Four-membered intermediate

The mechanism of the Hurd-Mori reaction has been discussed extensively in the review by Stanetty. The mechanism of the reaction was initially postulated by Hurd-Mori based on the isolation of intermediate 10. This intermediate was shown to transform into the desired thiadiazole upon heating in ethanol, either with or without acid. The reaction was thought to proceed via the four-membered intermediate 11, which would release the volatile ethylformate as a by-product. In 1995, Kobori and co-workers were able to isolate and determine crystallographically a very similar intermediate structure to 10 in their mechanistic studies of the reaction. ... 

Trifluoroacetonitrile and the iminophosphorane (193) gave the adduct (195) presumably via the four-membered intermediate (194). 

In comparison with aluminum, the number of known iminogallane and imi-noindane clusters is small. While the preparation of (PhNAlPh)4 dates back to 1962 [34], it was structurally characterized ten years later [40], the first imino-gallanes and iminoindanes were prepared in 1993 by the reaction of QF5NH2 with GaMe3 and InMe3, respectively [Eq. (18)] [47]. The four-membered intermediates,... 

The elemental processes involved such as P-H oxidative addition and P-C reductive elimination are precedented (see above). The telomer formation may stem from the four-membered intermediate, which can relieve the ring strain upon further insertion of acrylonitrile into the Pt-C bond. 

In a later study of these reactions, differently substituted thioureas were compared in relation to their effectiveness in producing four-membered rings. Compounds 177 and 178 produced four-membered intermediates that collapsed to the more stable chlorformamidines. This phenomenon is most likely the result of the greater basicity of the N-alkyl than that of the N-aryl group. 

The results in Table 3 were explained as shown in Scheme 4. From the fact that no kinetic isotope effect was observed in the reaction of phenyl-substituted disilenes with alcohols (Table 1), it is assumed that the addition reactions of alcohols to phenyltrimethyl-disilene proceed by an initial attack of the alcoholic oxygen on silicon (nucleophilic attack at silicon), followed by fast proton transfer via a four-membered transition state. As shown in Scheme 4, the regioselectivity is explained in terms of the four-membered intermediate, where stabilization of the incipient silyl anion by the phenyl group is the major factor favoring the formation of 26 over 27. It is well known that a silyl anion is stabilized by aryl group(s)443. Thus, the product 26 predominates over 27. However, it should be mentioned that steric effects also favor attack at the less hindered SiMe2 end of the disilene, thus leading to 26. 

With the help of DFT calculations the stability of the three-coordinate complexes g was recently confirmed, but no evidence has yet been found for the five-coordinate intermediates h [14,15]. Instead, Goo (Sen et al. disclosed the linear complex i as an alternative structure for the initial intermediate [15]. It offers a valid explanation for Jutand s findings and reinstates the original four-membered intermediates in the catalytic cycle, albeit starting from anionic precursors (Scheme 3, bottom). 

Ions containing a Si—C double bond can be generated in the gas phase via dissociative ionization of silacyclobutane derivatives. The silene radical cation [Me2Si=CH2]+ has been obtained by El ionization of 1,1-dimethyl-l-silacyclobutane145. The reactions with ethylene-dr and propene are suggested to occur by way of a [2 + 2] cycloaddition, leading to a cyclic four-membered intermediate. 

One atom incorporation reactions are discussed in Chapter II subdivided into carbon, nitrogen, and oxygen incorporation. A few of these reactions are discussed in other sections. Because of their special reactivity most of the three-membered ring compounds used for expansion are combined in Chapter III. Reactions with four-membered intermediates are collected in Chapter IV Reactions of the type 1/9 — 1/10 will be found in Chapter V and those of 1/7 —> 1/8 (see Scheme 1/1) in Chapter VII. Bicyclic starting materials will be discussed in... 

IV. Ring Expansion from Four-membered Rings or via Four-membered Intermediates... 

There is evidence that a concerted [2 + 3] step, via path (a), is favored over four-membered intermediates postulated for (b). With chiral ligands L the reaction proceeds enantioselectively.168... 

A four-membered intermediate ring can also be drawn. 

The reaction can proceed by way of a four-membered intermediate, and in a final step removal of the oxygen and heteroatom (E) converts the carbonyl group into the carbon-carbon double bond (Scheme 4.18). 

Scheme 5 Four-member intermediate present in the thione-thiols rearrangements...

In the first step, the triphenylphosphine reacts with an alkyl azide to form an iminophosphorane with loss of nitrogen Staudinger reaction). In the second step, the nucleophilic nitrogen of the iminophosphorane attacks the carbonyl group to form a four-membered intermediate (oxazaphosphetane) from which the product Schiff base and the byproduct triphenylphosphine oxide are released. 

Azides may be reduced to amines with triphenylphosphine in what is known as the Staudinger reaction. The probable mechanism involves attack of triphenylphosphine on the azide and formation of a phosphinimine via a four-membered intermediate—notice the similarity with the Wittig reaction ... 

The selective formation of the arylated or vinylated allylalcohol was rationalized by assuming a cationic four-membered intermediate, in which the hydrogen atom (H ) on the hydroxyl-bearing carbon is located unfavorably for a syn palladium hydride elimination. The cationic intermediates were generated from organic triflates, iodonium salts, or organic iodide/thaUium or silver salt combinations (Scheme 28). 

The mechanisms of these reactions bear marked similarities, in spite of the differences in their reactivities and selectivities. Thus, in certain cases, a four-membered intermediate similar to the 1,2-oxaphosphetane intermediate in the Wittig reaction appears in the Peterson reaction as a pentacoordinate 1,2-oxasiletanide. Reactions of transition metal carbene complexes with carbonyl compounds also proceed through the formation of a four-membered oxametallacycle, which was recently found to be an intermediate of some McMurry reactions. Carbonyl olefination utilizing dimetallic species of zinc or chromium is somewhat similar to the Julia reaction in that they both involve the process of ) -elimination. 

A similar mechanism to form ketenes from triketones without elimination of CO appears unlikely, as it involves a four-membered intermediate. 

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