Internal nucleophile

The rearrangement of thiophenes to the isomeric pyrroles has proven synthetically useful (Schemes 98a and 98b). In the absence of a suitable internal nucleophilic nitrogen, so-called degenerate ring transformations may occur (Schemes 98c and 98d). 

Acetal and hemiacetal groups are particularly common in carbohydrate chemistry. Glucose, for instance, is a polyhydroxy aldehyde that undergoes an internal nucleophilic addition reaction and exists primarily as a cyclic hemiacetal. 

M-Acyliminium cyclizations of optically active mono- and di-oxygenated hydroxylactam derivatives have been used in the synthesis of a number of natural products. In case of a five-membered lactam the oxygen function adjacent to the iminium carbon directs attack of the internal nucleophile from the least hindered side, opposite to the substituent. In the examples given the size of the newly formed ring is determined by the electronic bias of the alkene substituent. 

So far, only reactions in which the internal nucleophile is tethered to the nitrogen atom of the A -acyliminium ion have been discussed, however, cyclizations with nucleophiles attached to other positions are also possible. If the nucleophile is connected to the carbon atom adjacent to the carbonyl group, bridged azabicycloalkane derivatives are obtained in high yield by using the more reactive allyl- or propargylsilanes. 

Reactions of this type are mostly performed with internal nucleophiles attached to the carbon atom adjacent to the iminium nitrogen, thus leading to tropane-like azabicyclic systems. Both allyl- and propargylsilanes, activated and unactivated alkenes, and ketones have been successfully used as nucleophiles. The products 1 and 2 are also obtained via two consecutive C —C bond-forming reactions in a single operation. 

Some more complex examples of this reaction type from the field of natural product synthesis, using a ketone, a thioenol ether, or a phenyl function as an internal nucleophile, are found in references 171-173. 

Analogous results were obtained for enol ether bromination. The reaction of ring-substituted a-methoxy-styrenes (ref. 12) and ethoxyvinylethers (ref. 10), for example, leads to solvent-incorporated products in which only methanol attacks the carbon atom bearing the ether substituent. A nice application of these high regio-and chemoselectivities is found in the synthesis of optically active 2-alkylalkanoic acids (ref. 13). The key step of this asymmetric synthesis is the regioselective and chemoselective bromination of the enol ether 4 in which the chiral inductor is tartaric acid, one of the alcohol functions of which acts as an internal nucleophile (eqn. 2). 

A fluxional amido-salt (24), in which the y-nitrogen atom acts as an internal nucleophile, has been identified by variable-temperature n.m.r. spectroscopy. At — 63 °C two methyl signals are observed, one a singlet, one a doublet (J = 11 Hz) whereas at + 60 °C there is only one signal, a doublet with J = 5.5 Hz (the average of the low-temperature coupling constants). The solvent extraction of organophosphorus compounds has also been studied by and H n.m.r, ... 

Similarly, with reactants 2 and 3 conformational preference dominates in the selectivity between CO, and CH2OH as the internal nucleophile. This conformational preference even extends to C02CH3, which can cyclize in preference to CH2OH when it is in the conformationally preferred position.77... 

Reactants with internal nucleophiles are also subject to cyclization by electrophilic sulfur reagents, a reaction known as sulfenylcyclization.92 As for iodolactonization, unsaturated carboxylic acids give products that result from anti addition.93... 

Internal nucleophilic capture of seleniranium ion is governed by general principles similar to those of other electrophilic cyclizations.96 The stereochemistry of cyclization can usually be predicted on the basis of a cyclic TS with favored pseudoequatorial orientation of the substituents. 

Various electrophilic selenium reagents such as those described in Scheme 4.3 can be used. V-Phenylselenylphthalimide is an excellent reagent for this process and permits the formation of large ring lactones." The advantage of the reagent in this particular application is the low nucleophilicity of phthalimide, which does not compete with the remote internal nucleophile. The reaction of phenylselenenyl chloride or V-phenylselenenylphthalimide with unsaturated alcohols leads to formation of (3-phenylselenenyl ethers. 

Part B of Scheme 4.5 gives some examples of cyclizations induced by selenium electrophiles. Entries 9 to 13 are various selenyletherifications. All exhibit anti stereochemistry. Entries 14 and 15 are selenyllactonizations. Entries 17 and 18 involve amido groups as the internal nucleophile. Entry 17 is an 5-exo cyclization in which the amido oxygen is the more reactive nucleophilic site, leading to an iminolactone. Geometric factors favor N-cyclization in the latter case. 

Carbonylation can also be carried out under conditions in which the acylrhodium intermediate is trapped by internal nucleophiles. 

By contrast, O in (43) is sufficiently electronegative not to donate an electron pair (unlike Oe in ROe and RCO20 above), and hydrolysis of EtOCH2CH2Cl thus proceeds via ordinary SN2 attack by an external nucleophile—which is likely to be very much slower than the internal nucleophilic attack in (42) — (44). That a cyclic sulphonium salt such as (44) is involved is demonstrated by the hydrolysis of the analogue (45), which yields two alcohols (the unexpected one in greater yield) indicating the participation of the unsymmetrical intermediate (46) ... 

Support for the suggestion that Fig. 13.6 involves a change in actual reaction pathway is provided by acetolysis of the threo diastereoisomer (31) of the brosylate. Acetolysis leads to two different distinguishable, diastereoisomers whose relative proportion will depend on how much of the total reaction proceeds by external nucleophilic attack via the SN2 pathway (erythro product, 32), and how much by internal nucleophilic attack via a cyclic phenonium ion intermediate (threo product, 33) ... 

Internal cyclopropane ring-opening reactions have also been developed using hydroxyl-substituted cyclopropanes. A typical example of this strategy comes from the reaction of the polycylic alcohol 131, which reacts quantitatively with Hg(OAc)2 to yield the acetal 132 (Equation (45)).165 Carbonate moieties can also be used as internal nucleophiles as, for example, in the case of 133, which affords 134 in a 60% yield upon reaction with Hg(OCOCF3) (Equation (46)).166... 

Intramolecular interception of the ketene intermediate by an internal nucleophile (as available e.g. in the sulphimide 333 derived from 2-aminopyridine) gave rise to... 

The study of both carbonyl and carbon acid participation in ester hydrolysis has been used by Bowden and Last (1971) to evaluate certain of the factors suggested for important roles in enzymic catalysis. A first model concerns a comparison of the three formyl esters and shows that the proximity of the formyl to the ester group and internal strain increase in passing along the series, 1,2-benzoate, 1,8-naphthoate and 4,5-phenanthroate. The very large rate enhancements result from the proximity of the internal nucleophile once formed and from internal strain. Strain is increased or induced by the primary... 

A certain dualism is observable in carbonium ion-carbanion chemistry, a dualism rather like that of lines and points in projective geometry. The reader may recall that interchanging the words "line and "point in a theorem of projective geometry converts it into a statement that is also a theorem, sometimes the same one. For most carbonium ion reactions a corresponding carbanion reaction is known. The dualism can be used as a method for the invention of new, or at least unobserved, carbanion reactions. The carbanionic reaction corresponding to the carbonium ion rearrangement is of course the internal nucleophilic... 

Sulfur atom as internal nucleophile. In this area, it has been shown that the reaction of 8-bromo-l,3-dimethyl-7-(2,3-epithiopropyl)xanthine 147 with a range of aliphatic and aromatic amines generates efficiently 2-amino-substituted 2,3-dihydro-thiazolo[2,3-/]xanthine derivatives 148. The process involves a sequential amine-induced thiirane ring opening followed by thiolate z/MYi-substitution of chlorine atom (Equation 66) <1994PCJ647>. 

Interestingly, enamides or imines are produced in the absence of external or internal nucleophiles as shown by the formation of 342, 344, and 346, respectively, obtained from 341 <1998JOC44>, 343 <1998JOC1619>, and 345 <1996SC3471> in good to excellent yields (Scheme 94). 

In some examples - in contrast to the above cases - an internal nucleophilic attack of a triple bond can occur (in an exo-dig-fasYiion), resulting in the formation of a new six-membered ring. Such conversions are shown in Scheme 29. 

The benzoyl derivative, PhCOMn(CO)5 and related acyl derivatives have intrinsic interest because they can be prepared by alkyl group migration to coordinated CO, a formal internal nucleophilic attack. This reaction, which is often referred to as carbonyl insertion86), is reversible in certain instances to give the metal alkyl. The enthalpy of disruption for PhCOMn(CO)s (Table 13) can be divided up to give the b.e.c of the Mn-COPh bond if the values of T (Mn-CO) shown in Table 13 are used, then the Mn-COPh enthalpy lies in the range 105 10 kJ mol-1, the lower value being preferred as outlined earlier. 

The proposed mechanism involves the usual oxidative addition of the aryl halide to the Pd(0) complex affording a Pd(II) intermediate (Ar-Pd-Hal), subsequent coordination of allene 8 and migratory insertion of the allene into the Pd-C bond to form the jt-allylpalladium(II) species 123. A remarkable C-C bond cleavage of 123 leads by decarbopalladation to 1,3-diene 120 and a-hydroxyalkylpalladium species 124. /8-H elimination of 124 affords aldehyde 121 and the H-Pd-Hal species, which delivers Pd(0) again by reaction with base (Scheme 14.29). The originally expected cyclization of intermediate 123 by employment of the internal nucleophilic hydroxyl group to form a pyran derivative 122 was observed in a single case only (Scheme 14.29). 

It is worth noting that in most of the reactions involving allenes with an internal nucleophile, cr-vinyl complexes are formed but their further reaction usually lead to unwanted by-products. 

All these features have been initially interpreted102-104 in terms of a molecular mechanism involving two successive alkene-iodine complexes of 1 1 and 1 2 stochiometries the second of which evolves by internal nucleophilic attack of the uncomplexed double bond to the diiodo derivative (equation 87). The intramolecular attack of the second double bond has been regarded as rate determining, owing to the fact that the overall rate law is second order in iodine rather than the usual third order. Nevertheless more... 

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