Of nucleophilic species

In view of the successful preparation of so many homochiral sulfoxides via the reaction of nucleophilic species with sulfinate ester 19, it appears likely that the reaction is capable of extension to provide still more examples of potentially useful sulfoxides. 

The synthetic potential of the CMGL synthons is not limited to their ability to be easily opened in the presence of nucleophilic species. Indeed, subsequent functionalisation at position 2 provides 1,2-bisfunctionalised carbohydrates in an easy, general and competitive manner (Scheme 18).3e This offers an alternative to other methods, notably glycosylation reactions81 using intermediates such as 1,2-isopropylidene acetals,82 1,2-orthoesters,83... 

Scheme 11.51). Some of these reactions are even initiated by the addition of nucleophilic species, being formed by fragmentation of open-chain compounds, which are originally derived from C-5 adducts (see Section II,C,l,b). Thus, the number of molecules that react by addition of the amide ion to C-5 is certainly higher than can be derived from the percentage of the molecules given in Table II.IO. 

The activation of lactones by Bronsted acids was discussed in the section 2.3.3 dealing with cationic polymerization. An alternative relies on the use of nucleophilic species for the activation of lactones (Fig. 21). 

Dehydrobromination of bromotrifluoropropene affords the more expensive trifluoropropyne [237], which was metallated in situ and trapped with an aldehyde in the TIT group s [238]synthesis of 2,6-dideoxy-6,6,6-trifluorosugars (Eq. 77). Allylic alcohols derived from adducts of this type have been transformed into trifluoromethyl lactones via [3,3] -Claisen rearrangements and subsequent iodolactonisation [239]. Relatively weak bases such as hydroxide anion can be used to perform the dehydrobromination and when the alkyne is generated in the presence of nucleophilic species, addition usually follows. Trifluoromethyl enol ethers were prepared (stereoselectively) in this way (Eq. 78) the key intermediate is presumably a transient vinyl carbanion which protonates before defluorination can occur [240]. Palladium(II)-catalysed alkenylation or aryla-tion then proceeds [241]. 

Recent progress on the use of hypervalent iodine reagents for the construction of carbon-het-eroatom (N, O, P, S, Se, Te, X) bonds is reviewed. Reactions of aryl-A3-iodanes with organic substrates are considered first and are loosely organized by functional group, separate sections being devoted to carbon-azide and carbon-fluorine bond formation. Arylations and alkenyla-tions of nucleophilic species with diaryliodonium and alkenyl(aryl)iodonium salts, and a variety of transformations of alkynyl(aryl)iodonium salts with heteroatom nucleophiles are then detailed. Finally, the use of sulfonyliminoiodanes as aziridination and amidation reagents, and reactions of iodonium enolates formally derived from monoketones are summarized. 

Aryl-A3-iodanes 1 are electrophilic at iodine and undergo ligand exchange with a variety of nucleophilic species, including organic functional groups (Scheme 1). Such reactions may be regarded as nucleophilic substitutions at trivalent... 

The use of diaryliodonium salts for direct arylations of nucleophilic species is a well-established practice. Examples of C-heteroatom bond formation by this approach, including uncatalyzed arylations of dialkyl phosphite, thiocarboxy-late, arylthiosulfonate, dialkyl phosphorothiolate, arylselenolate, and aryltel-lurolate salts with symmetrical diaryliodonium halides, are shown in (Scheme 40) [110-115]. 

Unsaturated products 234, particularly ot,P-unsaturated carbonyl derivatives, are sometimes hardly obtainable due to additional side reactions readily taking place in the presence of nucleophilic species, as discussed in Sec. B (following) or to dimerization and polymerization, or even isomerization, as partially illustrated in Figs. 30 and 31 (Chap. 1, B.3). 

In the absence of nucleophilic species, the radical cations generated in the photo-induced electron-transfer reactions may undergo other reactions. Thus, 1-phenoxypropene undergoes cis-trans isomerisation on irradiation in the presence of electron acceptors such as dicyanobenzene (Majima et al., 1979). Some alkenes undergo dimerisation giving cyclobutanes on irradiation in the... 

In the preceding sections we outlined reactions of fluoroalkenes with many types of nucleophilic species, but reactions involving initial attack on fluoroalkenes by fluoride ion have been reserved for a separate discussion here. 

Reactions with nucleophiles The most striking feature about this alkyne is that it is extremely electrophilic in nature, and electrophilic additions are suppressed whilst nucleophilic additions proceed with ease in reactions with a wide range of nucleophilic species (cf. fluoroalkenes) [309, 310] (Figure 7.88). 

HCN + EtsAl H+ + [EtsAlCN]-Reactions of epoxides with Lewis acids in the absence of nucleophilic species lead to molecular rearrangements (see Chapter 8). 

The hrst example, reported by Craig and Munasinghem [135] and shown in Scheme 7.88, illustrates the types of strained fused-ring systems available from the delivery of nucleophilic species to adjacent centers. 

The HSAB-principle may be criticized as being too opportunistic the qualitative hard and soft assignments for reacting centers are used to explain rather small regio preferences, which reflect energy differences of only a few tenths of a Kcal/mol. However, the HSAB-principle is soundly based [41]. Basicity and polarisability may be used (with care) to predict qualitatively relative reactivities of a series of nucleophilic species. 

Using the above-mentioned hydrazone intermediates, derived from attack of nucleophilic species bearing carbonyl, cyano or carboxylate functions in a-position, many widely functionalized 1-aminopyrroles have been obtained (i.e. 3-substituted-l-aminopyrroles, l-amino-2,3-dihydropyrrol-2-ols, 1,2-diaminopyrroles, pyrrolo(2,3-6]pyrroles, 1-amino-l//-pyrrol-2(3/J)-ones, and 3-unsubstituted-l-aminopyrroles). The reaction pathway indicates an intramolecular interaction between the >C=N-NH- nitrogen atom and one of the above-mentioned functional groups followed by an appropriate molecular rearrangement and/or elimination, leading to the heterocyclization process. Scheme 1 shows the type of new pyrroles synthesized in our laboratory. 

The rates of addition to the unsaturated 1- and 1,1-disubsituted olefins are thought to be mainly determined by polar factors. Electron-withdrawing substituents will facilitate the addition of nucleophilic species, while electron-donating substituents will enhance the addition of electrophilic species. The addition of an initiating free radical to a monomer is called the initiation step, which is the first step of a chain reaction or propagation that ends through a termination reaction, in which two radicals interact in a mutually destructive reaction to form covalent bonds and cease propagation. 

One of the features of application of electrochemical methods in organic chemistry is that electrochemical synthesis can be carried out under controlled potential, enabling one to oxidize intermediate o -adducts and, at the same time, to avoid oxidation of nucleophilic species. 

It has recently been demonstrated that bromo-tns-pyrrolidino-phosphonium hexafluorophosphate (PyBroP) can be functiOTiing as a mild activator of azine N-oxide providing regioselective addition of Ai-nucleophiles (amines, sulfonamides, and NH-heterocycUc compounds) to pyridine, quinoline, and isoquinoline N-oxides (Scheme 53) [112,113]. A strong regiochemical preference for the orf/io-substitution pattern in aU these cases is likely caused by specific electrostatic attraction of nucleophilic species and the intermediate phosphonium salt 76. This synthetic procedure was successfully extended for other types of nucleophilic reagents (phenols, thiols, malonates). 

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