Carbonyl thermal activation

Rhodium and cobalt carbonyls have long been known as thermally active hydroformylation catalysts. With thermal activation alone, however, they require higher temperatures and pressures than in the photocatalytic reaction. Iron carbonyl, on the other hand, is a poor hydroformylation catalyst at all temperatures under thermal activation. When irradiated under synthesis gas at 100 atm, the iron carbonyl catalyzes the hydroformylation of terminal olefins even at room temperatures, as was first discovered by P. Krusic. ESR studies suggested the formation of HFe9(C0) radicals as the active catalyst, /25, 26/. Our own results support this idea, 111,28/. Light is necessary to start the hydroformylation of 1-octene with the iron carbonyl catalyst. Once initiated, the reaction proceeds even in the... 

The optically active 1,2-dioxetane of 2,4-adamantanedione (89) was synthesized. Thermal activation of 89 yielded chemiluminescence (Xmax = 420 nm characteristic of ketone fluorescence), pointing to intermediate 90 which is chiral only in its excited state due to the out-of-plane geometry of one of the two carbonyl groups. However, circular polarization of chemiluminescence measurement of 90 has not detected optical activity at the moment of emission. The authors have concluded that fast, relative to the lifetime of ketone singlet excited state, intramolecular n, it energy transfer caused racemization of 90196. 

Photochemical activation of transition metal carbonyls has been used as a preparative tool for substitution of carbonyl ligands by donor molecules or unsaturated hydrocarbons for many years (7-6). The advantage of photochemical activation in comparison with thermal activation is the possibility of conducting reactions at fairly low temperatures. Hence even thermolabile products can be prepared and isolated by appropriate treatment of the reaction mixtures. However, due to the various activation modes of transition metal carbonyls by UV light, often more than one product is obtained, and chromatographic separation is necessary. Limitations are set primarily by the amount of substance which can be irradiated in solution at one time. 

Photochemical activation (15) and thermal activation (11,16, 17) of iron carbonyl complexes In various zeolites have been reported. Part of our study Is to use Mossbauer spectroscopy to Investigate the behavior of Fe(C0)5 on several zeolites when activated photochemically and thermally. Another part of our study Is to Investigate the novel preparation method of Scherzer and Fort (18) that Introduces iron Into (in their study) zeolite NH Y as an anionic complex. Finally, we will report the preparation of ferrocene sublimed onto zeolite ZSM-5. The photochemical and thermal activation of these systems will be reported as well as preliminary results of the photochemical isomerization of olefins by Fe(C0)5 zeolites and the thermal activation of Fischer-Tropsch catalytic systems. It also should be noted here that our Mossbauer studies involve an in-situ pretreatment cell which can be heated to 500°C under various gaseous atmospheres. 

The reason that esters frequently give better yields of oxetane is not known, but it seems likely that reaction of hydroxide ion would u> primarily on the carbonyl carbon atom, to form an Intermediate of i fa> usual type- This might reasonably decompose either (a) with con carted attack on the halogeii-subetituted carbon atom, or (b) inii. -i 3-halogenoalkoxide ion which is thermally activated by tlw he i.l decomposition of the intermediate, thus fadlitating ring cloeure. 

Figure 4. Primary regions of surface composition during the thermal activation of a supported carbonyl complex.

Ene reactions involve the addition of a compound bearing a double bond (enophile) to an olefin possessing an allylic hydrogen atom (ene). They can be thermally activated, but, as the enophile, like the dienophile in the Diels-Alder reaction, should be electron deficient, complexation with a Lewis acid increases the reaction rate thus allowing to carry out the reaction under milder conditions. On the other hand Bronsted acids can also catalyze the reaction through protonation of the carbonyl group and rearrangement to form a more stable carbonium ion. 

Recent studies indicate that the primary photochemical event of a physisorbed, monomeric metal carbonyl is equivalent to that in fluid solution (17-19). However, the products derived from photoactivation of a surface-confined complex can be quite different frtHn those obtained either in the gas phase or in fluid solution (17-20). To a significant extent, these differences, which are particularly evident on hydroxylated supports, arise from the formal participation of the support in the secondary chemistry. Coordination to a surface functionality can stabilize the primary photoproduct, influence its surface mobility, and change its optical absorption characteristics (17-20). In addition, although not well understood at present, surface topology, can impose further constraints on adsorbate reactivity (22,23). Each or any combination of these changes modifies the secondary thermal and/or photochemical reactions. Consequently, photoactivation of an adsorbed metal carbonyl may lead to different chemistry from that found in fluid solution and, since photoactivation is generally at room temperature, from that observed in the thermal activation of the adsorbed complex. 

The substitution of a CO ligand by another 2-electron donor (e.g. PR3) may occur by photochemical or thermal activation, either by direct reaction of the metal carbonyl and incoming ligand, or by first replacing a CO by a more labile ligand such as THF or MeCN. An example of the latter is the formation of Mo(CO)5(PPh3) (equation 23.25) which is most efl ectively carried out by first making the THF adduct (23.34) in situ. 

As a crucial requisite for formation of electronically excited carbonyl product in a chemielectronic process, the reactant must store sufficient energy to chemienergize the product during its thermal decomposition. Since only about 20-25 kcal/mol of thermal activation are available for the 1,2-dioxetanes, the heats of reaction must be highly exothermic. In other words, the condition shown in Eq. (35) must continue, in which Ea... 

The parameter A, in turn, is inversely proportional to the rate constants for nucleation, nuclei and chains growth (Kgm, Kgs, Kgi) respectively. Thus, the analysis of non-isothermal reduction of nickel from NiO allows determination of the possible mechanism for this process, which is interpreted as quasi-chain reaction accompanied by the nuclei growth with equal probability of the size distribution. The proposed kinetic analysis can be used for other thermally activated processes from unstable precursors such as oxalates, metal carbonyls etc. 

The role of transition-metal carbonyls and particularly those of the Group 6 metals in homogeneous photocatalytic and catalytic processes is a matter of considerable interest [1]. UV irradiation especially provides a simple and convenient method for generation of thermally active co-ordinately unsaturated catalyst for alkenes or alkynes transformation. By using tungsten and molybdenum carbonyl compounds as catalysts, alkenes and alkynes can be metathesized, isomerised and polymerized. Photocatalytic isomerization of alkenes in the presence of molybdenum hexacarbonyl was observed by Wringhton thirty years ago [2]. Carbonyl complexes of molybdenum catalyze not only... 

The use of either an anionic carbonyl precursor or a suitable mixture of two anions, so as to set the metal/charge ratio from the outset, is not compulsory because the final metal/charge ratio can also be controlled or achieved by addition of a carefully calibrated amount of oxidizing or reducing agent to the reaction mixture. A recent example in which reductive conditions were introduced during thermal activation of a carbonyl anion precursor is shown in equation (3.5), [164]... 

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