exhibit different competition and selectivity for multiple reaction channels with this surface, determined by a combination of molecular electronic and structural factors. Isoxazole is chemically attached to Si(111)-7 x 7 through both dative-bond addition and [4 + 2]-like cycloaddition. Oxazole chemisorbs on Si(111)-7 x 7 through both dative-bond addition and [2 + 2]-like cycloaddition. The kinetically favored [2 + 2]-like cycloadduct at low temperature is thermally converted into the thermodynamically preferred [4 + 2]-like cycloadduct at a temperature higher than 300 K. Thiazole is chemically bound to this surface only through formation of a Si...N dative bond at low temperature. This dative-bonded molecule is thermally converted into a [4 + 2]-like cycloadduct. The reaction channels of the three five-membered aromatic molecules containing two different heteroatoms (isoxazole, oxazole, and thiazole) and of the aromatic molecules containing only one heteroatom (pyridine, pyrrole, furan, and thiophene) are compared and analyzed for a thorough understanding of the reaction mechanisms of various heterocyclic aromatic molecules on this surface. The intrinsic connection between surface reaction mechanism and molecular electronic structure is demonstrated. This includes the distribution of electron density on the molecular ring determined by the geometric arrangement of the heteroatoms, the electronegativity of the heteroatoms, and the electronic contribution of the heteroatoms to formation of aromatic pi conjugation, as well as the molecular polarity.     

Selective formation of cumulative double bonds (C[double bond]C[double bond]N) in the attachment of multifunctional molecules on Si(111)-7 x 7

The cumulative double bond (C[double bond]C[double bond]N), an important intermediate in synthetic organic chemistry, was successfully prepared via the selective attachment of acrylonitrile to Si(111)-7 x 7. The covalent binding of acrylonitrile on Si(111)-7 x 7 was studied using high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), scanning tunneling microscopy (STM) and DFT calculations. The observation of the characteristic vibrational modes and electronic structures of the C[double bond]C[double bond]N group in the surface species demonstrates the [4 + 2]-like cycloaddition occurring between the terminal C and N atoms of acrylonitrile and the neighboring adatom-rest atom pair, consistent with the prediction of DFT calculations. STM studies further show the preferential binding of acrylonitrile on the center adatom sites of faulted halves of Si(111)-7 x 7 unit cells.     

Reaction mechanism of cis-1,3-butadiene addition to the Si(100)-2 x 1 surface

A set of 40 finite temperature ab initio molecular dynamics trajectories is employed to investigate the distribution of addition products and underlying microscopic mechanism of the addition of 1,3-butadiene to the Si(100)-2 x 1 surface. The product yields are in good agreement with recent STM measurements and include a Diels-Alder [4 + 2] adduct with a surface dimer acting as the dienophile, a [4 + 2]-like adduct that bridges two dimers within a row, a [4 + 2]-like adduct that bridges two dimers in adjacent rows, and an interdimer [2 + 2]-like adduct. The trajectories indicate that a common mechanism underlies the distribution and is predominantly a nonconcerted stepwise mechanism that proceeds via an intermediate zwitterion composed of a carbocation bonded to a negatively charged surface dimer.     

Si-C(N) sigma linkages and N --> Si dative bonding at pyridine/Si(111)-7 x 7

We experimentally demonstrated that pyridine/Si(111)-7 x 7 can act as an electron donor/acceptor pair as a result of the charge transfer from the electron-rich N atom of pyridine to the electron-deficient adatom of the Si surface, evidenced by the upshift of 1.8 eV (state A) for the N(1s) core level upon the formation of a datively bonded complex compared to physisorbed molecules. Another state (B) whose N(1s) binding energy downshifts by 1.2 eV was assigned to an adduct through Si-C and Si-N covalent linkages, formed via a [4 + 2]-like addition mechanism on Si(111)-7 x 7. Binding molecules through the formation of the dative bond resulted from significant electron transfer opens a new approach for the creation of Si-based molecular architectures and modification of semiconductor interfacial properties with unsaturated organic molecules.     

[6+2]Cycloadditions catalyzed by titanium complexes

The Ziegler catalyst TiCl₄-Et₂AlCl and the arenetitanium(II) complex (η6-C₆H₆)Ti(II)(AlCl₄)₂ induce [6 + 2]cycloaddition reactions of cycloheptatriene with dienes and acetylenes. Addition to 1,3-butadiene affords 7 - endo - vinyl - bicyclo[4.2.1]nona - 2,4 - diene (main product) and bicyclo[4.4.1]- undeca - 2,4,8 - triene, a product of [6+4]cycloaddition. Isoprene reacts similarly, yielding mainly 7- endo - isopropenyl - bicyclo[4.2.1]nona - 2,4 - diene. 2,3 - Dimethyl - 1,3 - butadiene gives 8,9dimethylbicyclo [4.4.1]undeca - 2,4,8 - triene, a product of [6 + 4]cycloaddition, while [6 + 2]cross-adducts are minor products. The reaction of cycloheptatriene with norbornadiene gives mainly hexacyclo[6.5.1.0^2,7.0^3,12.^6,10.0^9,13]tetradec - 4 - ene via [6+2]cycloaddition followed by intramolecular Diels-Alder reaction. As a by-product, pentacyclo[7.5.0.0^2,7.0^3,5.0⁴⁸]tetradeca - 10,12 - diene is formed by a [2+2+2]mechanism. Addition of cycloheptatriene to diphenylacetylene and bis - (tri- methylsilyl)acetylene furnishes sustituted bicyclo[4.2.1]nona - 2,4,7 - trienes. Alkenes, E,E-2,4 - hexadiene and 1,3 - cyclooctadiene are unreactive. The [6+2]cycloaddition is made possible by coordination of cycloheptatriene to titanium, which changes the symmetry of the frontier orbitals in the triene. The reactivity of the trienophile is also enhanced by coordination to the catalyst.     

A convenient preparation of 2-fluoro-3-alkoxy-1,3-butadienes

[reaction: see text] 1-Chloro-1-fluoro-2-methoxy-2-methylcyclopropane eliminates HCl on heating in quinoline solution above 50 degrees C to give 2-fluoro-3-methoxy-1,3-butadiene in high yield. If an alcohol is added to the reaction then a 2-fluoro-3-alkoxy-1,3-butadiene is obtained in high yield. The dienes give smooth 4 + 2 cycloaddition reactions.     

Trifluoromethyl groups in crystal design of 1,4-diphenyl-1,3-butadienes for topochemical [2 + 2] photodimerization

[structure: see text] UV irradiation of the powdered crystalline sample of each of three (E,E)-1,4-di(trifluoromethyl-substituted)phenyl-1,3-butadienes (1-3) was found to yield a single [2 + 2] cycloaddition product in the solid state. Moreover, upon irradiation, the crystalline samples of two (E,E)-1,4-di(trifluoromethyl- and fluorine-substituted)phenyl-1,3-butadienes (4, 5) undergo a similar conversion to afford a [2 + 2] cycloaddition product, respectively. Our observations suggest that trifluoromethyl groups can be used to direct 1,4-diphenyl-1,3-butadiene molecules to form a parallel, offset-stacked orientation suitable for topochemical [2 + 2] cycloaddition.     

Photochemical reaction of 7-aminocoumarins. 9. [2+2]-Cycloadducts with trans,trans-1,4-diphenyl-1,3-butadiene

The photoreactions of 7-diethylaminocoumarin, 4-methyl-7-diethylaminocoumarin, 4-trifluoromethyl-7-diethylaminocoumarin, and 4-N-morpholino-7-diethylaminocoumarin with trans,trans-1,4-diphenyl-1,3-butadiene, which lead to the formation of [2+2]-cycloaddition products, were studied. It was established that photocycloaddition proceeds with the formation of adducts that have a 1-endo-styryl substituent and a 2-exo-phenyl group. The effect of the substituent in the 4-position of the 7-aminocoumarin molecule on the effectiveness of cycloaddition is discussed.See [1] for Communication 8.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1319–1325, October, 1990.     

Asymmetric Brønsted Base Catalyzed and Directed [3+2] Cycloaddition of 2‐Acyl Cycloheptatrienes with Azomethine Ylides

Conjugated cyclic trienes have the potential for different types of cycloaddition reactions. In the present work, we will, in a novel asymmetric cycloaddition reaction, demonstrate that the organocatalytic reaction of 2‐acyl cycloheptatrienes with azomethine ylides proceeds as a [3+2] cycloaddition, which is in contrast to the Lewis acid‐catalyzed reaction, in which a [3+6] cycloaddition takes place. In the presence of a chiral organosuperbase, 2‐acyl cycloheptatrienes react in a highly enantioselective manner in the [3+2] cycloaddition with azomethine ylides, providing the 1,3‐dipolar cycloaddition product in high yields and up to 99 % ee. It is also shown that the diene formed by the reaction can undergo stereoselective dihydroxylation, bromination, and cycloaddition reactions. Finally, based on experimental observations, some mechanistic considerations are discussed.     

Functionalization of diamond (001)‐2 × 1 surface by cycloaddition of 1,3‐cyclohexadiene: A theoretical study

Density functional theory calculations have been used to study [4 + 2] and [2 + 2] cycloaddition reaction between 1,3‐cyclohexadiene and diamond (001)‐2 × 1 surface. The calculations revealed four possible reaction pathways for 1,3‐cyclohexadiene with the surface dimmers of diamond. Full geometry‐optimized structures were obtained for all products, including intradimer and interdimer reaction products. These results were analyzed both in terms of the total energy values and the detailed optimized geometries. We found that the intradimer [4 + 2] product is energetically favored over the other products, and the barrier to intradimer [4 + 2] addition is lower than the other additions, so the intradimer [4 + 2] product is expected to be the dominant product on the surface. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Novel [4 + 2]-cycloaddition of 1-phenyl-1-benzothiophenium salts with dienes. Experimental evidence for a lack of aromaticity in the thiophene ring

The [4 + 2]-cycloaddition reaction of 1-phenyl-1-benzothiophenium triflates has been conducted for the first time. [4 + 2]-Cycloaddition with dienes such as cyclopentadiene and 1,3-diphenylisobenzofuran occurs successfully to give cycloadducts. This result indicates that the C=C bond of the thiophene ring acts as a 2pi electron component in the cycloaddition reaction. Cycloadducts were formed in high yields with high stereoselectivity. However, the cycloaddition with other less reactive dienes such as 2,3-dimethyl-1,3-butadiene did not take place. The structure and stereochemistry of cycloadduct 2a were analyzed by NMR techniques. Furthermore, reaction of the cycloadducts with sodium methoxide in methanol gave the ring-opened products in high yields.     

Cycloaddition of carbonyl compounds on Si(100): new mechanisms and approaches to selectivity for surface cycloaddition reactions

Density functional theory has been used to explore cycloaddition reactions of organic molecules containing carbonyl functional groups on the Si(100) surface. As with other pi bonds, carbonyl groups can add to the surface by a [2+2] cycloaddition with negligible activation barrier, as previously shown through experiment. However, the present calculations indicate that 1,2-dicarbonyls, such as glyoxal, may also react by means of a [4+2] addition to form a hetero-Diels-Alder product in which the organic ring stands normal to the surface. Calculations of [2+2] and [4+2] pathways indicate that both reactions proceed without significant barriers. This reactivity is analogous to that of conjugated dienes, in which evidence for both reactions has been observed. In contrast to unsaturated alkyl systems, which must react through the pi electron system, the reactions of carbonyls may proceed through a very different mechanism, in which the initial surface interaction is through the oxygen lone pair. The presence of lone pairs affects the geometry of the [4+2] adduct, and may alter the competition between [2+2] and [4+2] addition. Some potential rearrangement reactions of the initial binding products are described. Recent experimental studies of a 1,2-dicarbonyl on Si(100) are reinterpreted in light of these calculations, and found to be consistent with the presence of the [4+2] adduct. Finally, some molecules are suggested as cycloaddition reagents for experimental tests of the conclusions presented here.     

Competition and selectivity of organic reactions on semiconductor surfaces: reaction of unsaturated ketones on Si(100)-2 x 1 and Ge(100)-2 x 1

A combined experimental and theoretical study of a model system of multifunctional unsaturated ketones, including ethyl vinyl ketone (EVK), 2-cyclohexen-1-one, and 5-hexen-2-one, on the Si(100)-2 x 1 and Ge(100)-2 x 1 surfaces was performed in order to probe the factors controlling the competition and selectivity of organic reactions on clean semiconductor surfaces. Multiple internal reflection infrared spectroscopy data and density functional theory calculations indicate that EVK and 2-cyclohexen-1-one undergo selective [4 + 2] hetero-Diels-Alder and [4 + 2] trans cycloaddition reactions on the Ge(100)-2 x 1 surface at room temperature. In contrast, on the Si(100)-2 x 1 surface, evidence is seen for significant ene and possibly [2 + 2] C=O cycloaddition side products. The greater selectivity of these compounds on Ge(100) versus Si(100) is explained by differences between the two surfaces in both thermodynamic factors and kinetic factors. With 5-hexen-2-one, for which [4 + 2] cycloaddition is not possible, a small [2 + 2] C=C cycloaddition product is observed on Ge(100) and possibly Si(100), even though the [2 + 2] C=C transition state is calculated to be the highest barrier reaction by several kilocalories per mole. The results suggest that, due to the high reactivity of clean semiconductor surfaces, thermodynamic selectivity and control will play important roles in their selective functionalization, favoring the use of Ge for selective attachment of multifunctional organics.     

Theoretical study on reactions of nitroethylene with the Si(100)-2 x 1 surface

Possible reaction pathways of nitroethylene with the Si(100)-2 x 1 surface have been investigated by unrestricted density functional theory. The facile occurrence of the studied reactions was demonstrated by the low activation energies of the rate-determining steps (1.07-5.23 kcal/mol). It was found that the [4 + 2] cycloaddition reaction of nitroethylene is most kinetically favorable. The isomerization reactions of the addition products were also investigated. The [3 + 2] cycloaddition product may further undergo a rearrangement by overcoming a 12.37 kcal/mol activation energy barrier into an isomer, with an oxygen atom of the nitryl group inserted between two silicon atoms of the Si(100) surface.     

Stereochemistry of the [2+4] cycloaddition of cyclopentyne

The [2+4] cycloaddition of cyclopentyne with a pair of diastereomeric 1,3-dienes is found to occur with high stereoselectivity. The results support the applicability of the principles of orbital symmetry even in the case of this exceedingly reactive dienophile.     

Regiospecific topochemical reactions controlled by trifluoromethyl directing groups

A parallel, offset-stacked orientation was found in the crystal packing of E,E-1,4-di(o-trifluoromethyl)phenyl-1,3-butadiene. UV-irradiation of the powered crystalline sample resulted in a quantitative conversion to a single [2 + 2] cycloaddition product.     

Lewis acid initiated or high pressure promoted reactions of isoprene with phenylsulphinyl chloride

Lewis acid initiated reaction of isoprene with PhSOCl proceeds in a ene fashion with the formation of 2-phenylsulphinylmethyl-1,3-butadiene. High pressure promoted addition produces Z-1-phenylsulphinyl-4-chloroadduct presumably $\text{via}$ [4+2]cycloaddition.     

Synthesis of perhydroxazin-4-ones. Competitive Mukaiyama versus hetero Diels–Alder reaction in the cycloaddition of 2-aza-3-trimethylsilyloxy-1,3-butadiene and aldehydes

The reactions of 2-aza-3-trimethylsilyloxy-1,3-butadiene with carbonyl dienophiles are described. 2-Aza-1,3-butadienes participate as dienes in the [4+2] cycloaddition with aldehydes to afford perhydroxazin-4-ones in good yields. Experimental results, however, show that a Mukaiyama type two-step reaction must be taken into account. The cycloadducts obtained have proved to be useful intermediates in the synthesis of α-amino-β-hydroxy acids.     

Identifying Multinuclear Organometallic Intermediates in On‐Surface [2+2] Cycloaddition Reactions

We investigate the on‐surface [2+2] cycloaddition reaction of 2,3,6,7,10,11‐hexabromotriphenylene (HBTP) on Ag(111), Cu(111), Au(111), and Cu‐dosed Au(111) surfaces using STM and DFT simulation focusing on the organometallic intermediates. The fully debrominated HBTP molecules form an organo‐silver framework on Ag(111) and an organo‐copper framework on Cu(111), both incorporating multinuclear metal adatom clusters. The organo‐silver framework is converted into porous covalent networks via [2+2] cycloaddition above 240 °C. In contrast, the organo‐copper framework is very stable and does not undergo [2+2] cycloaddition even at 300 °C. On Au(111), no organo‐gold intermediate of [2+2] cycloaddition is observed. After loading Cu onto Au(111), the partially debrominated HBTP molecules bind to Cu adatom dimers to form multinuclear organo‐copper complexes at 100 °C which undergo [2+2] cycloaddition at 140 °C. This study shows that the choice of surface can direct the reaction pathway.