Nucleotides,Part LXVI , β‐Heteroarylethyl Groups – New Phosphate‐Protecting Groups for Phosphotriester Chemistry

The β‐heteroaryl‐substituted ethanols 6 – 10 were synthesized and, together with pyridine‐2‐ethanols and pyridine‐4‐ethanols, were tested as a new type of phosphate‐protecting groups in the synthesis of oligonucleotides by the phosphotriester approach. The synthesis of 5′‐O‐(monomethoxytrityl)thymidine 3′‐(β‐heteroarylethyl 2,5‐dichlorophenyl phosphates) 13 – 17 and 21 provided useful monomeric building blocks in which the various blocking groups could be removed selectively by acid (MeOTr), oximate (2,5‐dichlorophenyl phosphate), and 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU) (heteroarylethyl phosphate) treatment. The new, fully blocked dimers 38 – 41 , with β‐heteroarylethyl protecting groups in the phosphate moiety, were synthesized. The β‐heteroarylethyl groups show a broad range of stability towards base treatment in aprotic solvents depending upon the activation of the H−C(β) atoms by the heterocyclic moiety.     

Reactive Groups,Spacer Groups and Functional Groups in Macromolecular Design

Several aspects of modern design of macromolecular architecture are discussed: the influence and importance of functional groups which often dominate the characteristics of the macromolecular structure; the importance of the spacer groups that provide flexibility and allow the functional group to act independently from the main chain when the functional group is attached to the main chain.

Examples are given for the synthesis of reactive, telechelic polymers, for polymerizability of monomers whose polymerizable group is separated from the functional group by a flexible methylene spacer, and the reactivity of functional groups separated from the main chain by a spacer group.     

Nucleosides,Part LXIV,Base‐Labile Protecting Groups for the Oligoribonucleotide Synthesis

New labile protecting groups for the anticipated synthesis of oligoribonucleotides were developed and introduced via their carbonochloridates 8 – 11 at the 5′‐O position of thymidine ( 15 ) to form 16 , 18 , 21 , and 24 in good yields (Schemes 2 and 3). Similarly, the 5′‐O‐diphenylphosphinoyl(dpp)‐protected thymidine derivative 27 was synthesized with diphenylphosphinoyl chloride 14 as the reactive reagent. With the help of the model compounds 16 , 18 , 21 , 24 , and 27 , the deprotection rates of these functions towards base treatment were recorded to evaluate their usefulness as temporary protecting groups in RNA assembly (Table). Finally, the relative stability of the 2‐(4‐nitrophenyl)ethoxycarbonyl (npeoc) protecting group towards bases confirmed its use as a permanent blocking group in our npe/npeoc strategy.     

Electro-Deprotection—Electrochemical Removal of Protecting Groups

The reactions of electrochemical splitting may be successfully used for removal of protecting groups. Theoretical and preparative aspects of the method of electro-deprotection are discussed, and examples of its use are given. The removal often requires high potentials. The use of modified protecting groups (“inner activation”) or of catalysts (electron carriers) which facilitate electron transfer against the standard potential gradient (“external activation”), can greatly increase the scope of the electrochemical method.     

Synthesis,Electronic, and Morphological Properties of Tetrahedral Oligothiophenes with n‐Hexyl Terminal Groups

A series of tetrahedral oligothiophenes bearing n‐hexyl groups at the α‐positions of the terminal thiophene rings, (n‐C₆H₁₃(C₄H₂S)_n)₄C (Hex‐TnTM; n=1–4), has been synthesized by Kosugi–Migita–Stille coupling as a key reaction. Thanks to the improved solubility afforded by the terminal n‐hexyl groups, the largest homologue (n=4) was successfully obtained. Whereas the smaller derivatives (n=1, 2) were obtained as liquid substances, the larger derivatives (n=3, 4) were obtained as solids. Hex‐T3 TM partially adopts syn conformations between the adjacent thiophene rings in the crystal, probably owing to the packing force. Hex‐T3 TM not only appeared in the crystalline state but also the amorphous state, which was stable to up to 80 °C. Regardless of the terminal groups, the derivatives of n=2 exhibited a broad fluorescence with large Stokes shifts compared to the corresponding linear analogues, thereby suggesting the presence of intramolecular interactions between the bithiophene moieties. Interactions between terthiophene branches was also suggested in the radical cations of Hex‐T3 TM by cyclic voltammetry measurements.     

P‐Aminophosphaalkenes with C‐Isopropyldimethylsilyl Groups

Amination of the C‐isopropyldimethylsilyl P‐chlorophosphaalkene (iPrMe₂Si)₂C=PCl ( 1 ) leads to the P‐aminophosphaalkenes (iPrMe₂Si)₂C=PN(R)R′ (R, R′ = Me ( 2 ), R = H, R′ = nPr ( 3 ), R = H, R′ = iPr ( 4 ), R = H, R′ = tBu ( 5 ), R = H, R′ = 1‐Ada ( 6 ), R = H, R′ = CPh₃ ( 7 ), R = H, R′ = Ph ( 8 ), R = H, RR′ = 2,6‐iPr₂Ph (= DIP) ( 10 ), R = H, R′ = 2,4,6‐Me₃Ph (= Mes) ( 11 ), R = H, R′ = 2,4,6‐tBu₃Ph (= Mes*)] ( 12 ), R = H, R′ = SiMe₃ ( 13 ), and R, R′ = SiMe₂Ph (1 4 ). ³¹P‐NMR spectra confirm that phosphaalkenes 2 – 7 and 10 – 14 are monomeric in solution; the structures of 7 , 10 , and 12 were determined by X‐ray crystallography. Freshly prepared (iPrMe₂Si)₂C=PN(H)Ph ( 8 ) is a monomer that dimerizes with (N→C) proton migration within several hours to the stable diazadiphosphetidine [(iPrMe₂Si)₂CHPNPh]₂ ( 9 ). NMR‐scale reactions of deprotonated 5 and 13 with tBuiPrPCl provide by P–P bond formation the P‐phosphanyl iminophosphoranes [(iPrMe₂Si)₂C=](RN=)PPtBu(iPr) [R = tBu ( 15 ), R = Me₃Si ( 17 )]. Deprotonated 5 and Me₃GeCl deliver by N–Ge bond formation the aminophosphaalkene (iPrMe₂Si)₂C=PN(tBu)GeMe₃ ( 20 ), which with elemental selenium 5 undergoes (N→C) proton migration to form the alkyl(imino)(seleno)phosphorane [(iPrMe₂Si)₂CH](tBuN=)P=Se ( 21 ), which is a selenium‐bridged cyclic dimer in the solid state.     

Photoinitiators with Functional Groups, 8

Summary: Bimolecular type‐II photoinitiators for radical photopolymerization suffer from a diffusion‐controlled limitation of reactivity and from deactivation by back electron transfer. Here, a very efficient concept to increase the photoinitiator activity by the covalent binding of phenylglycine to benzophenone using a methylene spacer is presented. Photo‐DSC experiments proved that the rate of polymerization can be tripled in comparison to a physical mixture of the components or an industrially applied system with triethanolamine as coinitiator.

Structure of the new photoinitiator synthesized here.     

Silver‐Catalyzed Coupling of Two CH Groups and One‐Pot Synthesis of Tetrasubstituted Furans,Thiophenes, and Pyrroles

Silver‐catalyzed coupling of two C?H groups to form 1,4‐diketones have been developed for the first time. The resultant ketones then undergo cyclization to synthesize tetrasubstituted furans, thiophenes, and pyrroles from benzyl ketone derivatives in a one‐pot reaction process. This highly‐efficient synthetic method, which utilizes air as the terminal oxidant and readily accessible starting materials, displays a wide substrate scope and broad functional‐group tolerance.     

Introduction of Fluorine and Fluorine‐Containing Functional Groups

Over the past decade, the most significant, conceptual advances in the field of fluorination were enabled most prominently by organo‐ and transition‐metal catalysis. The most challenging transformation remains the formation of the parent C? F bond, primarily as a consequence of the high hydration energy of fluoride, strong metal—fluorine bonds, and highly polarized bonds to fluorine. Most fluorination reactions still lack generality, predictability, and cost‐efficiency. Despite all current limitations, modern fluorination methods have made fluorinated molecules more readily available than ever before and have begun to have an impact on research areas that do not require large amounts of material, such as drug discovery and positron emission tomography. This Review gives a brief summary of conventional fluorination reactions, including those reactions that introduce fluorinated functional groups, and focuses on modern developments in the field.     

meso‐Free BIII Subporphyrins with Electron‐donating Groups

meso‐Free B^III 5,10‐bis(p‐dimethylaminophenyl)subporphyrins were synthesized. They display red‐shifted absorption and fluorescence spectra, bathochromic behaviors in polar solvents, a high fluorescence quantum yield (Φ_F=0.57), and a small HOMO–LUMO gap mainly due to destabilized HOMO as compared with meso‐free B^III 5,10‐diphenylsubporphyrin. This subporphyrin serves as a nice precursor of various meso‐substituted B^III subporphyrins such as B^III meso‐nitrosubporphyrin, B^III meso‐aminosubporphyrin, and meso‐meso’ linked B^III azosubporphyrin dimer. Reactions of meso‐free B^III subporphyrins with NBS or bis(2,4,6‐trimethylpyridine)bromonium hexafluorophosphate gave meso‐meso′ linked subporphyrin dimers, often as a major product along with meso‐bromosubporphyrins.     

Pyrazole–Tetrazole Hybrid with Trinitromethyl,Fluorodinitromethyl, or (Difluoroamino)dinitromethyl Groups: High‐Performance Energetic Materials

High‐nitrogen‐content compounds have attracted great scientific interest and technological importance because of their unique energy content, and they find diverse applications in many fields of science and technology. Understanding of structure–property relationship trends and how to modify them is of paramount importance for their further improvement. Herein, the installation of oxygen‐rich modules, C(NO₂)₃, C(NO₂)₂F, or C(NO₂)₂NF₂, into an endothermic framework, that is, the combination of a nitropyrazole unit and tetrazole ring, is used as a way to design novel energetic compounds. Density, oxygen balance, and enthalpy of formation are enhanced by the presence of these oxygen‐containing units. The structures of all compounds were confirmed by XRD. For crystal packing analysis, it is proposed to use new criterion, Δ_OED, that can serve as a measure of the tightness of molecular packing upon crystal formation. Overall, the materials show promising detonation and propulsion parameters.     

Quantum Dot‐Catalyzed Photoreductive Removal of Sulfonyl‐Based Protecting Groups

This Communication describes the use of CuInS₂/ZnS quantum dots (QDs) as photocatalysts for the reductive deprotection of aryl sulfonyl‐protected phenols. For a series of aryl sulfonates with electron‐withdrawing substituents, the rate of deprotection for the corresponding phenyl aryl sulfonates increases with decreasing electrochemical potential for the two electron transfers within the catalytic cycle. The rate of deprotection for a substrate that contains a carboxylic acid, a known QD‐binding group, is accelerated by more than a factor of ten from that expected from the electrochemical potential for the transformation, a result that suggests that formation of metastable electron donor–acceptor complexes provides a significant kinetic advantage. This deprotection method does not perturb the common NHBoc or toluenesulfonyl protecting groups and, as demonstrated with an estrone substrate, does not perturb proximate ketones, which are generally vulnerable to many chemical reduction methods used for this class of reactions.