Electrophilic reducing agents

Sodium triacetoxyborohydride Na[BH(OAc)3] ° and hydrogenation (H2, Pd/C) are used as alternatives to sodium cyanoborohydride for the reductive amination of carbonyl compounds. Also, Zn[BH4]2 is a particularly effective agent for the reductive amination of a,p-unsaturated aldehydes and ketones.  

In situ reduction of tosylhydrazones by NaBHjCN provides an efficient method for the deoxygenation of carbonyl compounds to furnish the corresponding hydrocarbons (see also Section 3.4). In the case of tosylhydrazones derived from a,P-unsaturated carbonyl compounds, the reduction leads to a stereoselective migration of the double bond to give the corresponding tran -alkene. 

Diisobutylaluminum hydride (DIBAL-H) can be obtained commercially neat or in hydrocarbon solvents. Reductions with DIBAL-H must be carried out in the absence of air and moisture. DIBAL-H is a very versatile reagent for the selective reduction of 

In contrast to LAH reductions of esters, nitriles and lactones, where two hydrides are utilized, reductions of these compounds by DIBAL-H at low temperature can be stopped after the transfer of one hydride to the carbonyl carbon. Hydrolytic workup of the tetrahedral intermediates furnishes the corresponding aldehydes. 

The reduction of esters to aldehydes generally works best when alkoxy or amino functionality is in close proximity to the ester group, as in a- or -alkoxy esters. A neighboring alkoxy group will stabilize the tetrahedral intermediate through chelation and prevent overreduction. DIBAL-H-mediated mono-reduction of lactones delivers the corresponding lactols (hemiacetals).  

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Iodine azide, on the other hand, forms pure adducts with A -, A - and A -steroids by a mechanism analogous to that proposed for iodine isocyanate additions. Reduction of such adducts can lead to aziridines. However, most reducing agents effect elimination of the elements of iodine azide from the /mwj -diaxial adducts of the A - and A -olefins rather than reduction of the azide function to the iodo amine. Thus, this sequence appears to be of little value for the synthesis of A-, B- or C-ring aziridines. It is worthy to note that based on experience with nonsteroidal systems the application of electrophilic reducing agents such as diborane or lithium aluminum hydride-aluminum chloride may yet prove effective for the desired reduction. Lithium aluminum hydride accomplishes aziridine formation from the A -adducts, Le., 16 -azido-17a-iodoandrostanes (97) in a one-step reaction. The scope of this addition has been considerably enhanced by the recent... 

The most characteristic reaction of amine boranes is their conversion into aminoboranes and, subsequently, to borazine at higher temperatures.9,17 For complexes of low stability, the transfer of H from the boron to the donor is a characteristic process,11 40 as in the utilization of diborane as an electrophilic reducing agent. The neutral complexes of boranes are fairly stable towards hydrolysis. The key step of the hydrolysis was formerly assumed to involve displacement of BH3 by a proton, whereas in recent studies ionic intermediates, containing five-coordinated boron (R3N—BH4) are also taken into consideration.41,42 The hydrolytic... 

A detailed study of a series of a,a-halo ketones demonstrated that electrophilic reducing agents, notably borane, DIBAL-H and BH3 DMS, provided excellent yields of the alcohols without reduction of the halogen bond. In contrast, nucleophilic reagents (NaBH4, Li(Bu 0)3AlH and K-selectride) were unsatisfactory. The results for a representative a,a-dibromo ketone (87) being reduced to the mono- (89) and di-bromohydrin (88 equation 22) are shown in Table 4. 

The a-oxoketone dithioacetal reduction is reported [9] to follow different pathways, depending on the nature of reducing agents and reaction condition (Chart 25.8). The electrophilic reducing agent, 9-BBN effects the conjugate reduction to afford the p-methylthioketone [10]. 

Reduction of arenes by catalytic hydrogenation was described m Section 114 A dif ferent method using Group I metals as reducing agents which gives 1 4 cyclohexadiene derivatives will be presented m Section 1111 Electrophilic aromatic substitution is the most important reaction type exhibited by benzene and its derivatives and constitutes the entire subject matter of Chapter 12... 

Frontier Orbitals and Chemical Reactivity. Chemical reactions typically involve movement of electrons from an electron donor (base, nucleophile, reducing agent) to an electron acceptor (acid, electrophile, oxidizing agent). This electron movement between molecules can also be thought of as electron movement between molecular orbitals, and the properties of these electron donor and electron acceptor orbitals provide considerable insight into chemical reactivity. 

Hydrazinoseienazoles, electrophilic substitutions. in 5-position, 240 reaction with /S-ketoesters, 235 reducing agents, 234 table of pioducts, 234... 

The reductive couphng of imines can follow different pathways, depending on the nature of the one-electron reducing agent (cathode, metal, low-valent metal salt), the presence of a protic or electrophihc reagent, and the experimental conditions (Scheme 2). Starting from the imine 7, the one-electron reduction is facihtated by the preliminary formation of the iminiiim ion 8 by protonation or reaction with an electrophile, e.g., trimethylsilyl (TMS) chloride. Alternatively, the radical anion 9 is first formed by direct reduction of the imine 7, followed by protonation or reaction with the electrophile, so giving the same intermediate a-amino radical 10. The 1,2-diamine 11 can be formed from the radical 10 by dimerization (and subsequent removal of the electrophile) or addition to the iminium ion 8, followed by one-electron reduction of the so formed aminyl radical. In certain cases/conditions the radical 9 can be further reduced to the carbanion 12, which then attacks the... 

The successful deoxygenation of the sulfoxide 18a by either hexachlorodisilane as the reducing agent, or diiron nonacarbonyl according to the deoxygenation-complexation route can also be rationalized in terms of electrophilic attack of the reagents used on the nucleophilic sulfoxy oxygen. 

The electrophilicity of alane is the basis for its selective reaction with the amide group. Alane is also useful for reducing azetidinones to azetidines. Most nucleophilic hydride reducing agents lead to ring-opened products. DiBAlH, A1H2C1, and A1HC12 can also reduce azetinones to azetidines.100... 

Electrophilic catalysis may play an important role in the case of the similar benzylic carbon, too. For an O-benzyl system, it was found in a 1997 experiment that palladium oxide is a much more effective catalyst than palladium metal when the catalyst has been prereduced with chemical reducing agents. This finding shows very clearly that the electrophilic character of the unreduced metal ions plays an important role in the hydrogenolysis of the benzyl C—O bonds. Additional support for this mechanism is the fact that a small amount of butylamine can inhibit the hydrogenolysis of the benzyl C—O bond. 

Recently, the decomposition of N-sulfonyloxy-AAF under aqueous conditions has been further examined and appears to be consistent with this overall mechanism (50). That is, the major products appear to be 1- and 3-sulfonyloxy-AAF with small amounts of AAF, 4-hydroxy-AAF, and a dimer formed by addition of the electrophile onto the aromatic ring of another AAF molecule (51). Furthermore, the relative yields of AAF could be increased by addition of the reducing agent, ascorbic acid (52). 

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