High-order Time Expansion Path Integral Ground State

The feasibility of path integral Monte Carlo ground state calculations with very few beads using a high-order short-time Green's function expansion is discussed. An explicit expression of the evolution operator which provides dramatic enhancements in the quality of ground-state wave-functions is examined. The efficiency of the method makes possible to remove the trial wave function and thus obtain completely model-independent results still with a very small number of beads. If a single iteration of the method is used to improve a given model wave function, the result is invariably a shadow-type wave function, whose precise content is provided by the high-order algorithm employed.Comment: 4 page

Critical behavior of dissipative two-dimensional spin lattices

We explore critical properties of two-dimensional lattices of spins interacting via an anisotropic Heisenberg Hamiltonian and subject to incoherent spin flips. We determine the steady-state solution of the master equation for the density matrix via the corner-space renormalization method. We investigate the finite-size scaling and critical exponent of the magnetic linear susceptibility associated to a dissipative ferromagnetic transition. We show that the Von Neumann entropy increases across the critical point, revealing a strongly mixed character of the ferromagnetic phase. Entanglement is witnessed by the quantum Fisher information which exhibits a critical behavior at the transition point, showing that quantum correlations play a crucial role in the transition even though the system is in a mixed state.Comment: Accepted for publication on Phys. Rev. B (6 pages, 5 figures

Primordial Helium Production in a Charged Universe

We use the constraints arising from primordial nucleosynthesis to bound a putative electric charge density $|e|n_q$ of the universe. We find $|n_q/n_\gamma| \lesssim 10^{-43}$, four orders of magnitude more stringent than previous limits. We also work out the bounds on $n_q$ in models with a photon mass, that allows to have a charge density without large-scale electric fields

Asymmetric Synthesis of Secondary Alcohols and 1,2-Disubstituted Epoxides via Organocatalytic Sulfenylation

Enantioenriched secondary alcohols can be prepared via a short reaction sequence involving asymmetric organocatalytic sulfenylation of an aldehyde, organometallic addition, and desulfurization. This process provides access to enantioenriched alcohols with sterically similar groups attached to the alcohol carbon atom. The intermediate β-hydroxysulfides can also serve as precursors to enantioenriched 1,2-disubstituted epoxides via alkylation of the sulfur and subsequent base-mediated ring closure

Catalytic electrophilic halogenation of silyl-protected and terminal alkynes: trapping gold(I) acetylides vs. a bronsted acid-promoted reaction

In the presence of a cationic gold(I) catalyst and N-halosuccinimide, both trimethylsilyl-protected and terminal alkynes are converted into alkynyl halides. Further experiments showed that silyl-protected alkynes undergo electrophilic iodination and bromination under Brønsted acid catalysis, whilst terminal alkynes require a cationic gold catalyst. The former reactions probably proceed via activation of the electrophile, whilst the latter reactions proceed via a gold(I) acetylide intermediate. Gold-catalysed halogenation was further combined with gold-catalysed hydration and subsequent annulation to provide convenient routes to iodomethyl ketones and five-membered aromatic heterocycles

Novel approaches to medium rings, enantiomerically enriched alcohols and haloalkynes

Medium rings (8-12 atoms) are present in many naturally occurring molecules which in many cases have potentially useful biological activity. Unfortunately current synthetic methods to synthesise medium rings lack general applicability or require high dilution and/or tedious optimisation procedures. This thesis describes the investigation of a novel strategy in which a suitably functionalised linear precursor undergoes double cyclisation and fragmentation to afford a medium ring. The key to this proposed sequence is the generation of an intramolecular cyclic ylide which can react with a pendant electrophile generating a bicyclic intermediate that subsequently undergoes fragmentation, thus generating a medium ring. The use of sulfur ylides was initially investigated for the synthesis of the 8-membered ring natural product Cephalosporolide-D but all attempts to trigger the proposed sequence of events were unsuccessful leading only to side reactions or decomposition. Alternative metal catalysed approaches to the synthesis of sulfur ylides were also investigated with no success. Nitrogen and phosphorus cyclic ylides were also evaluated and the latter reaction gave small quantities of the target medium ring product. This thesis also describes the discovery that high levels of diastereoselectivity could be achieved by the addition of carbon-nucleophiles to α-sulfenylaldehydes, intermediates in the synthesis of precursors to medium rings. Enantiomerically enriched secondary alcohols could be subsequently obtained from the β-hydroxysulfides after Raney-Ni reduction of the carbon-sulfur bond. In combination with the studies on metal catalysed generation of ylides, this thesis reports a gold(I) catalysed mild procedure for the synthesis of haloalkynes from either terminal alkynes or trimethylsilylacetylenes initially discovered in our laboratories. Interestingly it was found that protic acids were able to catalyse the halogenation of trimethylsilylacetylenes but not of terminal alkynes. The reactions were successfully extended to a series of aromatic and aliphatic alkynes. Procedures were also developed for the conversions of terminal alkynes into α-iodoketones which were also used in situ for the synthesis of heterocycles