Nucleophilic reactions of the

A. Nucleophilic Reactions of the P=0 Group.—Tris(trifluoromethyl)-phosphine oxide (33) reacts with hexamethyldisiloxane to give a phos-phorane, whose n.m.r. spectrum at — 140 °C shows non-equivalent trifluoromethyl groups. Although this unusual reaction clearly involves nucleophilic attack of the phosphoryl oxygen on silicon at some stage of the reaction, a full study of the mechanism has not been published. Tertiary phosphine oxides can be converted cleanly into dichlorophos-phoranes (34) by treatment with two moles of phosphorus pentachloride. Alkylation of the sodium salt of tetraphenylmethylenediphosphine dioxide (35) with alkyl halides, in dimethyl sulphoxide, has been reported to... 

Two-step synthetic routes to poly(/i-aminoborazines) from /i-chloroborazines involve initial nucleophilic reaction of the /i-chloroborazine with appropriate linking reagents followed by a deamination reaction of the as-obtained /i-aminoborazine. The 5-tiichloroborazine undergoes nucleophilic attack by ammonia or amine derivatives on the boron atom linked to chlorine atoms. For the same reasons previously quoted a tertiary amine (e.g., Et3N) must be added to precipitate the corresponding hydrochloride. 

The stereochemistry of the nucleophilic reaction of the enolate ion of 1,4-dihydro-4-methoxycarbonyl-l-methylpyridine with and (5 )-(+)-2-bromobutane has... 

Hence, nucleophilic reactions of the superoxide ion are typical. This ion can be compared with the thiophenoxide and thiocyanate ions with respect to nucleophilicity. The cause of such high nucleophilicity lies in a so-called a-effect In 0—0 ion, an attacking site (O ) adjoins directly to a site (O ) with a significant electronegativity. This effect usually confers special activity to nucleophiles. The effect can be additionally enhanced by including the 0—0 group in sulfenate. 

A plausible mechanism for the formation of 4 is rationalized on the basis that photolysis of 3 results in [2-1-2] cyclization to thietane 4 and is subsequently followed by rearrangement to thiolactone 5 (Scheme 6). Ring opening of the initially formed thietane 4 leads to a zwitterion, which is facilitated by lone pair electrons of nitrogen and oxygen atoms, and nucleophilic reaction of the thiolate anion to carbonyl carbon gives 5. For the tricyclic thietane 4a, nucleophilic addition of the thiolate anion is difficult, and results in the formation of stable thietane 4a. 

Formation of hydroxamic acids via the reaction of the carbonyl group of aldehydes and a-oxo acids with the aromatic or aliphatic C-nitroso group belongs to the small number of nucleophilic reactions of the C-nitroso group. ... 

Interestingly, treating (>/4-cyclooctatetraene)Fe(CO)3 with acetyl chloride under Friedel-Crafts reaction conditions yielded unexpectedly222-223 the (>/2,>/3-8-e.x0-acetyl bicy-clo[3.2.1]octadienylium)Fe(CO)3 cation complex, presumably by rearrangement of the intermediate bicyclo[5.1.0]octadienylium isomer (Scheme 8). The structure of the rearranged cation was confirmed from the X-ray crystal structure and from the typical 1,3-cr.ji-allylic products obtained upon nucleophilic reaction with LiAlD4 and NaCN. The nucleophilic reaction of the more bulky iodide occurs, however, on the metal. 

Reactions of the Alkoxide Ion. The nucleophilic reactions of the alkoxide ion (RO ) are very similar to those of the hydroxide ion (OH ) with the exception that the latter has an extra ionizable proton which can lead to further reaction after the initial nucleophilic attack. 

The nucleophilic reaction of the cyanide ion on the carbonyl group is facilitated by protonat-ing the latter to a carboxonium ion. The addition of acid promotes the formation of cyanohydrins, but mainly for a thermodynamic reason. Under acidic conditions cyanohydrins equilibrate with the carbonyl compound and HCN. Under basic conditions they are in equilibrium with the same carbonyl compound and NaCN or KCN. The first reaction has a smaller equilibrium constant than the second, that is, the cyanohydrin is favored. So when cyanohydrins are formed under acidic or neutral (see Figure 9.8) instead of basic conditions, the reversal of the reaction is suppressed. 

Koizumi and co-workers used optically active allylic chloroselenuranes bearing a 2-exo-hydroxyl-lO-bornyl group to produce optically active allylic selenium ylides in situ [52]. The nucleophilic reaction of the corresponding chiral chloro-selenurane and selenoxide with an active methylene compound occurs in a highly stereoselective manner to give the corresponding chiral selenium ylides with retention of configuration [53] (Scheme 33). 

The next step then would be nucleophilic reaction of the carbanion with the electrophilic carbon of the thiocarbonyl group. This reaction joins carbons 1 and 3, as was predicted from the numbering scheme. 

A much less likely first step is nucleophilic reaction of the carbonyl oxygen of the isocyanate with the carbon attached to bromine ... 

Phenols react with chloroform in the presence of hydroxide ion in water to give o- and jp-hydroxybenzaldehydes. The steps of the reaction are (1) the formation of dichlorocarbene, as shown in Example 4.22 (2) nucleophilic reaction of the phenoxide with the electrophilic carbene and (3) hydrolysis. 

Because it is easier to protonate an aldehyde than a carboxylic acid (compare benzoic acid and benzaldehyde in Appendix C), the ring closure would be written best as protonation of the aldehyde oxygen followed by nucleophilic reaction of the carboxylic acid carbonyl oxygen. The carbonyl oxygen acts as the nucelophile because a resonance-stabilized cation is produced. If the hydroxyl oxygen acts as a nucleophile, the cation is not resonance-stabilized. 

Because C-3 becomes attached to C-2, a nucleophilic reaction of the nitranion at the electrophilic carbonyl carbon must take place. Sterically this carbon is the most accessible. This step is followed by an intramolecular nucleophilic reaction of the alcoholate with the adjacent carbon (numbered 1 in the previous equation). 

In the nucleophilic reaction of the carboxylic acid with the cation, 7-28, the carbonyl oxygen, not the C—O oxygen, acts as the nucleophile because only then is the resulting intermediate stabilized by resonance. 

There are two possible condensation reactions for the nitrile. One is nucleophilic reaction of the oxyanion with the carbon of the nitrile functional group. This carbon is activated by nitrogen and by the strongly electron-withdrawing trichloromethyl group. The other possibility, 8 2 displacement of chloride, is ruled out because there are three chlorines and four bonds in the product. After the nucleophilic reaction, a trace of methanol is needed to form a neutral product by protonation of the anion. 

Nucleophilic reactions of the spin-paired tris(o-phenanthroline) iron(II) ion are bimolecular 70-72). The tris complex is close to the spin-free complex in energy since dithiocyanatobis(o-phenanthroline) iron(II) exists in a spin-free = spin-paired equilibrium 53). The corresponding tris(o-phenanthroline)nickel(II) ion is unaffected by the same nucleophile, which probably rules out Sat2 attack on the organic ring as the predominant factor. 

The occurrence of general acid-catalyzed hydroxylaminolysis or methoxylaminolysis of thiol esters or amides has been described in Section IIB in terms of kinetically important tetrahedral intermediates. Two kinetically indistinguishable mechanisms for general acid-catalyzed aminolysis reactions are represented by transition states 42 and 43. Mechanism 42 involves a prior protonation of the ester followed by a general base-catalyzed aminolysis mechanism 43 is a general acid-assisted nucleophilic reaction of the amine. Mechanism 42 can be ruled out in the hydrazinolysis of phenyl acetates (Bruice and Benkovic, 1964) and in the hydrazinolysis of S-thiolvalerolactone (Bruice et al., 1963) on the basis of a calculated rate constant which is greater than the diffusion-controlled limit. Mechanism 43 is therefore correct. 

Although the reaction of 1,2-dichloro-3,3-difluorocyclopropene with thiocyanate leads simply to the replacement of the two chlorines by the nucleophile, reaction of the cyclopropene or of tetrachlorocyclopropene with sodium arylsulfinates gave an alkene in low yield (< 10%), apparently via the cyclopropenone, e.g. reaction of I. ... 

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