Higher membered rings

The secondary amines used in the preparation of enamines have been primarily simple dialkylamines or cyclic amines of five- or higher-membered rings. Azetidine (4) yields a stable enamine with cyclopentanone (28). No simple enamines formed by condensation of ethylenimine (5) or a substituted ethylenimine with an aldehyde or ketone have been reported. 

Ph2P(NH2)NP(NH2)Ph2]+Cl, 19, which is prepared by the reaction of Ph2PCl3 and ammonia (136). This synthon has a preformed N-P-N-P-N unit and can be readily cyclized to a six or higher membered ring upon reaction with an appropriate reagent (137). An important application of the use of the Bezman s salt approach has been the synthesis of metallocyclophosphazenes of the type 20, which contain an early transition metal in the framework of the cyclophosphazene skeleton [Fig. 15(A)] (138). A modification of the Bezman s salt to design a chiral synthon allowed the synthesis of an optically pure cyclophosphazene (139). 

Eight- and Higher-Membered Rings with N—CO—N or N-CS-N Units... 

Spiro- and Tricyclic Phosphoranes with Six- and Higher-Membered Rings... 

The enone system itself is usually part of a five- or six membered ring, although acyclic a,(3-unsaturated ketones and enols of P-diketones are also found to undergo cycloadditions under certain conditions. For seven- and higher membered rings the primary photochemical event is Z—E isomerization around the C—C double bond, the E-isomer then eventually undergoing further thermal reactions. 

It was demonstrated in Section 3 that spiroannulation of cyclopropane to four- and higher-membered rings does not cause an increase in the strain energy of the molecule. It is not surprising, then, that the chemistry of these compounds does not differ from that of simple alkyl substituted cyclopropanes. Since exciting reviews on the chemistry of cyclopropanes are available42, the authors of this chapter will limit the description of properties of SPC of these types to some of their interesting applications. 

Highly strained 3- and 4-membered rings polymerize practically irreversibly but polymerization of 5-, 6-, 7-, and higher member rings, important from both a basic and practical point of view, is highly reversible. Thus, in these systems, reversibility of propagation step introduces an additional factor which should be taken into account in kinetic and mechanistic studies and which has practical consequences discussed in more detail in Section II.B.l. 

Depending on the size of the ring it may proceed as reversible (5- and higher member rings) or practically irreversible (3,4-membered rings) reaction. 

The synthetic application of azetedine polymerization is limited, however, due to the difficulties with monomer synthesis (the closure of 4-membered ring is much more difficult than the closure of 3-, 5-, or higher member rings). 

Rings containing three silicate tetrahedra are observed in cyclosilicates (e.g., benitoite). The relative prevalence of three-, four-, and higher-membered rings has been discussed by Chakoumakos et al. (1981), and we will return to this topic in Chapter 5. 

This type of reaction is quite new and as yet undeveloped. The possibility that the dipolar intermediate involved in all /-amino-effect reactions discussed thus far reacts in other than the expected manner should be plausible given a suitable conjugated system. It is this type of possibility that allows extension of the /-amino effect to the formation of higher-membered rings. 

Further improved yields may be obtained in the presence of a tertiary amine catalyst, which are known to complex with carbonyl halides for the higher-membered rings, the addition of a catalytic amount of HCl is beneficial. With tetrahydrofuran, for example, the following sequence of reactions is believed to take place ... 

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