A-Bonded adducts

Endo adducts are usually favored by iateractions between the double bonds of the diene and the carbonyl groups of the dienophile. As was mentioned ia the section on alkylation, the reaction of pyrrole compounds and maleic anhydride results ia a substitution at the 2-position of the pyrrole ring (34,44). Thiophene [110-02-1] forms a cycloaddition adduct with maleic anhydride but only under severe pressures and around 100°C (45). Addition of electron-withdrawiag substituents about the double bond of maleic anhydride increases rates of cycloaddition. Both a-(carbomethoxy)maleic anhydride [69327-00-0] and a-(phenylsulfonyl) maleic anhydride [120789-76-6] react with 1,3-dienes, styrenes, and vinyl ethers much faster than tetracyanoethylene [670-54-2] (46). 

Addition of pyrazole to C—X double bonds is also common. Formaldehyde gives stable adducts (260) and (261) (69BSF2064), but in the addition to ketones, (262) is only observed at low temperatures (Section 4.04.1.3.3(i)). However, hexafluoroacetone forms a stable adduct (262 R = Cp3) that has been used as a chelating agent (Section 4.04.2.1.3(iv)). Addition of pyrazoles to aryl isocyanates affords (263) the addition is also reversible, but it requires high temperatures to dissociate the adduct (Section 4.04.1.5.1). 

Acid-base interactions in the most general Lewis sense occur whenever an electron pair from one of the participants is shared in the formation of a complex, or an adduct . They include hydrogen bonding as one type of such a bond. The bond may vary from an ionic interaction in one extreme to a covalent bond in the other. Acid-base interactions and their importance in interfacial phenomena have been reviewed extensively elsewhere [35,78] and will be described only briefly here. 

An ingenious approach to the synthesis of steroids incorporating a tropone A ring has been developed by Birch and co-workers. Addition of dibromocarbene to 3-methoxyestra-2,5(10)-dien-17-one 17-ethylene ketal (42) gives a monodibromocarbene adduct formulated as (43) accompanied by a minor amount of a bisadduct. This confirms earlier observations that electrophilic halocarbenes add mainly to 2,3- or 2,5-dihydroanisoles at the double bond bearing the methoxyl group. 

Specific alterations of the relative reactivity due to hydrogen bonding in the transition state or to a cyclic transition state or to electrostatic attraction in quaternary compounds or protonated azines are included below (cf. also Sections II, B, 3 II, B, 5 II, C and II, F). A-Protonation is often reflected in an increase in JS and therefore the relative reactivity can vary with the significance of JS in controlling the reaction rate. Variation can also result from rate determination by the second stage of the SjjAr2 mechanism or from the intervention of thermodynamic control of product formation. Variation in the rate and in the reactivity pattern of polyazanaph-thalenes will result when nucleophilic substitution [Eq. (10)] occurs only on a covalent adduct (408) of the substrate rather than on its aromatic form (400). This covalent addition is prevented by any 4-... 

Active carbonyl compounds such as benzaldehyde attack the electron-rich double bond in DTDAFs to give a dipolar adduct, which immediately undergoes dissociation with formation of two molecules of 146 (64BSF2857 67LA155).Tlie existence of by-products such as benzoin led to the synthetic application of thiazolium salts in the acyloin condensation. For example, replacement of the classic cyanide ion by 3-benzyl-4-methyl-5(/3-hydroxyethyl) thiazolium salts allowed the benzoin-type condensation to take place in nonaqueous solvents (76AGE639) (Scheme 57). 

Six-coordination is obtained in [PtMe3(acac)]2 by bidentate (0,0 ) behaviour and by a bond to the 7-carbon, a situation maintained in solution at room temperature (on warming, the bond to the 7-carbon breaks). In the bipyridyl adduct, it is the bond to the 7-carbon that completes the octahedral coordination [192],... 

Nucleophilic addition to acetylenic sulfoxides provides a,/ -ethylenic sulfoxides. Treatment of 181 with monoalkyl-copper afforded nearly quantitatively /J-alkylated a, / -ethylenic sulfoxides 182 through cis-addition to the triple bond. The reaction with lithium dimethylcuprate also afforded a similar adduct however, the reaction with lithium di-n-butylcuprate was found to give a small amount of ethyl n-butyl sulfoxide 183 besides the... 

In type A, the adduct loses water (or, in the case of addition to C=NH, ammonia, etc.), and the net result of the reaction is the substitution of C=Y for C=0 (or C= NH, etc.). In type B, there is a rapid substitution, and the OH (or NH2, etc.) is replaced by another group Z, which is often another YH moiety. This substitution is in most cases nucleophilic, since Y usually has an unshared pair and SnI reactions occur very well on this type of compound (see p. 434), even when the leaving group is as poor as OH or NH2. In this chapter, we shall classify reactions according to what is initially adding to the carbon-hetero multiple bond, even if subsequent reactions take place so rapidly that it is not possible to isolate the initial adduct. 

In—W bond. Use of Ph3Al leads to a complex in which the oxygen atom of a carbonyl ligand is the site of electron pair basicity in a WC=OAl link. Solutions of [n-Bu4N][Ph3GaCpW(CO)3] in CH2CI2 contain, in addition to free [CpW(CO)3], two isomeric complexes a metal-metal-bonded species and a C- and O-bonded adduct of the type found in the Ph3Al case. 

Boric acid (H3 BO3) and boron trifiuoride (BF3) exempiity the bonding patterns of boron compounds. As Figure 21-6 shows, both these compounds contain deiocaiized bonds. Each acts as a Lewis acid, readiiy adding an additionai anion to form [B (OH) ] and [BF4 ] adducts. Both adducts are tetrahedrai species in which the bonding can be described using four a bonds formed from sp hybrid orbitais on the boron atom. 

Although Lewis and Bronsted bases comprise the same species, the same is not true of their acids. Lewis acids include bare metal cations, while Bronsted-Lowry acids do not. Also, Bell (1973) and Day Selbin (1969) have pointed out that Bronsted or protonic acids fit awkwardly into the Lewis definition. Protonic acids cannot accept an electron pair as is required in the Lewis definition, and a typical Lewis protonic add appears to be an adduct between a base and the add (Luder, 1940 Kolthoff, 1944). Thus, a protonic acid can only be regarded as a Lewis add in the sense that its reaction with a base involves the transient formation of an unstable hydrogen bond adduct. For this reason, advocates of the Lewis theory have sometimes termed protonic adds secondary acids (Bell, 1973). This is an unfortunate term for the traditional adds. 

Triphenylsilanol also forms simple hydrogen-bonded adducts with ethers. For example, the crown ether 12-crown-4 forms a 2 1 adduct... 

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