Tin hydride reaction

Deuterated and tritiated tin hydrides have been used to prepare deuterated saccharides93 and tritiated steroids46 from alkyl bromides, (equations 68 and 69). It is important to note that isomerization has occurred at the chiral reaction centre in the saccharide reaction (equation 68). For the steroid, the tin hydride reaction is regiospecific, i.e. it only reacts at the more reactive bromide rather than the less reactive chloride site and does not react with the keto group, the hydroxyl group or the acetal group. 

For the primary and secondary a-alkoxy radicals 24 and 29, the rate constants for reaction with Bu3SnH are about an order of magnitude smaller than those for reactions of the tin hydride with alkyl radicals, whereas for the secondary a-ester radical 30 and a-amide radicals 28 and 31, the tin hydride reaction rate constants are similar to those of alkyl radicals. Because the reductions in C-H BDE due to alkoxy, ester, and amide groups are comparable, the exothermicities of the H-atom transfer reactions will be similar for these types of radicals and cannot be the major factor resulting in the difference in rates. Alternatively, some polarization in the transition states for the H-atom transfer reactions would explain the kinetic results. The electron-rich tin hydride reacts more rapidly with the electron-deficient a-ester and a-amide radicals than with the electron-rich a-alkoxy radicals. 

The tertiary a-ester (26) and a-cyano (27) radicals react about an order of magnitude less rapidly with Bu3SnH than do tertiary alkyl radicals. On the basis of the results with secondary radicals 28-31, the kinetic effect is unlikely to be due to electronics. The radical clocks 26 and 27 also cyclize considerably less rapidly than a secondary radical counterpart (26 with R = H) or their tertiary alkyl radical analogue (i.e., 26 with R = X = CH3), and the slow cyclization rates for 26 and 27 were ascribed to an enforced planarity in ester- and cyano-substituted radicals that, in the case of tertiary species, results in a steric interaction in the transition states for cyclization.89 It is possible that a steric effect due to an enforced planar tertiary radical center also is involved in the kinetic effect on the tin hydride reaction rate constants. 

Radical chain processes break down whenever the velocity of a termination reaction is comparable to the velocity of the rate-controlling step in a chain reaction. This situation would occur, for example, if one attempted to use EtsSiH as the hydrogen atom donor in the alkyl halide reduction sequence in Figure 4.6 and employed typical tin-hydride reaction conditions because the rate constant for reaction of the silane with an alkyl radical is 4 orders of magnitude smaller than that for reaction of Bu3SnH. Such a slow reaction would not lead to a synthetically useful nonchain sequence, however, because no radical is persistent in this case. In fact, a silane-based radical chain reduction of an alkyl halide could be accomplished successfully if the velocity of the initiation reaction was reduced enough such that it (and, hence, also the velocity of alkyl radical termination... 

The tin hydride reaction is a standard dehalogenation using the radical to sustain a chain reac -je (pp. 1040-1). The stereochemistry is that of the most stable product with the C02Me group on outside of the folded molecule. In fact, this stereochemistry is also unimportant as it disappear. s. the next reaction. 

Tin hydride reaction sequences are usually initiated by thermal decomposition of azo-Z>/5-isobutyrylnitrile (AIBN), which has a decomposition half-life of about 1 h at 80 °C. Although other initiators can be used at different temperatures, AIBN is... 

The Pd-catalyzed hydrogenoiysis of acyl chlorides with hydrogen to give aldehydes is called the Rosenmund reduction. Rosenmund reduction catalyzed by supported Pd is explained by the formation of an acylpalladium complex and its hydrogenolysis[744]. Aldehydes can be obtained using other hydrides. For example, the Pd-catalyzed reaction of acyl halides with tin hydride gives aldehydes[745]. This is the tin Form of Rosenmund reduction. Aldehydes are i ormed by the reaction of the thio esters 873 with hydrosilanes[746,747]. 

Dimethyl iodo(4-pentenyl)malonate (926) undergoes a Pd-catalyzed intramolecular radical-type reaction to form the alkyl iodides 927 and 928. rather than a Heck-type reaction product(775]. The same products are also obtained by a radical reaction promoted by tin hydride(776]. Although yield was low, a similar cyclization of the n-chloro ester 929 to form the seven-membered ring 930 was ob,served(777(. 

The nucleophilic displacement reactions with azide, primary amines, thiols and carboxylatc salts arc reported to be highly efficient giving high (>95%) yields of the displacement product (Table 9.25). The latter two reactions are carried out in the presence of a base (DBU, DABCO). Radical-induced reduction with tin hydrides is quantitative. The displacement reaction with phenolates,61j phosphines,6M and potassium phthalimide608 gives elimination of HBr as a side reaction. 

The tin hydrides find important applications as reducing agents. Many of their reactions (particularly the reduction of alkyl halides and the hydrostannation of simple alkenes and alkynes) arc known to proceed through RaSn- intermediates, and this aspect of their chemistry is referred to in Section II,G. 

In a modification of this method, the Sn-0 bonded compound can be generated in situ by partial acidolysis of a tin hydride, and, from the reaction between diphenyltin dihydride and carboxylic acids, a number of l,2-bis(acyloxy)-l,l,2,2-tetraphenylditins, (RC02)Ph2SnSnPh2(02CR) (e.g., R = CHj, CF3, PhaSi, or PhjGe), have been prepared (257, 258). 

The versatility, predictability and functional-group tolerance of free radical methodology has led to the gradual emergence of homolytic reactions in the armory of synthetic chemistry. Tin hydrides have been successfully employed in radical chemistry for the last 40 years however, there are drawbacks associated with tin-based chemistry. Organotin residues are notoriously difficult to remove from desired end products, and this, coupled with the fact that many organotin compounds are neurotoxins, makes techniques using tin inappro-... 

The readily available organotin compounds include tin hydrides (stannanes) and the corresponding chlorides, with the tri-n-butyl compounds being the most common. Trialkylstannanes can be added to carbon-carbon double and triple bonds. The reaction is usually carried out by a radical chain process,137 and the addition is facilitated by the presence of radical-stabilizing substituents. 

Disilenes readily add halogens14,66 and active hydrogen compounds (HX), such as hydrogen halides,63,66 alcohols, and water,27 63 as well as hydride reagents, such as tin hydride and lithium aluminum hydride.66 These reactions are summarized in Scheme 9. The reaction of the stereo-isomeric disilene (E)-3 with hydrogen chloride and alcohols led to a mixture of E- and Z-isomers, but the reaction with chlorine gave only one of the two possible stereoisomers, thus indicating that the former two reactions proceed stepwise while the latter occurs without Si—Si rotation. 

Photoreduction of aromatic and aliphatic nitro compounds gives hydroxylamines or amines, which is well reviewed.125 The radical reaction of primary nitro compounds with tin hydride does not give the denitrated product (see Chapter 7), but give the corresponding oximes (Eq. 

The nitro groups in Eqs. 7.88-7.90 are readily replaced by hydrogen with tin hydride under radical conditions as discussed already. However, the nitro groups in the a-nitrosulfides or (3-nitrosulfides are not replaced by hydrogen on treatment with tin hydride but the reaction affords desulfonated products (Eq. 7.51) and alkenes (Eq. 7.97) such radical elimination reactions are discussed in Section 7.3.1. (see Eqs. 7.91 and 7.92).138... 

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