Methods for the general formation of C(sp3)C(sp3) bonds by cross‐coupling reactions in the presence of functional groups (see Equation (1)) are important but uncommon synthetic organometallic tools. ...Recent developments in the metal‐catalyzed reactions of mild alkyl nucleophiles and alkyl electrophiles have yielded new methodologies for the formation of this type of CC bond. M=Ni, Pd. m=Li, Mg, Zn, Al, Sn, B, Si.
Olefins devoid of directing or activating groups have been dicarbofunctionalized here with two electrophilic carbon sources under reductive conditions. Simultaneous formation of one C(sp3)–C(sp3) ...and one C(sp3)–C(sp2) bond across a variety of unbiased π-systems proceeds with exquisite selectivity by the combination of a Ni catalyst with TDAE as sacrificial reductant. Control experiments and computational studies revealed the feasibility of a radical-based mechanism involving, formally, two interconnected Ni(I)/Ni(III) processes and demonstrated the different ability of Ni(I) species (Ni(I)I vs PhNi(I)) to reduce the C(sp3)–I bond. The role of the reductant was also investigated in depth, suggesting that a one-electron reduction of Ni(II) species to Ni(I) is thermodynamically favored. Further, the preferential activation of alkyl vs aryl halides by ArNi(I) complexes as well as the high affinity of ArNi(II) for secondary over tertiary C-centered radicals explains the lack of undesired homo- and direct coupling products (Ar–Ar, Ar–Alk) in these transformations.
Two for the price of one: The formation of two C(sp3)C(sp3) bonds can be achieved in a single operation by sequential cyclization and cross‐coupling of alkyl zinc bromides with iodoalkanes ...containing an alkene group (see scheme). The reaction is catalyzed by Ni(py)4Cl2 in the presence of tridentate nitrogen ligands, shows a high functional‐group compatibility, and seems to follow a radical mechanism.
There is significant interest in developing suitable nucleoside analogs exhibiting high fluorescence and triplet yields to investigate the structure, dynamics, and binding properties of nucleic acids ...and promote selective photosensitized damage to DNA/RNA, respectively. In this study, steady-state, laser flash photolysis, time-resolved IR luminescence, and femtosecond broad-band transient absorption spectroscopies are combined with quantum chemical calculations to elucidate the excited-state dynamics of 2-oxopurine riboside in aqueous solution and to investigate its prospective use as a fluorescent or photosensitizer analog. The Franck–Condon population in the S1 (ππ*) state decays through a combination of solvent and conformational relaxation to its minimum in 1.9 ps. The population trapped in the 1ππ* minimum bifurcates to either fluoresce or intersystem cross to the triplet manifold within ca. 5 ns, while another fraction of the population decays nonradiatively to the ground state. It is demonstrated that 2-oxopurine riboside exhibits both high fluorescent (48%) and significant triplet (between 10% and 52%) yields, leading to a yield of singlet oxygen generation of 10%, making this nucleoside analog a dual fluorescent and photosensitizer analog for DNA and RNA research.
The formation of catalytically active alkyl-Ni(
i
) complexes by comproportionation of diorgano-Ni(
ii
) precursors and Ni(0) species proceeds easily through triplet states by alkyl ligand exchange. ...The process involves inversion of the configuration at the carbon that is transferred.
Formation of catalytically competent alkyl-Ni(
i
) complexes involves comproportionation of diakyl-Ni(
ii
) and Ni(0) species and inversion of the configuration.
A synthesis of skipped 1,4-enynes through functionalization of the cyclobutene core with alkynes has been achieved, suggesting an unusual pathway of oxidative addition in tertiary iodoalkanes.
A ...Pd-catalyzed C(sp
3
)-C(sp) bond formation for the direct functionalization of cyclobutenes with alkynes has been achieved, suggesting (DFT and control experiments) an unusual pathway of oxidative addition in tertiary iodoalkanes.
New (triflyl)cyclobutenes have been prepared by palladium-catalyzed hydrodetriflylation reaction using water and deuterium oxide as convenient hydrogen and deuterium sources. In addition, an ...investigation of the possible mechanism for this Tsuji-Trost type reaction of bis(triflyl)cyclobutenes has been facilitated by labelling studies and density functional theory (DFT) calculations.
A palladium-catalyzed hydrodetriflylation reaction using water as a convenient hydrogen source has allowed the preparation of novel (triflyl)cyclobutenes from readily available precursors.
The development of new synthetic methods cannot avoid taking into account economic and environmental issues. In this sense, atom‐economical catalyzed cascade reactions fulfill the desired requisites, ...especially if they afford intermediate building‐blocks that can be used for further transformations. Metal‐catalyzed borylative cyclization reactions have shown to be useful synthetic tools as they provide complex molecules through formation of several bonds. The resulting boronates show high stability, good reactivity, versatility, low toxicity, and can be further functionalized on page 11239 ff.
Good to excellent reactivity and regiocontrol have been achieved in the Cu(I)-catalyzed borylation of dialkyl internal alkynes with bis(pinacolato)diboron. The presence of a propargylic polar group ...(OH, OR, SAr, SO(2)Ar, or NHTs), in combination with PCy(3) as ligand, allowed maximizing the reactivity and site-selectivity (β to the propargylic function). DFT calculations suggest a subtle orbitalic influence from the propargylic group, matched with ligand and substrate size effects, as key factors involved in the high β-selectivity. The vinylboronates allowed the stereoselective synthesis of trisubstituted olefins, while allylic substitution of the SO(2)Py group without affecting the boronate group provided access to formal hydroboration products of unbiased dialkylalkynes.
The cyclizations of enynes substituted at the alkyne gives products of formal 4+2 cyclization with Au(I) catalysts. 1,8-Dien-3-ynes cyclize by a 5-exo-dig pathway to form hydrindanes. 1,6-Enynes with ...an aryl ring at the alkyne give 2,3,9,9a-tetrahydro-1H-cyclopentabnaphthalenes by a 5-exo-dig cyclization followed by a Friedel−Crafts-type ring expansion. A 6-endo-dig cyclization is also observed in some cases as a minor process, although in a few cases, this is the major cyclization pathway. In addition to cationic gold complexes bearing bulky biphenyl phosphines, a gold complex with tris(2,6-di-tert-butylphenyl)phosphite is exceptionally reactive as a catalyst for this reaction. This cyclization can also be carried out very efficiently with heating under microwave irradiation. DFT calculations support a stepwise mechanism for the cycloaddition by the initial formation of an anti-cyclopropyl gold(I)-carbene, followed by its opening to form a carbocation stabilized by a π interaction with the aryl ring, which undergoes a Friedel−Crafts-type reaction.
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