Alkenes, viii

This sequence of events is in contrast with the reduction of 0(,j3-unsaturated ketones with diborane, a process that is known to start with the addition of diborane to the C=C and C=0 bonds that yields VII (see Scheme 8.2). This intermediate subsequently undergoes elimination to the alkene VIII wherein the C=C linkage appears shifted. Remember VI requires two equivalents of BH3 for its conversion to VIII, since the ketone does not contain the strategic components for its self-redox as compound I does.)... 

The hydrogenation of unsaturated aldehydes IV can be a complex transformation, as depicted in Scheme 2. Although the desired reactions are normally either the formation of allylic alcohol V, or saturated aldehyde VI, by 1,2 addition of hydrogen across the functional group, 1,4-addition across the conjugated functions can provide the enol, VII. Over-hydrogenation can result either in further saturation or, for allylic alcohols, hydrogenolysis to the alkene VIII (which can, in turn, be further saturated). 

Concerning consecutive reactions, a typical example is the hydrogenation of alkynes through alkenes to alkanes. Alkenes are more reactive alkynes, however, are much more strongly adsorbed, particularly on some group VIII noble metal catalysts. This situation is illustrated in Fig. 2 for a platinum catalyst, which was taken from the studies by Bond and Wells (45, 46) on hydrogenation of acetylene. The figure shows the decrease of... 

The relative rates of reaction of the silene Me2Si=C(SiMe3)2 with a series of amines, alcohols, phenols, thiophenols, dienes, and alkenes were obtained174 and are reported in Table VIII Section IV.C. 

Cycloaddition reactions between alkenes and noncarbohydrate, carbonyl compounds have been described in discussing the reactions of alkenes (see Table I and Scheme 1). The depiction of the excited carbonyl given in Scheme 6 is useful in understanding the regiochem-istry of the cycloaddition process, as it suggests that the electron-deficient oxygen atom in the excited carbonyl will react with the alkene to produce the (more-stable) 1,4-diradical. Table VIII lists cycloaddition reactions in which the excited carbonyl is part of a carbohydrate. 

The cationic polymerization of arylenes differs in some respects from that of alkenes. The most notable features are that the degree of polymerization is generally lower and that the proportion of unsaturated end groups is always small [21, 22] and often very variable [10]. In the system styrene-TiCl4-CCl3COOH-toluene low polymers are formed which have tolyl end groups [11]. It is not possible to decide at present whether the transfer reaction involved in this is (VIII) or (IX) ... 

Turning from iminophosphanes to alkylidenophosphanes (phospha-alkenes), the orientation of the [2 + 2]-cycloaddition is inverted, as far as phosphorus is concerned only one example has been worked out (product VIII) 19). The phosphaalkyne iBuC=P does not react with the iminoborane BuB=NtBu, which instead trimerizes (IS). An exotic [2 + 2]-cycloaddition is observed when the very reactive titanaethene... 

There have also been significant advances in the imido chemistry of ruthenium and osmium. A variety of imido complexes in oxidation states +8 to +6 have been reported. Notably, osmium (VIII) imido complexes are active intermediates in osmium-catalyzed asymmetric aminohydroxyl-ations of alkenes. Ruthenium(VI) imido complexes with porphyrin ligands can effect stoichiometric and catalytic aziridination of alkenes. With chiral porphyrins, asymmetric aziridination of alkenes has also been achieved. Some of these imido species may also serve as models for biological processes. An imido species has been postulated as an intermediate in the nitrite reductase cycle. " ... 

Fission of carbon-carbon double bonds with the combined use of an osmium catalyst and periodate as a comsumable reagent is an alternative to ozonolysis. In diis process a catalytic amount of osmium is oxidised to osmium(viii) by preiodate and converts the alkene to a glycol which is then cleaved by the periodate. In the electrochemical modification of this process, which uses a divided cell and aque-... 

In Fig. 1.6 a simplified mechanism for as -dihyroxylation of alkenes and ketohydroxylation of R CH=CHR by RuCl3/Oxone /aq. Na(HC03)/Et0Ac-CH3CN is shown. The cA-dihydroxylation route involves (3 + 2) cycloaddition of RuO to the alkene giving a Ru(VI) ester (1) which is oxidised by (HSOj) to the Ru(VIII) ester (2). Reversible nucleophilic addition of water to (2) gives the diol R CH(OH) CH(OH)R (3). Ketohydroxylation ensues when the activated Ru(VIII) ester... 

The first chapter concerns the chemistry of the oxidation catalysts, some 250 of these, arranged in decreasing order of the metal oxidation state (VIII) to (0). Preparations, structural and spectroscopic characteristics are briefly described, followed by a summary of their catalytic oxidation properties for organic substrates, with a brief appendix on practical matters with four important oxidants. The subsequent four chapters concentrate on oxidations of specific organic groups, first for alcohols, then alkenes, arenes, alkynes, alkanes, amines and other substrates with hetero atoms. Frequent cross-references between the five chapters are provided. 

Moreover, Fiirstner shotved that tvhereas complex VIII was inactive for RCM reaction, its PCy3 analog IX was, in contrast, very active in a variety of RCM reactions. The latter is now commercially available and currently used in alkene metathesis (see Section 8.3 for further applications). 

Two observations initiated a strong motivation for the preparation of indenylidene-ruthenium complexes via activation of propargyl alcohols and the synthesis of allenylidene-ruthenium intermediates. The first results from the synthesis of the first indenylidene complexes VIII and IX without observation of the expected allenylidene intermediate [42-44] (Schemes 8.7 and 8.8), and the initial evidence that the well-defined complex IX was an efficient catalyst for alkene metathesis reactions [43-44]. The second observation concerned the direct evidence that the well-defined stable allenylidene ruthenium(arene) complex Ib rearranged intramo-lecularly into the indenylidene-ruthenium complex XV via an acid-promoted process [22, 23] (Scheme 8.11) and that the in situ prepared [33] or isolated [34] derivatives XV behaved as efficient catalysts for ROMP and RCM reactions. 

This 6-hydrogen elimination in 2-rhoda oxetanes is apparently favored over reductive elimination to an epoxide. Moreover, the reverse step, i.e., the oxidative-addition of epoxides to Rh and Ir results in 2-rhoda oxetanes [85] and/or hydrido formylmethyl complexes [86]. Therefore, assuming that 2-metalla oxetanes are intermediates in the oxygenation of alkenes by group VIII transition metals, the reported reactivity would account for selectivity to ketones in the catalytic reactions based on these metals. 

Alkenes. Most Group VIII metals, metal salts, and complexes may be used as catalyst in hydrosilylation of alkenes. Platinum and its derivatives show the highest activity. Rhodium, nickel, and palladium complexes, although less active, may exhibit unique selectivities. The addition is exothermic and it is usually performed without a solvent. Transition-metal complexes with chiral ligands may be employed in asymmetric hydrosilylation 406,422... 

Phenyl-l-butene. With Co2(CO)8, hydroformylation of this alken-ylbenzene, as with allylbenzene, yields little hydrogenation product (Table VIII). 

Metal catalysed decomposition of diazocarbonyl compounds in the presence of alkenes provides a facile and powerful means of constructing electrophilic cyclopropanes. The cyclopropanation process can proceed intermolecularly or intramolecularly. Early work on the topic of intramolecular cyclopropanation (mainly using diazoketones as precursors) has been surveyed31. With the discovery of powerful group VIII metal catalysts, in particular the rhodium(II) derivatives, metal catalysed cyclopropanation of diazocarbonyls is currently the most fertile area in cyclopropyl chemistry. In this section, we will review the efficiency and versatility of the various catalysts employed in the cyclopropanation of diazocarbonyls. Cyclopropanations have been organized according to the types of diazocarbonyl precursors. Emphasis is placed on recent examples. 

Hydroperoxo species intervene as reactive species in the Group VIII metal-catalyzed oxidation of alkenes by 02 or H202. 

There are also several situations where the metal can act as both a homolytic and heterolytic catalyst. For example, vanadium complexes catalyze the epoxidation of allylic alcohols by alkyl hydroperoxides stereoselectively,57 and they involve vanadium(V) alkyl peroxides as reactive intermediates. However, vanadium(V)-alkyl peroxide complexes such as (dipic)VO(OOR)L, having no available coordination site for the complexation of alkenes to occur, react homolyti-cally.46 On the other hand, Group VIII dioxygen complexes generally oxidize alkenes homolytically under forced conditions, while some rhodium-dioxygen complexes oxidize terminal alkenes to methyl ketones at room temperature. 

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