How Electrophilic Are Cobalt Carbonyl Stabilized Propargylium Ions?

BiCl3-Catalyzed propargylic substitution reaction of propargylic alcohols with C-, O-, S- and N-centered nucleophiles

A general and efficient BiCl3-catalyzed substitution reaction of propargylic alcohols with carbon and heteroatom-centered nucleophiles such as allyl trimethylsilane, alcohols, aromatic compounds, thiols and amides, leading to the construction of C-C, C-O, C-S and C-N bonds, has been developed.

Ranking the Reactivity of Superelectrophilic Heteroaromatics on the Electrophilicity Scale

The kinetics of the reactions of a series of reference carbon nucleophiles, consisting of N-methylpyrrole A, indole B, N-methylindole C, and enamines D-G, with 10 electron-deficient aromatic and heteroaromatic substrates (1-10), resulting in the formation of stable anionic sigma-adducts, have been investigated in acetonitrile at 20 degrees C. It is shown that the second-order rate constants k(1) pertaining to the carbon-carbon coupling step of these processes fit nicely the three-parameter equation log k (20 degrees C) = s(N + E), allowing the determination of the electrophilicity parameters E of 1-10 and therefore the ranking of these neutral electron-deficient compounds on the comprehensive electrophilicity scale defined for cationic electrophiles by Mayr et al. (Mayr, H.; Kempf, B,; Ofial, A. R. Acc. Chem. Res. 2003, 36, 66). The E values of 1-10 are found to cover a range from -13 to -5, going from 1,3,5-trinitrobenzene 1, the least reactive molecule, to 4,6-dinitrotetrazolo[1,5-a]pyridine 8, 4-nitro-6-trifluoromethanesulfonylbenzofuroxan 3, and 4,6-dinitrobenzofuroxan 2, the three most reactive heterocycles. Of major interest is that the E value of 2 is essentially the same as that for 4-nitrobenzenediazonium cation (E = -5.1), approaching that of the tropylium cation family (E approximately -3 to -6) as well as a number of metal-coordinated carbenium ions. Such a ranking holds promise for expanding the range of coupling reactions which can be envisioned with such strongly electron-deficient neutral heteroaromatics as nitrobenzofuroxans and related compounds. Arguments are also given which exclude the possibility for the reactions studied to proceed via an electron-transfer mechanism.

Spontaneous generation and stereoselective coupling of Co2(CO)6-complexed propargyl radicals

[reaction: see text] The spontaneous generation and stereoselective coupling of Co(2)(CO)(6)-complexed propargyl radicals have been discovered. One- and two-step complementary methods (Method A: (1) HBF(4); (2) CH(2)Cl(2), 20 degrees C; Method B: Tf(2)O, CH(2)Cl(2), 20 degrees C) provided an easy access to synthetically useful d,l-3,4-diaryl-1,5-alkadiynes (de 74-98%).

Investigation of a stereoselective co-mediated rearrangement reaction

A stereocontrolled approach to alpha-alkyl beta-alkynyl cyclohexanones is reported through a Lewis acid mediated rearrangement reaction of enol ethers bearing an Co-alkyne moiety. The reaction proceeds with high levels of stereoselectivity in the presence of Ti- and B-Lewis acids to provide a range of alpha,beta-disubstituted cyclohexanones in high yield although the products are prone to epimerization at the alpha-position in the presence of the B-promoter system. The potential for an enantioselective variant of this process is outlined, and a rationale for the observed stereochemical trends and detailed structural analyses of the ketone products are described.

Pi-nucleophilicity in carbon-carbon bond-forming reactions

Which electrophiles react with which nucleophiles? The correlation log k(20 degrees Celsius) = s(E + N), in which electrophiles (carbocations, metal-pi-complexes, diazonium ions) are characterized by one (E) and nucleophiles are characterized by two parameters (N, s), proved to be applicable for a wide variety of electrophile-nucleophile combinations. Since the introduction of this correlation in 1994 (Angew. Chem., Int. Ed. Engl. 1994, 33, 938-957), numerous new reagents have been characterized, and in 2001 (J. Am. Chem. Soc. 2001, 123, 9500-9512), a new method of parametrization was proposed that facilitates a continuous extension of the data sets without the need for reparametrization of existing data. This Account adjusts the N and s parameters of all presently characterized pi-nucleophiles (arenes, alkenes, organometallics) to the new parametrization and illustrates how to employ the resulting reactivity scales for analyzing synthetic and mechanistic problems in organic and macromolecular chemistry. Predictions of absolute rate constants, inter- and intramolecular selectivities, and analyses of reaction mechanisms are discussed. We outline how new compounds can be added to the scales and present our view on the scope and limitations of this approach to polar organic reactivity.

Ruthenium-catalyzed propargylation of aromatic compounds with propargylic alcohols

A novel ruthenium-catalyzed propargylation of aromatic compounds with propargylic alcohols has been found to afford the corresponding propargylated aromatic products in good yields with complete regioselectivity. The catalytic reaction provides a potential usefulness for practical application in organic synthesis, because the selective propargylation of aromatic compounds with an aromatic C-H bond cleavage is generally difficult.

Relationships between carbocation stabilities and electrophilic reactivity parameters, E: quantum mechanical studies of benzhydryl cation structures and stabilities

Quantum chemical calculations of the affinities of benzhydryl cations (XC(6)H(4))(2)CH(+) for the methyl anion, hydroxide, and hydride anion have been performed up to the B3LYP/6-311++G(3df,2pd)//B3LYP/6-31G(d,p) level and compared with rate and equilibrium constants in solution. An excellent linear correlation between the empirical electrophilicity parameter E (by log k = s(N + E); Mayr, H.; Bug, T.; Gotta, M. F.; Hering, N.; Irrgang, B.; Janker, B.; Kempf, B.; Loos, R.; Ofial, A. R.; Remennikov, R.; Schimmel, H. J. Am. Chem. Soc. 2001, 123, 9500-9512) and the calculated methyl anion affinities ranging over 46.5 kcal mol(-1) is found that reproduces the electrophilicity parameters E which range from -10 to +6 with a standard deviation of +/-0.26 (11 points). The calculated OH(-) and H(-) affinities correlate with the calculated CH(3)(-) affinities with a slope of 1.00, indicating that the relative anion affinities of benzhydrylium ions are independent of the nature of the reference base. Linear correlations of the experimental pK((R)(+)) values and chloride ion affinities in solution with the calculated anion affinities in the gas phase indicate that solvation attenuates the differences of carbocation stabilization in the gas phase but does not affect the relative differences. Application of Marcus theory shows that only reaction series with alpha = partial differential DeltaG(++)/ partial differential Delta(r)G degrees = 0.5 corresponding to a slope parameter of s = 0.67 can have constant intrinsic barriers. The slope parameters s found for pi-nucleophiles and C-H hydride donors (s approximately equal 1) are interpreted by a decrease of the intrinsic barriers with increasing electrophilicities of the carbocations. On the other hand, a value of s < or = 0.67 as found for many n-nucleophiles as well as for Si-H, Ge-H, or Sn-H hydride donors is indicative of intrinsic barriers which are constant or slightly increase with increasing electrophilicity of the carbocation.

Reference scales for the characterization of cationic electrophiles and neutral nucleophiles

Twenty-three diarylcarbenium ions and 38 pi-systems (arenes, alkenes, allyl silanes and stannanes, silyl enol ethers, silyl ketene acetals, and enamines) have been defined as basis sets for establishing general reactivity scales for electrophiles and nucleophiles. The rate constants of 209 combinations of these benzhydrylium ions and pi-nucleophiles, 85 of which are first presented in this article, have been subjected to a correlation analysis to determine the electrophilicity parameters E and the nucleophilicity parameters N and s as defined by the equation log k(20 degrees C) = s(N + E) (Mayr, H.; Patz, M. Angew. Chem., Int. Ed. Engl. 1994, 33, 938-957). Though the reactivity scales thus obtained cover more than 16 orders of magnitude, the individual rate constants are reproduced with a standard deviation of a factor of 1.19 (Table 1). It is shown that the reactivity parameters thus derived from the reactions of diarylcarbenium ions with pi-nucleophiles (Figure 3) are also suitable for characterizing the nucleophilic reactivities of alkynes, metal-pi-complexes, and hydride donors (Table 2) and for characterizing the electrophilic reactivities of heterosubstituted and metal-coordinated carbenium ions (Table 3). The reactivity parameters in Figure 3 are, therefore, recommended for the characterization of any new electrophiles and nucleophiles in the reactivity range covered. The linear correlation between the electrophilicity parameters E of benzhydryl cations and the corresponding substituent constants sigma(+) provides Hammett sigma(+) constants for 10 substituents from -1.19 to -2.11, i.e., in a range with only very few previous entries.