Borane-carbonyl chemistry

Boranes in Organic Chemistry, 11, 1 Carboranes and Organoboranes, 3, 263 Catalyses by Cobalt Carbonyls, 6, 119... 

Even though qualitative bonding descriptions of metal atom clusters up to six or seven atoms can be derived and in some cases correlated with structural detail, it is clear that most structures observed for higher clusters cannot be treated thus. Nor do the structures observed correlate with those observed for borane derivatives with the same number of vertices. Much of borane chemistry is dominated by the tendency to form structures derived from the icosahedron found in elemental boron. However, elemental transition metals possess either a close-packed or body-centered cubic arrangement. In this connection, one can find the vast majority of metal polyhedra in carbonyl cluster compounds within close-packed geometries, particularly hexagonal close-packing. 

Just as for group 5, 6, and 7 ( -CsF MCU species, Fehlner has shown that BH3-THF or Li[BH4] react with group 8 and 9 cyclopentadienyl metal halides to result in metallaborane clusters, many of them having a metal boron ratio of 1 3 and 1 4, and much of the synthetic chemistry and reactivity shows close connections with the earlier transition metals. The main difference between the early and later transition metallaboranes that result is that the latter are generally electron precise cluster species, while as has been shown, the former often adopt condensed structures. Indeed, as has been pointed out by King, many of the later transition metallaborane clusters that result from these syntheses have structures closely related to binary boranes and, in some cases, metal carbonyl clusters such as H2Os6(CO)18.159... 

The foregoing examples show the relevance to metal-carbonyl cluster chemistry of the borane-oarborane structural and bonding pattern. Its relevance to other areas of chemistry may be explored readily using a systematic skeletal electron-counting procedure (161, 201). 

The field of organoboron chemistry pioneered by Brown [40] also provides a wealth of excellent transformations. Consider the asymmetric reduction of carbonyl compounds by Alpine-Borane [41]. Alpine-Borane is prepared by the following sequence ... 

It quickly became clear that the use of Itsuno chemistry for reducing the carbonyl group of XVII [with R-diphenylvalinol borane complex] would not be economic, again owing to the need for excesses of the borane complex. The catalytic elaborations of Itsuno s chemistry also failed. 

Although carbohydrates are cheap and readily available chiral compounds, their application in stereoselective synthesis was for a long time limited to ex-chiral-pool syntheses [3]. They have been considered too complex compared to other chiral auxiliaries, for example a-pinene in borane-chemistry [4] or BINAP-derivatives in reduction chemistry [5]. However, it has been shown during the past few years that carbohydrates can be successfully applied as stereodifferentiating tools in many different reaction types such as aldol- [6], hydrogenation- [7], carbonyl addition- [8], Michael- [9], Diels-Alder- [10], hetero-Diels-Alder [11], and rearrangement reactions [12]. 

Sec also the borane and carborane-manganese carbonyl compounds R. N. Grimes, in Comprehensive Organometallic Chemistry, ed, G. Wilkinson, F. G. A. Stone and E. W, Abel, Pergamon, Oxford, 1982, vol. 1, p. 459. 

Tris(pentafluorophenyl)borane [B(CgFj)3], is a powerful and selective Lewis acid catalyst used in many reactions in organic chemistry [1 ]. Parks and Piers [5] found that B(CgFj)3 catalyzes the hydrosilylation of carbonyl compounds. The silylation of alcohols with the formation of Ft as the only by-product [6] and the cleavage of silyl ether and ether bonds catalyzed by B(C F5)3 [7, 8] provide an... 

The allylation reactions of carbonyl compounds catalyzed by chiral Lewis acids represent a powerful new direction in allylmetal chemistry. Yamamoto and coworkers reported the first example of the catalytic enantioselective allylation reaction in 1991, using the chiral (acyloxy)borane (CAB) catalyst system (see below) [288]. Since then, several additional reports of the catalytic allylation reaction have appeared. To date, the most effective catalyst systems reported for the enantioselective reaction of aldehydes and Type II allyl- and crotylstannane and silane reagents include the Yamamoto CAB catalyst and catalysts complexes composed of various Lewis acidic metals and either the BINOL or BINAP chiral ligands [289-293]. Marshall and Cozzi have recently reviewed progress in the enantioselective catalytic allylation reaction [294, 295]. 

A particularly useful borane is the chiral reagent (-)-B-allyl(diisopinocampheyl)borane.l 3 Although the chemistry of acyl addition to aldehydes and ketones to give alcohols will not be formally discussed until Section 8.4.C, the addition of this chiral allylborane to carbonyl compounds will be presented here. In a simple example taken from Smith s synthesis of (-)-macrolactin A,n 156 was treated with (-)-B-... 

Metal triangles, tetrahedra, and octahedra form the basic building blocks of transition metal carbonyl clusters. The smaller clusters with between three and six metal atoms often adopt these pseudo-spherical deltahedral geometries, but as the nucle-arity of the cluster increases, condensed structures, built up from the smaller poly-hedra by vertex-, edge-, or face-sharing, tend to be favored in preference to the larger spherical deltahedra based on the Platonic and Archimedean solids. In general, this is in contrast to the structures foimd in borane chemistry. 

The adherence to close-packed structural arrangements lends support to the idea that these compounds can be used as models for metal surface chemistry—with respect to chemisorbed species and their mobility and reactions of substrates on these surfaces. It also indicates a marked deviation from the behavior of boranes and their derivatives. Structures based upon some polyhedra favored by boron, such as the pentagonal bipyramid, triangulated dodecahedron, and especially the icosahedron, are absent so far in metal-carbonyl cluster chemistry. In this connection, it has been mentioned that [M(CO)3],g compounds should be the closest analogs to On skeletal electron counting... 

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