Hai-Shan Dang

Polarity-reversal catalysis by thiols of radical-chain hydrosilylation of alkenes

Abstract The radical-chain hydrosilylation of alkenes by triethylsilane under very mild conditions is promoted by thiols, which act as polarity-reversal catalysts for the abstraction of electron-rich hydrogen from the silane by nucleophilic β-silylalkyl radicals.

ENE REACTIONS OF ALLYLIC TIN COMPOUNDS WITH SINGLET OXYGEN

Abstract Allyltin compounds react with singlet oxygen by metalloallylation to give stannyl peroxides, by hydroallylation to give hydroperoxides, and by rearrangement and cycloaddition to give 4-stannyldioxolanes.

The effect of ligands, solvent and temperature on the reactions of allyltin(IV) compounds with singlet oxygen

Abstract The reaction of singlet oxygen with a variety of allyltin compounds CH 2 CHCH 2 SnR 3 (R 3  Me 3 , Bu 3 , allyl 3 , (cyclo-C 6 H 11 3 , Ph 3 , allylBu 2 , Bu 2 Cl, Bu 2 OAc, allylCl 2 , allylCl 2 bipy) has been investigated, and the allylperoxytin compounds, 3-stannylallyl hydroperoxides, and 4-stannyl-1,2-dioxolanes which result from M-ene, H-ene and cycloaddition processes, respectively, have been identified by NMR spectroscopy. As the tin centre becomes more electropositive, as indicated by the 13 C NMR shift of the allylic CH 2 group, the proportion of the M-ene reaction increases, and when δCH 2 is above about 23.7, the allylperoxytin compound is the only product. An exception to this rule is tetraallyltin, δCH 2 16.13, which similarly shows only the M-ene reaction. This is tentatively ascribed to the special effect of hyperconjugation between the CSn σ-bond and the remaining π-systems. A polar solvent favours the M-ene reaction. The cycloaddition reaction is favoured by low temperature, and at − 70°C in a non-polar solvent it may become the major route. Diallylmercury and allylmercury chloride react with singlet oxygen to show only the M-ene reaction, but also undergo extensive photosensitized decomposition. With 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD), allylmercury chloride shows only the M-ene reaction.

Radical-chain cyclisation of unsaturated acetals and thioacetals in the presence of thiols as polarity-reversal catalysts

Abstract Polarity-reversal catalysis by thiols has been applied to promote the radical-chain cyclisation of 2-(pent-4-enyl)-substituted 1,3-dioxolanes, 1,3-dithianes and 1,3-dithiolanes to give spirocyclic products.

Deoxygenation of carbohydrates by thiol-catalysed radical-chain redox rearrangement of the derived benzylidene acetals

Five- or six-membered cyclic benzylidene acetals, derived from 1,2- or 1,3-diol functionality in carbohydrates, undergo an efficient thiol-catalysed radical-chain redox rearrangement resulting in deoxygenation at one of the diol termini and formation of a benzoate ester function at the other. The role of the thiol is to act as a protic polarity-reversal catalyst to promote the overall abstraction of the acetal hydrogen atom by a nucleophilic alkyl radical. The redox rearrangement is carried out in refluxing octane and/or chlorobenzene as solvent at ca. 130 degrees C and is initiated by thermal decomposition of di-tert-butyl peroxide (DTBP) or 2,2-bis(tert-butylperoxy)butane. The silanethiols (Bu(t)O)3SiSH and Pr(i)3SiSH (TIPST) are particularly efficient catalysts and the use of DTBP in conjunction with TIPST is generally the most effective and convenient combination. The reaction has been applied to the mono-deoxygenation of a variety of monosaccharides by way of 1,2-, 3,4- and 4,6-O-benzylidene pyranoses and a 5,6-O-benzylidene furanose. It has also been applied to bring about the dideoxygenation of mannose and of the disaccharide alpha,alpha-trehalose. The use of p-methoxybenzylidene acetals offers no great advantage and ethylene acetals do not undergo significant redox rearrangement under similar conditions. Functional group compatibility is good and tosylate, epoxide and ketone functions do not interfere; it is not necessary to protect free OH groups. Because of the different mechanisms of the ring-opening step (homolytic versus heterolytic), the regioselectivity of the redox rearrangement can differ usefully from that resulting from the Hanessian-Hullar (H.-H.) and Collins reactions for brominative ring opening of benzylidene acetals. When simple deoxygenation of a carbohydrate is desired, the one-pot redox rearrangement offers an advantage over H.-H./Collins-based procedures in that the reductive debromination step (which often involves the use of toxic tin hydrides) required by the latter methodology is avoided.

Homolytic reactions of ligated boranes. Part 19. Relationships between structure, reactivity and enantioselectivity for hydrogen-atom abstraction by chiral amine–boryl radicals

The molecular structures of optically active quinuclidine–isopinocampheylborane and of the polycyclic amine–borane formed by cyclisation of N-nopylpyrrolidine–borane have been determined by X-ray crystallography. These and related amine–borane complexes have been used previously as polarity-reversal catalysts to bring about kinetic resolution of racemic esters and ketones. The key step in these resolutions is enantioselective H-atom abstraction from an α-C–H group in the carbonyl compound by the chiral amine–boryl radical derived from the catalyst. Ab initio and semi-empirical molecular orbital calculations have been carried out for representative transition states involved in H-atom transfer to amine–boryl radicals and the roles of dipole–dipole interactions, stereoelectronic effects and hydrogen-bonding have been investigated. The steric demands of a variety of amine–boryl radicals in H-atom transfer reactions have been assessed by determining the relative rates of abstraction from the α-C–H bonds in diethyl malonate and diethyl methylmalonate.

Homolytic reactions of ligated boranes. Part 17. Amine–boranes as polarity reversal catalysts for radical chain reactions of esters with vinylic epoxides and with allylic tert-butyl peroxides

In the presence of an amine-borane catalyst, methyl acetate, dimethyl malonate and dimethyl methylmalonate (HZ) each react with 1 -methylene-2,3-epoxycyclo-hexanes or -pentanes at a C–H group a to the ester function to give allylic alcohols. The reaction proceeds by a radical chain mechanism and is initiated by UV photolysis of added di-tert-butyl peroxide at 30 °C; isolated yields are generally 50–70%. The amine–borane, usually quinuclidine-borane (QNB), acts as a polarity reversal catalyst to facilitate regioselective overall transfer of hydrogen from an α-C–H group of the ester to an allyloxyl radical to give the allylic alcohol. The α-alkoxycarbonylalkyl radical (Z˙) also formed in this reaction adds to the vinyl epoxide to give an oxiranylcarbinyl radical, subsequent ring opening of which regenerates the allyloxyl radical. In the presence of QNB, methyl acetate, dimethyl malonate, triethyl methanetricarboxylate and ethyl cyanoacetate each react at an α–C–H group with an allylic tert-butyl peroxide H2CC(R)CMe2OOBut(R = H or Me) to give the 2,3-epoxypropanation products ZCH2[graphic ommitted] in 50–80% yield. Again, a radical chain mechanism is followed and the amine–borane catalyses α-hydrogen-atom transfer from the ester to ButO˙, which is generated by an SHi reaction of the β-tert-butylperoxyalkyl radical formed by addition of Z˙ to the allylic peroxide.

Regioselectivity in the ring-opening β-scission of 2-phenyl-1,3-dioxan-2-yl radicals derived from bicyclic benzylidene acetals

The thiol-catalysed radical-chain redox rearrangement to benzoate esters of a number of cis- and trans-fused bicyclic benzylidene acetals derived from 1,3-diols has been investigated at ca. 130 °C in refluxing octane. The most generally effective and convenient combination of initiator and catalyst for this type of reaction consists of di-tert-butyl peroxide in conjunction with triisopropylsilanethiol. The benzoate esters are produced by β-scission of intermediate 2-phenyl-1,3-dioxan-2-yl radicals with fused cyclohexane or cyclopentane rings and there are two modes of cleavage for each radical, to give either a primary or a secondary 3-benzoyloxyalkyl radical. The regioselectivity of β-scission differs markedly depending on whether the ring junction is cis or trans, such that the trans-isomer gives preferentially the primary alkyl radical while the cis-isomer affords the secondary radical. Density functional calculations indicate that the β-scission proceeds through a product-like transition state in which the geometry at the emerging radical centre is quite close to planar. The regioselectivity observed in the β-scission of these bicyclic 1,3-dioxan-2-yl radicals can be understood in terms of the interplay between the thermodynamic driving force, charge-transfer stabilisation of the transition state and the degree of umbrella angle strain at the emerging radical centre.

Selective radical-chain epimerisation at CH centres α to oxygen under conditions of polarity-reversal catalysis

Polarity-reversal catalysis by tri-tert-butoxysilanethiol has been applied to promote radical-chain epimerisation selectively at carbon centres of the type R1R2C∗(H)OR.

Homolytic reactions of ligated boranes. Part 18. The scope of enantioselective hydrogen-atom abstraction by chiral amine–boryl radicals for kinetic resolution under conditions of polarity reversal catalysis

A variety of new and previously-known optically active amine–borane complexes have been used as polarity reversal catalysts for the kinetic resolution of representative racemic carbonyl-containing compounds. The key step involves enantioselective abstraction of hydrogen from a C–H bond α to the carbonyl function by optically active amine–boryl radicals derived from the catalyst by hydrogen-atom transfer to tert-butoxyl radicals generated by photolysis of di-tert-butyl peroxide. Chiral discrimination is generally not large, although enantioselectivity factors up to 8.8 were obtained at –74 °C in oxirane as solvent. The more reactive substrate enantiomer can generally be predicted by consideration of the steric interactions between the substituents attached to the boron atom and to the α-carbon atom in the diastereoisomeric transition states. However, hydrogen bonding and dipole–dipole interactions, together with stereoelectronic effects, may also play a part in determining enantioselectivity particularly when there is not marked steric asymmetry around the reacting centres.