Multicomponent reactions (MCRs) are fundamentally different from two-component reactions in several aspects. Among the MCRs, those with isocyanides have developed into popular organic-chemical reactions in the pharmaceutical industry for the preparation of compound libraries of low-molecular druglike compounds. With a small set of starting materials, very large libraries can be built up within a short time, which can then be used for research on medicinal substances. Due to the intensive research of the last few years, many new backbone types have become accessible. MCRs are also increasingly being employed in the total synthesis of natural products. MCRs and especially MCRs with isocyanides offer many opportunities to attain new reactions and basic structures. However, this requires that the chemist learns the “language” of MCRs, something that this review wishes to stimulate.

Multicomponent Reactions with Isocyanides.

ion. The oxidative addition mechanism was originally proposed22i because of the lack of a strong rate dependence on polar factors and on the acidity of the medium. Later, however, the electrophilic substitution mechanism also was proposed. Recently, the oxidative addition mechanism was confirmed by investigations into the decomposition and protonolysis of alkylplatinum complexes, which are the reverse of alkane activation. There are two routes which operate in the decomposition of the dimethylplatinum(IV) complex Cs2Pt(CH3)2Cl4. The first route leads to chloride-induced reductive elimination and produces methyl chloride and methane. The second route leads to the formation of ethane. There is strong kinetic evidence that the ethane is produced by the decomposition of an ethylhydridoplatinum(IV) complex formed from the initial dimethylplatinum(IV) complex. In D2O-DCl, the ethane which is formed contains several D atoms and has practically the same multiple exchange parameter and distribution as does an ethane which has undergone platinum(II)-catalyzed H-D exchange with D2O. Moreover, ethyl chloride is formed competitively with H-D exchange in the presence of platinum(IV). From the principle of microscopic reversibility it follows that the same ethylhydridoplatinum(IV) complex is the intermediate in the reaction of ethane with platinum(II). Important results were obtained by Labinger and Bercaw62c in the investigation of the protonolysis mechanism of several alkylplatinum(II) complexes at low temperatures. These reactions are important because they could model the microscopic reverse of C-H activation by platinum(II) complexes. Alkylhydridoplatinum(IV) complexes were observed as intermediates in certain cases, such as when the complex (tmeda)Pt(CH2Ph)Cl or (tmeda)PtMe2 (tmeda ) N,N,N′,N′-tetramethylenediamine) was treated with HCl in CD2Cl2 or CD3OD, respectively. In some cases H-D exchange took place between the methyl groups on platinum and the, CD3OD prior to methane loss. On the basis of the kinetic results, a common mechanism was proposed to operate in all the reactions: (1) protonation of Pt(II) to generate an alkylhydridoplatinum(IV) intermediate, (2) dissociation of solvent or chloride to generate a cationic, fivecoordinate platinum(IV) species, (3) reductive C-H bond formation, producing a platinum(II) alkane σ-complex, and (4) loss of the alkane either through an associative or dissociative substitution pathway. These results implicate the presence of both alkane σ-complexes and alkylhydridoplatinum(IV) complexes as intermediates in the Pt(II)-induced C-H activation reactions. Thus, the first step in the alkane activation reaction is formation of a σ-complex with the alkane, which then undergoes oxidative addition to produce an alkylhydrido complex. Reversible interconversion of these intermediates, together with reversible deprotonation of the alkylhydridoplatinum(IV) complexes, leads to multiple H-D exchange

Activation of c-h bonds by metal complexes

Strong Closed-Shell Interactions in Inorganic Chemistry.

The increase in atmospheric carbon dioxide is linked to climate changes; hence there is an urgent need to reduce the accumulation of CO2 in the atmosphere. The utilization of CO2 as a raw material in the synthesis of chemicals and liquid energy carriers offers a way to mitigate the increasing CO2 buildup. This review covers six important CO2 transformations namely: chemical transformations, photochemical reductions, chemical and electrochemical reductions, biological conversions, reforming and inorganic transformations. Furthermore, the vast research area of carbon capture and storage is reviewed briefly. This review is intended as an introduction to CO2, its synthetic reactions and their possible role in future CO2 mitigation schemes that has to match the scale of man-made CO2 in the atmosphere, which rapidly approaches 1 teraton.

The teraton challenge. A review of fixation and transformation of carbon dioxide

Two major energy-related problems confront the world in the
next 50 years. First, increased worldwide competition for
gradually depleting fossil fuel reserves (derived from past
photosynthesis) will lead to higher costs, both monetarily and politically. Second, atmospheric CO_2 levels are at their highest recorded level since records began. Further increases are predicted to produce large and uncontrollable impacts on the world climate. These projected impacts extend beyond climate to ocean acidification, because the ocean is a major sink for atmospheric CO2.1 Providing a future energy supply that is secure and CO_2-neutral will require switching to nonfossil energy sources such as wind, solar, nuclear, and geothermal energy and developing methods for transforming the energy produced by these new sources into forms that can be stored, transported, and used upon demand.

Frontiers, Opportunities, and Challenges in Biochemical and Chemical Catalysis of CO2 Fixation

Carbon dioxide fixation in its intact form is achieved from ''bifunctional'' complexes containing in their structure a nucleophilic cobalt(I) and an alkali cation: Co(R-salen)M (R-salen = substituted salen ligand; salen = N,N'-ethylenebis(salicylideneaminato); M = Li, Na, K, Cs). The fixation of CO/sub 2/ by such systems displays different degrees of reversibility depending on the reaction solvent and the nature of the alkali cation. The X-ray analysis carried out on (Co(n-Pr-salen)K-(CO)/sub 2/(THF)) shows carbon dioxide anchored to the cobalt through a Co-C sigma bond, while the oxygens interact with the alkali cation, in a polymeric structure. IR spectra are diagnostic for the presence of CO/sub 2/ bonded as a bent molecule in a reduced form displaying three strong bands in the following ranges: 1700-1650, 1300-1250, and 1250-1200 cm/sup -1/. The binding of CO/sub 2/ is partially prevented in the presence of a crown ether, which can complex the alkali cation. The reaction of (Co(salen)Na(CO/sub 2/)) and (Co(salen)Na(THF)) with dicyclohexano-18 crown-6(DCHC) gave a complex, (Co(salen)Na)/sub 2/(DCHC), whose structure shows the presence of the intact bifunctional unit where, however, all the coordination sites around the alkali cation are filled by the oxygens from salen and DCHC and are not further available for bindingmore » the oxygen atoms of the CO/sub 2/ molecule. Crystallographic details and the final R factor for specific observed reflections are given for (Co(n-Pr-salen)K(CO/sub 2/)(THF))/sub n/, (CO(n-Pr-salen)K(CO/sub 2/)(THF))/sub n/, (Co(salen)Na)/sub 2/(DCHC).« less

Carbon dioxide fixation: bifunctional complexes containing acidic and basic sites working as reversible carriers

Polynuclear aryl derivatives of Group 11 metals. Synthesis, solid state-solution structural relationship, and reactivity with phosphines

Carbon dioxide activation. Deoxygenation and disproportionation of carbon dioxide promoted by bis(cyclopentadienyl)titanium and -zirconium derivatives. A novel bonding mode of the carbonato and a trimer of the zirconyl unit

Bifunctional activation of carbon dioxide. Synthesis and structure of a reversible carbon dioxide carrier

Magnetic Properties Diagnostic for the Existence of Iron(II)-Iron(II) Bonds in Dinuclear Complexes Which Derive from Stepwise Insertion Reactions on Unsupported Iron-Aryl Bonds

Carlo Floriani

Papers

Carbon dioxide fixation: bifunctional complexes containing acidic and basic sites working as reversible carriers

Polynuclear aryl derivatives of Group 11 metals. Synthesis, solid state-solution structural relationship, and reactivity with phosphines

Carbon dioxide activation. Deoxygenation and disproportionation of carbon dioxide promoted by bis(cyclopentadienyl)titanium and -zirconium derivatives. A novel bonding mode of the carbonato and a trimer of the zirconyl unit

Bifunctional activation of carbon dioxide. Synthesis and structure of a reversible carbon dioxide carrier

Magnetic Properties Diagnostic for the Existence of Iron(II)-Iron(II) Bonds in Dinuclear Complexes Which Derive from Stepwise Insertion Reactions on Unsupported Iron-Aryl Bonds