Russian Chemical Reviews 68 (9) 765 ± 779 (1999) Synthesis and reactions of mixed halogenobuta-1,3-dienes R V Kaberdin, V I Potkin, V A Zapol'skii Contents I. Introduction II. Synthesis of mixed halogenobuta-1,3-dienes III. Chemical properties of mixed halogenobuta-1,3-dienes IV. Conclusion Abstract. The published data on the methods for the preparation of mixed halogenobuta-1,3-dienes are generalised and syste- matised with a special emphasis on compounds of high practical value. Chemical transformations of mixed halogenodienes are considered and broad possibilities of the use of these compounds for the synthesis of various valuable, but difficultly accessible polyfunctional products are demonstrated. The bibliography includes 115 references. I.Introduction The first representative of halogenobutadienes, viz., hexachloro- buta-1,3-diene, was obtained as early as 1876.1 However, in the subsequent 50 years studies on the synthesis of halogenobuta- dienes were episodic and only in the early 1930s did they become purposeful and systematic. The development of preparation methods and the search for possible uses of this class of compounds in organic synthesis were stimulated by the discovery that mono- and dichlorobuta-1,3-dienes easily polymerise. The first data on the synthesis of halogen derivatives of butadiene were generalised by Petrov in 1944.2 Later studies devoted mainly to the synthesis and transformations of chloro and fluoro derivatives of butadiene were systematised in a fundamental Houben ± Weyl monograph.3 Particularly intensive studies were carried out in the field of chloro derivatives of butadiene and their results were surveyed in reviews 4± 6 and monographs.7±9 At present, there are numerous publications devoted to the synthesis of mixed halogenobutadienes (MHB) containing atoms of different halogens, viz., fluorine, chlorine, bromine and iodine.Mixed halogenobutadienes possess a broad spectrum of useful properties: they are good dielectrics, refrigerants and heat-transfer agents; they are used as aerosols, lubricants and floatation agents and possess algicidal, bactericidal and fungicidal activities.10 ± 15A number of MHB manifest high antitumour activity.16 Chlor- ine ± fluorine-containing butadienes are employed as monomers for the preparation of valuable polymers and copolymers resistant to heat, light, chemical corrosion, etc.13, 17, 18 And, finally, MHB R V Kaberdin, V I Potkin, V A Zapol'skii Institute of Physical Organic Chemistry, National Academy of Sciences of Byelarus, ul.Surganova 13, 220072 Minsk, Byelarus. Fax (7-017) 284 16 79. Tel. (7-017) 284 16 00. E-mail: ifoch@ifoch.bas-net.by (R V Kaberdin, V A Zapol'skii). Tel. (7-017) 284 09 72. E-mail: potkin@ifoch.bas-net.by (V I Potkin) Received 2 March 1999 Uspekhi Khimii 68 (9) 845 ± 860 (1999); translated by V D Gorokhov #1999 Russian Academy of Sciences and Turpion Ltd UDC 547.413 765 765 774 778 are promising semi-products in organic synthesis. For example, the preparation of polyfluoromethine compounds is based on MHB, which opens a way to the synthesis of novel polymethine dyes.19 It should be noted that the introduction of atoms of different halogens into the butadiene molecule leads to a considerable increase in the number of theoretically possible isomers.Thus consideration of structural and geometric isomers shows that dichlorobuta-1,3-diene has nine isomers,7 while for butadiene derivatives with two different halogen atoms the number of possible isomers increases tenfold. From a theoretical aspect, mixed halogenobutadienes, like other compounds with the buta- diene system, are convenient subjects for studying the s-cis ± trans- isomerism associated with the appearance of conformationally non-equivalent structures resulting from the hindered rotation of the vinyl fragments around the central C7C bond.20 ± 22 They are also promising as model substrates for studying regularities of the processes of nucleophilic vinylic substitution (SNVin).This review surveys the literature data on the chemistry of mixed halogen- obuta-1,3-dienes published up to 1999. We presume that such a review will fill the gap in the relevant literature and will contribute to the further development of research into the chemistry of halogen derivatives of alkenes and alkadienes. II. Synthesis of mixed halogenobuta-1,3-dienes The basic methods for the preparation of MHB are: (1) Elimination reactions (dehydrohalogenation and dehalo- genation) of polyhalogenobutadienes and -butenes; (2) halogenation and hydrohalogenation of acetylenic hydro- carbons of the C4 series; (1) thermal decomposition of accessible polyhalogenated compounds.Other methods of the synthesis of individual representatives of MHB are also known. 1. Elimination reactions Numerous chlorofluoro- and bromochlorobutadienes were obtained by dehydrohalogenation and dehalogenation of poly- halogenobutanes and -butenes). Dehydrohalogenation of polyhalogenobutenes under the action of various dehydrohalogenation agents proceeds as the 1,4- or 3,4-elimination of a molecule of hydrogen halide depend- ing on the position of the C=C bond. Another conventional way for the synthesis of MHB is dehalogenation of polyhalogenobu- tanes and -butenes, which is carried out under the action of metals, zinc in most cases.766 a.Dihalogenobutadienes The simplestMHBcontaining two different halogen atoms can be obtained from trihalogenobut-1- and -2-enes. Thus the dehydro- bromination of 1,4-dibromo-2-chlorobut-2-ene (1), which is the main product of chloroprene bromination (2), resulted in 1-bromo-2-chlorobuta-1,3-diene (3) in preparative yield.23, 24 The diene 3 formed in this reaction is a mixture of geometric isomers with the predominance of the Z-isomer 3a.25 Br2 CH2 7HBr CClCH CH2 2 CH2BrCCl CHCH2Br 1 Cl Cl H Br + H H C C C C Br C C C C H H H H H 3b (E-isomer) 3a (Z-isomer) Papazyan et al. 26 have proposed performing the dehydrobro- mination of butene 1 in the presence of phase-transfer catalysts (PTC) that considerably simplified the process and made it more economical (the conditions for the synthesis of the diene 3, its yield and the data related to all subsequent syntheses of MHB by the elimination reaction are presented in Table 1).Chlorination of bromoprene (4) and subsequent dehydro- chlorination of 2-bromo-1,4-dichlorobut-2-ene (5) leads to a mixture of E- and Z-isomers of 2- (6) and 3-bromo-1-chloro- buta-1,3-diene (7).27, 28 Cl2 CH2 7HCl CH2ClCBr CHCH2Cl 5 CBrCH CH2 4CHCl CBrCH CH2+ CHCl CHCBr CH2 7 6 A more convenient synthesis of E- and Z-isomers of the diene 7 is based on the dehydrobromination of 3,4-dibromo-1-chloro- but-1-ene (8), which is the product of bromination of 1-chloro- buta-1,3-diene (9).29 Br2 7 7HBr CHCl CHCHBrCH2Br 8 CHCl CHCH CH2 9 Dehydrohalogenation of the by-products of the chlorine addition to the diene 4, viz., a mixture of 3- (10) and 2-bromo- 3,4-dichlorobut-1-enes (11), yielded a mixture of 2-bromo-3- chlorobuta-1,3-diene (12) and (E)-1,2-dichlorobuta-1,3-diene (13) (GLC).27 CH2ClCBrClCH CH2+ CH2 10 CBrCHClCH2Cl 11 CClCH CH2 7HBr (E)-CHCl 13 CH2 7HCl CClCBr CH2 12 The diene 12 was initially obtained by dehydrobromination of 3,4-dibromo-2-chlorobut-1-ene (14), which is a by-product of chloroprene 2 bromination.24 Br2 2 CH2 CH2 7HBr CClCBr CH2 12 CClCHBrCH2Br 14 The preparative synthesis of the diene 12 is based on the 1,4- elimination of hydrogen halide from 1,2-dibromo-3-chlorobut-2- ene (15) or 2-bromo-1,3-dichlorobut-2-ene (16) or a mixture of these butenes with an alkali (KOH or NaOH) in the presence of PTC.30 R V Kaberdin, V I Potkin, V A Zapol'skii 7HBr CH3CCl CBrCH2Br 15 12 7HCl CH3CCl CBrCH2Cl 16 Of the disubstituted chlorofluorobutadienes, the following three compounds are known: 1-chloro-1-fluoro- (17), 1-chloro-2- fluoro- (18) and 3-chloro-2-fluoro-buta-1,3-dienes (19) (see Table 1).The diene 17 was obtained as the result of dehydrochlorina- tion of 4,4-dichloro-4-fluorobut-1-ene (20),31, 32 whereas the diene (18) resulted from the dechlorination of 3,4,4-trichloro-3-fluoro- but-1-ene (21).33 The diene 19 is formed upon dehydrohalogena- tion of a liquid mixture of mono-, di- and trifluoro compounds which are the products of the reaction of 1,2,3,4-tetrachlorobu- tane with HF.34 Yet another route to the diene 19 consists in the dehydrohalogenation of 2,4-dichloro-1,3-difluorobutane (22), which is the main product of ClF addition to butadiene (see Table 1).35, 36 b.Trihalogenobutadienes The first representative of mixed trihalogenobutadienes, viz., 3-bromo-1,1-dichlorobuta-1,3-diene (23), was initially prepared by the thermal dehydrochlorination of 3-bromo-1,1,1,4-tetra- chlorobutane (24) 37 and later, of 2-bromo-4,4,4-trichlorobut-1- ene (25), which is the product of bromotrichloromethane addition to allene 38 (see Table 1). 3-Bromo-1,2-dichlorobuta-1,3-diene (26) was obtained by a multistep synthesis the last stage of which was dehydro- bromination of 3,4-dibromo-1,2-dichlorobut-1-ene (27), which is the main product of 1,2-dichlorobuta-1,3-diene (13) bromination.The latter is obtained by the dehydrochlorination of 1,2,4- trichlorobut-2-ene (28).39 Papazyan et al.26 simplified the syn- thesis of the diene 26 from the butene 28. The target product 26 was obtained by direct bromination of the butene 28 and subsequent dehydrohalogenation of the adduct formed, viz., 2,3- dibromo-1,2,4-trichlorobutane (29). The by-product of this reac- tion is 2-bromo-1,3-dichlorobuta-1,3-diene (30). Br2 CH2ClCBrClCHBrCH2Cl 29 CH2ClCCl CHCH2Cl 287HCl 7HCl, 7HBr CHCl CClCH CH2 13Br2 7HBr CHCl CClCHBrCH2Br 27 CHCl CClCBr CH2 26 Two methods for the synthesis of 1-bromo-2,3-dichlorobuta- 1,3-diene (31) are known.It is obtained by dehydrobromination of either 1,4-dibromo-2,3-dichlorobut-2-ene (32) 39 [the product of 2,3-dichlorobuta-1,3-diene (33) bromination] 39 or 1,2-dibromo- 2,3,4-trichlorobutane (34) [the product of bromine addition to the accessible 2,3,4-trichlorobut-1-ene (35)].26 CH2 CClCCl CH2 33 CH2ClCHClCCl CH2 35Br2 Br2 CH2ClCHClCBrClCH2Br 34 CH2BrCCl CClCH2Br 32 7HCl,7HBr 7HBrCHBr CClCCl CH2 31767 Synthesis and reactions of mixed halogenobuta-1,3-dienes Table 1. Synthesis of mixed halogenobuta-1,3-dienes by the elimination from halogenobutanes and -butenes. Ref. Yield (%) Halogenobutadiene Reaction conditions Elimination agent Starting compound 23, 24 25 KOH KOH CH2BrCCl=CHCH2Br (1) CH2BrCCl=CHCH2Br (1) 38 31.5, 1.5 10% NaOH NaOH CH2BrCCl=CHCH2Br (1) CH2ClCBr=CHCH2Cl (5) 27, 28 7 EtOH 96% EtOH, 0 8C, 14 h water, 1% katamin AB, EtOH, 40 8C, nitrosodiphenylamine 29 10% NaOH water, katamin AB, 40 8C, 3 h water, katamin AB the same EtOH, nitrosodiphenyl- 30 30 27 60.2 51.6 7 NaOH NaOH NaOH CHBr=CClCH=CH2 (3) (Z)-CHBr=CClCH=CH2 (3a), (E)-CHBr=CClCH=CH2 (3b) CHBr=CClCH=CH2 (3) 72 26 (E)-CHCl=CBrCH=CH2 (6a)+ +(Z)-CHCl=CBrCH=CH2 (6b)+ +CHCl=CHCBr=CH2 (7) (6a : 6b : 7=72 : 2 : 26) (E)-+(Z)-CHCl=CHCBr=CH2 (7) 66.8 (E:Z=82 : 18) CH2=CClCBr=CH2 (12) CH2=CClCBr=CH2 (12) CH2=CClCBr=CH2 (12)+ amine, 46 ± 48 8C +(E)-CHCl=CClCH=CH2 (13) (12 : 13=9:1) CFCl=CHCH=CH2 (17) CHCl=CFCH=CH2 (18) CH2=CClCF=CH2 (19) CCl2=CHCBr=CH2 (23) CHCl=CHCHBrCH2Br (8) (E:Z=3:1) CH3CCl=CBrCH2Br (15) CH3CCl=CBrCH2Cl (16) CH2=CHCBrClCH2Cl (10)+ +CH2=CBrCHClCH2Cl (11) (10 : 11=1:4) CFCl2CH2CH=CH2 (20) CHCl2CFClCH=CH2 (21) CH2FCHClCHFCH2Cl (22) CCl3CH2CHBrCH2Cl (24) 31 33 35, 36 37 53.5 7745.0 KOH Zn KOH 7 38 39 26 CCl3CH2CBr=CH2 (25) Et3 N KOH NaOH *90 65.0 80.0 CHCl=CClCHBrCH2Br (27) CH2ClCBrClCHBrCH2Cl (29) EtOH, boiling the same EtOH thermal dehydro- chlorination, 450 8C boiling, 2 h MeOH, 20 8C 80% EtOH, 40 ± 50 8C 39 26 26 54.0 47.0 90.0 KOH NaOH NaOH CH2BrCCl=CClCH2Br (32) CH2ClCHClCBrClCH2Br (34) CH2BrCBrClCHBrCH2Br (37) MeOH, 20 8C EtOH 80% EtOH CH2ClCBr=CBrCH2Cl (41) 58.0 777 42 43 43 16 47 Zn KOH KOH CCl3CH2CBrFCH3 (46a) Bu3 N CBr3CH2CBrFCH3 (46b) Bu3 N CBrF2CCl2CH=CH2 (48) CFCl=CFCH2CH2Br (50) (E:Z=44 : 56) CCl2=CClCHBrCH2Br (52) 26 93.0 NaOH EtOH, 25 8C 140 8C, 100 Torr the same EtOH EtOH (KOH : EtOH= =1:1) 70% EtOH, 20 ± 25 8C 49 water, TEBAa CHBrClCHBrCHClCHCl2 (57) KOH 91.0 43.5 717.0 the same EtOH, boiling the same EtOH 50 51 51 353 KOH Zn Zn Zn KOH CHBrClCHBrCH=CCl2 (58) CBrF2CFClCCl=CH2 (59) CBrF2CFClCHClCHCl2 (60) CH2=CBrCClFCBrF2 (64) CCl2=CHCBrClCHBrCl (65) 54 20% NaOH CCl2=CHCBrClCHBrCl (65) 55 10% NaOH CBr2ClCHBrCH=CCl2 (67) water, TEBA,a 40 ± 45 8C, 4 h water, TEBA,a 25 8C, 15 h the same 55 10% NaOH CBrCl2CHBrCH=CBrCl (69) 51 19.0 EtOH, boiling CBrF2CFClCCl=CHCl (71) PrnOH or PriOH, boiling CCl2=CHCBr=CH2 (23) CHCl=CClCBr=CH2 (26) CHCl=CClCBr=CH2 (26)+ +CHCl=CBrCCl=CH2 (30) (26 : 30=94.5 : 5.5) CHBr=CClCCl=CH2 (31) CHBr=CClCCl=CH2 (31) CHBr=CClCBr=CH2 (38)+ +CHBr=CBrCCl=CH2 (39) (38 : 39=93 : 7) CHCl=CBrCBr=CH2 (40) CCl2=CHCF=CH2 (42) CBr2=CHCF=CH2 (43) CF2=CClCH=CH2 (44) (E)-+(Z)-CFCl=CFCH=CH2 (45) 80.0 (E:Z=44 : 56) CCl2=CClCBr=CH2 (54)+ +CCl2=CClCH=CHBr (55) (54 : 55=10 : 1) (Z)-+(E)-CBrCl=CHCH=CCl2 (56) 60 (Z: E=4:1) CBrCl=CHCH=CCl2 (56) CF2=CFCCl=CH2 (61) CF2=CFCH=CHCl (62) CF2=CFCBr=CH2 (63) EtOH, 25 8C (Z)-+(E)-CCl2=CHCCl=CBrCl (66) 68.0 (Z: E=1:1) (Z)-+(E)-CCl2=CHCCl=CBrCl (66) 85 (Z: E=1:3) (E)-+(Z)-CBrCl=CBrCH=CCl2 (68) 55.0 (E:Z=7:3) (E)-+(Z)-CBrCl=CHCBr=CCl2 (70) 49 (E:Z=3:2) CF2=CFCCl=CHCl (73) CFCl=CHCF=CF2 (74) 64 56 47.7 73.0 Zn CFCl=CHCFClCF2Cl (72) ZnZn Zn EtOH, boiling EtOH CF2=CFCF=CFCl (75) (E)-+(Z)-CF2=CFCF=CFCl (75) CFCl2CFClCFClCF2Cl (76) CFCl2CFClCFClCF2Cl (76)+ 10, 12 57 +CFCl2CFClCFClCF2I (79) (E:Z=1:1) 10 60, 61 62 EtOH BuOH, 85 ± 95 8C dioxane, boiling Zn Zn Zn CF2ClCFClCCl2CF2Cl CFCl2CFClCFClCFCl2 (77) CFCl2CFClCFClCFCl2 (77) 748.0 82.5 CF2=CFCCl=CF2 (81) CFCl=CFCF=CFCl (82) CFCl=CFCF=CFCl (82) (E,E : E,Z:Z,Z=1 : 2.5 : 1.5)768 Table 1 (continued).Reaction conditions Elimination agent Starting compound EtOH Zn CFCl2CFClCFClCF2I (79)+ CF2ClCCl2CCl2CF2Cl (93) 58 +CFCl2CFClCF2CFClI (89) CF2 =CFCF=CFCl (75) 41 10, 69 70 EtOH water, 25 8C Zn CBrCl2CHBrCCl2CHCl2 (98) Et2 NH KOH 20% KOH, TEBAa (E)-CBrCl=CClCHBrCBrCl2 (101) a Triethylbenzylammonium chloride. 1,1-Dibromo-3-chlorobuta-1,3-diene (36) is obtained as an insignificant admixture in the synthesis of 1,1,3-tribromobuta-1,3- diene.40 Upon dehydrobromination of 1,2,3,4-tetrabromo-2-chloro- butane (37) [the product of exhaustive bromination of chloro- prene (2)], 1,3-dibromo-2-chlorobuta-1,3-diene (38) is mainly formed. In addition, a small amount of 1,2-dibromo-3-chloro- buta-1,3-diene (39) was detected by GLC.26 Br2 CH2 7HBr CH2BrCBrClCHBrCH2Br 37 CClCH CH2 2 CH2BrCBrClCBr CH2+CH2BrCCl CBrCH2Br 7HBr CClCBr CHBr 39 CHBr CClCBr CH2+ CH2 38 2,3-Dibromo-1-chlorobuta-1,3-diene (40) was obtained by of 2,3-dibromo-1,4-dichlorobut-2-ene dehydrochlorination (41) 41 (see Table 1).As regards the fluorine-containing trihalogeno derivatives of MHB, 1,1-dichloro-3-fluoro- (42), 1,1-dibromo-3-fluoro-(43), 2-chloro-1,1-difluoro- (44) and 1-chloro-1,2-difluorobuta-1,3- dienes (45) were obtained. The synthesis of the dienes 42 and 43 is based on dehydrohalo- of 3-bromo-3-fluoro-1,1,1,-trihalogenobutanes genation (46a,b).42 CX2 7HBr,7HX CHCF CH2 42, 43 CX3CH2CBrFCH3 46a,b X=Cl (42, 46a), Br (43, 46b).The butadiene 44 was first synthesised from 2-chloro-1,2- difluoro-4-iodobut-1-ene (47) 43 and later from 1-bromo-1,1- difluoro-2,2-dichloro-2-chlorobut-3-ene (48) 16 and 2,4-dichloro- 1,1,1,-trifluorobut-2-ene (49).44 CF2 7HI CClCH2CH2I 47 Zn CF2 7BrCl CClCH CH2 44 CBrF2CCl2CH CH2 48 Zn 7ClF CF3CCl CHCH2Cl 49 The butadiene 45 is formed as a mixture of E- and Z-isomers upon dehydrobromination of the isomeric forms of 4-bromo-1- chloro-1,2-difluorobut-1-ene (50) obtained by dehalogenation of 1,4-dibromo-1,2-dichloro-1,2-difluorobutane (51), which is the product of telomerisation of CBrFClCBrFCl with ethylene under the action of benzoyl peroxide. It was shown that the starting butene 50 and the target diene 45 have the same ratio of isomers, i.e., the elimination reaction occurs stereospecifically.45 R V Kaberdin, V I Potkin, V A Zapol'skii Ref.Yield (%) Halogenobutadiene 45 CFCl=CFCF=CFCl (82), 788.0 10.0 16 CF2=CClCCl=CF2 (92) CCl2=CBrCCl=CCl2 (99), CBrCl=CBrCCl=CCl2 (100) (E)-CBrCl=CClCBr=CCl2 (102) 60 71 (E,E)-+(E,Z)-CBrCl=CClCBr=CBrCl (103) (E,E : E,Z=2:3) CH2 CH2 CBrFClCBrFCl 7BrCl CBrFClCFClCH2CH2Br 51 7HBr CFCl CFCH2CH2Br 50 (E,Z)F F F Cl + H H C C C C C C C C F Cl H H H H 45b (E-isomer) 45a (Z-isomer) 1,1,3-Trihalogenobutadienes are convenient subjects for the elucidation of characteristic features of conformational behav- iour. The use of 1H NMR and UV spectroscopy made it possible to establish that the bulky substituents (Cl, Br) in the 1,3 (or 2,4) positions of the diene chain destabilise the s-trans-form, which is characteristic of haloprenes and disubstituted butadienes, hence the 1,1,3-trihalogenobuta-1,3-dienes 23 and 36 exist in a skew conformation in the form of a single conformer.40, 46 According to the 1H NMR and IR spectroscopic data, the dienes 42 and 43 having a fluorine atom in the position 3 exist as a mixture of two conformers, viz., the skew and s-trans-forms.42, 47 c.Tetrahalogenobutadienes Several examples of the synthesis of tetrasubstituted MHB based on the elimination reaction are given below. Dehydrobromination of 3,4-dibromo-1,1,2-trichlorobut-1-ene (52), which is the product of bromine addition to the accessible 1,1,2-trichlorobuta-1,3- diene (53),48 affords a mixture of 3- (54, the main product) and 4-bromo-1,1,2-trichlorobuta-1,3-dienes (55, the by-product).26 Br2 CCl2 CCl2 7HBr CClCH CH2 53 CCl2 CClCHBrCH2Br 52CClCH CHBr 55 CClCBr CH2+ CCl2 54 Two convenient methods have been developed for the syn- thesis of 1-bromo-1,4,4-trichlorobuta-1,3-diene (56).The first method consists in dehydrohalogenation of 1,2-dibromo-1,3,4,4- tetrachlorobutanes (57) in the presence of PTC. It was shown that the reaction product is a mixture of geometric isomers with the predominance of the Z-form.49 The second method of the syn- thesis of the diene 56 is based on dehydrobromination of 3,4- dibromo-1,1,4-trichlorobut-1-ene (58) under the conditions of phase-transfer catalysis in the presence of triethylbenzyl- ammonium chloride (TEBA).50Synthesis and reactions of mixed halogenobuta-1,3-dienes CHBrClCHBrCHClCHCl2 577HBr,7HCl H Br H Cl Cl C C Cl C C C C Cl C C Br 56b 56a Cl H H Cl 7HBr CHBrClCHBrCH CCl2 58 Information about tetra(chloro, fluoro) MHB can be found only in the patent literature.Thus dehalogenation of 4-bromo-2,3- dichloro-3,4,4-trifluorobut-1-ene (59) and 1-bromo-2,3,4,4,-tetra- chloro-1,1,2,-trifluoro-butane (60) result in 3-chloro-1,1,2-tri- fluoro- (61) and 4-chloro-1,1,2-trifluorobuta-1,3-dienes (62), respectively.51 CF2 7BrCl CFCCl CH2, 61 CBrF2CFClCCl CH2 59 CHCl. CF2 7BrCl, 7Cl2 CFCH 62 CBrF2CFClCHClCHCl2 60 There is only one known representative of the bromine- containing tetrahalogenobutadienes, viz., 3-bromo-1,1,2-tri- fluorobuta-1,3-diene (63), which was obtained in high yield by dehalogenation of 2,4-dibromo-3-chloro-3,4,4-trifluorobut-1-ene (64) (see Table 1).3 CH2 CH2 7ClBr CBrCClFCBrF2 64 CBrCF CF2 63 d.Pentahalogenobutadienes Some pentasubstituted chlorobromo and chlorofluoro derivatives of butadiene can be obtained by an elimination reaction. Thus dehydrobromination of 3,4-dibromo-1,1,3,4-tetrachlorobut-1- ene (65) (the product of bromine addition to the Z- or E-isomers of 1,1,3,4-tetrachloro-1,3-butadiene) affords a mixture of geo- metric isomers of 1-bromo-1,2,4,4,-tetrachlorobuta-1,3-diene (66), the yield of which depends to a considerable extent on the reaction conditions (see Table 1).52 ± 54CBrCl CCl2 7HBr CHCBrClCHBrCl 65 (E,Z) CClCH CCl2 66 (E,Z) Dehydrobromination of 3,4,4-tribromo-1,1,4-trichlorobut-1- ene (67) in the presence of PTC leads to a mixture of E- and Z-isomers of 1,2-dibromo-1,4,4-trichlorobuta-1,3-diene (68a,b) Similarly, a mixture of E- and Z-isomers of 2,4-dibromo-1,1,4- trichlorobuta-1,3-diene (70a,b) 55 was obtained from 1,3,4-tri- bromo-1,4,4-trichlorobut-1-ene (69).7HBr CBr2ClCHBrCH CCl2 67 Cl Br Br Br Cl C C Cl C C + C C Cl C C Br 68b 68a Cl H H Cl 769 7HBr CBrCl CHCHBrCBrCl2 69 Br H Cl H Cl Cl C C C C + C C Br C C Cl Cl 70b Br Cl 70a Br Dehalogenation of 4-bromo -1,2,3-trichloro-3,4,4-trifluoro- (71) and 1,3,4-trichloro-1,3,4,4-tetrafluorobut-1-ene (72) (see Table 1) yielded 3,4-dichloro-1,1,2-trifluorobuta-1,3-diene (73) and 4-chloro-1,1,2,4-tetrafluorobuta-1,3-diene (74),56 respecti- vely.e. Hexahalogenobutadienes The synthesis of hexasubstituted MHB is documented; in partic- ular, the methods for the preparation of some chlorine, fluorine- containing representatives are well developed. Thus 4-chloro- 1,1,2,3,4-pentafluorobuta-1,3-diene (75) was first obtained by dehalogenation of 1,2,3,4,4,-pentachloro-1,1,2,3,4-pentafluoro- butane (76), which is the reaction product of 1,1,2,3,4,4-hexa- chloro-1,2,3,4-tetrafluorobutane (77) with a slight excess of SbF3Cl2.10, 12 SbF3Cl2 CFCF CFCl Cl(CFCl)4Cl CF2 F(CFCl)4Cl 72Cl2 75 77 76 Later,57, 58 two new methods for the synthesis of the diene 75 have been proposed.The first method is based on the trans- formation of a 1 : 1 mixture of erythro- and threo-forms of 3,4,4- trichloro-1,1,2,3,4-pentafluoro-1-iodobutane (78), which is the product of free-radical addition of 1,2,2-trichloro-1,2-difluoro-1- iodoethane to trifluoroethylene (GLC). Dehydrohalogenation of this product and subsequent chlorination of intermediate poly- halogenobutenes yield a mixture of polyhalogenobutanes (76 and 79). Dehalogenation of these compounds gives high yield of the diene 75 as a mixture of E- and Z-isomers.57 CFCl2CFClI+CHF CF2 CFCl2CFClCHFCF2I 78 7HCl, 7HI Cl2 [CFCl2CFClCF CF2+CFCl2CF CFCF2I] [Cl(CFCl)4F+Cl(CFCl)3CF2I] 7Cl2,7ClI 79 76 F F F F + Cl F C C C C C C C C F F Cl F F F 75b (Z-isomer) 75a (E-isomer) Another way to the synthesis of the diene 75 is based on the transformation of 1,3,4-trichloro-1,2,3,4,4-pentafluoro-1-iodo- butane (80), which is the reaction product of 1,2-dichloro-1,2,2- trifluoro-1-iodoethane with 1-chloro-1,2-difluoroethylene.58 Suc- cessive dehydroiodination, chlorination and dechlorination of polyhalogenobutane (80) yields the diene 75. CFCl CHF CF2ClCFClCHFCFClI CF2ClCFClI + 7HI 80 Cl2 F(CFCl)4Cl CF2ClCFClCF CFCl 72Cl2 76 CF2 CFCF CFCl 75 The synthesis of 3-chloro-1,1,2,4,4-pentafluorobuta-1,3-diene (81) is also based on elimination reactions.10, 43770 CHF CClCHFCF2I CF2 CClI+CF2 CF2 7HI CF2ClCFClCCl2CF2Cl CF2 72Cl2 CClCF CF2 81 The synthesis of 1,4-dichloro-1,2,3,4-tetrafluorobuta-1,3- diene (82) has attracted much attention.The diene 82 was obtained for the first time by dechlorination of perhalogeno- butane 77,59 which in turn was synthesised by fluorination of hexachlorobuta-1,3-diene (83),60 the reaction of 1,2,3,4-tetra- chlorobuta-1,3-diene (84) with ClF3 (see Ref. 61) or chloroiodi- nation of 1,2-dichloro-1,2-difluoroethylene (85) with subsequent heating of the resulting 1,2,2-trichloro-1,2-difluoro-1-iodoethane (86) with acetic anhydride, dichloromethane and granulated zinc at 160 8C (Scheme 1). Scheme 1 CCl2 CFCl CFCl 85 CHCl CClCCl CHCl 84 CClCCl CCl2 83 ClF3 F2 ICl Zn CFCl2CFClI 86 Cl(CFCl)4Cl 7772Cl2 CFCl CFCF CFCl 82 7Cl2 7Br2,7Cl2 CFCl CFCFClCFCl2 87 CBrFClCBrFCFClCFCl2 88 Yet another method for the synthesis of the diene 82 is based on dechlorination of 1,3,4,4-tetrachloro-1,2,3,4-tetrafluorobut-1- ene (87), which is the product of thermal dimerisation of the ethylene 85.63, 64 The diene 82 is formed in preparative yield upon treatment of 1,2-dibromo-1,3,4,4-tetrachloro-1,2,3,4-tetrafluoro- butane (88), which is the product of bromine addition to the butene 87,64 with zinc in dioxane (Scheme 1).And, finally, there are data on the synthesis of diene 82 based on transformations of a mixture of perhalogenobutanes.58 Thus dehalogenation of a mixture of telomers Cl(CFCl)3CF2I (79) and Cl(CFCl)2CF2CFClI (89) or F(CFCl)4F (90), Cl(CFCl)4F (76) and Cl(CFCl)4Cl (77) affords a 1 : 1 mixture of the dienes 75 and 82 (in the ratio 1 : 1) in the former case or a mixture of dienes 75, 82 and hexafluorobuta-1,3-diene in the latter case.CFCl CFCl2CFClI+CF2 89 [Cl(CFCl)3CF2I+Cl(CFCl)2CF2CFClI] 79 CFCl CFCF CF2+ CFCl CFCF CFCl 75 82 Zn F(CFCl)4F+Cl(CFCl)4F+Cl(CFCl)4Cl 77 76 90 75+82+ CF2 CFCF CF2 91 Theoretically, the diene 82 can exist in the form of three geometric isomers. F F Cl F F F F C C F C C Cl C C C C F C C Cl C C Cl Cl F Cl F F F Z,Z-form E,Z-form E,E-form R V Kaberdin, V I Potkin, V A Zapol'skii Indeed, as was shown by Kremlev et al.,62 the diene 82 is an equilibrium mixture of E,E-, E,Z- and Z,Z-isomers in the ratio of 1 : 2.5 : 1.5.The 19F NMR (Ref. 65) and UV (Ref. 21) spectral data and the values of dipole moments 66 make it possible to conclude that the isomers are in the non-planar s-cis-conforma- tion similarly to hexachloro- (83) 67 and hexafluorobutadienes (91).68 Two methods for the synthesis of 2,3-dichloro-1,1,4,4-tetra- fluorobuta-1,3-diene (92) are known, viz., dehalogenation of 1,2,2,3,3,4-hexachloro-1,1,4,4-tetrafluorobutane (93) 10 and 1,4- dibromo-2,2,3,3-tetrachloro-1,1,4,4-tetrafluorobutane (94).60, 69 72Cl2 CF2ClCCl2CCl2CF2Cl 93 CF2 CClCCl CF2 92 72BrCl CF2BrCCl2CCl2CF2Br 94 Some other difficultly accessible MHB, viz., 3,4-dichloro- 1,1,2,4-tetrafluoro- (95), 1,2,3,4-tetrachloro-1,4-difluoro- (96) and 2,3,4-trichloro-1,1,4-trifluorobuta-1,3-dienes (97), were obtained similarly.10, 60 CF2 CF2ClCFClCCl2CFCl2 72Cl2 CFCl2CCl2CCl2CFCl2 72Cl2 CF2 CF2ClCCl2CCl2CFCl2 72Cl2 CFCCl CFCl 95 CFCl CClCCl CFCl 96 CClCCl CFCl 97 Bromine ± chlorine-containing hexahalogenobutadienes were synthesised by an elimination reaction by the authors of this review.Thus dehydrohalogenation of 1,2-dibromo-1,1,3,3,4,4- hexachlorobutane (98) with aqueous-ethanolic solutions of alka- lis yields two MHB, viz., 2-bromo-1,1,3,4,4-pentachloro- (99) and 1,2-dibromo-1,3,4,4-tetrachlorobuta-1-3-diene (100), in the ratio of 2 : 1. CCl2 CBrCl2CHBrCCl2CHCl2 98 7HCl, 7HBr CBrCCl CCl2 99 CBrCl CBrCCl CCl2 100 The use of an aqueous solution of diethylamine shifted the reaction towards the predominant formation of the diene 99.70 Dehydrohalogenation of (E)-1,3,4-tribromo-1,2,4,4-tetra- chlorobut-1-ene (101) under the conditions of phase-transfer catalysis yielded (E)-1,3-dibromo-1,2,4,4-tetrachloro- (102) and 1,3.4-tribromo-1,2,4-trichlorobuta-1,3-dienes (103) as a mixture of two geometric isomers.Since the b-bromo-a,b-dichlorovinyl group of the butene (101) having the E-configuration is not directly involved in the reaction, the formation of the two isomeric forms of the diene 103 occurs due to the E,Z-isomer- isation of the a,b-dibromo-b-chlorovinyl group in the molecule. Consequently, the diene 103 is a mixture of E,E- (103a) and E,Z- isomers (103b).71 Cl Cl Cl C C C C Br Cl Cl KOH Cl 102 Br C C Br CHBrCBrCl2 Cl Cl Cl Cl 101 + Cl C C Br C C C C Br C C Br Br Br Cl Br 103b (E,Z-isomer) 103a (E,E-isomer)Synthesis and reactions of mixed halogenobuta-1,3-dienes 2.Halogenation and hydrohalogenation of the acetylenic hydrocarbons of the C4 series Acetylenic hydrocarbons, mainly conjugated halogenobutenynes and dihalogenobutadiynes, have served as the starting com- pounds for the synthesis of the first representatives of MHB.2 The syntheses based on acetylene derivatives have not found wide uses because of their poor accessibility and explosiveness. None- theless, nowadays procedures for the preparative synthesis of a number of di-, tri- and hexasubstituted MHB have been devel- oped. It is also noteworthy that a new catalytic reaction of acetylene dimerisation has been recently discovered. This is carried out in methanolic solution of NaI in the presence of Pt(IV); it is accompanied by the addition of an iodine molecule and results in (E,E)-1,4-diodobuta-1,3-diene.72 PtIV (ICH=CH)2 HC:CH I7, I2/MeOH E,E-isomer This reaction seems to be promising for the synthesis of certain MHB.The currently used procedures for the synthesis of MHB by the addition of halogens and hydrogen halides to the car- bon ± carbon triple bond imply the use of the existing C4 carbon chain. a. Dihalogenobutadienes The first representative of the dihalogenobutadienes, viz., 2-chloro-1-iodobuta-1,3-diene (104), was prepared by hydro- chlorination of 4-iodobut-1-en-3-yne (105a) in the presence of catalysts CuCl+NH4Cl.73 In the reaction of this butenyne (105a) with HBr, the predom- inant direction is the reductive elimination of the iodine atom at the triple bond with the formation of vinylacetylene (106).Among the reaction products, a considerable amount of (E)-2-bromo-1- iodobuta-1,3-diene (107a) was also found as the addition product of BrI to the enyne (106). The yield of (Z)-2-bromo-1-iodobuta- 1,3-diene (107b), which is the addition product of HBr to the initial butenyne 105a, is insignificant.74 Table 2. Synthesis of mixed halogenobutadienes based on acetylene derivatives. Starting compound Reaction conditions Electrophilic agent (Hal2 or HHal) HCl HBr HI HI HBr CuCl+NH4Cl CuBr CuI CuI CuBr 740 8C, CHCl3 the same CH2=CHCCl=CHI (104) (Z)-CHCl=CBrCH=CH2 (6b) (Z)-CHCl=CICH=CH2 (108) (Z)-CHBr=CICH=CH2 (109) (E)-+(Z)-CHI=CBrCH=CH2 (107) CH2=CHCCl=CClI (112) CH2=CHCBr=CBrI (113) " CH2 =CHCBr=CBrCl (114) " CH2 =CHCCl=CBrCl (115) CCl4 CH2ClCH2Cl CH2=CHC:CI (105a) CH2=CHC:CCl (105b) CH2=CHC:CCl (105b) CH2=CHC:CBr (105c) CH2=CHC:CI (105a) CH2=CHC:CI (105a) Cl2 CH2=CHC:CI (105a) Br2 CH2=CHC:CCl (105b) Br2 CH2=CHC:CBr (105c) Cl2 CH2ClC:CCH2Cl (117) Br2 , 1,8-diazabicyclo- [5.4.0]undec-7-ene CH2ClC:CCH2Cl (117) I2, 1,8-diazabicyclo- [5.4.0]undec-7-ene ClI EtCl, 750 8C CH:CC:CH CuBr2 MeOH, 1 h CCl4, 25±60 8C CHCl3 CHCl3 CHCl3 CCl4 CCl4 dry ether CHCl=CClC:CH CCl:CC:CCl (121a) Br2 CCl:CC:CCl (121a) I2 CBr:CC:CBr (121b) I2 CI:CC:CI (121c) Br2 CCl2=CClC:CCl (128) Br2 CCl2=CClC:CBr CCl2=CClC:CI Br2 I2 771 CH2 CHC CI 105a HCl CH2 CHCCl CHI 104 H H BrI HBr I C C CH2 7BrI CHC CH 106 C C H H 107a Br H H HBr H C C C C H I 107b Br The addition of hydrogen halides to the conjugated enynes with a halogen atom at the triple bond in the presence of copper(I) halides is widely used for the synthesis of MHB.74 It was established that 4-halogenobut-1-en-3-ynes (105b,c) add HBr and HI at the triple C:C bond giving the corresponding dihalogenobutadienes. Thus hydrobromination of 4-chlorobut- 1-ene-3-yne 105b proceeds stereospecifically with the formation of the Z-form of 2-bromo-1-chlorobuta-1,3-diene (6b) in high yield.The reaction of hydroiodination of chloro- (105b) and bromo- substituted (105c) enynes proceeds similarly and results in the formation of (Z)-1-chloro- and 1-bromo-2-iodobuta-1,3-dienes (108 and 109), respectively (Table 2). HHal CH2 CHC CX 105b,c (Z)-H2C CHCHal CHX 6, 108, 109 6: X=Cl, Hal=Br; 108: X=Cl, Hal=I; 109: X=Br, Hal =I. The addition of mixed halogens (ClI and BrI) to the con- jugated butenyne 106 was studied.74, 75 It turned out that the 1,4- addition of halogens occurred predominantly. Only with the use of a large excess of enyne and in the presence of CuCl were the 1,2- addition products, viz., the dienes 104 and 107 in the form of E-isomers, obtained.74, 75 Ref.Yield (%) Halogenosubstituted butadiene 73 74, 75 74, 75 74, 75 74, 75 74 74 74 74 795 45.2 70.8 41.5 9.7 14.2 7.3 8.5 (Z)-CHCl=CBrCBr=CH2 (40) 91 76 (Z)-CHCl=CICI=CH2 (118) 71 7678 84.0 CHI=CClCCl=CHI (119) (Z,Z+Z,E+E,E ) CHCl=CClCBr=CBr2 (120) CBrCl=CBrCBr=CBrCl (122) CICl=CICI=CICl (123) CBrI=CICI=CBrI (125) CBrI=CBrCBr=CBrI (126) CCl2=CClCBr=CBrCl (100) CCl2=CClCBr=CBr2 (130) CCl2=CClCI=CI2 (131) 79 11 80, 81 80, 81 80, 81 42 42 42 50.0 *98 77784.0 77772 b. Trihalogenobutadienes An attempt to obtain mixed trihalogenobutadienes by the addi- tion of halogens to the triple bond of enynes 105a ± c did not give satisfactory results.74, 75 The reaction of chlorine and bromine with these enynes proceeded mainly as the addition of halogens to the double bond and the 1,4-addition of enynes with the formation of butynes 110 and allenes 111, respectively. The addition of one mole of a halogen to the triple bond of enynes, which would lead to the corresponding trihalogenobuta-1,3-dienes, is less prefera- ble, the yields of products 53, 112 ± 116 are low.74, 75 Y2 CH2+ CHC CX 105a ± c CH2YCH C CXY+CH2 111 CH2YCHYC CX+ 110CHCY CXY 53, 112 ± 116 X=I: Y=Cl (112), Br (113); X=Y=Cl (53); X=Cl, Y=Br (114); X=Br, Y=Cl (115); X =Y=Br (116).The addition of bromine or iodine to 1,4-dichlorobut-2-yne (117) yielded the corresponding halogeno-substituted butenes, which gave (Z)-2,3-dibromo-1-chloro- (40) and (Z)-1-chloro-2,3- diiodobuta-1,3-dienes (118) upon catalytic 1,4-dehydrochlori- nation.76, 77 CH2ClC CCH2Cl 117 Br2 I2 CH2ClCI CICH2Cl CH2ClCBr CBrCH2Cl 7HCl 7HCl CICI CHCl CBrCBr CHCl (Z)-CH2 (Z)-CH2 118 40 c.Tetra- and pentahalogenobutadienes The synthesis of tetra- and pentasubstituted buta-1,3-dienes is demonstrated in separate examples. 2,3-Dichloro-1,4-diiodobuta- 1,3-diene (119) was obtained as a mixture of Z,Z-, Z,E- and E,E- isomers by the reaction of diacetylene with chloroiodine.78 ClI CH CC CH Cl Cl Cl H I I I C C C C C C I + H + C C I C C H C C H H Cl H Cl I Cl 119c (E,E-isomer) 119b (Z,E-isomer) 119a (Z,Z-isomer) The reaction of copper(II) bromide with 1,2-dichlorobut-1-en- 3-yne gives 1,1,2-tribromo-3,4-dichlorobuta-1,3-diene (120) (see Table 2).79 d.Mixed perhalogenobutadienes In 1930, difficultly accessible perhalogenobutadienes (122 ± 127), including first representatives of MHB, were obtained by halo- genation of dihalogenobuta-1,3-dienes (121).80, 81 Y2 CXY CYCY CXY 122 ± 127 CX CC CX 121a ± c X=Cl (a), Br (b), I (c); Y=Br, I. 127 126 124 123 122 Compound 125 X Cl Cl Br Br I I Y Br I Br I Br I R V Kaberdin, V I Potkin, V A Zapol'skii According to the patent data,11 1,4-dichloro-1,2,3,4-tetrabro- mobuta-1,3-diene (122) is formed in quantitative yield upon bromination of dichlorobuta-1,3-diene (121a). Synthesis of some mixed perhalogenobutadienes was carried out based on the accessible perchlorobutenyne (128).42 Thus direct bromination of the enyne 128 gave 1,3,4,4-tetrachloro-1,2- dibromobuta-1,3-diene (100) in high yield, whereas the synthesis of 1,1,2-tribromo- (130) and 3,4,4-trichloro-1,1,2-triiodobuta-1,3- dienes (131) was carried out using copper trichlorovinyl acetyle- nide 129 (Scheme 2).42 Scheme 2 Br2 CCl2 CClCBr CBrCl 100 CCl2 CuCl, NH3 CClC CCl 128 CCl2 CClC CCu 129 Br2 I2 CClC CI CClC CBr CCl2 CCl2 I2 Br2 CCl2 CCl2 CClCBr CBr2 130 CClCI CI2 131 3.Synthesis of mixed halogenobutadienes by pyrolysis of polyhalogenated compounds A number of preparative methods for obtaining MHB containing from four to six halogen atoms in the molecule are based on the pyrolysis of cyclic halogen-containing compounds.Thus the passage of 4-chloro-3,3,4-trifluoro- (132a) or 4,4-dichloro-3,3- difluoro-tricyclo[4.2.1,02,5]non-7-enes (132b) through a red-hot tube filled with rings of quartz glass yielded 4-chloro-1,1,4- trifluoro- (133) and 4,4-dichloro-1,1-difluorobuta-1,3-dienes (134), respectively.82 F FX CXCl CHCH CF2 133, 134 Cl 132a,b X = F (132a, 133); X=Cl (132b, 134). A more convenient method for the preparation of the diene 133 is based on the pyrolysis of 1-acetoxy-2-chloro-2,3,3- trifluorocyclobutane (135) or 1-acetoxy-3-chloro-2,2,3-trifluoro- cyclobutane (136).83 F X AcO F CFCl CHCH CF2 133 Y 135, 136 135: X =Cl, Y =F; 136:X=F,Y=Cl. ManyMHBwere obtained by the pyrolysis of the correspond- ing halogeno-substituted cyclobutenes.Thus the pyrolysis of 1-chloro-3,3,4,4-tetrafluorocyclobut-1-ene (137) gives 2-chloro- 1,1,4,4-tetrafluorobuta-1,3-diene (138) in preparative yield (Table 3).13, 83 F Cl F CF2 F CClCH CF2 138 137 F The main pyrolysis products of 1,2,3,4-tetrafluoro-3,4-diiodo- and 4-chloro-1,2,3,4-tetrafluoro-3-iodocyclobutenes (139a,b) are 1,2,3,4-tetrafluoro-1,4-diiodobuta-1,3-diene (140), formed as aSynthesis and reactions of mixed halogenobuta-1,3-dienes Table 3. Miscellaneous methods for the preparation of mixed halogenobutadienes. Halogeno-substituted butadiene Starting compound Reaction conditions Method of preparation a 132a (see b) 132b (see b) 136 (see b) 137 (see b) 139a 700 8C (1 Torr) 700 8C (1 Torr) 700 8C (10 Torr) 700 8C (1 Torr) 580 8C (0.1 Torr) AAAAA CFCl=CHCH=CF2 (133) CCl2=CHCH=CF2 (134) CFCl=CHCH=CF2 (133) CF2=CClCH=CF2 (138) CFI=CFCF=CFI (140) (E,E : E,Z:Z,Z=21 : 16: 8) CFI=CFCF=CFCl (141) CFCl=CHCH=CH2 (17) Cu, 180 ± 190 8C heating AA 139b CFCl2SOnCH2CH=CH2 (n=1, 2) CHCl=CFCF2CF2COONa (142) A (Z)-CF2=CFCF=CHCl (143) CCl2=CHCH=CBr2 (149) heating with a burner (1 Torr) CCl2=CHCHO (145) B Ph3 P=CBr2, CH2Cl2, 0 8C the same " BB CCl2=CClCH=CBr2 (150) CCl2=CBrCH=CBr2 (151) CI2 =CClCH=CBr2 (152) (Z)-CHBr=CHCCl=CCl2 (55) CCl2=CClCHO (146) CCl2=CBrCHO (147) CI2=CClCHO (148) B " CCl2=CClCHO (146) B Ph3 P=CHCOOEt, B CCl2=CBrCH=CCl2 (154) CCl2=CBrCHO (147) CBr2=CClCH=CCl2 (155) CHCl=CClCCl=CClI (157) CBr2=CClCHO (153) CHCl=CClCCl=CClLi BC Br2 CCl2=C=O, dry ether the same I2, FeCl3, ether,7110 8C Cl Cl C SF4 , 140 8C CF2 =CHCH=CCl2 (134) COOH 158 C CFCl=CFI Cu, 145 8C CFCl=CFCF=CFCl (82) (E,E : E,Z:Z,Z=19 : 51 : 30) aA is pyrolysis; B is synthesis of MHB from polyhalogenoacroleins; C is other methods.b The starting compounds were obtained by the [2 + 2]- cycloaddition reaction. mixture of E,E-, E,Z- and Z,Z-isomers, and 4-chloro-1,2,3,4- tetrafluoro-1-iodobuta-1,3-diene (141), respectively.84 CFCl+ CFCl CF2 F2C X = I F F CF2 X CFI CFCF CFI 140 CClCFClCF2Cl 144 I X=Cl F Cu F CFI CFCF CFCl 141 139a,b 4. Synthesis of mixed halogenobutadienes based on polyhalogenoacroleins X = I (139a), Cl (139b).An original way of the preparation of some bromine, chlorine- containing MHB is based on the reaction of polyhalogenoacro- leins 145 ± 148 with dibromomethylenetriphenylphosphor- ane.88, 89 CHal CHal2 CXCHO CXCHO+ Ph3P CBr2 145 ± 148 Mixed halogenobutadienes are also formed by the pyrolysis of some aliphatic compounds. Thus heating of sulfoxide CFCl2SOCH2CH=CH2 or sulfone CFCl2SO2CH2CH=CH2 yielded 1-chloro-1-fluorobuta-1,3-diene as one of the reaction products (17).85 Hal=Cl: X=H (145, 149), Cl (146, 150), Br (147, 151); Hal = I: X = Cl (148, 152). Decarboxylation of sodium 5-chloro-2,2,3,3,4-pentafluoro- pent-4-enoate (142) in vacuo gives high yield of (Z)-4-chloro- 1,1,2,3-tetrafluorobuta-1,3-diene (143).86 The first representative of MHB containing three different F F H C C CHCl CFCF2CF2COONa 142 C C F halogen atoms, viz., 4,4-dibromo-2-chloro-1,1-diiodobuta-1,3- diene (152) was obtained in 1970 by the reaction of a-chloro-b,b- diiodoacrolein (148) with Ph3P=CBr2.88, 89 Cl 143 F A method of preparation of (Z)-4-bromo-1,1,2-trichlorobuta- 1,3-diene (55) from the acrolein 146 and phosphorylide Ph3P=CHCOOEt has been developed.89 The butadiene 81 is present among the pyrolysis products of chlorofluoroethylene.Presumably, this resluts from thrmal de- chlorination of butene 144, which appears in the reaction due to the addition of chlorofluorocarbene to the initial chlorotrifluoro- ethylene. 773 Ref. Yield (%) 82 82 83 83 84 7756.0 38.0 37.0 84 85 77 86 82.0 88, 89 76.0 88, 89 88, 89 88, 89 89 62.0 57.0 44.0 82.0 42 63.0 42 90 55.0 60.0 91 7 92, 93 49.0 CFCl CFCl CFCl CF2 7Cl2 CClCF CF2 81 CHal2 CXCH CBr2 149 ± 152774 CCl2 7P(O)Ph3 CClCHO+ Ph3P CHCOOEt 146 H COOH COOEt H Br2 H2O C C C CH CCl H CCl2 CCl2 CClCl Cl CClCHBrCHBrCOOH Br C C CCl2 C C Cl 7CO2, 7HBr H H (Z)-55 Yet another method of the synthesis of MHB is based on the reaction of halogenoacroleins with dichloro ketene.Starting from the acrolein 147 and b,b-dibromo-a-chloroacrolein (153), 2-bromo-1,1,4,4-tetrachloro- (154) and 1,1-dibromo-2,4,4-tri- chloro-buta-1,3-dienes (155) were obtained, respectively.42 C O CHal2 CCl2 CHal2 CXCHO+ 147, 153 CXCH CCl2 154, 155 147, 154: Hal=Cl, X=Br; 153, 155: Hal=Br, X=Cl.This reaction is supposed to proceed through the initial formation of a lactone 156, which is easily decarboxylated with the formation of the target products 154 and 155. CCl2C O CXCH CHal2 156 O The yields of the dienes 55, 149 ± 152, 154, 155 and conditions of their synthesis are listed in Table 3. 5. Miscellaneous reactions for the preparation of mixed halogenobutadienes It was shown 28 that chlorination of bromoprene (4) yields, along with different addition products, also 2-bromo-1-chlorobuta-1,3- diene (6) as a mixture of Z- and E-isomers. Cl2 CH2 CHCl CBrCH CH2 6 CBrCH CH2 4 The reaction of lithium 1,2,3,4-tetrachlorobuta-1,3-dien-1-ide with iodine yielded 1,2,3,4-tetrachlorobuta-1-iodo-1,3-diene (157).90 I2 CHCl CClCCl CClLi CHCl CClCCl CClI 157 The synthesis of 1,1-dichloro-4,4-difluorobuta-1,3-diene (134) in low yield by the reaction of sulfur tetrafluoride with 2,2- dichlorocyclopropanecarboxylic acid (158) has been reported.91 Cl Cl CF2 +SF4 CHCH CCl2 134 COOH 158 A convenient method for the preparation of 1,4-dichloro- 1,2,3,4-tetrafluorobuta-1,3-diene (82) is the reducive dimerisation of a mixture of Z- and E-isomers of 2-chloro-1,2-difluoro-1- iodoethylene in the presence of a copper powder.92, 93 The diene 82 is formed as a mixture of E,E-, E,Z- and Z,Z-isomers in approximately the same ratio as reported elsewhere 62 (see Table 3).Cu CFCl CFI CFCl CFCF CFCl 82 Unfortunately, the attempt to extend this reaction for the synthesis of other MHB has been unsuccessful because of inaccessibility of iodine-containing halogenoethylenes. R V Kaberdin, V I Potkin, V A Zapol'skii III. Chemical properties of mixed halogenobuta- 1,3-dienes 1. Addition reactions. Halogenation of mixed halogenobutadienes Despite the fact that MHB possess electrophilic properties, they add, like other diene compounds with conjugated double bonds, halogens to both the terminal positions of the diene system (1,4- addition) and to one of the double bonds (1,2- or 3,4-addition). The direction of this reaction is determined to a considerable extent by the nature of MHB and the halogenation agent; experimental conditions are also highly important.The 1,4- addition is characteristic of the dienes having a small number of bulky substituents (Cl, Br or I) and low asymmetry. In the MHB with a high extent of substitution with bulky substituents, the p,p- conjugation is disturbed so that these dienes react as alkenes. Sometimes, an exhaustive halogenation takes place at both C=C bonds and results in the formation of polyhalogenobutanes. It was shown that the bromination of a mixture of E- and Z-isomers of 1-bromo-1,4,4-trichlorobutadienes (56) (E:Z ratio=1 : 4) occurs at the 1,2- and 3,4-positions and results in a mixture of two products, viz., 3,4,4-tribromo-1,1,4-trichloro- and 1,3,4-tribromo-1,4,4-trichlorobut-1-enes in the ratio 70 : 30 with a total yield of 76 %.55, 94 CBr2ClCHBrCH CCl2 CBrCl2CHBrCH CBrCl CBrCl CHCH CCl2 56 Bromination of the E-isomer of the diene 66 (45 ± 50 8C, illumination with an incandescent 60 W lamp, 8 h) occurs exclusively at the b,b-dichlorovinyl group and results in (E)-1,3,4-tribromo-1,2,4,4-tetrachlorobut-1-ene in a yield of 93%.71 Cl Cl Cl Cl Br2 C C Cl C C Br C C Br CHBrCBrCl2 66 Cl H Monochloropentafluoro- and dichlorotetrafluorobuta-1,3- dienes (their structure has not been unequivocally established) add one molecule of bromine with the formation of the corre- sponding butenes of the general formulae C4ClBr2F5 and C4Cl2Br2F4 at 20 ± 150 8C.61 The diene 81 adds two molecules of bromine at 50 8C and gives 1,2,3,4-tetrabromo-3-chloro-1,1,2,4,4-pentafluorobutane, the structure of which was confirmed by the results of chromato- mass spectrometry.87 According to Petrov et al.,95 the chlorina- tion and bromination of the diene 81 occur exclusively as the 1,4- addition and lead to the corresponding but-2-enes.Thus treat- ment of the diene 81 with a mixture of N-bromosuccinimide (NBS) andHFinTHFyielded a mixture of E- and Z-isomers of 3-chloro- 1-bromo-1,1,2,4,4,4-hexafluorobut-2-ene (E:Z=5 : 1) in a total yield of 82 %. Fluorination of the diene 81 with VF5 from720 to 730 8C occurs predominantly as the 1,4-addition and results in 2-chloroheptafluorobut-2-ene as a mixture of Z- and E-isomers (yields, 20% and 36%, respectively).A small amount (8%) of 2-chlorononafluorobutane is formed as the product of exhaustive fluorination.96 VF5 CF3CCl CFCF3+CF3CFClCF2CF3 CF3CCl CFCBrF2 CClCF CF2 CF2 81 HF NBS 2Br2 CBrF2CBrClCBrFCBrF2 According to the patent literature,97 the diene 92 reacts with bromine (illumination with a 100 W lamp) to give 1,4-dibromo- 2,3-dichloro-1,1,4,4- tetrafluorobut-2-ene (1,4-addition),Synthesis and reactions of mixed halogenobuta-1,3-dienes Br2 CBrF2CCl CClCBrF2, CF2 CClCCl CF2 92 whereas with the diene 82 1,2-addition occurs, which results in 3,4- dibromo-1,4-dichloro-1,2,3,4-tetrafluorobut-1-ene. Its chlorina- tion gives 1,2-dibromo-1,3,4,4-tetrachloro-1,2,3,4-tetrafluoro- butane.64 Chlorination of the diene 82 affords 1,1,2,3,4,4- hexachloro-1,2,3,4-tetrafluorobutane.64 2Cl2 CFCl2CFClCFClCFCl2 Br2 CBrFClCBrFCF CFCl CFCl CFCF CFCl 82 Cl2 CBrFClCBrFCFClCFCl2 2.Cycloaddition reactions a. Reaction of mixed halogenobutadienes with difluorocarbene The addition of carbenes to MHB was studied only for the reaction of difluorocarbene with the diene 81. Petrov et al.98 used hexafluoropropylene oxide as the source of difluorocarbene. Heating of an excess of the epoxide with the diene 81 in an autoclave gave the halogeno-substituted cyclopentene 159 in high yield. The cyclopentene 159 is one of the pyrolysis products of chlorotrifluoroethylene.87 According to Petrov et al.,98 the difluorocarbene formed in the course of pyrolysis adds to the diene 81 to give polyhalogenovinylcyclopropanes, whose intra- molecular rearrangement yields the cyclopentene 159.87 CF2 CF2 CClCF CF2 81F2C CClCF CF2 CFCCl CF2 + F2C CF2 Cl F CF2 F F FF 159 b.Diels ± Alder syntheses involving mixed halogenobutadienes Chloro-substituted buta-1,3-dienes (mostly, mono- and dichloro- derivatives) are convenient subjects for studying the Diels ± Alder reaction and preparation of practically valuable compounds.7 Some MHB were also studied in the [4+2]-cycloaddition. Thus upon prolonged boiling of an excess the dienes 40 and 118 with methyl vinyl ketone in the presence of Na2CO3, the corresponding 1-acetyl-3,4-dihalogenocyclohexa-1,3-dienes 160 (X=Br) and 161 (X=I) were obtained. Upon treatment of the cyclohexa- dienes 160 and 161 with 1,8-diazabicyclo[5.4.0]undec-7-ene, they are readily aromatised into the corresponding halogenoacetophe- nones 162 and 163.77, 99 Cl X C H C CHCOMe + CH2 7HCl C CH2 X 40, 118 COMe X COMe X 7HX X 162, 163 160, 161 The [4+2]-cycloaddition of (ClSN)3 and FSN to the buta- diene 81 yielded the difficultly accessible and highly reactive 6- membered heterocycles 164 and 165, respectively, which are the derivatives of 1,2-thiazines (Scheme 3). Treatment of the 1,2- thiazine 164 with water yields the 1,2-thiazin-3-one-1-oxide 166.The reaction of methanol with the 1,2-thiazine 165 results in the 775 Scheme 3 O F F Cl Cl H2O NH N (ClSN)3 SCl SO F F F F F166 F164 CF2 KF CClCF CF2 81 F F F F Cl Cl N MeOH N FSN SF SOMe F F F F F167 F165 substitution of the methoxy group for the sulfur-bound fluorine atom by the methoxy group with the formation of the product 167.Thiazine 164 is easily fluorinated with KF and gives a high yield of the thiazine 165.100 3. Elimination reactions As has been shown above (see Section II.2), butenynes are widely used in the synthesis of MHB. In turn, the accessible MHB can be used in the synthesis of some highly reactive halogen-containing butenynes. Thus the preparation of 4-bromo-1,1,2-trichlorobut- 1-en-3-yne (168) has been developed based on dehydrobromina- tion of the dienes 55 and 150. The synthesis of the butenyne 168 (yield 60 %) from the diene 55 (Z-isomer) includes two stages: the elimination of HBr under the action of the methanolic solution of potassium carbonate and the substitution of bromine for a hydrogen atom in the 1,1,2-trichlorobut-1-en-3-yne obtained under the action of sodium hypobromite.The second method of the synthesis of the butenyne 168 consists in the dehydrobromi- nation of the diene 150. It was established that the yield of the target product depends to a considerable extent on the nature of the dehydrohalogenation agent. When the methanolic solution of potassium carbonate is used, the yield of butenyne 168 is 57.5 %; this amounts to 70% with the use of benzyltrimethylammonium hydroxide (Triton B).89, 101, 102 CCl2 CCl2 7HBr CClC CBr 168 CClCH CBr2 150 NaOBr CClC CH CCl2 CCl2 7HBr CClCH CBrH 55 It was shown that the dehydrobromination of the diene 40 under the action of finely ground KOH at 100 8C results in 2-bromo-4-chlorobut-1-en-3-yne in 51% yield.42 CBrC CCl CH2 CH2 7HBr CBrCBr CHCl 40 Dehydrochlorination of some pentahalogenosubstituted MHByielded the previously inaccessible tetrahalogenobutenynes. CXC CCl CHal2 CXCH CCl2 CHal2 7HCl Hal=Cl, X=Br; Hal=Br, X=Cl.4. Substitution reactions a. Interaction of mixed halogenobutadienes with organometallic compounds Organolithium and -magnesium reagents found rather limited uses in the chemistry of MHB. The pronounced `affinity' of these compounds to halogens, on the one hand, and the presence of a large number of halogen atoms in the MHB molecules, on the776 other hand, largely prevent selective nucleophilic substitution in these substrates, though such reactions are known (see below).The metal ± halogen exchange is more characteristic of MHB. Thus starting from 1,2,3,4-tetrachlorobuta-1,3-diene 84 and butyllithium, mono- and dilithium derivatives were obtained, based on which a number of fine syntheses were carried out.103, 104 Syntheses involving MHB were studied in the example of 1,4- dichloro-1,2,3,4-tetrafluoro-1,3-butadiene (82).62 Treatment of an isomeric mixture of the diene 82 (E,E :E,Z:Z,Z=1 : 2.5 : 1.5) with butyllithium and then with CO2 gave (E,Z)-5-chloro-2,3,4,5- tetrafluoropenta-2,4-dienoic acid (169) in 25% yield. Comparison of the ratio of isomeric forms of this diene and the yield of the acid shows that it is the Z,Z-isomer of the diene 82 that reacts.This reaction proceeds through the stage of exchange of an exo- chlorine atom for lithium (according to Kremlev et al.,62 the starting diene exists in a non-planar s-cis-conformation). F F F F C C C C 1) BuLi 2) CO2 COOH Cl C Cl Cl C C C F F F F (E,Z)-169 (Z,Z)-82 The reaction of organomagnesium compounds with MHB occurs in a different way. The reaction of 1-chloroperfluorobuta- 1,3-diene (75) (E :Z=1 : 1) with phenylmagnesium bromide (170) includes the nucleophilic substitution of the phenyl radical for the terminal fluorine atom of the trifluorovinyl fragment, which results in the formation of 4-chloro-1,2,3,4-tetrafluoro-1-phenyl- buta-1,3-diene (171) as a mixture of E,E-, E,Z- andZ,Z-isomers in the ratio 2 : 1 : 2 and in a total yield of 77%.57 Various para-substituted arylmagnesium bromides (172 ± 175) were used in this reaction, and this extended appreciably the synthetic possibilities of this method.105 It was shown that the resulting 1-aryl-4-chloro-1,2,3,4-tetrafluorobuta-1,3-dienes (176 ± 179) (yields of 40%± 49 %) represent mixtures of E,E-, E,Z- and Z,Z-isomers in the ratio 1 : 1 : 1 (NMR data).The crystalline E,E-isomer of compound 176 was isolated in individ- ual form. Metallation of products 171, 176 ± 179 with butyllithium and subsequent carbonisation yielded a series of (E,E)-5-aryl- 2,3,4,5-tetrafluoropenta-2,4-dienoic acids (180 ± 184).CFCF CFCl CF2 p-RC6H4MgBr 170, 172 ± 175 BuLi, CO2 p-RC6H4CF CFCF CFCl 171, 176 ± 179 p-RC6H4CF CFCF CFCOOH 180 ± 184 R = H (170, 171, 180), Me2N (172, 176, 181), MeO (173, 177, 182), Me (174, 178, 183), CF3 (175, 179, 184). 19F NMR studies of the electronic effect of substituents and determination of the acidity have shown that the 1,2,3,4-tetra- fluorobuta-1,3-diene system is involved in the transfer of the electronic influence from a substituent in the aromatic ring to the corresponding reaction centre, viz., the carboxyl group or a fluorine atom.105 Compounds 171, 176 ± 179 served as the basis for the synthesis of symmetrical a,o-diarylperfluoropolyenes (185).19 BuLi CFCF CFCl 2 p-RC6H4CF 171, 176 ± 179 91 2 p-RC6H4CF CFCF CFLi p-RC6H4(CF CF)6C6H4R-p 185 R V Kaberdin, V I Potkin, V A Zapol'skii Compounds of the type 185 with the (CF=CF)n groups are good conductors of the conjugation effect because the fluorine atoms do not create any steric hindrance to its transfer.This opens up a broad perspective for the synthesis of a novel type of polymethine dyes with a totally or partially fluorinated chain.19, 105 b. Nitration reactions Of all the principal methods for the preparation of nitrobuta- dienes, the leading part belongs to the nitration of dienes with nitric acid or nitration mixtures. Nitro- and especially polychloronitrobutadienes are widely employed in organic synthesis.106 For example, pentachloro-2- nitrobuta-1,3-diene, which was obtained by the reaction of pentachloro-2H-buta-1,3-diene with nitric acid, proved to be a versatile starting material for numerous syntheses of compounds belonging to different classes.107 ± 109 The first representative of nitroderivatives ofMHB[4-bromo- 1,1,3,4-tetrachloro-2-nitrobuta-1,3-diene (186)] was obtained by the reaction of 60% nitric acid with a 1 : 1 mixture of Z- and E-isomers of 1-bromo-1,2,4,4-tetrachlorobuta-1,3-diene 66 at 80 ± 85 8C.The nitrodiene 186 is a mixture of two isomers (GLC). Along with the nitration of the initial diene 66 at the b,b- dichlorovinyl group, competitive oxidative destruction takes place, which leads to the formation of 3-bromo-2,3-dichloro- acrylic acid (187) as a by-product.110 HNO3 CBrCl CClCH CCl2 66 CBrCl CClC CCl2+CBrCl CClCOOH NO2 187 186 Recently,54 the nitration of the diene 66 with nitrating mixtures HNO3±H3PO4 and HNO3±H2SO4 has been studied over a broad range of concentrations and temperatures.It was found that the optimum conditions for the nitration process are as follows: the temperature 95 ± 100 8C, the ratio of the components of the nitration mixture HNO3:H3PO4 (or HNO3:H2SO4) equal to 10 : 1 and the ratio of diene : nitration mixture equal to 1 : 3 or 1 : 4. Under these conditions, the reaction is completed in 23 h; the yield of the nitrodiene 186 reaches 56% upon nitration with the HNO3±H2SO4 mixture and 58% upon nitration with the HNO3±H3PO4 mixture. The nitration of the diene 66 (as a mixture of Z- and E-forms in the ratio 1 : 3) yields an isomeric mixture of the nitrodiene 186 in virtually the same ratio.Among by-products of this reaction, b-bromo-a,b-dichloroacrolein (yield 8%) and 3-bromo-2,3- dichloroacrylic acid (187) were detected. HNO3±H3PO4 or HNO3±H2SO4 CBrCl CClCH CCl2 66 CBrCl CClC CCl2+CBrCl CClCHO+CBrCl CClCOOH 186 187 NO2 The nitration of other pentasubstituted MHB, viz., 1,2- dibromo-1,4,4-trichloro- (68) and 1,3-dibromo-1,4,4-trichloro- buta-1,3-dienes (70) with nitric acid or nitration mixtures occurs similarly. Thus nitration of the E-isomer of the diene 68 with the 55%± 60% nitric acid, fuming nitric acid or a nitration mixture HNO37H3PO4 yielded (E)-3,4-dibromo-1,1,4-trichloro-2-nitro- buta-1,3-diene (188).Synthesis and reactions of mixed halogenobuta-1,3-dienes Br Br CH CCl2 C(NO2) CCl2 Y C C C C Br Br Cl Cl 68 Nitration agent Y 188 Yield of 188 (%) T /8C 40 45 50 80 ± 85 20 90 ± 95 55%± 60% HNO3 Fuming HNO3 HNO3±H3PO4 The nitration of the diene 70 with the above-cited reagents under similar conditions gives 1,3-dibromo-1,4,4-trichloro-2- nitrobuta-1,3-diene in yields of 35%, 43% and 48%, respectively.1-Bromo-1,4,4-trichlorobuta-1,3-diene (56) reacts differently with nitric acid and with nitration mixtures. In this case, the nitration proceeds in the two directions: (1) substitution of a hydrogen atom in the b,b-dichlorovinyl group and (2) substitution of a bromine atom in the b,b-bromochlorovinyl group resulting in the formation of 1,1,4-trichloro-1,3-dinitrobuta-1,3-diene (189).Another reaction product is 4-bromo-1,1,4-trichloro-3-nitrobut- 1-ene (190). Under the optimum conditions [the nitration mixture HNO37H3PO4 (10 : 1), 90795 8C, 2 h], the yields of 1,4-dinitro- buta-1,3-diene 189 and nitrobutene 190 are 18% and 40%, respectively.49 H + CHClBrCH(NO2)CH CCl2 CBrCl CHCH CCl2 56NO2 C C Cl C C Cl 190 189 Cl NO2 According to the IR spectroscopy and the X-ray diffraction analysis, the diene 189 is the Z-isomer and has a virtually planar s-trans-configuration.111, 112 5. Isomerisation reactions It was shown above (see Section II.3) that the accessible poly- halogenocyclobutenes are convenient starting compounds for the synthesis of some MHB.As early as 1949, the thermal cyclisation of hexafluorobuta-1,3-diene (91) at 150 ± 180 8C was carried out and resulted in hexafluorocyclobutene (191) and a mixture of dimers C8F12 and trimers C12F18.113 Later, these transformations were also performed for hexachlorobuta-1,3-diene (83), and hexachlorocyclobutene (192) was isolated by distillation.7 The isomerisation of halogenobutadienes into the corresponding cyclobutenes is a reversible reaction.X X t X CX2 X CXCX CX2 83, 91 X X 191, 192 X = F (91, 191), Cl (83, 192). The isomerisation of MHB was studied in the example of 1,4- dichloro-1,2,3,4-tetrafluorobuta-1,3-diene (82) and 2,3-dichloro- 1,1,4,4,-tetrafluorobuta-1,3-diene (92). It was shown that heating of the diene 82 at 250 8C in a sealed glass tube results in 3,4- dichlorotetrafluorocyclobutene (193) in 56% yield.Upon heating of this diene in a platinum crucible over activated carbon to 320 8C, the yield of the butene 193 reaches 74%. The reverse process (transformation of cyclobutene into diene) occurs at 380 8C at a lower rate, therefore, the content of butadiene in the thermolysis products of the cyclobutene 193 is only 14%.64 F F Cl 3208C Cl CFCl CFCF CFCl 82 3808C F F 193 777 Heating of a mixture of E,E-, E,Z- and Z,Z-isomers of the diene 82 in a nickel tube at 330 8C yielded three products, viz, 3,4- dichloro- (193), 1,4-dichloro- (194) and 1,2-dichlorotetrafluoro- cyclobutenes (195). Presumably, the cyclobutenes 194 and 195 are formed due to the double bond migration catalysed by the fluoride ion.114 The diene 92 undergoes only partial transformation when heated in a sealed steel tube at 170 8C for 44 h.The unreacted diene 92 was recovered, 1,2-dichlorotetrafluorocyclobutene (195) and a small amount of unsaturated dimers and a dimer 196 97 were isolated from the reaction mixture. The total conversion of the diene 92 into the cyclobutene 195 occurs after 89 h.97 F F Cl Cl F Cl F F F + 170 8C 44 h F F F Cl F Cl F CF2 F 195 Cl196 CClCCl CF2 92 195 170 8C 89 h It was supposed 64 that the reaction diene.cyclobutene is a general characteristic of the polyhalogenobutadienes. 6. Miscellaneous reactions The reduction of MHB was studied only in the example of (Z)-1- bromo-2-chlorobuta-1,3-diene (3).It was shown that the reduc- tion of Z-diene (3a) with LiAlD4 in diethyl ether occurs with high stereoselectivity and leads to the formation of a mixture of (E)- (197a), (Z)-2-chloro-1-deuteriobuta-1,3-diene (197b) and 2-chlo- robutadiene (2) in the ratio 94.2 : 2.2 : 3.6, respectively.25 Cl H H C C C C D H H 197a (E-isomer) Cl Br Cl D H C C LiAlD4 H C C C C H C C H H H H H 3a 197b (Z-isomer) CH2 CClCH CH2 2 In example of the reaction of pentachloro-2H-buta-1,3-diene (198) with fuming nitric acid it was shown that the reaction product is 3,4,4-trichloro-2-nitrobut-3-enoic acid (199), i.e., the b,b-dichlorovinyl group of the diene 198 is involved in the reaction and an earlier transformation into the CH(NO2)COOH group occurs.115 An analogous reaction giving 4-bromo-3,4-dichloro-2- nitrobut-3-enoic acid (200) in high yield also occurs upon interaction of fuming nitric acid with 1-bromo-1,2,4,4-tetrachlor- obuta-1,3-diene (66).110 HNO3 CClX CClCH(NO2)COOH 199, 200 CClX CClCH CCl2 66, 198 X=Cl (198, 199), Br (66, 200).The reaction of the E-isomer of the diene 66 with chlorosul- fonic acid proceeds in an unusual manner. Under the optimum conditions (70 ± 80 8C, the diene : acid ratio equal to 1 : 2; 5 h), the reaction gives (E)-1,3-dibromo-1,2,4,4-tetrachlorobuta-1,3-diene 102 in 31% yield as the main product. In addition, bromochloromaleic acid 201 (yield 6%) and hexachlorobuta-1,3-diene 83 (yield *2%) were also found among the reaction products.Dienes 102 and 83 are apparently778 formed upon halogenation of the starting compound 66 with chlorine and bromine, which appear in the reaction mixture upon destruction and resinification of the substances involved. Acid 201 is the hydrolysis product of the diene 102. Cl Cl HSO3Cl Cl C C C C Br 66 Cl H Br Cl CCl2+ + CCl2 C C CClBr CClCBr 102 CClCCl CCl2 83 HOOC COOH 201 When the diene 66 is used in the reaction in the form of E- and Z-isomers (1 : 1), the diene 102 is also formed as a mixture of two isomers in the ratio 1 : 1.71 IV. Conclusion Analysis of the published data shows that constant attention is paid to the synthesis of mixed halogenobuta-1,3-dienes. Nowa- days, there are rather well developed methods for the synthesis of hexasubstituted chloro-, fluoro- and chlorobromobutadienes, while the methods for the synthesis of MHB containing from 2 to 5 atoms of halogens are somewhat less developed.Most of the methods for the preparation ofMHBare based on the elimination reactions of accessible polyhalogenobutenes and -butanes, i.e. they rely on the use and transformations of the existing C4 carbon chain. At present, novel methods have been proposed for the synthesis ofMHBunder the conditions of phase- transfer catalysis, which makes their preparation simpler and more efficient. Virtually all of the synthesised MHB contain two kinds of the halogen atoms. At present, there is a convenient procedure for the synthesis of only one MHB representative containing three kinds of the halogen atoms.Unfortunately, until now no MHB have been prepared with four different halogen atoms (F, Cl, Br, I) which could be used as convenient models for studying regularities of competitive processes of nucleophilic vinylic substitution (SNVin) of different halogen atoms. Transformations of MHB are represented in the relevant literature to a lesser extent; nonetheless, the available data show that MHB possess broad possibilities for the development of new methods for the synthesis of polyfunctional compounds which are difficultly accessible or totally inaccessible by other ways. We hope that a stimulus for the further development of studies in this field will be the fact that many MHB possess a broad spectrum of useful properties. References 1.F Krafft Chem. Ber. 10 801 (1876) 2. A A Petrov Usp. Khim. 13 203 (1944) 3. 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