Hydrogen Donor Abilities of the Group 14 Hydrides

Recent Advances in the Chemistry of the Organotin Hydrides

Advances in the chemistry of the organotin hydrides during the last decade are reviewed.

Aminopyridine-Borane Complexes as Hydrogen Atom Donor Reagents: Reaction Mechanism and Substrate Selectivity

Lewis base-borane complexes are shown to be potent hydrogen atom donors in radical chain reduction reactions. Results obtained in ¹ H, ¹¹ B, and ¹³ C NMR measurements and kinetic experiments support a complex reaction mechanism involving the parent borane as well as its initial reaction products as active hydrogen atom donors. Efficient reduction reactions of iodides, bromides, and xanthates in apolar solvents rely on initiator systems generating oxygen-centered radicals under thermal conditions and pyridine-borane complexes carrying solubilizing substituents. In contrast to tin hydride reagents, the pyridine-boranes reduce xanthates faster than the corresponding iodides.

Understanding Initiation with Triethylboron and Oxygen: The Differences between Low-Oxygen and High-Oxygen Regimes

A unified kinetic theory for both initiation and autoxidation reactions of Et3B and O2 is put forth, and then divided into low-oxygen and high-oxygen experimental regimes for application of Et3B/O2 as an initiating system. In the low-oxygen regime, only long, efficient chains can be initiated. In the high-oxygen regime, less efficient chains can be initiated but they must compete with autoxidation. We apply the analysis along with new experimental results to show why AIBN and Et3B/O2 give different stereochemical results in hydrostannation reactions of propargyl silyl ethers. Counterintuitively, AIBN is the better initiator, initiating both the rapid chain hydrostannation and the subsequent slow E/Z isomerization. AIBN gives the thermodynamic results. Et3B/O2 is the poorer initiator, initiating only the hydrostannation and not the isomerization. Et3B gives the kinetic result. We further apply the analysis to understand recent results in Et3B/water reductions.

Preparation of optically active bicyclodihydrosiloles by a radical cascade reaction

Bicyclodihydrosiloles were readily prepared from optically active enyne compounds by a radical cascade reaction triggered by tris(trimethylsilyl)silane ((Me₃Si)₃SiH). The reaction was initiated by the addition of a silyl radical to an α,β-unsaturated ester, forming an α-carbonyl radical that underwent radical cyclization to a terminal alkyne unit. The resulting vinyl radical attacked the silicon atom in an S_Hi manner to give dihydrosilole. The reaction preferentially formed trans isomers of bicyclosiloles with an approximately 7:3 to 9:1 selectivity.

Hydrogermylation of 5-ethynyluracil nucleosides: formation of 5-(2-germylvinyl)uracil and 5-(2-germylacetyl)uracil nucleosides

A stereoselective radical-mediated hydrogermylation of the protected 5-ethynyluracil nucleosides with trialkyl-, triaryl,- or tris(trimethylsilyl)germanes gave (Z)-5-(2-germylvinyl)uridine, 2'-deoxyuridine, or ara-uridine as major products. Reaction of the β-triphenylgermyl vinyl radical intermediate with oxygen and fragmentation of the resulting peroxyradical provided also 5-[2-(triphenylgermyl)acetyl]pyrimidine nucleosides in low to moderate yields. Thermal isomerization of the latter in MeOH occurred via a four-centered activated complex, and subsequent hydrolysis of the resulting O-germyl substituted enol yielded 5-acetyluracil nucleosides in quantitative yield.

UV-initiated hydrosilylation on hydrogen-terminated silicon (111): rate coefficient increase of two orders of magnitude in the presence of aromatic electron acceptors

UV-initiated (254 nm) hydrosilylation of hexadecene on Si(111)-H has been studied in the presence of various aliphatic and aromatic molecules (additives). Many of these additives cause an enhancement in the pseudo-first-order rate coefficient (k(obs)) of hydrosilylation, some up to 200× faster than observed in neat hexadecene. It is proposed that these additives capture the photoejected electron from the surface, thereby increasing the probability of reaction of the alkene with the surface hole (h(+)), leading to Si-C bond formation. While the ability of these additives to increase k(obs) is related to their reduction potential, aromatic additives are particularly efficient; we suspect this is due to the relatively strong physisorption of the aromatic molecules leading to a favorable geometry for electron transfer. The presence of these additives permits the use of a much lower intensity of UV light (~30 μW/cm(2)), reducing the probability of photodegradation of the monolayer, and maximum coverage can be reached within minutes.

Polarity reversal catalysis in radical reductions of halides by N-heterocyclic carbene boranes

Otherwise sluggish or completely ineffective radical reductions of alkyl and aryl halides by N-heterocyclic carbene boranes (NHC-boranes) are catalyzed by thiols. Reductions and reductive cyclizations with readily available 1,3-dimethylimidazol-2-ylidene borane and a water-soluble triazole relative are catalyzed by thiophenol and tert-dodecanethiol [C(9)H(19)C(CH(3))(2)SH]. Rate constants for reaction of the phenylthiyl (PhS•) radical with two NHC-boranes have been measured to be ~10(8) M(-1) s(-1) by laser flash photolysis experiments. An analysis of the available evidence suggests the operation of polarity reversal catalysis.

Recent applications of the (TMS)3SiH radical-based reagent

This review article focuses on the recent applications of tris(trimethylsilyl)silane as a radical-based reagent in organic chemistry. Numerous examples of the successful use of (TMS)(3)SiH in radical reductions, hydrosilylation and consecutive radical reactions are given. The use of (TMS)(3)SiH allows reactions to be carried out under mild conditions with excellent yields of products and remarkable chemo-, regio-, and stereoselectivity. The strategic role of (TMS)(3)SiH in polymerization is underlined with emphasis on the photo-induced radical polymerization of olefins and photo-promoted cationic polymerization of epoxides.

Radical deoxygenation of xanthates and related functional groups with new minimalist N-heterocyclic carbene boranes

Minimalist N-heterocyclic carbene boranes 1,3-dimethylimidazol-2-ylideneborane and 2,4-dimethyl-1,2,4-triazol-3-ylideneborane are readily available and have low molecular weights. They exhibit superior performance to first-generation NHC-boranes, providing improved yields in reductions of xanthates and related functional groups.

N-heterocyclic carbene boryl radicals: a new class of boron-centered radical

Reduction of xanthates by N-heterocyclic carbene boranes (NHC-boranes) has been suggested to occur by a radical chain mechanism involving heretofore unknown NHC-boryl radicals. In support of this suggestion, both the expected borane dithiocarbonate product and an unexpected borane xanthate product have now been isolated. These are the first NHC-boranes with boron-sulfur bonds, and their structures have been secured by spectroscopic and crystallographic means. The first rate constants for H-atom transfer from an NHC borane complex were determined by using the ring opening of a substituted cyclobutylcarbinyl radical as a clock reaction. The rate constant for reaction of the NHC-borane with a secondary alkyl radical at ambient temperature is 4 x 10(4) M(-1) s(-1), and the Arrhenius function displayed an entropic term (log A term) that was typical for a bimolecular reaction. The B-H bond dissociation energy of an NHC-borane complex has been estimated at 88 kcal/mol. The putative NHC-boryl radical in these transformations has been detected by EPR spectroscopy. Spectral analysis suggests that it is a pi-radical, analogous to the benzyl radical.

Catalysis of stannane-mediated radical chain reactions by benzeneselenol

The discovery and development of the catalysis of stannane-mediated radical chain reactions by benzeneselenol, generated in situ by reduction of diphenyl diselenide with tributyltin hydride, are described. The catalytic sequence is discussed in terms of polarity reversal catalysis of radical chain reactions, and applications to synthesis are presented. These include the prevention of numerous radical rearrangement reactions, the ability to intervene in certain multistep radical rearrangements, especially aryl and vinyl radical cyclizations, at intermediate stages with advantages to the product profile, and the effective trapping of allyl-, benzyl-, and cyclohexadienyl-type radicals, permitting inter alia the isolation of aryl cyclohexadienes and their application in synthesis.

Convergent Assembly of Polycyclic Ethers via Acyl Radical Addition to Unactivated Enol Ether

A new convergent strategy for assembling 6/6- and 6/7-fused ether ring systems was developed. The key features in our method include Ag+-promoted facile formation of chemically labile enol ether from O,S-acetal and addition of an acyl radical to unactivated enol ether to cyclize a six- or seven-membered ether ring. [reaction: see text]

Reaction of the 2'-silyl and 2'-stannyl derivatives of 6-(bromomethyl)dimethylsilyl-1',2'-unsaturated uridine under radical conditions

The mode of cyclization (5-exo versus 6-endo) of 2-sila-5-hexen-1-yl radicals generated from 2'-tributylstannyl- and 2'-trimethylsilyl-6-(bromomethyl)dimethylsilyl-1',2'-unsaturated uridines (8 and 9) was investigated. Although the actual structure of the reaction products differ from each other reflecting the ease of elimination of the 2'-substituent, it was found that both substrates prefer the 5-exo-cyclization pathway.