Carbon dioxide stereochemistry

The stereochemistry of the carboxylation of 4-substituted ( + )-(/ S)-fra ,v-1-(4-mcthylphcnyl-sulfinylmethyl)cyclohexane after metalation with methyllithium and quenching with carbon dioxide was reported64. The results listed in Table 1 show that the d.r. of around 75 25 under kinetic control changes to 25 75 under thermodynamic control. This is the result of the equilibration of the two diastereomeric metalated species. As shown by the experiment in hexamethylphosphoric Iriamide (IIMI A) (d.r. = 57 43 under kinetic control) an electrophilic assistance of the lithium cation to the electrophilic approach is probably involved. 

Allenamide ( )-13 was prepared by trapping the corresponding lithioallene with carbon dioxide, followed by conversion of the carboxylate to the amide. Chromatographic resolution of the enantiomers of 13 was easily accomplished on a 10x250mm Chiralcel OD HPLC column. Addition of vinyllithium 14 to (+)-13, followed by quenching the reaction with aqueous NaH2P04, led to cyclopentenone (—)-15 in 64% yield with >95% chirality transfer (Eq. 13.4). The absolute stereochemistry of (-)-5 is consistent with the mechanistic hypothesis put forth in Eq. 13.3 [8]. 

Yields are variable, but the reaction appears to be promoted by the presence of alkoxymethyl substituents on the oxirane ring. Reactions with p-propiolactones give lower yields (<10%). Cyclic carbonates can also be obtained from the zinc(II) promoted reaction of oxiranes with carbon dioxide [62] the stereochemistry of substituents on the oxirane are retained in the carbonate. 

Quinoline364 and isoquinoline behave much like pyridine toward DMAD, and only the more interesting reactions will be noted. Isoquinoline in ether at -60° with DMAD and carbon dioxide gives, via the postulated intermediate 295, zwitterion 296 (m.p. 83°-84°), which is far more stable than its pyridine analogs (cf. 2).365 Its stereochemistry has not been established but its IR spectrum shows absorption due to the carboxylate anion and excludes the cyclic formulation 297. Replacing the carbon dioxide by ethyl carbonate gives 298.366 In wet ether the... 

Further evidence for the stereochemistry of the SO2 insertion reaction [reaction (j)] has been obtained . The insertion of carbon dioxide into metal-carbon bonds is being developed with renewed interest, in part because of its role in the greenhouse effect . 

Related photochemical reactions in which carbon dioxide is eliminated are summarized in Table 10. As well as y-aryl, y-enone substituents effectively promote the reaction. A remarkable example is provided by dihydro-l,2-(6j5,ll/ )-santonine (3). This crystalline solid slowly liquefied on exposure to normal laboratory lighting and then recrystallized. The product, norepimaalienone (4) has lost the elements of carbon dioxide and the stereochemistry of the methyl-bearing carbon is inverted. Simple derivatives of compound 3 underwent the same reaction. The cyclopropanes produced are themselves photosensitive and if more forcing conditions are employed, alkene rather than cyclopropyl products are isolated. ... 

The stereochemistry of the direct attack can be expected to resemble the corresponding reactions at a saturated carbon (Sections 5.1.1.1 and 5.1.1.2)—inversion for nucleophilic substitution, and retention, or perhaps occasionally inversion, for electrophilic substitution. In practice, SN2 reactions at trigonal carbon are rare,503 and their stereochemistry, where inversion is known,504 barely established. Electrophilic attack, like the reactions of vinyllithium reagents with protons, aldehydes and carbon dioxide, takes place invariably... 

REACTIONS OF COORDINATED LIGANDS - dealing with the chemistry of molecules such as oxygen, nitric and nitrous oxide, carbon dioxide, oximes, and nitriles STEREOCHEMISTRY, STRUCTURE, AND CRYSTAL ENGINEERING structure and stereochemistry involving lone pair effects, outer sphere interactions, and hydrogen bonding. 

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