NH3 reduction

The straight chain congeners of GT-2208 and GT-2209 were synthesized from aldehyde 49 which is readily available from urocanic acid. Wittig chemistry provides the cis olefin 50 from 49 (Figure 24). The irons olefin 51 is obtained from Na (lig. NH3) reduction of acetylene 52 (Figure 24). 

Russian workers have recently described an interesting synthesis of 1,3-disubstituted pyrroles which consists of the following 3-step procedure, illustrated for the preparation of 1-benzyl-3 -ethylpyrrole (from which 3-ethylpyrrole is easily available by Na/NH3 reductive removal of the benzyl group) ... 

Impregnation and physically mixed Re catalysts were much less active and much less selective for the phenol synthesis (Table 2.4). The CVD catalyst was almost 18 times more active than the conventional impregnation catalyst. In the physically mixed and impregnated catalysts, the Re7 + precursors partly aggregated as ReOx like Re02 in the presence of the NH3 reductant and such ill-defined Re aggregates decreased both activity and phenol selectivity as shown in Table 2.4. 

Metal-ammonia reduction of ketones. Swiss chemists have reported a detailed study of the mechanism of this reaction, using for the most part (-l-)- 3,3-D2] camphor as substrate. The conclusions drawn have some useful practical applications. The choice of metal (Li, Na, or K) has little effect on the course of reduction to the thermodynamically more stable diastereoisomeric alcohol. The most important conclusion is that pinacol reduction can be suppressed completely in Na-NH3 reductions by use of ammonium chloride as the proton source, a finding first reported by Murphy and Sullivan.2 This salt also partially suppresses pinacol formation in Li-NHj reductions. It also suppresses reduction of enolates, and thus should decrease racemization in reduction of chiral ketones. 

Regioselectivity of the Birch reductive alkylation of polysubstituted biaryls is affected by the electronic nature of substituents on both aromatic rings. The electron-rich 3,5-dimethoxyphenyl moiety is selectively reduced and then alkylated, whereas phenols and aniline are not dearomatized under these conditions. Biaryls possessing a phenol moiety are alkylated on the second ring, provided that the acidic proton has been removed prior to the Li-NH3 reduction.300... 

The rate-determining step in the Na/NH3 reduction of alkynes is the protonation of the radical anion A. The next step, the reaction of the alkenyl radical C to the alkenyl-sodium inter-... 

Methyl iodide, reactions with dialkyl-amino-thiazoles, 32. See also Alkylation 4-Methylthiazole, preparation of, from Na/NH3 reduction of 4-methyl-A-4-thiazoline-2-thione, 397 2-Methylthio-3-methylthiazolium salts, as catalyst for methylthiothiazole rearrangement, 406 Methylvinylketone, reaction of, with... 

Birch reduction of phenanthrene or 9-alkylphenanthrenes results in tetrahydro derivatives, but 9,10-dihydro derivatives are obtained in high yield by Li/NH3 reduction of phenanthrenes substituted by a 9-trimethylsilyl group (equation I). The trimethylsilyl group of 1 can be removed in quantitative yield with Bu4NF in... 

The rate-determining step in the Na/NH3 reduction of alkynes is the protonation of the radical anion A. The next step, the reaction of the alkenyl radical C to the alkenyl-sodium intermediate B, determines the stereochemistry. The formation of B occurs such that the substituents of the C=C double bond are in trims positions. This trans-selectivity can be explained by product-development control in the formation of B or perhaps also by the preferred geometry of radical C provided it is nonlinear at the radical carbon. The alkenylsodium compound B is protonated with retention of configuration, since alkenylsodium compounds are configurationally stable (cf. Section 1.1.1). The Na/NH3 reduction of alkynes therefore represents a synthesis of fnms-alkencs. 

Williamson et al. (56) showed that Cu2 + Y was an active catalyst for the oxidation of NH3 to N2 and H20. A mechanism was proposed involving the intermediate formation of an amine complex [Cu(NH3)4]2+. The NH3 reduction of Cu2+ and Cu+ in this complex was proposed as the slow step with reoxidation via 02 being very rapid. This mechanism was consistent with the kinetic expressions which were shown to be first order in NH3 and zero order in 02. 

Nitrogenases are very versatile enzymes. They reduce, in addition to N2, a lot of other substrates, for example, protons, acetylene, azide, nitriles, and isonitriles. All of these substrates are reduced by multiples of [2 H+/2 e ] reductions. Both CO and NO inhibit nitrogenase activity. The apparent [2 H+/2 e ] multiplicity of substrate reductions and a couple of other findings strongly suggest diazene and hydrazine to be intermediates of the N2 —> NH3 reduction. 

IMH3 has a boiling point of -33 °C, making it a gas at room temperature. To carry out a Na, NH3 reduction, NH3 gas is condensed into a fiask kept at -78 °C by a cooiing bath of soiid CO2 in acetone. When Na is added to the iiquid NH3, a briliiant biue soiution is formed. 

All five Group I metals (Li, Na, K, Rb and Cs) and three Group II metals (Ca, Sr, Ba) have been used in NH3 to effect the reduction of ketones to secondary alcohols. " In addition, it has been reported that Yb-NH3 reduction of a,p-unsaturated ketones affords the saturated alcohol as the major product, which presumably arises via reduction of the intermediate saturated ketone.Reduction of (+)-camphor with Yb-NH3 both in the absence and presence of NH4CI affords the same 86 14 ratio of bomeol (2) to iso-bomeol (3). In the presence of NH4CI, the ketone is completely consumed and dimeric reduction products are not observed. Excess Yb-THF-HMPA effects bimolecular reduction of aromatic ketones, but aliphatic ketones are apparently inert to Yb-THF. - ... 

In the Na/NH3 reduction only the ring-opened 123 was observed, while in the rather slow NaN reduction besides little cyclopropane 49 (6%) mostly the ring-opened 124 (58 %) and 123 (17 %) were formed. It has been pointed out before that the NaN reduction of 5(1-52 results only in the cyclopropane 49. As far as the formation of the cyclopropane 49 and the acid 99 in the Li reduction of 122 is concerned, a similar mechanism has been discussed as outlined in Scheme 15 for the reduction of 50-52 and 118-Hal in homogeneous solution. The marginal retentions of 49 (2.5 % o.p.) and 99 (2.6 % o.p.) are in line with the expected instability of the corresponding fluoride containing radical anion. 

From the very different reduction products from cis- and trans-145, respectively, with sodium in NH3 one can conclude that a possible trimethylene radical anion intermediate is not reversibly formed. Otherwise, both stereoisomers cis- and trans-145 should lead to the same reduction products. An irreversible ring-opening has similarly been observed in the Na/NH3 reduction of (+ )-(R)-49 as shown earlier. Comparable results to those of cis-... 

The function of the alcohol in the metal -NH3 reduction is to provide a proton source that is more acidic than ammonia to ensure efficient quenching of the radical anion and pentadienyl anion species. Furthermore, the presence of alcohol represses the formation of the amide ion NH2 , which is more basic than RO M and is capable of isomerizing the 1,4-cyclohexadiene product to the thermodynamically more stable conjugated 1,3-cyclohexadiene. 

The synthesis recommended by the program differs in some small respects from the one actually achieved by Stork and Isobe. The most important point of difference is that lithium in ammonia is used to reduce the ketone at C-9. It may certainly be questioned as to whether the Li/NH3 reduction would be as stereoselective as the method actually used by Stork, i.e. 

Staley and Rocchio 50) observed in Li/NH3 reductions of cis-50 that the C-l-C-2 bond is cleaved 70 times faster than the C-2-C-3 bond, while in trans-51 the C-2-C-3 bond is cleaved with a 320-fold preference. 

It was recommended that alkali-metal-NH3 reduction of ketones should be conducted in the presence of NH4+ this follows from the finding that reduction of [3,3-2H2]camphor and quenching in the absence of the ion gave epimeric alcohols with one or two atoms of tracer and complex products formed by disproportionation, abstraction of H from the medium, and pinacolic coupling. In the presence of NH4C1, exclusively [3,3-2H2]-alcohols were formed with Li, Na, or K. It was concluded that a mechanism proposed by House (1972) predominated when NH4+ was the proton source.120... 

Electrochemical reduction of Mo(NH3) is fully reversible in both TH F and PhF. Interestingly, at slow scan rates ( 10mVs ), waves corresponding to the MoN couple become discernible (near —1.8 V in THF). Therefore exchange of ammonia in Mo(NH3) vide infra) with dinitrogen takes place to an observable extent after the begirming of the Mo(NH3) reduction sweep (lOmVs" ), a period of 30-40s. Since the potentials of CrCp 2 ° and Mo(NH3) in fluorobenzene are both -1.63 V, CrCp 2 is a viable reductant of Mo(NH3). ... 

The organomercuric compound 13 was transformed into diketone 14 by hydroxylation with oxygen in the presence of NaBH4 followed by Jones oxidation. LAH reduction and acetylation provided diacetate 15 as a mixture of diastereomers, and which was converted to ( )-baiyunol (4) by U/NH3 reduction followed by hydrolysis. Thus the aglycone 4 was obtained as a racemate by rather simple operation in good overall yield. ... 

Figure 4.15 (a) SECM arrangement (not to scale) for feedback measurement at pure (left) and SWNTs-lm-sandwiched (right) water/chloroform interface (b) Experimental approach curves (CHI 900) for a tip in aqueous solution approaching SWNTs-Im-sandwiched (I) and pure (II) water/chloroform interface. Currents are normalized to the steady-state diffusion limiting current, /jco and distance to tip radius. The aqueous solution contained 0.5 mM Ru(NH3) and 100 mM KCI. The tip (Pt, 10 mm radius, RG = 10) was held at -0.35 V vs. Ag AgCl (saturated KCI) for Ru(NH3) reduction and approached at 1 pm/s. The counter electrode was Pt wire [58]. Reproduced by permission of The Royal Society of Chemistiy. 

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