Aryl derivatives mechanisms

The basicities of some phosphinamides (84) have been measured and the acid-catalysed hydrolysis studied. Unsubstituted and A -alkyl derivatives follow an A2 mechanism of reversible protonation followed by ratedetermining water attack. However, the rates for the A -aryl derivatives follow Hq (but with a slope of 0.5), and an A mechanism was suggested as most consistent with this fact and the solvent isotope effect. The anomalous dependence on Ho, together with the large negative value of A5, while not necessarily excluding an ionization mechanism, leaves the question in some doubt. 

Studies of the structures of cuprate species were initiated to elucidate the mechanisms by which they interact with substrates and to understand their special reactivities. In the early days these investigations were restricted to solution studies by spectroscopic techniques. It was not until 1982 that the first example of a cuprate species - [(Cu5Ph6)(Li(THF4))j - was structurally characterized by X-ray crystal structure determination [100] (vide infra). It should be noted that most of these studies, reviewed previously [29, 45, 101], were limited to simple alkyl and aryl derivatives. 

Betaines 261 are stable crystalline compounds. Knowledge of their chemical reactions is still limited. Alkaline hydrolysis of aryl derivatives (261 R = Ar) gives the 2-azobenzoic acids 266 but the mechanism of this rearrangement is unknown. Reduction by tin and hydrochloric acid gives the hydrazides 267. Thermolysis of the p-tolyl compound (261 R = p-MeC6H4) (120 C at 0.1 mm Hg) gives the isomeric triazine (268 R = >-MeC6H4). Phosphorus pentasulfide converts the 2-methyl derivative (261 R = Me) into 2-methyl l,2,3-benzotriazinium-4-thiolate (272 R = Me) (Section in,B,15). 

Bis(r 6-aryl) derivatives of molybdenum have been prepared by the co-condensation of the arene and molybdenum atoms at 77 K.305 Friedel-Crafts reactions catalysed by [(r 6-arene)Mo(CO)3] complexes have been developed further, and such reactions with organic halides shown to proceed via an ionic mechanism.306... 

There is a discrepancy in the literature concerning 1,2-dihydro-isoquinoline itself. Thus, Huckel and Graner54 report its trimerization to 31 (m.p. 138°, the same melting point that Packham and Jackman11 ascribe to the monomer). It is thought63 that in a nonpolar solvent, 1,2-dihydroisoquinoline is relatively stable, but in methanol, protonation and trimerization occur certainly the mass spectrum of the compound (m.p. 138°) described by Packham and Jackman indicates that it is trimeric.63 The series of 1,2-dialkyl-1,2-dihydroisoquinolines described by Bradley and Jeffry30 was purified by distillation under reduced pressure. The stability of these compounds is quite remarkable in view of the known tendency for 1,2-dihydroisoquinolines to undergo disproportionation. Some other 1,2-dialkyl-1,2-dihydroisoquinolines have been described,7 as well as 1-aryl derivatives.7 The derivative (32) when heated with triethyl phosphite is transformed by an unknown mechanism, in 37% yield, into 33.64... 

Several possible mechanisms of polar nucleophilic photosubstitution in an aryl derivative 238 are portrayed in Scheme 6.93. The first, unimolecular nucleophilic photosubstitution mechanism (SN1 Ar where 1 denotes first-order kinetics), in which an excellent leaving group (X) is heteroly tic ally detached from excited state to form a relatively unstable aryl cation and is subsequently attacked by a nucleophile, is rarely observed.836 838... 

One of the first reactions which occurs is exchange of groups between the two metal centers, resulting in the formation of alkyl or aryl derivatives of the transition metal. These compounds are unstable and ultimately decompose, liberating a hydrocarbon fragment with simultaneous reduction of the transition metal The mechanism of these reduction reactions may proceed by way of free radicals in some cases and by concerted paths in other cases. Discussions of this process (6) have been... 

The reactions of 2-amino-2-deoxyaldoses and 1-amino-l-deoxyketoses, and their A-alkyl and A -aryl derivatives, with /3-dicarbonyl compounds give pyrrole derivatives whose nature depends somewhat upon the experimental conditions used. Accounts of these reactions, and discussions of the structures and properties of the products, are given in this Section. The mechanisms are dealt with in Section IV. 

Isoflavonoids are a characteristic and very important subclass of flavonoids. Their structures are based on the 3-phenylchromen skeleton that is chemically derived from the 2-phenylchromen skeleton, by an aryl-migration mechanism. Structurally, isoflavonoids can be classified according to the oxidation of the C15 skeleton, their complexity and the internal formation of heterocyclic rings [6],... 

Since the mechanism involves electron transfer to a heteroatom rather than carbon, vinyl, and aryl derivatives can be easily reduced under these conditions. Many functional groups can be reduced, depending on their ability to accept an electron. Cyclopropane rings are opened under these conditions. The bridging cyclopropane moiety in 498, for example, was opened with lithium and ammonia to give 499 in 85% yield, as... 

The phenyl complex 12 of a similar structure is kinetically more resistant to C-O reductive elimination than the methyl analog 9 as different reductive elimination mechanisms are expected to be operational in these two cases, concerted three-center in the case of the aryl derivative and an Sn2 mechanism in the case of the alkyl complex. In particular, the symmetric phenyl complex (dpms)Pt Ph(OH)2, 12, in Fig. 6 is completely inert in acidic aqueous solutions at 100°C [25], whereas its methyl analog 9, (dpms)Pt Me(OH)2, eliminates methanol via an Sis/2 mechanism at room temperature [9]. 

The mechanisms of Mizoroki-Heck reactions performed from aryl derivatives are presented herein by highlighting how the catalytic precursors, the bases and the ligands may affect the structure and reactivity of intermediate palladium(O) or palladium(II) complexes in one or more steps of the catalytic cycle and, consequently, how they may affect the efficiency and regioselectivity of the catalytic reactions. 

At ambient temperature, the paUadium-catalyzed arylation of indoles proceeds with reagents [Ar2l]BF4 to give C-2 arylated derivatives 70 (Scheme 11.23) [77, 78]. This reaction was proposed to proceed by a different mechanism involving Pd(II)/ Pd(IV) via complexes 71 and 72. Importantly, the reaction of [Ph2l]OTfwith Pd(II) and Pt(II) has been reported to give Pd(IV) species by formal transfer of Ph [79]. 

The first examples of cine-substitution in which the anionic a -adduct is quenched by electrophiles (other than a proton) before elimination takes place have been reported (Scheme 9). Insight into the reaction mechanism enabled the direct transformation of 2-benzylpyridazin-3(2i/)-one and 2-benzyl-6-chloropyridazin-3(2//)-one into the corresponding C(4) alkyl and aryl derivatives (when bromine was used as the electrophile). 

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