Proton affording

Since double bonds may be considered as masked carbonyl, carboxyl or hydroxymethylene groups, depending on whether oxidative or reductive methods are applied after cleavage of the double bond, the addition products from (E)-2 and carbonyl compounds can be further transformed into a variety of chiral compounds. Thus, performing a second bromine/lithium exchange on compound 4, and subsequent protonation, afforded the olefin 5. Ozonolysis followed by reduction with lithium aluminum hydride gave (S)-l-phenyl-l,2-ethanediol in >98% ee. 

Evans found that molecular hydrogen was efficiently generated by the reaction of a simple diiron complex [CpFe(CO)2]2 (Fp2) with acetic acid (pA a = 22.3) in acetonitrile [202]. Electrochemical simulations revealed that Ep2, [CpEe(CO)2] (Fp ), and [CpFe(CO)2H] (FpH) were key intermediates in this catalytic mechanism (Scheme 61). Reduction of Fp2 produces both an Fp anion and an Fp radical, which is further reduced to give an Fp anion. The oxidation of the Fp anion by proton affords FpH. This protonation was found to be the rate-limiting step. The dimerization of the FpH generates Fp2 and H2. Alternatively, the FpH is reduced to afford the FpH anion, which is subsequently protonated... 

The proposed mechanism of H2 evolution by a model of [FeFeJ-hydrogenases based upon DFT calculations [204-206] and a hybrid quanmm mechanical and molecular mechanical (QM/MM) investigation is summarized in Scheme 63 [207]. Complex I is converted into II by both protonation and reduction. Migration of the proton on the N atom to the Fe center in II produces the hydride complex III, and then protonation affords IV. In the next step, two pathways are conceivable. One is that the molecular hydrogen complex VI is synthesized by proton transfer and subsequent reduction (Path a). The other proposed by De Gioia, Ryde, and coworkers [207] is that the reduction of IV affords VI via the terminal hydride complex V (Path b). Dehydrogenation from VI regenerates I. 

The only other alkenyl carbenoid with a proton trans to the halide that can readily be generated by deprotonation is the parent 1-lithio-l-chloroethene 57 [43] (Scheme 3.13). Insertion into organozirconocenes arising from hydrozirconation of alkenes and alkynes, followed by protonation, affords terminal alkenes and ( )-dienes 59, respectively [38]. The latter provides a useful complement to the synthesis of 54 in Scheme 3.12 since the stereocontrol is >99%. 

Closely related to both allyl carbenoids and the allenyl carbenoids discussed above, propargyl carbenoids 101 are readily generated in situ and insert into zirconacycles to afford species 102 (Scheme 3.27), which are closely related to species 84 derived from allenyl carbenoids [65], Protonation affords a mixture of allene and alkyne products, but the Lewis acid assisted addition of aldehydes is regioselective and affords the homopropargylic alcohol products 103 in high yield. Bicydic zirconacyclopentenes react similarly, but there is little diastereocontrol from the ring junction to the newly formed stereocenters. The r 3-propargyl complexes derived from saturated zirconacycles are inert towards aldehyde addition. 

Attempts by Fish and Johnson to effect a steroid synthesis using a standard epoxide-initiated pentacyclization of a polyene afforded complex mixtures [69]. Alternatively, the allyl alcohol 326 was synthesized and treated with TFA (Scheme 19.60). Protonation affords a symmetrical tetramethylallyl cation that undergoes cyclization to give pentacycle 327 in 31% yield. Simultaneous cleavage of the isopropylidene and vinylidene groups was carried out to furnish the diketone 328 in 88% yield, which was then converted to sophoradiol (329). 

The second result relates to an electronation procedure in which field emission into liquids provides a source of low energy electrons which are capable of effecting selective chemical changes (Noda et al., 1979). Thus dissociative electron capture by alkyl halides, and electron capture followed by protonation, afford alternative routes to specific spin adducts, as exemplified in Scheme 8. 

Replacement of the chloro substituents and hydroboration of the benzoannelated diborafulvene 21 with LiBH4 leads to an anionic product, which after protonation affords the l-carbapentaborane(lO) 22 in 36% yield (Scheme 3.2-12) [34],... 

The conjugate addition of organometallic reagents R M to an electron-deficient alkene under, for instance, copper catalysis conditions results in a stabilized car-banion that, upon protonation, affords the chiral yS-substituted product (Scheme 7.1, path a). Quenching of the anionic intermediate with an electrophile creates a disubstituted product with two new stereocenters (Scheme 1, path b). With a pro-chiral electrophile, such as an aldehyde, three new stereocenters can be formed in a tandem 1,4-addition-aldol process (Scheme 1, path c). 

Z-151, and hydroboration/protonation affords -cycloalkene. In 2002, Trost reported an alternative procedure for the synthesis of the -olefin from an alkyne by hydrosihlation... 

While protonation affords the vinylidene expected by H migration from the original 1-alkyne, use of other electrophiles provides a convenient route to disubsti-tuted vinylidenes. The stereospecificity of this reaction with Re(C=CR)(NO)(PPh3)... 

The reaction of PH3 with Me2N(CH2)2Cl in superbasic media (KOH/ DMSO) was investigated. The tertiary phosphane, upon protonation, affords the extremely water-soluble ligand P[(CH2)2NHMe2]3 Cl3 (solubility 1450 g/1000 ml of water at room temperature) (41). Cationic guanidino-phosphanes (42) and tertiary arylphosphanes containing one or two benzylic amino groups have also been synthesized (43). 

The mechanism of these reactions has been studied by several authors. The first product of the reaction between aldehydes and hydrazine or monosubstituted hydrazines is the hydrazone (378), which can be isolated in a few cases. The dimerization of the hydrazones proceeds at room temperature and is catalyzed by acids. As mentioned above, protonation affords the intermediate (379) which dimerizes to yield (377 R1 = H). 

In the case of the anionic manganese complexes, subsequent protonation affords the neutral alkylcarbene complexes. Intramolecular addition of deprotonated dppe to the a-carbon occurs on reaction of [Fe(=C=CMe2XdppeX C5H5)J+ with base [NaN(SiMe3)2 or KOH in tet-... 

Electrophilic attack on //-vinylidene complexes can occur either on the methylene carbon, or at the metal-metal bond. With the manganese complexes (45, R = H or Me), protonation affords the//-carbyne complexes (46), which in the case of R = Me, exist in the stereoisomeric forms shown (57). Interconversion of the two forms is slow at room temperature ... 

Under oxygen in the absence of water, toluene will transfer an electron to the positive hole, concurrently with electron transfer from the conduction band to oxygen, to give a toluene radical cation. On the other hand, in the presence of water, both toluene and water will transfer an electron to the positive holes. The resulting toluene radical cation may subsequently lose a proton affording a benzyl radical, which will be oxidized with oxygen or the superoxide anion to benzyl alcohol and benzaldehyde, as proposed for the reactions of Fenton s reagent with toluene (57). 

Interestingly, zirconacyclopentane 246 formed by the reaction of 1,6-heptadiene with the Zr complex has the firms ring junction mainly [108]. It should be noted that the preparation of the trans ring junction in the bicyclo[3.3.0]octane system by other means is difficult. Carbonylation of 246 affords trans-fuzed bicyclo[3.3.0]octanone 247 [109,111]. The diacetoxy compound 248 is obtained by oxidative cleavage of 246. Protonation affords the frans-dimethylcyclopentane skeleton. Similar reactions occur with 1,6-enynes, and Pauson Khand-type cyclopentenone synthesis is possible by carbonylation. 

Note that the first-formed product from the cycloaddition is actually the sodium tetrazolate salt 8.32. Protonation affords the neutral tetrazole 8.31. Prolonged acidic hydrolysis accomplishes several transformations hydrolytic removal of both the phthalimide and acetyl nitrogen protecting groups, and... 

A PET reaction between excess phthalimide (in equilibrium with its conjugate base) and an alkene led to a clean phthalimidation of nonactivated double bonds. Here, the singlet excited state of phthalimide acts as the oxidant and a radical ion pair is formed. The olefin cation radical is trapped by the phthalimide anion, and back electron transfer, followed by protonation, affords the photoaddition products [40], Protected phenethylamines are readily accessible in this way. This reaction has been carried out by using NaOH as the base it has been shown that the amounts (usually equimolar with the alkene) must be carefully chosen in order to avoid the undesired competition with [2 + 2] photocycloaddition. 

A variety of cobalt-containing mixed-metal butterfly alkyne clusters are known and will be described in approximate order of decreasing cobalt content. The dehalogenation of [RuCl2(Ph2PC=CBut)(p-cymene)] by [Co2(CO)8] followed by protonation affords fRuCo3(/x4-t -Bu QH)(/i-... 

Anionic acyl-iron complex (8) may be intercepted by ethylene, which inserts into the acyl-iron bond to produce a S-metalloketone that rearranges to an a-metalloketone (Scheme 4). Protonation affords the ethyl ketone. Intramolecular (see Intramolecular) versions of this process are known as well, and provide an approach to cychc ketones. That the cyclization is not always regioselective (see Regioselectivity) is shown in an example used in the total synthesis of ( )-aphidicolin (Scheme 5). 4i... 

Similarly, cis-halopentacarbonylmetallate anions are formed upon treatment of (CO)sMX (M = Mn, X = Br M = Re, X = Cl, Br, I) at - 78°C. The thermally stable anions can be protonated, affording the cis-halohydroxycarbene complexes (for M = Mn see also ref. 52). Further reaction ofcis-bromo[hydroxy(methyl)carbene](tetra-... 

The hydration of alkenes (Scheme 2.10a) is a useful way of generating alcohols. In the acid-catalysed hydration of an alkene, the initial electrophilic attack by a proton affords the more stable carbenium ion (car-bocation), which is then neutralized by the attack of a water molecule. In the case of an unsymmetrical alkene this hydration follows the Markownikoff rule This rule states that the electrophilic part of the addendum becomes attached to the carbon atom that bears the greater number of hydrogen atoms. This generates the more stable, more highly substituted carbenium ion. 

Reaction of the carbocation with H2O as a nucleophiie affords the substitution product (Reaction [1]). Aiternativeiy, H2O acts as a base to remove a proton, affording the elimination product (Reaction [2]). Two products are formed. 

Treatment of 2,4-diphenylquinazoline with methyllithium provides the addition product 13 which on subsequent protonation affords the 3,4-dihydro derivative 14. However, alkylation of 13 with iodomethane produces two dimethyl-substituted 2,4-diphenyldihydroquinazolines 15 and 16. ... 

In the primary reductive step, oxygen from air or Fe(III) ions function as electron acceptor. The radical cation l+ , generated in the oxidative counterpart, loses a proton, affording the strongly reducing a-amino radical (Figure 10), which injects an electron into the conduction band of CdS. Hydrolysis of the resulting iminium salt leads finally to the secondary amine. Repetition of this multi-step reaction affords the completely demethylated product 2. 

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