Wolff rearrangement

In 1902, Wolff discovered that treating diazoacetophenone with silver (I) oxide and water resulted in formation of phenylacetic acid. Similarly, treatment with silver (I) oxide and ammonia formed phenylacetamide.^[3] A few years later, in an independent study, Schröter observed similar results.^[5] The reaction is occasionally called the Wolff-Schröter rearrangement.^[2] The Wolff rearrangement was not commonly used until 20 years after it was discovered, as facile diazo ketone synthesis was unknown until the 1930s.^[2] The reaction has proven useful in synthetic organic chemistry and many reviews have been published.^[1][2]

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The mechanistic pathway of the Wolff-rearrangement has been the subject of much debate, as there are often competing concerted and stepwise mechanisms.^[2] However, two aspects of the mechanism can be agreed upon. First, α-diazocarbonyl compounds are in an equilibrium of s-cis and s-trans-conformers, the distribution of which may influence the mechanism of the reaction. Generally, under photolysis, compounds in the s-cis conformation react in a concerted manner due to the antiperiplanar relationship between the leaving and migrating groups, whereas compounds in the s-trans conformation react stepwise through a carbene intermediate or do not rearrange. Second, regardless of the reaction mechanism, the rearrangement gives a ketene intermediate, which can be trapped by a weakly acidic nucleophile, such as an alcohol or amine, to give the corresponding ester or amide, or an olefin, to give a [2+2] cycloaddition adduct. Strong acids do not rearrange, but rather protonate the α-carbon and give S_N2 products.

Understanding the stereochemistry of α-diazo ketones is essential in elucidating the mechanism of the Wolff rearrangement. α-diazocarbonyl compounds are generally locally planar, with large rotational barriers (55–65 kJ/mol) due to C-C olefin character between the carbonyl and α-carbon, illustrated in the rightmost resonance structure.^[6] Such a large barrier slows molecular rotations sufficiently to lead to an equilibrium between two conformers, an s-trans and s-cis-conformer. s-cis-Conformers are electronically favored due to Coulombic attraction between the oxygen with a partial negative charge and the cationic nitrogen, as seen in the rightmost resonance structure.^[1] If R₁ is large and R₂ is hydrogen, s-cis is sterically favored. If R₁ and R₂ are large, s-trans is sterically favored; if both substituents are sufficiently large, the steric repulsion can outweigh the Coulombic attraction, leading to a preference for s-trans. Small and medium cyclic substrates are constrained in the s-cis conformation.

When the α-diazo ketone is in the s-cis conformation, the leaving group (N₂) and the migrating group (R₁) are antiperiplanar, which favors a concerted mechanism, in which nitrogen extrusion occurs concurrently with 1,2-alkyl shift. There is evidence this mechanism occurs in both thermolytic and photolytic methods, when the s-cis-conformer is strongly favored.^[7]

CIDNP studies show that photochemical rearrangement of diazoacetone, which largely exists in the s-cis-conformer, is concerted.^[8] Product ratios from direct and triplet-sensitized photolysis have been used as evidence for proposals that claim that concerted products arise from the s-cis-conformer and stepwise products occur through the s-trans-conformer.^[9]

s-trans-α-Diazo ketones do not have an antiperiplanar relationship between the leaving and migrating group, and thus are thought to generally rearrange stepwise. The stepwise mechanism begins with nitrogen extrusion, forming an α-ketocarbene. The α-ketocarbene can either undergo a 1,2-alkyl shift, to give the ketene product, or can undergo a 4π electrocyclic ring closure, to form an antiaromatic oxirene. This oxirene can reopen in two ways, to either α-ketocarbene, which can then form the ketene product.

There are two primary arguments for stepwise mechanisms. The first is that rate constants of Wolff rearrangements depend on the stability of the formed carbene, rather than the migratory aptitude of the migrating group.^[10] The most definitive evidence is isotopic scrambling of the ketene, as predicted by an oxirene intermediate, which can only occur in the stepwise path. In the scheme below, the red carbon is ¹³C labelled. The symmetric oxirene intermediate can open either way, scrambling the ¹³C label. If the substituents R₁ and R₂ are the same, one can quantify the ratio of products stemming from the concerted and stepwise mechanisms; if the substituents are different, the oxirene will have a preference in the direction it opens, and a ratio cannot be quantified, but any scrambling indicates some reactant is going through a stepwise mechanism.^[1] In photolysis of diazo acetaldehyde, 8% of the label is scrambled, indicating that 16% of product is formed via the oxirene intermediate.^[11] Under photolysis, the biphenyl (R1=R₂=phenyl) substrate shows 20–30% label migration, implying 40–60% of product goes through the oxirene intermediate.^[12] α-diazocyclohexanone shows no label scrambling under photolytic conditions, as it is entirely s-cis, and thus all substrate goes through the concerted mechanism, avoiding the oxirene intermediate.^[13]

Isotopic labeling studies have been used extensively to measure the ratio of product stemming from a concerted mechanism versus a stepwise mechanism.^[14] These studies confirm that reactants that prefer s-trans conformations tend to undergo stepwise reaction. The degree of scrambling is also affected by carbene stability, migratory abilities, and nucleophilicity of solvent. The observation that the migratory ability of a substituent is inversely proportional to amount of carbene formed, indicates that under photolysis, there are competing pathways for many Wolff reactions.^[14] The only Wolff rearrangements that show no scrambling are s-cis constrained cyclic α-diazo ketones.^[13]

Under both thermolytic and photolytic conditions, there exist competing concerted and stepwise mechanisms. Many mechanistic studies have been carried out, including conformational, sensitization, kinetic, and isotopic scrambling studies. These all point to competing mechanisms, with general trends. α-Diazo ketones that exist in the s-cis conformation generally undergo a concerted mechanism, whereas those in the s-trans conformation undergo a stepwise mechanism.^[1] α-diazo ketones with better migratory groups prefer a concerted mechanism.^[1] However, for all substrates except cyclic α-diazo ketones that exist solely in the s-cis conformation, products come from a combination of both pathways.^[1] Transition metal mediated reactions are quite varied; however, they generally prefer forming the metal carbene intermediate.^[2] The complete mechanism under photolysis can be approximated in the following figure:

The mechanism of the Wolff rearrangement is dependent on the aptitude of the migratory group. Migratory abilities have been determined by competition studies. In general, hydrogen migrates the fastest, and alkyl and aryl groups migrate at approximately the same rate, with alkyl migrations favored under photolysis, and aryl migrations preferred under thermolysis.^[15] Substituent effects on aryl groups are negligible, with the exception of NO₂, which is a poor migrator.^[15] In competition studies, electron deficient alkyl, aryl, and carbonyl groups cannot compete with other migrating groups, but are still competent.^[16][17][18] Heteroatoms, in general, are poor migratory groups, because their ability to donate electron density from their p orbitals into the π* C=O bond decreases migratory ability.^[1] The trend is as follows:^[1]

Photochemical reactions: H > alkyl ≥ aryl >> SR > OR ≥ NR₂

Thermal reactions H > aryl ≥ alkyl (heteroatoms do not migrate)

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While known since 1902, the Wolff rearrangement did not become synthetically useful until the early 1930s, when efficient methods became available to synthesize α-diazocarbonyl compounds. The primary ways to prepare these substrates today are via the Arndt-Eistert procedure, the Franzen modification to the Dakin-West reaction, and diazo-transfer methods.

The Arndt–Eistert reaction^[19] involves the acylation of diazomethane with an acid chloride, to yield a primary α-diazo ketone. The carbon terminus of diazomethane adds to the carbonyl, to create a tetrahedral intermediate, which eliminates chloride. The chloride then deprotonates the intermediate to give the α-diazo ketone product.

These α-diazo ketones are unstable under acidic conditions, as the α-carbon can be protonated by HCl and S_N2 displacement of nitrogen can occur by chloride.

The Dakin–West reaction is a reaction of an amino acid with an acid anhydride in the presence of a base to form keto-amides. The Franzen modification^[20] to the Dakin–West reaction^[21] is a more effective way to make secondary α-diazo ketones. The Franzen modification nitrosates the keto-amide with N₂O₃ in acetic acid, and the resulting product reacts with methoxide in methanol to give the secondary α-diazo ketone.

Diazo-transfer reactions are commonly used methods, in which an organic azide, usually tosylazide, and an activated methylene (i.e. a methylene with two withdrawing groups) react in the presence of a base to give an α-diazo-1,3-diketone.^[22] The base deprotonates the methylene, yielding an enolate, which reacts with tosylazide and subsequently decomposes in the presence of a weak acid, to give the α-diazo-1,3-diketone.

The necessary requirement of two electron withdrawing groups makes this reaction one of limited scope. The scope can be broadened to substrates containing one electron withdrawing group by formylating a ketone via a Claisen condensation, followed by diazo-transfer and deformylative group transfer.^[23]

One of the greatest advantages of this method is its compatibility with unsaturated ketones. However, to achieve kinetic regioselectivity in enolate formation and greater compatibility with unsaturated carbonyls, one can induce enolate formation with lithium hexamethyldisilazide and subsequently trifluoroacylate rather than formylate.^[24]

Wolff rearrangements can be induced under thermolytic,^[3] photolytic,^[4] and transition-metal-catalyzed conditions.^[3]

Thermal conditions to induce rearrangement require heating to relatively high temperatures, of 180 ˚C, and thus have limited use.^[3] Many Wolff rearrangement products are ring-strained and are susceptible to ring-open under high temperatures. In addition, S_N2 substitution of the diazo group at the α-carbon can take place at lower temperatures than rearrangement, which results in byproducts. The greatest use of thermal Wolff rearrangements is the formation of carboxylic acid analogs, by interception of the ketene with high boiling solvents, such as aniline and phenol.^[3]

Transition metals greatly lower the temperature of Wolff rearrangements, by stabilization of a metal-carbene intermediate. However, these carbenes can be so stable, as to not undergo rearrangement. Carbenes of rhodium, copper, and palladium are too stable and give non-Wolff products (primarily carbene insertion products).^[2] The most commonly used metal catalyst is silver(I) oxide, although silver benzoate is also common. These reactions are generally run in the presence of a weak base, such as sodium carbonate or tertiary amines.^[2]

Whereas thermal and metal mediated Wolff rearrangements date back to 1902,^[3] photolytic methods are somewhat newer, with the first example of a photolytic Wolff rearrangement reported in 1951.^[4] α-diazo ketones have two absorption bands, an allowed π→π* transition at 240–270 nm, and a formally forbidden π→σ* transition at 270–310 nm.^[4] Medium or low-pressure mercury arc lamps can excite these respective transitions. Triplet sensitizers result in non-Wolff carbene byproducts, and thus are not useful in synthetic applications of the Wolff rearrangement.^[2] However, they have been used to probe the mechanism of the Wolff rearrangement.