Elimination reactions complexes

Fortunately, in the presence of excess copper(II)nitrate, the elimination reaction is an order of magnitude slower than the desired Diels-Alder reaction with cyclopentadiene, so that upon addition of an excess of cyclopentadiene and copper(II)nitrate, 4.51 is converted smoothly into copper complex 4.53. Removal of the copper ions by treatment with an aqueous EDTA solution afforded in 71% yield crude Diels-Alder adduct 4.54. Catalysis of the Diels-Alder reaction by nickel(II)nitrate is also... 

Extraction of hemiceUulose is a complex process that alters or degrades hemiceUulose in some manner (11,138). Alkaline reagents that break hydrogen bonds are the most effective solvents but they de-estetify and initiate -elimination reactions. Polar solvents such as DMSO and dimethylformamide are more specific and are used to extract partiaUy acetylated polymers from milled wood or holoceUulose (11,139). Solvent mixtures of increasing solvent power are employed in a sequential manner (138) and advantage is taken of the different behavior of various alkaUes and alkaline complexes under different experimental conditions of extraction, concentration, and temperature (4,140). Some sequences for these elaborate extraction schemes have been summarized (138,139) and an experimenter should optimize them for the material involved and the desired end product (102). 

Another type of elimination reaction is the reaction of 1 -(5-methyl-1 -phenyl- H-pyrazol-4-yl)ethanone with a Vilsmeier complex to give 3-chloro-3-(5-methyl-l-phenyl-l//-pyrazol-4-yl)propenal, which in turn undergoes dichloroformylation... 

Alkyl halides are encountered less frequently than their oxygen-containing relatives alcohols and ethers, but some of the kinds of reactions they undergo—nucleophilic substitutions and eliminations—are encountered frequently. Thus, alkyl halide chemistry acts as a relatively simple model for many mechanistically similar but structurally more complex reactions found in biornolecules. We ll begin in this chapter with a look at how to name and prepare alkyl halides, and we ll see several of their reactions. Then in the following chapter, we ll make a detailed study of the substitution and elimination reactions of alkyl halides—two of the most important and well-studied reaction types in organic chemistry. 

Elimination reactions are more complex than substitution reactions for several reasons. There is, for example, the problem of regiochemistry. What... 

Table 2. x./TUnsaturated Iron-Acyl Complexes 3 from 2 via Elimination Reactions CFe R1 base/THF [FeL R1 [Fe] ... 

Elimination reactions have been particularly studied in the case of dialkyls. They depend on the alkyl groups being cis trans-complexes have to isomerize before they can eliminate, and a complex with a trans-spanning diphosphine ligand is stable to 100°C (Figure 3.56). 

Recently the first examples of complexes between the four-membered amidinato-Group 13 metal(l) heterocycles and transition metal fragments were reported. Complexes of the type CpFe(CO)2[M(X) But(NR)2 ] (M = Al, Ga, In X = Cl, Br R = Pri, Gy) were formed in salt-elimination reactions between Na[CpFe(CO)2] and [But(NR)2]MX2. A series of complexes between the four-membered amidinato-Group 13 metal(l) heterocycles and Group 10 metal(O) fragments have been prepared according to Scheme 35. ... 

As already pointed out, the presence of a macrocyclic ligand, which occupies the four equatorial coordination sites of an octahedral complex, will tend to limit insertion and elimination reactions of the Co—C bond to those which require only that single coordination site. The presence of unidentate ligands in the pentacyanides, on the other hand, will offer greater opportunities for reactions which require a second, adjacent site. 

Of special Interest as O2 reduction electrocatalysts are the transition metal macrocycles In the form of layers adsorptlvely attached, chemically bonded or simply physically deposited on an electrode substrate Some of these complexes catalyze the 4-electron reduction of O2 to H2O or 0H while others catalyze principally the 2-electron reduction to the peroxide and/or the peroxide elimination reactions. Various situ spectroscopic techniques have been used to examine the state of these transition metal macrocycle layers on carbon, graphite and metal substrates under various electrochemical conditions. These techniques have Included (a) visible reflectance spectroscopy (b) laser Raman spectroscopy, utilizing surface enhanced Raman scattering and resonant Raman and (c) Mossbauer spectroscopy. This paper will focus on principally the cobalt and Iron phthalocyanlnes and porphyrins. 

Besides dissociation of ligands, photoexcitation of transition metal complexes can facilitate (1) - oxidative addition to metal atoms of C-C, C-H, H-H, C-Hal, H-Si, C-0 and C-P moieties (2) - reductive elimination reactions, forming C-C, C-H, H-H, C-Hal, Hal-Hal and H-Hal moieties (3) - various rearrangements of atoms and chemical bonds in the coordination sphere of metal atoms, such as migratory insertion to C=C bonds, carbonyl and carbenes, ot- and P-elimination, a- and P-cleavage of C-C bonds, coupling of various moieties and bonds, isomerizations, etc. (see [11, 12] and refs, therein). 

A recent study has also found evidence of reductive elimination of 2-propenylimidazolium salt from a [Pd(7i-allyl)(NHC)]" complex, probably following allyl rearrangement (Scheme 13.3) [29], The elimination reaction generated L-Pd(O) in situ, which could then catalyse a novel C-C couphng process. 

There are a number of ways of generating carbenes that will be discussed shortly. In some cases, the reactions involve complexes or precursors of carbenes rather than the carbene per se. For example, carbenes can be generated by a-elimination reactions. Under some circumstances the question arises as to whether the carbene has a finite lifetime, and in some cases a completely free carbene structure is never attained. 

The chemistry of tantalum is different. The tantalum pentaalkyl complex does not exists, because it transforms easily into a carbene complex by a-elimination. This complex reacts also with silica, leading to a supported tantalum complex.262,263 Their reaction proceeds first by the addition of the silanol OH group across the tantalum-carbon double bond followed by elimination of an alkane (Scheme 7.19). 

The complex OsHCl(CO)(P Pr3)2 reacts with n-BuLi in hexane at room temperature to give the dihydride OsH2(CO)(r 2-CH2=CHEt)(P,Pr3)2, which is isolated as a colorless oil in quantitative yield. The reaction most probably involves the replacement of the CP anion by a butyl group to give OsH( -Bu)(CO)(P Pr3)2, which evolves into the dihydride derivative by a hydrogen (3-elimination reaction (Scheme 19).57 Treatment of OsHCl(CO)(P Pr3)2 with C3H5MgBr affords OsH( n3-C3H5)(CO)(P,Pr3)2.58... 

An additional prerequisite in this reaction, however, is inhibition of a premature P-hydrogen elimination. Reaction of 6/4-56 and 6/4-57 led to 6/4-58 with 41 % yield. Again, one can assume that first a Ni-complex 6/4-59 is formed, which gives the bicyclic 6/4-60 followed by formation of the triquinane skeleton 6/4-58 via 6/4-61 with a P-hydride elimination being the last step (Scheme 6/4.15). 

Simple tertiary carbocations represent a benchmark against which to compare the reactivity of other a-methyl carbocations. Therefore, it is necessary to deal with complex questions about the mechanism for substitution and elimination reactions at tertiary aliphatic carbon in order to evaluate the rate constant... 

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