Anionic intramolecular

Ding C F, Wang X B and Wang L S 1998 Photoelectron spectroscopy of doubly charged anions intramolecular Coulomb repulsion and solvent stabilization J. Phys. Chem. A 102 8633... 

The perfect diastereoselectivity observed in these reactions lead to the assumption that they do not proceed via a simple anionic intramolecular cyclization (i.e. carbozincation) as was proposed in earlier61 publications. Cyclizations of non-propargylic substrates15,62 were shown to proceed with significantly lower diastereoselectivity (cis/trans typically 75/25). Thus, in the case of propargylic compounds, e.g. 114, a simple carbozincation process is unlikely to be operative. It was therefore reasoned that the active species in these reactions is not 115-ZnBr but rather its allenic isomer 120, which undergoes a metallo-ene-allene reaction in a chair-like transition state as depicted in equation 5659a. 

Reactions of 2-(3-hydroxy-3-phenylpropylseleno)benzoxazole with KH in tetrahydrofuran (THF) give selenetane 31. In the cases of /< rt-alcohols (R1 = C2H5 or CH2Ph), complex mixtures of products and no expected selenetane 31 are obtained because of steric hindrance at the reaction site. The formation of a selenetane is explained by a spiro intermediate which is converted into a selenolate anion. Intramolecular displacement of 30 gives the selenetane 31 (Scheme 7) <1998H633>. 

We can also make rings using alkoxide nucleophiles, and in this example the phenol (hydroxy-benzene) is deprotonated by the sodium methoxide base to give a phenoxide anion. Intramolecular attack on the conjugated ketone gives the cyclic product in excellent yield. In this case, the methoxide (pkaH about 16) will deprotonate the phenol (pKa about 10) completely, and competitive attack by MeCT acting as a nucleophile is not a problem as intramolecular reactions are usually faster than their intermolecular equivalents. 

A slightly modified synthesis of dihydrobenzo[c]furan was achieved, in which the key reaction involved the exposure of a dibromide to dry alumina and one equivalent of water in toluene instead of in hexanes <07TL3039>. Another synthesis of dihydrobenzo[c]furans was reported by Hashmi and Teles <07SL1747> and an example is depicted below. Base-promoted cycloaddition of 1-aryl- or l-aryl-7-substituted-4-oxahepta-1,6-diynes in DMSO were found to involve an anionic intramolecular Diels-Alder reaction, and also resulted in the formation of dihydrobenzo[c]furans <07JA4939>. 

O- -H- -O (anion) intramolecular HB [88]. The (X-fN) study reveals an asymmetric arrangement in the former case the H-atom is covalently bonded only to one of the N-atoms, strongly polarizes the VSCC of the other, but the N- -H BCP is located in a region of positive Laplacian. Although V Pbcp < both... 

Benzyne also reacts with compounds containing nucleophilic carbon atoms such as enolates and aryl anions. Intramolecular nucleophilic addition to an aryne by the ortho ring carbon atom of another benzene ring substituted... 

Tertiary amines have been reported to undergo addition to singlet trans-stilbene product selectivity appears to be determined by the orientation of deprotonation of the aminium radical intermediate by the stilbene radical anion. Intramolecular addition has been observed in 5-2-(r-methylalkyl)-aniline (220) on irradiation in methanol to give the 25,3/ -2,3-dimethylindoline... 

The reaction is believed to involve initial anionic intramolecular attack of an n-lithio group on a lithioethynyl group, the reaction possibly proceeding through a 2,3-dilithiobenzofuran. If the terminal alkyne is replaced by a substituted group, then direct reaction of the lithium species with the alkyne leads to the bicyclic anion, which can be quenched to yield the benzofuran. 

There are two distinct paths for cleavage of H-H (and X-H) bonds homolysis to a dihydride (as above) and heterolysis, that is, breaking the H-H bond into H and H fragment. Both paths have been identified in catalytic hydrogenation and are available for other cr-bond activations. A proton can split off from the H2 and migrate to either an external Lewis base (intermolecular) or a ligand/anion (intramolecular) as in Scheme 3. 

Numerous carbo- and heterocyclic compounds of various sizes have been prepared by HR-type monocyclization. Cyclopropanes are formed only when neopentylpalladium intermediates are formed in the absence of anions. Intramolecular carbopalladation of the alkenyl triflate 201 generates the neopentylpalladium 202, and its 3-exo cyclization and jd-H elimination afford the cyclopropane 203 [95]. Intermolecular carbopalladation of the alkyne 204 generates 205. Its 5-exo cyclization provides the neopentylpalladium 206, which undergoes 3-exo cyclization to construct the three-membered ring 207. Finally y3-H elimination gives rise to the cyclopropane 208 [96]. 

The first practical synthesis of indoles using aryne technology was that of Caubere and colleagues [8-12]. A selection of these anionic intramolecular indolizations is shown in Scheme 2, all of which utilize the complex superbase NaNH -i-BuONa [13]. One can alkylate the intermediate cyclized anion on nitrogen (equation 3). 

C.-F. Ding, X.-B. Wang, and L.-S. Wang,/. Phys. Chem. A, 102, 8633-8636 (1998). Photoelectron Spectroscopy of Doubly Charged Anions Intramolecular Coulomb Repulsion and Solvent Stabilization. 

Synthesis.—In general, the previously known methods of constructing sulphur-containing rings, such as intramolecular displacements by thiolate anions, intramolecular addition of thiol radicals to carbon-carbon multiple... 

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