Reversible carbon hydrides

This becomes especially apparent in hydroformylation reactions of internal alkenes, since not only does (E)/(Z)-isomerization take place, but -aldehydes are obtained. Thus, in the hydroformylation of ( )-4-octene by Co2(CO)g, n-nonanal (78%), 2-methyloctanal (10%), 2-ethylheptanal (6%) and 2-pro-pylhexanal (6%) are obtained. This isomerization is supressed with the phosphine-modified catalysts, in the presence of excess phosphine and at high CO pressures. Both carbon monoxide and phosphine can react with a 16-electron complex to provide an 18-electron complex (e.g. 4 — 5 Scheme 2), the reverse (3-hydride elimination is prevented, a requirement for this elimination being the presence of a vacant co-... 

Similarly, vinylstannanes can also yield products of cine-substitution (Scheme 8.17), specially if tin and an electron-withdrawing or aryl group are bound to the same carbon atom [40, 137-141]. It has been suggested that formation of these products proceeds via intermediate formation of a palladium carbene complex [138, 140] or via reversible /3-hydride elimination [141], and can be avoided by addition of Cu(I) salts [142], which increase the rate of Stille coupling, or by protecting vinylic C-H groups by transient silylation [143]. 

A disadvantage of the widespread use of methanol is its high toxicity to humans and the environment. Methanol s high solubility in water raises concerns that well-water contamination could arise from the widespread use of methanol as an automotive fuel. In general, CO2-releasing fuels address the principle of an emission-free fuel. In the best case, the emitted CO2 is reused for methanol production, for example by plants. There are other carbon hydrides, which can be used as on-board reversible hydrogen storage materials. 

Hydrogen can be bonded to unsaturated carbon hydrides. This reaction takes place reversibly for several chemical systems at moderate temperatures with a relatively high storage capacity ... 

Also noteworthy are some alkylidenes that exemplify rare reactivity for metal hydrides. The first is the cyclic carbene complex 565, the formation of which is itself unusual, proceeding as it does from the interaction of Bp Rh(CO)(py) (566) and methyl iodide. This is proposed to involve the oxidative addition of Mel and subsequent migratory insertion of CO, though at what stage the B-H activation occurs remains to be determined. More significant, however, is that on heating to 45 °C, 565 irreversibly evolves into the alkyl complex 567 via a rare reverse a-hydride migration onto the alkylidene carbon (Scheme 55, Section II-D.2). 

Palladium Pd(II)-catalysed hydroalkylation of Al-protected allylic amines PG(R )N-CH(R )C=CH2 (PG = protecting group) by Bu"ZnBr and other alkylzinc reagents has been reported to afford anti-Markovnikov products PG(R )N-CH(R )CH2CH2-Bu". Mechanistic studies suggest that a reversible jS-hydride elimination/hydride insertion process furnishes the primary Pd-alkyl intermediate, which undergoes transmetallation followed by reductive elimination to form a new sp -sp carbon-carbon bond. ° DFT PBE/3z calculations have been employed to elucidate the solvent effect on hydroxymethoxycarbonylation of cyclohexene catalysed by (Ph3P)2Pd. ... 

Complexes 79 show several types of chemical reactions (87CCR229). Nucleophilic addition may proceed at the C2 and S atoms. In excess potassium cyanide, 79 (R = R = R" = R = H) forms mainly the allyl sulfide complex 82 (R = H, Nu = CN) (84JA2901). The reaction of sodium methylate, phenyl-, and 2-thienyllithium with 79 (R = R = r" = R = H) follows the same route. The fragment consisting of three coplanar carbon atoms is described as the allyl system over which the Tr-electron density is delocalized. The sulfur atom may participate in delocalization to some extent. Complex 82 (R = H, Nu = CN) may be proto-nated by hydrochloric acid to yield the product where the 2-cyanothiophene has been converted into 2,3-dihydro-2-cyanothiophene. The initial thiophene complex 79 (R = R = r" = R = H) reacts reversibly with tri-n-butylphosphine followed by the formation of 82 [R = H, Nu = P(n-Bu)3]. Less basic phosphines, such as methyldiphenylphosphine, add with much greater difficulty. The reaction of 79 (r2 = r3 = r4 = r5 = h) with the hydride anion [BH4, HFe(CO)4, HW(CO)J] followed by the formation of 82 (R = Nu, H) has also been studied in detail. When the hydride anion originates from HFe(CO)4, the process is complicated by the formation of side products 83 and 84. The 2-methylthiophene complex 79... 

The intramolecular Michael addition11 of a nucleophilic oxygen to an a,/ -unsaturated ester constitutes an attractive alternative strategy for the synthesis of the pyran nucleus, a strategy that could conceivably be applied to the brevetoxin problem (see Scheme 2). For example, treatment of hydroxy a,/ -unsaturated ester 9 with sodium hydride furnishes an alkoxide ion that induces ring formation by attacking the electrophilic //-carbon of the unsaturated ester moiety. This base-induced intramolecular Michael addition reaction is a reversible process, and it ultimately affords the thermodynamically most stable product 10 (92% yield). 

There are also reactions in which hydride is transferred from carbon. The carbon-hydrogen bond has little intrinsic tendency to act as a hydride donor, so especially favorable circumstances are required to promote this reactivity. Frequently these reactions proceed through a cyclic TS in which a new C—H bond is formed simultaneously with the C-H cleavage. Hydride transfer is facilitated by high electron density at the carbon atom. Aluminum alkoxides catalyze transfer of hydride from an alcohol to a ketone. This is generally an equilibrium process and the reaction can be driven to completion if the ketone is removed from the system, by, e.g., distillation, in a process known as the Meerwein-Pondorff-Verley reduction,189 The reverse reaction in which the ketone is used in excess is called the Oppenauer oxidation. 

Fig. 30. Mechanism for C-C activation of propene. Decay of the allyl hydride complex may proceed via migration of the metal-bound H atom to the /3-carbon atom in the allyl moiety (i.e. reverse /3-H migration), leading to formation of the same metallacyclobutane complex implicated in the Y + cyclopropane reaction. The dynamically most favorable decay pathway is to YCH2 + C2H4.

The A-frame hydride [Pt2H2(/i-H)(/i-dppm)2] undergoes reductive elimination of H2 in the presence of tertiary phosphine ligands, L, to give the platinum(I) dimer, [Pt2HL(//-dppm)2]. Hill and Puddephatt have shown that this occurs via the intermediate [Pt2II2(/i-H)L(//-dppm)2] (14).99 Carbon monoxide reacts rapidly and reversibly with [PtH(/r-PP)2Pt(CO)]+, PP = R2P-CH2-PR2, R = Et or Ph, to give [PtH(/i-PP)2Pt(CO)2]+ and [PtH(CO)(/u-PP)2Pt(CO)2]+, the first reported mixed valence, platinum(0)-platinum(ll) complexes.100... 

A catalyst used for the u-regioselective hydroformylation of internal olefins has to combine a set of properties, which include high olefin isomerization activity, see reaction b in Scheme 1 outlined for 4-octene. Thus the olefin migratory insertion step into the rhodium hydride bond must be highly reversible, a feature which is undesired in the hydroformylation of 1-alkenes. Additionally, p-hydride elimination should be favoured over migratory insertion of carbon monoxide of the secondary alkyl rhodium, otherwise Ao-aldehydes are formed (reactions a, c). Then, the fast regioselective terminal hydroformylation of the 1-olefin present in a low equilibrium concentration only, will lead to enhanced formation of n-aldehyde (reaction d) as result of a dynamic kinetic control. 

Hydride transfer from carbon to a carbonyl carbon atom occurs, reversibly, in the above reaction of which the classical example is the reduction of ketones, e.g. (43), with Al(OCHMe2)3 (44) in propan-2-ol,... 

This reaction of aromatic aldehydes, ArCHO, resembles the Cannizzaro reaction in that the initial attack [rapid and reversible—step (1)] is by an anion—this time eCN—on the carbonyl carbon atom of one molecule, the donor (125) but instead of hydride transfer (cf. Cannizzaro, p. 216) it is now carbanion addition by (127) to the carbonyl carbon atom of the second molecule of ArCHO, the acceptor (128), that occurs. This, in common with cyanohydrin formation (p. 212) was one of the earliest reactions to have its pathway established— correctly —in 1903. The rate law commonly observed is, as might be expected,... 

The relationship between the electronegativities of silicon, carbon, and hydrogen X(Si) < X(H) < X(C) is the reason for the polarity of the Si H bond in the sense Si+H which is reversed with respect to C-H. Therefore, silicon hydrides can easily be decomposed by hydrolysis. A fundamental conclusion from electronegativity can be formulated as follows. 

The compound [HRh(C0)(TPPTS)3] is a "precatalyst" and dissociates to the 16e species [HRh(C0)(TPPTS)2]. This oxo-active complex initiates the hydroformylation cycle. Under oxo conditions (presence of CO/H2, H20, and a surplus of TPPTS) the hydroxo complex [(HO)Rh(CO)(TPPTS)2] may be formed and again reversibly converted to [HRh(C0)(TPPTS)3] (equilibrium lies almost completely towards the hydride). However, higher carbon monoxide partial pressures may cause the displacement of TPPTS by CO according to Equation 5.8. 

Thus, the role of zinc in the dehydrogenation reaction is to promote deprotonation of the alcohol, thereby enhancing hydride transfer from the zinc alkoxide intermediate. Conversely, in the reverse hydrogenation reaction, its role is to enhance the electrophilicity of the carbonyl carbon atom. Alcohol dehydrogenases are exquisitely stereo specific and by binding their substrate via a three-point attachment site (Figure 12.7), they can distinguish between the two-methylene protons of the prochiral ethanol molecule. 

Condensation between the allenic aldehydes 25 and the allenylboranes 24, derived from the allenylsilanes 23, also exhibited high diastereoselectivity (Scheme 20.7) [33-35], However, unlike 17, a reversal of diastereoselectivity in favor of the RR/SS pair of the a-silyl alcohols 26 occurred. Consequently, treatment of 26 with potassium hydride to promote the syn elimination furnished the enyne-allenes 27 having predominantly the E configuration (fc Z>% 4) for the central carbon-carbon double... 

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