Polar reagents, 1,2-addition

The general reactivity of higher a-olefins is similar to that observed for the lower olefins. However, heavier a-olefins have low solubihty in polar solvents such as water consequentiy, in reaction systems requiting the addition of polar reagents, apparent reactivity and degree of conversion maybe adversely affected. Reactions of a-olefins typically involve the carbon—carbon double bond and can be grouped into two classes (/) electrophilic or free-radical additions and (2) substitution reactions. 

A variety of polar reagents add to P=C or P=N bond when more electronegative part is bonded to phosphorus. Addition of two water molecules on P=C bond in [l,4,2]diazaphospholo[4,5-a]pyridine (76) and P=N bond in [l,2,3]diazaphos-pholo[ 1,5-aIpyridinc (2) causes ring opening (Scheme 23) [16, 68], In the case of 76 (R=C02Et), further hydrolysis to pyridinium salt 78 was observed [73],... 

Scheme 24 Addition of polar reagents on N=P or C=P bond of diazaphospholopyridines...

When a polar reagent adds to the P=C bond of a heterophosphole the more electronegative part generally becomes bonded to the phosphorus. The addition reactions are often reversible and the addition equilibria are more or less balanced. 

Table 6-1 shows the results of electrophilic addition of polar reagents to ethylene. 

Substitution rather than addition reactions with polar reagents such as HNO, H2SO4 and Btj. these reactions the aromatic unsaturated ring is preserved. 

The polarity of the carbonyl bond facilitates addition of water and other polar reagents relative to addition of the same reagents to alkene double bonds. This we have seen previously in the addition of organometallic compounds... 

Exercise 16-3 The foregoing discussion explicitly refers to addition of polar reagents to carbonyl groups. Therefore an ionic mechanism is implied. Consider whether the same reactivity differences would be expected forethene and methanal intherad/ca/-chain addition of hydrogen bromide to methanal and ethene initiated by peroxides. What about the relative equilibrium constants Show your reasoning. (Review Section 10-7.)... 

It seems clear that cluster-substituted carbonium ions are easily generated, are very stable thermally, and are of sufficient electrophilic reactivity to be useful in the synthesis of many new functional cluster compounds. Before we consider further their exceptional stability and the mode of their stabilization, it is of interest to mention some chemical consequences of the high stability of a positive charge generated a to the apical carbon atom of the (OC)9Co3C cluster. For instance, one might expect the addition of the positive part of an ionic or polar reagent, X+Y-, to the... 

The saturated five-membered cyclic ether resembles furan but has four additional hydrogen atoms. Therefore, it is called tetrahydrofuran (THF). One of the most polar ethers, tetrahydrofuran is an excellent nonhydroxylic organic solvent for polar reagents. Grignard reactions sometimes succeed in THF even when they fail in diethyl ether. 

The first reaction that you met in this book, in Chapter 2, was the nucleophilic addition to a carbonyl group. Yet we have shown you no examples of radicals adding to carbonyl groups. This typical reaction of polar reagents is really quite rare with radicals. 

The addition reactions of the polar reagents HCN and formaldehyde occur via the facile formation of the adducts AD4..8 and AD4-9. Moderate activation energies of 17.7 and 49.3 kJ/mol respectively lead to the transition states of the cycloaddition (Scheme 4). The optimized stmctures of AD4.9 and... 

Scheme 4. Energy diagram (in kJ/mol) for the cycloaddition of polar reagents with 4 (top addition of OCH2 bottom addition of HCN).

Magnesio-ene reaction, 46 47, 356 357 Mechanism of Barbier reaction, 406 407 Mechanism of Grignard reagents addition reactions. 219 248 concerted (polar), 221, 237 242 cyclic transition state see Mechanism of Grignard reagents, addition reactions, concerted [polar]) development of reactivity spectrum. 225-235, 246... 

Our results and related literature data (7) clearly show that the P=E double bond in two-coordinate phosphines undergoes facile addition of polar reagents to give three-coordinate P derivatives. It is also well known that the three-coordinate pin center is relatively nucleophilic and that, in molecules containing an electrophilic site, further chemical transformations are usually observed. Thus, in certain reactions of the two-coordinate phosphines, it is possible to find both types of reactions electrophilic addition to the P=E bond, followed by nucleophilic attack by the three-coordinate P atom on an internal electrophilic center. For example, in the enol form of a ketone, the O-H moiety is capable of undergoing addition to the P=E double bond of the two-coordinate phosphines. Acetylacetone and related reagents react rapidly with the phosphinimine at low temperature (ca. -50 C) and with the (methylene)phosphine at 0 °C to give the new 2,3-dihydro-... 

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