Carbon, electron-deficient migration

A related unprecedented double insertion of electron-deficient alkynes has also been reported in the reactions of the linear Pt2Pd heterotrimetallic complex 64 with 65 (RO2CCSCR) (Scheme 24) [95,96]. A series of unsymmetri-cal A-frame clusters 68 has thus been obtained in which a first insertion of the alkyne takes place site-selectively into the Pt-Pd bond vs the Pt-Pt bond (66). After a zwitter-ionic polar activation of the resulting inserted alkene (67), a subsequent reaction with the phosphine unit of the dpmp allows one to obtain the products 68 via the nucleophilic migration of the terminal P atom from the Pd center to the CH terminal carbon (formation of the P-C bond). 

Photorearrangement of 4-/>-cyanophenyl-4-phenylcyclohexene (69) took place mainly byp-cyanophenyl migration/67,70,74 The conclusion could then be drawn that the rearranging excited state is not electron deficient at the /3-carbon atom, since one would not expect a cyanophenyl group to migrate to a positive carbon. The/3-carbon was proposed to have odd electron character ... 

The rearrangements that we have considered to date all have one feature in common the migration of an alkyl or aryl group, with its electron pair, to a carbon atom which, whether it be a carbocation or not, is electron-deficient. Another atom that can similarly become electron-deficient is nitrogen in, for example, R2N or RN (a nitrene, cf. carbenes above), and it might be expected that alkyl or aryl migration to such centres would take place, just as it did to R3C and R2C this is indeed found to be the case. 

Whereas lactone annulation invokes a relief of strain of the four membered ring by migration of the ring bond to an electron deficient oxygen, a similar migration to an election deficient carbon creates a cyclopentanone synthesis (Eq. 73). The release of approximately 84 kJ/mole (20 kcal/mole) provides a strong driving force. Thus, the 1,1-cyclobutanone annulation of ketones translates into a 1,1-cyclopentanone annulation. 

A simplified mechanism for the Beckmann rearrangements and important related reactions is shown hi Scheme 9. Summarizing the mechanism section, the key step of the reaction is the migration of an a-carbon group to the electronically deficient nitrogen atom of the oxime. A nitrilium ion in some cases or an imidate in others are key intermediates in the reaction. Their destiny determines the course of the transformation. Basically, three different pathways may be possible and can be synthetically exploited ... 

Many carbenes, like carbocations, rearrange to more stable structures by the migration of a neighboring group to the electron-deficient carbon. Thus phenylmethylcarbene rearranges to ethenylbenzene (styrene) ... 

In contrast to the results obtained in reactions of M—CF3 complexes with BF3 (see Section III,B,1), the reaction of the perfluoronickelacyclopentane complex 49 appears to proceed via fluoride abstraction, followed by phosphine migration to the electron-deficient carbon to afford the a-fluorophos-phonium ylid complex 50 and related derivatives (117). 

It should be noted that amines may be formed by hydrolysis of amides arising from the intramolecular Beckmann rearrangement of ketoximes (p. 1047) this rearrangement is a further example of the migration of a nucleophilic carbon species from a carbon to an electron-deficient nitrogen. 

During arylations of carbon nucleophiles with aryl halides in the presence of palladium triarylphosphine complexes products containing the aryl group of the phosphine can result (Scheme8.16). These reactions proceed via reversible arylation of the Pd-bound phosphine, which occurs at temperatures above 50 °C, particularly readily in the presence of iodide [11] (see Section 8.2). Electron-deficient aryl groups usually migrate less readily than electron-rich groups [23, 25],... 

Incidentally, the oxidation step presumably involves a standard syn shift to an electron-deficient oxygen with retention at carbon. The [1,2] Stevens rearrangement also goes with retention of the migrating group (Zimmerman, 1963). 

The formation of the dioxolanes in the photo-oxygenations of allylic stannanes with electron rich tin centers (i.e., compare 16 and 20) can be attributed to the ability of tin to stabilize and migrate to an electron deficient P carbon (Sch. 9). The reduced yield of dioxolane in the reaction of 22 in comparison to 20 or 21 can be attributed to a steric effect operating in conjunction with an electronic effect of the carbomethoxy group in the bridged (or perhaps open) intermediate 23 which promotes hydrogen abstraction in lieu of sterically more demanding nucleophilic attack (Sch. 9). 

As the bromine leaves, the nitrogen becomes electron deficient, so the carbon group migrates to the nitrogen. The nitrogen uses a pair of electrons to stabilize the carbonyl carbon. 

The Hofmann rearrangement also involves a migration to an electron-deficient nitrogen. In this case, an amide is treated with Cl2 or Br2 in aqueous base, resulting in the formation of an amine with one less carbon. The original carbonyl carbon is lost as carbon dioxide. The mechanism is shown in Figure 22.7 and an example is provided by the following equation ... 

The migration of alkyl groups to carbene centres has much in common with the migration of alkyl groups to cationic centres discussed in Chapter 37—after all, both carbenes and carbocations are electron-deficient species with a carbon atom carrying only six electrons in its outer shelJ. 

Nitrenes, like carbenes, are immensely reactive and electrophilic, and the same Wolff-style migration takes place to give an isocyanate. The substituent R migrates from carbon to the electron-deficient nitrogen atom of the nitrene. Isocyanates are unstable to hydrolysis attack by water on the carbonyl group gives a carbamic acid which decomposes to an amine. 

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