Diazaborolidines

Results of the asymmetric 2-propenylborations of several chiral a- and /i-alkoxy aldehydes are presented in Table 11 74a-82 84. These data show that diisopinocampheyl(2-propenyl)borane A and l,3-bis(4-methylphenylsulfonyl)-4,5-diphenyl-2-propenyl-l,3,2-diazaborolidine C exhibit excellent diastereoselectivity in reactions with chiral aldehydes. These results are in complete agreement with the enantioselectivity of these reagents in reactions with achiral aldehydes (Section 1.3.3.3.3.1.4.). In contrast, however, the enantioselectivity of reactions of the tartrate 2-propenylboronate B (and to a lesser extent the tartrate (/i)-2-butenylhoronate)53b is highly... 

Several chiral heterocyclic borylating agents have been found useful for enantio-selective aldol additions. The diazaborolidine 14 is an example.136... 

The possibility of using chiral auxiliaries or chiral catalysts to achieve enantio-selective Claisen rearrangements has been explored.253 One approach is to use chiral boron enolates. For example, enolates prepared with the chiral diazaborolidine bromide O lead to rearranged products of more than 95% enantiomeric excess.254... 

Entry 6 is an example of application of the chiral diazaborolidine enolate method (see p. 572). Entry 7 involves generation of the silyl ketene acetal by silylation after conjugate addition of the enolate of 3-methylbutanoyloxazolidinone to allyl 3,3,3-trifluoroprop-2-enoate. A palladium catalyst improved the yield in the rearrangement... 

The reaction of diazaboroline 10 selectively furnishes only the regioisomer ll,245 whereas that of diazaborolidine 9 with 1-phenyl-1-propyne results in the formation of a mixture of regioisomers 11 and 12 (85 15) (Equation (98)).246... 

Chiral diazaborolidines catalysts in asymmetric reductions have been less described than the corresponding oxaza-borolidines. Although not isolated, the formation of compound 47 has been characterized by nB NMR spectroscopy with the detection of a signal at 24 ppm (from BF3.Et20 as an external standard) <2000TA4329>. 

C NMR data in support of the betaine structure of the diazaborolidine 56 indicate that delocalization of the positive charge causes a downfield shift for C-5 (ca. 13 ppm) and C-3a/C-6a (ca. 4 ppm) <2004CC1860>. 

The electrochemical properties of the imidazo[4,5-,7 [l,3,2]diazaborolidines 56, especially the AgEM constants (10 ), are close to the boron-bridged violenes indeed, the values obtained for 56 are on a par with those of the acceptor part in organic metals derived from TCNQ or -dicyanochinodiimine (DCNQI) (AgEM 10 -10 ) <2004CC1860>. 

The boron substructure was eliminated from the imidazo[4,5-i/][l,3,2]diazaborolidine framework 56 following ultrasonic reduction with metallic lithium in THE and anions of the imidazoles 90 formed. The observed reactivity mled out the possibility of isolating the intermediates from these single-electron-transfer reactions <2004CC1860>. 

The imidazo[4,5- [l,3,2]diazaborolidines 56 were formed from 47/-imidazoles 90 on reaction with boranes in the presence of NEts (Equation 16) <2004CC1860>. 

A further example of synthesis of the imidazo[4,5-rf [l,3,2]diazaborolidine ring system involving reaction between 4-imino-5-aminoimidazoles, analogous to 90, and boranes has been reported (see Section 10.05.9.1.2(viii)) <2006ZN(B)437>. 

The typical preparation of the cyclic boroles from acyclic starting materials usually involves acid base chemistry or transacetalization protocols. 1,3,2-Diazaborolidines, such as 311 and 313, are readily available via the reaction of the corresponding amines 310 and 314 with boron tribromide (BBr3) and boron halide 312 (Scheme 51) <2006JCD3777>. 

Diazaborolines, isoelectronic with C5H5-, have been obtained by dehydrogenation of diazaborolidines (79) and from a,/3-diimines and halo-genoboranes (80). From UB- and 14N-NMR, UV, and PE studies, it was concluded that compounds of type 13 possess a 7r -electron system and therefore should be able to complex the Cr(CO)3 fragment. This was proved by the formation (91%) of the yellow complex 64 (80a) (air-stable decomp, above 170°C). As observed in borazine and diazadiborine complexes, only a small upfield shift (13, 8 = 26.2 64, 8 = 18.3 ppm) occurs in the B-NMR spectrum. Its constitution has been derived from lH and 13C-NMR, IR [jmass spectra data. (See Scheme 5.)... 

The reaction of N-t-Bu-N -trimethylsilylcarbodiimide with 2-chloro-1,3-dimethyl-1,3-diazaborolidine affords N-t-butyl-N -diazaborolidin-2-ylcarbodiimlde, bp 57 °C in high yield. ... 

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