Nucleophilic complexes

Benzimidazole 3-oxides, e.g. (189), react with phosphorus oxychloride or sulfuryl chloride to form the corresponding 2-chlorobenzimidazoles. The reaction sequence involves first formation of a nucleophilic complex (190), then attack of chloride ions on the complex, followed by rearomatization involving loss of the fV-oxide oxygen (191 -> 192). 

Better yields are attributed to intimate association of the basic nitrile group at the surface of the mtrosomum salt causing nitrosative decomposition of the azide to occur in close proximity to the weakly nucleophilic complex fluoride anion Fluorination yields can be further enhanced to 59-81% by lengthening the azido nitrile chain, but the reaction is accompanied by pronounced secondary fluoronitnle formation arising from rearrangement [100, 101] (Table 8)... 

A tentative mechanism includes ruthenium-induced isomerization of the initial allylic alcohol via (hydrido) (7T-allyl)ruthenium complex 167 to the corresponding Ru-bound enol 168. This in. ( ////-generated nucleophile complex can then add to aldehydes or imines under formation of the desired products. 

N-diazeniumdiolates are an interesting class of compounds, presently under development, which can deliver NO specifically to a target site. Diazeniumdiolates are N O nucleophile complexes capable of releasing NO in an aqueous environment or in... 

Several experiments were carried out to test equation 32 and the four constants could be evaluated the determined values are shown in equation 33172. It can be observed that catalysis by a HBA-nucleophile complex is more important than for the nucleophile itself, as expected on the basis of the dimer nucleophile . 

Some Schrock-type carbene complexes, i.e. high-valent, electron-deficient, nucleophilic complexes of early transition metals, can undergo C-H insertion reactions with simple alkanes or arenes. This reaction corresponds to the reversal of the formation of these carbene complexes by elimination of an alkane (Figure 3.36). 

In the absence of nucleophile, neither the 412 nm species nor the formation of the radical anion, nor that of the photosubstitution product is found. It is concluded therefore that the 412 nm species results from some kind of interaction between the (excited) aromatic compound and the nucleophilic reagent. The character of this aromatic compound-nucleophile-complex is as yet unknown. However, in our present view, the nature of the complex has to allow for the formation of both the radical anion and the photosubstitution product(s). An attractive possibility for this complex remains the a-complex, in formal analogy with the Meisenheimer complexes in the thermal nucleophilic reactions with aromatic compounds. An exciplex forms another possibility. 

Owing to the small difference between the pulse duration and the triplet lifetime in solutions containing the nucleophile, direct evidence could not be obtained for the formation of the aromatic compound-nucleophile-complex from the triplet state. However, we have no indications against this route and for the moment we wish to adopt it since it gives a simple picture consistent with all experimental data. 

Shibasaki and coworkers have applied heterobimetalhc gallium complexes to the desymmetrization of meso epoxides using phenolic nucleophiles. Complex 35 is... 

Reagents of choice for reduction of epoxides to alcohols are hydrides and complex hydrides. A general rule of regioselectivity is that the nucleophilic complex hydrides such as lithium aluminum hydride approach the oxide from the less hindered side [511, 653], thus giving more substituted alcohols. In contrast, hydrides of electrophilic nature such as alanes (prepared in situ from lithium aluminum hydride and aluminum halides) [653, 654, 655] or boranes, especially in the presence of boron trifluoride, open the ring in the opposite direction and give predominantly less substituted alcohols [656, 657,658]. As far as stereoselectivity is concerned, lithium aluminum hydride yields trans products [511] whereas electrophilic hydrides predominantly cis products... 

The next extension of preparatively useful ester enolate chemistry was the deconjugative a-alkylation of a./J-unsaturatcd esters20,21. A Michael addition of LDA was avoided by the use of one equivalent of HMPA21, which forms a non-nucleophilic complex with the former. The yields of the mono- and disubstituted products are all in the region of 90% 21,22. 

Pyridine and quinoline /V-oxides react with phosphorus oxychloride or sulfuryl chloride to form mixtures of the corresponding a- and y-chloropyridines. The reaction sequence involves first formation of a nucleophilic complex (e.g. 270), then attack of chloride ions on this, followed by rearomatization (see also Section 3.2.3.12.5) involving the loss of the /V-oxide oxygen. Treatment of pyridazine 1-oxides with phosphorus oxychloride also results in an a-chlorination with respect to the /V-oxide groups with simultaneous deoxygenation. If the a-position is blocked substitution occurs at the y-position. Thionyl chloride chlorinates the nucleus of certain pyridine carboxylic acids, e.g. picolinic acid — (271), probably by a similar mechanism. 

Within the past 20 years, ferrates, i.e. anions possessing iron as the center atom, have found increasing application as nucleophilic complexes in substitution chemistry. In these reactions, the ferrate replaces the leaving group X in a first nucleophilic substitution event. A transfer of one ligand from the metal atom (i.e. a reductive elimination, path A, Scheme 7.2) or substitution of the metal atom via external attack of the nucleophile (path B) concludes this mechanistic scenario. However, the exact mechanism in ferrate-catalyzed nucleophilic substitutions is still under debate. Apart from the ionic mechanism, radical processes are also discussed in the literature. 

Fischer-type complexes such as 1 were first prepared in 1964 and their chemical properties studied [1], Schrock-type nucleophilic complexes such as 2 were prepared later [2], They are formed by coordination of strong donor ligands such as alkyl or cyclopentadienyl with no 7i-acccptcr ligand to metals of high oxidation states. The nucleophilic carbene complexes show Wittig s ylide-type reactivity and the structures may be considered as ylides (eq. 8.1)... 

The most recently discovered template effect is the one that makes use of anions. Vogtle and coworkers [12] have found that a phenolate equipped with one stopper can bind in the cavity of the tetralactam macrocycle by two strong hydrogen bonds. Then this nucleophile complex is reacted with an electrophilic semi-axle to obtain rotaxane in very high yields up to 95%. 

As mentioned above, the electrophilic metal carbene complexes are stabilised by the presence of heteroatoms or phenyl rings at the divalent carbon atom, while hydrogen or alkyl groups stabilise the nucleophilic complexes. Therefore, there is a distinction between carbenoids and alkylidenes when designing carbene ligands corresponding to the former or the latter class. 

Figure 6.4 Schematic presentation of metal-carbon bonding in (a) a transition metal carbene electrophilic complex and (b) a transition metal alkylidene nucleophilic complex...

Arguments for the intermediacy of penta- or hexacoordinate silicon intermediates have been discussed in detail in earlier reviews (10-13,247), but little attention has so far been devoted to Eq. (40). There is, however, a body of evidence that nucleophilic displacement at silicon may follow pathway (40). Evidence for this mechanism includes the following (1) the common existence of compounds postulated to be intermediates or modeling the intermediates i.e., positively charged ionic silane-nucleophile complexes containing tetracoordinate silicon (2) the dynamic behavior of these compounds when mixed with their components, and the behavior... 

Important evidence of the ionic structure of many silane-uncharged nucleophile complexes in solution is their high electrical conductance, characteristic of strong electrolytes (239,252,254-256,263). The 1 1 stoichiometry of certain complexes in solution was proved by conductometric titration (Fig. 1). The stoichiometry in solids was, in many cases, confirmed by elemental analysis (239,260). 

These mechanisms have been the subject of several reviews (10-13), and there is no room to discuss them in detail. A short comparison with the silylonium intermediate pathway should, however, be included. Both of these conflicting mechanisms assume the formation of a substrate-nucleophile complex in a fast preequilibrium step, and the complex reacts with a nucleophilic reagent to give the product in the rate-limitng step. It is, therefore, appropriate to compare how easily the corresponding complexes are formed and how readily they react toward the product [Eq. (71)]. The... 

Excess nucleophile is often needed in polymerization of more nucleophilic monomers. For example, esters, ethers, and amines afe used in large excess over aluminum halides and alkylaluminum halides to control polymerization of vinyl ethers [269]. The original Lewis acid is no longer available and covalent species are activated by the Lewis acid/nucleophile complex. Carbenium ions are additionally deactivated by excess nucleophile. 

The second strategy involves the attack of chiral nucleophile to the imine. The mechanism of this reaction involves the formation of chiral nucleophilic complex from nucleophile and chiral ligands 3.39. 

One means of stereoselective cleavage of biaryl lactones [53] is activation of the carbonyl group with a Lewis acid and subsequent attack with a chiral nucleophile. Conversely, activation can be effected with a chiral Lewis acid followed by attack of an achiral nucleophile. Complexation of a biaryl lactone to the chiral fragment [CpRe (NO)(PPh3)j then reduction with K(s-Bu)3BH (K-selectride) and ring opening of the intermediate rhenium lactolate gives the metalated aldehyde (dr = 75 25) which is converted to the alcohol without essential loss of optical purity (Sch. 6) [54]. 

The alkylation of amides by alkyl halides or simple sulfonic acid esters is usually of little importance because the alkylation equilibrium is placed on the side of the starting compounds. This is not the case, however, in either the alkylation of vinylogous amides (which has been achieved even with alkyl iodides ) or if intramolecular alkylation is possible, e.g. in -(2-haloethyl)amides. In the latter case cyclic iminium compounds (81 equation 51) are readily available by replacing the more nucleophilic halide by less nucleophilic complex anions, which can be achieved by addition of Lewis acids or AgBF4. °-2 ... 

Nucleophilic complex bases with good nucleophiles as activating agents. In this group, nucleophilic properties are masked but they do not completely disappear. 

IV. 2.1.3.1. Introduction. As we said before, ketone enolates generally are good activating agents for NaNH210, n and lead to nucleophilic complex bases. In other words, it is possible to use the base part to generate a dehydrobenzene and the nucleophile part to condense on the reactive intermediate. 

IV. 2.1.3.4. 1-Bro mo naphthalene. Nucleophilic complex base NaNH2-enolate very easily generate 1-naphthyne 124 from l-bromonaphthalene8S ... 

Vinylic Halogeno Cyclohexenes. Encouraging results described above led us to think that, as in the aromatic series, it would be possible to condense ketone enolates on cyclohexyne generated by reaction of nucleophilic complex bases NaNH2-ketone enolates on 1-halogeno cyclohexene. 

We can conclude that nucleophilic complex base NaNH2-enolates are able to generate cyclohexynes. Ketone enolates condense on these to give chiefly alcohols 153 and/or ketones 151 and 152. 

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