Photochemical Strain-Release-Driven Cyclobutylation of C(sp3)-Centered Radicals

A new photoredox-catalyzed decarboxylative radical addition approach to functionalized cyclobutanes is described. The reaction involves an unprecedented formal Giese-type addition of C(sp³)-centered radicals to highly strained bicyclo[1.1.0]butanes. The mild photoredox conditions, which make use of a readily available and bench stable phenyl sulfonyl bicyclo[1.1.0]butane, proved to be amenable to a diverse range of α-amino and α-oxy carboxylic acids, providing a concise route to 1,3-disubstituted cyclobutanes. Furthermore, kinetic studies and DFT calculations unveiled mechanistic details on bicyclo[1.1.0]butane reactivity relative to the corresponding olefin system.     

Photochemical Strain‐Release‐Driven Cyclobutylation of C(sp3)‐Centered Radicals

A new photoredox‐catalyzed decarboxylative radical addition approach to functionalized cyclobutanes is described. The reaction involves an unprecedented formal Giese‐type addition of C(sp³)‐centered radicals to highly strained bicyclo[1.1.0]butanes. The mild photoredox conditions, which make use of a readily available and bench stable phenyl sulfonyl bicyclo[1.1.0]butane, proved to be amenable to a diverse range of α‐amino and α‐oxy carboxylic acids, providing a concise route to 1,3‐disubstituted cyclobutanes. Furthermore, kinetic studies and DFT calculations unveiled mechanistic details on bicyclo[1.1.0]butane reactivity relative to the corresponding olefin system.     

Structure, strain, and degenerate rearrangement of tricyclo[2.1.0.0 1,3]pentasilane and related molecules

We theoretically investigate a highly strained tricyclic silane (tricyclo[2.1.0.0 1,3]pentasilane (4b), an isomer of pentasila[1.1.1]propellane (3b)) composed of three fused three-membered rings. The central ring is distorted. One of the fusion bonds in the central ring is shorter than the normal Si-Si single bond (2.350 A) whereas the other is as long as the fusion bonds in bicyclo[1.1.0]tetrasilane (2b) (2.860 A) and 3b (2.778 A). The tricyclic silane is less strained than the carbon congener and more strained than the isomer 3b. The electron delocalization between one of the fusion bonds and the geminal Si-Si ring bonds elongates the fusion bond and stabilizes the molecules to reduce the strain. The silanes composed of the fused three-membered rings are less strained than the carbon congener. A degenerate rearrangement of a three-membered ring is predicted. The enthalpy of activation of the rearrangement of the distorted central ring is low (7.2 kcal/mol) for 4b, but appreciable (22.3 kcal/mol) for the germanium congener, tricyclo[2.1.0.0(1,3)]pentagermane (4c). We investigate the effects of the substituents on the distortion of the central three-membered ring and the degenerate rearrangement.     

Thermal behavior of [2.1.1]propellane: a DFT/ab initio study

Density functional and ab initio molecular orbital calculations have been used to search for the low energy path of the thermal isomerization of [2.1.1]propellane 1. Three reaction modes were considered: ring opening of the bicyclo[1.1.0]butane unit in 1 to give 1,2-dimethylenecyclobutane 21, opening of the four-membered ring of 1 to afford 1,3-dimethylenecyclobutane 20, and breaking of the [2.1.1]propellane central bond and one of the bicyclo[1.1.0]butane side bonds to form carbene 17. At the CAS(12,12)PT2N/6-31G(d) level of theory, the activation barrier of the latter route was lowest in energy. Further investigation of this process at the QCISD(T)/6-311G(d,p)//QCISD/6-31G(d) and B3PW91/6-311G(d,p)// B3PW91/6-311G(d,p) level of theory indicated that the barrier of isomerization of 1 --> 17 amounts to 29 kcal/mol and that 17 is stabilized by hydrogen migration to give dienes 18 and 19.     

The addition of methyllithium to 2,2,4,4-tetramethylcyclobutan-1-one-3-thione. The generation and capture of a bishomoenolate anion.

Methyllithium added to 2,2,4,4-tetramethylcyclobutan-1-one-3-thione to produce lithium 3-methylthio-2,2,4,4-tetramethylbicyclo[1.1.0]but-1-oxide. This bishomoenolate was alkylated on carbon by methyl iodide, but retained the bicyclo[1.1.0]butane skeleton when silated with chlorotrimethylsilane. The ease of oxidation of a series of 1,3-diheteroatom substituted bicyclo[1.1.0]butanes was determined.     

Isomeric metamorphosis: Si3E (E = S, Se, and Te) bicyclo[1.1.0]butane and cyclobutene

Group 14 and 16 hybrid heavy bicyclo[1.1.0]butanes (tBu2MeSi)4Si3E (E = S, Se, and Te) 2a-c have been prepared by the [1 + 2] cycloaddition reaction of trisilirene 1 and the corresponding chalcogen. Bicyclo[1.1.0]butanes 2 have exceedingly short bridging Si-Si bonds (2.2616(19) A for 2b and 2.2771(13) A for 2c), a phenomenon explained by the important contribution of the trisilirene-chalcogen pi-complex character to the overall bonding of 2. Photolysis of 2a and 2b produced their valence isomers, the heavy cyclobutenes 3a and 3b, featuring flat four-membered Si3E rings and a planar geometry of the Si=Si double bond. The mechanism of such isomerization was studied using deuterium-labeled 2a-d6 to ascertain the preference of the pathway, involving the direct concerted symmetry-allowed transformation of bicyclo[1.1.0]butane 2 to cyclobutene 3.     

Preparation and crystal structure of two bicyclo[1.1.0]butan-2-ones: a hybrid oxyallyl-cyclopropanone motif

The first X-ray crystal structures of two different bicyclo[1.1.0]butanones show a very long transannular bond of 1.69 A and a large carbonyl pyramidal distortion of 0.26 A out of the plane of the three carbons. These features are those expected of molecules with a hybrid cyclopropanone-oxyallyl structure, and these structures differ markedly from those of the parent bicyclo[1.1.0]butane skeleton.     

Unsubstituted bicyclo[1.1.0]but-2-ylcarbinyl cations

A synthesis for the unsubstituted bicyclo[1.1.0]but-2-ylmethanols (endo- and exo-9) from 1,3-butadiene has been developed. Solvolyses of their sulfonates 10 and 11 took entirely different courses, as the endo compound 10 gave rise exclusively to rearranged products such as cyclopent-3-en-1-ol (14), while the exo compound 11 underwent only the substitution of the tosylate group with complete retention of the exo-bicyclo[1.1.0]but-2-ylmethyl skeleton. Under solvolytic conditions, 10 reacted at very similar rates to the corresponding monocyclic substrate, that is, cyclopropylcarbinyl mesylate (19); in contrast, 11 reacted only three times as fast as n-butyl tosylate and about 1000-fold slower than 10. The nature of the bicyclo[1.1.0]but-2-ylcarbinyl cations has been probed by quantum chemical calculations. Whereas, the exo isomer (exo-18) corresponds to a local energy minimum, the endo isomer is only a transition state [endo-18(TS)] for an automerization of the nonclassical cyclopent-3-en-1-yl cation (13) and converts into 13 by a Wagner-Meerwein rearrangement. The most favorable isomerization of exo-18 also leads to 13 but via a transition state resembling the 2-vinylcycloprop-1-yl cation [25(TS)]. On the introduction of methyl groups at positions 1 and 3 of exo-18, the cation is no longer an energy minimum and it becomes a transition state [27(TS)] for an automerization of the nonclassical 1,3-dimethylcyclopent-3-en-1-yl cation (28). The large effect of the methyl substitution rationalizes the puzzling results of the previous product and rate studies, which utilized various substituted derivatives of bicyclo[1.1.0]but-2-ylcarbinyl sulfonates as substrates.     

A comparative study of bent-bond vs. Walsh model in strained systems. Bicyclo(1.1.0)butane

Bent bond and Walsh models for the electronic structure of the highly strained bicyclo(1.1.0)butane were critically examined and compared with the available PE spectrum. It is found that the bent bond scheme is by far more superior to the widely used Walsh semilocalized molecular orbitals. Hence the local hybrid orbitals provide a useful aid in assigning and interpreting of PE spectra. Present results lend support to the hybrid model in bicyclo(1.1.0)butane proposed by Schulman.     

Isolable silylene, disilenes, trisilaallene, and related compounds

Our recent studies of synthesis, structure, and reactions of an isolable silylene, stable novel cyclic and conjugated disilenes, a trisilaallene are summarized. Due to the distinctive electronic and steric effects of trialkylsilyl substituents, tetrakis(trialkylsilyl)disilenes showed interesting structural features around SiSi bonds, electronic spectra, and reactions. The tetrasilyldisilenes were useful reagents for the synthesis of novel types of organosilicon compounds such as η²-disilene transition metal complexes and a 1,3-disilabicyclo[1.1.0]butane. Photochemical and thermal interconversion among Si₄R₆ isomers including a cyclotetrasilene, a silylcyclotrisilene, and a bicyclo[1.1.0]tetrasilane occured without apparent participation of the corresponding tetrasila-1,3-diene. The first spiropentasiladiene was thermally very stable and showed remarkable spiroconjugation between the two ring π systems. An isolable dialkylsilylene was found to be well-protected sterically from dimerization but least perturbed electronically. Using the silylene, a trisilaallene, the first stable compound with formal sp-hybridized silicon atom, was synthesized. In contrast to carbon allenes, the skeleton of the trisilaallene was significantly bent and remarkably fluxional.     

Promotion of phosphaalkyne cyclooligomerisation by a Sb(v) to Sb(iii) redox process

A high yield of the tetraphosphaladderene, anti-tetraphosphatricyclo[4.2.0.0(2,5)]octa-3,7-diene, is obtained from reaction of the zirconocene 1,3-diphosphabicyclo[1.1.0]butane with Ph(2)SbCl(3) in THF or CH(2)Cl(2). Exploration of the reaction pathway using density functional theory suggests that an envelope-type adduct of Ph(2)SbCl and 1,3-diphosphabicyclo[1.1.0]butane plays a pivotal role in the reaction. The zwitterionic character of this intermediate species allows it to act simultaneously as both an ene and an eneophile, and a symmetry-allowed bimolecular reaction leads to the tetraphosphaladderene species via a spirocyclic intermediate.     

1,3-Diborata-2,4-diphosphoniocyclobutane-1,3-diyls communicate through a para-phenylene linker

The presence of a second 1,3-diborata-2,4-diphosphoniocyclobutane-1,3-diyl in the para-position of a phenylene linker favors the diradical form over the 1,3-diborata-2,4-diphosphoniobicyclo[1.1.0]butane bond stretch isomer, as long as the two planar diradical units are coplanar with the linker. This demonstrates that two BPBP diradicals communicate through the phenyl ring.     

A Tetrasilicon Analogue of Bicyclo[1.1.0]but‐1(3)‐ene Containing a Si=Si Double Bond with an Inverted Geometry

Bicyclo[1.1.0]tetrasil‐1(3)‐ene 1 , a tetrasilicon analogue of bicyclo[1.1.0]but‐1(3)‐ene that contains a formal double bond between bridgehead silicon atoms in an inverted geometry, was synthesized and isolated in the form of thermally stable orange crystals. The distance between the bridgehead Si atoms in 1 is much longer than those in typical Si=Si bonds, but still shorter than that of a previously reported pentasila[1.1.1]propellane. DFT calculations suggest that the bridgehead bond in 1 comprises a σ bond with an inverted geometry and a π bond. This notion is supported by the UV/Vis spectrum of 1 , which exhibits several absorption bands in the UV/Vis region. While 1 is stable toward typical trapping agents for Si=Si double bonds, 1 reacts with carbon tetrachloride to furnish a hexachlorotetrasilane.     

Maximum-overlap hybridization in bicyclo [2.1.0] pent-2-ene

Applying the method of maximum overlap, the hybridization in bicyclo[2.1.0]pent-2-ene is determined. In addition, upon comparing these results with those of cyclobutene, cyclopropane, Dewar benzene and bicyclo[1.1.0]butane, it is found that the hybrids calculated by the method of maximum overlap are transferable between parts of molecules for which the structural features are similar.     

Reactivity of 13,13-dibromo-2,4,9,11-tetraoxadispiro[5.0.5.1]tridecane toward organolithiums: remarkable resistance to the DMS rearrangement

Reactions of dibromocyclopropane 2a, containing two spiro-fused 1,3-dioxane rings, with MeLi gave only the methylation products 8 and 9 even at elevated temperatures. In contrast, the cyclohexane analogue 2b treated with MeLi underwent a smooth rearrangement to bicyclo[1.1.0]butane 11b at -78, -10, or +35 degrees C. Treatment of 2a with PhLi gave the alpha-Ph anion 13 as the only product, which underwent smooth methylation with MeI to give 14. Under the same conditions, 2b with PhLi gave bicyclo[1.1.0]butane 11b accompanied by bromophenyl derivative 8b. Treatment of either dibromide with t-BuLi gave a mixture of products including debrominated cyclopropanes 12. Experimental results were augmented with DFT calculations for salts 23 and MP2//DFT-level calculations for carbenes 22. They demonstrated a higher stability of the dioxane alpha-bromo anion with respect to alpha-elimination by 4.8 kcal/mol and also a lower tendency of the carbene 22a to undergo rearrangement by 4.0 kcal/mol than the cyclohexane analogues. These differences have been attributed to the inductive effect of the four oxygen atoms, which results in lower LUMO energy, the higher positive charge at the carbenic center, and the overall more electrophilic character of carbene 22a as compared to the cyclohexane derivative 22b. The rearrangement of carbenes 22 to the corresponding allenes 1, the thermodynamic products, requires a higher activation energy DeltaG(double dagger)(298) by 4.2 kcal/mol for dioxane and 6.4 kcal/mol for cyclohexane derivatives than for the formation of the bicyclo[1.1.0]butanes 11. The DeltaG(double dagger)(298) for intramolecular insertions to the CH bond is low and calculated as 6.0 kcal/mol for dioxane 22a and 2.0 kcal/mol for the formation of cyclohexane 22b.     

Unusual carbon shielding effects of cyclopropanes and double bonds in strained bicyclo[3.1.0]hexanes and cyclopentenes

Carbon-13 shieldings and one-bond ¹³C? H coupling constants of bicyclo[2.1.1]hexane, bicyclo[2.1.1]hex-2-ene, tricyclo[3.1.1.0^2,4]heptane and benzvalene are presented and compared to the data of related compounds. If a bicyclo[3.1.0]hexane system is part of a rigid skeleton, the cyclopropane ring exerts specific γ substituent effects of two kinds. In the case of the bicyclohexane boat form an upfield shift of the C-3 signal is observed and in the case of the chair form a downfield shift of 15–20 ppm. Compared to the corresponding cyclopentanes the double bond in strained cyclopentenes causes downfield shifts of the C-4 absorption. This effect increases with increasing strain, reaching a 45.9 ppm maximum in benzvalene. Hence it is the only known bicyclo[1.1.0]butane having a reversed order of carbon shieldings. The downfield shifts are explained by means of simple orbital interaction schemes.     

9,9-Dimethyl-8,10-dioxapentacyclo[5.3.0.0.0.0]decane and naphthotetracyclo[5.1.0.0.0]oct-3-ene: new substituted [1.1.1]propellanes as precursors to 1,2,3,4-tetrafunctionalized bicyclo[1.1.1]pentanes

Two new substituted [1.1.1]propellanes have been generated from the corresponding bicyclo[1.1.0]butanes in either single-step (1a) or four-step procedures (1b). The observed degree of double lithiation of the bicyclo[1.1.0]butanes is discussed in the context of DFT computational results. Addition reactions across the central C(1)-C(3) bonds of the propellanes were studied. Only the propellane 1b gave the biacetyl addition product.     

Synthesis of a Bicyclobutane Fatty Acid Identified from the Cyanobacterium Anabaena PCC 7120

By design: A carbanion-mediated cyclization reaction cascade serves as the key final step in the total synthesis of a novel oxylipin, which features a strained bicyclo[1.1.0]butane conjugated to a labile vinyl epoxide.     

1,3-di(methythio)-2,2,4,4-tetramethylbicyclo[1.1.0]butane

1,3-Di(methylthio)-2,2,4,4-tetramethylbicyclo[1.1.0]butane has been synthesized by two alternate routes and has been characterized by single crystal X-ray crystallography. This report corrects earlier, erroneous discussions of the title compound which have appeared in the literature.     

Consecutive carbon-carbon bond formation approach in tandem cyclization reactions

The conventional tandem cyclization reactions involve the formation of alternating carbon-carbon bonds, whereas the newly developed cyclization reactions involve the formation of consecutive carbon-carbon bonds, in which N-aziridinylimines have been utilized as geminal radical acceptor and donor equivalents in a single operation. This unprecedented tandem cyclization approach becomes feasible by the successful generation of 5- and 6-membered ring radicals by radical cyclizations of N-aziridinylimines. The same notion can be applied to the anionic cyclizations of N-aziridinylimines, thereby allowing anionic consecutive carbon-carbon bond formation. This approach has great synthetic potential, particularly for the construction of quaternary carbon centers, and it provides highly efficient routes for the synthesis of natural products.