Stille coupling Variations

Recent developments in Stille coupling Variations in Stille coupling Suzuki coupling... 

The Stille coupling involves heating a halide, a stannane and a catalyst in a suitable solvent, such as toluene or DMF. No base is required. The conditions are relatively straightforward, with little overall variation, apart from the catalyst. However, if a triflate is used instead of a halide, the reaction may not succeed, as transmetallation of aryltin compounds with arylpalladium triflates is often difficult. Addition of a halide source, such as lithium chloride, usually solves this problem as it allows the formation of the arylpalladium chloride, which can undergo transmetallation. 

Prakash and Thompson used monodisperse poly(4- and poly(2-vinylpyridine) nanospheres as stabilisers to support 1-4 nm Pd nanoparticles in a study concerning the Heck, Suzuki and Stille couphng reactions [14]. The material was found to be very active. However, the reactions were Umited to the couphng of the highly reactive 4-bromo-nitrobenzene with, respectively, butyl acrylate, phenylboronic acid and phenyl-trimethylstannane. No noticeable influence of the nature of the stabilisers was reported in this study. The authors argue that the material was stable under the reaction conditions, but this conclusion is only based on macroscopic TEM analyses. In view of the low Pd concentration which was required to perform effective Heck, Suzuki or Stille coupling reactions, it is not surprising that no major variation of the Pd particle size was observed. 

Ketones can be synthesized by a variation of the Stille coupling that involves coupling in the presence of carbon monoxide. The following reaction is an example. 

The variations and improvements of the Stille coupling are numerous in literature and each of the parameters that can potentially influence the mechanism of the reaction has been studied. In this paragraph, we do not have the pretension to give an exhaustive list of the numerous modifications that have been published but instead we will focus on presenting a representative set of examples. This will allow emphasis on the logical path taken to bring the reaction to where it stands now, 30 years after its discovery. 

Coupled cluster calculations give variational energies as long as the excitations are included successively. Thus, CCSD is variational, but CCD is not. CCD still tends to be a bit more accurate than CID. 

Two general procedures have been used to obtain AH values. The first involves the measurement of log K values over a range of temperatures the observed variation may be used to derive the required AH value. However, because of the usual errors inherent in log K determinations coupled with the limited temperature range normally possible, AH values obtained in this manner tend to be somewhat unreliable. In contrast, the direct determination of AH using calorimetry commonly results in values which are considerably more accurate. Nevertheless, such calorimetric determinations may still not be easy for particular macrocyclic systems. Difficulties can arise in measuring the total heat evolved for metal complexation when long equilibration times are necessary. To lessen such problems, sensitive calorimeters have been used which are able to integrate the heat released over an extended time. 

The significant relative mass difference (c. 16%) between the two stable isotopes of Li (approximately Li 7.5%, Li 92.5%), coupled with broad elemental dispersion in Earth and planetary materials, makes this a system of considerable interest in fingerprinting geochemical processes, determining mass balances, and in thermometry. Natural mass fractionation in this system is responsible for c. 6% variation among materials examined to date (Fig. 1). Although the modem era of Li isotope quantification has begun, there are still many questions about the Li isotopic compositions of fundamental materials and the nature of fractionation by important mechanisms that are unanswered (e.g., Hoefs 1997). 

The scheme we employ uses a Cartesian laboratory system of coordinates which avoids the spurious small kinetic and Coriolis energy terms that arise when center of mass coordinates are used. However, the overall translational and rotational degrees of freedom are still present. The unconstrained coupled dynamics of all participating electrons and atomic nuclei is considered explicitly. The particles move under the influence of the instantaneous forces derived from the Coulombic potentials of the system Hamiltonian and the time-dependent system wave function. The time-dependent variational principle is used to derive the dynamical equations for a given form of time-dependent system wave function. The choice of wave function ansatz and of sets of atomic basis functions are the limiting approximations of the method. Wave function parameters, such as molecular orbital coefficients, z,(f), average nuclear positions and momenta, and Pfe(0, etc., carry the time dependence and serve as the dynamical variables of the method. Therefore, the parameterization of the system wave function is important, and we have found that wave functions expressed as generalized coherent states are particularly useful. A minimal implementation of the method [16,17] employs a wave function of the form ... 

---