Ethylene hydroborated, reaction

Boron Bromide. Approximately 30% of BBr produced in the United States is consumed in the manufacture of proprietory pharmaceuticals (qv) (7). BBr is used in the manufacture of isotopicaHy enriched crystalline boron, as a Etiedel-Crafts catalyst in various polymerization, alkylation, and acylation reactions, and in semiconductor doping and etching. Examples of use of BBr as a catalyst include copolymerization of butadiene with olefins (112) polymerization of ethylene and propylene (113), and A/-vinylcarbazole (114) in hydroboration reactions and in tritium labeling of steroids and aryl rings (5). 

Sm(III)-catalyzed olefin hydroboration reaction by catecholborane has recently been theoretically investigated [28], The stationary structures on the model reaction path considering ethylene as the olefin, Cp2SmH as the active catalyst, and HB(OH)2 as the model borane were obtained at the RHF and MP2 levels. MP4SDQ energy calculations were also carried out at the MP2 structures. 

The addition of a gas to a reaction mixture (commonly the hydrogen halides, fluorine, chlorine, phosgene, boron trifluoride, carbon dioxide, ammonia, gaseous unsaturated hydrocarbons, ethylene oxide) requires the provision of safety precautions which may not be immediately apparent. Some of these gases may be generated in situ (e.g. diborane in hydroboration reactions), some may be commercially available in cylinders, and some may be generated by chemical or other means (e.g. carbon dioxide, ozone). An individual description of the convenient sources of these gases will be found under Section 4.2. 

Impressive calculations for several sequences of reaction steps of the catalytic hydroboration of ethylene by HB(OH)2 using a model Wilkinson catalyst, RhCl(Ph3)2, have been done by Musaev et al. [64]. Different mechanisms were compared involving more than 30 intermediates and transition states. Full-geometry optimizations were performed at the MP2 level. This study shows that it is now possible to understand catalytic reactions at a very detailed level. The hydroboration reaction is just one example, although a particularly spectacular one, of the type of detailed studies that have been performed by Morokuma et al. For earlier work along similar lines, see Ref. 5. 

Hydroboration attracted several theoretical studies in 1978. Publications which have appeared include an ab initio study of the reaction of BHs with ethylene, a CNDO/2 study of the nature of the ethylene-borane complex, the transition state in the hydroboration reaction, a MNDO study of hydroboration of alkenes and alkynes, and a MNDO study of hydroboration and borohydride reduction with implications concerning cyclic conjugation and pericyclic reactions. Finally, HaBCHO was one of the molecules which has been studied with regard to banana bonds of the carbonyl group. ... 

Only a minimum of mechanistic attention has been paid to the catalytic effect that ethers have on the hydroboration reaction. After determining that the hydroboration of olefins with l,2-bis(3-methyl-2-butyl)diborane is first order in olefin and also first order in the substituted diborane, it was suggested that the role of the solvent is to coordinate with the dialkylborane monomer, the leaving group in this particular reaction 101). However, the room temperature hydroboration of ethylene with borine carbonyl 102) intimates, by analogy, that a rather loosely held adduct may account for acceleration of B—H addition to unsaturated systems in ether solvents. The use of a molybdena-aluminum catalyst has been moderately effective in converting an ethylene-diborane mixture to a mixture of ethyldiboranes at room temperature 103). 

In a reaction reminiscent of the ether-catalyzed hydroboration reaction (Sections II,A,l,d and II,A,2,d), the B—H bonds of BH3CO add to ethylene at ambient temperatures to form triethylboron with the release of carbon monoxide (102). 

An alternative method of hydroboration is to use diisopinocampheylborane (12) (Scheme 4). This reaction is particularly useful for sterically hindered alkenes. Diisopinocampheylborane (12) is prepared from borane-dimethyl sulfide and (+)-pinene.[23-24] Treatment of 4-meth-ylenecyclohexanone ethylene ketal with diisopinocampheylborane (12) gives the borane 13.[25] Further treatment with 2 equivalents of an aldehyde results in the elimination of pinene and the formation of a new dialkyl boronate, e.g. treatment of 13 with acetaldehyde gives the diethyl cyclohexylmethylboronate 14J261 The dialkyl boronates thus produced can be transesterified with pinanediol to give 15[26] or with other cyclic diols. 

A series of polyether macrocycles (40)-(47) that contain a coordinated reducible transition metal redox-active 16-electron molybdenum nitro-syl Mo(NO) 3+ group have been prepared by us in collaboration with McCleverty and Jones (86,87) (Scheme 14). Compounds (40)-(44) were synthesized from the reactions between [Mo(NO)LX2] (L = tris(3,5-dimethylpyrazolyl)hydroborate X = Cl or I) and the appropriate amine-substituted benzo crown ether. Compounds (45)—(47) were prepared from reactions between [Mo(NO)LI2] and tetra-, penta-, or hexa-ethylene glycol, respectively, in the presence of triethylamine. A crystal structure determination of (45) (88) is shown in Fig. 14. 

The reactions of hindered alkenes with thexylborane are generally accompanied by a significant amount of dehydroboration to give 2,3-dimethyl-2-butene. Thus, a very large excess of the latter is needed in order to produce (monomeric) dithexylborane. Hydroboration of a-pinene (1 1 ratio) with thexylborane results in ca. 15% dehydroboration.However, alkenes of somewhat lower steric requirements, such as simple 1,2-disubstituted ethylenes, can be converted fairly cleanly into the corresponding thexylmonoalkylboranes e.g. equation 13). These are mixed dialkylboranes and behave like other fairly hindered dialkylboranes (Section 3.10.4.3). They can hydroborate alkenes of lower steric requirements to give totally mixed trialkylboranes. 

The selectivities exhibited by dibromoborane-dimethyl sulfide are also interesting. In contrast to the situation with 9-BBN-H, internal alkynes are hydroborated faster than either terminal alkynes or terminal alkenes. Unlike reactions with disiamylborane, 1,1-disubstituted ethylenes are hydroborated in preference to simple 1-alkenes. These properties can lead to interesting possibilities for selective hydroboration of polyunsaturated molecules (e.g. Schemes 8 and 9)." ... 

Dibromoborane-dimethyl sulfide exhibits regioselectivity in hydroboration of alkenes which is comparable to that exhibited by dialkylboranes. Excess alkene should be avoided in reactions of trisubstituted ethylenes, otherwise hydrogen bromide liberated during work-up may add to the excess ethylene and cause problems. 

Reactions of the anti-van t Hoff/le Bel titanium complex Cp2Ti(PhC)2(HBEt2) with ethylene, acetylenes, phospa-cetylenes, acetone, and nitriles give metallacycle or bimetallic compounds (Scheme 521). In some cases, hydroboration of the G-C or C-heteroatom multiple bond in the substrates is observed.1331 (C5Me5)2Ti[r/2-CH2=CHB(Cat)]1303... 

This synthesis has been adapted to make the natural (5 -isomer of 114, reducing the initially formed aldehyde to (S)-114 with baker s yeast. [This same synthesis has been adapted to make (S)-callosobruchic acid (115), see below]. Julia s synthesis of a-geraniol (73) was also extended to make 114 by hydro-boration. Hydroboration of 73 with diisopinocampheylborane (made from (- )-a-pinene [(-)-116]) gave only a small ee however. A synthesis of ( )-114 started with the reaction of 2-methylpropiolactone and the ethylene acetal of 3-oxobutylmagnesium bromide. The methyl ester of the acid 117 thus prepared was chain-lengthened by reaction with acetylene and rearrangement with a vanadium catalyst of the ynol thereby obtained. The aldehyde 118 was then reduced with lithium aluminum hydride to 114. ... 

According to MNDO and ab initio results for the addition of BH3 to ethylene (1), the reaction is highly exothermic and proceeds via a reactant-like transition state with the double bond elongated to 138-140 pm and little pyramidalization (254). It therefore seems unlikely that release of strain will be noticeable in hydroborations of distorted double bonds. The low regioselectiv-ity observed in hydroboration of 69 and the structurally related 282 may therefore reflect similar hybridizations of both carbon atoms of the double bond. 

A reaction known as hydroboration is of especial preparative importance for the conversion of olefins into alcohols.70-73 This consists in treating simple olefins such as ethylene, 1- or 2-pentene, cyclohexene, or styrene with sodium tetrahydridoborate in the presence of aluminum chloride at 25°, wherepon... 

The insertion of alkenes and alkynes into M-E (E = SiRa, SnR3, BR2) is not substantially different from the insertion into M-H or M-C bonds. The same orbital considerations hold in these cases (see Section 6.2.1) and theoretical methods have been applied to analyze these processes, generally in the context of mechanistic studies of catalytic reactions where this may be an important step (hydrosilation, hydroboration, diboration, silylstannation, etc.). Table 6.11 contains some calculated activation energies for insertion of ethylene and alkynes into M-ER bonds of comparable systems. The balance of bond breaking/bond making is less favorable for M-Si than for M-C or M-H bonds. Moreover, the directionality of the 0(M-Si) bonds is detrimental for insertion as discussed when... 

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