Hydroboration complex

This complex exhibits only modest activity in the catalytic hydrogenation of 1-hex-ene (3 turnovers/hr at 1 atm), in a dilference from other aryl oxide-based hydrido derivatives as effective arene hydrogenation catalysts [422], Similar alkoxide tetra-hydroborate complexes of U(IV) complexes are reported [423]. 

This chapter focuses on the chemistry ofbiomimetic copper nitrosyl complexes relevant to the NO-copper interactions in proteins that are central players in dissimilatory nitrogen oxide reduction (denitrification). The current state of knowledge of NO-copper interactions in nitrite reductase, a key denitrifying enzyme, is briefly surveyed the syntheses, structures, and reactivity of copper nitrosyl model complexes prepared to date are presented and the insight these model studies provide into the mechanisms of denitrification and the structures of other copper protein nitrosyl intermediates are discussed. Emphasis is placed on analysis of the geometric features, electronic structures, and biomimetic reactivity with NO or NOf of the only structurally characterized copper nitrosyls, a dicopper(II) complex bridged by NO and a mononuclear tris(pyrazolyl)hydroborate complex having a Cu(I)-NO formulation. 

Table 1 Selected nB chemical shift (ppm) of cyclic hydroborate complexes...

Dicarbonyl complexes of mixed cyclopentadienyl bis(phosphine)amide zirconium compounds have also been prepared by ligand-induced disproportionation of (775-C5H5)ZrBH4[N(SiMe2CH2PPr12)2] 47, yielding the bis(tetra-hydroborate) complex (775-C5H5)Zr(BH4)2[N(SiMe2CH2PPr12)2] 48 and the dicarbonyl derivative... 

Zm complex (45) provided a model for the Zm -activated serine of AP (see Scheme 10). An alkoxide group in (45b) attacked BNPP to give a phosphorylserine intermediate (46), which was susceptible to further hydrolysis by the intramolecular Zn -bound hydroxide in (46b) to give (47) (Scheme 33). For the first step, the alcohol group is deprotonated by the proximate Zn (p7 a = 7.5) to an alkoxide complex (45b), which was 125 times more effective as nucleophile to the phosphate substrate than was the Zn -activated water of the reference compound (34). For the subsequent step, the nucleophilic Zn species (46c) was generated with a pXa value of 9. This intramolecular hydrolysis is 45,000 times faster than the intermolecular hydrolysis of ethyl 4-nitro-phenyl phosphate (ENP) with (34b). These results imply an advantage of the intramolecular arrangement of two Zn ions in AP (as shown in Scheme 10) over mononuclear Zn hydrolases. Toward any mononuclear Zn phosphatase model, phosphomonoesters were not substrates, but instead were inhibitors, as shown by isolation of the stable complexes (43), (44), and (47). The tris(pyrazolyl)-hydroborate complexes such as (35) and (38) hydrolyze phosphodiesters to phosphomonoesters, which similarly bind to Zn to become inert to further hydrolysis. ... 

Complexes IrH2(BH4)L2, where L is a bulky tertiary phosphine, have been prepared. They are the first examples of metal-hydroborate complexes in which the BH4 ligand is non-fluxional. Thus, the structure is (5), and H n.m.r. signals were seen separately for the bridge and terminal hydrogen atoms. °... 

Scheme 43. Unit composition of /i-bora-alkyne ditungsten Cp hydroboration complex.

Substituted tetrahydroborates will participate in simple exchange reactions and, thus, are useful starting materials for the preparation of several higher hydroborate complexes [7]. 

Versatile [3 + 2]-cydoaddition pathways to five-membered carbocydes involve the trimethylenemethane (= 2-methylene-propanediyl) synthon (B.M. Trost, 1986). Palladium(0)-induced 1,3-elimination at suitable reagents generates a reactive n -2-methylene-l,3-propa-nediyl complex which reacts highly diastereoselectively with electron-deficient olefins. The resulting methylenecyclopentanes are easily modified, e. g., by ozonolysis, hydroboration etc., and thus a large variety of interesting cyclopcntane derivatives is accessible. 

Another possibility for asymmetric reduction is the use of chiral complex hydrides derived from LiAlH. and chiral alcohols, e.g. N-methylephedrine (I. Jacquet, 1974), or 1,4-bis(dimethylamino)butanediol (D. Seebach, 1974). But stereoselectivities are mostly below 50%. At the present time attempts to form chiral alcohols from ketones are less successful than the asymmetric reduction of C = C double bonds via hydroboration or hydrogenation with Wilkinson type catalysts (G. Zweifel, 1963 H.B. Kagan, 1978 see p. 102f.). 

We can consider the hydroboration step as though it involved borane (BH3) It sim phfies our mechanistic analysis and is at variance with reality only m matters of detail Borane is electrophilic it has a vacant 2p orbital and can accept a pair of electrons into that orbital The source of this electron pair is the rr bond of an alkene It is believed as shown m Figure 6 10 for the example of the hydroboration of 1 methylcyclopentene that the first step produces an unstable intermediate called a tt complex In this rr com plex boron and the two carbon atoms of the double bond are joined by a three center two electron bond by which we mean that three atoms share two electrons Three center two electron bonds are frequently encountered m boron chemistry The tt complex is formed by a transfer of electron density from the tt orbital of the alkene to the 2p orbital... 

The mechanistic complexity of hydroboration-oxidation stands m contrast to the simplicity with which these reactions are carried out experimentally Both the hydrobo ration and oxidation steps are extremely rapid reactions and are performed at room tern perature with conventional laboratory equipment Ease of operation along with the fact that hydroboration-oxidation leads to syn hydration of alkenes and occurs with a regio selectivity opposite to Markovmkov s rule makes this procedure one of great value to the synthetic chemist... 

Diborane [19287-45-7] the first hydroborating agent studied, reacts sluggishly with olefins in the gas phase (14,15). In the presence of weak Lewis bases, eg, ethers and sulfides, it undergoes rapid reaction at room temperature or even below 0°C (16—18). The catalytic effect of these compounds on the hydroboration reaction is attributed to the formation of monomeric borane complexes from the borane dimer, eg, borane-tetrahydrofuran [14044-65-6] (1) or borane—dimethyl sulfide [13292-87-0] (2) (19—21). Stronger complexes formed by amines react with olefins at elevated temperatures (22—24). 

Borane—dimethyl sulfide complex (BMS) (2) is free of these inconveniences. The complex is a pure 1 1 adduct, ca 10 Af in BH, stable indefinitely at room temperature and soluble in ethers, dichioromethane, benzene, and other solvents (56,57). Its disadvantage is the unpleasant smell of dimethyl sulfide, which is volatile and water insoluble. Borane—1,4-thioxane complex (3), which is also a pure 1 1 adduct, ca 8 Af in BH, shows solubiUty characteristics similar to BMS (58). 1,4-Thioxane [15980-15-1] is slightly soluble in water and can be separated from the hydroboration products by extraction into water. 

Borane—triethylamine complex (4) is used when slow Hberation of borane at elevated temperatures is advantageous, eg, in the cycHc hydroboration of trienes to avoid the formation of polymers (59). 

A number of less hindered monoalkylboranes is available by indirect methods, eg, by treatment of a thexylborane—amine complex with an olefin (69), the reduction of monohalogenoboranes or esters of boronic acids with metal hydrides (70—72), the redistribution of dialkylboranes with borane (64) or the displacement of an alkene from a dialkylborane by the addition of a tertiary amine (73). To avoid redistribution, monoalkylboranes are best used /V situ or freshly prepared. However, they can be stored as monoalkylborohydrides or complexes with tertiary amines. The free monoalkylboranes can be hberated from these derivatives when required (69,74—76). Methylborane, a remarkably unhindered monoalkylborane, exhibits extraordinary hydroboration characteristics. It hydroborates hindered and even unhindered olefins to give sequentially alkylmethyl- and dialkylmethylboranes (77—80). 

The products are Hquids, soluble in various solvents and stable over prolonged periods. Monochloroborane is an equiUbtium mixture containing small amounts of borane and dichloroborane complexes with dimethyl sulfide (81). Monobromoborane—dimethyl sulfide complex shows high purity (82,83). Solutions of monochloroborane in tetrahydrofuran and diethyl ether can also be prepared. Strong complexation renders hydroboration with monochloroborane in tetrahydrofuran sluggish and inconvenient. Monochloroborane solutions in less complexing diethyl ether, an equiUbtium with small amounts of borane and dichloroborane, show excellent reactivity (88,89). Monochloroborane—diethyl etherate [36594-41-9] (10) may be represented as H2BCI O... 

Primary dialkylboranes react readily with most alkenes at ambient temperatures and dihydroborate terminal acetylenes. However, these unhindered dialkylboranes exist in equiUbtium with mono- and ttialkylboranes and cannot be prepared in a state of high purity by the reaction of two equivalents of an alkene with borane (35—38). Nevertheless, such mixtures can be used for hydroboration if the products are acceptable for further transformations or can be separated (90). When pure primary dialkylboranes are required they are best prepared by the reduction of dialkylhalogenoboranes with metal hydrides (91—93). To avoid redistribution they must be used immediately or be stabilized as amine complexes or converted into dialkylborohydtides. 

Although dichloroborane reacts direcdy with alkenes in the gas phase (118), its complexes with diethyl ether and dimethyl sulfide are so strong that direct hydroboration does not proceed (119,120). The addition of a decomplexing agent, eg, boron trichloride, is necessary for hydroboration. 

Catalytic Asymmetric Hydroboration. The hydroboration of olefins with catecholborane (an achiral hydroborating agent) is cataly2ed by cationic rhodium complexes with enantiomericaHy pure phosphines, eg, [Rh(cod)2]BE4BINAP, where cod is 1,5-cyclooctadiene and BINAP is... 

A more convenient hydroborating agent is the borane-tetrahydrofuran complex (H3B-THF). It is very reactive, adding to alkenes within minutes at 0°C, and is used in tetrahydrofuran as the solvent. 

Hydroboration of heterocycles catalyzed by transition metal complexes 97T4957. 

The conversion of a thiolactone to a cyclic ether can also be used as a key step in the synthesis of functionalized, stereochemically complex oxacycles (see 64—>66, Scheme 13). Nucleophilic addition of the indicated higher order cuprate reagent to the C-S double bond in thiolactone 64 furnishes a tetrahedral thiolate ion which undergoes smooth conversion to didehydrooxepane 65 upon treatment with 1,4-diiodobutane and the non-nucleophilic base 1,2,2,6,6-pentamethylpiperidine (pempidine).27 Regio- and diastereoselective hydroboration of 65 then gives alcohol 66 in 89 % yield after oxidative workup. Versatile vinylstannanes can also be accessed from thiolactones.28 For example, treatment of bis(thiolactone) 67 with... 

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