Iodine Lewis acids

In the case of the four-parameter equation, the enthalpies of interaction of a large number of acids and bases were determined calorimetrically in an inert solvent. With these values being known, a value of 1.00 was assigned for EA and CA for the Lewis acid iodine. The experimental enthalpies for the interaction of iodine with several molecular Lewis bases were fitted to the data to determine EB and CB values for the bases. Values were thus established for the four parameters for many acids and bases so that they can be used in Eq. (9.112) to calculate the enthalpies of the interactions. The agreement of the experimental and calculated enthalpies is excellent in most cases. However, the four-parameter approach is used primarily in conjunction with interactions between molecular species, although extensions of the approach to include interactions between charged species have been made. Table 9.7 gives the parameters for several acids and bases. 

Iodine is unique among the halogens in that it initiates polymerization of the more reactive monomers even in the absence of a Lewis acid. Iodine not the actual initiator when it is used in the absence of a Lewis... 

Breaking Carbon-Heteroatom Bond Dehydration. The dehydration of alcohols to alkenes is a particular example of a wide range of fS (or 1,2-) eliminations (103-105). The reaction in the homogeneous phase can be characteristically promoted by Br0nsted acids, but certain Lewis acids (iodine, ZnCl2) may also be used. 

Guiheneuf, G., Laurence, C. and Wojtkowiak, B. (1971) Substituent effects on the properties of the carbonyl group. II. Determination of the relative basicity of the carbonyl group towards the Lewis acid, iodine. Bull. Soc. Chim. Fr., 1157—1163. 

The most widely used reactions are those of electrophilic substitution, and under controlled conditions a maximum of three substituting groups, e.g. -NO2 (in the 1,3,5 positions) can be introduced by a nitric acid/sul-phuric acid mixture. Hot cone, sulphuric acid gives sulphonalion whilst halogens and a Lewis acid catalyst allow, e.g., chlorination or brom-ination. Other methods are required for introducing fluorine and iodine atoms. Benzene undergoes the Friedel-Crafts reaction. ... 

The irradiation of calciferol in the presence of iodine leads to the formation of 5,6-/n7 j -vitaniin D2 [14449-19-5] (31) or [22350 1-0] (32) (67,68). 5,6-/ra j -Vitainin D as well as vitamin D (2) or (4) can be converted to isovitamin D by treatment with mineral or Lewis acids. Isocalciferol (35) [469-05-6] or (36) [42607-12-5] also forms upon heating of 5,6-/ -vitamin D. Isotachysterol (33) [469-06-7] or (34) [22350-43-2] forms from isocalciferol or vitamin D upon treatment with acid, and its production appears to be the result of sequential formation of trans- and isocalciferol from calciferol. These reactions are the basis of the antimony trichloride test for vitamin D (69—72). 

In the presence of halogen Lewis acids, such as metal haUdes or iodine, aromatic hydrocarbons are halogenated on the ring (24). 

Aromatic Ring Reactions. In the presence of an iodine catalyst chlorination of benzyl chloride yields a mixture consisting mostly of the ortho and para compounds. With strong Lewis acid catalysts such as ferric chloride, chlorination is accompanied by self-condensation. Nitration of benzyl chloride with nitric acid in acetic anhydride gives an isomeric mixture containing about 33% ortho, 15% meta, and 52% para isomers (27) with benzal chloride, a mixture containing 23% ortho, 34% meta, and 43% para nitrobenzal chlorides is obtained. 

Complex [(CXI )Ir(/j,-pz)(/i,-SBu )(/j,-Ph2PCH2PPh2)Ir(CO)] reacts with iodine to form 202 (X = I) as the typical iridium(II)-iridium(II) symmetrical species [90ICA(178)179]. The terminal iodide ligands can be readily displaced in reactions with silversalts. Thus, 202 (X = I), upon reaction with silver nitrate, produces 202 (X = ONO2). Complex [(OC)Ir(/i,-pz )(/z-SBu )(/i-Ph2PCH2PPh2)Ir(CO)] reacts with mercury dichloride to form 203, traditionally interpreted as the product of oxidative addition to one iridium atom and simultaneous Lewis acid-base interaction with the other. The rhodium /i-pyrazolato derivative is prepared in a similar way. Unexpectedly, the iridium /z-pyrazolato analog in similar conditions produces mercury(I) chloride and forms the dinuclear complex 204. 

Treatment of commercially available and symmetrical 3,4,5-tri-methoxytoluene (37) with iodine, periodic acid, and acetic acid under the conditions of Suzuki19 results in the formation of symmetrical diiodide 38 in 93 % yield. Although only one of these newly introduced iodine atoms is present in intermediate 13, both play an important role in this synthesis. Selective monodemethylation of 38 with boron trichloride furnishes phenol 39 in 53% yield together with 13 % of a regioisomer. Evidently, one of the Lewis-basic iodine substituents coordinates with the Lewis-acidic boron trichloride and directs the cleavage of the adjacent methyl ether... 

Lewis acids have been found to increase the rate of hydrogen exchange in halogen acids, and thus iodine increases the rate of hydrogen exchange in hy-... 

The equilibrium between propargyl- and allenyl-tin compounds is not spontaneous, but it occurs in the presence of Lewis acids or coordinating solvents, and an ion-pair mechanism has been proposed (159). Substitution by iodine, or addition to chloral, occurs with propargyl/al-lenyl rearrangement (160, 161), analogous to the allylic rearrangement already mentioned. 

The introduction of the halogens onto aromatic rings by electrophilic substitution is an important synthetic procedure. Chlorine and bromine are reactive toward aromatic hydrocarbons, but Lewis acid catalysts are normally needed to achieve desirable rates. Elemental fluorine reacts very exothermically and careful control of conditions is required. Molecular iodine can effect substitution only on very reactive aromatics, but a number of more reactive iodination reagents have been developed. 

The attempted exchange reaction between 27 and phenylbismuth diiodide gave only intractable products. Apparently the bismole 31 is unstable under these Lewis acidic conditions. However, iodination of 27 gave 30 which, on lithiation followed by reaction with phenylbismuth diiodide, afforded the desired l-phenyl-2,5-dimethylbismole (31)," which could be... 

Negishi previously reported that a wide variety of Lewis-acidic compounds catalyzed hydrozirconation of alkenes such as 1-decene 108 with /-BuZrCp2Cl.126 It was found that the reaction of 1-decene with 1.1 molar equiv. of TIB A, in the presence of 2-5 mol% of chlorine-containing late transition metals, led to the formation of 1-iododecane 110 after treatment of the product with iodine (Table 9). 

In the highly competitive arena surrounding the Pfizer compounds CP-263,114 and CP-225,917 (Figure 4.2), Nicolaou and co-workers employed a hydrozirconation—iodination sequence to produce vinyl iodide 4 [17]. Lithium—halogen exchange and subsequent conversion to enone 5 sets the stage for a Lewis acid assisted intramolecular Diels—Alder reaction affording polycyclic 6 as the major diastereomer (Scheme 4.3). 

A third method of estimating solvent basicity is provided by the donor number concept 14 ). The donor number of a solvent is the enthalpy of reaction, measured in kcal per mole, between the solvent and a Lewis add such as antimony (V) chloride. (Other Lewis acids, such as iodine or trimethyltin chloride, may be used, but the scale most often reported is that for SbCl5.) Available values for the SbCls donor number have been included in Table 1. Plots of the Walden product versus solvent basicity (A//SbC1 ) for several solvents are shown for lithium, sodium, and potassium ions in Fig. 10 and for the tetraalkylammon-... 

Lewis acid catalyst is normally required when ammonium polyhalides are used, although recourse does not have to be made to strong acids, such as aluminium trichloride. Bromination and iodination reactions are normally conducted in acetic acid in the presence of zinc chloride [32], but chlorination using the ammonium tetrachloroiodate in acetic acid does not require the additional presence of a Lewis acid [33]. Radical chlorination of toluenes by benzyltrimethylammonium tetrachloroiodate in the presence of AIBN gives mixtures of the mono-and dichloromethylbenzenes [34], Photo-catalysed side-chain chlorination is less successful [35], Radical bromination using the tribromide with AIBN or benzoyl peroxide has also been reported [36, 37],... 

A stereoselective tandem iodination and aldol-type condensation has been described for the reaction of methyl propiolate and carbonyl compounds in the presence of a stoichiometric amount of tetra-n-butylammonium iodide and zirconium chloride to yield Z-3-iodo-2-(l-hydroxyalkyl)propenoates, as the major products [48]. No reaction occurs in the absence of the Lewis acid. There does not appear to be any control on the chirality of the hydroxyl centre. 

Iodine has been reported to possess a mild Lewis acidity and can activate carbonyl groups. It can for example catalyze the addition of pyrroles to cf,]3-unsaturated ketones (Scheme 85) [224], A mixture of pyrrol and 3 equiv. of ketone gave disub-stituted products in up to 92% yield in 10 min with 10 mol% of iodine. In cases when only 1.1 equiv. of ketone was apphed in the reaction, mono- and disubstituted products were isolated in few minutes in up to 95% yield in a ratio between 1 1 and up to 1 5. A-Alkylated pyrroles also participated in the reaction in good yields. 

Next to iodine there is also another class of neutral Lewis acids known. Tetracyanoethylene, dicyanoketene acetals and derivatives can catalyse reaction due to their tt-Lewis acid properties. They promoted the alcoholysis of epoxides [238], tetrahydropyranylation of alcohols [239], monothioacetahzation of acetals [240], and carbon-carbon bond formation of acetals [241,242] and imines [243] with silylated carbon nucleophiles. 

---