Organic reaction mechanisms. 1998 / 15

acids,349,350 and pinacol has been converted into pinacolone and 2,3-dimethylbuta- 1,4-diene at relatively mild temperatures over metal-substituted aluminophosphate molecular sieves.351 An efficient pinacol rearrangement, mediated by trialkyl orthoformate, has been developed352 (see Scheme 73). It has been shown353 that a pinacol rearrangement occurs during photo-excitation of 9, 9 -bifluorene-9, 9 -diol (218). The reaction proceeds via initial C−O bond heterolysis to give a substituted 9-fluorenyl cation, which undergoes rearrangement and deprotonation to yield spiro[9H -fluorene- 9, 9 (10 -OH)phenanthren]-10 -one (219). A novel chromium(0)-promoted 6π –4π - cycloaddition–pinacol rearrangement strategy that delivers substituted nine-membered carbocycles with complete control of substituent stereochemistry has been described,354 as shown in Scheme 74. An interesting stereo-controlled approach to highly substituted

MeCF2 R

SCHEME 78

Monoand bi-cyclic cyclopentanes, known precursors of variety of sesquiterpenes, have been prepared357 by the acid-catalysed rearrangement of 1-methylcyclo- butylmethanols. An acid-catalysed rearrangement (see Scheme 77) has been found to afford a practical method for converting a bicyclo[4.2.0]octene system (221) into a bicyclo[3.2.1]octene framework (222) in a recent synthesis of verrucarol.358

The thallium trinitrate-mediated ring contraction of trans-decal-2-ones has opened up a new route to the hydrindane system,359 and fluorinative ring contraction of cyclic alkenes to afford difluorocycloalkanes has been induced by iodotoluene difluoride and Et3N–HF. A possible mechanism360 is shown in Scheme 78. The double bond of the cyclohexene ring is attacked by iodotoluene difluoride activated by HF from the axial direction, followed by the addition of a fluoride ion from the trans direction. Reductive elimination of iodotoluene from the resulting adduct, ring contraction and the addition of the fluoride ion to the carbocation stabilized by fluorine then take place to give the ring-contracted difluorinated product.

The reaction of different substituted 2-norbornanones with triflic anhydride in the presence of nitriles has been carried out in order to study the factors that influence the different reaction possibilities of 2-norbornyl carbocations.361 The chemistry of 2-norbornyl cations with spiro-annellated cyclobutane rings has been found to deviate strongly from that of the cyclopropane analogues.362 A cyclobutane ring spiro-annellated to the 6-position does not undergo ring expansion, whereas a cyclopropane ring does. On the other hand, a cyclobutane ring spiro-annellated to the

3-position expands readily giving rise to a uniquely endo-selective tertiary cation (223), whereas an analogously positioned cyclopropane ring remains intact. The main product of the acid-catalysed hydrolysis of 3-methyl-3-nortricyclanol (224) has been identified363 as endo-2-methyl-exo,exo-norbornane-2,5-diol (225). Acid hydrolyses of 2-exo-arylfenchyl alcohols have been found to afford the corresponding cyclofenchones as the kinetic products. These on prolonged treatment with acid are converted into Wagner–Meerwein products via equilibration with the stabilized fenchyl carbocations. These stabilized, sterically unhindered carbocations are proposed to react with water to give 2-endo-arylfenchyl alcohols that are stereoelectronically set up for a Wagner–Meerwein rearrangement. The presence of ortho substituents on the aryl ring hinders the Wagner–Meerwein rearrangement through decreased resonance stabilization of the carbocation and steric encumbrance to attack by external nucleophiles. However, when the ortho substituent itself is a nucleophile, the barrier to Wagner–Meerwein rearrangement is overcome and the authors364 have suggested that this is due to internal trapping of the carbocation from the exo-side to give a reactive intermediate that is stereoelectronically predisposed to concerted bond migration. Epoxide (226), on treatment with trifluoroacetic acid, has been found to undergo a regioselective ring opening, followed by a Wagner–Meerwein-type rearrangement, to give the 6,9-bis(trifluoroacetoxy) derivative (228). The intermediacy of the 2H - pyridazinium ion (227) has been invoked365 for the transformation. The possibility of the intervention of a 2H -pyrazinium ion to account for the formation of the skeletally

SCHEME 80

the tetrahydropyrans (241) with high trans-selectivity. A plausible mechanism for the formation of (241) involves attachment of a proton to the hydroxyl group of (237) to form the oxonium ion (238), shift of the proton from the oxygen atom to the α-carbon, and a 1,2-silyl migration of the β-silyl carbocation (239) to yield another β-silyl carbocation (240). Intramolecular attack of the oxygen from the side opposite to the silyl group will then give trans-(241). A highly stereospecific skeletal rearrangement involving a syn-1,2-silyl shift and the elimination of a trimethylsilyl group has been invoked379 to account for the formation of enantiomerically enriched propargylsilanes (and allylsilanes) from the reaction of oxasilacycloalkanes with acid (see Scheme 80). Acidic treatment of the (1S,1 S,2 R)-α-hydroxycyclopropylsilane (242) has been found to yield, via the unprecedented α-silyl cation (243), a mixture of rearranged products which are composed of the ring-opened (S)-vinylsilane (244), the tandem (1,2)-carbon–carbon bond migration product, (1S,2R,1 S)-silylcyclopropane (245: R3 = H, R4 = OH) and its 1 R isomer (245; R3 = OH, R4 = H), respectively.380

In the presence of strong acids, 1-hydroxyalkyltris (trimethylsilyl) silanes (246) have been found381 to isomerize by a 1,2-trimethylsilylhydroxy exchange to afford trimethylsilylmethylsilanols (247). The reaction of acylpolysilanes with silylbistriflimides has been found382 to lead to novel silanols via a pathway involving two 1,2-migrations of trimethylsilyl groups from silicon to carbon and one migration of a R3SiO unit from carbon to silicon (see Scheme 81).

A detailed comparison of the rearrangement of 1,3-radical cations and carbocations derived from tricyclo [3.3.0.02,4] octanes has shown (by electron-transfer oxidation and protonation, respectively) that electronic substituent effects on the diyl sites profoundly influence the regioselectivities of the Wagner–Meerwein 1,2-shifts. The

regioselectivity of the electron-transfer oxidation has been rationalized383 in terms of a qualitative MO interaction diagram, whereas that of the protonation is considered to follow the relative stability of the initially formed carbocation. Ab initio computational studies of methanethiol and dimethyl sulfide radical cations have demonstrated384 that both of these groups of compounds have similar decomposition paths that involve rearrangement and fragmentation of initially formed radical cations. Two different types of intermediates, a bisected trimethylenemethane cation radical and a diradical have been directly observed385 during the photochemical electron-transfer degenerate methylenecyclopropene rearrangement, (248) → (249). The recently discovered photochemical single electron-transfer-induced rearrangement of allyl phosphites (250) has been applied386 to the preparation of allyl phosphonates (251). A number of persistent dihydrobenzofuranyl cations have been investigated by 1H NMR and UV–visible spectroscopy and by cyclic voltammetry, and for the first time a selective and high-yield rearrangement proceeding via radical dications has been unambiguously established.387

A density functional study has been made of the competition between the Wolff rearrangement and 1,2-hydrogen shift in β-oxy-α-diazocarbonyl compounds.388 A report has appeared389 which shows that fiveand six-membered acyclic ethers can be prepared enantioselectively from achiral diazo ketones, using chiral copper complexes as catalysts (see Scheme 82). A highly efficient protocol for the chain elongation of fluorenylmethoxycarbonyl-protected α-amino acids by a Ag+-catalysed ultrasound-promoted Wolff rearrangement of the corresponding α-diazo ketones has been described.390 The Wolff rearrangement of diazo ketones derived from N -p-tolylsulfonyl-protected α- and β-amino acids has been investigated.391 Several different reaction pathways, including direct carbene N−H insertion, appear to be possible, depending on the nature of the N -protecting group, the substrate structure and the solvent. The thermolysis of α-diazo-β-keto-phosphonates (252) has been shown to afford 1-(disubstituted)-amino-1H -2-benzopyrans (253) which can be transformed into 1H -2-benzopyran derivatives by the action of various nucleophilic reagents. The extension of this reaction to pyridine and thiophene α-diazo-β-ketophosphonate