Reactivity associative substitutions

M. M. Bhadbhade, and D. Srinivas, Effects on molecular association, chelate conformation and reactivity toward substitution in Cu(5-X-salen) complexes, salen = N,N -ethylenebis (salicyli-denaminato), X = H, CH3O and Cl synthesis, X-ray structures and ERR investigations, Inorg. Chem. 32, 5458-5466 (1993). 

Nucleophilic substitution reactions usually proceed readily at both trivalent and pentava-lent phosphorus centres. An important exception is provided by the phosphine oxides, R3P=0 these do not enter into substitution reactions and show little or none of the chemical reactivity associated with their carbon analogues the ketones, R2C=0. Phosphonium compounds are particularly sensitive to attack by nucleophilic reagents, although they are stable to electrophilic attack. 

An especially favorable form of slippage has been used by Basolo and co-workers to explain the unusually high associative substitution lability of Rh(T -indenyl)(CO)2, which is about 10 times more reactive than the (ti -CjHj) analogue. This has come to be known as the indenyl ect. It is proposed that the intermediate formed by slippage is stabilized by the gain in resonance energy in the six-membered ring, as shown in Scheme 5.1. 

The associative substitution reactions of metal carbonyls, and the preparations, bonding and reactivity of thiocarbonyl complexes were also reviewed in 1985. 

Methanol shares chemical properties with other primary aliphatic alcohols, with most of its reactivity associated with the hydroxyl group. Many reactions of methanol involve the cleavage of either the C-OH bond or the 0-H bond, leading to the substitution of the OH group or the proton. Methanol is an important chemical for the synthesis of a wide range of organic compounds. Table 6 lists... 

Quinone Methides. The reaction between aldehydes and alkylphenols can also be base-cataly2ed. Under mild conditions, 2,6-DTBP reacts with formaldehyde in the presence of a base to produce the methylol derivative (22) which reacts further with base to eliminate a molecule of water and form a reactive intermediate, the quinone methide (23). Quinone methides undergo a broad array of transformations by way of addition reactions. These molecules ate conjugated homologues of vinyl ketones, but are more reactive because of the driving force associated with rearomatization after addition. An example of this type of addition is between the quinone methide and methanol to produce the substituted ben2yl methyl ether (24). 

The meaning of the word aromaticity has evolved as understanding of the special properties of benzene and other aromatic molecules has deepened. Originally, aromaticity was associated with a special chemical reactivity. The aromatic hydrocarbons were considered to be those unsaturated systems that underwent substitution reactions in preference to addition. Later, the idea of special stability became more important. Benzene can be shown to be much lower in enthalpy than predicted by summation of the normal bond energies for the C=C, C—C, and C—H bonds in the Kekule representation of benzene. Aromaticity is now generally associated with this property of special stability of certain completely conjugated cyclic molecules. A major contribution to the stability of aromatic systems results from the delocalization of electrons in these molecules. 

Even polyalkoxy-s-triazines are quite prone to nucleophilic substitution. For example, 2,4,6-trimethoxy-s-triazine (320) is rapidly hydrolyzed (20°, dilute aqueous alkali) to the anion of 4,6-dimethoxy-s-triazin-2(l )-one (331). This reaction is undoubtedly an /S jvr-4r2 reaction and not an aliphatic dealkylation. The latter type occurs with anilines at much higher temperatures (150-200°) and with chloride ion in the reaction of non-basified alcohols with cyanuric chloride at reflux temperatures. The reported dealkylation with methoxide has been shown to be hydrolysis by traces of water present. Several analogous dealkylations by alkoxide ion, reported without evidence for the formation of the dialkyl ether, are all associated with the high reactivity of the alkoxy compounds which ai e, in fact, hydrolyzed by usually tolerable traces of water. Brown ... 

The reactivity of Ce, C7, Cg aromatics is mainly associated with the benzene ring. Aromatic compounds in general are liable for electrophilic substitution. Most of the chemicals produced directly from benzene are obtained from its reactions with electrophilic reagents. Benzene could be alkylated, nitrated, or chlorinated to important chemicals that are precursors for many commercial products. 

Depending on the nature of the substituent R, the radical 76 (Scheme 3.53) may be slow to add to double bonds and primary radical termination can be a severe complication (see 3.2.9).30 40The problems associated with formation of a relatively stable radical are mitigated with certain tx-alkoxy (77) and a-alkanesulfonyl derivatives (79).280 In both cases the substituted benzyl radicals formed by a-scission (78 and 80 respectively) can themselves undergo a facile fragmentation to form a more reactive radical which is less likely to he involved in primary radical termination (Scheme 3.55, Scheme 3.56). 

---