Borylation

SCHEME 24S9 Representative example of arene borylation. 

Three routes have been preferentially used to prepare ferrocene compounds that contain heteroelements directly attached to the metallocene unit borylation, mercuration, and lithiation. 

The dihaloboryl-substituted ferrocenes can be used to prepare various other boryl derivatives thus, FC-BX2 compounds (X = Cl, Br or I) have been converted into FC-BR2 (R = NMe2 [4, 7], NEt2, OEt, SMe, Me [4, 5]). As an example, the synthesis 

It has been shown that borylation of ferrocene with BBrj or BIj in hexane solution proceeds in a stepwise manner [9]. Excess BXj leads to mixtures containing essentially l,3,r-tris- and l,3,r,3 -tetrakis(dihalogenoboryl) ferrocene the intermediates are FC-BX2 and fc(BX2)2, although small amounts of l,3-bis(diahalogenoboryl) ferrocene have also been detected in the predominant 1,1 isomer, fc(BX2)2 (X = Br, I). 

Although nBuLi metallates ferrocene to give a mixture of both Fc-Li and fcLi2 under almost any conditions (see above), the isomeric tBuLi gives exclusively and cleanly monolithio-ferrocene, Fc-Li, in Et20 solution [38, 282] (see Appendix). 

Since the pioneering studies of Hartwig and colleagues [52] and Smith and coworkers [53], the transition-metal-catalyzed activation and functionalization of unreac-tive C—H bonds of hydrocarbons [54] have led to increasing attention being paid 

For catalytic hydrohoration reaction (a) Mannig, D. and Noth, H. (1985) Angewandte Chemie-International Edition in English, 24, 878  

Hulmes, D.I., Knight, E.I., Layzell, T.P. and Lloyd-Jones, G.C. (1999) Transition Metal-Catalysed Reactions (eds S.I. Murahashi and S.G. Davies), Blackwell Science, Oxford, UK, p. 465  

4 Hoel, E.L and Hawthorne, M.F. (1973) Journal of the American Chemical Society, 

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Boron haUdes have also been shown to insert into B—B bonds to give initial products with the new boryl moiety in a bridge position (80). 

Iminoboianes have been suggested as intermediates in the formation of compounds derived from the pyrolysis of azidoboranes (77). The intermediate is presumed to be a boryl-substituted nitrene, RR BN, which then rearranges to the amino iminoborane, neither of which has been isolated (78). Another approach to the synthesis of amino iminoboranes involves the dehydrohalogenation of mono- and bis(amino)halobotanes as shown in equation 21. Bulky alkah-metal amides, MNR, have been utilized successfully as the strong base,, in such a reaction scheme. Use of hthium-/i /f-butyl(ttimethylsilyl)amide yields an amine, DH, which is relatively volatile (76,79). 

Synthesis, reactivity, and structure of transition metal boryl compounds, derivatives of B,N- and B,0-heterocycles with B—M bond 98CRV2685. 

For the construction of oxygen-functionalized Diels-Alder products, Narasaka and coworkers employed the 3-borylpropenoic acid derivative in place of 3-(3-acet-oxypropenoyl)oxazolidinone, which is a poor dienophile in the chiral titanium-catalyzed reaction (Scheme 1.55, Table 1.24). 3-(3-Borylpropenoyl)oxazolidinones react smoothly with acyclic dienes to give the cycloadducts in high optical purity [43]. The boryl group was converted to an hydroxyl group stereospecifically by oxidation, and the alcohol obtained was used as the key intermediate in a total synthesis of (-i-)-paniculide A [44] (Scheme 1.56). 

One of the most general preparative routes to allyl- and 2-butenylboranes involves the reaction of an allylic organometallic species and an electrophilic borylating reagent. Various esters of allylboronic acid have been prepared in this way2,4-5. 

Recent optimization studies reveal that the yield of 2-(2-propenyl)-1,3,2-dioxaborolane-4,5-di-carboxylate esters (i.e., the tartrate ester modified allylboronates) is improved by using triiso-propyl borate as the borylating agent1. The improved yields are directly related to the increased efficiency of the preparation of the intermediate allylboronic acid. 

A second route to a-substituted allylboronates involves the functionalization of substituted allylorganometallic precursors with a suitable borylating agent. Several examples of this method are summarized below. 

Racemic l-methyl-2-butenylboronates (E)- and (Z)-3 may be prepared selectively via reactions of the l-methyl-2-butenyl Grignard reagent with the appropriate borate ester. Use of triisopropyl borate provides a 96 4 mixture of (E)-3l(Z)-3 on a 0.36 mol scale15. Use of a bulkier borylating agent, such as 2-isopropyloxy-4,4,5,5-tetramethyl-l,3,2-dioxaborolane, reverses the selectivity, enabling a 91 9 mixture of (Z)-3/( )-3 to be obtained on a 0.5 mol scale. The diastereomeric purity of this mixture may be enhanced to 95 5 by treatment with 0.15 equivalents of benzaldehyde, since ( )-l-mcthyl-2-butenylboronatc ( )-3 is more reactive than (Z)-3. Repetition of this process provides (Z)-3 that is 98% isomerically pure. 

I4 Boron enolates derived from oxazolidinone 3 arc reported to give either syn- or imp-adducts depending on the amounl of boryl Inflate and base employed, the character of the base, and the structure of the aldehyde see H. Danda, M. M. Hansen. C. H. Heathcock, J. Org. Chem. 55,173... 

Bei Anwendung der theoretischen Menge Diboran lauft die Reduktion liber das durch Disproportionierung gebildete Carbonsaureanhydrid und Bis-[diacyloxy-boryl]-oxid1-3 ... 

Keywords diene boryl Diels-Alder cycloaddition... 

One year later Van der Eycken and Dehaen described the smooth microwave-assisted borylation of 4, 5, 6 and 7-bromo-lff-indole using PdCl2(dppf) as a precatalyst and KOAc as a base (Scheme 30) [48]. With 5, 6, and 7-bromo-lH-indole, DMSO was used as solvent at a temperature of 150 °C (with a set power of 150 W) for 17-27 min, resulting in the corresponding boronate esters in good yields. For 4-bromo-lH-indole, DME gave a better result at the same temperature (with a set power of 250 W). 

Borylated Bis(dioxime)metal Complexes and Related Compounds... 

Tris(dioxime) complexes that are capped with a boryl group BR at only one end of the molecule are also known, e.g., with technetium(III) [227, 228] and rhenium(III) [229]. 

The formation of boron-group IB bonds succeeds in two ways by transfer of a boryl group from metal-boron compounds to other metals, and by reaction of anionic boranes or carboranes with transition-metal halides. 

NH-Phosphinous amides can be metallated at nitrogen, via their corresponding anions [78,112,113] as well as borylated [114] and silylated [22,115, 116]. In this last case, the isomeric P-silylphosphazenes are also occasionally obtained. 

The proposed mechanism for Fe-catalyzed 1,4-hydroboration is shown in Scheme 28. The FeCl2 is initially reduced by magnesium and then the 1,3-diene coordinates to the iron center (I II). The oxidative addition of the B-D bond of pinacolborane-tfi to II yields the iron hydride complex III. This species III undergoes a migratory insertion of the coordinated 1,3-diene into either the Fe-B bond to produce 7i-allyl hydride complex IV or the Fe-D bond to produce 7i-allyl boryl complex V. The ti-c rearrangement takes place (IV VI, V VII). Subsequently, reductive elimination to give the C-D bond from VI or to give the C-B bond from VII yields the deuterated hydroboration product and reinstalls an intermediate II to complete the catalytic cycle. However, up to date it has not been possible to confirm which pathway is correct. 

The stoichiometric insertion of terminal alkenes into the Cu-B bond of the (NHC)Cu-B(cat) complex, and the isolation and full characterisation of the p-boryl-alkyl-copper (I) complex has been reported. The alkyl complex decomposes at higher temperatures by P-H elimination to vinylboronate ester [67]. These data provide experimental evidence for a mechanism involving insertion of alkenes into Cu-boryl bonds, and establish a versatile and inexpensive catalytic system of wide scope for the diboration of alkenes and alkynes based on copper. 

The complexes [Cu(NHC)(MeCN)][BF ], NHC = IPr, SIPr, IMes, catalyse the diboration of styrene with (Bcat) in high conversions (5 mol%, THF, rt or reflux). The (BcaO /styrene ratio has also an important effect on chemoselectivity (mono-versus di-substituted borylated species). Use of equimolecular ratios or excess of BCcat) results in the diborylated product, while higher alkene B(cat)j ratios lead selectively to mono-borylated species. Alkynes (phenylacetylene, diphenylacety-lene) are converted selectively (90-95%) to the c/x-di-borylated products under the same conditions. The mechanism of the reaction possibly involves a-bond metathetical reactions, but no oxidative addition at the copper. This mechanistic model was supported by DFT calculations [68]. 

The enantioselective P-borylation of a,P-unsaturated esters with (Bpin) was studied in the presence of various [CuCl(NHC)] or [Cu(MeCN)(NHC)] (NHC = chiral imidazol-2-ylidene or imidazolidin-2-ylidene) complexes. The reaction proceeds by heterolytic cleavage of the B-B bond of the (Bpin), followed by formation of Cu-boryl complexes which insert across the C=C bond of the unsaturated ester. Best yields and ee were observed with complex 144, featuring a non-C2 symmetric NHC ligand (Scheme 2.31) [114]. 

Scheme 2.31 Copper-catalysed asymmetric borylation of conjugated enones...

Further insight into the P-borylation reaction of a,P-enones (Scheme 2.32) showed that the reaction can be carried out in THF, and the catalyst generated in situ from CuCl (5 mol%), the imidazolium salt (5 mol%), and NaO Bu (10 mol%), to form the [Cu(O Bu) (NHC)] as the catalysis initiating species. In this case, stable boron enolate products are formed which need to be hydrolysed by methanol to the ketone products [114]. 

One most important observation for the mechanistic discussion is the oxidative addition/insertion/reductive elimination processes of the iridium complex (31) (Scheme 1-10) [62]. The oxidative addition of catecholborane yields an octahedral iridium-boryl complex (32) which allows the anti-Markovnikov insertion of alkyne into the H-Ir bond giving a l-alkenyliridium(III) intermediate (34). The electron-... 

A catalytic cycle proposed for the metal-phosphine complexes involves the oxidative addition of borane to a low-valent metal yielding a boryl complex (35), the coordination of alkene to the vacant orbital of the metal or by displacing a phosphine ligand (35 —> 36) leads to the insertion of the double bond into the M-H bond (36 —> 37) and finally the reductive elimination to afford a hydroboration product (Scheme 1-11) [1]. A variety of transition metal-boryl complexes have been synthesized via oxidative addition of the B-H bond to low-valent metals to investigate their role in cat-... 

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