Propargyl anion system

The kind of dipoles that feature in the 1,3-DPCAs are isoelectronic with an allyl or propargyl anion system. They have a 7u-electron system consisting of two filled and one empty orbital, and both ends of the dipole have nucleophilic as well as electrophilic properties. 1,3-Dipoles of the allyl anion type are bent as the system has four electrons in three parallel p -orbitals perpendicular to the plane of the dipole. On the other hand, the presence of a double bond orthogonal to the delocalized 7r-system in the propargyl/aUenyl anion type causes linearity to the dipole (Figure 5.9). 

In Chapter 10 of Part A, the mechanistic classification of 1,3-dipolar cycloadditions as concerted cycloadditions was developed. Dipolar cycloaddition reactions are useful both for syntheses of heterocyclic compounds and for carbon-carbon bond formation. Table 6.2 lists some of the types of molecules that are capable of dipolar cycloaddition. These molecules, which are called 1,3-dipoles, have it electron systems that are isoelectronic with allyl or propargyl anions, consisting of two filled and one empty orbital. Each molecule has at least one charge-separated resonance structure with opposite charges in a 1,3-relationship, and it is this structural feature that leads to the name 1,3-dipolar cycloadditions for this class of reactions.136... 

Dilithiodibenzylacetytenebis(tmeda) (49) (68) is monomeric. Each lithium atom is bonded to a benzylic carbon and has further interactions with a phenyl C(l) and the acetylenic v system. The structure can be understood as being derived from a perpendicular 2-butyne-l,4-diyl dianion, "CH2—C=C—CH2, in which each negative charge is stabilized in its own orthogonal tt system. The lithiums bridge each of these propargyl (or allenyl) anion systems in a 1,3 manner. 

The most important of the concerted cycloaddition reactions is the Diels-Alder reaction between a diene and an alkene derivative to form a cyclohexene. The alkene reactant usually has a substiment and is called the dienophile. We discuss this reaction in detail in Section 10.2. Another important type of [2+4] cycloaddition is 1,3-dipolar cycloaddition. These reactions involve heteroatomic systems that have four rr electrons and are electronically analogous to the allyl or propargyl anions. 

It is noteworthy that the indenyl complex RuCl(ri -C9H7)(PPh3)2l4 provides an efficient catalyst precursor for the anti-Markovnikov hydration of terminal alkynes in aqueous media, especially in micellar solutions with either anionic (sodium dode-cylsulfate (SDS)) or cationic (hexadecyltrimethylammonium bromide (CTAB)) surfactants [38]. This system can be applied to the hydration of propargylic alcohols to selectively produce P-hydroxyaldehydes, whereas RuCl(Cp)(PMe3)2 gives a,P-unsat-urated aldehydes (the Meyer Schuster rearrangement products)(Scheme 10.8) [39]. 

This synthesis of a variety of five-membered heterocycles involves the reaction of a neutral 4rr-electron-three-atom system, the dipole, with a 2TT-electron system, the dipolarophile (Scheme 2). Table 3 illustrates 1,3-dipoles with a double bond and with internal octet stabilization, the propargyl-allenyl anion type. 

All 1,3-dipoles contain an allyl anion type n system, i.e., four electrons delocalized over three parallel atomic n orbitals, but, in addition, 1,3-dipoles of the propargyl-allenyl type contain an additional n bond in the plane perpendicular to the allyl anion MO (Fig. 1). 

Copper salts such as CuCN, CuBr-SMc2 and CuCN-2LiCl were demonstrated to mediate the regioselective allylation, alkylation, and propargylation of the lithium anion of n-butylthiooxazole 128 providing 2,5-disubstituted systems 129. Desulfurization with deactivated W2-Raney nikel produced 5-substituted derivatives <03TL7395>. 

However, not only heterocyclic but also carbocyclic systems can be made via nucleophilic attack at propargylic centers. This is usually done intramolecularly as shown in Eq. (7), when a malonic ester anion displaces a propargylic chloride [16]. 

A 1,3-dipole as shown in Schemes 6-5 and 6-6 corresponds to a system with three parallel atomic p-orbitals, i.e., to an allyl anion, but without net charge. It is, therefore, called an allyl-type 1,3-dipole. The system may contain, however, an additional 7i-bond in the plane perpendicular to the allyl anion type molecular oribtal, and then belongs to the propargyl - allenyl type. Normally, 1,3-dipoles of this type are linear, whereas those of the allyl type are bent. The term 1,3 relates to the reactivity in these positions, not to formal charges. A series of theoretical studies (e. g., by Hiberty and Leforestier, 1978 Yamaguchi et al., 1980 see review of Houk and Yamaguchi, 1984) clearly show, however, that some of these 1,3-dipoles have considerable biradical character (e.g., O3 53% and CH2N2 28% in ab initio calculations at the 4-3IG level). We will return to biradicals in the mechanistic discussion of Sect. 6.3. 

Nitrogen Systems.—Monoaza-compounds. Treatment of the azapentalene anion (745) with dichloromethane and butyl-lithium yields a mixture of indolizine (746 R = H) and compound (747) it has been established that the latter is not a precursor of indolizine.3-Diethylaminoindolizine (746 R = NEt2) is formed by the reaction of 2-bromopyridine with propargyl alcohol and diethylamine in the presence of bis(triphenylphosphine)palladium(ll) dichloride and copper(i) iodide. The pyridinium ylide (748) undergoes 1,5-dipolar cyclization to the tran -dihydroindolizine (749). As in previous years, there have been several reports on the synthesis of the indolizine ring system from pyridinium ylides the methide (750 R = H, R = Ph) adds benzylideneacetophenone to form the tetrahydroindolizine (751), the action of phenyl vinyl sulphoxide. 

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