Boranes bicyclic

The formation of a strong intramolecular dative P—B bond in the hydro-borylation reaction of diallylphenylphosphine with triethylaminophenyl-borane results in a bicyclic product, l-borata-5-phosphoniabi-cyclo[3,3,0]octane (204) [Eq. (145)] (64JA5045). The presence of the dative bond in this compound is indicated by an NMR study and by its stability to hydrolysis and oxidation. 

In the first step, borane coordinates to the nitrogen atom in oxazaborolidine 79b from the less hindered side of the fused bicyclic system, resulting in the... 

This observation has led to the preparation of more effective bicyclic oxaza-borolidines such as 1, prepared from (S)-(-)-2-(diphenylhydroxymethyl)pyrrolidine and BH3 (la) or methylboronic acid (lb). Both reagents catalyze borane reduction of alkyl aryl ketones to furnish (R)-alcohols in > 95% ee, by face-selective hydride transfer within a complex such as B. Catalyst lb is somewhat more effective than... 

Monosodium L-glutamate (MSG), 12 49 Monosodium phosphate (MSP), 18 833 manufacture of, 18 853, 857-858 thermal dehydration of, 18 846 Monostatic optical arrangement, 23 139 Monosubstituted boranes, 13 635-636 Monoterpenoid alcohols, 24 500-528 bicyclic, 24 527-528 monocyclic, 24 509-527 Monoterpenoid aldehydes, 24 529-536 Monoterpenoid ethers, 24 528-529 Monoterpenoid hydrocarbons,... 

An impressive solid-phase synthesis of bicyclic guanidines has been communicated, and the approach is outlined in Scheme 20 45,46 iV-acylated dipeptides 18 were reduced to triamines by exhaustive borane reduction, then reacted with thiocarbonyldiimidazole. An intermediate thiocarbonyl was cyclized to the guanidine product in this process. 

Support-bound 1,2-diamines can be readily converted into imidazolidinones by treatment with carbonyl diimidazole [128,129]. The required diamines have been prepared on cross-linked polystyrene by reduction of peptides bound to MBHA resin with borane. Similarly, bicyclic imidazolines have been prepared from triamines and thiocarbonyl diimidazole (Entry 10, Table 14.3). Dehydration of polystyrene-bound monoacyl ethylene-1,2-diamines yields 4,5-dihydroimidazoles (cyclic amidines, Entry 5, Table 13.18). Several groups have reported the synthesis of 2-aminoimidazol-4-ones from resin-bound amino acid derivatives (e.g., Entry 6, Table 15.11). Most of these compounds are, however, unstable, and slowly decompose if dissolved in DMSO (Jesper Lau, private communication). 

An especially selective hydroborating reagent is prepared from 1,5-cyclooctadiene and borane. The product is a bicyclic compound of structure 1 (often abbreviated as 9-BBN), in which the residual B-H bond adds to unhindered alkenes with much greater selectivity than is observed with other hydroborating reagents. It is also one of the few boranes that reacts sufficiently slowly with oxygen that it can be manipulated in air. 

Two dialkyl boranes arc in common use. The bicyclic 9-borabicyclo[3.3.1] nonane (9-BBN), introduced in Chapter 34 as a reagent for diastereoselcctive aldol reactions, is a stable crystalline solid. This is very unusual for an alkyl borane and makes it a popular reagent. It is made by hydroboration of cyclo-octa-1,5-diene. The second hydroboration is fast because it is intramolecular but the third would be very slow. The regioselectivity of the second hydroboration is under thermodynamic control. 

Bicyclic enamines were prepared using the bicyclic ketone, tris(dimethylamino)borane, dimethylamine and potassium carbonate in an autoclave heated to 95-105 °C494. 

Two possible alcohols might be formed by hydroboration/oxidation of the alkene shown. One product results from addition of BH3 to the top face of the double bond (not formed), and the other product results from addition to the bottom face of the double bond (formed). Addition from the top face does not occur because a methyl group on the bridge of the bicyclic ring system blocks approach of the borane. 

Mono- and bicyclic phosphorus heterocycles 199, 200, 202, and 203 were synthesized starting from the bifunctional phosphorylating agent bis(diisopropylamino)ethynyl phosphine 195 via ring-closing enyne metathesis using 4,5-dihydroimidazol-2-ylidene ruthenium benzylidene complex, as a catalyst. Bicyclic phosphorus oxides 199 were obtained in 66-83% yield, whereas phosphorus borane derivative 202 was isolated in 74% yield <2001TL8231>. 

Thermal rearrangements starting from cyclononatetraenylborane. All known cyclooctatetraenes above 0°C rearrange quantitatively to corresponding dihydroindenes. This reaction pathway also takes place in the case of borane 6, but only as a side process.8,11,12 The main product of rearrangement of 6 at 0°C is bicyclic borane 7. The most probable reaction pathway is outlined by Eq. 7. Ci s-exo-dipropylboryl 9 was characterized by NMR as a metastable intermediate.12... 

Thus, the properties of two structurally related bicyclic boranes 7 and 18 show that incorporation of a boron atom with an empty 2p-AO into a bicyclic unsaturated framework results in structurally very labile molecules with complex dynamic behaviour and unusual chemical properties. 

Zwitterionic bicyclic compounds 5a-c are obtained when 64 is allowed to react with di-methoxyborane or di(isopropoxy)borane at temperatures below -10°C. The constitution of 5c is established by the crystal structure shown in Figure 1 (left side). The similarity of the NMR data of 5a and 5b to those of 5c (relevant chemical shifts are included in the formulas through-out this report 8I3C in bold and 8MB in plain text) proves the constitutions of 5a and 5b. 

The hydroboration of simple, open-chain dienes can be somewhat complicated, leading to mixtures of regioisomers, cyclic and open-chain structures, and oligomeric structures of different sizes, all depending on reaction conditions and/or reactant proportions. - However, by control of stoichiometry, thermal isomerization of product mixtures, control of temperature during the hydroboration stage, subsequent treatment of a trialkylborane with further borane, or similar tricks , it is often possible to arrive at an unique product. Several important cyclic dialkylboranes can be made in this way, as shown in equations (6), (7) " and (8). Similar treatment of trienes can lead to bicyclic systems (e.g. equation 9). -24..to... 

In this model the sense of asymmetric induction is controlled by two principle factors (i) coordination of borane on the least-hindered face of the bicyclic ring system and (ii) coordination of the Lewis acid syn to the small group (R ). The latter point is in good agreement with the structural data that has been presented in this chapter, and is further supported by results from the asymmetric reduction of oxime ethers (Figure 51). As predicted by the model (75 and 78), (E) and (Z)-oxime ethers afford enantiomeric amines upon reduction by the reagent derived from (-)-norephedrine and borane (2 equiv.). Here, Lewis acid coordination is dictated by the ( )/(Z) stereochemistry of the oxime ether rather than by the rule of coordination syn to the small group. 

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