Alkenes, viii additions

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). 

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... 

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... 

The [2+2] Mechanism Already in 1977 Sharpless proposed a stepwise [2+2] mechanism for the osmylation of olefins in analogy to related oxidative processes with d°-metals such as alkene oxidations with CrO,Cl2 [23, 24], Metallaoxetanes [25] were suggested to be formed by suprafacial addition of the oxygens to the olefinic double bond. In the case of osmylation the intermediate osmaoxetane would be derived from an olefm-osmium(VIII) complex that subsequently would rearrange to the stable osmium(VI) ester. 

Recently, several studies have been made of the photolysis of disilanes or polysilanes in the presence of an electron-deficient alkene using a photosensitizer (such as phenanthrene) and acetonitrile as solvent. These conditions result in the addition of silyl groups to one end of the alkene double bond and hydrogen to the other end (equation 18) and evidently involve the reaction of the radical anions of the electron-deficient silene with silyl radicals67 (see also Section VIII.A). 

Alkenes can be dihydroxylated cis-selectively by reaction with a stoichiometric amount of yV-mcthylmorpholinc-yV-oxidc (NMO for a preparation, see Figure 14.30), a catalytic amount of a suitable Os(VIII) reagent, and in the presence of water. This reaction, the cis-vic dihydroxylation, was not discussed in the section on cis-selective additions to C=C double bonds in alkenes (Section 3.3), and it also was not discussed in the context of cycloadditions (Chapter 12). From a preparative point of view, this reaction is closely related to oxidative cleavages, and for this reason it is introduced now (Figure 14.16). 

When bulky alkenes are used, addition of a second alkene molecule to produce an oxo-bis(l,2-diolato)osmate(VI) 7 is preferred. Subsequent osmium oxidation and hydrolysis releases the 1,2-diol and regenerates the trioxo(l,2-diolato)osmate(VIII) 4 that can reenter the catalytic cycle. In this latter case amine addition has no major effect. Proof for the existence of species of type 7 (X-ray) as well as for the catalytic properties of 4 have recently been provided38. 

Whereas hydrolysis of epoxides leads to the trans diaxial addition of water and the formation of trans glycols (1,2-diols), cis glycol formation involves the addition of osmium(VIII) oxide (osmium tetroxide, OsO ) or cold dilute aqueous potassium manganate(Vll) (potassium permanganate) to an alkene. 

The basic mechanism of the Heck reaction (as shown below) of aryl or alkenyl halides or triflates involves initial oxidative addition of a pal-ladium(O) species to afford a a-arylpalladium(II) complex III. The order of reactivity for the oxidative addition step is I > OTf > Br > Cl. Coordination of an alkene IV and subsequent carbon-carbon bond formation by syn addition provide a a-alkylpalladium(II) intermediate VI, which readily undergoes 3-hydride elimination to release the product VIII. A base is required for conversion of the hydridopalla-dium(II) complex IX to the active palladium(O) catalyst I to complete the catalytic cycle. 

The addition of silanes across multiple bonds occurs in the presence of catalysts, mostly complexes of VIII B group elements (e.g. Co, Ni, Pd, Pt) (equation 15). Concerning the course of reaction it can be generally said that, predominantly, (a) the hydrosilylation of simple alkenes and alkynes places the silicon atom at the less substituted carbon atom and (b) via catalysts and reaction conditions a stereospecific course of reaction can be accomplished. Furthermore, as a very positive side-effect, asymmetrical hydrosilylation can be realized if chiral catalysts are employed33a 33bf 33c. For further details we recommend comprehensive review papers on this subject33a c. 

The rhodium (O) complex, (VIII), reacts with an alkyne to form a 1 1 addition complex which is catalytically active for the hydrogenation of alkynes and (weakly) alkenes. Complexes of types (III), but with chloro-ligands, and (VII, X = Cl) also form complexes with acetylenes, which can be subsequently hydrogenated. A series of complexes of structure (VH) with or... 

Applied to benzene the residual affinities were expressed as in (VIII) and it was suggested that the neutralisation of the partial valencies all around the ring led to their lack of reactivity in typical addition reactions of alkenes. 

Various oxidation reactions represented by Patterns 2 and 3 in Scheme 3 are discussed in Sect. VIII.3. However, the Wacker oxidation and related reactions, in which nucleophilic attack on Pd-alkene 7r-complexes plays an important role, are discussed in Sect. V.3. Most of the allylic substitution reactions represented by Pattern 4 are generally preceded by reduction of allylic electrophiles (via oxidative addition). Consequently, the overall processes do not generally involve net oxidation or reduction. So, they are discussed in Sect. V.II. 

Oxidation of alkenes with osmium(Vin) tetroxide (OSO4) (Chapter 6, Scheme 6.10) involves a cyclic intermediate and results in the addition of two hydroxyl groups to the same face [d5-, syn-, (Z)- or suprafacial] of the reacting double bond. However, it is usually desirable to avoid stoichiometric concentrations of expensive, heavy metal poisons. Therefore, the observation that catalytic quantities of osmium(Vni) tetroxide (OSO4) could effectively be used in the presence of a less noxious oxidizing agent (the N-oxide of N-methylmorpholine, NMO) was particularly important. The function of the latter then is to convert the osmium(VI) trioxide (OsOs), produced when the osmium(VIII) tetroxide is reduced (as it performs the oxidation), back to osmiiun(Vni) tetroxide (OSO4), that is,... 

The trajectory of nucleophilic attack upon the double bond of VIII is quite close to that of the addition of nucleophiles to the carbonyl bond. For the addition reaction of the hydride-ion (Y = H) to ethylene (R = Z = H), and propene (R = CH3) the angle d is 123° (according to the ab initio 3-21G calculations in Ref. [23]. The driving force of the reaction is the charge transfer from the electron lone pair orbital of the nucleophile Y to the 7i -orbital of olefine. The latter has, unlike the orbital n Q (see Fig. 4.1), no loop on the a-atom of carbon and is delocalized, which diminishes the overlap integral and thereby the energy of interaction AE from Eq. (4.10) of the nucleophile with alkene as compared to the energy of interaction with the carbonyl... 

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