Y- bonds

The broken bonds (boldface= dissociated fragment) (boldface=recommended data reference in parentheses) Methods (reference in parentheses)  

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Both types of processes, 7r -assisted y, -bond cleavage and P -bonding, have been invoked to operate in the phototransformations of the aldehyde-ketone (153) to products (155), (156) and (158). The conversions have been observed at room temperature in dioxane, t-butanol, ethanol and benzene using light of wavelengths 2537 A or above 3100 A or sensitization by acetophenone. The phosphorescing excited triple state of (153) is very similar to that of testosterone acetate (114), but its reactions are too rapid... 

Ternary imiiiium salts have been prepared by cleavage of a covalent C—Y bond in a... 

If X-Y bond fission occurs, the product is a 6-coordinate iridium(III) complex (Table 2.6) otherwise a 5-coordinate (or pseudo-5-coordinate) adduct is obtained in which Ir formally retains the (+1) state (Table 2.7). This distinction can be somewhat artificial IrCl(02)C0(PPh3)2 can be regarded as an iridium(III) peroxo complex. 

A strictly entropically controlled tendency for statistical ligand distribution was discussed 150) for ligand exchange when the sum of the Sb—X and Sb—Y bond energies remains constant. Calculations show that due to the electronic interaction in the entire molecule an energetic tendency also exists to form Lewis acids with mixed ligand spheres ... 

Sn2 stands for substitution nucleophilic bimolecular. The lUPAC designation (p. 384) is AnDn- In this mechanism there is backside attack The nucleophile approaches the substrate from a position 180° away from the leaving group. The reaction is a one-step process with no intermediate (see, however, pp. 392-393 and 400). The C—Y bond is formed as the C—X bond is broken ... 

Both the Se2 (front) and Se2 (back) mechanisms are designated DeAe in the lUPAC system. With substrates in which we can distinguish the possibility, the former mechanism should result in retention of configuration and the latter in inversion. The reaction of allylsilanes with adamantyl chloride and TiCU, for example, gives primarily the antiproduct via a Se2 reaction. When the electrophile attacks from the front, there is a third possibility. A portion of the electrophile may assist in the removal of the leaving group, forming a bond with it at the same time that the new C—Y bond is formed... 

As is outlined for ene reactions of singlet oxygen in Scheme 15, the prototypical ene reaction starts with the electron delocalization from the HOMO of propene to the LUMO of X=Y. The delocalization from the HOMO, a combined n and orbital with larger amplitude on n, leads to a bond formation between the C=C and X=Y bonds. Concurrent elongation of the bond enables a six-membered ring transition stracture, where partial electron density is back-donated from the LUMO of X=Y having accepted the density, to an unoccupied orbital of propene localized on the bond. As a result, the partial electron density is promoted (pseudoex-cited) from the HOMO (it) to an unoccupied orbital (ct n ) of alkenes. This is a reaction in the pseudoexcitation band. 

Early experimental spectroscopic investigations on Rg- XY complexes resulted in contradictory information regarding the interactions within them and their preferred geometries. Rovibronic absorption and LIF spectra revealed T-shaped excited- and ground-state configurations, wherein the Rg atom is confined to a plane perpendicular to the X—Y bond [10, 19, 28-30]. While these results were supported by the prediction of T-shaped structures based on pairwise additive Lennard-Jones or Morse atom-atom potentials, they seemed to be at odds with results from microwave spectroscopy experiments that were consistent with linear ground-state geometries [31, 32]. Some attempts were made to justify the contradictory results of the microwave and optical spectroscopic studies, and... 

Starting from 63, the carbonylation may proceed via coordination and insertion of CO into the vinyl-C-Pd bond to provide an a,P-unsaturated acyl complex. This complex reacts with (ArY) 2, and subsequently the C-Y bond is formed by reductive elimination to give 64 (Scheme 7-14). Because the compound 64 could be directly converted into the corresponding enal 65 by the Pd-catalyzed reduction with BujSnH, this sequence is synthetically equivalent to the regio- and stereoselective thioformy-lation and selenoformylation of alkynes (Eq. 7.49) [53, 54]. 

The proposed mechanism (Scheme 7-20) includes (a) oxidative addition of Y-Ge (Y = S or Se) bonds to Pd(PPh3)n, (b) insertion of acetylene into the Pd-Y bond to give 92 or insertion of acetylene into the Pd-Ge bond to form 93, (c) formation of 91 by either a G-Ge or a G-Y bond-forming reductive ehmination with regeneration of Pd(PPh,),. 

This generalized reaction sequence consumes the halide, the stannane, and the reactant X=Y, and effects addition to the organic radical and a hydrogen atom to the X=Y bond. The order of reactivity of organic halides toward stannyl radicals is iodides > bromides > chlorides. 

Catalyst 70 is very effective for the reaction of terminal alkenes, however 1,1-disubstituted olefins provide hydrosilylation products presumably, this is due to steric hindrance [45]. When a catalyst with an open geometry (78 or 79) is employed, 1,1-disubstituted alkenes are inserted into C-Y bonds to give quaternary carbon centers with high diastereoselectivities (Scheme 18). As before, initial insertion into the less hindered alkene is followed by cyclic insertion into the more hindered alkene (entry 1) [45]. Catalyst 79 is more active than is 78, operating with shorter reaction times (entries 2 and 3) and reduced temperatures. Transannular cyclization was possible in moderate yield (entry 4), as was formation of spirocyclic or propellane products... 

Three different, simple mechanisms can be envisaged for 1,2-eliminations, differing from each other in the timing of H—C and C—Y bond-breaking. This could (a) be concerted,... 

The observed order of ability did not, however, correlate with the P Ka of YH, with the strength of the C—Y bond, or with the polar effect of Y Clearly, leaving group ability even in this simple reaction is a highly complex attribute. 

The El/S l ratio is, of course, substantially independent of the leaving group Y, but this is not the case with E2/Sjv2, where breaking of the C—Y bond is involved in each alternative T.S. The following rough sequence, in order of increasing promotion of elimination, is... 

Saal, D., Dong, Y., Bond, A., Malenka, R.C. Drugs of abuse and stress trigger a common synaptic adaptation in dopamine neurons. Neuron. 37 577, 2003. 

There is considerable multiple character in the carbene carbon-X(Y) bonds. An internal comparison of carbon-X(Y) bond lengths in each of the structures tabulated reveals a substantial shortening of carbene car-bon-X(Y) relative to other carbon-X(Y) bonds in the molecule. Carbene carbon-nitrogen distances are found to be shorter in the mono-N-substituted carbenes than in the bis-N-substituted compounds. 

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