Stannanes Research Articles

Synthesis and Some Reactions of Dibutyltin (S)- and (R)-Camphorsulfonyl Hydrides

The synthesis and physical properties of dibutyltin (S)-camphorsulfonyl hydride (1) and dibutyltin (R)-camphorsulfonyl hydride (2), and diphenyltin (S)-camphorsulfonyl hydride (3) as well as that of their organotin precursors are described. Their reactivity with different amines as triethylamine, morpholine and pyridine has been compared with other mixed hydrides as dibutyltin chloride hydride, dibutyltin acetate hydride and dibutyltin dihydride. It has been studied also the possibility of using of dibutyltin (R)- or (S)-camphorsulfonyl hydrides for the stereoselective reduction of different ketones as acetophenone, menthon, camphor and cyclopropyl-(4-metoxyphenyl)-methanone. The reduction of acetophenone with studied camphorsulfonyl hydrides carried out in benzene at room temperature afforded 1-phenylethanol with relatively low enantioselectivity. Addition of 10 equiv. of MnCl(2)*4H(2)O or ZnCl(2) to the reduction mixture involving dibutyltin (S)-camphorsulfonyl hydride (1) and acetophenone and carried out in methanol and tetrahydrofuran, respectively, resulted in remarkable increase in enantioselectivity. The comparative kinetic studies of reduction of acetophenone by different hydrides proved that dibutyltin camphorsulfonyl hydride is significantly more reactive in comparison with dibutyltin chloro hydride and dibutyltin acetate hydride. Analogous results have been obtained from kinetic studies for different tin hydrides with chosen amines. The outcome of these studies supported by theoretical calculations led to the conclusion that the order of reactivity of the studied hydrides correlates with the rate of their homolytic decomposition at room temperature.

Investigation of Cyclization Reactions of Dicyclohexyl-6-iodo- and -6-tosylhexenylborane. A Facile Radical Cyclization Diverted to a Rearrangement−Cyclization with Base

Dicyclohexyl-6-iodohexenylborane efficiently undergoes radical cyclization at room temperature using tri-n-butyltin hydride as a hydrogen donor and without the aide of a radical initiator. Efforts to develop environmentally friendly reagents using hypophosphites as substitutes for tin hydrides in this reaction provided a 60% yield of cyclopentanemethanol when tetrabutylammonium hypophosphite was used. Air was necessary as an initiator when hypophosphites were used. During investigations of the radical cyclization reactions, it was discovered that excess tetrabutylammonium hydroxide provided the rearrangement-cyclization product in excellent yield. Since this product was chiral, efforts were focused on achieving enantioselectivity. Oxazaborolidines made from chiral amino alcohols and 6-tosyl-1-hexenylboronic acid were treated with methyllithium to give 1-cyclopentylethanol after oxidation in 60% ee, demonstrating that oxazaborolidines were effective chiral directors for this reaction.

Reactions of Tin(II) Hydride Species with Unsaturated Molecules

Nützliches Zinn: Das Zinn(II)-hydrid [LSnH] (L=HC{CMeN(2,6-iPr2C6H3)}2) reagiert mit einer Vielzahl an Verbindungen, die ungesättigte C-O-, C-C- oder C-N-Bindungen enthalten, unter gleichzeitiger Übertragung von Wasserstoff und {LSn} auf das organische Substrat – ein eleganter Zugang zu Zinn(II)-Verbindungen. Diese einzigartigen Stannylene verfügen über ein freies Elektronenpaar, das sich für die Komplexbildung mit Übergangsmetallen eignet.

Reactions of Tin(II) Hydride Species with Unsaturated Molecules

Reactions of Tin(II) Hydride Species with Unsaturated Molecules

Intermolecular Nonreductive Alkylation of Enamides via Radical−Polar Crossover

A carbon-carbon bond construction method is disclosed which involves radical addition of alpha-haloesters or iodoacetonitrile to enamides. Despite the presence of tri-n-butylstannane, nonreductive addition was predominant; H-atom transfer from tin hydride was not observed. Rapid iodine atom transfer to (or electron transfer from) the radical adduct, resulting in an iminium ion intermediate and radical chain propagation, is consistent with the observed reactivity.

Regioselective Synthesis of 1,8-Naphthyridinone-Annulated Oxygen and Sulfur Heterocycles by Tri-n-butyl Tinhydride–Mediated Aryl Radical Cyclization

The tin hydride–mediated cyclizations of a number of ethers, sulfides, and sulfones under mild, neutral conditions have been investigated. While the 2-bromobenzyloxy ethers were prepared in 62–65% yields by the alkylation of 4-hydroxy-1-phenyl-1,8-naphthyridin-2(1H)-one with 2-bromobenzyl bromides in refluxing acetone in the presence of anhydrous potassium carbonate, the sulfides were derived from 4-mercapto-1-phenyl-1,8-naphthyridin-2(1H)-one and 2-bromobenzyl bromides in 82–84% yields by a phase-transfer catalysis (PTC) reaction. The corresponding sulfones were prepared by treatment of the sulfides with m-CPBA in refluxing dichloromethane. The ethers, sulfides, and the sulfones were treated with n Bu3SnH-AIBN to give regioselectively 1,8-naphthyridinone-annulated oxygen and sulfur heterocycles in 70–78% yields.

Designing new free-radical reducing reagents: Theoretical study on Si–H bond dissociation energies of organic silanes

Bond dissociation energies of a series of substituted silanes were studied with the density functional theory methods. The performances of six different density functional methods including B3LYP, B3P86, BH&HLYP, B1LYP, PBE1KCIS, and TPSSLYP1W were examined for the prediction of Si–H bond dissociation energies. The results showed that B3P86 was the most accurate theoretical procedure among these six DFT methods. Using the B3P86 method, we then carried out a systematic study about the substituent effects on Si–H bond dissociation energies, with a focus to identify the possible approaches to weaken the Si–H bond strength. On the basis of the knowledge learned from the systematic study on model systems, we proposed some new silicon-based radical reducing reagents which may be used to replace toxic tin hydride reagents.

Intramolecular homolytic substitution at the sulfur atom: an alternative way to generate phosphorus- and sulfur-centered radicals

Two efficient procedures involving tin hydride or thiophenol-mediated intramolecular homolytic substitution at the sulfur atom are reported. They lead to the generation of varied P(V)-centered radicals from the corresponding aryl or alkyne thiophosphorus substrates. The radical formed can be trapped by an olefin via an intermolecular addition, leading to the construction of C–P bonds. Thiophosphination of triple bonds was also achieved using a radical cycloisomerization process. Extension of the methodology to sulfur-containing species was examined.

ChemInform Abstract: Intramolecular Cyclization Reaction — Palladium(0)‐Catalyzed Cyclization is More Effective than Tin Hydride Mediated Reaction.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Complexes of Borane and N-Heterocyclic Carbenes: A New Class of Radical Hydrogen Atom Donor

Calculations suggest that complexes of borane with N-heterocyclic carbenes (NHC) have B-H bond dissocation energies more then 20 kcal/mol less than free borane, diborane, borane-THF, and related complexes. Values are in the range of popular radical hydrogen atom donors like tin hydrides (70-80 kcal/mol). The resulting prediction that NHC borane complexes could be used as radical hydrogen atom donors was verified by radical deoxygenations of xanthates by using either AIBN or triethylborane as initiator.

ChemInform Abstract: Spurring Radical Reactions of Organic Halides with Tin Hydride and TTMSS Using Microreactors.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

How Do Analogous α-Chloroenamides and α-Iodoenamides Give Different Product Distributions in 5-EndoRadical Cyclizations?

5-Endo cyclizations of N-alkenyl carbamoylmethyl radicals provide gamma-lactam radicals, which in turn evolve to reduced or non-reduced (alkene) products depending on reagents and reaction conditions. Several groups have made surprising observations that chlorides are better radical precursors than iodides in such cyclizations. Here is described a detailed study of tin and silicon hydride-mediated radical cyclizations of N-benzyl-2-halo-N-cyclohex-1-enylacetamides. The ratios of directly reduced, cyclized/reduced, and cyclized/non-reduced products depend not only on the reaction conditions and reducing reagent but also on the precursor. Prior explanations for the precursor-dependent product ratios based on amide rotamer effects are ruled out. The precursor-dependent behavior is further dissected into two different effects: (1) the ratio of cyclized/reduced products to cyclized/non-reduced products depends on the ability of the radical precursor to react with the product gamma-lactam radical in competition with tin hydride (iodides can compete, chlorides cannot), and (2) the occurrence of large amounts of directly reduced (noncyclized) products in the case of iodides is attributed to a competing ionic chain reaction by which the precursor is reductively deiodinated with HI. This side reaction is not available to chlorides, thereby explaining why the chlorides are better precursors in such reactions. The ability of the iodides to provide cyclized products can be largely restored by adding base. The chlorides and iodides then become complementary precursors, with chlorides giving largely cyclized/reduced products and iodides giving largely cyclized/non-reduced products.

Intramolecular cyclization reaction — Palladium(0)-catalyzed cyclization is more effective than tin hydride mediated reaction

Intramolecular CH arylation of pyrone, pyridone, uracil, imidazole, and benzimidazole derivatives are carried out in the presence of a Pd catalyst to form potentially bioactive fused heterocyclic compounds, whereas the n-Bu3SnH-mediated aryl radical cyclization of the precursors 3 afforded mainly halogen-reduced uncyclized products.Key words: Pd(0) catalyst, CH arylation, intramolecular cyclization, regioselectivity.

Spurring Radical Reactions of Organic Halides with Tin Hydride and TTMSS Using Microreactors

Tributyltin hydride-mediated radical reactions of organic halides were successfully carried out in a continuous flow system using a microreactor. The reactions proceeded within a very short period of time, coupled with quickly decomposing radical initiators such as V-65 and V-70. The continuous flow reaction system was applied to gram scale synthesis of a key intermediate for furofuran lignans.

Regioselective synthesis of bioactive heterocycles by radical cyclization

In recent years free radical cyclization has emerged as one of the versatile and simplest methodology for the construction of carbon-carbon bond in organic synthesis. In multi-step synthesis its use is significant as the key step of the synthesis involve the formation of carbon-carbon or carbon-hetero bond employing radical cyclization strategy. Regioselective synthesis of heterocycles viz. uracil, pyrone, coumarin, quinolone indole, benzofuran-annulated oxygen, nitrogen and sulfur heterocyclic compounds by tri-n-butyl tin hydride and thiol-mediated radical cyclization will be presented.

Recent developments in the chemistry of low valent Group 14 hydrides

This Perspective discusses the synthesis and reactivity of low valent Group 14 hydrides of germanium and tin. The use of hindered terphenyl ligands has facilitated the isolation of a number of unusual new main group hydride structural types (each predicted by computational work), and has culminated in the use of these species as precursors to new cluster archetypes.

Isomeric Forms of Heavier Main Group Hydrides: Experimental and Theoretical Studies of the [Sn(Ar)H]2 (Ar = Terphenyl) System

A series of symmetric divalent Sn(II) hydrides of the general form [(4-X-Ar')Sn(mu-H)]2 (4-X-Ar' = C6H2-4-X-2,6-(C6H3-2,6-iPr2)2; X = H, MeO, tBu, and SiMe3; 2, 6, 10, and 14), along with the more hindered asymmetric tin hydride (3,5-iPr2-Ar*)SnSn(H)2(3,5-iPr2-Ar*) (16) (3,5-iPr2-Ar* = 3,5-iPr2-C6H-2,6-(C6H2-2,4,6-iPr3)2), have been isolated and characterized. They were prepared either by direct reduction of the corresponding aryltin(II) chloride precursors, ArSnCl, with LiBH4 or iBu2AlH (DIBAL), or via a transmetallation reaction between an aryltin(II) amide, ArSnNMe2, and BH3.THF. Compounds 2, 6, 10, and 14 were obtained as orange solids and have centrosymmetric dimeric structures in the solid state with long Sn...Sn separations of 3.05 to 3.13 A. The more hindered tin(II) hydride 16 crystallized as a deep-blue solid with an unusual, formally mixed-valent structure wherein a long Sn-Sn bond is present [Sn-Sn = 2.9157(10) A] and two hydrogen atoms are bound to one of the tin atoms. The Sn-H hydrogen atoms in 16 could not be located by X-ray crystallography, but complementary Mössbauer studies established the presence of divalent and tetravalent tin centers in 16. Spectroscopic studies (IR, UV-vis, and NMR) show that, in solution, compounds 2, 6, 10, and 14 are predominantly dimeric with Sn-H-Sn bridges. In contrast, the more hindered hydrides 16 and previously reported (Ar*SnH)2 (17) (Ar* = C6H3-2,6-(C6H2-2,4,6-iPr3)2) adopt primarily the unsymmetric structure ArSnSn(H)2Ar in solution. Detailed theoretical calculations have been performed which include calculated UV-vis and IR spectra of various possible isomers of the reported hydrides and relevant model species. These showed that increased steric hindrance favors the asymmetric form ArSnSn(H)2Ar relative to the centrosymmetric isomer [ArSn(mu-H)]2 as a result of the widening of the interligand angles at tin, which lowers steric repulsion between the terphenyl ligands.

Synthesis and Characterization of Linear Poly(dialkylstannane)s

A comprehensive study was made of the synthesis of a spectrum of poly(dialkylstannane)s by catalytic dehydropolymerization of dialkylstannanes (dialkyltin dihydrides, R2SnH2, prepared by reduction of R2SnCl2), with R = ethyl, propyl, butyl, pentyl, hexyl, octyl, and dodecyl. The polymerization reactions were followed by 1H and 119Sn NMR spectroscopy, IR spectroscopy (disappearance of the Sn−H vibration), and quantitative measurement of H2 which evolved during the reaction. Among the numerous metal complexes employed as catalyst, [RhCl(PPh3)3] was found to be particularly suited for the preparation of these inorganic polymers. The reaction parameters temperature, solvent, R2SnH2 concentration, and [RhCl(PPh3)3]/R2SnH2 ratio were varied, with the most prominent influence on the monomer conversion being the temperature. The number-average molar masses of the polystannanes were in the range of 1 × 104 to 1 × 105 g/mol, depending on the reaction conditions. For the generic case of the polymerization of Bu2SnH2 with [RhCl(PPh3)3] as catalyst, it was demonstrated that poly(dibutylstannane) did not form by a random polycondensation, but by growth at a rhodium complex, whereby only a minor part of [RhCl(PPh3)3] was converted to catalytically active species by reaction with tin hydrides. The polymers featured phase transitions into liquid-crystalline states, on occasion at remarkably low temperatures. A particularly high phase transition temperature was observed for poly(dipropylstannane), which also was characterized by a high density, indicative of a particularly favorable packing of the propyl groups.

Regiocontrol of the Palladium‐Catalyzed Tin Hydride Addition to Z‐Enynols: Remarkable Z‐Directing Effects.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Regioselective Synthesis of Benzofuran‐Annulated Six‐Membered Sulfur Heterocycles by Aryl Radical Cyclization

The tin hydride–mediated cyclization of a number of sulfides and sulfones under mild and neutral conditions has been investigated. The sulfides were in turn derived from 3(2H) benzothiofuranone and 2‐bromobenzyl bromides by phase‐transfer‐catalyzed reaction, and the corresponding sulfones were prepared by treatment of the corresponding sulfides with m‐CPBA at room temperature. The sulfides and sulfones were then reacted with n Bu3SnH‐AIBN to afford regioselectively benzofuran‐annulated six‐membered sulfur heterocycles.