Diborane carbonyl compounds

Treatment of Fischer-type carbene complexes with different oxidants can lead to the formation of carbonyl compounds [150,253]. Treatment with sulfur leads to the formation of complexed thiocarbonyl compounds [141]. Conversion of the carbene carbon atom into a methylene or acetal group can be achieved by treatment with reducing agents. Treatment of vinylcarbene complexes with diborane can also lead to demetallation and formation of diols [278]. The conversion of heteroatom-substituted carbene complexes to non-heteroatom-substituted carbene complexes... 

Cobalt hydrocarbonyl, diborane, and aluminum hydrides add, I think, to all of these carbonyl compounds. Of course, there is the well known Grignard reagent and the alkyllithium additions to carbonyl compounds. Aluminum alkyls add, and we could have listed all the other alkali metal alkyls. Recent work has shown that the tin alkoxides add readily to all these derivatives, and similarly, a tin amide adds to most of these carbonyl compounds. 

BROWN, HERBET C. (1912-). An English-born chemist who was the recipient of the Nobel prize for chemistry with Wuug. Georg in 1979. Via his work in organic synthesis, he discovered new routes to arid substituents to olefins selectively. His early education was irregular and disjointed as a result of family circumstances and the economic depression of the 1930s. He eventually received his Ph.D. from the University of Chicago. The reduction of carbonyl compounds with diborane was the topic of... 

Hydroboration, reduction of G=0. This complex is as efficient as BH3 THF lor hydroboration it forms diborane when dissolved in benzene. It reduces carbonyl compounds to alcohols at 0° in hexane or CH2C12. The stereoselectivity resembles that of BH3-THF. It reduces carboxylic acids to primary alcohols in 60-75% yield. 

Although catalytic hydrogenation in the presence of H2 and a catalyst such as Pt, Pd, Ni or Ru, reaction with diborane, and reduction by lithium, sodium or potassium in hydroxylic or amine solvents have all been reported to convert carbonyl compounds into alcohols, the most common reagents used for the reduction of carbonyl compounds are hydride donors. 

A variety of reducing agents have been used for the conversion of imines to amines.The examples in equations (50) and (51) are typical. The relatively slow destruction of sodium cyanoboro-hydride in slightly acid aqueous solutions allows the process of imine formation to be carried out simultaneously with reduction. Apparently, the protonated imine is the species that is reduced and this process occurs significantly faster than reduction of the original carbonyl compound by this reagent. A recent report by Nose indicated that diborane in methanol will reduce only imines and no other functionality. Apparently, the diborane reacts rapidly with methanol to form a derived reagent. 

Reduction. The reaction of diborane with acetone (2) and other simple carbonyl compounds to form dialkoxyboranes may involve the equilibrium BjHj 2 BHi, and a possible formulation of the first step is shown in (5). The monoalkoxyborane... 

Hydroboration, the addition of borane (or diborane) to n-donor substrates such as alkenes, acetylenes, carbonyl compounds, and so on, is a most useful synthetic reaction and was developed by the extensive work of Brown et al. 2 "52 In the absence of more nucleophiUc n- and n-donor sites, borane will also attack a bonds in alkanes, silanes, and aromatic compounds to yield addition, cleavage, and rearrangement products. The interaction of borane with hydrocarbons involves 3c-2e bonded pentacoordinate carbons. 

Carbonyl compounds (after ref. 5]. McMurry58 desired to selectively reduce an olefinic double bond with diborane in the presence of a keto group. This was successfully achieved by conversion to the dinitrophenylhydrazone, hydroboration and removal of the protective group by ozonolysis in ethyl acetate at — 78°. Oximes and oxime acetates are reduced by BH3. Attempted protection by ketalization in this case was unsuccessful because of simultaneous migration of the double bond. 

Chiral boranes can be used as reducing agents for carbonyl compounds. 30 Brown prepared a variety of chiral boranes in connection with extensive studies of hydroboration reactions (sec. 5.4.B). Reaction of diborane and a-pinene, for example, gave (-)-( / ,25,3/ ,5/ )-diisopinocampheylborane (206), which reduced carbonyl derivatives to alcohols with high asymmetric induction.33 Trialkylboranes are also capable of... 

Other uses of diborane include the preparation of boron nitride by the reaction of diborane with ammonia, as a catalyst for polymerization, and for the conversion of olefins to trialkyl bo-ranes. It is also used in the conversion of amines to amine boranes and as a selective reducing agent with carbonyl compounds such as aldehydes and ketones to form alcohols. 

My thesis was completed in 1938 and the results were published in 1939 Hydrides of Boron. XI. The Reaction of Diborane with Organic Compounds Containing a Carbonyl Group (3). 

Diborane also has a useful pattern of selectivity. It reduces carboxylic acids to primary alcohols under mild conditions that leave esters unchanged.77 Nitro and cyano groups are relatively unreactive toward diborane. The rapid reaction between carboxylic acids and diborane is the result of formation of a triacyloxyborane intermediate by protonolysis of the B-H bonds. The resulting compound is essentially a mixed anhydride of the carboxylic acid and boric acid in which the carbonyl groups have enhanced reactivity toward borane or acetoxyborane. 

Other reagents which have occasionally been used to cleave hydrazides include diborane (which also reduces the carbonyl groups), sodium naphthalenide, 0,0-diethyldithiophosphoric acid, (EtO)2PS2H, - and sulfur monochloride. Nickel-aluminum alloy in aqueous methanolic potassium hydroxide is a good reagent for reductively cleaving a number of N—N bonded compounds, such as A -methyl-A -phenylhydrazine and Af/Z-dimethylnitrosamine. - Nitrosamines have also been cleaved with titanium(IV) chloride-sodium borohydride and lithium aluminium hydride. 

The compound borane-carbonyl occupies a special position in any discussion of molecules derived from boron hydrides with boron-carbon bonds. Carbon monoxide reacts with diborane (20 atm/room temperature) to give an adduct of the borane group which does not rearrange or lose hydrogen (39) The adduct OC BH3 does, however, dissociate into carbon monoxide and diborane at ordinary temperatures. No carbonyls of other Group III acceptor molecules have been prepared so far 133), and it has been suggested 27, 58) that borane-carbonyl owes its existence to the ability of the hydrogen atoms of borane to transfer electrons to a vacant p,-orbital in carbon monoxide. 

Diborane reduction is summarized in Scheme 37. Diketo bicycles of this type in some instances provide products of the carbonyl group reduction and N—N bond splitting. For example, dioxo derivative (235) and tetraoxo derivative (237) provide (236) and (238), respectively, in high yields <78TL543, 79JOC4473). On the Other hand, tetrabromo compound (239) is reduced to (240) which is isolated in 60% yield <75JOC47>. 

Attempts to prepare the required bicyclic azetidines by selective reduction of the C-7 carbonyl bond in penicillins failed [66, 67,68] treatment with diborane in THF gave exclusively the aminoalcohols 107 resulting from the cleavage of the four-membered ring (Scheme 34). However, 3-hydroxypropylamine derivatives can be transformed into azetidines, using for instance a modified Mitsonobu reaction [69, 70]. Nevertheless, this transformation was unsuccessful with compound 107b because cyclisation occurred onto the acylamino side-chain to yield dihydrooxazole 108 (Scheme 35). 

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