Carbonyl Compounds and Alkenes

Ketones and aldehydes can undergo photochemical [2-1-2] cycloaddition reactions with alkenes to give oxetanes. This is called the Paterno-Buchi reaction. For alkyl carbonyl compounds both singlet and triplet excited states seem to be involved, but for aromatic compounds the reaction occurs through the triplet state.The regiochemistry can usually be accounted for on the basis of formation of the most stable 2-oxa-1,4-diradical. For example, styrene and benzaldehyde give 2,3- not 2,4-diphenyloxetane.  

The same generalization can also account for the reversal of orientation between furan and dihydrofuran.  

The Paterno-Buchi reaction is ordinarily not stereospecific, but instead favors the more stable adduct for either alkene isomer, indicating the involvement of a relatively a long-lived diradical intermediate.  

An exception to this generalization has been noted for E- and Z-cyclooctene. This reaction is nearly stereospecific at low temperature. This result is attributed to conformationally distinct 1,4-diradical intermediates that undergo intersystem crossing and cyclization faster than stereochemical interconversion. 

At higher temperature, some stereorandomization occurs as the result of competition between rotational processes and fragmentation of the 1,4-diradical intermediate. 

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Photocycloaddition Reactions of Carbonyl Compounds and Alkenes. Photocycloaddition of ketones and aldehydes with alkenes can result in formation of four-membered cyclic ethers (oxetanes), a process often referred to as the Paterno-Buchi reaction.196... 

Scheme 6.11. Photocycloaddition Reactions of Carbonyl Compounds and Alkenes...

By analogy with the formation of dihydropyrans from unsaturated carbonyl compounds and alkenes (see Section 2.24.2.7.l(i)), the synthesis of 4//-pyrans from the [4 + 23-cycloaddition of unsaturated carbonyl compounds and alkynes would seem to offer some potential. Such a reaction has indeed proved of value, but examples are largely restricted to the use of ynamines as the dienophile (76BSF987). 

The photochemical reaction of carbonyl compounds and alkenes, which is referred to as the Paterno-Buchi (PB) reaction, was developed in 1909 [13], and is currently one of the most widely used methods for oxetane synthesis (Scheme 7.4). As exemplified in the PB reaction of benzophenone with 2-methylpropene [14], a selective formation of the oxetane is possible even when the photochemical reaction involves highly unstable molecules that is, the excited state of carbonyls. Due to its synthetic importance and mechanistic interest, the PB reaction is the most extensively studied synthetic method for oxetanes. Thus, several extensive reviews describing the PB reaction have been published since 1968, and the reader is directed towards these for further information [15]. In this chapter, methods that allow for the control of the regioselective and stereoselective formation of synthetically important oxetanes will be described. 

A major problem in the reaction of a,/3-unsaturated carbonyl compounds and alkenes proves to be the competition between hetero Diels-Alder and ene reactions. Intramolecular cycloadditions of 1,6- and 1,7-dienes with ester and cyano groups at the double bond yield the ene product nearly exclusively, but with alkylidene- and benzylidene-ketoesters and 1,3-diketones the Diels-Alder reaction is preferred under thermal conditions, however under Lewis acid catalysis also ene reactions occur [12]. 

Paternd-Biichi reactions [152] this competition has been investigated for electron-rich alkene substrates for several combinations of carbonyl compounds and electron-donors, e.g. a-diketones and ketene acetals [153], aromatic aldehydes and silyl ketene acetals, and enol ethers. In polar solvents, the assumption of a 1,4-zwitterion as decisive intermediate is reasonable. This situation then resembles the sequence observed for ET-induced thermal [2 -I- 2]-cycloaddition reactions [154]. Both regio- and diastereoselectivity are influenced by this mechanistic scenario. The regioselectivity is now a consequence of maximum charge stabilization and no longer a consequence of the primary interaction between excited carbonyl compound and alkene. Whereas 3-alkoxyoxetanes are preferentially formed from triplet excited aldehydes and enolethers, 2-alkoxyoxetanes result from the reaction of triplet excited ketones or aldehydes and highly electron-rich ketene silylacetals (Scheme 40) [155]. 

Figure 6.27 Experimental heats of hydrogenation for selected carbonyl compounds and alkenes. ...

Makihara,N., Ogo, S. and Watanabe, Y., pH-selective hydrogenation of water-soluble carbonyl compounds and alkenes with [Cp Ir(III)(H20)3] (Cp = tj -CsMes) as a catalyst precursor in very acidic media, Organometallics, 2001, 20, 497. 

Strong acids produce carbocations from a variety of functional molecules. Protonation of alcohols, epoxides, carbonyl compounds, and alkenes does so. Lewis acids such as anhydrous aluminum chloride can combine with these substrates and can also remove halide ions from carbon to give carbocations. Diazotization of primary amines in acid solution is another source. 

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 car bonyl 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... 

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... 

Oxaziridines are generally formed by the action of a peracid on a combination of a carbonyl compound and an amine, either as a Schiff base (243) or a simple mixture. Yields are between 65 and 90%. Although oxygenation of Schiff bases is formally analogous to epoxidation of alkenes, the true mechanism is still under discussion. More favored than an epoxidation-type mechanism is formation of a condensation product (244), from which an acyloxy group is displaced with formation of an O—N bond. 

The photochemical reactions of organic compounds attracted great interest in the 1960s. As a result, many useful and fascinating reactions were uncovered, and photochemistry is now an important synthetic tool in organic chemistry. A firm basis for mechanistic description of many photochemical reactions has been developed. Some of the more general types of photochemical reactions will be discussed in this chapter. In Section 13.2, the relationship of photochemical reactions to the principles of orbital symmetry will be considered. In later sections, characteristic photochemical reactions of alkenes, dienes, carbonyl compounds, and aromatic rings will be introduced. 

Lewis-acid catalyzed inverse electron-demand Diels-Alder reactions between conjugated carbonyl compounds and simple alkenes and enolethers also allow dihydropyranes to be prepared. SnCU-Catalyzed cycloaddition of... 

Alkyl substituents accelerate electrophilic addition reactions of alkenes and retard nucleophilic additions to carbonyl compounds. The bonding orbital of the alkyl groups interacts with the n bonding orbital, i.e., the HOMO of alkenes and raises the energy (Scheme 22). The reactivity increases toward electron acceptors. The orbital interacts with jt (LUMO) of carbonyl compounds and raises the energy (Scheme 23). The reactivity decreases toward electron donors. 

Scheme 6 Mechanistic spectrum of [2+2] cycloaddition of carbonyl compounds with alkenes and atkynes...

A second important reaction type considered in this chapter is conjugate addition, which involves addition of nucleophiles to electrophilic double or triple bonds. A crucial requirement for this reaction is an electron-withdrawing group (EWG) that can stabilize the negative charge on the intermediate. We focus on reactions between enolates and a,(3-unsaturated carbonyl compounds and other electrophilic alkenes such as nitroalkenes. 

As discussed in Section 10.4 of Part A, concerted suprafacial [2tt + 2tt] cycloadditions are forbidden by orbital symmetry rules. Two types of [2 + 2] cycloadditions are of synthetic value addition reactions of ketenes and photochemical additions. The latter group includes reactions of alkenes, dienes, enones, and carbonyl compounds, and these additions are discussed in the sections that follow. 

In a, P-unsaturated carbonyl compounds and related electron-deficient alkenes and alkynes, there exist two electrophilic sites and both are prone to be attacked by nucleophiles. However, the conjugated site is considerably softer compared with the unconjugated site, based on the Frontier Molecular Orbital analysis.27 Consequently, softer nucleophiles predominantly react with a, (i-unsaturated carbonyl compounds through conjugate addition (or Michael addition). Water is a hard solvent. This property of water has two significant implications for conjugate addition reactions (1) Such reactions can tolerate water since the nucleophiles and the electrophiles are softer whereas water is hard and (2) water will not compete with nucleophiles significantly in such... 

Catalytic Reductive Coupling of Alkenes and Alkynes to Carbonyl Compounds and Imines Mediated by Hydrogen... 

Ihmels H, Otto D (2005) Intercalation of Organic Dye Molecules into Double-Stranded DNA - General Principles and Recent Developments. 258 161-204 Iida H, Krische MJ (2007) Catalytic Reductive Coupling of Alkenes and Alkynes to Carbonyl Compounds and Imines Mediated by Hydrogen. 279 77-104 Imai H (2007) Self-Organized Formation of Hierarchical Structures. 270 43-72 Indelli MT, see Chiorboli C (2005) 257 63-102 Inoue Y, see Borovkov VV (2006) 265 89-146 Ishii A, Nakayama J (2005) Carbodithioic Acid Esters. 251 181-225 Ishii A, Nakayama J (2005) Carboselenothioic and Carbodiselenoic Acid Derivatives and Related Compounds. 251 227-246... 

The reactions of allylmetal reagents with carbonyl compounds and imines have been extensively investigated during the last two decades [1], These carbon—carbon bondforming reactions possess an important potential for controlling the stereochemistry in acyclic systems. Allylmetal reagents react with aldehydes and ketones to afford homo-allylic alcohols (Scheme 13.1), which are valuable synthetic intermediates. In particular, the reaction offers a complementary approach to the stereocontrolled aldol process, since the newly formed alkenes may be readily transformed into aldehydes and the operation repeated. 

Allyltitanium complexes derived from a chiral acetal have been reacted with carbonyl compounds and imines [63], While the chiral induction proved to be low with carbonyl compounds, high induction was observed with imines. This complex represents the first chiral homoenolate equivalent that reacts efficiently with imines. Finally, the reactions with electrophiles other than carbonyl compounds and imines, namely a proton source, NCS, and I2, furnished the corresponding alkene, chloro, and iodo derivatives in good yields [64]. 

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