Diborations

Dicesium tetraborate pentahydrate [12228-83-0] 5H20, and dicesium diborate heptahydrate [66634-85-3] Qs O 7H20, also exist. 

Fig. 9. The borate glass stmctural groups where = boron Q — oxygen (a) boroxol (b) pentaborate (c) triborate and (d) diborate (142).

Traces of water have been removed by refluxing with tetraacetyl diborate (prepared by warming 1 part of boric acid with 5 parts (w/w) of acetic anhydride at 60, cooling, and filtering off), followed by distn [Eichelberger and La Mer J Am Chem Soc 55 3633 1933],... 

Aromatische Aldehyde und Ketone lassen sich auch durch Dibor an unter elektrophiler Katalyse von Bortrifluorid-Atherat zu Kohlenwasserstoffen reduzieren8. Die Methode ist z.B. besonders zur Herstellung von Benzyl-cyclopropanen geeignet, die auf andere Weise nur schlecht zuganglich sind9. 

A crystalline sample of diborate 79 that is bridged via three 1,4-alkanedioxy moieties has been obtained accidentally during the reduction of trans-1,2-cyclopropanedicarboxylic acid with H3B THF in a yield of 22% (Fig. 21). 

Diborate 80 is produced from a 3 2 mixture of l,l -bi-2-naphthol and boric acid in refluxing benzene as a racemic mixture of RRR and SSS combinations... 

Fig. 21. Diborates 79 and 80 are propeller compounds with a cage-like cavity...

Abstract The use of A-heterocyclic carbene (NHC) complexes as homogeneous catalysts in addition reactions across carbon-carbon double and triple bonds and carbon-heteroatom double bonds is described. The discussion is focused on the description of the catalytic systems, their current mechanistic understanding and occasionally the relevant organometallic chemistry. The reaction types covered include hydrogenation, transfer hydrogenation, hydrosilylation, hydroboration and diboration, hydroamination, hydrothiolation, hydration, hydroarylation, allylic substitution, addition, chloroesterification and chloroacylation. 

The metal catalysed hydroboration and diboration of alkenes and alkynes (addition of H-B and B-B bonds, respectively) gives rise to alkyl- or alkenyl-boronate or diboronate esters, which are important intermediates for further catalytic transformations, or can be converted to useful organic compounds by established stoichiometric methodologies. The iyn-diboration of alkynes catalysed by Pt phosphine complexes is well-established [58]. However, in alkene diborations, challenging problems of chemo- and stereo-selectivity control stiU need to be solved, with the most successful current systems being based on Pt, Rh and An complexes [59-61]. There have been some recent advances in the area by using NHC complexes of Ir, Pd, Pt, Cu, Ag and Au as catalysts under mild conditions, which present important advantages in terms of activity and selectivity over the established catalysts. 

The Ag complex 74 catalyses the diboration of internal and terminal aUcenes to 1,2-bis-diboronate esters by bis(catecholato)diboron, (Bcat), in THF at room temperature. Variable conversions (30-90%) were obtained at 5 mol% loading after 60 h. 

Fig. 2.12 Silver, gold and platinum complexes with monodentate NHC ligands as catalysts for the diboration of alkenes and alkynes...

Diboration of terminal alkenes has also been studied with other d " metals (Fig. 2.12) including the Ag and Au complexes 75-77 and the Pt" complexes 78-79. Styrene is diborylated with 100% selectivity and good conversions in THF (46% for 75 and 94% for 77 at 5 mol%, 60 h) using equimolecular amounts of (Bcat)j. The difference in activity between the Ag and Au complexes has been ascribed to the increased lability of the Ag-NHC bond, which may lead to catalyst decomposition under the reaction conditions, hi both catalytic systems it is believed that the active species involves only one coordinated NHC ligand. Complex 77 is less active than 74 and 75, possibly due to steric reasons. The enantioselectivity of 77 in the diboration of prochiral alkenes is very low [63]. 

Diboration of terminal and internal alkynes by (Bcat) [B(cat)2 alkene = 1 1] was also achieved by using complexes 78 and 79 at room temperature in THF. Best activity was observed with 78 containing the less electron donating triazolyhdene carbene hgand. Terminal alkenes can also be diborylated under the same conditions, however the selectivity for the diborylated product was much lower (up to 65%) [64]. 

The palladium (II) NHC complexes 81 and 82 (Fig. 2.13) have also been used as catalysts in the diboration of styrene. In the presence of NaOOCCHj, (1 equiv.. 

Fig. 2.13 Diboration catalysts based on palladium and iridium NHC complexes...

The Ir complexes 83 or [lr(lMes)Cl2Cp ], in the presence of NaOAc and excess of (Bcat), catalyse the diboration of styrene, at high conversions and selectivities for the diborated species, under mild conditions. Other terminal alkenes react similarly. The base is believed to assist the heterolytic cleavage of the (cat)B-B(cat) bond and the formation of Ir-B(cat) species, without the need of B-B oxidative addition [66]. 

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]. 

Magnesium diborate Magnesium perborate Boron oxide Titanium diboride Boron carbide... 

I 7 Hydroboration, Diboration, Silylboration and Stannylboration Tab. 1-3 Isomerization to the Terminal Carbon 1. HBX2/catalyst... 

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