The structures of the lead halide perovskites CsPbCl3 and CsPbBr3 have been determined from X-ray powder diffraction data to be orthorhombic with Pnma space group symmetry. Their structures are distorted from the cubic structure of their hybrid analogs, CH3NH3PbX3 (X = Cl, Br), by tilts of the octahedra (Glazer tilt system a – b + a –). Substitution of the smaller Rb+ for Cs+ increases the octahedral tilting distortion and eventually destabilizes the perovskite structure altogether. To understand this behavior, bond valence parameters appropriate for use in chloride and bromide perovskites have been determined for Cs+, Rb+, and Pb2+. As the tolerance factor decreases, the band gap increases, by 0.15 eV in Cs1–x Rb x PbCl3 and 0.20 eV in Cs1–x Rb x PbBr3, upon going from x = 0 to x = 0.6. The band gap shows a linear dependence on tolerance factor, particularly for the Cs1–x Rb x PbBr3 system. Comparison with the cubic perovskites CH3NH3PbCl3 and CH3NH3PbBr3 shows that the band gaps of the methylammonium perovskites are anomalously large for APbX3 perovskites with a cubic structure. This comparison suggests that the local symmetry of CH3NH3PbCl3 and CH3NH3PbBr3 deviate significantly from the cubic symmetry of the average structure.

The rearrangement of allylic trichloroacetimidates is a well-known transformation, but the corresponding rearrangement of benzylic trichloroacetimidates has not been explored as a method for the synthesis of benzylic amines. Conditions that provide the trichloroacetamide product from a benzylic trichloroacetimidate in high yield have been developed. Methods were also investigated to transform the trichloroacetamide product directly into the corresponding amine, carbamate and urea. A cationic mechanism for the rearrangement is implicated by the available data.

Alcohols are effectively converted to their corresponding diphenylmethyl (DPM) ethers by reaction with O-diphenylmethyl trichloroacetimidate in refluxing toluene without the requirement of a catalyst or other additive. A number of acid and base sensitive substrates were protected in excellent yield using this new method without disturbing pre-existing functionality present in these molecules. This reaction is the first example of the formation of an ether from stoichiometric amounts of a trichloroacetimidate and an alcohol without the addition of a Brønsted or Lewis acid catalyst.

Diphenylmethyl trichloroacetimidate is a useful reagent for the protection of carboxylic acids as their corresponding diphenylmethyl esters. These esterifications proceed rapidly without the need for an added catalyst or promoter. A variety of carboxylic acid substrates undergo esterification in excellent yields with the trichloroacetimidate reagent, including substrates possessing acid-or base-sensitive functionality. Protection of a carboxylic acid with a highly enolizable α-stereocenter using diphenylmethyl imidate was also accomplished without racemization.Esters are common protecting groups for carboxylic acids and are often utilized in multistep organic synthesis. A popular choice is the diphenylmethyl (DPM) ester, as it can be removed using aqueous base, by hydrogenation, or under acidic conditions. 1 This flexibility is advantageous, as the different conditions provide options should incompatibilities with other functional groups be encountered during removal. Diphenylmethyl esters are also popular because they do not introduce new stereogenic centers or overly complicate NMR spectra. For these reasons, the diphenylmethyl protecting group has been used extensively in the synthesis of peptides, 2 β-lactam antibiotics 3 and complex natural products. 4Formation of diphenylmethyl esters with simple substrates is typically effected using acid catalysis with diphenylmethanol, 5 however, these conditions do not tolerate complex substrates with delicate functionality. With sensitive substrates, diphenylmethyl esters are typically prepared by exposing the carboxylic acid to diphenyldiazomethane. 6 The unstable and toxic nature of this reagent 7 has led to a number of surrogate reactions based on the in situ formation of diphenyldiazomethane from diphenylmethyl hydrazone. Unfortunately, these procedures typically depend on strong oxidizing reagents or environmentally hazardous metal salts. 3b,7a,8 The scope of these esterifications is also restricted due to the powerful oxidizing conditions required to form the diphenyldiazomethane. Given the limitations associated with known methods, especially with carboxylic acids possessing sensitive functionality, esterification reagents that show improved convenience, practicality and safety are desirable. Ideally, any new reagent for the generation of diphenylmethyl esters would: 1) form the ester under mild conditions at room temperature, 2) not require stoichiometric amounts of toxic metal salts or powerful oxidizing agents, 3) react selectively with the carboxylic acid without disturbing other functionality in the molecule, and 4) be relatively stable with a long shelf-life.Recently, we began to evaluate alternatives for the formation of diphenylmethyl esters under mild conditions from the corresponding trichloroacetimidate. Trichloroacetimidates are excellent alkylating agents under acidic conditions, as the alkylation is driven by the loss of the imidate and the formation of trichloroacetamide. This process provides a substantial thermodynamic driving force, facilitatin...

Convenient Formation of Diphenylmethyl Esters Using Diphenylmethyl Trichloroacetimidate. -Diphenylmethyl trichloroacetimidate is a useful reagent for the rapid protection of carboxylic acids. No racemization of highly enolizable -chiral acids occur. Mild deprotection can be achieved by hydrogenation in the presence of Pd-C. -(ADHIKARI, A. A.; SHAH, J. P.; HOWARD, K. T.; RUSSO, C. M.; WALLACH, D. R.; LINABURG, M. R.; CHISHOLM*, J. D.; Synlett 25 (2014) 2, 283-287, http://dx.