13.6.3. N-Nitrosoamines
N-Nitrosoamines are reduced easily to the hydrazine and, if continued, to the amine (62). Early workers ruled out cleavage of dimethylhydrazine as the source of dimethylamine in hydrogenation of N-nitrosodimethylamine since little ammonia was found; the tetramethyltetrazene was implicated in the hydrogenolysis (131). Palladium-on-carbon under mild conditions is used for industrial production of dialkyl hydrazines from JV-nitrosoamines.
Hydrogenolysis can be decreased, if necessary, by the addition of any of a variety of salts that increase the ionic strength of the medium (162). Iron salts have been used specifically for this purpose (102,173). Yields may depend markedly on the conditions of the reaction (92).
13.6.4. C-Nitroso Compounds
C-Nitroso compounds with an a-hydrogen atom rearrange readily to the corresponding oxime (171) and perhaps to the unsaturated hydroxylamine (145). Reduction of these is discussed in the chapter on oximes.
Aromatic nitroso compounds usually are considered to be intermediates in the hydrogenation of a nitroaromatic compound to the aromatic hydroxyl- amine or amine. However, nitroso compounds do not accumulate in these reductions, suggesting that they are reduced more easily than are nitro compounds. Catalysts effective for the nitro group should also be effective for nitroso.
Formation of azo-type products might be troublesome. These by-products, arising from reduction of aromatic nitro compounds, usually are assumed to be derived from the coupling of intermediate nitroso and hydroxylamine compounds. The coupling problem is accentuated in reduction of nitroso compounds because of much higher concentrations. It can be alleviated by dropwise addition of the substrate to the hydrogenation and use of acidic media.
13.7. Hydrogenolysis of the Carbon-Carbon Bond
Carbon-carbon bonds are not easily cleaved under mild conditions unless weakened by strain (3,86,182) or activation. The most common examples of carbon-carbon bond cleavage occur in cyclopropanes.
13J.1. Cyclopropanes
Cyclopropanes are now readily available and have become useful, through hydrogenolysis, for synthesis of compounds containing quaternary carbons, 0em-dialkyl, r-butyl, and angular-methyl substituents (179), compounds often available only with difficulty otherwise (27,53,58,150,156). Cyclopropanes can be formed in good yields by hydrogenation of cyclopropenes (26).
An a priori determination of the direction of ring opening is not always easy, for it is difficult to decide which of several controlling factors is operative.
Various generalities concerning the direction of opening have been suggested.
Cyclopropanes carrying only phenyl substituents are cleaved exclusively at the bonds adjacent to the phenyl substituent (87), whereas alkyl substituents favor cleavage at the bond opposite the substituent (1220).
Electron-attracting substituents are often cleaved at the bond adjacent to the substituent (63,79,94,146,154), but there are exceptions (113,176).
More highly substituted nonfunctional Cyclopropanes open variously at the carbon with the most hydrogens (736,1080), least hindrance (39aJ47c), or greatest strain (51aJ22aJ56\ Additional strain allows facile ring opening.
Hydrogenolysis of the cyclopropane ring in the strained compound 36, occurred at ambient conditions to give seven parts of 4-homoproto- adamantane (37) and one part of 38 (85).
EtOAc 250C, I a U n H2
(36) (37)
In contrast, hydrogenation of 39 gives only 40.
(38)
(39) (40)
The catalyst exerts some influence on the bonds broken in hydrogenolysis of saturated Cyclopropanes (118), but in vinyl and alkylidene Cyclopropanes the effect is pronounced. Platinum or palladium are used frequently. In one case, Nishimura's (1240) catalyst, rhodium-platinum oxide (7:3), worked well where platinum oxide failed (28). An impressive example of the marked influence of catalyst is the hydrogenation of the spirooctane 42, which,
13.7. HYDROGENOLYSIS OF THE CARBON-CARBON BOND 175
depending on catalyst and solvent, gives 41,43, or 44 in excellent yields (186).
H2 (Ph3P)3RhCl
COOCH3
(41) 100%
2O0C COOCH3
(42)
10%Pd-on-C,20°C,H2
T H F r H2O l : !
COOCH3
(43) 97%
COOCH3
(44) 84% plus 16% 41
The above results have a precedent. Homogeneous catalysis usually gives hydrogenation and not hydrogenolysis, and ethylcyclohexane is formed over palladium in similar spiro systems in amounts that depend on the solvent (17Pa). Formation of 43 also might be expected since spiro [2.5] octane itself gives 1,1-dimethylcyclohexane (1596). Alkylated vinylpropanes usually give extensive hydrogenolysis over palladium (72a,98aJ74d), whereas platinum or rhodium tend to give hydrogenation initially (340,113).
13.7.2. Cyclobutanes
Cyclobutanes are cleaved less readily than are cyclopropanes, but, none- theless, fission occurs without difficulty if the ring has additional strain (113,1746,176), adjacent unsaturation (726,1530), or aromatic substituents (26c,82b).
Hydrogenation of 5,10-diazabenzo[b]biphenylene with Raney nickel in hot ethanol gave 2-phenylquinoxaline in 78% yield (9). Similar fission of the four- membered ring occurs with biphenylene itself and with substituted bi- phenylenes (8).
13.7.3 Aromatization
Hydrogenolysis of the carbon-carbon bond occurs readily when one fragment is a good leaving group and, as a result of its loss, the other fragment becomes aromatic (7e). Extensive hydrogenolysis is apt to occur when an allyl or, more especially, a benzyl group is attached to a quaternary carbon in a conjugated cyclohexadienone. Polar and hydrogen-bonding solvents favor hydrogenolysis (7a,26b,115a).