Displacement of alkoxide

Owing to the instability of a-halogenoaldehydes it is occasionally preferable to use more stable derivatives, such as enol acetate prepared according to Bedoukian s method (204) and a-bromoacetals (4, 8, 10, 16, 22, 67, 101, 426). An advantage is said to be in the yield however, this appears to be slight. The derivatives react in the same sense as the aldehydes themselves, that is, the acetal group as the more polarized reacts first and enters the C-4 position. It is likely that the condensation and cyclization occur by direct displacement of alkoxide ions. Ethyl-a,/3-dihalogeno ethers (159, 164, 177, 248) have also been used in place of the free aldehydes in condensation with thioamides. 

The driving force for this transformation is the fact that the less electronegative nitrogen atom is a better Tr-donor than oxygen and can form a stronger bond with the carbene carbon atom. Hence displacement of alkoxide by amines and thiols is commonly observed, but the reverse reactions are seldom seen. 

Alternatively, the displacement of alkoxide by attack of a nucleophile at the carbonyl unit of complexed magnesium S-ketoester can proceed smoothly according to a Claisen-type reaction, as described in the transformation of xantophanic enol 79 into resorcinol 80 (equation 101) . 

Solvent for Displacement Reactions. As the most polar of the common aprotic solvents, DMSO is a favored solvent for displacement reactions because of its high dielectric constant and because anions are less solvated in it (87). Rates for these reactions are sometimes a thousand times faster in DMSO than in alcohols. Suitable nucleophiles include acetyUde ion, alkoxide ion, hydroxide ion, azide ion, carbanions, carboxylate ions, cyanide ion, hahde ions, mercaptide ions, phenoxide ions, nitrite ions, and thiocyanate ions (31). Rates of displacement by amides or amines are also greater in DMSO than in alcohol or aqueous solutions. Dimethyl sulfoxide is used as the reaction solvent in the manufacture of high performance, polyaryl ether polymers by reaction of bis(4,4 -chlorophenyl) sulfone with the disodium salts of dihydroxyphenols, eg, bisphenol A or 4,4 -sulfonylbisphenol (88). These and related reactions are made more economical by efficient recycling of DMSO (89). Nucleophilic displacement of activated aromatic nitro groups with aryloxy anion in DMSO is a versatile and useful reaction for the synthesis of aromatic ethers and polyethers (90). 

The advantages of titanium complexes over other metallic complexes is high selectivity, which can be readily adjusted by proper selection of ligands. Moreover, they are relative iaert to redox processes. The most common synthesis of chiral titanium complexes iavolves displacement of chloride or alkoxide groups on titanium with a chiral ligand, L ... 

Chloro-5-arylisoxazoles undergo nucleophilic displacement with alkoxide ion. Halogen atoms in the 5-position of the isoxazole nucleus are readily displaced if an activating group is present in the 4-position (63AHC(2)365). 

An explanation for the stereoselectivity of the reaction involves optimal overlap of the 7t-orbital of the carbonyl with the developing electron rich p-orbital on C2 during the Sj,j2 displacement of the chloride by the alkoxide (24). Thus, orbital overlap imposes conformational constraints in the transition state that leads to nonbonding interactions disfavoring transition state 15P... 

An aryloxypyrimi done has been described as an anti ulcer agent this activity is of note since the agent does not bear any structural relation to better known anti ulcer drugs. Displacement of halogen on the acetal of chloro-acetaldehyde by alkoxide from m-cresol gives the intermediate This affords enaminoaldehyde when subjected... 

Although the Koenigs-Knorr reaction appears to involve a simple backside S 2 displacement of bromide ion by alkoxide ion, the situation is actually more complex. Both a and /3 anomers of tetraacetyl-o-glucopyranosyl bromide give the same /3-glycoside product, implying that they react by a common pathway. 

Nucleophilic displacement of bromide from 5-acetyl-10-bromo-5//-dibenz[7>,/]azepine (41) by alkoxide,132 and by cyanide ion in dimethylformamide,212 has been noted. However, replacement of bromide by cycloalkylamines (e.g., piperidine) to give the 10-cycloalkylamino derivatives. e.g. 44, is best accomplished in the presence of potassium ferf-butoxide, a result which suggests that the aminodebromination proceeds via an elimination-addition (EA) pathway involving an azepyne intermediate 43 (see Section 3.2.1.5.7.) rather than by the more usual addition-elimination (AF.) mechanism.118... 

The proposed mechanism of the catalytic reaction involves the formation of the Cu(l) alkoxide 68 by displacement of either the chloride or the NHC from 65-67, followed by conversion to the hydride 69 by metathetical exchange of the tert-butoxide by the H of the silane (Fig. 2.10). 

Transfer of the hydride from the Cu to the electrophilic carbon and cleavage of the copper alkoxide by the silane regenerates 69. Recent reports point to the influence of the type of the counter ion X" of the homoleptic 66-67 on the activity, the BF being superior to the PF analogue this effect has been attributed to differences in the rate of active catalyst generation from the homoleptic [Cu(NHC)2] X and NaO Bu due to solubility differences of the inorganic salts formed during the displacement of the NHC by BuO" [54] (Scheme 2.10). 

---