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Amides deprotonation

Kinetic resolution can also be accomplished via eliminative pathways. Thus, the enantiomerically enriched allylic alcohol 102 can be prepared from the meso epoxide 96 with up to 96% ee by the action of LDA in the presence of the chiral diamine 101 and 1,8-diazabicyclo-[5.4.0]undec-7-ene (DBU). The DBU is believed to function as an aggregation modifier, and the active catalyst is theorized to be a heterodimer of the lithium amide (deprotonated 101) and DBU, although some nonlinear effects have been observed at low DBU concentrations <00JA6610>. Dipyrrolidino derivatives (e.g., 104) have also demonstrated utility with regard to kinetic resolution <00H1029>. [Pg.63]

Amide deprotonation is normally observed in copper(II), nickel(II) or cobalt(II) complexes of amides, but it has now been demonstrated that [Zn(LH)2]2 (H3L = H2NC0CH2N(CH2C02H)2) may be further deprotonated, and that the equilibria in Scheme 1 are established. The ability of the d10 zinc(II) ion to promote such equilibria suggests that a major requirement for deprotonation of an amide is a positively charged centre, since the d10 ion cannot show ligand field effects.390... [Pg.946]

The above studies indicate that metal ions catalyze the hydrolysis of amides and peptides at pH values where the carbonyl-bonded species (25) is present. At higher pH values where deprotonated complexes (26) can be formed the hydrolysis is inhibited. These conclusions have been amply confirmed in subsequent studies involving inert cobalt(III) complexes (Section 61.4.2.2.2). Zinc(II)-promoted amide ionization is uncommon, and the first example of such a reaction was only reported in 1981.103 Zinc(II) does not inhibit the hydrolysis of glycylglycine at high pH, and amide deprotonation does not appear to occur at quite high pH values. Presumably this is one important reason for the widespread occurrence of zinc(Il) in metallopeptidases. Other metal ions such as copper(II) would induce amide deprotonation at relatively low pH values leading to catalytically inactive complexes. [Pg.426]

A variety of N-O-chelated glycine amide and peptide complexes of the type [CoN4(GlyNR R2)]3+ have been prepared and their rates of base hydrolysis studied.169 The kinetics are consistent with Scheme 8. Attack of solvent hydroxide occurs at the carbonyl carbon of the chelated amide or peptide. Amide deprotonation gives an unreactive complex. Rate constants kOH are summarized in Table 16. Direct activation of the carbonyl group by cobalt(III) leads to rate accelerations of ca. 104-106-fold. More recent investigations160-161 have dealt with... [Pg.431]

Lithium amide deprotonation of epoxides is a convenient method for the preparation of allylic alcohols. Since the first deprotonation of an epoxide by a lithium amide performed by Cope and coworkers in 19585, this area has received much attention. The first asymmetric deprotonation was demonstrated by Whitesell and Felman in 19806. They enantioselectively rearranged me.vo-cpoxidcs to allylic alcohols for example, cyclohexene oxide 1 was reacted with chiral bases, e.g. (S,S) 3, in refluxing TFIF to yield optically active (/ )-2-cyclohexenol ((/ )-2) in 36% ee (Scheme 1). [Pg.412]

Figure 3.54 Amide deprotonation during the synthesis of amino functionalised silver ( ) carbene complexes. Figure 3.54 Amide deprotonation during the synthesis of amino functionalised silver ( ) carbene complexes.
The work described above suggests that carbonyl-bonded amides and peptides when coordinated to Co Rh and Ir will undergo base hydrolysis ca. 10 times faster than the free ligand, the rate acceleration arising primarily from a more positive value of AS. At high pH, amide deprotonation occurs (Scheme 8) leading to catalytically inactive complexes. Much higher rate accelerations can be obtained if intramolecular attack by coordinated water or hydroxide ion can take place (ca. 10 -10"-fold). [Pg.433]

The amide deprotonation step involves a prior coordination complex between the amide (or amidine) and the butyllithium base. - As shown in Scheme 3, formation of a coordination complex between an amide or amidine substrate and butyllithium is observed by IR shortly after mixing, but tefore the deprotonation. A recent mechanistic study has shown that this coordination complex is between the amide and the 5-butyllithium aggregate, and that the effect of the coordination is to increase the reactivity of the complex. " In fact, the aggregate may be activated by adding TMEDA prior to the amide, with a similar increase in reactivity. However, because these studies were conducted in cyclohexane, extrapolation of these observations to ethereal solvents is somewhat problematic. ... [Pg.67]

Unlike system 4, sensor 5 is totally insensitive to Fe" and Co". The higher selectivity is related to the feet that amide deprotonation is promoted only by metals forming very strong coordinative interactions, a privilege reserved to metals late in the 3d series. In this way, system 5 does not recognize or sense size or shape, but merely the position in the Periodic Table. [Pg.106]

The reduction of ergoline alkaloid 5 can be performed in the presence of aniline as the proton source. Addition of lithium amide deprotonates the urea side chain. Under these reaction conditions, cleavage of the urea side chain is suppressed and the reduction product 6 is formed in a trims cis ratio of >95 59. [Pg.994]

A number of bases may be used for deprotonation, but the most important ones are lithium amide bases such as those illustrated in Figure 3.3. Although other alkali metals may be used with these amides, lithium is the most common. Amide bases efficiently deprotonate virtually all ctirbonyl compounds, and do so regioselectively with cyclic ketones such as 2-methylcyclohexanone i.e., C2 vs. C6 deprotonation) and stereoselectively with acyclic carbonyls (i.e., E(O)- vs. Z(O)- enolates. If the carbonyl is added to a solution of the lithium amide, deprotonations are irreversible and kinetically controlled [36-38]. Under such conditions, the con-... [Pg.78]

A similar mechanism was proposed by Lindow et al. (II) for the biphenyl system. However, biphenyl provides much less dialkylation (Table I) than either naphthalene or anthracene under normal quenching conditions. Perhaps the amide deprotonation is slower in this case because of the reduced acidity of the doubly allylic system (i.e., 1,4-dihydrobiphenyl) in comparison with the allylic-benzylic or doubly benzylic systems (i.e., dihydronaphthalene or dihydroanthracene, respectively). [Pg.83]

Activated complexes involved in lithium amide deprotonation of 2 have been determined to contain one molecule of 2. Gibbs free energy difference between the two most stable TSs at 298 K. [Pg.12]

In a similar way, enolates can be prepared from esters or carboxyUc amides. Deprotonation of simple esters (e.g. methyl or teit-butyl propionate with LDA) generates the rran -enolate with good selectivity however, this is not translated into a good selectivity for the anti aldol product and approximately an equal mixture of the two diastereomeric products is normally formed. One solution to this is to use a bulky aromatic ester group, which generates predominantly the anti aldol product (1.67). [Pg.34]

Cr(CO)3-complexed benzylic ethers and sulfides are acidic enough to imder-go hthium amide deprotonation, a to the heteroatom, followed by electrophile trapping the more elaborate base 41 [prepared via the addition of PhMgCl to the enantiomerically pure bis-imine derived from gjyoxal and (i )-a-methylben-zylamine] usually giving the best ees (up to 97%), e.g.. Scheme 23 [87]. [Pg.17]

Construction of the Dolby model system began with epoxidation of cyclohexene 209 employing mCPBA in chloroform. The epoxide product was then subjected to a base-promoted cyclization via amide deprotonation and oxirane opening to provide benzamide 210 (Scheme 23). Hydroxyl oxidation using the Jones conditions next furnished cyclohexanone 211. A Fischer indole synthesis on this scaffold, followed by amine deprotection using sodium hydroxide, led to the desired l,3-(iminoethano)carbazole 83. While the yield of this route was low, it was the first example of a method to fashion the 2-azabicyclo[3.3.1]nonan-8-one system 211, and importantly demonstrated that indoHzation on such a scaffold was possible. [Pg.210]

Very strong bases (such as sodium amide) deprotonate terminal acetylenes to form carbanions called acetylide ions (or alkynide ions). Hydroxide ion and alkoxide ions are not strong enough bases to deprotonate alkynes. Internal alkynes do not have acetylenic protons, so they do not react. [Pg.388]

Allyl chloride anions, generated in situ via lithium diisopropyl-amide deprotonation of the corresponding allyl chioride, have been... [Pg.81]

When halobenzene derivatives are heated with powerful bases such as sodium amide, deprotonation is followed by elimination to give a benzyne, which reacts rapidly with the nucleophile. [Pg.1030]


See other pages where Amides deprotonation is mentioned: [Pg.70]    [Pg.749]    [Pg.101]    [Pg.85]    [Pg.945]    [Pg.433]    [Pg.313]    [Pg.3608]    [Pg.5192]    [Pg.300]    [Pg.105]    [Pg.303]    [Pg.3607]    [Pg.5191]    [Pg.5818]    [Pg.181]    [Pg.139]   
See also in sourсe #XX -- [ Pg.3 , Pg.65 , Pg.66 ]

See also in sourсe #XX -- [ Pg.65 , Pg.66 ]

See also in sourсe #XX -- [ Pg.3 , Pg.65 , Pg.66 ]




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