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.     

Valence Isomerization of a 1,3-Diphosphacyclobutane-2,4-diyl: Photochemical Ring Closure to 2,4-diphosphabicyclo[1.1.0]butane and Its Thermal Ring Opening to gauche-1,4-Diphosphabutadiene

The bond stretch isomer 1,3-diphosphacyclobutane-2,4-diyl 1 was transformed photochemically to give the previously unknown 2,4-diphosphabicyclo[1.1.0]butane 2 , which itself can be converted thermally into gauche-1,4-diphosphabutadiene 3 . The crystal structures of these three energy-rich valence isomers of 1,2-diphosphete have been determined. R=SiMe₃; Mes*=2,4,6-tBu₃C₆H₂.     

Sterically crowded bicyclo[1.1.0]butane radical cations

The variability of carbon-carbon single bonds by steric and electronic effects is probed by DFT calculations of sterically crowded bicyclo[1.1.0]butanes and their radical cations. The interplay of sterics and electronics on the gradual weakening and breaking of bonds was studied by investigating bridgehead substitution in 1,3-di-tert-butylbicyclo[1.1.0]butane and 2,2',4,4'-tetramethyl-1,3-di-tert-butylbicyclo[1.1.0]butane and geminal substitution in 2,2'-di-tert-butylbicyclo[1.1.0]butane and 2,2',4,4'-tetra-tert-butylbicyclo[1.1.0]butane. Bridgehead substitution leads to a lengthening of the central bond, whereas bisubstitution on the geminal carbon leads to a shortening of this bond due to a Thorpe-Ingold effect. Although the character of the central bond can be modulated by substitution and electron transfer over a range of 0.35 A, the state forbidden ring planarization does not occur. Sterically crowded bicyclo[1.1.0]butane radical cations are therefore promising candidates for the investigation of extremely long carbon-carbon single bonds.     

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.     

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.     

Tricyclo[2.1.0.01,3]pentane

The reaction of 1,1-dibromo-2,3-bis(chloromethyl)cyclopropane with methyllithium leads initially to ring closure to 1-bromo-2-chloromethylbicyclo[1.1.0]butane. Further reaction leads to an unstable compound which reacts with phenylthiol to give 2-vinyl-1-cyclopropyl phenyl sulfide and undergoes thermal rearrangement at ∼ -50°C to give cyclopentadiene. Strong evidence is presented which suggests that tricyclo[2.1.0.01,3]pentane may be the intermediate.     

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.     

The reaction of 1,2,2-trimethylbicyclo[1.1.0]butane with excited state 1-cyanonaphthalene

1,2,2-Trimethylbicyclo[1.1.0]butane reacted with excited state 1-cyanonaphthalene at a diffusion-controlled rate in methanol to produce cis- and trans-1-methoxy-2,2,3-trimethylcyclobutane and 1-methoxy-2, 2-dimethyl-3-methylenecyclobutane as simple methanol adducts of the starting bicy-clo[l.1.0]butane. In addition, 1:1:1 adducts of the starting bicyclo[1.1.0]butane, 1 -cyanonaphthalene, and methanol were isolated and characterized. Products were explained on the basis of a single electron transfer process from 1,2,2-trimethylbicyclo[1.1.0]butane to excited state 1-cyanonaphthalene to initially produce the cation radical of the bicyclo[1.1.0]butane and the anion radical of 1-cyanonaphthalene.     

Skeletal isomerism within 1,3-disilabicyclo[1.1.0]butane and 2,3-disilabuta-1,3-diene derivatives

A bis-adamantane-spiro-fused 1,3-bis(triisopropylsilyl)-1,3-disilabicyclo[1.1.0]butane equilibrates with the corresponding 2,3-bis(triisopropylsilyl)-1,3-disilabuta-1,3-diene with a ratio of 1:19. The 1,3-disilabuta-1,3-diene was fully characterized by a combination of multinuclear NMR and UV-VIS spectroscopies, elemental analysis, and single-crystal X-ray diffraction analysis.     

The synthesis of octavalene (tricyclo[5.1.0.02,8]octa-3,5-diene) and several substituted octavalenes

The first synthesis of octavalene (1a) is reported. The starting material is homobenzvalene (5), to which monobromocarbene is added. The resulting compound 3a takes up bromine across the central bicyclo[1.1.0]butane bond to form the tribromide 7a which undergoes a cyclopropyl bromide-allyl bromide rearrangement on heating. From the product (1Oa) HBr is eliminated to give a 1,3-dibromocyclobutane with a 1,3-butadiene bridge across its 2- and 4-position (11a). Finally, t-butyllithium removes the two Br atoms from 11a and converts it into a 4:1 mixture of 1a and cyclooctatetraene. This reaction sequence represents the first application of protective group strategy in bicyclo[1.1.0]butane chemistry. Octavalene (1a) is shown to rearrange to cyclooctatetraene at 50°. Deuterium-labeled 1a ([1,8-D₂] 1a) is used to prove that a [1,5]-sigmatropic shift does not occur in 1a. Utilizing the above methodology 4-bromooctavalene (1b) and 3-phenyl-5-bromooctavalene (1c) are synthesized from the dibromocarbene adducts 3b and c of homobenzvalene (5) and 5-phenylhomobenzvalene (6), respectively. Surprisingly, 1c was accompanied by a small quantity of 3-bromo-1-phenyloctavalene (1d). Possible mechanisms for the addition of bomine to the bicyclo[1.1.0]butane system of compounds 3 and for the formation of the octavalenes 1 are discussed. In the ¹³C-NMR spectra of 1 and 11 chemical shifts at unexpectedly high field are observed for C-6 of the 1,3-cycloheptadiene moieties.     

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.     

[1.1.1]Propellane: Reaction with free radicals

[1.1.1]Propellane is more reactive towards free radicals than bicyclo[1.1.0]butane, and much more reactive than bicyclo[2.1.0]pentane. Therefore, the reactivity is not determined by strain energy relief or the HOMO energy. The addition of acetaldehyde is unique in that a 1:2 adduct is formed. A number of other additions are described, and provide convenient routes to 1,3-disubstituted bicyclo[1.1.1]pentanes.     

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.     

Mass spectral fragmentation of (S,S)-2-substituted 4,4-diphenyl-3,1-oxazabicyclo[3.3.0]octanes: ring contraction of pyrrolidine and 1,3-oxazolidine in mass spectrometry

The mass spectral behaviour of (S,S)-2-substituted 4,4-diphenyl-3,1-oxazabicyclo[3.3.0]octanes has been studied with the aid of mass-analyzed ion kinetic energy spectrometry and accurate mass measurements under fast atom bombardment (FAB) and electron impact (EI) ionization conditions. Under FAB ionization, all compounds show a tendency to form protonated aldehyde or benzophenone ions and to form protonated 1-azabicyclo[3.1.0]hexane ions, which can further lose an ethylene or cyclopropane from the pyrrolidine ring to produce protonated 1-azabicyclo[1.1.0]butane ions and 3H-azirine ions, respectively. Under EI ionization, a similar fragmentation to that under FAB ionization was observed. The title compounds also show a tendency to yield oxirane ions and oxirenium ions by loss of pyrrolidine and pyrrolidine plus H. Ring contractions of 1,3-oxazolidine by loss of an aldehyde or ketone and of pyrrolidine by loss of an ethylene or cyclopropane were observed under both FAB and EI ionization conditions.     

Theoretical group 14 chemistry. Part 2. Si4R6: a theoretical approach

The relative energies of the isomeric compounds of Si₄R₆ (R=H, Me, Ph, 2,6-dimethylphenyl, 2,6-diisopropylphenyl) were explored with different theoretical methods including semiempirical, DFT and perturbation theory calculations. The results are compared with previous calculations and with experimental findings. On going from small substituents to bulky aryl groups the stability of the isomers is reversed from tetrasilabicyclo[1.1.0]butane (1) and tetrasilacyclobutene derivatives to the s-cis and s-trans conformers of tetrasilabuta-1,3-dienes. The concept of bond stretch isomerism of 1 and its dependence on the substituents at the central silicon atoms is also discussed.     

Ring-opening reactions of 3-substituted 1-azabicyclo[1.1.0]butane with dichlorocarbene

Reaction of 3-ethyl-1-azabicyclo[1.1.0]butane ( 1a ) with chloroform-potassium tert-butoxide afforded a ring-opened product, 1,1-dichloro-2-aza-4-ethylpenta-1,4-diene ( 4a ), which was characterized via conversion to the corresponding N-substituted 5-chloro-1,2,3,4-tetrazole, Sa . Reaction of 3-phenyl-1-azabicyclo-[1.1.0]butane ( 1b ) with “Seyferth's reagent” (PhHgCCl₂Br) afforded 1,1-dichloro-2-aza-4-phenylpenta-1,4-diene ( 4b ), which also was characterized via conversion to a tetrazole derivative, i.e., 5b . Finally, the reaction of 1b with dichlorocarbene generated under phase transfer conditions (chloroform-sodium hydroxide-TEBA) was studied. At short reaction times (0.5 hour), the major reaction product was 4b . However, at longer reaction times (20–30 hours), two secondary products, 8 and 9 , were formed which resulted via subsequent dichlorocyclopropanation of 4b .     

Synthesis and characterization of triazenide and triazene complexes of ruthenium and osmium

Triazenide [M(eta2-1,3-ArNNNAr)P4]BPh4 [M = Ru, Os; Ar = Ph, p-tolyl; P = P(OMe)3, P(OEt)3, PPh(OEt)2] complexes were prepared by allowing triflate [M(kappa2-OTf)P4]OTf species to react first with 1,3-ArN=NN(H)Ar triazene and then with an excess of triethylamine. Alternatively, ruthenium triazenide [Ru(eta2-1,3-ArNNNAr)P4]BPh4 derivatives were obtained by reacting hydride [RuH(eta2-H2)P4]+ and RuH(kappa1-OTf)P4 compounds with 1,3-diaryltriazene. The complexes were characterized by spectroscopy and X-ray crystallography of the [Ru(eta2-1,3-PhNNNPh){P(OEt)3}4]BPh4 derivative. Hydride triazene [OsH(eta1-1,3-ArN=NN(H)Ar)P4]BPh4 [P = P(OEt)3, PPh(OEt)2; Ar = Ph, p-tolyl] and [RuH{eta1-1,3-p-tolyl-N=NN(H)-p-tolyl}{PPh(OEt)2}4]BPh4 derivatives were prepared by allowing kappa1-triflate MH(kappa1-OTf)P4 to react with 1,3-diaryltriazene. The [Os(kappa1-OTf){eta1-1,3-PhN=NN(H)Ph}{P(OEt)3}4]BPh4 intermediate was also obtained. Variable-temperature NMR studies were carried out using 15N-labeled triazene complexes prepared from the 1,3-Ph15N=N15N(H)Ph ligand. Osmium dihydrogen [OsH(eta2-H2)P4]BPh4 complexes [P = P(OEt)3, PPh(OEt)2] react with 1,3-ArN=NN(H)Ar triazene to give the hydride-diazene [OsH(ArN=NH)P4]BPh4 derivatives. The X-ray crystal structure determination of the [OsH(PhN=NH){PPh(OEt)2}4]BPh4 complex is reported. A reaction path to explain the formation of the diazene complexes is also reported.     

Titanium(III) trisamidotriazacyclononane: reactions with C60 and radicals

The reaction of titanium trisamidotriazacyclononane, [Ti{N(Ph)SiMe2}3tacn] (1), with C60 led to the synthesis of [Ti{N(Ph)SiMe2}3tacn]C60 (2) in high yield. Treatment of 2 with PhCH2Br led to the formation of [Ti{N(Ph)SiMe2}3tacn]Br and the radical PhCH2C60 (3). The reaction of CH3I with 1 gives two products. One is [Ti{N(Ph)SiMe2}3tacn]I (4), which results from the oxidation of 1 by an I radical. The other product, 5, resulting from a multistep reaction scheme that involves redox and nucleophilic reactions, presents an imido ligand formed by ligand rearrangement upon C-N bond cleavage. In solution, an exchange process that corresponds to a reversible 1,3-silyl shift between two Ti-bonded N atoms leads to isomers 5a and 5b. This equilibrium transforms an imido (TiNPh) into an amido ligand (Ti{NPh}SiMe2CH2Ph) with concomitant generation of an anionic moiety in the originally neutral triazacyclononane ring. In solution, either 5a or 5b displays additional fluxional processes that consist of its corresponding racemization processes.     

A combined mechanistic and computational study of the gold(I)-catalyzed formation of substituted indenes

Substituted indenes can be prepared after a sequence [1,3] O-acyl shift-hydroarylation-[1,3] O-acyl shift. Each step is catalyzed by a cationic NHC-Gold(I) species generated in situ after reaction between [(IPr)AuOH] and HBF(4)·OEt(2). This interesting silver-free way is fully supported by a computational study justifying the formation of each intermediate.