Application of Intramolecular Carbenoid Reactions in Organic Synthesis

Orthogonene: A computational study of a strongly twisted alkene

The strongly twisted (calculated dihedral angle 84°) alkene orthogonene (tetracyclo[8,2,2,02,7,03,10]tetradecene-2(3)) was studied at the DFT (B3LYP/6-31G*) and ab initio (HF/6-31G* and MP2(fc)/6-31G*) levels with a view to exploring the question of how realistic a synthetic objective it is, particularly in view of the fact that an attempt was made to synthesize it. Orthogonene is a relative minimum on the potential energy surface at the B3LYP/6-31G* and MP2(fc)/6-31G* levels, but is predicted to have a very low barrier (1.2 kJ mol–1 from the B3LYP calculations) to rearrangement by a 1,2-carbon shift to a carbene, which in turn isomerizes to a cyclopropane and (or) an alkene (the carbene is perhaps not a stationary point on the potential energy surface). Thus, very likely orthogonene could be observed only at temperatures approaching absolute zero, if at all.Key words: orthogonene, tetracyclo[8,2,2,02,7,03,10]tetradecene-2(3), twisted double bond, orthogonal double bond, carbene-strained alkene isomerization, ab initio, DFT.

Palladium-catalyzed carbonylative cyclization of unsaturated aryl iodides and dienyl triflates, iodides, and bromides to indanones and 2-cyclopentenones

Indanones and 2-cyclopentenones have been successfully prepared in good to excellent yields by the palladium-catalyzed carbonylative cyclization of unsaturated aryl iodides and dienyl triflates, iodides, and bromides, respectively. The best results are obtained by employing 10 mol % of Pd(OAc)(2), 2 equiv of pyridine, 1 equiv of n-Bu(4)NCl, 1 atm of CO, a reaction temperature of 100 degrees C, and DMF as the solvent. This carbonylative cyclization is particularly effective on substrates that contain a terminal olefin. The proposed mechanism for this annulation includes (1) Pd(OAc)(2) reduction to the active palladium(0) catalyst, (2) oxidative addition of the organic halide or triflate to Pd(0), (3) coordination and insertion of carbon monoxide to produce an acylpalladium intermediate, (4) acylpalladation of the neighboring carbon-carbon double bond, (5) reversible palladium beta-hydride elimination and re-addition to form a palladium enolate, and (6) protonation by H(2)O to produce the indanone or 2-cyclopentenone.

High selectivity from configurational match/mismatch in carbon-hydrogen insertion reactions of steroidal diazoacetates catalyzed by chiral dirhodium(II) carboxamidates

Diazo decomposition of steroidal diazoacetates, where the point of attachment is the 3-position of the steroid A-ring, catalyzed by chiral dirhodium(II) carboxamidates results in products from carbon-hydrogen insertion in high yield and selectivities. Use of S-configured catalysts shows a distinctive preference for insertion into the 3-position to form beta-lactone products. The R-configured catalysts direct insertion preferentially to the equatorial C-H bond at the 2-position. Substituents or functional groups at the 5/6-position prevent C-H insertion from taking place at the 4-position. Even in the best case with the 5/6-positions fully saturated, however, insertion into the 3-position remains competitive with insertion into the 4-position. Corresponding 3-substituted phenyldiazoacetates give only beta-lactone products, and selectivity here is highest with chiral dirhodium(II) prolinate catalysts. A model is presented to explain these results. Overall, this methodology is versatile for functionalization of the steroid A-ring at positions 2 and 3.

Catalytic asymmetric cyclopropanation of heteroaryldiazoacetates

Rh(2)(S-DOSP)(4)-catalyzed decomposition of heteroaryldiazoacetates in the presence of styrene results in highly diastereoselective and enantioselective cyclopropanations. Heteroaryldiazoacetates containing both electron-rich and electron-deficient heterocycles, such as thiophene, furan, pyridine, indole, oxazole, isoxazole, and benzoxazole, are effective in this chemistry. These studies broaden the range of diazo compounds containing both electron-withdrawing and electron-donating groups, which undergo highly diastereoselective cyclopropanations.

Selectivity in reactions of allyl diazoacetates as a function of catalyst and ring size from gamma-lactones to macrocyclic lactones

Catalytic reactions of diazoacetates tethered through zero, one, two, and three ethylene glycol units to an allyl group have been investigated for chemoselectivity, diastereoselectivity, and enantioselectivity. Results from cyclopropanation, carbon-hydrogen insertion, and oxonium ylide generation are compared from reactions of achiral and chiral catalysts of copper(I) and dirhodium(II) carboxylates and carboxamidates. Relative to results from intermolecular reactions of ethyl diazoacetate with allyl ethyl ether, intermolecular reactions show a diversity of selectivities including preference for the opposite configurational arrangement from the one preferred in corresponding intermolecular cyclopropanation reactions. Enantioselectivities for cyclopropanation are dependent on the catalyst ligands in a manner that reflects divergent trajectories of the carbon-carbon double bond to the reacting carbene center. Enantioselectivity increases as a function of ring size with chiral copper catalysts, but the reverse occurs with chiral dirhodium(II) carboxamidates. Mechanistic implications, including those related to the conformation of the reacting metal carbene, offer a new dimension to understanding of enantioselectivity in catalytic asymmetric cyclopropanation reactions.

Synthesis of Polycyclic Ring Systems Using Transition Metal Catalyzed Cyclizations of Diazo Alkynyl Ketones

The rhodium(II)-catalyzed reaction of α-diazo ketones bearing tethered alkyne units represents a new and useful method for the construction of a variety of substituted cyclopentenones. The process proceeds by addition of the rhodium-stabilized carbenoid onto the acetylenic π-bond to give a vinyl carbenoid intermediate. The resulting rhodium complex undergoes a wide assortment of reactions including cyclopropanation, 1,2-hydrogen migration, CH-insertion, addition to tethered alkynes and ylide formation. When 2-alkynyl-2-diazo-3-oxobutanoates were treated with a Rh(II)-catalyst, furo[3,4-c]furans were formed in excellent yield.

Rhodium(II)-catalyzed carbocyclization reaction of alpha-diazo carbonyls with tethered unsaturation

o-Alkynyl-substituted alpha-diazoketones undergo internal cyclization to produce indenone derivatives upon treatment with catalytic quantities of Rh(II)-carboxylates. A variety of structural influences were encountered by varying the nature of the substituent group attached to the diazo center. The cyclization reaction involves addition of a rhodium-stabilized carbenoid onto the acetylenic pi-bond to generate a cycloalkenone carbenoid. The cyclized carbenoid was found to undergo both aromatic and aliphatic C-H insertion as well as cyclopropanation across a tethered pi-bond. Subjection of diazo phenyl acetic acid 3-phenylprop-2-ynyl ester to Rh(II) catalysis furnished 8-phenyl-1, 8-dihydro-2-oxacyclopenta[a]indenone in high yield. The formation of this compound involves cyclization of the initially formed carbenoid onto the alkyne to produce a butenolide which then undergoes C-H insertion into the neighboring aromatic system. When a vinyl ether is added, the initially formed rhodium carbenoid intermediate can be intercepted by the electron-rich pi-bond prior to cyclization. Different rhodium catalysts were shown to result in significant variation in the product ratios. The competition between bimolecular cyclopropanation, 1,2-hydrogen migration, and internal cyclization was probed using several enol ethers as well as diazoesters which possess different substituent groups on the ester backbone. The specific path followed was found to depend on electronic, steric, and conformational factors.

Three- to six-carbon ring-enlargement reaction of cyclic ortho esters bearing a diazocarbonyl side chain. Use of the intramolecular formation of tricyclooxonium ylides

Two types of bicyclic ortho esters 14 and 18, which are tethered to a diazocarbonyl group by polymethylene linkages--(CH2)n--of different lengths (n = 1-3 for 14 and 1-4 for 18), were prepared and catalytically decomposed by treatment with Rh2(OAc)4 either in the presence or absence of a protic nucleophile (MeOH, PhOH, AcOH) to give ring-enlargement product lactones 25 and 30 of different sizes. With 14, the enlargement took place when n = 1 or 2, but not when n = 3. With 18, in which the diazo carbon is substituted with a methoxycarbonyl group, the length of the chain can be extended further to n = 4 to obtain ring-enlargement products or their derivatives. All of these reactions could be explained in terms of the intermediacy of tricyclooxonium ylides 22 and 28. The ylides form an equilibrium with the corresponding ring-opened zwitterions 22' and 28', respectively, which, after protonation by a protic nucleophile, undergo mainly ring-enlargement to form medium-sized or large oxalactones rather than 1,2-rearrangement.