Alkyl iodides, 184 Table

Allenes are deprotonated by organolithium bases to yield allenyllithium intermediates. Subsequent treatment of these intermediates with various reactive carbon electrophiles can follow several pathways. An early study showed that terminal allenes bearing a free CH2 substituent afford mainly the direct SE2 substitution product A upon treatment first with BuLi and then with various unbranched alkyl iodides (Table 9.1) [5], A negligible amount of the SE2 propargylic product C was formed under these conditions Small amounts of regioisomeric allene alkylation products B were presumed to arise from 1,3-dilithioallenes. 

As well as conventional kinetic methods, the shock-tube method and the toluene vapour-flow method have also been used for studying the unimolecular elimination of hydrogen iodide from alkyl iodides. Table 12 summarises Arrhenius Parameters for these reactions. 

Polarizability indicates how readily an electron cloud can be distorted. The larger the atom, the more loosely it holds the electrons in its outermost shell, and the more they can be distorted. The more polarizable the atom, the stronger are the van der Waals interactions. Therefore, an alkyl fluoride has a lower boiling point than an alkyl chloride with the same alkyl group. Similarly, alkyl chlorides have lower boiling points than alkyl bromides, which have lower boiling points than alkyl iodides (Table 2.6). 

The reaction of alkyl halides with metal nitrites is one of the most important methods for the preparation of nitroalkanes. As a metal nitrite, silver nitrite (Victor-Meyer reaction), potassium nitrite, or sodium nitrite (Kornblum reaction) have been frequently used. The products are usually a mixture of nitroalkanes and alkyl nitrites, which are readily separated by distillation (Eq. 2.47). The synthesis of nitro compounds by this process is well documented in the reviews, and some typical cases are listed in Table 2.3.92a Primary and secondary alkyl iodides and bromides as well as sulfonate esters give the corresponding nitro compounds in 50-70% yields on treatment with NaN02 in DMF or DMSO. Some of them are described precisely in vol 4 of Organic Synthesis. For example, 1,4-dinitrobutane is prepared in 41 -46% yield by the reaction of 1,4-diiodobutane with silver nitrite in diethyl ether.92b 1-Nitrooctane is prepared by the reaction with silver nitrite in 75-80% yield. The reaction of silver nitrite with secondary halides gives yields of nitroalkanes of about 15%, whereas with tertiary halides the yields are 0-5%.92c Ethyl a-nitrobutyrate is prepared by the reaction of ethyl a-bromobutyrate in 68-75% yield with sodium nitrite in DMF.92d Sodium nitrite is considerably more soluble in DMSO than in DMF as a consequence, with DMSO, much more concentrated solutions can be employed and this makes shorter reaction times possible.926... 

Reactions of alcohols, ethers, and aliphatic halides with carbon monoxide were described as far back as 1948-1953 (173, 195). High pressure and temperature were required, however, for these processes. The use of alkaline media allowed carbonylation of alkyl iodides under mild conditions (example 22, Table VII). More recently, carbonylation of alkyl-nickel complexes was reported (example 26, Table VII). 

The conventional procedure involves 5 steps via telomerization of perfluoro-alkyl iodide with perfluoroethylene and ethylene followed by oxidation. The estimated scale of perfluorination of C8H17COOH or CH3S03H at a Ni anode in liquid HF in the US, Japan and UK amounts to about 100 tonnes per year cf. Table 8. 

Ohtomi et al. (1976) have studied the catalytic effect of polypode ligands, such as [ 116] on the reactions of alkyl halides under liquid-liquid phase-transfer conditions (Table 34). Primary alkyl iodides are seen to be more reactive than the corresponding bromides. In contrast, the reactivity towards CN- declines in the order RBr > RI > RC1. It is interesting to note that this order differs from that observed in solid-liquid two-phase systems catalysed by crown ethers (Cook et al., 1974). 

Table 17 Secondary deuterium KIEs for the Menshutkin reactions of deuterated pyridines and alkyl iodides in nitrobenzene at 25°C.a...

Primary alkyl iodides and bromides are excellent substrates for the Victor Meyer reaction, providing a route to both substituted and unsubstituted nitroalkanes (Table i. i).63,65,70,7i formation of the corresponding nitrite ester is a side-reaction and so the nitroalkane is usually isolated by distillation when possible. The reaction of primary alkyl chlorides with silver nitrite is too slow to be synthetically useful. Secondary alkyl halides and substrates with branching on... 

The scope of these reactions is illustrated by the variety of functionalized nitrate esters which can be prepared (Table 3.1). In general, primary and secondary alkyl iodides and bromides... 

Table 1 Polyfunctional products obtained by the Ni(ll)-cat zedcross<50UFling of Arylzinc bromides and alkyl iodides in the presence of 4-(trifluoromethyl)styrene. 

The H.T.-Cl can also be a catalyst for alkyl iodide production in toluene. Thus, benzyl iodide could be obtained by the reaction of benzyl chloride (33 mmol) with butyl iodide (33 mmol) at 373 K (Table 1). 

Finally, alkyl iodides, some of which have natural sources, are of interest for both the troposphere and stratosphere. Figure 4.46 shows the absorption spectra of some simple alkyl iodides and Table 4.42 the absorption cross sections for CH3I (Roehl et al., 1997). 

The radicals were generated in the photolysis of the appropriate alkyl iodide in the presence of excess C02 to minimize hot radical and wall effects. The analysis was identical to that of Christie s work with CH3I, previously described (Sect. VII-B). The ratios found for k2i/ (k22 + k23) were 7, 11, and 22, respectively, for C2H5, n-C3H7, and z-C3H7 radicals. Lower limits for reaction (24) are known, and thus minimum values for k22 + k23 could be estimated (Table 7-3). 

Ketones can be a-brominated on solid phase by treatment with synthetic equivalents of bromine, such as pyridinium tribromide (Entry 2, Table 6.1) or phenyltri-methylammonium tribromide (DCM, 20 °C, 3 h [10]). Resin-bound organometallic compounds, such as vinylstannanes [11] or organozinc derivatives [12], react cleanly with iodine to yield the corresponding vinyl or alkyl iodides (see also Section 3.13). Additions of halogens or their synthetic equivalents to C=C double bonds on cross-... 

The Mitsunobu reaction is usually only suitable for the alkylation of negatively charged nucleophiles rather than for the alkylation of amines, and only a few examples of such reactions (mainly intramolecular N-alkylations or N-benzylations) have been reported (Entry 15, Table 10.2). Halides, however, are very efficiently alkylated under Mitsunobu conditions, and it has been found that the treatment of resin-bound ammonium iodides with benzylic alcohols, a phosphine, and an azodicarboxylate leads to clean benzylation of the amine (Entry 9, Table 10.3). Unfortunately, alkylations with aliphatic alcohols do not proceed under these conditions. The latter can, however, also be used to alkylate resin-bound aliphatic amines when (cyanomethyl)-phosphonium iodides [R3P-CH2CN+][r] are used as coupling reagents [62]. These reagents convert aliphatic alcohols into alkyl iodides, which then alkylate the amine (Entry 10, Table 10.3). 

Table 4.18. 13C Chemical Shifts of Alkyl Iodides in Different Solvents (<5C in ppm) [253].

Br —4, and 1+16 kcal mole-1)119 reflect the decreasing bond dissociation energies for H—X and C—X bonds in the series F, Cl, Br, I, and the relatively constant Z)(X—X) (Table 9.4). The highly exothermic fluorination requires no external initiators and occurs violently and uncontrollably on mixing fluorine with a hydrocarbon either in the gas or liquid phase. Chlorination must be initiated, but proceeds readily, whereas bromination frequently requires elevated temperatures. Iodination is rarely successful, and indeed is more likely to occur in the reverse direction as reduction of alkyl iodides by HI.120... 

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