Iminoboranes 4 + 2 -cycloadditions

For reason of symmetry conservation, a thermal concerted [2-1-2]-cycloaddition is forbidden as far as D2h symmetry can be assumed for the activated complex. This is not necessarily the case for the concerted dimerization of iminoboranes, in which the symmetry requirements seem to be essentially lowered. Nevertheless, a two-step mechanism according to Eq. (21) must be taken into account. The assumed intermediate in Eq. (21) contains a sextet boron atom with a linear... 

Thermal or catalytic cyclodimerization of iminoboranes is obviously a [2 -I- 2]-cycloaddition (Section IV). A mixture of two different iminoboranes may be stabilized by formation of three different cyclodimers. If the relative stability of the two iminoboranes, however, differs distinctly, the mixed cyclodimer will be formed preferentially by... 

Reaction of aldehydes and ketones with iminoboranes has been widely investigated. Conditions for the [2 + 2]-cycloaddition between XBNR and R R"CO are relatively good stability of the iminoborane and lack of enolic protons in the oxo compound [Eq. (46)] 14, 19). Relatively less stable iminoboranes, but in some cases the stable ones too, may react with 0X0 compounds by a total opening of the B=N triple bond [Eq. (43)], presumably via a [2 + 2]-cycloaddition [Eq. (42)] (Section V,D). A relatively stable iminoborane and a ketone containing enolic protons may yield an open-chain product, probably through a six-membered cyclic transition state [Eq. (46b)] 19). 

Turning from iminophosphanes to alkylidenophosphanes (phospha-alkenes), the orientation of the [2 + 2]-cycloaddition is inverted, as far as phosphorus is concerned only one example has been worked out (product VIII) 19). The phosphaalkyne iBuC=P does not react with the iminoborane BuB=NtBu, which instead trimerizes (IS). An exotic [2 + 2]-cycloaddition is observed when the very reactive titanaethene... 

Alkynes have been well explored as dipolarophiles in the [3 -t- 21-cycloaddition with almost all possible 1,3-dipoles (78), whereas the reaction of iminoboranes as dipolarophiles has focused on covalent azides as 1,3-dipoles. Most well-characterized iminoboranes were reacted with phenyl azide, according to Eq. (52) (11-14,17, 20). 

The formation of Dewar borazines from iminoboranes and their cyclodimers is also a [4 + 2]-cycloaddition, whether or not the Dewar borazines are the final products or are rearranged to normal borazines (Section IV,E). 

Cycloaddition phosphathioketenes, 33 332 reactions, in preparation of fluoroalicyclic derivatives, 15 327-328 reactions, of iminoboranes, 31 159-165 [2-1-2] Cycloaddition phosphaalkenes, 33 271-272 phosphathioketenes, 33 334 [2-t-l] Cycloaddition, phosphaalkenes, 33 269-271... 

As indicated in Scheme 9.4, the formation of Dewar borazines involves the dimerisation of an iminoborane followed by the [4 + 2] cycloaddition of the cyclodimer thus formed with an iminoborane. For smaller R/R groups, the isomerisation of the bicyclic Dewar borazine to a monocyclic borazine occurs readily. 

Transformations to the cyclotrimeric boiazines and cyclotetrameric tetraza-2,4,6,8,l,3,5,7-tetraboracanes also occur. The rate of dimerization for amino iminoboranes has been shown to be stabilized by bulky substituents (76,79,83). This stabilization through dimerization is essentially a [2 + 2] cycloaddition. There are a number of examples of these compounds forming cycloadducts with other unsaturated polar molecules (78). Iminoboranes can add to electron-deficient carbene complexes of titanium such as (C5H5)2Ti(CH2) [84601-70-7] by [2 + 2] cyclo addition, yielding the metallacycle shown in equation 26 (84). 

Sakurai et al. were successful in reacting even tetramethylsilene (Me2Si=SiMe2) with benzene in a photochemical [2 + 4] cycloaddition at 10 K in an argon matrix [472]. With mentioning the suitability of iminoboranes [473] and selenoaldehydes [474] to serve as dienophiles in hetero Diels-Alder reactions, this enumeration of exemplary, less usual hetero Diels-Alder reactions shall be completed. 

It shonld be stressed that in some cases, for example, chloroboranations, alkylideno-iminoboranes give different prodnct types when compared to amino-iminoboranes. Some additional reactivity studies are summarized for t-BuB=N-t-Bu (30) and tmpB=N-t-Bu (31) in Schemes 6 and 7. Note that the ontcome of silylene cycloaddition to (30) is dependent on the size of the organic fragment on the silylene, and the addition of the cyclostannylene occurs... 

Cycloaddition reactions of two-co-ordinate phosphorus compounds described this year includes the [2 + 2] cycloaddition of an iminoborane to aminoimino-phosphines to give 1,3,2,4-diazaphosphaboretidines (125). In an attempt to prepare hydrazino-bis-phosphaalkenes (126) as new candidates for hetero-Cope rearrangements, the tricyclic compounds (127) were obtained. The result is explained by a Diels-Alder rearrangement of (126), as shown. 

The four-membered heterocycles 72 and 73 were obtained by organometallation of amino-iminoboranes and by cycloaddition of aminoalkylideneborane to the same moiety [17]. 

Iminoboranes t-BuB NR result from Cl(t-Bu)B=N(R)SiMe3 in the gas phase and react with t-BuHC=TaCpCl2 by [2+2] cycloaddition while t-BuHC=Ta(CH2Bu-t)3 reacts with... 

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