Boranes conversion

This is one of the few methods available for the direct and efficient conversion of an acid, via the acid chloride, to an ortho ester. The preparation of the oxetane is straightforward, and a large number of oxetanes have been prepared [triol, (EtO)2CO, KOH]." In addition, the -butyl analogue has been used for the protection of acids. During the course of a borane reduction, the ortho ester was reduced to form a ketal. This was attributed to an intramolecular delivery of the hydride. ... 

Careful control of temperature, pressure and reaction time enables the yield of the various intermediate boranes to be optimized. Eor example, B4H10 is best prepared by storing B2H6 under pressure at 25° for 10 days this gives a 15% yield and quantitative conversion according to the... 

Reduction of 3,5,5-tris-aryl-2(5// )-furanones 115 (R, R, R = aryl) with dimethyl sulfide-borane led to the formation of the 2,5-dihydrofurans 116 in high yields. However, in the case of 3,4-diaryl-2(5//)-furanones 115 (R, R = aryl R = H or r = H R, R = aryl), the reduction led to a complicated mixture of products of which only the diarylfurans 117 could be characterized (Scheme 36) (88S68). It was concluded that the smooth conversion of the tris-aryl-2(5//)-furanones to the corresponding furan derivatives with the dimethylsulfide-borane complex in high yields could be due to the presence of bulky aryl substituents which prevent addition reaction across the double bond (88S68). 

Borane 2 adds to carbon-carbon double bonds without the need of catalytic activation. This reaction has been discovered and thoroughly investigated by H. C. Browm, and is called hydroboration It permits a regioselective and stereospecific conversion of alkenes to a variety of functionalized products. 

No matter which of the electrophilic methods of double-bond shifting is employed, the thermodynamically most stable alkene is usually formed in the largest amount in most cases, though a few anomalies are known. However, there is another, indirect, method of double-bond isomerization, by means of which migration in the other direction can often be carried out. This involves conversion of the alkene to a borane (15-16), rearrangement of the borane (18-11), oxidation and hydrolysis of the newly formed borane to the alcohol (12-28), and dehydration of the alcohol (17-1) ... 

Several other methods for the conversion of boranes to tertiary alcohols are also known. 

For another conversion of trialkylboranes to ketones, see 18-26. Other conversions of boranes to secondary alcohols are also known. 

Conversion of Boranes to Primary Aicohois, Aidehydes, or Carboxylic Acids... 

For the amino-borane dehydrocoupling using [Rh(l,5-cod)(p-Cl)]2 as starting catalyst, an induction period and a sigmoid-shaped kinetic curve (plot of substrate conversion versus time) were also observed, consistent with metal-particle formation. But, for Ph2PH BH3... 

Several other oxidants can be used to effect the borane to alcohol conversion. Oxone (2K2S05 KHS04 K2S04) has been recommended for oxidations done on a... 

Advances in the use of anionic stereogenic phosphorus have been interesting. Acylation of lithium o-anisylphenylphosphide with chloro-formates bearing chiral alkyl groups provided a diastereomeric mixture that could be induced to undergo an inversion at phosphorus (at relatively low temperature) to form the more favorable diastereoisomer in a crystalline lattice.186 Subsequent conversion to the quaternary phos-phonium species was followed by removal of the acyl group and isolation of the chiral tertiary phosphine as the borane derivative (Equation 3.10). 

Among early and group III transition metals, the yttrocene catalysts have been studied in greatest detail. However, related metallocenes show great promise as catalysts for reductive cyclization. Neodymocene-catalyzed cyclization of 1,5- and 1,6-dienes 14a and 15a proceeds readily in the presence of silane to afford cyclopentanes 14b and 15b.37 Lutetocenes and samarocenes also catalyze silane-mediated cyclization of 1,5-diene 14a to cyclopentane 14b.38 39 In the case of the samarium-based metallocenes, the feasibility of borane-mediated cyclization has been established, as demonstrated by the highly diastereoselective conversion of phenyl-substituted diene 16a to cyclopentane 16b (Scheme ll).40... 

Purification. Two laboratories noted that commercial samples of KH12 and NaH2 are ineffective for conversion of hindered trialkylboranes into the corresponding borohydrides. Both groups find that treatment of the aged metal hydrides with lithium aluminum hydride in THF results in highly active hydrides that react readily even with such hindered trialkylboranes as tris(3-methyl-2-butyl)borane. 

The trifluorovinylxenonium salt (22, 27) and a series of c/.s-X-l,2-difluoroalkenylxenonium salts were successfully synthesized and isolated in 30-60% yields (22). It is noteworthy that the xenodeborylation reaction of highly acidic boranes, CF2=CFBF2 (fluoride affinity 80.3 kcal mol 1) and cis-CnF2n+iCF=CFBF2 (fluoride affinity 90.0 kcal mol 1 for n = 1) is accompanied by their partial conversion into perfluoroalkyltrifluoroborates, whereas in case of X = Cl the anion, [C1CF2CF2BF3] was not observed (eq 6). 

Scheme 3.2-23. Conversion of the 1-carba-orachno-penta-borane(lO) 30 into the 2,4-dicarba-n/do-hexaborane(8) 43, followed by deprotonation yields borate 44.

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