Substituents endocyclic

Isoxazolines with alkyl substituents are also all liquids (or low melting solids) and incorporation of aryl substituents results in crystallinity. Introduction of carboxy substituents and endocyclic carbonyl or imino groups also has the anticipated effect, with crystalline products being isolated. These trends are illustrated by the data compiled in Table 2. 

Tertiary pyrrolines (49, = 1) and piperideines (49, = 2) (if R = H and the enamine can exist in the monomeric form or if R = aryl) evidently possess an endocyclic -double bond (79,155,156). The stretching frequency of the double bond can be lowered to 1620-1635 cm by conjugation with an aromatic substituent. The double bond of an analogous compound with aliphatic substituents in position 2 may occupy either the endo or the exo position. Lukes and co-workers (157) have shown that the majority of the five-membered-ring compounds, traditionally formulated with the double bond in a position, possess the structure of 2-alkylidene derivatives (50) with an exocyclic double bond, infrared absorption at 1627 cm . Only the 1,2-dimethyl derivative (51) is actually a J -pyrroline, absorbing at 1632 cm . For comparison, l,3,3-trimethyl-2-methylene pyrrolidine (52) with an unambiguous exocyclic double bond has been prepared (54). 

The following types of thiopyran isomerizations have been reported in the last decade valence-bond tautomerism, endocyclic hydrogen shifts and migration of nonhydrogen substituents. Thermal processes will be mentioned here and photochemically induced isomerizations will be discussed in Section V,I. 

Gas phase reactivity toward allyltrimethylsilane was used to compare the reactivity of several cyclic A-acyliminium ions and related iminium ions.203 Compounds with endocyclic acyl groups were found to be more reactive than compounds with exocyclic acyl substituents. Five-membered ring compounds are somewhat more reactive than six-membered ones. The higher reactivity of the endocyclic acyl derivatives is believed to be due to geometric constraints that maximize the polar effect of the carbonyl group. 

The colored form of spironaphthopyran 32 absorbs at A,max of ca. 450 m,70 and the closed spiro form is colorless, which has no absorption band above 400nm. Bulky substituent group is especially important for photochromic sunglass. Introduction of the spiroadamantane or spirobi-cyclo[3.3.1]heptane into the 2-position of naphthopyran increases the resistance to photo-fatigue reaction, since endocyclic double bond induced by 1,7-hydrogen shift in the colored form cannot be formed in 2-adamantyl or 2-bicycloheptanyl group. 

We have recently investigated X-ray structure of 3-methoxycarbonyl-[l,4,2] diazaphospholo[4,5-a]pyridine as first example of molecular structure determination for [l,4,2]diazaphosphole ring [84], The ester substituent lies strictly in the molecular plane with carbonyl group in the trans orientation with the formal C=P bond. Endocyclic P-N and P-C bonds are averaged between respective single and double bond lengths. 

Six-membered cyclic nitronates can undergo rearrangements accompanied by cleavage of the endocyclic N-0 bond or by cleavage of the endocyclic C-0 bond. The latter fact is evidently associated with the presence of substituents, which efficiently stabilize the positive charge, at the C-6 atom. This occurs upon treatment of nitronates (154a,b) with silicon dioxide (Scheme 3.123) (117). 

For unsaturated lactones containing an endocyclic double bond also the two previously described mechanisms are presumably involved and the regio-selectivity of the cyclocarbonylation is governed by the presence of bulky substituents on the substrate. Inoue and his group have observed that the catalyst precursor needs to be the cationic complex [Pd(PhCN)2(dppb)]+ and not a neutral Pd(0) or Pd(II) complex [ 148,149]. It is suggested that the mechanism involves a cationic palladium-hydride that coordinates to the triple bond then a hydride transfer occurs through a czs-addition. Alper et al. have shown that addition of dihydrogen to the palladium(O) precursor Pd2(dba)3/dppb affords an active system, in our opinion a palladium-hydride species, that coordinates the alkyne [150]. 

A comment is appropriate here about a-effects of endocyclic substituents. Such effects are not defined according to eq. [11], since they replace carbon instead of hydrogen atoms in the parent compound. Thus, these SCSs represent differences between the actual shift effects and that of a carbon fragment in the same position. 

The p-carbon signals in heterocyclohexanes 52 (143) respond to the electronegativity of endocyclic substituents similarly to those of carbon atoms in p positions with respect to exocyclic substituents. The slopes of chemical shift vs. electronegativity plots for the p-methylene groups in 52 and 53 are negative (Table 10). For the quaternary P-carbon atom in 53, however, a slight positive slope is observed (164). 

Thus, in the endocyclic case, interaction effects can at best be estimated, whereas if exocyclic substituents are involved and the molecular framework is conformationally rigid, a reliable quantitative measure is obtainable. 

In compounds containing endocyclic substituents the individual SCSs will be referred to the corresponding saturated carbocyclic analog. 

Substituent effects on the endocyclic cleavage of glycosides by trimethylaluminium have been explained in terms of a cyclic C-H O hydrogen-bonded intermediate. ... 

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