Boranes in Organic Chemistry 3 . α-, β-and γ-Haloalkylboranes : The Perspective Vehicles for Organic Synthesis

The methods of synthesis of α− and β−haloalkylboranes, including chloration of alkylboronic acid esters, additive bromation and chloration of esters of vinyland alkenylboronic acids, addition of bromine to trivinylborazines have been considered. The reactions of radical addition of polyhaloidmethanes to vinylboranes, α− and β−unsaturated boronic esters, B-vinyl-B-arylboronic esters, B-trivinyl-B-triarylborazines were discussed. The hydroboration of acetylenic halogenides of dicycloalkylboranes, which led to halocontaining derivatives of dialkylvinylborane was separately considered. The examples of hydroboration of halogenides of allyl and vinyl types are presented. The reaction of dienic synthesis, which takes place between vinylchloroboranes or vinylboronic esters and tetraor hexachlorocyclopentadienes has been discussed. The reaction of alkenes and allenes with boron tribromide was described. The Markovnikov and nonMarkovnikov hydrobromation of boron vinylderivatives has been envisaged. The approaches to the synthesis of perfluoroalkylboranes on the base of hydroboration of perfluoroalkenes have been discussed. The methods of the synthesis of boronates, containing halogetaryl substituents, have been performed. The reactions of hydroboration of halogenides of allylic and propargylic types by 9-borabicyclononane have been shown. The regioand stereoselectivity of the reaction has been discussed. The examples of the synthesis of boranes of the norbornene type were presented. The reaction of boroallylilation of allyland propargylhalogenides leading to the derivatives of 3-bora-bicyclo[3,3,1]-nonane has been discussed. Some directions of using of haloidalkylboranes in the synthesis have been discussed. The examples of nucleophilic substitution leading to oxyalkyland azidoalkylboranes have been presented. The route of obtaining of alcohols from α−haloidalkylboranes has been shown. The general scheme of synthesis of α− aminoboronic acids was perfomed. The general approach to the synthesis of allenes on the base of hydroboration products of propargyl halogenides has been discussed. The schemes of synthesis of 1,4disubstituted-1,2,3-butatrienes are presented. The wide using reaction of introducing of vinylic group into substituent, bonding with boron atom in molecules of dialkylvinylboranes, was discussed. The reactions of new C-C bonds formation, based on the action of iodine on the alkylvinylboronates leading to 1,3-dienes and alkylidencyclanes have been shown. Τhe route of the synthesis of cyclopropanes from β− haloidalkylboranes has been discussed. Introduction α-, β-, γ-Haloalkylboranes are rare groups of synthetic compounds with common structures such as B–C–X, B–C–C–X, B–C–C–C–X (X = F, Cl, Br or I) are mainly intermediate compounds in hydroboration reactions partly given in some reviews [1-9]. α-Haloalkylboranes Reactions of chlorine with boranes Chlorine reacted with trimethylboranes at minus 95°C to give α-chloro-α-boron derivative 1 [10]. Compound 1 reacted with lithium azide and with water to form the corresponding azide 2 and alcohol Eurasian ChemTech Journal 5 (2003) 83-108 Boranes in Organic Chemistry (3) 84 3 [11] (Scheme 1). esters is common for all derivatives of dibutyl propene-2-boronate, in the presence of azobisisobutyronitrile as the initiator, as resulting are α-haloalkylboranes 12 (Scheme 3) [15]. 1-Bromo-3,3,3-trichloropropane-1-boronic acid 15 could be obtained from 1-bromo-3,3,3-trichloro-dibutylpropane-2-boronate 13 by recrystallization from water, and 13 could be converted to its o-phenylenediamine 14 [16]. Lightinitiated additon of bromotrichloro-methane to butyl B-phenyl-B-vinylborinate 16 proceeded readily butyl B-phenyl-B-(1-bromo-3,3,3-trichloro-1-propyl)borinate 17 [17,18]. Formation of amidoboranes N-Trimethylsilylamides 18 reacted with bromodiorganylboranes quantitatively under formation of the corresponding amidoboranes, in certain cases these are in equilibrium with the dimeric forms [19]. Among these reactions in one case could be forming α-haloalkyl compound 19 (Scheme 4). Thus, N-phenyl-Ntrimethylsilyldichloroacetamide reacted with bromodimethyl-borane to form 19 [19]. Hydroboration of cyclovinylic derivatives Pasto and Snyder also studied a series of vinyl halides to determine the placement of B – H in tetrahydrofuran with borane [14]. Hydroboration of 4t-butyl-1-chlorocyclohexene at 25°C gave 60% 1chlorocyclohexyl borane 20, and 40% of the boron atom introduced at the β-position 21 (Scheme 5). B CH3 H3C CH3 Cl2 B H3C CH3 Cl + HCl B H3C CH3 N3 B H3C CH3 OH LiN3 H2O α 1


Formation of amidoboranes
N-Trimethylsilylamides 18 reacted with bromodiorganylboranes quantitatively under formation of the corresponding amidoboranes, in certain cases these are in equilibrium with the dimeric forms [19].Among these reactions in one case could be forming α-haloalkyl compound 19 (Scheme 4).Thus, N-phenyl-Ntrimethylsilyldichloroacetamide reacted with bromodimethyl-borane to form 19 [19].

Hydroboration of cyclovinylic derivatives
Pasto and Snyder also studied a series of vinyl halides to determine the placement of B -H in tetrahydrofuran with borane [14].Hydroboration of 4t-butyl-1-chlorocyclohexene at 25°C gave 60% 1chlorocyclohexyl borane 20, and 40% of the boron atom introduced at the β-position 21 (Scheme 5).

Synthesis of haloalcohols via α α α α α-haloalkylboranes
The action of diborane on halogenated derivatives of unsaturated hydrocarbons was investigated for the first time by Stone et al. [12,13].Treatment α-halogenated boranes 4 -7 proceeded corresponding haloalcohols 8 -11 (Scheme 2).Pasto and Snyder [14] also found that in the hydroboration of vinyl halides, the boron atom was completely or predominantly in the α-position.

Bromination of dioxaborolane derivatives
A series of brominations of dioxaborolane derivatives has been described by Coindard et al. [21].Bromine reacted at minus 80

Scheme 10
Matteson and Liedtke [24] showed that the dibutyl ester of (E)-(1-methyl-propenyl)-boronic acid reacted with bromine and ethanol in benzene to form

Hydroboration of Acetylenes
Dihydroboration of acetylenes proceeds to place two boron atoms preferentially on the terminal carbon atom has been investigated [27].A careful reexamination of the dihydroboration of 1-hexyne using deuterium as a tracer has confirmed the original conclusion [28].These vinylboranes evidently have rich possibilities for organic synthesis.Thus, the hydroboration of substituted propargyl chlorides provides the convenient route for terminal allenes were βhaloalkyl boranes 55, are the postulated intermediates, in yields of 64-73% (Scheme 16) [29].Hydroboration of functional alkynes is also possible, and when the functionality is close to the triple bond, further reaction of the boranes may lead to useful synthetic procedure, [30][31][32] where intermediate 56 is β-chloroalkylborane.β-Heterosubstituted alkylboranes can induce β-elimination 57 to form allenes 58 and 59 [33].

Intramolecular transfer reactions of alkenylboranes with inorganic electrophiles
The predilection of organoboranes to undergo to  presence of excess base, usually sodium hydroxide, gave configurationally inverted alkenes [45], and formed as intermediate β-haloalkyl boranes com-pounds 76A and 76B (Scheme 23).A major drawback of the iodine induced alkene synthesis is that it utilizes one or two alkyl groups.

Scheme 24
According to Pasto and Hickman [39] who carried out detailed studies of the hydroboration of 3chlorocyclohexene with diborane in diethyl ether and tetrahydrofuran, the boron atom attacks mainly in 2 position with the formation of the trans-isomer 80 (85-87%) and cis-isomer 81 (10%), and two minor  The presence of substituents can introduce marked directive influences on the reaction of hydroboration [38,46,47].Hydroboration of 3-chlorocyclopentene with tetraethyldiborane in butyl ether at 0-10°C leads mainly to 2-chlorocyclopentylboranes 82 (Scheme 26) [48].
Tanigawa et al. [68] have described the synthesis of two stannan derivatives such as trans-cinnamyltriethyl-and trans-cinnamyltriphenyltin and their reactions with tribromoborane.The reaction of transcinnamyltriethyl-and/or trans-cinnamyl-triphenyltin 114 with tribromoborane proceeded with the preferential transfer of the trans-cinnamyl group transi-

Markovnikov hydroboration in the formation of β β β β β -fluoroalkylboranes
Fluoro-containing compounds are widely being used in biology and medicine, analytical and organic chemistry [71].Recently Brown et al. [72] have demonstrated the Markovnikov hydroboration of perfluoroalkylehylenes 127 -132 (Scheme 42).In all cases investigated, authors have been found that boron atom incorporated in β-position of fluorine only.

Reactions with allyl chlorides
If the chloride atom located in a γ-position then hydroboration of alkenes involves predominant placement of the boron atom at the least substituted site of the double bond when diborane or tetraalkyldiboranes were added [7,8,9].During reaction of hydroborationoxidation of chloroalkenes usually forming of different 3-butenyl derivatives via γ-boryl alkyl chlorides [47].
Tetraalkyldiboranes react with 3-haloolefins more selectively than diborane.According to Koster et al. [41,42] in reaction of γ-haloolefines with tetraethyland also tetrapropyldiborane, the boron atom incorporated predominantly to the terminal carbon atom as indicated below (Scheme 48).
The organoboranes obtained from tetraalkyldiboranes and/or allyl chlorides react with aqueous alkali, or metal hydrides and form cyclopropane rings 143 with different alkyl fragments (X = halides) according to following general reaction (Scheme 49) [5,6,41].Matos and Soderquist [77] using propargyl bromide for dihydroboration with 9-BBN-H followed by treatment of the adduct with aqueous sodium hydroxide obtained the hydroxy(cyclopropyl)borate complexes, which underwent efficient palladiumcatalyzed cross-coupling to produce a variety of aryl and vinyl cyclopropanes in good to excellent yields via formation of γ-bromoalkylborane 146 [78].
Brown and Rhodes [65] reported that 9-BBN reacted with appropriate branched allylic chlorides, to undergo cyclization with aqueous NaOH and to form the corresponding cyclopropane derivatives 164 via γ-boryl alkyl chloride 165 (Scheme 56).The cycliza-tion of hydroborated allylic chlorides to form the corresponding cyclopropanes was discovered by Hawthorne and Dupont [40] and subsequently applied to the synthesis of a variety of cyclopropanes by Hawthorne [64].