Electrophilic addition and cyclization reactions of allenes

Progress in fluoroalkylation of organic compounds via sulfinatodehalogenation initiation system

The sulfinatodehalogenation reaction represents one of the most important methodologies to incorporate fluorine into organic molecules. Using inexpensive sulfur-containing reactants such as Na(2)S(2)O(4) under mild conditions, per- and polyfluoroalkyl halides (R(F)X, X = Br, I, CCl(3)) can be transformed smoothly into the corresponding sulfinate salts. This method is also used for the perfluoroalkylation of alkenes, dienes, alkynes and aromatics. Notably, after 1998, the sulfinatodehalogenation of perfluoroalkyl chlorides (R(F)Cl) has been realized by using dimethylsulfoxide (DMSO) as a solvent instead of CH(3)CN/H(2)O in the Na(2)S(2)O(4)/NaHCO(3) initiation system. Perfluoroalkyl chlorides, ethyl chlorofluoroacetates and chlorodifluoroacetates, and even nonfluorinated compounds, such as ethyl chloro- or dichloroacetates and chloroform, were either converted into the corresponding sulfinate salts or alkylated alkenes, alkynes and aromatics (including porphyrins). The sulfinatodehalogenation reaction has remarkable advantages. With the increasing demands to utilize the unique properties of fluorine and fluorinated functional groups in medicinal, agricultural and material sciences, we believe that there will continue to be useful developments in sulfinatodehalogenation chemistry and it will be applied more widely in the future.

Synthesis of conjugated allenes through copper-catalyzed γ-selective and stereospecific coupling between propargylic phosphates and aryl- or alkenylboronates

A Cu-catalyzed γ-selective coupling reaction between propargylic phosphates and aryl- or alkenylboronates afforded aryl- or alkenyl-conjugated allenes. The reaction showed excellent functional group compatibility in both the propargylic substrates and the boronates. The reaction of an enantioenriched propargylic phosphate proceeded with excellent chirality transfer with 1,3-anti stereochemistry to give axially chiral aryl- and alkenylallenes.

Reaction of cis-3-chloroacrylic acid dehalogenase with an allene substrate, 2,3-butadienoate: hydration via an enamine

cis-3-Chloroacrylic acid dehalogenase (cis-CaaD) catalyzes the hydrolytic dehalogenation of cis-3-haloacrylates to yield malonate semialdehyde. The enzyme processes other substrates including an allene (2,3-butadienoate) to produce acetoacetate. In the course of a stereochemical analysis of the cis-CaaD-catalyzed reaction using this allene, the enzyme was unexpectedly inactivated in the presence of NaBH(4) by the reduction of a covalent enzyme-substrate bond. Covalent modification was surprising because the accumulated evidence for cis-CaaD dehalogenation favored a mechanism involving direct substrate hydration mediated by Pro-1. However, the results of subsequent mechanistic, pre-steady state and full progress kinetic experiments are consistent with a mechanism in which an enamine forms between Pro-1 and the allene. Hydrolysis of the enamine or an imine tautomer produces acetoacetate. Reduction of the imine species is likely responsible for the observed enzyme inactivation. This is the first reported observation of a tautomerase superfamily member functioning by covalent catalysis. The results may suggest that some fraction of the cis-CaaD-catalyzed dehalogenation of cis-3-haloacrylates also proceeds by covalent catalysis.

Asymmetric synthesis of trisubstituted allenes: copper-catalyzed alkylation and arylation of propargylic phosphates

Asymmetric synthesis of trisubstituted allenes is accomplished by copper-catalyzed alkylation and arylation of propargylic phosphates using organoboron nucleophiles. Excellent chirality transfer and regioselectivity, together with good functional group compatibility, were observed in reactions with both alkyl boranes and arylboronic esters.

Palladium-catalyzed cyclization of propargylic compounds

Many groups have explored the scope of the palladium-based cyclization of propargylic compounds since Tsuji's first report in 1985. Through the proper positioning of an internal nucleophilic center and the judicious selection of an appropriate external nucleophile, the synthetic chemist can effectively assert control over the course of the reaction and its products. However, initial investigations were very limited: only heterocyclic compounds were originally synthesized. We have found the palladium-catalyzed cyclization of propargylic compounds to be a very efficient method for producing both carbocyclic and heterocyclic compounds. In this Account, we discuss the cyclization reactions of functionalized propargylic compounds with a variety of nucleophiles that we have developed over the past few years. We also review similar reactions reported by other groups. We focus here on the cyclization of functionalized propargylic compounds containing a carbon nucleophilic center that is in close proximity to the propargylic moiety. We conducted a detailed investigation of their cyclizations with carbon nucleophiles, with nitrogen nucleophiles, with oxygen nucleophiles, and without nucleophiles. We have developed several efficient and useful methods for the synthesis of indenes, naphthalenes, polycycles, and spirocyclic compounds. All of these reactions proceed satisfactorily under very mild conditions; high regio- and stereoselectivity have been observed as well. In the course of our studies, we provided the first demonstration of a novel tandem C-H activation/bis-cyclization reaction of propargylic compounds with terminal alkynes. In addition, we used external nucleophiles to investigate the cyclization of functionalized propargylic compounds that bear an unsaturated carbon-carbon or carbon-heteroatom bond. We presented the first report of the use of external nucleophiles to initiate a novel cyclization of functionalized propargylic compounds containing an electrophile. This revelation provided a fresh perspective through the discovery of a new type of domino cyclization of propargylic compounds. Metal-catalyzed cyclization of propargylic compounds can provide indenes, cyclopentanones, cyclic carbonates, benzofurans, and a range of other cyclic molecules. A thorough understanding of the mechanisms involved in this class of reaction affords exceptional synthetic control, as shown here by our development of efficient procedures and reagents for palladium-catalyzed propargylic cyclizations.