Crossover, stereochemical

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. 

The stereospecificity of the reactions of the trans- and cis-propenyl complexes (52b) and (55) with cyclopentadiene has been investigated. The rrans-propenyl complex (52b) affords the endo and exo adducts as a 90 10 mixture, in which the trans stereochemistry present in the carbene complex is completely retained in both cycloadducts. As a point of comparison, the reaction of /rons-methyl crotonate with cyclopentadiene gives a 54 46 endo exo ratio in the thermal reaction and a 93 7 ratio in the aluminum chloride mediated reaction. The stereospecificity of these reactions was established when it was found that the cis-propenyl carbene complex (55) was found to give the ci s-derived endo and exo adducts (56) and (57) in an 84 16 ratio. Thus, no evidence for stereochemical crossover could be found in the reactions of (52b) or (55) in their reactions with cyclopentadiene. 

The stereochemical control exerted by migrating methyl in the deamination of l-ethyl-2-methylbutylamine (661) is also informative457. Since the decomposition of the diazonium ion is unassisted, only 28% of the 4-methyl-3-hexanol arises by a 1,2-methyl shift (concerted ionization leading directly to the methyl-bridged ion (662) would require 50%). An excess of (35,4S)-(664) over (3R, 4R)-(664) is obtained (72 28) and substantial crossover from threo- amine to erj> thro-alcohol occurs. The ery thro-alcohol, however, is enantiomerically pure (3R, 4S)—(664). Although the... 

Reaction of the triarylsulfonium salts with vinyllithium or its derivatives led to good to high yields of the corresponding styrenes. With stereochemically pure substituted vinyllithium, the integrity of the stereochemistry was retained. Cis- and rra/i5-propenyllithium (18) and (20) ai orded the corresponding CIS- and rrans-propenylbenzene (19) and (21) respectively with no crossover. 3,14... 

In the chelating mode they frequently stabilize the metal centre in an unusually high apparent formal oxidation state, e.g. [Fe (S2CNR2)3)]+ and rNi" (S2CNR2)3j+. They also have a propensity for stabilizing novel stereochemical configurations, unusual mixed oxidation states (e.g. of Cu), intermediate spin states (e.g, Fe S = I), and for forming a variety of tris chelated complexes of which lie at the T2 — spin crossover (p, 1096). ... 

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. 

Labeled vinyllithiums retain their stereochemical integrity. Thus, cis-and [Pg.15]

Stereochemical crossover in the pyrolysis of 3,5-disubstituted pyrazolines was proposed. The observation of a stereochemical crossover phenomenon stimulated a consideration of the mechanism from a different viewpoint. The loss of molecular nitrogen in the pyrolysis of a cis-3,5-disubstituted pyrazoline, cis-8, might be expected to give a trimethylene... 

As expected for this mechanism, the reactivity falls off as the alkyl group becomes more bulky and as the substituents become more electron-attracting. By carrying out a crossover reaction on a mixture of RS and SR isomers of [CpFe (CO)L CH2C H(Me)Ph ], which is chiral at both Fe and the carbon shown, very little of the crossover products, the R,R and 5,5 isomers of the sulfinate complex, are seen. This shows that the postulated ion pairs must stay together, and that the intermediate iron cation also has stereochemical stability. Ion pairing is very common in organic solvents, and the ions probably rarely break free from the ion pair- "... 

Returning to the platinacyclobutane problem, Casey and coworkers [72] provided two definitive stereochemical experiments that relate to the reaction mechanism. First, a crossover experiment showed that the reaction was intramolecular. Second, 21.119... 

---