4- Membered rings geometry

In general, M—Sb heterocycles either adopt four- or six-membered ring geometries of the type [R2MSbR x (x - 2, 3). The only structurally characterized Al—Bi heterocycle, [Me2AlBi(Tms)2]3 43, also forms a six-membered ring. A1—E heterocycles. The central structural parameters of Al—Sb heterocycles are summarized in Table 9. 

M-E heterocycles of the type [R2MER2]X generally adopt either four- or six-membered ring geometries (x = 2, 3), as was shown already for a large number of M-N heterocycles. This is illustrated in Table XVII, summarizing the number of structurally characterized four- and six-membered heterocycles [R MER (E = N, P, As).94... 

The strength of the hydrogen bond in the enol is 1.7 times the hydrogen bond strength of the alcohol (3.2 vs. 1.9 kcal/mol). This is probably due to the better planar five-membered ring geometry for intramolecular bonding that can be attained in the enol. The OH - F bond... 

The enzyme catalyzed reactions that lead to geraniol and farnesol (as their pyrophosphate esters) are mechanistically related to the acid catalyzed dimerization of alkenes discussed m Section 6 21 The reaction of an allylic pyrophosphate or a carbo cation with a source of rr electrons is a recurring theme m terpene biosynthesis and is invoked to explain the origin of more complicated structural types Consider for exam pie the formation of cyclic monoterpenes Neryl pyrophosphate formed by an enzyme catalyzed isomerization of the E double bond m geranyl pyrophosphate has the proper geometry to form a six membered ring via intramolecular attack of the double bond on the allylic pyrophosphate unit... 

This stereoelectronic requirement would lead to a large distortion of the normal geometry of a five-membered ring and introduce strain. It is this distortion and strain that disfavor the 5-endo-trig cyclization. In contrast, 5-endo-dig cychzation is feasible because the acetylenic system provides an orbital that is available for a nearly planar mode of approach. 

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