Carbonyl groups alkenes

The tin hydride method is reductive, and the cyclic radical is almost always trapped by a hydrogen atom. In simple cyclizations, both the radical precursor and the alkene are lost during tin hydride reduction, and this sometimes results in underfunctionalized products, necessitating the introduction of extra functional groups for subsequent transformations. However, in the synthesis of simple molecules, this is often an advantage as steps to remove residual alkenes, carbonyl groups and the like, left by ionic methods of C—C bond formation, are not required. Work-up requires separation of the desired products from the tin by-products (see Section 4.1.6.2.1). 

The Alder-ene reaction is an atom-economic reaction which forms a new carbon carbon-bond from two double bond systems (alkenes, carbonyl groups, etc.) with double bond migration [5]. This reaction follows the Woodward-Hoffmann rules if the reaction is performed under thermal conditions. However, when transition metal catalysts are involved, thermally forbidden Alder-ene reactions can also be realized (Scheme 9.1). Examples of such processes are the formal [4 + 4]-Alder-ene reaction catalyzed by low-valent iron catalysts. 

Unless the indole nitrogen atom is substituted by an acyl group, the photocycloaddition reactivity of indoles is very limited. In the absence of an N-acyl group, the only reactions reported are the triplet-sensitized 2-1-2 addition of alkynes to AT-methylindoles and the electron-transfer-sensitized 4-1-2 addition of dienes to indoles. When an AT-acyl substituent is present, the C2-C3 bond appears to be activated and will undergo photocycloaddition reactions with a variety of partners such as alkenes, carbonyl groups, and the C2-C3 bond of ground-state N-acylindoles. 

Migration of a hydride ligand from Pd to a coordinated alkene (insertion of alkene) to form an alkyl ligand (alkylpalladium complex) (12) is a typical example of the a, /(-insertion of alkenes. In addition, many other un.saturated bonds such as in conjugated dienes, alkynes, CO2, and carbonyl groups, undergo the q, /(-insertion to Pd-X cr-bonds. The insertion of an internal alkyne to the Pd—C bond to form 13 can be understood as the c -carbopa-lladation of the alkyne. The insertion of butadiene into a Ph—Pd bond leads to the rr-allylpalladium complex 14. The insertion is usually highly stereospecific. 

Another feature of the Pd—C bonds is the excellent functional group tolerance. They are inert to many functional groups, except alkenes and alkynes and iodides and bromides attached to sp carbons, and not sensitive to H2O, ROH, and even RCO H. In this sense, they are very different from Grignard reagents, which react with carbonyl groups and are easily protonated. 

The final step can involve introduction of the amino group or of the carbonyl group. o-Nitrobenzyl aldehydes and ketones are useful intermediates which undergo cyclization and aromatization upon reduction. The carbonyl group can also be introduced by oxidation of alcohols or alkenes or by ozonolysis. There are also examples of preparing indoles from o-aminophcnyl-acetonitriles by partial reduction of the cyano group. 

The carbonyl group makes aldehydes and kefones rafher polar molecules dipole momenfs fhaf are subsfanfially higher fhan alkenes... 

The carbonyl carbon of a ketone bears two electron releasing alkyl groups an aldehyde carbonyl group has only one Just as a disubstituted double bond m an alkene is more stable than a monosubstituted double bond a ketone carbonyl is more stable than an aldehyde carbonyl We 11 see later m this chapter that structural effects on the relative stability of carbonyl groups m aldehydes and ketones are an important factor m then rel ative reactivity... 

In general aldehydes and ketones have higher boiling points than alkenes because they are more polar and the dipole-dipole attractive forces between molecules are stronger But they have lower boiling points than alcohols because unlike alcohols two carbonyl groups can t form hydrogen bonds to each other... 

It might be noted that most (not all) alkenes are polymerizable by the chain mechanism involving free-radical intermediates, whereas the carbonyl group is generally not polymerized by the free-radical mechanism. Carbonyl groups and some carbon-carbon double bonds are polymerized by ionic mechanisms. Monomers display far more specificity where the ionic mechanism is involved than with the free-radical mechanism. For example, acrylamide will polymerize through an anionic intermediate but not a cationic one, A -vinyl pyrrolidones by cationic but not anionic intermediates, and halogenated olefins by neither ionic species. In all of these cases free-radical polymerization is possible. 

The initial bond formation between the -> ir excited carbonyl compound and an alkene can occur by interaction of the half-filled n -orbital of the [I CO] with the ir-system of the alkene, in a sense transferring a tt-electron to the -orbital and making a bond between an alkene carbon and the carbonyl oxygen. In this process (common for electron rich olefins) the plane formed by the alkene carbons and their four substituents is perpendicular to the plane of the carbonyl groups and its two substituents (Figure 1). In the... 

The carbonyl group makes aldehydes and ketones rather polar molecules, with dipole moments that are substantially higher than alkenes. 

To identify the carbonyl compound and the ylide required to produce a given alkene, mentally disconnect the double bond so that one of its carbons is derived from a carbonyl group and the other is derived from an ylide. Taking styrene as a representative example, we see that two such disconnections are possible either benzaldehyde or formaldehyde is an appropriate precursor. 

Olefins conjugated with electron-withdrawing groups other than a carbonyl group undergo reactions with enamines in a manner similar to the carbonyl-conjugated electrophilic alkenes described above. Namely, they condense with an enamine to form a zwitterion intermediate from which either 1,2 cycloaddition to form a cyclobutane ring or simple alkylation can take place. 

The reaction starts with the nucleophilic addition of a tertiary amine 4 to the alkene 2 bearing an electron-withdrawing group. The zwitterionic intermediate 5 thus formed, has an activated carbon center a to the carbonyl group, as represented by the resonance structure 5a. The activated a-carbon acts as a nucleophilic center in a reaction with the electrophilic carbonyl carbon of the aldehyde or ketone 1 ... 

The initial step of the coupling reaction is the binding of the carbonyl substrate to the titanium surface, and the transfer of an electron to the carbonyl group. The carbonyl group is reduced to a radical species 3, and the titanium is oxidized. Two such ketyl radicals can dimerize to form a pinacolate-like intermediate 4, that is coordinated to titanium. Cleavage of the C—O bonds leads to formation of an alkene 2 and a titanium oxide 5 ... 

Products 7a and 7c, with the substituent R a to the carbonyl group, are by far predominantly formed. This regioselectivity is a result of the preferential approach of the alkene 2 to the dicobalthexacarbonyl-alkyne complex 5 from the side opposite to the substituent R of the original alkyne. The actual incorporation of the alkene however is less selective with respect to the orientation of the olefinic substituent R, thus leading to a mixture of isomers 7a and 7c. 

Peroxyacids transfer an oxygen atom to the alkene with syn stereochemistry—both C-0 bonds form on the same face of the double bond-through a one-step mechanism without intermediates. The oxygen atom farthest from the carbonyl group is the one transferred. 

I The region from 2000 to 1500 cm"1 is where double bonds (0=0, C=N, and C=C) absorb. Carbonyl groups generally absorb in the range 1680 to 1750 cm-1, and alkene stretching normally occurs in the narrow range 1640 to 1680 cm-1. 

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