Ethylene and acetylene complexes

Ethylene and Acetylene Complexes (COjjTM-CjHp and Cl4TM-C2H (TM = Cr, Mo, W)... 

The EDA results for the metal-ligand interactions in the metallacyclic compounds CI4TM-C2H, (TM = Mo, W), which are shown in Table 7.7, are very different from the data for the ethylene and acetylene complexes. There are no results for the chromium compounds because of SCF problems [34]. The electron-sharing... 

Although collision-stabilized reaction complexes take part in chain propagation, the complex spectra of ions observed for ethylene and acetylene suggest that this mechanism undoubtedly must compete with consecutive reactions of species produced by unimolecular dissociation of the complexes and by collisional dissociation of other ions. ... 

Many of the Lewis structures in Chapter 9 and elsewhere in this book represent molecules that contain double bonds and triple bonds. From simple molecules such as ethylene and acetylene to complex biochemical compounds such as chlorophyll and plastoquinone, multiple bonds are abundant in chemistry. Double bonds and triple bonds can be described by extending the orbital overlap model of bonding. We begin with ethylene, a simple hydrocarbon with the formula C2 H4. 

The indirect cyclisation of bromoacetals via cobaloxime(I) complexes was first reported in 1985 [67], At that time the reactions were conducted in a divided cell in the presence of a base (40yo aqeous NaOH) and about 50% of chloropyridine cobaloximeflll) as catalyst precursor. It was recently found that the amount of catalyst can be reduced to 5% (turnover of ca. 50) and that the base is no longer necessary when the reactions are conducted in an undivided cell in the presence of a zinc anode [68, 69]. The method has now been applied with cobaloxime or Co[C2(DOXDOH)p ] to a variety of ethylenic and acetylenic compounds to prepare fused bicyclic derivatives (Table 7, entry 1). The cyclic product can be either saturated or unsaturated depending on the amount of catalyst used, the cathode potential, and the presence of a hydrogen donor, e.g., RSH (Table 7, entry 2). The electrochemical method was found with some model reactions to be more selective and more efficient than the chemical route using Zn as reductant [70]. 

SCS-MP2 and the new perturbative B2-PLYP density functional methods provide accurate reaction barriers and outperform MP2 and B3-LYP methods when applied to the 1,3-dipolar cycloaddition reactions of ethylene and acetylene.39 Phosphepine has been shown to catalyse the asymmetric 3 + 2-cycloaddition of allenes with a variety of enones (e.g. chalcones) to produce highly functionalized cyclopentenes with good enantiomeric excess.40 The AuPPh3SbF6 complex catalysed the intramolecular 3 + 2- cycloaddition of unactivated arenyne- (or enyne)-yne functionalities under ambient conditions.41 A review of the use of Rh(I)-catalysed 3 + 2-cycloadditions of diaryl-and arylalkyl-cyclopropenones and aryl-, heteroaryl-, and dialkyl-substituted alkynes to synthesise cyclopentadienones for use in the synthesis of natural products, polymers, dendrimers, and antigen-presenting scaffolds has been presented.42... 

Many cyclization reactions via formation of metallacycles from alkynes and alkenes are known. Formally these reactions can be considered as oxidative cyclization (coupling) involving oxidation of the central metals. Although confusing, they are also called the reductive cyclization, because alkynes and alkenes are reduced to alkenes and alkanes by the metallacycle formation. Three basic patterns for the intermolecular oxidative coupling to give the metallacyclopentane 94, metallacyclopentene 95 and metallacyclopentadiene 96 are known. (For simplicity only ethylene and acetylene are used. The reaction can be extended to substituted alkenes and alkynes too). Formation of these metallacycles is not a one-step process, and is understood by initial formation of an tj2 complex, or metallacyclopropene 99, followed by insertion of the alkyne or alkene to generate the metallacycles 94-96, 100 and 101-103 (Scheme 7.1). 

Munakata and coworkers (1991) have recently characterised copper complexes with ethylene, acetylene, carbon monoxide and cod and studied the structures by X-ray analysis to investigate the bonding involved. Ethylene and acetylene are sideways bonded to copper (Fig. 5-18). Sigma donation from ethylene to copper predominates and ti-back donation is very weak. 

At present, there is an opinion that the first stage of the reaction proceed by another pathway. First of all, the reaction for ethylene and acetylene proceeds through formation of an intermediate weakly-bound complex, which was determined experimentally and theoretically [12-14, 16, 18]. Similar complexes were determined by methods of quantum chemistry for ozone with cis- and trans-butene-2 [19]. In [20], by approximation of the restricted Hartree-... 

In this section we review studies of atomic and molecular adsorbates on alloy surfaces. The section is organized from simplest adsorbates to most complex. Within each subsection we have organized the papers reviewed from most general to most specific. We begin with hydrogen in section 3.1 and oxgyen in 3.2. Carbon monoxide, carbon dioxide and other small molecules such as NO are presented in 3.3. Hydrocarbons such as ethylene and acetylene are discussed in 3.4. Finally, carbon and sulfur are presented in section 3.5. 

For the hydrogenation of ethylene and acetylene with chloroplatinic acid and stannous chloride as catalyst, a number of steps have been outlined which are characteristic of the mechanism 305). These steps are (1) competitive formation of a rr-ethyleneplatinum and a hydroplatinum complex (2) formation of the hydro-rr-ethyleneplatinum complex (3) rearrangement by insertion to form an ethylplatinum complex and (4) attack of the protonic hydrogen on the metal-carbon bond to form ethane and the catalyst. The reduction of acetylene to ethane proceeds via the intermediate formation of ethylene. 

Evidence for resonances in the cross sections for electron scattering from polyatomic molecules. Including hydrocarbons, can be found In the literature as far back as the late 1920 s (1,2). The authors of these papers, however, were unaware that the pronounced low energy peaks in the cross sections of molecules such as ethylene and acetylene were due to temporary negative Ion formation. Haas (3), In 1957, was apparently the first to observe that strong vibrational excitation accompanied such a peak, and to Invoke an unstable negative Ion complex as the means through which the excitation takes place. 

In these reactions, the ligand coupling mechanism is not involved. A different mechanism operates the formation of an intermediate cationic species which is likely to result from the initial formation of a Tc-complex (106).212 Such a mechanism has been invoked to explain the perfluoroalkylation of a number of ethylenic and acetylenic compounds.213-215... 

A review of the methods for the generation of cyclic carbonyl ylides from intramolecular carbene additions has recently appeared [64]. This intermediate was first exploited as the An component for cycloaddition reactions by Ibata [65]. ort/io-Disubstituted carboalkoxy aryl diazoketones such as 54 were decomposed by copper complexes, generating six-membered ring carbonyl ylides. These transient intermediates underwent subsequent intermolecular cycloadditions in the presence of ethylenic and acetylenic reagents to give predominantly exo products containing the oxabicyclo[3.2.1] nucleus, Eq. 38. 

H n.m.r. spectra of complexes of unsymmetrical acetylenes and phosphines show coupling to two nonequivalent phosphorus atoms showing that the square planar environment is maintained in solution 3>. However rotation of both ethylene and acetylene in zerovalent complexes has been predicted 115T... 

Insertion of ethylene and acetylene into the Zr-R bond of the Cp2Zr(R)Cl complex (where R = H and CH3), the so-called hydrozirconation reaction, has been studied by Endo, Koga, and Morokuma (EKM) [64] at the RHF and MP2 levels of theory. They have investigated two different paths, path 1 and 2, of the reaction shown in Fig. 20. In path 1, olefin attacks between the hydride and the chloride ligand, whereas in path 2, it attacks from the... 

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