Ring expansion silver

The structure of the product obtained by the silver(I) ion catalyzed ring expansion of anti-1-chloro-2,3-benzo-7-azabicyclo[2.2.l]hepta-2,5-diene, formulated tentatively as 1-methoxy-l/f-1-benzazepine,140 has been shown to be 2-methoxy-6,7-benzo-l-azabicyclo[3.2.0]hept-3-ene... 

Azetidin-2-one can be synthesized by treating 1-ethoxy-1-hydroxycy-clopropane with aqueous sodium azide at pH 5.5 (Scheme 8.7a). This type of construction has wider applications and A-substituted derivatives are formed from 1-amino-1-hydroxycyclopropanes in two steps first A-chlorination with tert- miy hypochlorite [2-methylpropan-2-yl chlo-rate(I)], and then treatment with silver ion in acetonitrile (ethanenitrile) to release chloride ion and trigger ring expansion of the tricycle (Scheme 8.7b). 

The reactive cyclopropylidene cyclopropylcarbinyl cation 37 was generated from the corresponding bromide and silver hexafluoroantimonate, and was reacted in situ with olefins such as cyclohexene to form the addition products (equation 33). On the other hand, under the same conditions, the isopropylidene cyclopropylcarbinyl cation 41 rearranged spontaneously by ring expansion (equation 34)71. 

Silver is often used as a halophile. In the context of six-electron species, the role of silver atoms in carbene, nitrene, and silylene transfer reactions, including aziridination, CH insertion, ring expansion, and silacyclopropanation, has been reviewed.9... 

Cyclohexyldienyl complexes, with Ti(IV), 4, 327 Cyclohexylisocyanides, with gold(I) halides, 2, 281 Cyclohexylphosphine, for semiconductor growth, 12, 9 Cyclohexyl selenides, preparation, 9, 480 Cyclohydrocarbonylation alkenes, 11, 515 alkynes, 11, 522 dienes, 11, 522 overview, 11, 511-555 for ring expansion, 11, 527 Cycloisomerizations, via silver catalysts, 9, 558 Cyclomanganation, product types, 5, 777-778 Cyclometallated azobenzenes, liquid crystals, 12, 251 Cyclometallated complexes for OLEDs... 

Another way to activate l-(l-iodoalkyl)-l-silacyclobutanes toward ring expansion is to use silver acetate in acetic acid. In this case, the reaction is believed to proceed via formation of a carbocation a to the silicon. The acetate counterion acts as a nucleophile, attacking the activated SCB with C-Si bond migration (Scheme 35) <1991TL6383>. Silver tetrafluoroborate in dichloromethane induces ring enlargement as well, but shows much lower efficiency (30% yield upon treatment with MeLi) <1994BCJ1694>. 

Oxidative ring expansion of tetrahydrofurans can be promoted by silver(i) salts to afford tetrahydropyrans in moderate yield (Equations 161 and 162) <1996J(P1)413, 1996TL213>. Zinc salts can also effect this transformation <1999TL2145>. 

Although usually described as driven by halide abstraction by silver ion, dibromo-cyclopropanes readily rearranged in the presence of silver salts in a process similar to those mentioned above. Such rearrangements usually led to ring expansion products. 

Scheme 4.5. Silver(I)-promoted solvolysis reaction with ring expansion.

In many instances, however, solvolysis of a halocyclopropane is deliberately accomplished in order to install an essential vinyl halide or hindered olefin. In 2000, Murphy and coworkers performed a silver ion-mediated ring expansion of gem-dibromocyclopropane 18 in wet acetone to afford the allylic alcohol 19 in 82% yield (Scheme 4.5).16 Under these conditions the desired cyclononene product was obtained as an inseparable mixture of E- and Z-isomers (7 93). Interestingly, two sets of peaks observed in the1H NMR spectrum indicated that the Z-isomer existed as two separate conformers at room temperature. This intermediate was subsequently used in Murphy s approach to the radical-based preparation of tricyclic indoles. 

TABLE 8.11. Scope of Silver-Catalyzed Cyclopropyldiazoacetate Ring Expansion... 

Similar ring expansions are observed when 3,3 -bicyclopropenyls are treated with silver ion (see section 9). Reaction of the propanoic acid derivative (352) with oxalyl chloride also leads to a three carbon ring expansion, although the detailed mechanism of the reaction is not clear 212) ... 

Product analysis shows that the thermal and metal induced reactions follow similar courses. The former reaction may proceed by opening of one ring to produce a formal vinyl carbene, followed by ring expansion the latter may be rationalised in terms of electrophilic attack by silver ion at one of the cyclopropenes to produce a carbenium ion (355) — that is a silver-coordinated equivalent of the carbene — followed by ring expansion of the second cyclopropene. 

Of lesser relevance to this discussion are halogenation methods involving the modification of the carbon skeleton (synthesis and degradation). The Hunsdiecker reaction, as applied to certain heterocyclic acids, has had limited application for the synthesis of halogen derivatives. The preparation of 3-bromo-4,6-dimethyl-2-pyridone from the silver salt of the respective 3-carboxylic acid by treatment with bromine in carbon tetrachloride is a rare example of success.13 The interaction of carbenes with heterocycles also has been employed infrequently, but recent advances in carbene generation may reactivate this approach.14 The Ciamician-Dennstedt ring expansion of pyrrole to / -halopyridines is a case in point18 [Eq. (4)] ... 

One notable example of silver mediated ring rearrangements is Eaton and Halpem s (166) 1970 report, which used silver to convert cubane into cuneane. Lactam formation through a ring expansion of alkoxycyclopropylamines was studied by Wasserman et al. (167) and appears to occur either through a nitrenium species or a concerted process (Fig. 41). 

Ring expansion reactions of the type shown in equations (17) and (18), in which a more equitable distribution of carbon atoms between two rings of a polycyclic hydrocarbon is produced, provide the link between the decyclization reactions above and the annulation reactions considered later. It would seem that the greater the strain in the reactant, the larger the number of potential catalysts. The complexes of seven different metals catalyze the reaction shown in equation (17). Of the possible catalysts, only the silver (I) salts bring about any byproduct formation. The isomerization of [l.l.ljpropellane (equation 18) is catalyzed by [RhCl(PPh3)3]. ... 

With compound 64 available, vanadyl acetylacetonate catalyzed epoxidation [44] accompanied by simultaneous cyclization, afforded the corresponding tetrahydrofuran and its diastereomer in a 4 1 ratio (Scheme 12). Ring expansion of the corresponding mesylate 65 with silver (I) carbonate afforded compound 66 in a 42% yield for the two steps [45]. Extension of the side chain in six steps, followed by an asymmetric epoxidation, gave product 67 stereoselectively. The cyclization of 67 with titanium tetraisopropoxide in a manner consistent with model studies [27d], afforded bicyclic ether 68 in 65% yield. Transformation to the epoxide under standard conditions afforded fragment 69 ready to be coupled with the D-ring side chain. 

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