Retro-Wittig decomposition

According to quantum-chemical calculations, decompositions of two last types are possible for betaines of type II (see Section 6). Retro-Wittig decomposition (ii) is the process inverse to their formation. The direction (iii) resulting in the formation of elementaolefins is much more interesting (Scheme 20). 

C5D5N above 80 °C, it gains the characteristic color of phosphorus ylide, and signals of phosphorus ylides and cyclosilathianes appear in the H, 13C, and 31P NMR spectra. In the presence of an equivalent amount of benzo-phenone in this solution, 1,1-dimethyl-2,2-diphenylethylene and Ph3PO are formed in 53% yield at 100 °C for 10min. This indicates that the retro-Wittig decomposition of 20a occurs in the solution (Scheme 23, equilibrium a). Probably, phosphorus ylide is also formed in the equilibrium bimolecular reaction between two betaine molecules (Scheme 23 equilibrium b). The ratio of the contributions of these two reactions is strongly determined by the solvent and temperature. 

Reductive decoupling, 181, 187 Reductive elimination, 171-174, 190-197 refined catalytic cycle, 206-211 and selectivity, 212-214 steric and electronic factors, 202-205, 217 Retro-Wittig decomposition, 37, 78, 79-83 thiobetaines, 57-58, 60, 65 Ring cleavage, 105, 276, 281 Ring closure in oligomerization, 172, 174, 190-197... 

This accounts for the considerable discrepancy between the alkene Z/E ratio found on work-up and the initial oxaphosphetan ais/trans ratio. By approaching the problem from the starting point of the diastereomeric phosphonium salts (19) and (20), deprotonation studies and crossover experiments showed that the retro-Wittig reaction was only detectable with the erythreo isomer (19) via the cis-oxaphosphetan (17). Furthermore, it was shown that under lithium-salt-free conditions, mixtures of (19) and (20) exhibited stereochemical drift because of a synergistic effect (of undefined mechanism) between the oxaphosphetans (17) and (18) during their decomposition to alkenes. 

As mentioned above (see Scheme 1), three main directions of the decomposition of intermediates that formed are possible when phosphorus and arsenic ylides react with compounds bearing C=X bonds 5,6,19,63,64,88 (i) elimination of R3E15=X to form olefins (Wittig type reaction) (ii) retro-Wittig type decomposition and (iii) elimination of R3E15 and formation of three-membered cycles (Corey-Chaykovsky type reaction). According to the data of Erker and coworkers,12,13,51 under kinetic control, the reaction of phosphorus ylides with thiocarbonyl compounds also affords phosphines and thiiranes, whose further transformations lead to olefins and R3PS under thermodynamic control. 

Available experimental data suggest that the decomposition of betaines I occurs via direction (iii) with R3E15 elimination giving three-membered heterocycles or via retro-Wittig type (ii) to eliminate R3E15=CR1R2 leading to the compounds with an E14=X bond (Scheme 19). 

Thermolysis of betaines 20 is not so selective as their photolysis.89 Decomposition of the Corey-Chaykovsky mode, which is predominant under irradiation, occurs in parallel with retro-Wittig type fragmentation. 

Among four main directions of the chemical transformations of betaines of the first group, which are presented in Scheme 38, isomerization to metallated ylides (direction A), retro-Wittig type decomposition (direction... 

However, the Me2E14=X (E14 = Si, Ge, Sn X = 0, S, NMe, C5H4) compounds formed during the retro-Wittig type decomposition are... 

As can be seen from the data presented, the high energies of complex formation decrease sharply the endothermicity of the retro-Wittig type decomposition and, moreover, fundamentally change the reaction mechanism. As has been shown for betaines (")X-E14Me2-CH2-E15( + )Me3 (X = S, Se E14 = Si, Ge E14 = P, As), the reaction occurs as bimolecular nucleophilic substitution at the E14 atom. For silicon betaines, the transition states TS-b-pyr with pentacoordinate silicon and nearby them no deep local minima corresponding to the C-b complexes can be localized in the reaction coordinate. 

Both of the above teehniques can be reinforeed by erossover experiments (method C) (12-14,20,2Ic,22,23a). An excess of CIC H CHO (ArCHO) is added to the solution below the temperature for oxaphosphetane decomposition. If stereochemical equilibration occurs exclusively by a retro-Wittig process to give the ylide 33, then an excess of the crossover aldehyde must produee the crossover products. Since 33 would be intercepted by excess ArCHO faster than it can recombine with the original aldehyde, the conversion from one oxaphosphetane diastereomer into the other (i.e., from 31 to 32) by way of any retro-Wittig mechanism will be suppressed using method C. However, it is essential to prove that the oxaphosphetane has not already decomposed prior to the addition of the crossover aldehyde. Otherwise, there is the risk of a false-negative crossover result. 

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