Preparation Organoaluminium chemistry

1 preparation

1.1 alkyl halides , aluminium
1.2 hydroalumination
1.3 carboalumination
1.4 laboratory preparations

preparation
from alkyl halides , aluminium

industrially, simple aluminium alkyls of type al2r6 (r = me, et) prepared in two-step process beginning alkylation of aluminium powder:

2 al + 3 ch3ch2cl → (ch3ch2)3al2cl3

the reaction resembles synthesis grignard reagents. product, (ch3ch2)3al2cl3, called ethylaluminium sesquichloride. term sesquichloride refers fact that, on average, cl:al ratio 1.5. these sesquichlorides can converted triorganoaluminium derivatives reduction:

2 (ch3ch2)3al2cl3 + 6 na → (ch3ch2)6al2 + 2 al + 6 nacl

this method used production of trimethylaluminium , triethylaluminium.

hydroalumination

aluminium powder reacts directly terminal alkenes in presence of hydrogen. process entails 2 steps, first producing dialkylaluminium hydrides. such reactions typically conducted @ elevated temperatures , require activation trialkylaluminium reagents:

3 al + 3/2 h2 + 6 ch2=chr → [hal(ch2chr)2]3

for nonbulky r groups, organoaluminium hydrides typically trimeric. in subsequent step, these hydrides treated more alkene effect hydroalumiunation:

2 [hal(ch2chr)2]3 + 3 ch2=chr → 3 [al2(ch2chr)3

diisobutylaluminium hydride, dimeric, prepared hydride elimination triisobutylaluminium:

2 i-bu3al → (i-bu2alh)2 + 2 (ch3)2c=ch2

carboalumination

organoaluminum compounds can react alkenes , alkynes, resulting in net addition of 1 organyl group , metal fragment across multiple bond (carboalumination). process can proceed in purely thermal manner or in presence of transition metal catalyst. uncatalyzed process, monoaddition possible when alkene substituted. ethylene, carboalumination leads poisson distribution of higher alkylaluminum species. reaction regioselective 1-alkenes. so-called zaca reaction first reported e.-i. negishi example of asymmetric carboalumination of alkenes catalyzed chiral zirconocene catalyst.

the methylalumination of alkynes in presence of cp2zrcl2 employed synthesis of stereodefined trisubstituted olefin fragments, common substructure in terpene , polyketide natural products. synthesis of (e)-4-iodo-3-methylbut-3-en-1-ol shown below typical application of reaction:

for terminal alkynes, reaction proceeds regioselectivity (>90:10 rr) , complete syn selectivity, in presence of propargylic or homopropargylic heteroatom substituents. unfortunately, extension of zirconocene-catalyzed methylalumination alkylalumination higher alkyls results in lower yields , poor regioselectivities.

laboratory preparations

although simple members commercially available @ low cost, many methods have been developed synthesis in laboratory, including metathesis or transmetalation. metathesis of aluminium trichloride rli or rmgx gives trialkyl:

alcl3 + 3 buli → bu3al + 3 licl


transmetalation


2 al + 3 hgph2 → 2 alph3 + 3 hg




^ michael j. krause, frank orlandi, alfred t. saurage , joseph r. zietz aluminum compounds, organic in ullmann s encyclopedia of industrial chemistry, 2005, wiley-vch, weinheim. doi:10.1002/14356007.a01_543
^ barry m. trost; martin f. semmelhack; ian fleming (1992). comprehensive organic synthesis: additions , substitutions @ c-c[pi]-bonds. pergamon. isbn 9780080405957. 
^ negishi, ei-ichi (2011). discovery of zaca reaction : zr-catalyzed asymmetric carboalumination of alkenes . arkivoc. 2011 (viii): 34–53. 
^ negishi, ei-ichi; wang, guangwei; rao, honghua; xu, zhaoqing (2010-05-14). alkyne elementometalation−pd-catalyzed cross-coupling. toward synthesis of conceivable types of acyclic alkenes in high yields, efficiently, selectively, economically, , safely: green way . journal of organic chemistry. 75 (10): 3151–3182. doi:10.1021/jo1003218. pmc 2933819 . pmid 20465291. 
^ negishi, ei-ichi (2002). organometallics in synthesis: manual (ed. m. schlosser). chichester, west sussex, uk: wiley. pp. 963–975. isbn 0471984167. 
^ rand, cynthia l.; horn, david e. van; moore, mark w.; negishi, eiichi (2002-05-01). versatile , selective route difunctional trisubstituted (e)-alkene synthons via zirconium-catalyzed carboalumination of alkynes . journal of organic chemistry. 46 (20): 4093–4096. doi:10.1021/jo00333a041.