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SCF+CI calculations are performed to investigate the potential energy surface associated with the thermal C-C ring opening of oxirane (CH 2 OCH 2 ). Conrotatory, disrotatory and non-synchronous processes are studied, and the results discussed with respect to experimental data (Huisgen and toll.) which indicate a loss of stereoselectivity in the ring opening reactions of some substituted oxirane molecules.

Resume L'influence des substituants alkyles sur l'acidite des alcools et des amines est traitee par une methode perturbationnelle. L'effet d'un substituant alkyle peut se decomposer en deux termes : un terme repulsif qui est plus grand dans les molecules neutres (alcools, amines) que dans les bases conjuguees correspondantes (RO − , RNH − ), un effet attractif qui est important uniquement dans les anions. On montre de plus que, meme si l'on ne tient pas compte des effets steriques, la solvatation augmente l'acidite des molecules non substituees plus que celle des molecules substituees. On peut ainsi rationaliser l'ordre d'acidite en phase gazeuse et en solution. “L'effet inductif” des groupes alkyles est egalement discute. On montre qu'un groupe alkyle peut etre attracteur d'electrons meme attache a un atome electronegatif. Un raisonnement analogue permet de rationaliser les ordres de basicites des alcools et des amines en phase gazeuse et en solution. Le traitement perturbationnel est confirme par des calculs ab initio (STO-3G).

Organic and organometallic haptotropic shifts in cyclopentadienyl compounds CpX are analyzed by studying the motion of an X+ across the face of a C5H5- ring. The ligand in transit, X+, may be H+, CH3+, XH3+ (X = Si, Ge, Sn), Mn(CO)3+, Li+, CUR+, and CH22+ and CH2, the latter serving as models for BR2+, AIR2+ and NR2+, PR2+ as well. The Cp- offers donor orbitals of a + e symmetry. The interactions of the ligand may be dominated by an acceptor orbital of symmetry type a (H+, CH3+) in which case 7' and q2 geometries are favored, or a set of acceptor orbitals of a + e symmetry (MII(CO)~+, Li+, CUR+), the e component of which greatly stabilizes the most symmetrical q5 coordination. The lower the energy of the e acceptor set and the better the overlap with its Cp counterpart, the more are q2 and q5 stabilized relative to 7'. In the case of XH3+ (X = C, Si, Ge, Sn) an e acceptor orbital moves to lower energy as one proceeds down the group, and this is responsible for the decreasing barrier for sigmatropic shifts in CpXH3. The XH2 case, which yields a number of interesting collapse struc- tures, is analyzed in detail for X, a main group center.

Resume It is shown that, given a diene substituted with a donor D and a dienophile substituted with an attractor A. the reaction rate will generally decrease when the addends exchange their substituents, i.e. A going to the diene and D to the dienophile. Donor substituents are more effective in raising an HOMO than an attractor in lowering a LUMO of a conjugated substrate. It follows that for thermal symmetry-allowed cycloadditions between acyclic systems, the greatest rate increase is expected when the donor substituent is put on the compound with the higher HOMO (usually the more conjugated compound) and the attractor on the compound with the lower HOMO. This does not mean “put the donor on the donor and the attractor on the attractor”: butadiene is simultaneously a better donor and a better acceptor than ethylene. Some kinetic effects of bridging groups in cyclic systems are discussed.

Germyl- and silyl-phosphines R3MPR′2(M = Ge, Si) generally add in 1-4 position to α-ethylenic aldehydes and ketones with formation of phosphorus alkenoxy-germanes or -silanes. The α-ethylenic aldehydes always lead to 1-4 Z and E adducts (Z preponderant), α-ethylenic ketones exclusively to the Z isomer. This particular stereochemistry is explained by a mechanism involving initial nucleophilic attack of the metal phosphine on carbon 4, steric hindrance of the reaction intermediate and participation of a 6-center pseudo-cyclic complex. On the contrary 1-2 addition on the carbonyl group of the 9-fluorenone is exculsively observed. The orientation of the additions (1–2 or 1–4) is interpreted in terms of Pearson (HSAB) theory.

The most usual difficulties encountered in the study of frontier-controlled reactions by approximate methods (aromaticity rules, frontier-orbital approximation and correlation diagram) are reviewed.

Es werden die Gruppen Wasser-Methanol und Ammoniak-Methylamin sowie ihre entsprechenden Anionen gegenubergestellt.

Application of the “hard and soft” concept to the Woodward–Katz model allows simple and accurate prediction of the structure of the major product in Diels–Alder reactions.