Carbene | EPFL Graph Search

In organic chemistry, a carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons. The general formula is or where the R represents substituents or hydrogen atoms. The term "carbene" may also refer to the specific compound , also called methylene, the parent hydride from which all other carbene compounds are formally derived. Carbenes are classified as either singlets or triplets, depending upon their electronic structure. Most carbenes are very short lived, although persistent carbenes are known. One well-studied carbene is dichlorocarbene , which can be generated in situ from chloroform and a strong base. Structures and bonding The two classes of carbenes are singlet and triplet carbenes. Singlet carbenes are spin-paired. In the language of valence bond theory, the molecule adopts an sp2 hybrid structure. Triplet carbenes have two unpaired electrons. Most carbenes have a nonlinear triplet ground state, except f

New Vinylation and Alkynylation Strategies with Hypervalent Iodine Reagents and Diazo Compounds

Guillaume Dominique Pisella

The development of a sequential copper-catalyzed oxy-alkynylation/intramolecular [4+2] cycloaddition of allenes and arenes was investigated at first. This one-pot protocol allowed the construction of complex polycylic architectures with high efficiency from aryldiazo esters and ethynylbenziodoxolone reagents (EBX). The mild reaction conditions (23 to 90 Â°C) for the dearomatization step was unusual compared to previous literature reports and DFT computations showed that substituents on the aryl ring resulted in favorable dispersive interactions, decreasing the activation energy barrier of the cycloaddition. The bicyclo[2.2.2]octadiene products were successfully applied as ligands for rhodium. The prepared diene-rhodium complexes were extensively characterized by NMR spectroscopy and X-ray crystallography. Finally, an enantiopure ligand having a pseudo C2-symmetry was used in the conjugate addition of phenylboronic acid to cyclohexenone and furnished the Î²-functionalized ketone in good yield and with high enantioselectivity.The vinylation reactions of diazo compounds are scarce. A successful copper-catalyzed vinylation of diazo compounds with vinylbenziodoxolone reagents (VBX) as partners was reported. The transformation allowed an unprecedented gem-oxy-vinylation at the carbene center, providing functionalized Î±-vinyl-hydroxyacid derivatives in high atom and step economy. The products were obtained in excellent yields and contained synthetically versatile functional groups for post-modifications. The development of an enantioselective version of this reaction was limited and further investigations will be required to obtain high enantioselectivities. In addition, a practical synthesis of alkyl-substituted VBX reagents has been established; VBX reagents being limited to aryl substituents until then. Multicomponent reactions provide efficient means to rapidly access molecular diversity. A copper(I)-catalyzed threeâcomponent reaction utilizing diazo compounds, alcohols as nucleophiles and vinyl- or ethynylbenziodoxole reagents, was successfully developed. The transformation allowed the synthesis of highly functionalized allyl and propargyl ethers. The scope is broad, and extensive variations of the three partners of the reaction is possible, leading to maximal structural diversity. Extension of this work to N-nucleophiles was not as straightforward as anticipated, but good results could be obtained with anilines, trifluorodiazoethane and EBX reagents as components.Questions about the reaction mechanisms of the copper-catalyzed difunctionalizations of diazo compounds with VBX and EBX reagents have been frequent all-along this research work. Preliminary results of in-depth mechanistic investigations have been obtained. The initial assumption of an internal alkyne transfer step was first invalidated by DFT computations and was latter refuted experimentally. A complexation between the carboxylate of ethynylbenziodoxolone reagents and the copper catalyst was characterized by NMR spectroscopy and contrasted to a alkyne-copper interaction observed with ethynylbenziodoxole reagents. These two possible complexes were supported by DFT calculations. Finally, non-classical sigmoidal kinetic profiles of the oxy-alkynylation reaction were obtained by in situ 19F NMR monitoring and the Cu-EBX complex is presumably involved in the observed induction periods.

EPFL2021

PX4+, P2X5+, and P5X2+ (X = Br, I) salts of the superweak Al(OR)4- anion [R = C(CF3)3]

Lutz Müller, Ines Raabe

PX4+[Al(OR)4]- (X = I: 1a, X = Br: 1b) was prepd. from X2, PX3, and Ag[Al(OR)4] [R = C(CF3)3] in CH2Cl2 at -30 Deg in 69-86% yield. P2X5+ salts were prepd. from 2PX3 and Ag[Al(OR)4] in CH2Cl2 at -30 Deg yielding almost quant. P2X5+[Al(OR)4]- (X = I: 3a, X = Br: 3b). The P-rich P5X2+ salts arose from the reaction of cold (-78 Deg) mixts. of PX3, P4, and Ag[Al(OR)4] giving P5X2+[Al(OR)4]- (X = I: 4a, X = Br: 4b) with a C2v-sym. P5 cage. Silver salt metathesis presumably generated unstable PX2+ cations from PX3 and Ag[Al(OR)4] (X = Br, I) that acted as electrophilic carbene analogs and inserted into the X-X (P-X/P-P) bond of X2(PX3/P4) leading to the highly electrophilic and CH2Cl2-sol. PX4+ (P2X5+/P5X2+) salts. Reactions that aimed to synthesize P2I3+ from P2I4 and Ag[Al(OR)4] instead led to anion decompn. and the formation of P2I5(CS2)+[(RO)3Al-F-Al(OR)3]- (5). All salts were characterized by variable-temp. soln. NMR studies (3b also by 31P MAS NMR), Raman and/or IR spectroscopy as well as x-ray crystallog. (with the exception of 4a). The thermochem. vols. of the P-X cations are 121 (PBr4+), 161 (PI4+), 194 (P2Br5+), 271 (P2I5+), and 180 .ANG.3 (P5Br2+). The obsd. reactions were fully accounted for by thermochem. calcns. based on (RI-)MP2/TZVPP ab initio results and COSMO solvation enthalpy calcns. (CH2Cl2 soln.). The enthalpies of formation of the gaseous P-X cations were derived as +764 (PI4+), +617 (PBr4+), +749 (P2I5+), +579 (P2Br5+), +762 (P5I2+), and +705 kJ mol-1 (P5Br2+). The insertion of the intermediately prepd. carbene analog PX2+ cations into the resp. bonds were calcd., at the (RI-)MP2/TZVPP level, to be exergonic at 298 K in CH2Cl2 by DrG(CH2Cl2) = -133.5 (PI4+), -183.9 (PBr4+), -106.5 (P2I5+), -81.5 (P2Br5+), -113.2 (P5I2+), and -114.5 kJ mol-1 (P5Br2+). [on SciFinder (R)]

2002

New Vinylation and Alkynylation Strategies with Hypervalent Iodine Reagents and Diazo Compounds

Guillaume Dominique Pisella

EPFL2021