Reduction of Unsaturated Hydrocarbons

The relative reactivities toward diimide cover a range of -10, from 1,2-dimethylcyclohexene to norbornene (ref. 6). Electron attractive substituents increase the reactivity of the double bond towards diimide although the data to place compounds such as maleic acid or acrylonitrile on the scale for Garbisch s hydrocarbons is lacking (ref. 21b). Garbisch et al. found that the main factors that contribute to the observed reactivities in diimide reductions of unsaturated hydrocarbons, eqn. (3), are torsional strain, bond angle... 

In the finely divided condition each of the metals can act as a catalyser, as, for example, in the reduction of unsaturated hydrocarbons. Thus acetylene in the presence of excess of hydrogen on... 

Reduction of unsaturated hydrocarbons and alcohols is similarly retarded by Cl , Br , and I ions 34) to the extent that reaction may become diffusion controlled 246). The anions SO4 and Cl affect hydrocarbon and alcohol oxidations by shifting the formation of C HpOq intermediate to more positive potentials 199, 247). Of course, the Type II oxygenated intermediate does itself deactivate the electrocatalyst due to its low reactivity. 

Reduction of unsaturated hydrocarbons. Gardner and co-workers4,5 have found... 

Solutions of alkali metals in liquid ammonia constitute strongly nucleophilic reagents, but nevertheless their efficiency for reduction of unsaturated hydrocarbons barely surpasses that of the systems described above. 

SCHEME 10 Examples of reduction of unsaturated hydrocarbons by Ndl2 and Dyl2-... 

Purpose. The oxidation of an alkene to an alcohol is investigated via the in situ formation of the corresponding trialkylborane, followed by the oxidation of the carbon-boron bond with hydrogen peroxide. The conditions required for hydroboration (a reduction) of unsaturated hydrocarbons are explored. Alkylboranes are particularly useful synthetic intermediates for the preparation of alcohols. The example used in this experiment is the conversion of 1-octene to 1-octanol in which an anti-Markovrukov addition to the double bond is required to yield the intermediate, trioctylborane. Since it is this alkyl borane that subsequently undergoes oxidation to the alcohol, hydroboration offers a synthetic pathway for introducing substituents at centers of unsaturation that are not normally available to the anti-Markovnikov addition reactions that are based on radical intermediates. 

The course of reduction of unsaturated and aromatic ketones is more complicated. Diaryl ketones, alkylaryl ketones and some aryl alcohols are smoothly reduced to the corresponding hydrocarbons. The recommended way of performing these reductions is to add an equimolar mixture of aluminum chloride and the ketone in ether to an equimolar mixture of aluminum chloride and LiAlH4 in ether. 

The electrochemical reduction of pure hydrocarbons without functional groups is almost exclusively restricted to unsaturated compounds. The reason is that aliphatic hydrocarbons have extremely low electron affinities that render their reduction impossible, despite a gain of solvation energy within the stability limits of conventional solvent-electrolyte systems. 

Reduction of unsaturated aromatic aldehydes to unsaturated hydrocarbons poses a serious problem, especially if the double bond is conjugated with the benzene ring or the carbonyl or both. In Clemmensen reduction the a,)8-unsaturated double bond is usually reduced [160], and in Wolff-Kizhner reduction a cyclopropane derivative may be formed as a result of decomposition of pyrazolines formed by intramolecular addition of the intermediate hydrazones across the double bonds [280]. The only way of converting unsaturated aromatic aldehydes to unsaturated hydrocarbons is the reaction of... 

Reductions. Hydrogenation of Mono- and Polyolefins and Acetylenes. Catalysis of hydrogenation of unsaturated hydrocarbons has been studied widely. Catalysis by a variety of metal complexes including those... 

Recently, it was found that the triangular skeleton can be easily destroyed in reactions which most likely include an electron transfer step. For example, the reduction of gem-dichloro[3]triangulane 66 with lithium in f-butanol (which is the standard method for the synthesis of cyclopropanes) unexpectedly gave 50% of unsaturated hydrocarbon (equation 45)88. An even more complex mixture of hydrogenolysis products was obtained in the reduction of dichloride 67. ... 

Reduction of the three isomeric pyridylcarbinols (29-31) (Scheme 10) leads to products similar to reduction of the hydrocarbon parents 15, 16, and 19 (Scheme 5).58 One should note, however, that the product ratios are different (Table IV). In the case of the 3-isomer (30), the unsaturated product (32) is exocyclic in this report, whereas it is endocyclic (18) in a previous report of the reduction of -picoline (16). Ferles and Tesarova give IR evidence for 32 here and identify 18 by derivatization.35 The IR evidence would seem to be more valid. In contrast to 30, the 3-pyridylmethylcarbinol (33) gave endocyclic piperideines 34 and 35 (Scheme 11). For the pyridylmethylcarbinols 36 and 37, the product ratios were different and the 3-isomer (36) gave a mixture of exocyclic and endocylic piperideines, but mostly exo-(38) (Scheme 12). 

Note. (1) The products of most Clemmensen reductions contain small amounts of unsaturated hydrocarbons. These can be removed by repeated shaking with 10 per cent of the volume of concentrated sulphuric acid until the acid is colourless or nearly so each shaking should be of about 5 minutes duration. The hydrocarbon is washed with water, 10 per cent sodium carbonate solution, water (twice), dried with magnesium sulphate or anhydrous calcium sulphate and finally fractionally distilled over sodium. 

Chapter 4 centers on two key transformations in organic synthesis (1) oxidation of alcohols and of unsaturated hydrocarbons (i.e., alkenes and alkynes) to carbonyl compounds (2) reduction of various carbonyl compounds to alcohols. 

Walk rearrangements have been observed in many carbo- and heterocyclic norcaradiene cycloheptatriene systems upon thermal as well as photochemical excitation. Table 4 contains selected examples. In this connection, the very different thermal stability of ll,ll-dimethyl-l,6-methano[10]annulene and its radical anion is worth mentioning (59). The neutral hydrocarbon rearranges to 7,7-dimethyl-l,2-benzocycloheptatriene at temperatures between 150 and 190°C [Table 4, entry 3, X = C(CH3)2 log A = UA Ea = 35.9 kcal/mol]. The corresponding rearrangement of the radical anion produced by reduction of the hydrocarbon with potassium occurs at -110°C already. The activation energy is lowered here by about 25 kcal/mol over that of the hydrocarbon. The norcaradiene walk has also gained some synthetic importance, for example, in the preparation of the unsaturated bicycle 44, a precursor of heptalene (60). 

In 2007, synthesis and complexation of a PSiP-pincer hgand, in which a sihcon atom and two phosphorus atoms are tethered by a phenylene group, was first reported by Turculet and coworkers [11-19]. They reported that the PSiP-ruthenium complex exhibited catalytic activity for transfer hydrogenation of ketones [11], and the PSiP-platinum and -palladium complexes efficiently catalyzed reduction of COj to methane by silanes [18[. Shortly after the Turculefs first report in 2007, we reported the first example of utilization of the phenylene-bridged PSiP-pincer complex in carbon-carbon bond formation reactions of unsaturated hydrocarbons [20[. 

---