Dichlorides, from alkenes

Sulfur dichloride is difficult to handle it disproportionates readily, has an unpleasant odor, and tends to introduce extra, unwanted chlorine atoms. Several reagents have been developed, especially by Harpp and co-workers, in which these undesirable features have been modified or eliminated. Succinimide-N-sulfenyl chloride (28) and phthalimide-N-sulfenyl chloride (29) are both stable crystalline compounds which undergo many of the reactions of the sulfur dichloride itself. They can, for example, be used in a facile, high yield synthesis of thiiranes from alkenes.33... 

Table 12. /(-Chloroalkyl-Substituted Methylselenium Dichlorides from Addition of Methylselenium Trichloride to Alkenes...

The analogy with alkene chemistry can be continued in that alkynes react with bromine, chlorine, or iodine, but only one of the two Ji-bonds is used. The reaction is known as dihalogenation of alkynes. When 2-hexyne reacts with chlorine (CI2), the alkyne reacts as a Lewis base and the isolated product is the vinyl dichloride 110 (J5-l,2-dichloro-l-pentene). Formation of this product is explained by an intermediate vinyl-chloronium ion, 109, which is analogous to the halonium ions formed from alkenes in Section 10.4.1. As with alkenes, the chloronium ion reacts with the nucleophihc chloride ion (CL) via anti attack... 

The mechanisms of formation of Tr-allyl-palladium complexes, both from tetrachloropalladate(n) and allyl alcohol in acidic aqueous solution and from palladium dichloride and alkenes in, for example, dimethyl-formamide, have been discussed. Allylpalladium halide dimers react... 

It is possible to prepare 1-acetoxy-4-chloro-2-alkenes from conjugated dienes with high selectivity. In the presence of stoichiometric amounts of LiOAc and LiCl, l-acetoxy-4-chloro-2-hutene (358) is obtained from butadiene[307], and cw-l-acetoxy-4-chloro-2-cyclohexene (360) is obtained from 1.3-cyclohexa-diene with 99% selectivity[308]. Neither the 1.4-dichloride nor 1.4-diacetate is formed. Good stereocontrol is also observed with acyclic diene.s[309]. The chloride and acetoxy groups have different reactivities. The Pd-catalyzed selective displacement of the chloride in 358 with diethylamine gives 359 without attacking allylic acetate, and the chloride in 360 is displaced with malonate with retention of the stereochemistry to give 361, while the uncatalyzed reaction affords the inversion product 362. 

Reaction of HOCl, formed from calcium hypochlorite and CO2, with highly substituted alkenes in CH2CI2 is a convenient route to aHyUc chlorides (111). Ketones are chlorinated to a-chloroketones by reaction with HOCl Acetone initially gives CH2COCH2CI (112). Methyl ethyl ketone gives 78% CH3CHCICOCH3, 15% CH3CH2COCH2CI, and 7% dichlorides (113). 

The reagent titanocene dichloride reduces carboxylic esters in a different manner from that of 10-86, 19-36, or 19-38. The products are the alkane RCH3 and the alcohol R OH. The mechanism probably involves an alkene intermediate. Aromatic acids can be reduced to methylbenzenes by a procedure involving refluxing first with trichlorosilane in MeCN, then with tripropylamine added, and finally with KOH and MeOH (after removal of the MeCN). The following sequence has been suggested ... 

AICI3 is a moisture-sensitive and strong Lewis acid. It is a first choice for Friedel-Crafts-type reactions, which provide numerous important transformations in laboratory and industry. It can also be applied to the transformation of alkenes to ketones via alkylaluminum halides.303 Hydrozirconation of an olefin and subsequent transmetalation from zirconium to aluminum gives the corresponding alkylaluminum dichloride, and the subsequent acetylation by acetyl chloride affords the corresponding ketone in high yield (Scheme 66). 

Negishi reported the zirconium-catalyzed enantioselective carboalumination of alkenes, which consisted of a hydroalumination/alkylalumination tandem process.133-135 This permits the asymmetric syntheses of methyl-substituted alkanols and other derivatives, typically with >90% ee, which represents an increase in ee value by 15% from the previously obtained 70-80%.136-138 The hydroalumination/zirconium-catalyzed enantioselective carboalumination of alkenes was carried out using (—)-bis(neomenthylindenyl)zirconium dichloride as the catalyst (Table 15).133... 

The formation of 2-chloroalkylphosphonyl dichlorides (105) from tertiary alcohols and (101) has been ascribed to the intermediate formation of alkenes, as shown.88 Tertiary alcohols are chlorinated by (101), in a mild, efficient procedure which usually occurs with retention of configuration,89 as for (106). 

Reactions of 3-methylthio-4-trimethylsilyl-l,2-butadiene with electron-poor monosub-stituted and disubstituted alkenes were promoted by a catalytic amount of ethylaluminum dichloride, affording the corresponding methylenecyclobutanes with high selectivities and with yields ranging from 37% for methyl crotonate to 97% for methacrylonitrile15. 

Atkenes from sic-dihaiides. The reaction of /c-dihalides with NaSeCH3 or NaSeC6H5 in ethanol or THF-HMPT (3 1) results in alkenes. The reaction involves a formal a rr-elimination in the case of wc-dibromides or uie-chloroiodidcs, but a formal xyn-elimination in the case of w c-dichlorides. Elimination to form alkenes also occurs with /K-haloalkyl phenyl selenides. 

Thiiranes can be formed directly and stereospecifically from 1,2-disubstituted alkenes by addition of trimethylsilylsulfenyl bromide, formed at -78 C from reaction of bromine with bis(trimethylsilyl) sulfide (Scheme 7).12 A two-step synthesis of thiiranes can be achieved by addition of succinimide-A/-sulfe-nyl chloride or phthalimide-A -sulfenyl chloride to alkenes followed by lithium aluminum hydride cleavage of the adducts (Scheme 8).13 Thiaheterocycles can also be formed by intramolecular electrophilic addition of sulfenyl chlorides to alkenes, e.g. as seen in Schemes 914 and 10.13 Related examples involving sulfur dichloride are shown in Schemes 1116 and 12.17 In the former case addition of sulfur dichloride to 1,5-cyclooctadiene affords a bicyclic dichloro sulfide via regio- and stereo-specific intramolecular addition of an intermediate sulfenyl chloride. Removal of chlorine by lithium aluminum hydride reduction affords 9-thiabicyclo[3.3.1]nonane, which can be further transformed into bicyclo[3.3.0]oct-1,5-ene.16... 

Aviv and Gross developed an interesting insertion reaction of diazo compounds into a secondary amine-hydrogen bond in the presence of Fe-corrole complexes (Scheme 7.8) [12], Competition experiments performed in the presence of an amine and an alkene revealed the N—H-insertion reaction to be much faster than the cyclopropanation of the C=C bond. Apart from this chemoselectivity issue, the reactions are characterized by their very short reaction times most insertion reactions were completed within 1 min at room temperature. Most recently, Woo s group reported on a similar process using commercially available iron tetraphenyl-porphyrin [Fe(TPP)] dichloride [13]. 

Dimethyltitanocene (213), called the Petasis reagent, can be used for alkenation of carbonyls (aldehydes, ketones, esters, thioesters and lactones). This reagent is prepared more easily than the Tebbe reagent by the reaction of titanocene dichloride with MeLi. However, this reagent may not be a carbene complex and its reaction may be explained as a nucleophilic attack of the methyl group at the carbonyl [67], Alkenylsilanes are prepared from carbonyl compounds. Tri(trimethylsilyl)titanacyclobutene (216), as a... 

An alternate route to formation of alkyl monolayers is via Lewis acid catalyzed reactions of alkenes with the hydrogen terminated surface. In this approach, a catalyst such as ethyl aluminum dichloride is used to mediate the hydrosilylation reaction of an alkene (or alkyne), resulting in the same type of product as in the case of the photochemical or thermal reactions. This type of reaction is well known based on molecular organosilane chemistry and has also been used successfully to alkylate porous silicon [31]. Although this route has been shown to work on H/Si(lll), the resulting monolayers are found to have lower coverages than those achieved using the photochemical or thermal approach [29], Another concern with this approach is the possibility of trace metal residues from the catalyst that could adversely affect the electronic properties of these surfaces (even when present at levels below the detection limit of most common surface analysis techniques). 

The reaction of the dimethyl-derivative (27) with butoxide ion might be expected to produce the chlorocyclopropene (28) however, in practice two eliminations occur to produce (31) and the carbene (30), which can be trapped by an added alkene. Both products may be derived from (28), by a 1,4- or a formal 1,2-elimination respectively a study using a 14C-label at C-l of (27) showed that the carbene (30) was formed with the label exclusively at C-l, suggesting elimination via (29)32). However, in a related study, the isolated cyclopropene (28) labelled with 12C at C-l has been shown to react with methyl lithium to produce the carbene (30) labelled only at C-2 this suggests either that the reaction of (28) with butoxide follows a completely different course to that with methyl lithium, or that (28) is not involved in the reaction of (27) with base33). In a similar reaction the dichloride (32) has been shown to react with t-butoxide in DMSO to produce the allene (33) the product may be explained in terms of initial elimination to produce (34), followed either by rearrangement to the alkyne (35) and then elimination or by direct 1,4-elimination as in (36), followed in either case by a prototropic shift. Whatever the mechanism, a 12C-label at Ca in (32) is found at Ca in (33) 33). 

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