Type-reduction

Low-valent nitrogen and phosphorus compounds are used to remove hetero atoms from organic compounds. Important examples are the Wolff-Kishner type reduction of ketones to hydrocarbons (R.L. Augustine, 1968 D. Todd, 1948 R.O. Hutchins, 1973B) and Barton s olefin synthesis (p. 35) both using hydrazine derivatives. 

Addition Reactions. The addition of nucleophiles to quinones is often an acid-catalyzed, Michael-type reductive process (7,43,44). The addition of benzenethiol to 1,4-benzoquinone (2) was studied by A. Michael for a better understanding of valence in organic chemistry (45). The presence of the reduced product thiophenyUiydroquinone (52), the cross-oxidation product 2-thiophenyl-1,4-benzoquinone [18232-03-6] (53), and multiple-addition products such as 2,5-(bis(thiophenyl)-l,4-benzoquinone [17058-53-6] (54) and 2,6-bis(thiophenyl)-l,4-benzoquinone [121194-11-4] (55), is typical ofmany such transformations. 

It is important to exclude air in all hydrazone-type reductions involving olefins (otherwise, over-reduction occurs due to diimide formation) in the above example, as an added precaution cyclohexene was used as a solvent. 

Because of the many examples of such activation of metal powders by TCS 14 only a limited and arbitrary number will be discussed here. The Clemmensen-type reduction of ketones such as cyclohexanone with Zn powder in the presence of TCS 14 affords, via 2082, 2084, and 2085, cyclohexene and, via 2082, O-silylated pinacol 2083 [19, 20]. Ketones such as 5a-cholestan-3-one 2086 are reduced by Zn dust-TCS 14 in TFIF, in ca 65-70% yield, to give 5a-cholest-2-ene 2087 and ca 5% 5a-cholest-3-ene [21] (Scheme 13.8). 

The scope and limitations for transfer hydrogenation employing either the iron porphyrin system or the combination of iron compound/terpy/PPhs are listed in Table 8. In most cases, the FeCVterpy/PPhs system displays a higher activity. Except for chloromethyl- and cyclopropyl-acetophenone, the desired products were obtained in good to excellent yields. It should be noted that a ring opened product was not observed when cyclopropyl acetophenone was employed. Hence, a radical-type reduction pathway was excluded and a hydride mechanism appeared to be reasonable. 

Zr compounds are also useful as Lewis acids for oxidation and reduction reactions. Cp2ZrH2 or Cp2Zr(0 Pr)2 catalyze the Meerwein-Ponndorf-Verley-type reduction and Oppenauer-type oxidation simultaneously in the presence of an allylic alcohol and benzaldehyde (Scheme 40).170 Zr(C)1 Bu)4 in the presence of excess l-(4-dimethylaminophenyl) ethanol is also an effective catalyst for the Meerwein-Ponndorf-Verley-type reduction.1 1 Similarly, Zr(0R)4 catalyze Oppenauer-type oxidation from benzylic alcohols to aldehydes or ketones in the presence of hydroperoxide.172,173... 

Chart 4.2 Wurtz-type reduction preparation of optically active polysilanes. 

Modified MPV-type reductions carried out with chiral magnesium alkoxides and with chiral Grignard reagents have been discussed in detail (1). These reagents differ from the aluminum alkoxides since the Grignard reaction is essentially irreversible. Chiral alkali metal alkoxides have also been used to effect asymmetric reductions (1). 

Because of the highly negative reduction potentials ( —3.0 V vs. SCE) [32], the electroreduction of esters of aliphatic carboxylic acids to primary alcohols by direct electron transfer from the cathode is very difficult and the electrochemical Birch-type reduction of aliphatic esters in MeNH2 or liquid NH3 has not been reported until recently (Scheme 15) [33, 34]. This reaction is not a reduction by direct electron transfer from the cathode to the C=0 bonds of the ester but the reduction by a solvated electron. 

Scheme 15 Cathodic Birch-type reduction of aliphatic esters to alcohols R alkyl, yields 95%.

Figure 1.26. Asymmetric Meerwein-Ponndorf-Verley-type reduction of ketones catalyzed by a Sm complex.

Regioselective synthesis of 2-substituted pyrroles using oximinocyanoacetate esters or related compounds in a Knorr-type reductive condensation with diketones was described . Thus, oximinocyanoacetates 63 reacted with pentane-2,4-diones 64 in hot... 

A 51 was disclosed by a British team (Scheme 9)3 The highly functionalized diene 49 was prepared from iodo compound 48 via Vasella-type reductive ring opening followed by Julia-Kocienski methylenation of the resulting aldehyde. The ring closure to cyclopentene 50 in the presence of catalyst D proceeded smoothly, despite the complex functionalization. 

Exercise 11-8 Consider that it is necessary to synthesize pure samples of d.l-hexane-3,4-D2 and meso-hexane-3,4-D2. Show how this might be done both with diimide and catalytic-type reductions, assuming that any necessary deuterium-iabeled reagents and six-carbon organic compounds are available. 

Meerwein-Ponndorf-Verley-Type Reduction Reduction of ketones by 2-propanol or related alcohols, known as Meerwein-Ponndorf-Verley (MPV) reduction, is promoted by various metal alkoxides, typically aluminum 2-propoxide [2a,d,281]. The C2 hydrogen of 2-propanol is transferred directly to the carbonyl carbon through a six-membered pericyclic transition state [284], Earlier, a stoichiometric quantity of a metal alkoxide was required for this purpose, but recently, lanthanide [285] and aluminum [286] complexes acting as excellent catalysts have been reported. 

Cathodic reduction of arylsilanes in methylamine using LiCl as a supporting electrolyte in an undivided cell gives 1,4-cyclohexadiene derivatives. The reaction seems to proceed in a manner similar to the Birch-type reduction. The cathodic reduction in a divided cell provides desilylation products (equation 53)68. 

Finally, it is noteworthy that Lewis base adducts of gallane (I.GalL) reduce cyclic ketones, enones and w-haloketones to the corresponding alcohols in excellent yields254. These reagents show some promise as a new extension of the boron-type reductions of carbonyl compounds. 

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