Man-Wing Tse

Properties of the β-carbonylalkyltin chlorides: Evidence for intramolecular coordination

Abstract The Mossbauer spectra and the solvent effect on the 1 H NMR spectra support the conclusions from infrared spectroscopy that the compounds Cl 3 SnCR 2 CRHCOX and the Cl 2 Sn(CR 2 CRHCOX) 2 (R = Me or H; X = Me, OH, or OAlkyl) are intramolecularly coordinated in both the solid state and in solution. The compounds Cl 3 SnCR 2 CRHCOX (X = OH or OAlkyl) react with t-butoxyl radicals in solution to show the ESR spectra of the displaced radicals CR 2 CRHCOX.

An electron spin resonance study of the reactivity of alkylchlorotin radicals, RnCl3 –nSn˙(n= 0–3) towards alkenes, alkyl bromides, and biacetyl. The spectra and structures of the alkylchlorotin derivatives of butane-2,3-semidione, RnCl3 –nSnOCMeCMeO˙

The radicals BunCl3 –nSn˙ were generated by photolysis of the appropriate cyclopentadienyltin compounds, Bun(C5H5)SnCl3 –n, and their reactivity towards alkenes and alkyl bromides was monitored by e.s.r. spectroscopy. Towards both reagents, the reactivity decreases as the number of chloro ligands increases, and it is tentatively suggested that this results from the reduced interaction of the SOMO of the radical and the LUMO of the alkene or alkyl bromide. All the radicals BunṠnCl3 –n(n= 0–3) react with biacetyl to show the e.s.r. spectra of the tin derivatives of butane-2,3-semidione, BunCl3 –nSnOCMeCMeO˙, and these radicals have also been generated by a variety of other methods. Below about +20 °C, when n= O or 1, the adducts Cl3SnOCMeCMeO˙ and BuCl2SnOCMeCMeO˙ show hyperfine coupling by two non-equivalent methyl groups and one unique chlorine atom; above +20 °C, the methyl groups become magnetically equivalent, and coupling is by more than one chlorine atom. [graphic omitted] This is interpreted to imply that the adducts have the structures (A) and (B) respectively, in which the ligands about the tin are approximately trigonal bipyramidal, and, at low temperatures, hyperfine coupling is by the apical chlorine atom; at higher temperatures, positional exchange between the ligands confers on the radicals C2v symmetry on the e.s.r. time scale. When n= 2, the two methyl groups are non-equivalent from –50 to 0 °C, with no hyperfine coupling from chlorine, probably implying the static monodentate structure (C). When n= 3, two radicals have been identified. The first, with a spectrum which is a regular binomial septet, is thought to be the rapidly fluxional cis-monodentate compound (D). The second, which displays a septet spectrum with a severe alternating line width effect, is thought to be the more slowly fluxional trans-compound (E).

Homolytic organometallic reactions. Part 14. Homolytic reactivity of β-C–H groups in organotin compounds. An alternative source of trialkyltin radicals

Rate constants (at –84°) for the reaction of t-butoxyl radicals with tetraethyltin at the α-methylene group to give the radical Et3SnCHCH3, and at the β-methyl group to give the radical Et3SnCH2ĊH2, are 1.2 × 104 and 4.8 × 103 l mol–1 s–1, respectively ca. 103 and 5 × 102 times greater (per hydrogen atom) than the reaction of t-butoxyl radicals with ethane. The abstraction of hydrogen from the β-position in alkyltin compounds is followed by reversible elimination to give an alkene and a trialkyltin radical, and, if di-t-butyl peroxide is photolysed in the presence of trimethylisobutyltin, the e.s.r. spectrum of the trimethyltin radical can be observed [reaction (i)]. This reaction Me3SnCH2CHMe2+ Me3CO·→ Me3SnCH2ĊMe2→ Me3Sn·+ CH2CMe2(i) provides a useful alternative source of trialkyltin radicals for e.s.r. studies, particularly for the generation of alkyl radicals by the reaction with alkyl bromides [reaction (ii)]. This formation of a β-trialkylstannylalkyl radical and Me3Sn·+ RBr → Me3SnBr + R·(ii) thence of a trialkyltin radical and alkene can constitute two of the three steps of a radical chain reaction with the overall equation Me3SnCH2CHMe2+ X–Y → Me3SnY + CH2CMe2+ HX, and examples of these reactions have been established where X–Y = Cl–CCl3, Br–CCl3, and Cl–OCMe3.

Homolytic organometallic reactions : XII. Homolytic dealkylation of di-n-butyl-t-butyltin chloride: preferential displacement of the n-butyl radical

Abstract t-Butoxyl radicals react with di-n-butyl-t-butyltin chloride to show the ESR spectrum of the n-butyl rather than the t-butyl radical. Cumyloxyl radicals behave similarly, but trimethylsiloxyl and benzoyloxyl radicals react to give both n-butyl and t-butyl radicals. The unexpected preferential formation of the less stable n-alkyl radical is tentatively ascribed to steric constraints in a 5-coordinate intermediate.

An electron spin resonance study of 3-oxypropenoyl radicals derived from glycidols

Glycidols with blocked OH groups (A; M = alkyl or trialkylsilyl) react with t-butoxyl radicals to show the e.s.r. spectra of the corresponding 3-oxypropenoyl radicals (D), and 24 examples of these acyl radicals are reported. The [graphic ommitted] reaction is thought to proceed through the formation of the allyloxyl radicals (B), which, in part, are converted into the aldehyde (C) which is very reactive towards loss of hydrogen to give the acyl radical (D). Glycidyl pivalate (A; M = COCMe3) reacts cleanly in this way, but glycidyl acetate (E; R = Me) also undergoes intramolecular 1,5-transfer of the acyl group to show the spectrum of the enoxyl radical (F). Glycidyl propionate and butyrate do not undergo this acyl transfer, but show the spectra of the radicals OĊCHCHOCOCH2R′ and [graphic omitted]HCH2OCOĊHR′(R′= Me or Et). [graphic ommitted]

The photolysis of cyclopentadienyl compounds of tin and mercury. Electron spin resonance spectra and electronic configuration of the cyclopentadienyl, deuteriocyclopentadienyl, and alkylcyclopentadienyl radicals

Cyclopentadienyl derivatives of tin(IV) and mercury(II) and alkylcyclopentadienyl derivatives of mercury(II) are photolysed in solution to show the e.s.r. spectra of the appropriate radicals RC5H4·(R = H, D, Me, Et, Pri, or But). The C5H5· radical is a planar π-radical with average D5h symmetry, and its spectrum is broadened in the presence of organic bromides, perhaps by a charge-transfer mechanism. The introduction of alkyl groups breaks the degeneracy of the ψA and ψS molecular orbitals of the π-system by electron release, destabilising the ψS MO, and the e.s.r. spectrum reflects the spin density distribution in the configuration ψA2ψS1. Deuterium has a small but detectable perturbing effect: the ψA MO is destabilised by ca. 100 J mol–1, and thermal mixing of the two energy levels results in the configuration ψS1.515ψA1.485. This is compatible with the model of a vibrational perturbation of the resonance integral β, rather than of the Coulomb integral α.

The characterisation of 1,1-diphenylstannacyclopentane and 1,1,6,6-tetraphenyl-1,6-distannacyclodecane. An X-ray diffraction study of the distannacyclodecane

The first oligomeric stannacycloalkane, 1,1,6,6-tetraphenyl-1,6-distannacyclodecane (A) has been isolated, together with the monomer, 1,1-diphenylstannacyclopentane (B), from the reaction between butane-1,4-diyl- [graphic omitted] dimagnesium dibromide and diphenyltin dichloride. These compounds have been characterised by molecular weight measurements and by i.r., mass, 13C, and 119Sn n.m.r. spectroscopy. In (A), the 13C n.m.r. signal of C-3 shows two pairs of satellites from coupling to the two tin nuclei, 2J 19.4 and 3J 38.0 Hz. In (B), the 13C signal of C-3 shows one pair of satellites, 2/3J 19.6 Hz, which can be regarded as resulting from the algebraic sum of the two-bond and three-bond mediated couplings. The structure of the distannacyclodecane (A) has been determined by single-crystal X-ray diffraction. It has a boat-chair-boat structure, similar to that of cyclodecane itself, with the Ph2Sn groups replacing those methylene groups which are not subject to steric hindrance. The principal features of the structure of the stannacyclopentane (B) are apparent from a preliminary X-ray study. It has a twisted conformation of C2 symmetry, with a twist angle of ca. 15° and an endocyclic angle at tin of 95°.

Photolysis of di-t-butyl peroxide under acid conditions

Photolysis of di-t-butyl peroxide in the presence of trifluoroacetic acid gives a species which can fragment to give a methyl radical, or can add to an alkene; this species is tentatively identified as the t-butyl alcohol radical cation, But[graphic omitted].

The homolytic reactivity of stannacyclopentanes

t-Butoxyl radicals react with 1,1,-dialkylstannacyclopentanes by an SH2 ring-opening reaction at the tin centre, with a rate constant of 2.1 x 106 l mol− s−1 at 243 K for the dibutyl derivative.

Photolysis of cyclopentadienyltin compounds: a new source of tin-centred radicals

η 1 -Cyclopentadienyltin(IV) compounds and bis-(η5-cyclopentadienyl)tin(II) are photolysed to give the cyclopentadienyl radical, and the former process provides a useful new route to tin(III) radicals.